Sedimentology and development of barrier islands, ebb-tidal deltas, inlets and backbarrier areas of the Dutch Wadden Sea more

65 Senckenbergiana maritima 24 65-115 Frankfurt am Main, 20. 6. 1994 Sedimentology and Development of Barrier Islands, Ebb-tidal Deltas, Inlets and Backbarrier Areas of the Dutch Wadden Sea With 49 Text-Figures and 1 Table ALBERT P. OOST & POPPE L. DE BOER ABSTRACT rOOST, A. P. & BOER, P. L. DE (1994): Sedimentology and development of barrier islands, ebb-tidal deltas, inlets and backbarrier areas of the Dutch Wadden Sea. - Senckenbergiana marit., 24 (1/6): 65-115, 49 figs., 1 tab.; Frankfurt a. M.] This paper presents an overview of the Dutch Wadden Sea from a sedimentological point of view. After the pioneering work of scientists like, amongst others, VAN STRAATEN and REINECK in the fifties and sixties, new impulses to this kind of research are being given by the need for detailed recent analogues of fossil hydrocarbon-containing rock successions and by the great concern about the future of our coastline in relation to accelerated sea-level rise. After many studies of a descriptive nature in the past, there is now a growing tendency to a more dynamical view to the Wadden Sea system. There is a strong interdependence between various tidal sub-environments within individual inlet systems. Together these sub-environments form so-called Sand Sharing Systems, whose behaviour is largely defined by the tidal prism and the wave climate. Such a dynamical approach may greatly facilitate the research and understanding of fossil barrier-related sediments. Apart from the physical processes the abundant biota plays also an important role in the sedimentological development of the Wadden Sea. The large amount of data on the development of the Wadden Sea in pre-historical and historical times, moreover, allows to test hypotheses about the evolution of the system on the scale of centuries to millennia. Contents Introduction Sedimentation: Sources and Sinks .................................................... ..... , ........................................................... " .. . . ... Sands Clays ............ .............................................................. Morphological Elements and Sedimentation Patterns .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. Barrier Islands .................................................................... The North Sea Barrier Island Coast .............................................. Dunes .......................................................................... Tidal Marshes and Washover Channels ........................................... Ebb-tidal Deltas and Inlets ...................................................... " Ebb-tidal Deltas . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Inlets .................................. '" . . . . .. . . . . . . . . . .. .. . . . . . .. . . . . . . . . . .. The Ebb-tidal Deltas and Inlets of the Dutch Barrier Islands ...... . . . . . . . . . . . . . . .. Cyclic Morphological Changes of Positions and Orientations of Shoals and Channels Sedimentary Facies and Sequences .......... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 66 67 67 67 68 68 70 74 74 75 75 75 77 79 80 Authors' Address: ALBERT P. OOST, POPPE L. DE BOER, Comparative Sedimentology Division, Faculty of Earth Sciences, Utrecht University, P.O. Box 80.021, NL-3508 TA Utrecht, The Netherlands Key words: Tides, tidal flat, tidal inlet, ebb-tidal delta, barrier island, Wadden Sea, tidal marshes, hydrocarbons, sea-level rise. - Running title: Dutch Wadden Sea 66 Backbarrier Area .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. C hannels . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Shoals and Gullies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . High Shoals and Salt Marshes .......... ..... .... .. ... . . ......... ...... . . ......... Sand-sharing System .. .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Relationships between Flora/ Fauna and Sediments .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. Bacteria . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. Diatoms ....... .. ............ . ... ....... ...... ... . ..... .... . .... . .. ............. Mytilus edlliis L (Blu e Mussel) ............. . ...... .... ......... .. . .. ............. Arenicola marina (L) (Lugworm) .. . . .......... .. ........ . . . . ... .. . . .. .... Pkn~ 82 82 84 91 92 93 93 93 94 95 ~ ..................... .. .. . .. .. . .................. . . ..... ... ........ ..... .. Sedimentary History of the Dutch Wadden Sea .... ...... ...... .............. . 95 Pleistocene ..................... . . . ......... . ............................ .. ...... . 95 H olocene 96 Pre-histo ric Sea-level Ri se and Sedimentation ... .... ...... .... . .. ............ . 96 Historical Development : Man and Flooding ............ ... .... ........ ...... . 98 Future Development: Gree nh ouse Effect and Sea-level Rise .......................... . 108 Acknowledgem ents .... .... ........ .. ....... .... ............. . . . .............. . .... . 109 References ............................. . . . . .. .. . . . . .. . . .. .... . . . .. . .. . ............ . 109 Introduction The obj ective of this paper is to give an overview of the sedimentology of the Dutch Wadden Sea barrier coast based on the many studies of the last decades. The Dutch Wadden Sea was studied in th e fifties, th e sixties and the seventies in great detail. In order to assess the potential impact of sea-level rise over the next decades, extensive studies are being undertaken to gain a better understanding of the interrelations between the hydrod ynamics and the morphology / sedimentology of the area. These studies lead to a new and dynamic view of the sedimentology of the Wadden Sea. Barrier islands, tidal inlets, tidal deltas, lagoons and tidal flats commonly occur along the coasts of marine basins with a micro- and especially mesotidal regime and a limited supply of sediment (Fig. 1; HAYES 1979). Their origin is usually related to a rise of sea level which, on the one hand, leads to a decrease of gradient on the land and a consequent decrease of sediment supply and~ on the other hand, to the shoreward transport and accumulation of marine sediments. Although tidal flats and salt marshes also occur along macrotidal coasts, barrier islands are commonly absent there. The Wadden Sea, wi th a tidal range from 1.4 to 3.5 m (Fig. 2), frin ges the Dutch, German and Danish coasts over a distance of nearly 500 km with a maximum width of approximately 35 km. Towards the North Sea it is bordered by some 20 large and many small barrier islands and sandy shoals. Behind these islands lies the largest tidal flat area in Europe. The notion 'wadden' indicates the intertidal flats, as they occur extensively in the Wadden Sea. The mainland coast consists of dykes, some salt marshes and a few Pleistocene cliffs. In the Dutch Wadden Sea the tidal regime is microtidal in the southwestern part, becoming mesotidal towards the east. In some of the deep channels between the larger islands the water depth may reach several tens of metres. During low tide about half of the area is exposed. Supply of fresh water and sediment by rivers is of minor importance. The bulk of the sediment is supplied from the North Sea; mino r amounts of carbonates and organic matter are produced within the Wadden Sea. Generally, the salinity approaches that of the coastal North Sea, except in parts of the western Dutch Wadden Sea where fr esh water from the lake 'IJsselmeer' and from small local rivers and manmade canals dilutes the coastal waters. Shallow coastal waters such as the Wadden Sea are among the richest food supplying marine ecosystems, supporting a ri ch flora and fauna . A large number of marine animals lives in the Wadden Sea during their juvenile stages and thus the area serves as an important nursery ground for the North Sea fauna. Fig. 1. Relat ion between the morph ology of th e coastline and the tid al range (HAYES 1979). 67 ( REINECK Fig. 2. Tidal parameters of the North Sea and th e British Isles. - A. Co-tidal lines; B. Co-range lines 1982a) . Sedimentation: Sources and Sinks Sands In the Dutch Wadden Sea the net deposition of sand amounts to 8-21 x 10 6 m 3 /yr (ANONYMUS 1981). This sand is mostly derived from erosion of the shoreface, beaches and dunes of the North Sea coast of the barrier islands and northern Holland (d. WINKELMOLEN & VEENSTRA 1974; VEENSTRA & WINKELMOLEN 1976), but most probably also from the adjacent North Sea floor (OOST & DE HAAS 1993). Deposition takes place mainly on the tidal flats , 87.5% of which comprise sandy sediments. The remaining 12.5% consist of fin e-grained muddy sediments. To compensate for the relative sea-level rise, deposition of several million m 3 /yr is needed in order to maintain the same altitude with regard to the mean sea level. In addition, substantial amounts of sand which are extracted from the Wadden Sea for building and construction purposes have to be compensated for. The recent pattern of sedimentation and erosion in the western Dutch Wadden Sea is still adapting to the change of tidal flow patterns, resulting from the closure of the 'Zuiderzee' embayment in 1932 by the construction of the 'Afsluitdijk'. Deposition in the wake of this enclosure was, and still is, exceptionally large (Fig. 3): old channels changed their direction, while new channels and tidal flats were formed . The sands in the Wadden Sea are largely composed of quartz (more than 80%) with some feldspar, CaC0 3 (mainly mollusc shell fragments (CADEE, in press) and mica (Fig. 4; VAN STRAATEN 1964). The sand deposited in the Wadden Sea is somewhat finer (Md: 170-190 JLm) than the sands along the North Sea coast (usually greater than 200 JLm) . Due to sorting processes the coarser sand does not pass the inlets. Sorting takes place towards the mainland (WINKELMOLEN & VEENSTRA 1974) and in the direction of th e long-shore current. As a result the grain size along the coast of the barrier islands and in the backbarrier areas decreases gradually from W to E (VEENSTRA & WINKELMOLEN 1976). Apart from the mineralogical similarity of the sands in the Wadden Sea and th ose in the adjacent North Sea, the influx from the North Sea is reflected by th e occurrence of mollusc shell fragments, sea urchin spines and foraminifera (d. VAN VOORTHUYSEN 1960). Clays The fine-grained material deposited in the Wadden Sea is also brought in from the North Sea (WIGGERS 1960; VAN STRAATEN 1960). It is a mixture of material from different sources: the Channel, the Flemish Banks, sea-floor erosion, and the river Rhine (VAN VEEN 1936). Suspended matter from the Rhine comprises 10-20% of the total supply. The fine-grained sediments consist predominantly of clay minerals, organic matter and CaC0 3 (mainly detrital; Fig. 4; VAN STRAATEN 1964). Approximately a net amount of 1-3 xl0 9 kg (dry weight) of suspended material is deposited yearly in the Dutch Wadden Sea, mainly along the inner margins, near the coast of the mainland, on tidal watersheds and in the Ems-Dollard estuary (EYSINK 1979; EISMA 1981; DIJKEMA et al. 1988). 68 240 C') size class ---E 0 21 0-80~ 200 160 120 80 40 10 20 30 years Cumulative sedimentation in the backbarrier area of Texel inlet after closure of the 'Zuiderzee' Cumulative sedimentation in the backbarrier area of the 'Zoutkamperlaag' inlet after closure of the 'Lauwerszee' 80-16~ 16-2~ <D % 90 80 c c '';::; 0 ..c Q) CO E '6 Q) (/) - - - - -- 40 50 60 70 60 50 40 30 20 10 calcite and aragonite dolomite mica and clay minerals muscovite and illite O~~~~~ Fig. 3. Effects of closures: gradual decreasing, but long-lasting sedimentation ( - - ) after the closure of the 'Zuiderzee' in the backbarrier area of Texel Inlet and (---) after the closure of the 'Lauwerszee' in the Zoutkamperlaag-backbarrier area (VROOM et al. 1989). glauconite, heavy minerals, stone fragments and biogenic silica Fig. 4. Average composition of Wadden Sea sediments 1964). (VAN STRAATEN Morphological Elements and Sedimentation Patterns The Wadden Sea is characterized by a series of adjacent tidal inlet systems. Each inlet system consists of an ebbtidal delta with the inlet proper, parts of the barrier islands adjacent to the inlet, and the back barrier area. These elements, i.e. the barrier islands, the inlets with their ebbtidal deltas and the backbarrier areas form the most important morphological units of the Wadden Sea. Barrier Islands On its seaward side the Dutch Wadden Sea is bordered by barrier islands (Fig. 5). These are formed by the combined transport action of wind, waves and tides. The wind strength varies seasonally. Average wind velocities in winter reach 15 ml s, whereas in summer velocities attain 7 m/s. The tidal wave in the North Sea moves from the SeW) to the N(E), that is from Den Helder in Holland to Esbjerg in Denmark (Fig. 2). The vertical range is largest (over 3 m) in the German Bight, because there the distance to the amphidromic point is greater than elsewhere along the Wadden Sea coast. Smallest ranges are found near Den Helder (1.3 m) and Esbjerg (DI}KEMA et al. 1980). HAYES (1975) stated that the length of barrier islands depends on the mean tidal range: the larger the tidal range, the smaller the barrier island length. Indeed, for the barrier islands of the Wadden Sea such a relationship exists (Fig. 6; WOLFF 1986). At present the islands do not migrate strongly. This is mainly the result of artificial coastal protection. Moreover, some of the islands and shoals are fixed by erosion-resistant Pleistocene or early Holocene cores. The barrier islands consist of the following sedimentary subenvironments: the lower and upper shoreface, the intertidal beach along the North Sea coast, the dunes, and the tidal marshes which merge with the back-barrier tidal flats. Under natural conditions the islands are cut by wash over channels. Parts of the tidal marshes were moulded into polders by the construction of dykes, and wash overs were blocked by artificially stimulating the growth of dunes. The barrier island Schiermonnikoog provides the best example of the original, natural situation on the barriers, as it has been left relatively undisturbed by man (Fig. 7). To the west the island is bordered by the large Zoutkamperlaag Inlet and to the east by the small Eilanderbalg Inlet. t " ,. .. ' ,'_ '" •__y North Sea • A,' BORKUM , ,'~ 1','!7,», ,."." ~~ ,,"~ _~" ",," ,~~J~s.~1JW' ;:..:~".~; :-:-7!.:,~.c."P?-'~f \d' .~~;.'>4. ,,<" ,, __, AMELAN6" ,,' '. ~,~ , • "" __ , ""'SCH •.••.••• ,IER ,"-"" ' :::::::: .... __ .. , __ ",. ,'" TERSCHELLING'/'" : -:::. ~ ,~,w'"~0;<';f'f~;~f;!~; ~:',~'~'"""~r.:i,:;~ ",r " "<j, , ;: ,,0-,, "Pi' i' ,", ,,' 1,',:,';{;::),;'," >': ,'j,"',~""""",.~',.'.4l'; '0'; ,~'Yfo' , {~,<,-:.:'t<t :~ d~ Xl>'J0Ji"" , ;: j ;.)' .. ;" '; , "l '" ,,'~ , /".'."""..,:,.-. ,V ,'4 .' ' ~' _/r.....~""'~;. ' " _". .... .. ,.' . 'J ' ~","" , ,,,' • MONNIKOOG,,' " ' .- ,'i;":; "-" "r-;.o{:_,,-,,*'i .,...•••""( ••"""J"'" "",J""'" ' "..,.' '-;:>f', ?, '~'" " , " ,:, ", y; .. P0 Q'" "',, " ,;Z.~~:i,", ~;': f:S~' ' ' ). ~- .. r:::: .l~" Q!?;tt~{-~"'~"1\ (i!:~·~~.;.3~"~,..:;,." ~:.'\,;.". "W,,~., ,~.:.. "', \\ .,'~,' ,,: ","- <, " " " " , "";",,,y, 0 @'_'~-;rv;-:c;''C:J!..;./'7 '7r'JoI" .-,. ~~ :>__ -', .... ,. _T.t.··'·-··"'· " ",', '"'", ... ' ,,",,' C, •• ,.-"'-V,~ ,-:r., 'M ¢ ,,-' ,}1;", ' , _ _ _ , _,,' ··t· :;... ' ' \, .... ' " "'l ,,'-' ""," ," ":." ='" " O W , " S Z E E P O ; t :i ,;, . . ,,,,::>.!, .",,;.; 4"" " M HA'UNGEN 0 LEEUWARDEN a WE'''''~ . "'~~~ D",,">~''':::';$i i ''f,' i' " MOEN ' ,'" i ; I1 U r-.... :,;;;"'<.. GRONINGEN ...:;- ,<"""" ~ ~. :::_, :"ON'-roorzoncl '~ ..' ~ ..,: ; ::': ; : ..... : -: . ' LvtJ9~woot"d .- -r.' ; t'. r,J.'r .....~ '.DEN HELDER ... "~I :/ 'JSSELMEER Mean high water Dutch Ordnance Level (DOL) Mean Low Water - 10 ill DOL Fig. 5. Physiography of the Dutch Wadden Sea (STEIJN 1991). 0' '-0 70 The tidal amplitude near Schiermo nnikoog is 2.3 m, and the regime thus is mesotidal (HAYES 1979; POSTMA & DIJKE MA 1982). Do minant wave heights are 0.5-1.5 m at 20 m water depth (BI EG EL 1991b). T he coast of this barrier island is thus classified as a mixed, tide·dominated energy shoreline, influenced by both tides and waves (d. HAYES 1975, 1979). The island has the t ypical 'drumstick ' form, which is characteristi c for mesotidal coasts (HAYES 1979) . The North Sea Barrier Island Coast For a full discussio n of the lowe r and upper shoreface the reader is referred to FLEMMING & DAVIS (1992, 1994). Sedimentation and erosio n on the intertidal beaches of the islands is mainly bro ught about by wave·action, coast· parallel, east ward directed residual curre nts (tidally and wind·induced) and wind·driven aeolian processes (the lat· ter occurring especially above the high· water mark). Over a longer time· span, sites of erosion and sedimenta· tion shift in position along the N orth Sea barrier coast from the adjacent inlets towards the centre of the island in a wave· like mode (Fi g. 8). Loci of erosio n and sedimenta· tio n move slowly and migration of an erosio n / sedimenta· tion wave from the inlets towards the centre of the islands covers periods of 30 to 70 years. Such patterns largely domin ate sedimentation and erosion al ong the island coasts and show significant periods of 18 to 19 years. It has been inferred that they are caused by regular changes of the tidal amplitude induced by the lunar nodal cycle, which is the variation of the orient ation of the Moon's orbit with reference to the orbit of the Earth aro und the sun (O OST et al. 1993). On an annual scale, higher fr equency sedimentatio n patterns can be observed. During the summer a coastal pro· file is built up, which is largely destructed durin g the stor· my winter season (Fig. 9). The development and positi on of beach ridges and the locations where rip. current chan· 3 r-----------------------------~--, E .€ C1l -; 21-- - ----:;>-L....---u - - ·- - - -----1 0.. E i= u Cii o Fig. 6. The relation betwee n tidal amplitud e and the length of the Wadden Sea barrier island s (WOLFF 1986) . Fig. 7. The Zoutkamperlaag and the barrier island of Schiermonnikoog. - Arrow shows pos ition of washove r channel. 71 28 25 21 19 17 18 20 29 km 26 27 22 23 24 15 16 \3 12 10 11 14 Years , 3 5 6 7 8 9 ~ N 0 Annual Coastline Change in m D D > +20 } progradation +20 to 0 o to -20 } retreat > 5 10 km -20 Fig. 8. Patterns of erosion and sed im entation as reflected by coastal retreat and progradation in m lyr alon g the North Sea coast of Terschellin g (OOST et al. 1993). nels develop is largely determined by wave action on the sandy coast. The beach ridges are formed by breaki ng waves during the high tide and the swash during low tide. Sediment transport along the beach takes place in three zones (Fig. 10) : 1. The swash zon e on the berm during high water; 2. The zone on the ridge, where sand brought in by waves is transported longshore, via the runnel by the longshore current and subsequently brought out by the rip current. Few much sediment is exchanged between zones 1 and 2, except during the winter when the beach profile is partly eroded (BLEUTEN 1971); 3. The aeolian zone where, depending on the wind force and direction, large amounts of sediment can be transported, can extend to the low-water line. In the intertidal zone a sediment layer several em thick can be eroded or deposited by the wind during th e period between two successive high waters. Calculations show that J.eo lian transport is probably as important as coast parallel transport (STEIJN 1991). Sand transport rates within these zones are high. Over a period of only 8 days with one weak storm (26-8-1970 to 3-9-1970): 72 breakerbar ------ -~~ ¢= Winter: seaward transport dominates ..... Summer: coastward transport dominates Fig. 9. Schematic beach profile: build-up in summer (-), destruction in winter (---). - I I Fig. 10. Sediment transport pattern on and along a barred beach. - The triangular arrows indicate aeolian transport in various directions, depending on the wind direction (zone of aeolian transport at low tide indicated). All other arrows indicate directions of sediment transport by water at high tide. For explanation of numbers see text. - After BLEUTEN (1971). - a landward shift of the berm in the period from neap- to springtide; - a net eastward sand transport of 70 m 3 on the beach r idge during one storm period (i.e., a net sedimentation of an average surface layer of 2 mm thickness during one storm); - a flattening of the berm profile and an eastward shift of the rip current channel during the storm were observed within an area of 150 x200 m2 on the barrier island Schiermonnikoog (Fig. 11) by BLEUTEN (1971). The great variety of sedimentary structures on a ridge and runnel dominated beach plain is discussed in detail by 73 ---- -,------- A C. tt=========:~~~~~~~~~~~' -----------1 ' -- - -- - \ , / --- -- ----- - ......, .... ;,,-'.------ --": / I " I /1/ ,// " ~ " rot --_._------- -... ,0 ..\ I B. _----- - ... - ... ~~-:-----;\------ /'" ~ _---, _--"'.... I "., I // t --- - ..... - - ..... , I ~r / I I //'" . t..::. ___ -------,. .... " ;/----, / 1/ ,/// // ,,/ /\. / {~.:~// '" --- -- _---- oil' -_._----- ----- --- -II --"'..- / o. = n =-----.::::=:====-:"..,.-----:::====----------------------(j-------------------------;;;; 9 III IV SOm V 8 I / / " c· L-__-----------------------------------------------. .~----------c--y,"<' _____ ------------t--~__ I (_ ... " /' /~ ----- ---_ .. --.. -~ ------ ---- --A. ------ ---------'--- --- _----r------~ _-----o.=========-___=====------"=======:-------.. ~------------------- ------------.... , ('-=~_ ;,,,' / ... ---- --- - ---- - -- -- --- ----:-------~-----------'\ ~ " _20/ _-----------0 ' I . '\''--_//' I - - - - -------- 50m II III IV V Fig. 1l. Development of a beach section on Schiermonnikoog. - Elevation in dm. A. Situation on 26 August 1970; B. Situation on 3 September 1970 (BLEUTEN 1971). 74 several authors (e.g. DAVIS et al. 1972). It should be realized, however, that sedimentary structures in intertidal sandy deposits will not always be preserved. Air filled cavities, up to 1 em in diameter, are formed within fine-grained sand; at rising tide the ground water level does not rise quickly enough to replace the interstitial air before flood water covers the sediment surface. Air is trapped in the sand between the ground water level and the sediment-water interface. Because of the weight of the overlying water column and the slow downward movement of water due to capillary action, the entrapped air is compressed and eventually may attain a pressure which enables it to lift a thin layer of overlying sediment. At this moment air cavities (cavernous sand) are formed, often to a depth of 20 cm (P. L. DE BOER 1979), by reorientation of grains with flat surfaces (WUNDERLICH 1979). The cavities are sustained by the friction of the grain fabric and the surface tension of the sand-water-air contacts. Observation and experiments show that, in layers of fine sand, bubbles develop preferentially in better sorted and coarser zones. This is probably because capillary forces are greater in finer-grained and less sorted sand. Thus, water penetrates by preference into the latter, pressing the interstitial air into the better sorted and coarser sand. The high content of air cavities in certain layers can result in a complete homogenisation of the structures in such layers. It can also provide a density instability responsible for deformational processes which can lead to the formation of convolute laminations. In the intertidal zone this appears to be a slow process that covers a number of ebb and flood cycles. Such structures can be formed in all intertidal areas where some relief is present and it may contribute to the instability of, amongst others, channel walls. On the island Schiermonnikoog, which is a National Park, the vegetation of one dune area to the NE of the village was removed in order to allow a natural development. This dune area rapidly evolved into actively moving dune complexes, with areas locally eroded down to the ground water table. The ground water table depends, amongst others, on the size of the dune complex (Fig. 12) and on the sea level (VAN OOSTEN 1986). Under natural conditions, aeolian deposits below the ground water table are preserved from erosion. A rise of sea level and an increase of the surface area of dune complexes therefore favour the preservation potential of dune deposits. Tidal Marshes and Washover Channels On most Dutch Wadden Sea islands wash over channels transect the dunes where these are not protected by man, i.e., especially in the eastern parts of the islands. During storms, water and sand are transported through the wash over channels from the North Sea into the backbarrier area or into small embayments on the islands. In the natural parts the higher intertidal zones behind the dunes consist of extensive tidal marshes, whereas the protected parts mainly comprise polders. For a discussion of the salt marshes, see below. Again, Schiermonnikoog is one of the most natural examples. At present, the eastern part of the barrier island is largely undisturbed by man (Fig. 7). In the period 1950-1978 a sand dyke was constructed by stimulating aeolian deposition in the eastern part of Schiermonnikoog. This artificial dune blocked the southern part of a broad beach plain with some isolated dunes. As a result salt marsh vegetation developed on top of the sandy beach deposits. Towards the North Sea, the salt marsh sediments wedge out. Originally the artificial dune extended further to the east, but part of it was destroyed by storm floods. The government repaired the dunes several times, but stopped doing so after 1984, so that washovers became active again. The channel at the eastern side of the island, near beach marking point No. 10 (Fig. 7), is an active washover channel during storms, especially in the winter season. Through such washover channels the whole outer-dyke tidal marsh area of Schiermonnikoog is flooded several times a year. The channels are flanked by sandy, vegetated levees which are locally more than 0.5 m high. They consist of North Sea beach sands brought in during the floods. These sandy deposits pinch out laterally and interfinger with fine-grained silty and clayey overbank deposits, which have settled from the seawater that stands above the tidal marsh for a number of days after floods. During the more quiet summer season the washover channel is partly filled and blocked at the North Sea side by aeolian deposits such as sand sheets, aeolian ripples and small dunes. Since repairs of the artificial dune have been stopped, the washover channel has become deeper and has extended gradually towards the North (Fig. 7). This has led to a slight panic among the island inhabitants, who became afraid that the washover might become an active channel, splitting the island into two parts. However, the washover channel enters the Wadden Sea close to the tidal watershed, thus greatly diminishing the risk of it evolving into a tidal inlet channel (d. EHLERS & KUNZ 1993). Dunes Along the North Sea side of most islands an almost continuous dune belt separates the beach from the rest of the island. The dunes are partly natural and partly man-made. Parts of the dunes on Schiermonnikoog, for example, have been formed as an artificial aeolian sand dyke, which was constructed by trapping sand blown inland from the beach between and behind sticks and branches stuck upright into the ground. Other parts of the dunes are natural, forming large parabolas which have developed in different periods. IZ2.:'SJ sand. ~sand. present eroded ~ sweet groundwater ---- groundwater table E3 seawater .......... former groundwater table r:=J salt groundwater-- Interface salt/sweet Fig. 12. Freshwater lens in the dunes (VAN OOSTEN 1986). 75 The washover channel near beach marking point 10 ends in the Wadden Sea on the tidal watershed south of the island (see arrow in Fig. 7). The washover-lobe deposited there consists mainly of sand. Biota flourish in this quiet part of the back barrier area and bioturbation is intensive. Lugworms and burrowing bivalves rework the bottom sediment continuously. Especially at the turn of the high tide shrimps, crabs and flat fish destroy the small-scale ripples, which form during the ebb and the flood. At low tide birds visit their feeding grounds to prey for invertebrates, thereby strongly bioturbating the tidal flats (HERTWECK 1982; EHLERS 1988; CADEE 1990). In places where the diatom/ algal mats are destroyed, erosion is stronger and may result in deep erosion scours. Such bioturbation, in combination with intensive reworking by waves and currents results in a complete destruction of the washover delta-lobe structures. Ebb-tidal Deltas and Inlets Ebb-tidal Deltas Ebb-tidal deltas are situated on the seaward side of the inlet throats between neighbouring barrier islands. They are the result of sedimentation by the tidal currents flowing through the inlets, wave action and the tidal wave along the coast (OERTEL 1975; HAYES 1975, 1979, 1980; HUBBARD et al. 1977; N UMMEDAL et al. 1977; N UMMEDAL & FISCHER 1978; FITzGERALD et al. 1984; SHA 1989a-d, 1990a, b; SHA & DE BOER 1991). Mesotidal regimes in particular are considered to favour their evolution and maintenance (HAYES 1979, 1980), although it should be noted that the tidal prism rather than the tidal amplitude defines the strength of the tidal currents within and adjacent to tidal inlets (SHA 1989c). The tide-induced currents and the morphology of ebb-tidal deltas are strongly interrelated (SHA 1989a, 1990a). A general empirical relation quantifying the sand volume of ebb-tidal deltas (outside the line along the barrier island coast) was proposed by DEAN & WALTON (1975), WALTON & ADAMS (1976) and STEIJN (1991) as: towards the inlet throats, where they reach values of 1-2 m/ s at maximum. Only during storms higher velocities have been measured. Empirical relations have been proposed for the cross-sectional area in the throat of the inlet and the tidal prism (e.g. O'BRIEN 1969; JARRETT 1976; DIECKMANN et al. 1988; GERRITSEN 1990; HUME & HERDENDORF 1990; NIEMEYER 1990; SHA 1990a; STEIJN 1991; BIEGEL 1991 b; EYSINK & BIEGEL 1992). For 162 inlets along the American coast the general empirical relation in metric units, with or without jetties is GARRETT 1976): Ac = 158x 1O.6psO.95 where Ac is the cross-sectional area and P s is the tidal prism at spring tide. DIECKMANN et al. (1988) showed a similar relation based on 26 tidal inlets along the Wadden Sea: Ac = 372x 10-6pO.915 where Ac is the cross-sectional area and P the mean tidal prism. v = 65.6x 1O-4P1.23 where V is the sand volume of the ebb-tidal delta and P is the mean tidal prism. The ebb-tidal delta volume decreases with increasing wave influence (Fig. 13; DEAN 1988). The Dutch ebb-tidal deltas plot on the line of strong wave action (EYSINK & BIEGEL 1992). They consist of large volumes of sediment, which are of the same order of magnitude as the adjacent barrier islands and thus play an important morphological role in the Wadden Sea system. Studies of tidal deltas have revealed that the morphologies of ebb-tidal deltas in different geographic places are quite similar and generally fit well to models of the kind as, for example, given by OERTEL (1972) and HAYES (1980). Typical components of an ebb-tidal delta are the main ebb channel, marginal flood channels, channel margin linear bars, swash platform and swash bars (Fig. 14). M E 0 "" ::=. "0 ~ 0; - - - - - low wave energy - - - average wave energy .... ------ high wave energy , ..,,' ./ , .' :g .D .0 :S "0 (f) Cii 100 "" ;:::::,:.., OJ OJ OJ ~' "'~.>/' ~/ E > 10 (5 ::J "0 c (f) C1l E Inlets The barrier islands are separated by tidal inlets, through which the tidal water passes to and from the back barrier area in the course of a tidal cycle. Inlets consist of one or more main channels separated by sandy shoals. The strongest currents in barrier systems generally occur in the inlets (and in the larger channels of the back barrier area). Tidal current velocities increase from the open North Sea ~ ::J ,, ,,'~.»/. .. , ,','..../ . ·s UJ CY ,<..-/ ," . ./ 100 Tidal prism (10 6 m3) 1000 Fig. 13. Relationship between the sand volume of the ebb-tidal delta (with reference to the normal barrier coast), the tidal prism and the influence of wave climate. - After DEAN (1988). 76 about 1 km Swash bar ..... ,.. .. --- .. .. ,. fJ! I I I .I I I' \ I ~ \ Fig. 14. Morphology of a typical ebb-tidal delta. - Arrows indicate direction of dominant tidal currents (SHA 1990a). The main ebb channel is mostly the deepest element in the ebb-tidal delta because the maximum velocities during the ebb commonly occur at lower water levels than during the flood (VAN STRAATEN 1964). SHA (1990a) demonstrated that, for the Dutch Wadden Sea, the maximum depth of the main inlet channel is linearly related to the tidal pnsm (Fig. 15), with the relation: The flood waters enter the inlet over a large front and together with wave-induced currents (wave-pump-concept; BRUUN & VIGGOSSON 1977) they transport sediment into the back barrier area. The asymmetry of tidal current velocities influences the development of tidal deltas. When the water level starts rising after low water, the ebb current in the main ebb channel is still flowing strongly, forcing h= 13.3+P/30 where h is the maximum depth (m) and P the mean tidal prism. Flemming (1991) found a relation between the local catchment area and the width of the inlets for the East Frisian Wadden Sea: A = 50 37.67 LO. 8 E Q) 45 40 T,L,,,~ / Texe~ where A is the catchment area (km 2) and L is the inlet width (km). Also, in other tidal areas relations between tidal prism and maximum depth of inlets have been found (amongst others DIECKMANN et al. 1988; HUME & HERDENDORF 1990; EYSINK & BIEGEL 1992). The rather large scatter in the data from different areas suggests that universally applicable relationships between channel widths and mean or maximum depths as a function of tidal prism cannot be given. The quantities probably depend strongly on local flow and bed conditions (EYSINK & BIEGEL 1992), as well as on the intensity of wave action. The ebb current is concentrated in the main ebb channel, draining the backbarrier in form of an ebb-jet. The ebb channel is flanked on either side by levee-like marginal linear bars, formed by the interaction of ebb and flood currents in combination with waves and wave-generated currents (HAYES 1976; SHA 1989b, 1990a). ~ (]) .r:: 30 £: 35 '0 li (]) "0 /~Ieland V I / V E 25 ,-Roltumerplaat :J E Schiermonnikoog 'x 20 Simons ~ <ll ,ek 200 I 15 10 Za: ~lielaf"\d ~ Engelsmanplaat-Pinkegat I e Rottu~eroo~ o I I 400 600 800 1000 Tidal prism (10 6 m3 ) Fig. 15. Relationship between the maximum depth of an inlet channel and the tidal prism (SHA 1990a) 77 A' SOO 10 D.OL -10 -20 -30L--200LO---L--~17~50~-L--'~50LO--J-~1~25~O--~--'~OO~O--~~7~50~~~~50~O~-L--~25~O---L--~--~--~2~~~~~~ -30 distance (m) g E AMELAND 1<: ----------"1 A profile ------------------------------ - - -- - - ---- -- 1935 1939 1946 1952 1958 1980 Fig. 16. Lateral shift of the Ameland inlet, situated to the west of Ameland. - After DE BOER et al. (1991). the flood currents into the marginal flood channels. The main channel is thus ebb-dominated and the marginal channels are flood-dominated, the latter being located laterally of the main ebb channel, adjacent to the barrier islands and between the swash platforms (d. HAYES 1976). The swash platforms are broad sheets of sand topped by isolated swash bars which are built by wave action. If such bars grow high enough, as in case of the shoal 'Noorderhaaks/Razende BoI' in the Texel Inlet, aeolian processes may contribute to the accumulation of sediment. As a result a new barrier island may develop, or, more commonly, the platform may migrate and connect to the adjacent downdrift barrier island (e.g., Texel: SHA 1989a, 1990a). OERTEL (1972) pointed out that swash platforms are generally located along the axis of, and on the highest parts of onshore migrating shoals, parallel to the direction of the wave-bore approach. The orientation of swash bars on the platform is generally perpendicular to the incident wave-bore. Geological and historical developments show that several of the Dutch and German inlets tend to move in the direction of the resulting sand transport along the coast, i.e., towards the east (Fig. 16; LUCK 1975; LIGTENDAG 1990; SHA 1992; OOST & DIJKEMA 1993; OOST, in prep. a). This migration is most likely not continuous through time and space, but is influenced by the underlying palaeotopography (SHA 1992) as well as the morphological developments in the backbarrier area (FLEMMING & DAVIS 1992; OOST & DE HAAS 1992; VAN DER SPEK, in press), by the variations in tidal amplitude (SHA 1990; FLEMMING & DAVIS 1992; OOST et al. 1993) and by variations in sea-level rise. Eastward directed sediment transport occurs in the ebb-tidal delta mainly as a result of the tide and waves, by migration of channels and shoals and by sand transport through the channels (OOST & DE HAAS 1992). Many inlets have today been artificially stabilized. The Ebb-tidal Deltas and Inlets of the Dutch Barrier Islands Morphological Patterns: Asymmetry and Orientation The larger ebb-tidal deltas have an updrift asymmetry, which is greatest in the ebb-tidal delta of Texel Inlet (tidal prism about 1050x 10 6 m 3) and the one of Terschelling Inlet (about 850x 10 6 m 3). Tidal prisms through inlets with downdrift oriented ebb-tidal deltas are considerably smaller. The asymmetrical orientation of smaller ebb-tidal deltas along the Wadden Sea coast and the downdrift (to the east) orientation of their main ebb channels (Fig. 17) is commonly ascribed to the fact that wind-driven waves dominantly attack the ebb delta from the west. Together with the residual tidal current they produce an easterly longshore drift which forces a downdrift, i.e. eastward directed asymmetry on the ebb-tidal deltas (LUCK 1976; NUMMEDAL & PENLAND 1981; SHA 1990a). The updrift orientation of the larger ebb-tidal deltas is due to the influence of tidal currents which pass through the inlet, as is clearly illustrated in Fig. 18. The morphodynamic character and orientation of ebb-tidal deltas is thus defined by the relative influence of waves from the open sea and tidal currents through the inlets. Waves and longshore drift tend to force the ebb-tidal deltas into a down-drift asymmetry, whereas the tidal wave tends to force the ebb-tidal delta into an updrift asymmetry. Because of the relatively large influence of waves and longshore drift, most of the ebb-tidal deltas of the Wadden Sea face to the Nand NE, i.e., they are downdrift-asymmetrical. Only the inlets with a relatively large tidal prism, such as the one of Texel, are oriented up drift (Fig. 18; SHA 1990a). 78 5 km '------' 6 N 2 m below MSL 6 m channel deeper than Fig. 17. Location and shape o f the ebb-tidal deltas along the East Frisian Islands. - After FITZGERALD et al. (1984). Note that the ebb-tidal deltas and their main ebb channels are asymmetrical to the east (SHA 1989c). NORTH SEA SCHIERMONNIKOOG • N /~ Islands . ';\'l>(\ AMELAND Terscheillng Inlet VLlELA~. () o / ' . ., - .... .,~~ :.. . . .. ... .. ,'f, ,.. ' " ~~O 0'<,.~ FRIESLAND LEGEND , " I 10 10 m below MSL channel oeeper than 10m SmMSL TEXEl 20 30km IJSSELMEER NOORD-HOLLAND Fig. 18. Locati on and shape of the ebb-tidal deltas of Texel Inlet (Marsdiep), Terschelling Inlet (Vli estroom) and Ameland Inl et (Borndiep). - Th e -10 m depth contour outlines th e position and shape of the ebb-deltas; black areas are channels within the ebb-tidal deltas dee per than 10 m below MSL (SHA 1989c). The IJsselmeer is the former 'Zuiderzee' which was blocked by the 'Afsluitdijk' in 1932. 79 6N 6 TEXEl N 5m o 2 3 km I ---.. 0.5 m I sec current averaged over 0-3 mdeplh 6 6 hours before HW at Hoek of Hoiland .p A B -3 Fig. 19. A. Schematic current and sand transport pattern in the ebb-tidal delta of Texel Inlet. Arrows indicate the net sand transport directions. - B. Tidal current roses for a tidal cycle in the ebb-tidal delta of Texel Inlet integrated from the "Stroomatlas van Nederland" (M.M.A.H. 1963). - Numbers near current roses indicate time in hours before or after high water at the Hook of Holland. Arrow length represents current velocities, averaged from the water surface to 3 m below the surface (SHA 1989a). Texel Inlet A large-scale study of depositional character and sediment transport in the ebb-tidal delta of Texel Inlet was made by SHA (1989a-d, 1990a, b). He found a flooddominated net sediment transport through the minor tidal channels and over the shoals (Fig. 19A), and an ebbdominated net sand transport in the larger and deeper channels. Sand, which is transported to the seaward margin of the ebb-tidal delta through the southwest-directed main ebb channels, is carried north by flood-dominated residual currents along the ebb delta front. Landward sediment transport dominates the northern part of the ebb-tidal delta. Sand is also deposited north of the ebb-delta shoal 'Noorderhaaks' owing to weak, rotational tidal currents (Fig. 19B). From here the sand is transported shoreward by waves in the form of swash bars. Part of the sand returns to the inlet through the flood-dominated channel in the north (Fig. 19A). The lower and higher intertidal parts of shoals between the major channels are dominated by (storm) waves and wind. The larger subtidal (parts of the) shoals are dominated by waves and have swash bars superimposed. Flood-dominated channels to the NE of the shoals tend to silt up and are abandoned upon attachment of the shoals to the island in the NE (SHA 1990a). Cyclic Morphological Changes of Positions and Orientations of Shoals and Channels An important feature of ebb-tidal deltas along the Wadden Sea coast is a clockwise rotation and translation of channels, together with the onshore movement of lower and higher intertidal shoals OOHNSON 1919; SHA 1989a, 1990a; OOST & DE HAAS 1992, 1993). Each cycle, covering a variable number of decades, begins with the development of a new main (ebb )channel, or by the transformation of a flood-dominated channel into an ebb-dominated one, which then gradually translates and rotates clockwise until it degenerates into a marginal flood channel, while a new main ebb channel develops in the updrift direction. This process can result in fast migration of the channels: on the island Schiermonnikoog two light houses built in 1853/54 were positioned in such way that, if kept in one line, one could safely enter into the Zoutkamperlaag from the North Sea through the main outer channel. However, the constructors did not anticipate the fast migration of the outer ebb-delta channels, so that the light-line became obsolete soon after it had been built. The southern light house is today used as a water tower. Upon rotation and northward migration of tidal channels, shoals tend to attach to the downstream island. The repetitive attachment of shoals contributes to the drumstick-form of the islands, i.e., being thin at the eastern head and much broader at the western head (d. Fig. 18). Such cyclic morphological changes are explained by the tendency of the system to be gradually pushed to a more downdrift orientation (SHA & DE BOER 1991) by wave action and longshore currents. In this way the length of the main (ebb) channel and the pathway of the tidal currents entering and leaving through the main inlet channel increases (Fig. 20). As a result, the tidal currents through this channel become weaker, and when the length of ebb (and flood) currents (of decreasing strength) through the main ebb channel reach a certain limit, the channel is 80 transformed into a marginal flood-dominated channel, an updrift channel taking over the role of the main ebb channel. As a consequence, other flood-dominated channels further down drift diminish in importance, wave and storm attack pushing the ebb delta shoals towards the island across the abandoning downdrift marginal flood channel. In this process the channels rotate clockwise, thereby producing characteristic, one-directional lateral migration patterns in the depositional record. Mid-flood - .- ~\/ }I Lagoon • ~--- \ I - ------+- -- - --+ Island I) \"" Mid-ebb - Barrier Island --Barrier Island and flood-dominated channels. Coarsest sediments are found in ebb-dominated channels which are deeper than flood channels because they are fewer in number and are fed by fast-flowing water draining from the backbarrier area throu h the relative! narrow inlet throat durin the ebb phase. Typica c anne sequences are t us c aractenzed by ebb-dominated channel deposits (dunes) in the deeper parts and increasingly smaller and more flooddominated structures higher up in the sequence. Upon abandonment of channels, as in the case of shoal attachment to the downdrift island, the development of the (fining-upward) channel sequence will show a break, with bioturbated organic-carbon-rich mud and intercalated finegrained sandy ripples forming the top of the sequence. Between the channels, swash bars form relatively shallow zones where waves are dominant and tidal currents are weak. Due to the strong wave action, the sands are clean and well sorted. If such shoals build up to the intertidal level, wind may cause further vertical accretion, thus producing supratidal shoals that are dominated by beach processes and aeolian transport. The ebb-delta lobes represent the terminal lobes of the ebb-tidal channels, i.e., accumulations of sand in the area where the ebb-tidal channels loose their transport capacity due to increasing depth and width. The active shallower parts of the lobes are affected by waves and tidal currents, while the more distal parts are finer grained and influenced by longshore tidal currents, rather than by waves and tides which pass through the inlet. In these distal parts of the ebb-delta lobes, storm deposits and bioturbation structures are preserved. Due to lateral channel migration, part of the ebb delta may become abandoned and subject to wave reworking and erosion. Hummocky cross-bedding and shell lags may develop in the abandoned parts, bioturbation structures being abundant because of the low rate of sedimentation and erosion. For a more detailed description of the distribution of surface sediments in the ebb-tidal delta of Texel Inlet see SHA (1989b; 1990a, b) and SHA & DE BOER (1991). Sequence Models Lateral migration of channels and ebb-delta lobes, and decreases and increases of tidal prism and of wave attack may produce different sedimentary sequences. The three most characteristic sequences are (5HA & DE BOER 1991): a) Progradational ebb-delta-lobe sequences (Fig. 21A), deposited from reworked inner-shelf to distal ebbdelta-lobe sediments, coarsening upward into active ebbtidal delta-lobe sediments with diminishing bioturbation towards the top. b) Abandoned ebb-tidal delta sequences (Fig. 21B), formed under conditions of a relative sea-level rise. Along the Wadden Sea islands such successions have formed during the Holocene due to the glacio-eustatic rise of sea level (d. SHA 1990a). Such sequences are truncated at the top by a ravinement surface formed during shoreface retreat. A reworked, inner-shelf tidal sand sheet covers the sequence. If the rise of sea level continues, the series may ultimately be covered by shelf mud deposits. SHA (1990a) demonstrated that the preservation potential of ebb-tidal delta deposits depends mainly on the maximum depth of shoreface erosion and the depth of the seaward limit of the ebb-tidal deltas. Into the backbatrier, in the throat of the inlet, flood-dominated dunes have been observed in Texel Inlet (Fig. 19A; Sba, 199Oa). Such flood dominance is enhanced by the assymmetry of the tidal wave in the North Sea and the low percentage of intertidal shoals in the drainage basin (Sha, 1989b; Steijn, 1991). /! / Lagoon Fig. 20. Schematic current patterns at mid-flood and mid·ebb showing the effect of the interaction of tidal currents parallel to the coast with the tidal currents through the inlet. - Strong, bidirectional currents are produced on the left (SW) side of the ebb-tidal deltas and a weak, rotational tidal current pattern occurs on the right (NINE) side. - See also Fig. 19B (SHA 1989c). Sedimentary Facies and Sequences Facies in the Ebb-tidal Deltas The character of surface sediments in ebb-tidal deltas is largely defined by the local tidal and wave energy, water depth, and location with respect to ebb and flood channels. Large amounts of sand are transported through the ebb. 81 A 4 I t Phi 2 0 I -2 ! ! j I Bloturbation Facies Bio- Facies B VW W MW'" P turbation L........! '\ Top of the active delta lobe Shelfrrud Inner-shelf tidal sand sheet RaV1'lElOlE!lllt~ace Lobe front Swash bar and platform Migrational shoal Dlatal lobe BaM 01 the abb-detta klbe Marginal flood Inner-shelf Hdal sand sheet (,,,,,,crl<oo 00:1 depOSIts) Channel (Ibod-ooentedl Phi LEGEND 0 -2 I I c 2 ! ! Bio- Facies sand • mud ! turbation Eolian dune Beach ridge/spit (wave-domated) YWW WWW .. ------.. ...". mud drape T ., o wave ripple small- scale ripple horizontal lamination (Mga tiaIaO shoal --"- Marginal flood channel (fIoo<>'or;eoted) = §§§ e low angle lamination megarlpple cross bedding djfJl Main Inlet channel ~ " large-scale cross bedding mud pebbl. atone pebble " Inlet channel floor . ~- shell and shell fragment wood ,(? Fig. 21. Hypothetical vertical sedimentary successions in inlet and ebb-delta deposits. - A. Active progradational ebb-tidal delta lobe. - B. Abandoned ebb-tidal delta lobe facies. - C. Migrating inlet. - Paleocurrent directions in the lower part are commonly ebbdominated, whereas higher in the sequence flood-oriented structures are dominant (SHA 1990a). c) Inlet and channel sequences (Fig. 21C), form especially in the proximal parts of the ebb-tidal delta, where migrating channels produce successions with ebbdominated channel deposits at the base, topped by flooddominated channel deposits. Depending on the local conditions, the top of the sequence may consist of abandoned channel deposits or shoal deposits. In all cases, preservation in the fossil record depends on the location with respect to the depth of shoreface erosion which, in turn, depends on the ratio between lateral and vertical migration of the shoreline in relation to the depth to which waves rework and erode the shoreface sediments. Preservation of Ebb-delta Deposits Ebb-delta deposits can be preserved in the course of a transgression. On seismic profiles off Terschelling and Ameland, the remnants of ebb-delta/ inlet deposits can be recognized (SHA 1990, 1992). They show low-angle, seaward-dipping foresets of the ebb-delta lobe and lowangle inclined beds formed by the inlet channel, migrating laterally parallel to the coast. As indicated above, a prerequisite for preservation is, of course, that the maximum erosional depth of the shoreface is less than the depth of part of the ebb-tidal delta deposits. With other words, the ravinement surface formed during shoreface retreat should be positioned above the lower bounding surface of the ebbtidal delta. The lower bounding surface is generally produced by migrating channels or is formed by the pre-existing sea bed in the more distal parts. The relative positions of these lower and upper (erosional) bounding surfaces depend on the rate of relative sea-level rise, the rate of sediment supply as a function of the tidal prism draining a particular inlet, and the wave erosion base. Early Holocene ebb-tidal-delta deposits in the subsurface offshore of the West Frisian Islands show that preservation of ebb-tidal delta deposits during a rise of the relative sea level does occur (SHA & DE BOER 1991). In the offshore of the Dutch Wadden Sea the deposits of tidal inlets and the related tidal deltas constitute a relatively large part of the offshore subfossil barrier-related sediments indeed (Fig. 21; SHA 1992). 82 Backbarrier Area Waves from the North Sea (which may reach a height of 10 m during severe storms) enter the Wadden Sea mainly around high tides. The influence of waves entering from the North Sea into the back barrier area (the Wadden Sea) decreases rapidly due to refraction and strong decrease of water depth towards the back barrier area. Indeed, direct observations show that the wave energy is strongly reduced right after the passage of the inlet throat (NIEMEYER 1986). Most waves in the Wadden Sea are locally generated, reaching heights of up to 2 m (max. approx. 4 m). These waves influence sedimentation on the intertidal shoals and in the salt marshes. Since the tide is asymmetrical due to the morphology of the Wadden Sea, especially that of the tidal flats, the ebb phase lasts longer than the flood phase. As the latter thus is stronger, it can transport larger quantities of sediment (VAN STRAATEN 1964; POSTMA 1967; DRONKERS 1986). The resulting net sediment transport is thus directed towards the back barrier area. In this process the Wadden Sea acts as a 'sorting machine'. The large differences in current velocity (Fig. 22) combined with the ebb and flood movements and the influence of waves on the various parts of the back barrier area efficiently sort the sediment with respect to its transportability (mainly grain size and shape). In the backbarrier area of the Wadden Sea clearly defined flood-tidal deltas, as known from other barrier island systems, e.g. those along the east coast of North America, do not occur. This may be due to the relatively strong currents which supply and redistribute so much sediment that the larger part of the Wadden Sea is filled up to the intertidal level. As a result, the term flood-tidal delta could be applied to the Wadden Sea as a whole. The backbarrier region is commonly subdivided into three zones, often referred to as sub-, inter-, and supratidal. Actually, the terms lower intertidal and upper intertidal are to be preferred over 'inter-' and 'supratidal' because during (very) high tides the latter zone is also subject to flooding. The backbarrier environment, which constitutes the Wadden Sea proper, can thus be divided into: 1. a zone below low water springs: channels; 2. a zone between low water springs and mean high water: shoals and gullies; 3. a zone above mean high water: high shoals (mainly at the North Sea side) and salt marshes. where Ac is the cross-sectional area of a channel and P is the mean tidal prism. VAN DER SPEK (in press) furthermore proposes an empirical relationship between the maximum depth of the backbarrier channels and the tidal prism: h = 1.48p 046 where h pnsm. IS the maximum depth and P is the mean tidal This and earlier numerical relationships have all been defined empirically; the exact nature of the generating physical mechanisms is still subject to debate (d. VAN DEN BERG 1986; DIECKMANN et al. 1988; GERRITSEN 1990; HUME & HERDENDORF 1990; MISDORP et al. 1990; NIEMEYER 1990; SHA 1990; BIEGEL 1991b; FLEMMING 1991; STEIJN 1991; VAN KLEEF 1991; EYSING 1992; EYSINK & BIEGEL 1992; BILSE 1993; DE VRIEND & BAKKER 1993; EYSINK 1993; VAN DER SPEK, in press). In smaller channels is the maximum current velocity lower. In such cases the clay content of the channel sediments is inversely related to the size of the channel. Local sediment transport depends on the local dominance of the ebb or the flood current. In contrast to estuaries, where ebb and flood currents often have separate channels, tidal channels in the Wadden Sea mostly have both flood and ebb dominated sections. Lateral channel migration is one of the most prominent mechanisms controlling erosion and deposition (Fig. 23). The rate of lateral accretion can be quite fast, reaching several 100 m / yr. The sediments in the channels are generally not or only poorly bioturbated. Depending on the rate of lateral migration relative to the general rate of subsidence and sediment accumulation, a considerable part of the inter-channel sediment deposited in the intertidal and subtidal zones may be eroded. Consequently, the ratio of channel to inter-channel deposits in the rock record may exceed the ratio in recent environments by several orders of magnitude. The lateral migration of channels is partly restricted by ebb- and flood-dominated tidal chutes (Fig. 24; VAN STRAATEN 1954). Due to inertia effects, flowing water has a tendency to flow in a straight line if possible. In the bends of channels the water flow is pushed against the outer bend. During the flood, water eventually rises above the channel banks. The flooding water tends to maintain its straight course and thus may create a secondary channel: a flood chute. In this way bends in tidal channels can be Channels In the Wadden Sea the inlet channel splits up into an extensive drainage network (Fig. 5). Through these channels the tidal waters flow to and from the intertidal shoals, thus forming the main pathway for water and sediment (WINKELMOLEN & VEENSTRA 1974) transfer. The current velocity in the major channels is almost everywhere similar (maxima around 1 m/ s, Fig. 22), the cross-section decreasing with decreasing tidal volume. For the larger channels of one of the tidal systems of the Dutch Wadden Sea (the Vlie system) it was shown that an empirical relation exists betw~'en the tidal prism and the cross-sectional area of the channels (GERRITSEN & DE lONG 1985; GERRITSEN 1990), i.e.,: Fig. 22. Typical tidal flow velocity curves (thick line) along the mesotidal Dutch/ German coast. Thin line indicates water level. Map shows locations. - (a) Flow velocity curves for the intertidal flats of Germany (modified after REINECK 1982a); (b) Flow velocity curve for an intertidal gully south of Ameland (modified after VAN STRAATEN 1954): (c) Flow velocity curve for a shallow subtidal channel (modified after VAN STRAATEN 1954); (d) Flow velocity curve for an inlet channel (modified after POSTMA 1954); (e) Ellipsoidal flow velocity curve at the sediment / water interface in the Dutch offshore region (modified after SHA 1990a). 83 LEGEND U /'-.. h~ C 1.0 0.8 0.6 3 2 0.4 a 0.4 0.2 0 0.4 00 0.2 .1 --.so 000.2 :J-O.2 -0.4 0 2 4 6 8 10 h (rn) :J -0.2 -011 0 2 4 6 8 10 12 --E -- 0 0 h (m) t (11) t (h) 12 1.0 0.8 0.6 b 2 2.0 ti) 0.4 :J -- --E 1.5 0 0.2 0 -0.2 -0.4 1.0 h 0.5 -0.6 -0.8 0 2 4 (m) t (h) 6 8 10 --.so :J -0.5 00 0 a· .. -1.0 -1.5 h (m) 0 2 4 North Sea The Nelherlands t (h) 6 8 10 12 e Germany N E Belgium s 84 cut off. Often the current is not strong enough to create a new cut-off channel, so that these flood chutes rapidly become shallower and narrower in the direction of the flow and vanish completely downstream, often developing a lobe of sediment at the end. Similar processes can be observed in the ebb currents (Fig. 24). Because of meander cut-offs and take over of drainage of one channel system by another, (parts of) channels can become abandoned. Abandoned channels are filled with sediment (clay, sand or a combination of both) in relatively short periods, i.e., in months to years (Fig. 25). Channel lags concentrated along the channel axis consist of (rare) stones, clay pebbles, relatively coarse sands and shells (for a discussion of this see FLEMMING et a!. 1992). Above the channel lag, especially in the inner bends of the channel, sand and fines are deposited. When channels are sufficiently deep and wide, dunes may develop within them. In general, the dimensions of such dunes increase with the depth of the channel (DE BOER et a!. 1991; OOST & DE HAAS 1992). Heights vary from several decimetres to several metres and spacings may reach several hundreds of metres in extreme cases. Parts of the dunes are ebb-dominated because the highest ebb-velocities are reached when the water level has dropped below the channel banks. On the other hand, the flood current is strongest at higher water levels. The dunes may register the tidal influence in the form of double mud drapes, thick- thin alternations at foreset laminae, due to the diurnal inequality of the tide, and bundle sequences formed by neap-spring cycles (VISSER 1980; DE BOER et aI. 1989). Shoals and Gullies Shoals are fully exposed during low water springs and flooded during mean high water. They form the larger part of the Wadden Sea. Up to 80% of the back barrier parts of the inlet systems of the Dutch Wadden Sea may consist of intertidal shoals (Fig. 26). This value is lower for the Texel Inlet and Vlie Inlet. According to EYSINK & BIEGEL (1992) there is a relation between the relative size of the intertidal shoals and the total size of the back barrier area. An alternative possibility is that the Texel and VEe systems have not yet reached an equilibrium because of the relatively young age of these basins (around 1000 years) and also because of the closure of the 'Zuiderzee' in 1932 (see historical section). In the channels the maximum current velocity during the flood occurs just before or around the time that the shoals are being submerged (VAN PARREEREN 1980; POSTMA & DIJKEMA 1982). As soon as the flood water spreads over the lower intertidal shoals its velocity decreases and accordingly its sand transport capacity decreases strongly. As a result, sandy levees with flood-oriented dunes often form " sedimentation mm. year· 1 ero<;ion mrn. year .1 ~~.~~ Fig. 23. Erosion and sedimentation in the southwestern part of the Wadden Sea over the Ia~t decades, caused mainly by the shifting of channels close to the inlets (ElsMA & WOLFF 1980). 85 along the channel margins. Such levees can be several dm higher than the surrounding shoals. Due to the relatively high resistance (a thin layer of water over a large area) tidal current velocities over these shoals are generally low (Fig. 22) . Sediment transport on the shoals in the lee of the levees is also somewhat flood-dominated. A~B Fig. 24. Development of ebb and flood chutes. - Bottom : section from A to B (VAN STRAATEN 1964). The ebb current reaches its maximum velocity after the water level has fallen below the level of the shoals. The water flowing from and (as groundwater) out of the flats is concentrated in shallow, meandering gullies that drain into the subtidal channels. Current velocities in the gullies may reach 1 m/ s around low water. Sedimentation in the gullies is thus dominated by falling stage currents (VAN STRAATEN 1964). The relation between water level and current velocities in the gullies is shown in Fig. 22. Lateral shift of the shoals occurs mainly by lateral migration of channels and gullies. Due to the weakness of muddy deposits, channel and gully erosion may be accompanied by various types of soft sediment deformation along the channel walls in the intertidal zone (Fig. 27). In the case of sandy layers, the instability of the channel walls is likely to be enhanced by repeated submergence/emergence and the intrusion of air (d. WUNDERLICH 1967; P. L. DE BOER· 1979). Slow net vertical accretion, of the order of mm I yr, occurs over large areas. The hydrodynamic conditions and the high-water level, however, restrict vertical accretion. With increasing height of the sandy shoals the influence of waves on the bottom increases. When the shoals become sufficiently high, the major part of sediment transport is accomplished by waves, this resulting in a strongly diffuse spreading of the sediment (EVSINK. 1979). Moreover, tidal currents can transport the sediment suspended by waves, whereas wave action prevents re-settling. Finally, storms and drift-ice, occurring mainly in autumn and winter, may level the shoals. Depending on the phase of the tide, the sediment thus eroded will be transported towards the subtidal areas, to the open sea or to the tidal marshes. Sand transport into and within the Wadden Sea has been estimated from volume changes based on repeated depth soundings (DE BOER et al. 1991; OOST & DE HAAS 1992, 1993). It appears that the net sediment transport for indivividual inlet systems of the Wadden Sea is of the order of millions of cubic metres per year. Based on direct measurements, extrapolations have been made (continuous measurements over a whole year have not been carried out), which show that the total annual sediment volume imported and exported through the inlets must amount to many millions of cubic metres per year (EVSINK 1993). Mean sedimentation rates in the Wadden Sea are of the order of 1 mm to 2 mml yr. This value has been determined on the basis of repeated soundings, dating by pollen, 21°Pb and by 14C determinations. Locally, over shorter timespans (year) vertical sedimentation rates vary from several em to 0.5 m I yr over large areas. By the above processes of erosion and sedimentation, a dynamic equilibrium is established (HEVNIS et al. 1987; NICHOLS 1989; EISMA et al. 1989; .EVSINK & BIEGEL 1992; OOST & DIJKEMA 1993; OOST & DE HAAS 1993). For the Dutch Wadden Sea the maximum elevation of shoals is 0.3 m below mean high water (except for the Dollard), whereas the mean height is 1.3 m below mean high water. The mean shoal height in particular seems to keep pace with the mean high water level (EVSINK 1979; EVSINK & BIEGEL 1992). Together with the more or less stable percentage of the intertidal areas this suggests that sedimentation keeps up with relative sea-level rise. The same can also be concluded from historical data (OOST & DIJKEMA 1993; OOST, in prep. a). The slow vertical accretion of intertidal areas gives ample opportunity for bioturbation. Layers of shells and shell fragments (CADEE, in press) buried at 15-20 em below the tidal flat surface are formed by the burrowing activity of the lugworm (A renicola marina; VAN STRAATEN 1954). Layers of shells can also form where dense populations of shells die in situ (for instance Mya arenaria). Such mass mortality layers are formed during extremely cold winters (d. VAN DEN BERG 1981). Important morphological elements of the backbarrier flats are the tidal watersheds separating the tidal basins of adjacent inlet systems. Morphologically a watershed forms a slightly elevated ridge comprising lower intertidal and upper subtidal sediments connecting the mainland with the barrier island. The watersheds can commonly be crossed by foot during low tide. In the Wadden Sea a morphological 86 E g .0 0 0 g> Grain size "0 C (I) (I) E ~ Q) g 'in ." (I) c 0 '0 c (I) ::J 0 Q) .0 £ a. Q) 0 i: ." "0 c: "0 8' >- g >." i3 'in Q) - g. "0 .r:. g. 'in c (U !!? <= lU Q) C t:: 8 Q) ~ '0 iii Q) "0 > >- '0 iii Q) Location: Zou/kamperfaag Core No. 2G.025 Oa/e:30- tO-t980 Top: -B.45m DOL (below MSL) >- 0 1980 "$-1979 1979 ...:t!. ...:t!. 5 5 w 1978 1977 2 ...:t!. s _ dark medium 1976 3 ...:t!. ...:t!. 4 5 s-- 1975 1975 1974 Dllghl 1 ::::1sand 1:::::::1 silt ~::::;:} clay ...:t!. 5 1973 1972 5 ...:t!. 5 1'-"-" I Haser bedding [::;] clay pebbles ...:t!. 6 5 1971 o 1-" I x-lamination climbing ripple -5-1970 7 w ~ parallel lamination 1970 ...:t!. 5 [JJ bloturbatlon S summer winter 1969 8 1969 Closureo! Lauwerszee W 9 10 Fig. 25. Infill of the partly abandoned main backbarrier channel of the Zoutkamperlaag. - Couplets of fine silt (summer) alternating with fine sand (winter) represent the seasonal infill of the channel. Years are indicated and, were available, depth data from depth soundings (OOST et al. 1993). 87 !U (ij <1> Q; 1.0 .;:: (ij ~ .0 ~ 0.8 s EB . . PG .0 '0 ~ <1> U ::;) EG 0.6 0.4 L- 2)I M I VJ ! !U (ij (ij <1> nM 5000 n :rg '1:: 0.2 C .~ <1> '0 0.0 o 50 100 500 1000 Total surface area (10 6 m2 ) - - 1) before cfosure of the 'Lauwerszee' 2) after closure of the 'Zuiderzee' ('IJsselmeer') - Backbarrier area ZZ ~ Zuiderzee M ~ Marsdiep / Zeegat van Te xe l EG = Eijerlandse Gat V = Vlie BD = Borndiep PG = Pinkegat ZL = Zoutkamperlaag EB ~ Eilanderbalg L = Lauwers S = Schild Fig. 26. Relation between percentage of intertidal area and tot al surface of catchment areas of different inlets o f the Dutc h Wadden Sea (STIVE & EYSINK 1989). - Note that the 'Zuiderzee' (ZZ) represents a sp ecia l case, w her eas before its closure by the 'Afsluitdijk' in 1932, interfere nce between e ntering and leaving tidal waves led to a damping of tidal current velocities and a consequent decrease of sedi ment transport capacity. With respect to the Texel (Marsdiep) and Vlie inlets it is questionable if these catchm ent areas have reached equilibrium after their origin about 1000 years ago and the interference caused by the closu re of the 'Afsluitdijk' ,in 1932. watershed is formed where the tidal wave enteri ng through a westerly inlet meets the wave entering through the next, i.e. more easterly inlet. At the watersheds low-velocity rotational currents (whirls) develop so that sediments (sands and fines) are easily deposited. Because the tidal wave approaches from th e west, the tidal wave enters each Fig. 27. Soft sediment deform ation in relation to late ral migrati o n of channels (REINECK 1982b) . consecutive inlet with a certain time lag. As a result the watersheds occur asymmetrically displaced towards the east between the islands and the mainland (Fig. 28). Since the positions of these watersheds continually shift due to inlet . migration, changes in tidal currents and wind effects (M. DE BOER 1979; FITZGERALD 1988; DE BOER et al. 1991; OOST & DE HAAS 1992, 1993), the formation of higher intertidal areas along the watersheds is prevented (EYSINK 1987). As stated before, the grain size of the sediment decreases from the open North Sea towards the mainland and the higher parts of the shoals. The deeper parts of the Wadden Sea are subject to high energy conditions and thus consist mainl y of sand. On the higher parts of the shoals, on the other hand, energy levels are lower due to di ssi pation of wave energy (EHLERS & KUNZ 1993), shorter duration of water cover and a general decrease in current velocity. This favours the sedimentat ion of silts and clays. There is a great variety of sedimentary structures on sandy and mixed tidal flats (d. VAN STRAATEN 1954; REINECK 1982b, c; WUNDERLICH 1982a, b; EHLERS 1988; OOST & BAAS, in press). Towards the higher, muddier parts no t only population densities of burrowing organisms increase, but algal mats and h ighe r plants (Zoste'ra) are also common. 88 Fig. 28. Physiography of the Wadden Sea south of Terschelling and Arneland. - Depth contours: 0 rn, 5 rn , and 10 rn below the local low-water line (VAN STRAAT EN 1964). Note the abundance of tidal channels close to th e inlets and th eir near-abse nce along the tidal watersheds. These grain size trends are brought about mainly because current velocities are reduced towards the higher shoals. Sand settles mostly in the early stages, being preferentially deposited on the lower parts of the shoals. Moreover, there is an increase in the concentration of suspended material from the inlets towards the more sheltered and shallower parts of the back barrier area which results in higher sedimentation rates of fine sediments. This increase in concentration is brought about by the following mechanisms: a) The average water depth (in the areas which are covered by water) is, paradoxically, much less at high tide, when the water covers the entire flats, than at low tide when the water only fills the channels (Fig. 29). The probability of suspended matter settling to the bottom over the tidal flats at high tide is therefore much greater than over the channel beds at low tide (VAN STRAATEN & KUENEN 1958). b) In the inner part of the Wadden Sea the tidal wave becomes asymmetrical in such a way that around low water the tide turns much more quickly than around high tide (d. Fig. 22). The time span during which low current velocities prevail, allowing suspended matter to settle, is therefore much greater at high tide than at low tide (POSTMA 1961). c) Higher current velocities are needed to erode particles from the bottom once they have settled, than the velocity at which these particles settle from suspension (scour lag or Hjulstrom-effect (HJULSTROM 1935 ; VAN STRAATEN & KUENEN 1958). This effect is enhanced if the deposited particles have had some time to consolidate, a process significantly affecting the erodability after as little as an hour (CREUTZBERG & POSTMA 1979). d) Particles will start to settle as soon as the current velocity drops below the settling velocity, the particles being carried along some distance before they reach the bottom (settling lag effect; Fig. 30; VAN STRAATEN & KUENEN 1958). The flood-current will transport a particle from its original starting position (1) in the direction of the land and it will settle somewhat landward (5) from the point where the current velocity becomes too low for transport (3). As a consequence the particle can only be eroded during the subsequent ebb by a water mass (B), which was landward from the water mass (A), which brought the particle in during the flood. Because of this, and because of the effects mentioned above at points band c, the particle will settle out landward (9) from its original starting position (1) during the second slack water after ebb (VAN STRAATEN & KUENEN 1958). e) Suspended matter is transported by waves in the direction of wave propagation which is mostly directed inward into the Wadden Sea (EHLERS & KUNZ 1993). Suspended matter deposited during calm periods can be resuspended and transported by waves. Since this is less common in the more sheltered areas, fine-grained sediments preferentially accumulate there. Indeed, the western Wadden Sea, which is more exposed to wave action than the eastern part, is poor in mud deposits. £) Organisms also play an important role in the deposition of suspended matter. They filter particles from the water and aggregate them into faeces and pseudo-faeces (e.g. Mytilus, Cemstoderma sp.; KAMPS 1962). Other animals eat freshly deposited mud and aggregate it into faeces, pseudo-faeces and bind sediments by the secretion of mucus (e.g. Macoma baltica, worms). Furthermore diatoms bind particles below and in algal mats. On the salt marshes the suspended matter (and during storms also sand) is retained between the vegetation. g) Furthermore, river water influx causes locally a seaward directed residual current at the surface and a residual saltwater current along the bottom. Suspended matter from both currents is trapped by this circulation and concentrated at the convergence zone in so-called 'turbidity maxima'. This effect is especially important in estuaries (POSTMA 1967) and contributes, e.g., to the largescale deposition of mud in the Ems-Dollard Estuary. 89 Tidal lIal High lid. Crttk Channtl a a a b a - a high tide level b - b low tide level Fig. 29. Difference in average depth between high and low-water level (VAN STRAATEN & KUENEN 1958). s [nltt 2, / ------------lt7 --------------- 10~~ . ~ .c en o ; Y1 A 1 Fig. 30. Settling lag effect. - Numbers indicate the successive pathways of suspended particles during a number of successive tidal cycles; letters indicate the pathway of the successive water masses capable of (eroding and) transporting the particles (VAN STRAATEN & KUENEN 1958). - See text for further explanation. The Dollard Embayment The grain-size distribution in the Dutch Wadden Sea illustrates the effects of the concentration of fines on sedimentation. One of the best examples can be observed in the Ems-Dollard region, which is a tidal embayment in the northeastern part of the Dutch Wadden Sea (Fig. 31). It consists of muddy tidal flats intersected by one main channel and some smaller branches. The area is bordered by dykes. In the southwest, in the south and in the east salt marshes are present. These receive some fresh water from a small (canalised) river in the south, the 'Westerwoldse A'. In the north the Dollard Estuary is separated from the river Ems by a dam (Fig. 31). The sediments in the Dollard are very fine-grained. The median grain size is mostly less than 150-200 JIm; even medians of 50 JIm occur in a large part of the estuary (WIGGERS 1960; VAN HEUVEL 1991) as a consequence of the sorting processes described above (VAN STRAATEN 1960). Moreover, part of the clay is derived from the river Ems (FAVEJEE 1960). Measurements show that when the flood 90 + G ;PUII: 5KM + 91 waters start to cover the intertidal flats, suspended sediment concentrations become very high as a result of erosion of the surface layer (DE HAAS & EISMA 1992). As a result, fine-grained sediments are transported to and concentrated on the intertidal flats. Sedimentation occurs mainly during summer and erosion during winter, especially during storms. 210Pb and pollen dating have revealed that sedimentation rates in the Dollard lie between 1.4 and 2.8 mm/yr (EISMA et at. 1989; HEYNIS et at. 1987). The relative rise of the sea level in the Dollard is about 1-2 mm I yr. Since this is less then the average rate of deposition, it confirms that the Dollard is an area of net sedimentation, a feature also evident from the historical records of land reclamation and dyke construction during the last centuries. The clays, which form the bulk of the fine-grained sediments, are fairly resistant to erosion. Lateral channel migration and the formation of tidal chutes is thus strongly restricted. This is one plausible explanation why channel patterns in this muddy embayment are relatively stable, as opposed to sandy intertidal areas (in a way comparable to anastomosing rivers versus meandering rivers). Sedimentary Sequence of Backbarrier Sediments Independent of the relative vertical sea-level movements, tidal flat deposits locally tend to show regressive successions due to the vertical accretion of channel deposits and also to the strong sediment trapping capacity of tidal flat systems. As stated above, the deeper parts of channels are often dominated by the ebb current, whereas flood dominance prevails in the shallower parts. This is due to the asymmetry of the tide, with the maximum flood-current velocities during the second half of the flood, and the maximum ebb-current velocity during the later stage of the ebb period. Indeed, fossil tidal deposits frequently show this pattern with ebb directions in the deeper parts and flood directions higher in the sequence. High Shoals and Salt Marshes High shoals, like Noorderhaaks (Fig. 19) and Richel, are mostly situated in, or in front of inlets on the North Sea side. They are mainly formed as a result of wave transport and aeolian processes. Their development depends directly on the development of the adjacent inlet channels (see Chapter "Ebb-tidal Deltas and Inlets"; SHA 1990a; SHA & DE BOER 1991). Exceptions are Griend and Engelsmanplaat, which top older fine-grained deposits acting as erosion resistant nuclei. The higher intertidal shoals are flooded only during spring tide andlor storms. Natural tidal marshes are today largely restricted to the barrier islands. Along the mainland coast most tidal marshes are artificial, their growth being stimulated and protected by shields of twigs. This is largely the result of dyke construction over the past centuries. Land reclama- Fig. 31. Geography and physiography of the Dollard Estuary. tion obstructs the natural extension of tidal environments in the direction of the mainland (DIJKEMA 1987a). Salt marshes are characterized by dense vegetation, comprising halophytes and algae. The vegetation reduces the currents, thus allowing sands, silts and clays to settle out. This sediment is then stabilized by the root systems of the marsh plants. The tidal marshes are drained by a system of small intertidal channels (creeks). When the height of the marsh increases relative to the low-water line, this upper intertidal zone is flooded less frequently than the lower intertidal zone. Sedimentation and erosion, and the wet, salty and poorly ventilated bottom, produce rather hostile conditions for higher plants (DIJKEMA et at. 1990). On the lower intertidal flats only algae and sea grasses (Zostera) can exist. Where sedimentation increases the height of the area to several decimetres below mean high water, pioneer plants (Salicornia dolichostachya, Spartina anglica) settle, followed with increasing height by Suaeda maritima and Aster tripolium (DIJKEMA et at. 1990). Apart from the elevation, successful settlement also depends on the wave energy and the firmness of the sediment (KONIG 1948; VAN EERDT 1985; GROENENDIJK 1986; DUKEMA 1987b). The pioneer zone is flooded almost every day (Fig. 32). Salicornia does not significantly enhance sedimentation, but enables other plants to settle (KAMPS 1962). Spartina, on the other hand, is known to enhance sedimentation (CHRISTIANSEN & MILLER 1983). When the height of the pioneer zone increases, the vegetation cover of pioneers becomes denser and other plants start to settle (VAN OOSTEN 1986). The border between the pioneer zone and the lower marsh, i.e. the area around or above the mean high-water level is characterized by the appearance of marsh grasses (e.g. Puccinella maritima). The lower marsh vegetation can only exist if the bottom is well ventilated (oxygenated; DIJKEMA et at. 1990). The vegetation of, amongst others, Limonium vulgare (prevalent in rdatively sandy areas), Pucci nella maritima, Halimione portulaeoides, Spergularia marttlma, Trigloehin maritima, Plantago marttlma, A triplex prostrata, eochlearia angliea and Aster tripolium enhance sedimentation rates to maximum values (DIJKEMA et at. 1988; 1990). In addition, erosion is strongly reduced (KAMPS 1962; VON WEIHE 1979). The denser vegetation, especially Puceinella maritima, also triggers the formation of creek systems (VAN STRAATEN 1964; DIJKEMA et al. 1990). The water flow is concentrated in the lower areas between the patches of plants, where it restricts sedimentation or even scours the bottom. When vegetation becomes denser, the originally randomly distributed lower-lying areas develop into strongly meandering creek systems. Lateral migration of these creeks is slow or absent (VAN STRAATEN 1964; pers. obs.). The levees of the creeks consist of an alternation of clay and sand layers, several mm to em thick, which accumulate through settling from suspension. Deformation processes comparable to those in lower intertidal gullies may occur in the creeks. The lower marsh is flooded several hundred times per year (Fig. 32). The creeks strongly enhance the drainage of the area and improve the ventilation of the sediment, thus promoting further the colonisation by plants (KAMPS 1962; VAN DIGGELEN 1988; DIJKEMA et at. 1990). When the marsh becomes even higher, sedimentation rates decrease strongly due to· the decreasing number of 92 frequency of tidal floodings (numbers/year) 400 300 200 100 0 sea wall +1.0m -1.0 (.:~' -.; mud zone Zostera diatoms pioneer zone Salicornia Spartina lower marsh Puccinellia Halimione middle marsh Festuca Juncus _ _-+-2.0 duration of tidal flooding hours/tide Fig. 32. Zonation of the tidal marshes in relation to the duration and frequency of flooding (DIJKEMA et al. 1990). floods and also due to the smaller sediment supply with each flood. As a result, marsh deposits typically show a fining and thinning upward of the sediment layers (VAN STRAATEN 1964). Due to sedimentation the lower marsh zone evolves into a middle marsh zone: Puccinella maritima disappears and plants like Festuca rubra, Juncus gerardii, Agrostis stolonifera, A rmeria marttzma, Artemisia aritimae, Elymus pychanthus and Glaux maritima appear (DIJKEMA & BOSSINADE 1990). Above this zone the upper marsh commences with normal grassland plants, such as Elymus repens, Leontodon autumnalis, Lolium perenne and Potentilla anserina (VAN OOSTEN 1986; DIJKEMA & BOSSINADE 1990). Normally deposition of fine silt and clay only occurs in the middle and higher marshes during (very) high water. Beds of sand and shells, deposited during storms, may be continuous over many kilometres. Shells of over a dm in length and other animal remains can also be transported over small distances by strong winds. During this process left/right sorting can take place (CADEE 1992). More substantial shell hash deposits, situated further inland, are accumulated in the upper intertidal zone by birds such as Eiderduck, Oystercatchers, Gulls and Crows. Up to several percent of the annual shell production is probably transported in this way (GOETHE 1937, 1958; REMANE 1951; LEOPOLD et al. 1984; OOST & LEOPOLD 1988; CADEE 1989, 1992, in press). For a more extensive discussion of Dutch tidal marshes the reader is referred to VAN STRAATEN (1954), VAN OOSTEN (1986), DIJKEMA (1987a, b), DIJKEMA et al. (1988) and DIJKEMA & BOSSINADE (1990). Sand-sharing System The different morphological elements constituting the Wadden Sea show a strong mutual interaction. All elements influence the local tidal currents and the wave regime and thus also the local sedimentation patterns. During the last decades many empirical relationships have been recognised for the different parts of the system. Especially the tidal prism greatly influences the character of the different morphological elements, as well as the interaction between them. For this reason each individual tidal inlet system can be considered to form a separate sand-sharing-system (d. DEAN 1988). As a first approximation each sand-sharing-system can be considered to comprise an inlet, the related ebb-tidal delta, the island points at either side of the inlet and the related backbarrier drainage area between the watersheds behind neighbouring islands. However, it should also be realized that adjacent drainage areas can influence each other across the watersheds (OOST & DIJKEMA 1993). By definition, all parts of a sand-sharing-system are coupled and are in a dynamic equilibrium with each other. Changes in any part of the system will primarily be compensated by sediment transport to or from other parts of the same system. When changes are temporary, the old dynamic equilibrium will eventually be restored. If changes are more permanent (e.g. by loss of drainage area), a new equilibrium will be established. In both situations sediments can be imported or exported from or to areas outside the normal sand-sharing system. 93 Sedimentologists studying barrier-related tidal deposits should thus realize that observations of any part of the system may provide important clues to other parts of the system. For instance, observed changes in channel size and depth may be related to changes in the (backbarrier) drainage area (when tidal range is constant) which, in turn, will also be expressed in the ebb-tidal delta volume (e.g. FLEMMING & DAVIS 1992; OOST & DE HAAS 1992, 1993; OOST, in press; VAN DER SPEK, in press). Relationships Between Flora/Fauna and Sediments The influence of biota on siliciclastic sedimentation is commonly ignored. This is certainly not justified for the Wadden Sea. As is the case in many back barrier areas, the biomass of the Wadden Sea is very large compared to other marine environments. Biota influence erosion, transport and sedimentation of all sediments and, moreover, they form part of it. The high nutrient supply enables a high primary production. Important primary producers are floating microscopic algae (phytoplankton) and, most important, microscopic benthic algae (micro-phytobenthos). Besides the local primary production, the influx of plankton and detritus from the North Sea is also important. This food supply is utilized by the primary consumers (or secondary producers). The majority of the fauna (molluscs, polychaetes) lives in the sediment as deposit feeders or suspension feeders. This fauna is predated upon by crustaceans, molluscs, echinoderms, fishes and birds (CREUTZBERG & KUIPERS 1984). As in most back barrier areas, the faunal diversity is low in the Wadden Sea. Normally four species [A renicola marina (lugworm), Mytilus edulis (blue mussel), Cerastoderma edule (cockle) and Mya arenaria (soft-shell clam)] each make up more then 15% of the total benthic macro-fauna (together even 75%1). Some 40 species constitute the other 25% (BEUKEMA 1977). This is partly due to the low variability of the substrate, allowing only a small range of ecological niches. More important, however, are the extremely large variations of the environmental conditions in time (CREUTZBERG & KUIPERS 1984). Particularly on the tidal flats, large fluctuations in temperature, humidity, current velocity, salinity and sediment supply occur in the course of a year. Only a few species can cope with these extreme variations. Some authors argue that estuaries have not existed long enough in geological time, or occur too isolated in space and time, to permit the evolution of a complete euryhaline estuarine fauna (NYBAKKEN 1982). This view is conjectural: other researchers state that the animals living on the tidal flats, also occurring in fully marine environments, are not particularly adapted to the extreme conditions on the tidal flats (HERTWECK 1990, 1992). The marine benthic community in the Wadden Sea consists primarily of species that have a free-swimming larval stage. In general, as is also the case in the Wadden Sea (e.g. BEUKEMA et al. 1977; BUHR & WINTER 1977), benthic communities are relatively stable over large areas through time. This stability is partly maintained because the larvae are able to detect if the sediment is suitable for settlement or not (NYBAKKEN 1982). Larvae often preferentially settle where the adults are already living (SEED 1976) and many larvae are able to detect the presence of adults of their own species by certain pheromones released into the water (CRISP & MEADOWS 1962). Larvae may also respond to physico-chemical factors such as light, pressure and salinity (THORSON 1966). In the early stages of their larval life large parts of the free-floating larvae reside in the upper, fastermoving water where dispersal is the strongest. Later, when time for settlement approaches, they migrate to the bottom. Other larvae are especially sensitive to pressure and light, hence being restricted to particular levels of the water column. These larvae can only settle if the water in which they reside comes into contact with the seabed, an important mechanism in the establishment of non-randomly distributed community patterns (NYBAKKEN 1982). Furthermore, in the Wadden Sea it has been noted that mass mortality during severe cold winters is compensated for by greater amounts of larvae in the following year, ensuring a quick re-settlement in places which have become devoid of life (SEED 1976; BEUKEMA 1982). The influence of some specific elements of the biota on the sediment is briefly discussed below. For a more extensive discussion on faunal influence the reader is referred to DORJES (1982) and HERTWECK (1982, 1992). Bacteria Sections through Wadden Sea sediments commonly show a light-coloured to rusty brown layer some mm to cm thick at the sediment surface. This layer is aerobic and contains algae, bacteria and ferric ions. Aerobic bacteria oxidize the organic compounds and produce hydroxides. Where erosion has occurred recently, or where the concentration of organic matter is extreme, this layer may be missing locally. This upper aerobic layer is followed by a darker, often black-coloured layer. This is the anoxic zone where anaerobic bacteria flourish. Some of these bacteria can reduce sulphate ions to form sulphur-hydrogens. These react with iron-hydroxides to form colloidal iron-sulphides which have an intense black colour. For a fuller treatment of this subject the reader is referred to GERDES et al. (1985) and VAN GEMERDEN (1993). It is still not clear which effect these bacterial processes have on sedimentation, especially on diagenetic processes. The strong diagenetic alterations around burrows, which have been attributed to the influence of anoxic-aerobic transitions (e.g. ALLER 1982; ZIJLSTRA 1994), indicate that the influence may be considerable. Diatoms Diatoms live on sandy shoals, in the wet troughs of ripples, and on mudflats. These. siliceous algae are rarely fossilized because the slightly alkaline waters tend to 94 dissolve the silica skeletons. Although benth ic diatoms hardly produce any sediment, they nevertheless have an important effect on sedimentation. EPIP ELIC diatoms move over the sediment surface and leave a trail of sticky slime. Where diatoms are abundant (e.g. at the sediment-water interface) they bind the sediment which has been deposited after a period of high current velocities. Moreover, sediment grains are glued together by epipsammic diatoms or by the mucus produced by diatoms. In case of very dense algal mats, grains may even become part of the organic carpet. Moreover, abundant mucus attached to the sand grains may decrease the bottom roughness. This mech anism is most effective in the case of poorly rounded, angular grains. Field experiments in an intertidal area of the Oosterschelde (SW Netherlands) have shown that in higher energy parts of the intertidal flats, where the activity of algae cannot be recognized on a macroscopic scale, strong erosion may occur on places where the algal cover is chemically destroyed (DE BOER 1981). Both epipsammic and epipelic diatom populatio ns may increase the threshold velocity for the erosion of sands in temperate climatic zones, as in the case of the Wadden Sea. Laborato ry experiments have revealed that a diato m population that was allowed to re-establish itself for 24 hours after stirring, increased the threshold velocity by several tens of percents as compared to freshly stirred and redeposited sand; sandy sediment surfaces with a rich epipeJic diatom populatio n may even show an increase of more than 100% (Vos et al. 1988). Stabilization by diatoms can therefore be an important facto r in the stabilization of sediment in intertidal and shallow subtidal areas. Mytilus edulis L. (Blue Mussel) The blue mussel lives on the surface and filters seawater for organic particles (suspension feeder) . The part of the filtrate which is indigestible is compressed into mud faeces and pseudo-faeces. The animal excretes faeces stri ngs which break up into individual pellets approximately 1 mm long. These pellets are hydraulically equivalent to sand particles of about 60 p,m in diameter. They are fairly resistant to erosion and can be transported over several tides before complete disintegration occurs (OOST, in prep. b). As o bserved on the tidal flats, small-scale ripples may locally consist predominantly of mud in the form of faecal pellets. Large amounts of mud are also deposited in mussel colonies, forcing them to move upward in order to avoid suffocation by their own pseudo-faeces. Mussels attach themselves with byssus threads to avoid being swept away by waves and currents. In doing so, the colon ies form a semi-rigid framework which tends to bind the sediment, thereby protecting it from erosion (cf. FLEMMING et al. 1993). N A Nor. t h sea 1 0 - - - -- _ _ --------10 Former L a uwer szee "10-. -10 m DOL ~ Sandy intertidaillats « 5% clay) _ _ Muddy intertidal flats (25% clay) Musselbed Friesland o I ~ Higher intertidal (above MHW) islands and shoals 5km I _ Higher intertidal (above MHW) mainland san marsh Fig. 33 . Mud-flats in the surroundings of mussel colonies consist partly of fae ces and pse udo-faeces produced by the mussels. - After DIJKEMA (1989). 95 In the more protected parts of the Wadden Sea, biodeposits of mussels may form an important element of the fine-grained sediments at some distance from the mussel colonies (Fig. 33). The amount of sediment filtered by the mussels in the Dutch part of the Wadden Sea is estimated to be up to 2 x l0 lO kg/yr, depending on the size of the population (DANKERs & KOELEMAIJ 1989; DANKERs et al. 1989; OOST, in prep. b). Theoretically this could form a layer of dry fine-grained sediment of about 1 mm/yr covering the entire surface of the Dutch Wadden Sea. Most of the biodeposies that are not resuspended, accumulate in the more sheltered parts of the Wadden Sea. The blue mussel is thus of major importance in the deposition of fine-grained sediments in the Wadden Sea. this way extensive layers of shells, especially of Hydrobia spp., are formed (VAN STRAATEN 1954, 1964). U -burrows of the type produced by A renicola spp. are found throughout the stratigraphic column, being considered typical for intertidal sediments. This does not, of course, imply that all V-burrows in the rock record relate to the same polychaete species. The strategy of making Vburrows allows animals to continuously ingest sediment without exposing themselves at the sediment surface to predators or hostile physical and chemical conditions. This favourable way of life in the intertidal domain has been adapted by a large variety of different animal species (see e.g., BROMLEY 1990). It should be noted, however, that such burrows are not exclusively restricted to intertidal environments, but can also be found in subtidal deposits (HERTwEcK 1992). Arenicola marina (L.) (Lugworm) Plants On the higher parts of the tidal flats, where migrating gullies are rare, organisms have sufficient time to completely modify the original depositional structures. The combined action of burrowing organisms works like a powerful conveyor-belt in all directions. One of the most important bioturbators is the lugworm A renicola marina, which forms V-burrows. The animal lives at depths of 15 to 20 cm, almost continuously ingesting sand (deposit feeder). There is no need for A renicola to migrate because sand is sliding down to the mouth by gravitational forces, creating a small pit at the sediment surface in the process. From time to time the animal excretes the indigestible sand in spaghetti-like strings at the surface (VAN STRAATEN 1954, 1964; BEUKEMA 1977; HERTwEcK 1982). It has been calculated that the top 30 em of the tidal flat sediments can be reworked completely by Arenicola spp. each year (CADEE 1976). The grains that are too large to be swallowed are concentrated at the deepest point of the V-burrow. In In upper intertidal flats and coastal dunes plants are the principal biotic component influencing sedimentation next to man. In tidally influenced areas a clear zonation can be observed from the lower intertidal to above the higher intertidal zone (see Fig. 32). The different plants are adapted to the extreme conditions of the salt marsh environment. Some influence sedimentation by decelerating the flow of the current during flooding. In this way large amounts of sediment are retained and the marsh accretes vertically (see also Chapter "High Shoals and Salt Marshes"). In the coastal dunes plants form a succession which mainly depends on the availability of water. Of these plants Ammophila arenaria plays an important role in the fixation of aeolian sediments with its strong vertical straws and long horizontal root system. The hardy character of this plant enables it to survive and even to propagate in this barren sand environment. Sedimentary History of the Dutch Wadden Sea Pleistocene From a geological point of view the Wadden Sea is relatively young. The Dutch Wadden Sea is underlain by an irregular surface of glacial till and fluvial sediments dating from the Riss (Saalian) glacial stage (approx. 180,000-130,000 BP). Most of these glacial deposits have been covered by younger sediments (Eemian interglacial, Wiirm/Weichselian glacial and post-glacial). Locally, e.g. on the island of Texel, the Riss deposits crop out as small (ice-pushed) moraine hills that reach a height of 10 m to 20 m above present mean sea level (Fig. 34). The area between Den Helder (south of Texel) and the 'Afsluitdijk' also contains Pleistocene outcrops, mainly comprising glacial moraines and fluvial sediments. The deposits form a highly irregular micro-relief in the otherwise flat Dutch landscape (Fig. 34). These Pleistocene deposits form the backbone of the western Wadden Sea and have strongly influenced the morphological development of the area. The development of Texel Inlet, i.e. the tidal inlet between the island of Texel and the mainland, for example, was strongly hampered by Pleistocene deposits in the subsurface. Also, the position of this island is stabilized by its Pleistocene core. During the Eemian (Riss-Wiirm) interglacial sea level was relatively high (e.g. approx. 75,000 years BP), low-lying areas being submerged. Shallow marine and tidal flat sediments with a characteristic fauna were deposited in the embayments. During the last glacial stage (Wiirm-Weichsel, which ended about 10,000 years BP) the icecap did not reach the Dutch region. Sea level was about 100 m lower than at present and extensive aeolian deposits (,cover sands' and loess) were formed on the emerged land surface. In many places these deposits form the substratum of the Holocene sediments. In general, the Pleistocene subsurface dips gently in the seaward direction. 96 c IJlrntion of iu Ilow Ju mJr ginil/ YJfI~y of !Ilt' Vee'" --- . . =",.. fJrl y (Ovrl{, of tilt' ice mJrgillJ1 YJlley of til! Vecllt U r / / Hoarn A/A-mt Jr . !.CilstricV", Fig. 34. Ice-pushed ridges dating fro m the Riss glacial. Holocene Pre-historic Sea-level Rise and Sedimentation During the earl y parts of the H olocene transgression, mean sea level rose at a rate of approx. 1 em / yr, mostly because of melting of the icecaps, but partly also by subsidence of the North Sea basin (Fig. 35). After 6,000 BP subsidence (at a rate of 0.10- 0.1 5 cm/yr) began to dominate, eustatic sea level now rising at a rate of only about 0.1 em/yr. Tidal deposits buried below the Recent marine sands in the southern North Sea and the D.O.L. -2 -4 -6 -8 MHW (beach plains S of the Old Rhine-estuary) MSL ~ 0 0 -.i -10 E E -12 OJ 'iii 2 I -14 -16 -18 MSL curves Louwe Kooijmans (1976) Jelgersma (1979) Van de Plassche -20 8000 7000 6000 5000 14C 4000 3000 2000 1000 o years BP VAN DE PLASSCHE Fig. 35 . The relative rise of sea level in Th e Netherlands according to different authors. - After (1982). Sediment was also derived from erosion of headlands, old ebb-tidal deltas and from the North Sea (Beets et aI., 1992). The dominance of sediment supply by rivers may further have 97 been enhanced by changes iu wave climate and tidal amplitude (Zagwijn, 1985). widespread occurrence of tidal flat mollusc shells suggest Western Area: Coast of South Holland that much of the southern North Sea basin comprised tidal and North Holland flats for some time in the period of 10,000 to 5,000 BP. During the Holocene large amounts of sediment were In the western area (coast of Holland) sediment supply transported to the modern Wadden Sea area, derived from by various large rivers overruled the effect of the sea-level the rivers and from the North Sea. It is assumed that a large rise, causing the coast to prograde. The intertidal areas part of the Wadden Sea sediment was derived from a high behind the coastal zone became shallower and tidal channel area west of Texel and, during the last millennia, from the systems were eventually abandoned (5,500-3,300 BP; coast of North Holland (SCHOORL 1973; ZAGWIJN et a!. BEETS et al. 1992; VAN DER SPEll. & BEETS 1992). By that 1985). time the coastline had been closed and large surfaces During the early sea-level rise the coast retreated and behind it were logged by fresh water. As a result, a second the ground water table rose. The coastal areas became marperiod of large-scale peat formation (Holland Peat) comshy and a peat layer was formed on the Pleistocene subsur. menced, becoming especially important face. From land to sea the following zones succeeded each ~ This was followed by a period characterized by a shorother: a peat layer (Basal Peat), a fine-grained zone of wadtage of sediment, so that large parts of the coastal zone den deposits, brackish lagoons and salt marshes and a sandy were once again submerged. coastal zone consisting of beach plains and dunes. During between the Middle Subboreal to Lower Subatlantic (Roe lethe landward retreat of the coast, the sediments formed in veld, 1974; Zagwijn, 1985). these zones were deposited on top of each other. The transWestern Dutch Wadden Sea gression continued until approx. 5,000 BP, after which the Dutch coastline became more or less stable. The topoIn the western Wadden Sea (Texel-Terschelling) the graphy defined by the Pleistocene subsurface dominated Pleistocene surface was at a relatively high elevation, thus the evolution of the area (Fig. 36). Around 5,000 BP a tidal strongly reducing the influence of the sea (Fig. 36). Until basin was formed in the west (Zeeland-Northern Holland), 3,700 BP this area developed in the same way as the western being separated by a Pleistocene high (Texel-Vlieland) from one (see above), except for TexeI High, where no the precursor of the present Wadden Sea (ZAGWIJN et al. submergence and formation of peat took place. After 3,700 1985). Each area experienced a different evolution: peat around groundwater table peat above groundwater table . ~ clays (brackish-marine) ~older ~Iocalpeat clay, ~wadden 1>/, " dunes and beaches, '." older creeks C:=J open sea Fig. 36. Reconstruction of the Wadden Sea region around 5300 BP kilometres further to the north than indicated in the figure. (ZAGWIJN et a!. 1985). - Position of.the barrier islands was several 98 BP part of the area east of the Vlie channel (EDELMAN 1964; EISMA & WOLFF 1980) gradually changed into a Wadden area. West of this line the surface was covered with peat until the Early Middle Ages. In the east the small Boorne river debouched, whereas in the west a river was situated at the site of the present Texel Inlet channel (SCHOORL 1973). The sea entered this part relatively late (approx. 1,000 BP), and after the peat layer was eroded this area too became tidally influenced (EDELMAN 1964; SCHOORL 1973; EISMA & WOLFF 1980). Eastern Dutch Wadden Sea In the eastern Dutch Wadden Sea the Pleistocene surface was lower, with a number of small rivers incised (amongst others Boorne and Hunze Valleys). The sea flooded the region before 5,300 BP (between 6,000 and 5,000 BP; SHA 1992) and barrier islands, channels, tidal flats and lagoons were formed. Low sediment supply in combination with continued sea-level rise forced the islands to shift landward. Sediment was needed to compensate the relative rise of the sea, i.e., to maintain the barrier islands and the Wadden Sea and to replace eroded Pleistocene and early Holocene fine-grained and peaty deposits. This sediment was largely derived from the island coasts. As a result, the islands shifted coastward (11 km for Ameland and 15 km for Schiermonnikoog in approx. 5,000 years; SHA 1992). Although shifts of the islands towards the east may have been partly caused by human interference (FLEMMING & DAVIS 1992), geological and historical evidence in the Dutch Wadden Sea clearly demonstrates that the tips of the islands also tend to shift eastward due to the coast-parallel eastward directed drift, caused by residual tidal and wavedriven currents. In the area of the present eastern Dutch Wadden Sea, lower rates of sea-level rise often did not result in peat formation. On the contrary, clayey deposits were formed, suggesting a larger marine influence than in the areas where peat was formed (Fig. 37). This is probably due to the presence of large tidal inlets at relatively short distance and the rather small width of the tidal area, as compared to the tidal area in the western area (coast of Holland). There, the inner flats were more sheltered behind long coastal barriers and peat formation was far more extensive. An important additional effect has probably been the much greater terrestrial run-off into the western part of The Netherlands as compared to the rather small fluvial input especially in the eastern part of the Wadden Sea. Historical Development: Man and Flooding The development of the western and eastern Dutch Wadden Sea in historical times was also different and will therefore be discussed separately here. Western Dutch Wadden Sea 1. In Roman times and during the early Middle Ages the coastline of northern Holland, up to the present Vlie Inlet (between Texel and Vlieland), was rather continuous, interrupted by relatively few river mouths and tidal inlets, peat marshes and woodlands covering large parts of the western Dutch Wadden Sea (Fig. 38). During the following centuries a major part of the coast in the western part was destroyed and the land behind it was flooded, while the coast as a whole retreated (Fig. 39). Also the 'Zuiderzee' (the embayment which was closed in 1932 by the 'Afsluitdijk') came into being during the Middle Ages, the Wadden Sea thus reaching its greatest extension in historical times. New inlets were formed. The evolution of the morphology of the ebb-tidal delta of Texel Inlet in relation to the evolution of the hydrodynamic regime can be explained by the mechanisms discussed in Chapter "Ebb-tidal Deltas and Inlets". Texel Inlet probably formed during a severe storm surge in 1170 A.D. It could not be navigated until about 1300 A.D. (SCHOORL 1973). Historical maps show that the maximum inlet depth increased from 25 m in about 1583 (earliest reliable map) to about 50 m at present (SHA 1990a). Based on the relationship between the maximum inlet depth and the tidal prism, the historical development of the tidal prism was reconstructed by SHA (1990a). He concluded that the tidal prism through Texel Inlet increased from about 240xl0 6 m 3 in 1583 to 1050xl0 6 m 3 in 1970. The increase of the tidal prism in historical times is related to the rise of sea level and the resulting increase of tidal amplitude together with an increase of drainage area. Reconstructions by SHA (1990a) on the basis of historical navigation charts since the 16th century, show that the 2. \ 1800 B.C. ReI. seal. -3m 3. . -1m ffiilll Peat ~ Clay Be (MAZURE Fig. 37. Sedimentation in the period 3500-200 al. 1974). et 99 8' CillID 2 8' @2 ~3 ~4 c:::::> 0 _ 3 4 05 5 Fig. 38. Reconstruction of North Holland and the southwestern part of the Wadden Sea region during the early Middle Ages (EISMA & WOLFF 1980). - 1. Pleistocene outcrops; 2. Dunes and beaches; 3. Clay deposits; 4. Oligotrophic peat; 5. Eu- and mesotrophic peat. Fig. 39. Reconstruction of the situation in the 14th century (EISMA & WOLFF 1980). - 1. Pleistocene outcrops; 2. Dunes and beaches; 3. Tidal flats and salt marshes; 4. Low-lying cultivated and inhabited polder area; 5. Dams across streams and inlets. ebb-tidal delta of Texel Inlet was asymmetrically oriented north to northwest before the middle of the 18th century (Fig. 40). It became symmetrical between the late 18th century and the early 19th century, and since the early 19th century it has been directed towards the southwest (Fig. 40). This is a clear development from a down-drift, relatively more wave-dominated to an updrift tide-dominated ebb delta, which is in close accordance with the observed increase in tidal prism. Much of the flooding of the area was enhanced by the exploitation of the land behind the coast: ditches and small canals were dug to drain the lowlands and peat was dug for fuel and salt extraction (EDELMAN 1963; EISMA & WOLFF 1980). The marshland subsided due to levelling, dewatering and oxidation upon aeration, and thus became vulnerable to floodings. Counter measures against further flooding started around 1000 A.D., when dykes were constructed to enclose sheltered areas and to reclaim lost land. In about 1250 A.D., a large dyke protecting a larger part of the inhabited area at the mainland which bordered the western part of the Wadden Sea was established (SCHOORL 1973). The former dune coast or barrier beach along the western coast had been breached during the Middle Ages and former land had been flooded, leaving only a few small islands. From the 16th century onward, a number of such islands were 100 Wadden Sea ..... , ... .. i " ' ,,-! ...... ....... .-:" ~-_~. about 3 km I about 3 k.m Wadden Sea Wadden Sea ( / J . ----- / . ~,II ) / . ) // ----.: ~'! ". J ~ Fig. 40. Evolution of Texel Inlet from a downdrift orientation in the 16th century (A) via a more symmetrical orientation in the 17 / 18th century (B) and 18/19th century (C) to the present-day situation with an updrift orientation of the ebb-tidal delta (D) (Sha 1990a), connected by sand dykes which were constructed by trapping aeolian sand between branches and mats of reed. This technique is still in use. Along the mainland, coastal protection works were intensified from the early 19th century onward, and at present the mainland coast of the Wadden Sea is almost everywhere bordered by dykes. Despite the dykes and other coastal protection works, the Dutch Wadden Sea is still a relatively undisturbed area where natural sedimentary processes proceed freely. During the Middle Ages loss of land generally dominated over reclamation. This process was gradually reversed during the early 17th century when larger areas were reclaimed, culminating in the large reclamations of the 19th and 20th centuries. The present extension of the Dutch Wadden Sea was reached in this century after the enclosure of the 'Zuiderzee' by the 'Afsluitdijk' and the enclosure of the much smaller 'Lauwerszee'. The history of land reclamation in the northwestern Netherlands is shown in Fig. 41. The 'Afsluitdijk' The 'Afsluitdijk' has a length of 32 km. It was built in 1932 and is one of the world's longest causeways across a back barrier area (Fig. 18). South of it the lake 'IJsseimeer' (before the closure: Zuiderzee) is situated. Before the dyke was built, the 'Zuiderzee' was a marine to brackish water backwater with an open connection to the Wadden Sea. It was influenced by the tidal wave entering it. The size of the Zuiderzee basin was such, that the reflected outgoing tidal wave interfered with the incoming tidal wave, resulting in a standing wave of low tidal amplitude (KLOK & SCHALKERS 1980). As a consequence, the tidal prism through the nearby Texel Inlet was also low (Fig. 42A). After closure, the tidal amplitude increased considerably (approx. 20% near Texel Inlet; Fig. 42B) and the catchment area of Texel Inlet within the remaining part of the Wadden Sea became larger (Fig. 43). In this way the tidal prism of Texel Inlet and, as a consequence, also its depth increased (SHA 1990a). The closure of the large Zuiderzee tidal basin also blocked the tidal channels draining it. In these channels the tidal currents were reduced to almost zero. The infill of one of these ~hannels, the Vlieter (Fig. 44), was studied by BERGER et al. (1987). Above the coarse sandy/shelly channel bed of 1932, a sequence of 1-3 cm thick mainly undisturbed layers formed, consisting of fine sandi silt, alter-, nating with silt/ clay and occasionally intercalated medium sand layers. Based on 210Pb measurements, average sedimentation rates were calculated to have been 6.5 cm/ yr (BERGER et al. 1987; E[SM~ et al. 1989). When compared 101 Eastern Dutch Wadden Sea From Greek, Roman and Early Medieval sources it can be concluded that at least since 200 B.C. the eastern parts of the coast consisted of barrier islands and intertidal flats. The early historical sources are thus in accordance with the geological reconstructions (DE JONG 1984; SHA 1992; BOSCH & Vos 1992). The landscape of the Province of Friesland, east of the 'Afsluitdijk', is quite flat because it largely consists of old tidal flat and tidal marsh deposits. Prehistoric man entered the area in several waves: the first around 500 B.C., the second around 50 B.C., the third 600-700 A.D., and the last between 900-1100 A.D. Man started to influence the morphology of the area by constructing dwelling mounds on which their settlements were protected against storm floods. These so-called 'terpen' or 'wierden' were built between 300 B.C. and 1200 A.D. The total amount of earth and dung piled up in these artificial mounds is estimated at about 30 times the amount of building material used in the Great Pyramid (BRUUN 1986). The 'terpen' were often situated on the levees of the larger creeks in the higher intertidal marshes. In later times, much of the fertile earth of these dwelling mounds was removed for agricultural purposes. Many of the churches in the centre of the old villages are still standing on the top of such mounds, the remnants of larger 'terpen'. Since at least 12 B.C. peat has been dug for heating and later also for salt production. Because of this and also for agriculture, the area was drained. As in the western Wadden Sea, a large part of the mainland was flooded, being at least partly caused by the mining of peat (EDELMAN 1974; GRIEDE 1978). In the period of ca. 800-1300 A.D. the 'Middelzee' (VAN DER SPEK, in press), the 'Lauwerszee' (GRIEDE 1978) and the Dollard estuary were formed. Loss of land in the Dollard continued up to the 16th century. Dyke construction began in the 10th century A.D., initially to protect land from the sea, but later also to reclaim land. Sedimentation in the higher intertidal marshes was actively encouraged in order promote land reclamation. Remnants of the old dykes can be observed in various places of the landscape. The total area of the dyked land is considerable (Tab. 1). In this way the tidal catchment _ 3 Fig. 41. Land reclamation in the northwestern part of Holland and the SW Wadden Sea. Numbers refer to year of reclamation (EISMA & WOLFF 1980). - 1. reclamation of lakes; 2. reclamation of tidal areas; 3. sand dykes. Tab I e 1. Dyke construction in the Dutch Wadden Sea, 'Zuiderzee' (now lake 'IJsselmeer') and 'Dollard' in km. - Data MAZURE (1974). Period (Century) Original character of the dyked area Intertidal! Marshes Subtidal flats Water covered Total 52 421 401 388 3871 5133 with partially abandoned tidal channels, such as in the Oosterschelde (see Fig. 22) and the Zoutkamperlaag, where the rate of sedimentation is of the order of dml yr, the Vlieter is being filled quite slowly (cm/yr). This must have been due to the drastic reduction of tidal currents through the Vlieter channel, the ability of the system to transport sediment being a major constraint on the infill of such channels (OOST et al. 1993). 11th-12th 13th-16th 17th-18th 19th 20th Total 52 421 242 313 115*) 1143 159 56 1650 1865 19 2106 2125 *) In the 20th century general land reclamation was reduced in favour of the closure of the 'Zuiderzee' and 'Lauwerszee' (ANONYM US 1981). 102 Schiermonnikoog A Ameland ~ Lau- 1'0>0 160 ~:~~'~~.::::::" "",.,00 \ ~ ~: ...... # fSO ....... --- .... ~-- ....... -' -:-;-;':_' '-:-':-:':-:' :-'. ------ C\J 0 : , 150 150- , , . ,,,, , , .. D , , ... _------- 160 .-- SchiermOhnikoog r#' B o 225 V Lauwers- oog · · · N 20km Fig. 42. Mean tidal amplitude in em before (A) and after (B) the closure of the Zuiderzee. Data: A: KLOIt & SCHAUERS (1980); B: VROOM et al. (1989). 103 area decreased strongly. Simultaneously the inlet channels, the channels in the back barrier area and the ebb-tidal deltas became smaller, due to an almost linear relationship between these features and the tidal prism (see above). As the tidal prism decreased, the ebb-tidal deltas were strongly eroded by wave action. Part of the eroded sand served as infill for the inlet channels and the channels in the backbarrier areas (d. OOST & DE HAAS 1992; VAN DER SPEK, in press), the process vividly illustrating the concept of the sand-sharing system. The formation of new wash over channels on the islands was strongly reduced by the protection of the vegetation cover (since at least 1354). This led to the stabilisation of dunes on the barrier islands and probably helped to limit the coastward retreat of the islands (EHLERS & KUNZ 1993; OOST & DIJKEMA 1993). A Wate.·shed Catchment area of Texel Inlet Area will. standing tidal wave B Watershed Catchment area uf Texel Inlet Fig. 43. Catchment area of Texel inlet before (A) and after (B) the closure of the Zuiderzee (KLOK & SCHALKERS 1980). 104 ~~!!/f ~~ ~.~ Mle~ ~G)"'\l SChiermonni. ko. og ~. 1970 1800 1650 1550 Fig. 44. Sedimentation (in m) in the former Vlieter channel based on a comparison of soundings in 1928/34 and 1975. The dot indicates the place of the corings (BERGER et al. 1987). Around 1300 A.D. the following islands and inlets were probably present from west to east: island Terschelling, Ameland Inlet (Bordena, 1297), island Ameland (Ammeland, 1307), Zoutkamperlaag Inlet (Scudbalwe, 1309), island Schiermonnikoog (1323), Lauwers Inlet (Lavicam, 1287), island Bosch (mentioned 1535), Schilt Inlet (?), island Rottummeroog (1354) and the Ems River. In the eastern Wadden Sea several small islands were situated (Cornasant, 1323; Bant and Heffesant; OOST, in prep. a). 1500 Take-over of the Drainage of the 'Lauwerszee' One of the most dramatic changes in the back barrier region of the eastern Dutch Wadden Sea has been the takeover of drainage of the 'Lauwerszee' embayment. Around 1300 A.D. the watershed of the barrier island Schiermonnikoog was situated on the coast of the province of Friesland (Fig. 45). The precursor of the Zoutkamperlaag Inlet (west of Schiermonnikoog) was small and had no connection with the 'Lauwerszee' embayment, which was entirely drained by the, at that time, large Lauwers Inlet situated east of Schiermonnikoog (d. BOSCH & Vas 1992). The watershed of Schiermonnikoog was probably breached in the period 1350-1450. Around 1500 A.D. both the Lauwers Inlet and the Zoutkamperlaag Inlet drained the 'Lauwerszee'. Later the drainage was rather rapidly taken over fully by the Zoutkamperlaag which, as a result, increased in cross-sectional area. Judicial information shows that by 1556 the Lauwers Inlet had lost its connection with the 'Lauwerszee' (FoRMsMA 1954, 1958). After the breaching, the western inlet (Zoutkamperlaag) took over the drainage of the 'Lauwerszee'. Schier- 1300 Fig. 45. Physiographic evolution of the eastern Dutch Wadden Sea (OOST, in prep. a). monnikoog became approx. 8 km (1300-1850) longer at its eastern head, the Lauwers Inlet also shifting eastward, while gradually becoming smaller. At the same time the small inlet at the western side of Schiermonnikoog and its ebb-tidal delta grew. Erosion in the west of Schiermonnikoog, amounting to approx. 4.5 km over the period 1300-1850, further facilitated the connection of the Zoutkamperlaag with the 'Lauwerszee'. Around 1700 A.D., the hamlets 'Westerburen', 'de Dompen' and 'Oosterbun;n' were situated west of the present village of Schiermonnikoog. Due to the migration of 105 dunes towards the east, the village of 'Oosterburen' eventually had to be abandoned. Strong erosion occurred at the western side of the island, especially during the storms of 1717, 1720, 1756 and 1760, and the hamlets 'Westerburen' and 'de Dompen', as well as the castle of the island ruler had to be given up. The present village was built in the period 1721-1761. Erosion continued until 1856, when the low-water line reached the present dune area. A new tidal flat on the SW side developed after 1859, becoming quite high (mainly higher intertidal) after the closure of the 'Lauwerszee' in 1969 (OOST & DE HAAS 1992). Around 1538/45 the backbarrier island Cornasant merged with the barrier island Bosch. The All Saints Flood of 1570 changed the barrier island Bosch and the back barrier island Heffesant into sandy shoals, the latter disappearing shortly afterwards. Although man tried to restore the dunes by planting grass, the island Bosch did not recover from this major blow. The shoal started to migrate to the south at about 1650 and disappeared in the 19th century (LANG 1958). Later a new shoal developed at the place where Bosch had been situated in 1600 (OOST, in prep. a). The changes which resulted from the take-over of drainage of the 'Lauwerszee' are a good illustration of how a dynamic equilibrium (Lauwers Inlet drains 'Lauwerszee') can change in a rather short time to a new dynamic equilibrium (Zoutkamperlaag Inlet drains 'Lauwerszee'). In contrast to Schiermonnikoog, the more western barrier island of Ameland migrated less strongly to the east. Some erosion occurred at the western head of the island (1.5 km in the period 1650-1970) and sedimentation at the eastern head (3.5 km in the period 1650-1991, especially in the period 1800-1991). The difference to Schiermonnikoog can be explained by the fact that the inlet of Ameland was not located as asymmetrically to the drainage area as was the case for the Zoutkamperlaag. Also, there was no inlet east of Ameland which could decrease in size, as was the case for Schiermonnikoog. Sedimentation in a Sheltered Embayment: The Infill of the Dollard Along the Wadden Sea coast several semi-enclosed embayments have existed and still exist (in the Dutch Wadden Sea: 'Zuiderzee', 'Middelzee', 'Lauwerszee' and 'Dollard'). Such embayments, which are enclosed by the mainland on three sides, strongly favour sedimentation. A good example is the Dollard embayment (Fig. 31). The present morphology of the Dollard is the result of: 1. 2. 3. 4. 5. the Holocene rise of sea level; tectonic subsidence; compaction of peat and clay; erosion of peat; human activities (land reclamation). As a result of (1) and (2) relative sea level rose and the coastline shifted landwards until about 1000 A.D. At this time the people who lived in this area began to build dykes. The marshy land behind the dykes was drained in the interest of agriculture, which resulted in an increased compaction of clay and peat. Man thus stimulated an increase of subsidence in a period of rising sea level. The dykes were breached from time to time by severe storms and huge areas were flooded. As a result, large amounts of clay and peat were eroded from the areas behind the dykes. In this way the formation of the Dollard estuary began in the 13th century. The Dollard had its largest extension around 1520 A.D.' . DOLl.ARD ......... ", .... ........ Win'cholcll" .', Fig, 46. Land reclamation in the Dollard Estuary, - Numbers show the year of reclamation. 106 - above +1 m +1.0 0.0 -O.S ·1.0 to to to to 0.0 ·O.S ·1.0 ·2.S m m m m .2.5 to ·S.O to .7.Sm ·10.0 to ·lS.0 m ·10.0 to ·lS.0 m below .lS.0 m ~ N 4km Fig. 47. Reconstruction of the ebb-tidal delta and th e backbarrier area of the Zoutkamperlaag in 1967. - Colours give different depth intervals with reference to DOL, which approximates mean sea level. - After OOST & DE HAA S (1993) in cooperation with Min. of Waterworks. +1.0 0.0 ·0.5 ·1.0 to to to to 0.0 -0.5 ·1.0 ·2.S m m m m - ·2.S to ·S.O to ·7.5 m ·]0.0 to ·]S.O m ·10.0 to .lS.0 m below ·lS.0 m ~ N 4km 107 - _ _ > +4.0 m +3.0 to +4.0 m +2.0 to +3.0 m +1.0 to +2.0 m 0.0 to +1.0 m -2.0 to -1.0 m -3.0 to -2.0 m -4.0 to -3.0 m > -4.0 m ~ N 4km Fig. 49. Reconstruction of erosion and sedimentation in the ebb-tidal delta and the backbarrier area of the Zoutkamperlaag in the period 1967-1987. - Colours give different intervals: positive = sedimentation; negative = erosion in m. Prepared in cooperation with Min. of Waterworks. The relatively sheltered embayment allows the settling of large amounts of sediments (clay to very fine sand). Sedimentation rates are high and cause a gradual silting up of the embayment. This was encouraged by man by the construction of small sedimentation fields, protected by rows of sticks, by which wave action was damped. Subsequently more and more of the land was reclaimed, until the Dollard reached its present shape (Fig. 46; VAN VOORTHUYSEN & KUENEN 1960; STREIF 1982a, b). The decreasing size of the embayment resulted in a reduction of the tidal prism and hence in strong infilling of the main channel (reduction of cross-sectional area). Lateral migration of channels in sheltered embayments is strongly restricted by the clayey sediments (e.g. IRION 1992), but also because the drainage area itself can not shift (much), in contrast to other parts of the Wadden Sea. Gradual infilling of the main channel(s) will thus often result in nested sandy channel-fill complexes, as was the case in the 'Lauwerszee' (OOST, in prep. a). Sedimentary Effects of the Closure of the 'Lauwerszee' Tidal systems are able to react rapidly to changes in the hydrodynamic regime, whereas the strong tidal and wave forces, which dominate the system, allow the rapid transfer of massive amounts of sediments so that old equilibria can be restored or new ones can be reached (sand-sharing-system). This is illustrated in great detail by the effects of the closure of part of the back barrier system of the Zoutkamperlaag (Fig. 7; OOST, in press; BIEGEL & HOEKSTRA, in press). Fig. 48. Reconstruction of the ebb-tidal delta and the back barrier area of the Zoutkamperlaag in 1987. - Colours give different depth intervals with reference to DOL, which approximates mean sea level. - After OOST & DE HAAS (1992) in cooperation with Min. of Waterworks. 108 Due to the closure of the 'Lauwerszee' in 1969, the tidal prism of the Zoutkamperlaag was reduced from 305 x 10 6 m 3 to 200x 10 6 m} (VAN SIjP 1989a, b). As a consequence of this permanent change, the system was no longer in equilibrium with the hydrodynamic conditions and both the ebb-tidal delta and the back barrier area started to change towards a new morphodynamic equilibrium. The ebb-tidal current diminished, as a result of which the ebbtidal delta of the Zoutkamperlaag was affected by wave erosion. Erosion was dominant between -4 m DOL (Dutch Ordnance Level = about Mean Sea Level) and -12 to -13 m DOL, which is the local storm-erosion base (Figs. 47 -49). From the total amount of sediment eroded in the period 1970-1987 (26 x 10 6 m 3) a small part was transported offshore by storms. The larger part of this sand was transferred into the backbarrier area in the period 1970-1987. A relatively fast rotation of the outer channels was brought about by an increase in sedimentation at the western side and an increase of erosion at the eastern side of the channels, caused by the decrease in tidal volume and the relative increase of wave influence (OOST & DE HAAS 1992). Within the ebb-tidal delta, sediment was concentrated by waves into a large intertidal hook or spit with a length of 4 km in the E-W direction and 4 km in the N -S direction. This spit already started to form before closure of the 'Lauwerszee', after abandonment of an outer tidal channel directly west of Schiermonnikoog. The large size of the spit (deposition of 6x 10 6 m 3 above the low-water line in the period 1970-1987) is due to the large amount of sand which became available by the partial erosion of the ebbtidal delta (BIEGEL 1991a; OOST & DE HAAS 1992). The large dimensions of the hook affected the direction of wave approach over the period 1970-1987, aligning them more or less perpendicular to the hook. The waves, in their turn, contributed to the build-up of the hook by accumulating large amounts of sand. Originally the hook surrounded a small embayment. After the hook was breached during a series of storms (1990), strong erosion by tidal currents occurred and gullies of up to -5 m DOL were formed. Moreover, storms and aeolian erosion tended to erode the hook, as a result of which it has no significant preservation potential. As stated above, a large part of the sediment (mainly sand) was transferred to the backbarrier area in the period 1970-1987 (OOST & DE HAAS 1992). Strong sedimentation took place in the main gorge due to the deceleration of the current (BIEGEL & HOEKSTRA, in press). In the back barrier area the change in tidal volume caused rapid vertical sedimentation in the main channel. The sediments consist of fine sand, clay or an alternation of both. The alternations have a rhythmic appearance (winter/summer couplets), with climbing ripple structures, linzen, loadcasts and bioturbation (Fig. 25). In the first period (1970-1975) the sediments were partly derived from the surrounding tidal flats (WINKELMOLEN & VEENSTRA 1974). After 1979 sediments were mainly derived from the ebb-tidal delta. In total, some 30x 10 6 m 3 of sediment (fine sand and clay) was deposited in the back barrier area during the period 1970-1987 (Figs. 47-49; OOST & DE HAAS 1992, 1993). The deposits in the channels largely represent a vertical build-up. This results in nested channel deposits and vertical infill with clays and sands, features commonly observed in the rock record. Under the conditions of a decreased tidal prism, any newly established channels will be shallower than the previous ones. The preservation potential of these older channel deposits is therefore rather good (OOST, in press). Another result of the closure of the 'Lauwerszee' was that the position of the watershed south of Schiermonnikoog was shifted. In the period 1970-1979 the watershed slowly shifted towards the east; after 1979 the eastward migration accelerated. To the west of the watershed new, small channels were formed (Figs. 47-49). After closure of the 'Lauwerszee' the large width of the Zoutkamperlaag main back barrier channel resulted in a faster transport of water towards the watershed (VAN PARREEREN 1980; POSTMA & VAN PARREEREN 1982). No significant changes have been observed in the inlet system east of the watershed. The lateral shift of the watershed must therefore have been generated mainly by the closu~e of the 'Lauwerszee' (OOST & DE HAAS 1992). Due to the migration of the watershed, the (abandoned) channel deposits of the inlet system east of Schiermonnikoog were covered, thereby improving their preservation potential. Future Development: Greenhouse Effect and Sea-level Rise In the Wadden Sea the relative rise of sea level amounted on average to 17 cm over the last 100 years. About half of this was due to the eustatic rise of sea level and the other half to subsidence and compaction. Due to rising concentrations of carbon dioxide and some trace gases (CH 4, N 20, CFK-ll, CFK-12) in the atmosphere, the radiation of heat from the atmosphere to outer space is reduced (HOUGHTON et al. 1990). As a result, a global warming of the lower atmosphere is anticipated in the near future: the Greenhouse effect. It is expected that this warming will cause thermal expansion of ocean water, melting of small icecaps, the retreat of glaciers on Greenland and in high mountains, and increased accumulation of snow on Antarctica (WARRICK & OERLEMANS 1990). The net effect will be an increase in the volume of sea water, resulting in an estimated global sealevel rise of 0.31 to 1.10 m by the year 2100 (WARRICK & OERLEMANS 1990). To date it has not been possible to conclusively demonstrate an acceleration in global sea-level rise (EKMAN 1988; WOODWORTH 1990; WARRICK & OERLE- . MANS 1990). The rates of future sea-level rise predicted above are comparable to early Holocene rates. The important difference is that during the early Holocene the complete coastal system was mobile, being freely able to retreat landwards in response to the rise of sea level. At present, however, dykes at the mainland block landward migratio~ of the coastal system. Extensive research is presently in 109 progress to assess the effects of a relative sea-level rise along the Dutch Wadden Sea coast (BRUUN 1986; NICHOLS 1989; DIJKEMA et al. 1990; MISDORP et al. 1990; EYSINK 1992, 1993; EYSINK & BIEGEL 1992; VAN DER SPEK & BEETS 1992; ANONYM US 1993; OOST & DIJKEMA 1993). Over a short time period, i.e. in the coming centuries, it is expected that sea-level rise will be compensated by increasing sedimentation on the backbarrier tidal flats (ANONYMUS 1993; EYSINK 1993; OOST & DIJKEMA 1993). Since sedimentation is commonly lagging behind a sea-level rise, it is expected that under conditions of accelerated sea-level rise a small percentage of the intertidal flats will change into subtidal areas (EYSINK 1993). The material needed to compensate sea-level rise will be derived from the coastal zone (to approx. -20 m DOL) of the barrier islands (ANONYMUS 1993) and probably to a small extent also from deeper offshore parts (OOST & DIJKEMA 1993). Thus, under the conditions of an artificially fixed mainland coastline, the anticipated response of the barrier and backbarrier systems to a further rise of sea level would be a retreat of the barrier islands and a narrowing of the backbarrier tidal basins. Artificial fixation of the Wadden Sea barrier islands in their present position might, in the long run, lead to a sediment deficiency (EHLERS & KUNZ 1993; OOST & DIJKEMA 1993) and thus to a permanent drowning of large parts of the intertidal back barrier region. Furthermore, it has been suggested that the Greenhouse effect will result in an increased number of storms. Thus the mixed energy coast may get a more wavedominated character, resulting in more washover channels (d. HAYES 1979). Also, stronger wave action in the backbarrier area may decrease the height of the tidal flats (see also chapter: "Shoals and Gullies"). Acknowledgements This contribution is dedicated to the members of the former Geological Institute of Groningen University, especially Prof. Dr. PH. H. KUENEN, Prof. Dr. L. M. J. U. VAN STRAATEN, Dr. H. VEENSTRA and DR. A. M. WINKELMOLEN. In an early stage they realized the importance of comparative sedimentalogy and put it into practive by comparing the recent Wadden Sea sedimentation with fossil equivalents. We thank ORSOLYA SZTANO for her useful comments on earlier versions of this paper and DOEKE EISMA, HENK DE HAAS and SHA LIPING for stimulating discussions and data on the Dollard Estuary. Furthermore we thank the members of the Coastal Genesis Working Group for the many hours of fruitful discussions on the dynamics of the Wadden Sea, and BURG FLEMMING and GUNTHER HERTZWECK for critically reviewing this paper. FRED TRAPPENBURG, JACO BERGENHENEGOUWEN, JAAP VAN DEN BOODGERT and ISAAK SANTOE are thanked for technical support. References ALLER, R. C. (1982): The effects of macrobenthos on chemical properties of marine sediment and overlying water. In: MCCALL, P. L. & TEVESZ, M. J. S. [Eds.]: AnimalSediment Relations: 53-102; New York, London (Plenum Press). ANONYMUS (1981): Zandwinning in de Waddenzee, resultaten van een hydrografisch-sedimentologisch onderzoek. Stuurgroep, Rijkswaterstaat, Directie Friesland, 40 pp. (1993): Invloed van gaswinning op de Waddenzee. Pub!. of Rijkswaterstaat, The Hague, 33 pp. BEUKEMA, J. J. & CADEE, G. C. & JANSEN, J. J. M. (1977): Variability of growth rate of Macoma balthica (L.) in the Wadden Sea in relation to availability of food. In: KEEGAN, B. F. & CEIDIGH, P. O. & BOADEN, P. J. S. [Eds.]: Biology of Benthic Organisms, 11th Eur. Symp. on Mar. Biology, Galway, Oct. 1976: 69-77; London (Pergamon). BIEGEL, E. J. (1991a): De ontwikkelingen van de ebgetijde delta en het kombergingsgebied van het Friesche Zeegat in relatie tot de sluiting van de Lauwerszee. - Univ. of Utrecht, Fac. of Geogr. Sci., GEOPRO, 1991.07: 57 pp. - - - (1991b): Equilibrium relations in the ebb tidal delta, inlet and back barrier area of the Frisian inlet system. Univ. of Utrecht, Fac. of Geogr. Sci., GEOPRO, 1991.028/GWAAO-91.016: 79 pp. BIEGEL, E. J. & HOEKSTRA, P. (in press): Morphological response characteristics of the Zoutkamperlaag, Frisian Inlet, The Netherlands. - Proc. Tidal Clastics '92 Symposium, Wilhelmshaven: 30 pp. BILSE, D. P. (1993): A large scale model of the morphological behaviour of the outer delta of tidal inlets. - Delft Hydraulics, Report H 1215: 42 pp. BLEUTEN, W. (1971): Longshore en ripcurrents op het strand van Schiermonnikoog. - Int. Rep. Fac. of Geogr. Sci., Univ. of Utrecht: 16 pp: Utrecht. BEETS, D. J. & VAN DER VALK, L. & STIVE, M. J. F. (1992): Holocene evolution of the coast of Holland. - Mar. Geo!., 103: 423-443. BERGER, G. W. & EISMA, D. & VAN BENNEKoM, A. J. (1987): 210pb derived sedimentation rate in the Vlieter, a recently filled-in tidal channel in the Wadden Sea. Nether!' J. Sea Res., 21: 287-294. BEUKEMA, J. (1977): Dierlijk leven in en op de bodem. - In: ABRAHAMSE,J. & JOENJE, W. & VAN LEEUWEN'SEELT, N. [Eds.]: Waddenzee, 2nd. pr.: 125-133; Harlingen. (1982): Annual variation in reproductive success and biomass of the major macrozoobentic species living in a tidal flat area of the Wadden Sea. - Nether!' J. Sea Res., 16: 37-45. J. - - - 110 BOSCH, A. & VOS P. C. (1992): Paleogeografische reconstructie van het Lauwersmeergebied, concept. - State Geol. SurveyProject 40009: 22 pp. BROMLEY, R. G. (1990): Trace Fossils; Biology and Taphonomy. 280 pp.; London (Unwin Hyman). BRUUN, P. (1986): Worldwide impact of sea-level rise on shore lines. - Rijkswaterstaat, Dienst Getijdewateren, The Hague, Pub!. 86-8-713: 81-111. BRUUN, P. & VIGGOSSON, G. (1977): The wave pump: conversion of wave energy to current energy. - Proc. ASCE, J. waterway, port, coast. ocean div., 103: WW4. BUHR, K. E. (1977): Distribution and maintenance of a Lanice conchilega association in the Weser Estuary (FRG), with special reference to the suspension-feeding behaviour of Lanice conchilega. - In: KEEGAN, B. F. & CEIDIGH, P. O. & BOADEN, P. J. S. [Eds.]: Biology of Benthic Organisms, 11th Eur. Symp. on Mar. Biology, Galway, Oct. 1976: 101-113; London (Pergamon). DE BOER, M. & KOOL, G. & LIESHOUT, M. F. (1991): Erosie en sedimentatie in de binnendelta van het Zeegat van Ameland, 1926-1984, Deelonderzoek no.4. - Rijkswaterstaat, Directie Noord-Holland, Rep. ANVX91.H202: 42 pp. DE BOER, P. L. (1979): Convolute lamination in modern sands of the estuary of the oosterschelde, the Netherlands, form- . ed as the result of entrapped air. - Sedimentol., 26: 283-294. (1981): Mechanical effects of micro-organisms on intertidal bedform migration. - Sedimentol., 28: 129-132. J. & WINTER, J. DE BOER, P. L. & OOST, A. P. & VISSER, M. J. (1989): The diurnal inequality of the tide as a parameter for recognizing tidal influences. - ]. sediment. Petrol., 59: 912-921. DE HAAS, H. & EISMA, D. (1992): Suspended sediment transport in the Ems-Dollard estuary. - Nether!' J. Sea Res., 31: 37-42. DE JONG, J. (1984): Age and vegetational history of the coastal dunes in the Frisian Islands, The Netherlands. - Geo!. en Mijnb., 63: 269-276. DE VRIEND, H. ]. & BAKKER, W. T. (1993): Sedimentary processes and morphological behaviour models for mixed-energy tidal inlets. - Nether!' Centre coast. Res. Delft, Rep. H 1887: 25 pp. DEAN, R. G. (1988): Sediment interaction at modified coastal inlets: processes and policies. - In: AUBREY, D. G. & WEISHAR, L. [Eds.]: Hydrodynamics and Sediment Dynamics of Tidal Inlets. - Lecture Notes on Coastal and Estuarine Studies, 29: 412-439; New York (Springer). DEAN, R. G. & WALTON, T. L. (1975): Sediment transport processes in the vicinity of inlets with special reference to sand trapping. - In: CRONIN, L. E. [Ed.]: Estuarine Research, Vol. 2: Geology and Engineering: 129-150; New York (Academic Press). DIECKMANN, R. & OSTERTHUN, M. & PARTENSCKY, H. W. (1988): A comparison between German and North American tidal inlets. - Proc. 21st Coast. Engng. Con£., ASCE, 3, 199: 2681-2691. DIJKEMA, K. S. (1987a): Changes in salt-marsh area in the Netherlands Wadden Sea after 1600. - In: HUISKES, A. H. L. & BLOM, c. W. P. M. & ROZEMA, J. [Eds.]: Vegetation between Land and Sea: 42-49; Dordrecht Gunk). (1987b): Geography of salt marshes in Europe. - Z. Geomorphol., N.F., 31: 489-499. CADEE, G. C. (1976): Sediment reworking by Arenicola marina on tidal flats in the Dutch Wadden Sea. - Neth. J. Sea Res., 10: 440-460. (1989): Size-slective transport of shells by birds and its palaeoecological implications. - Palaeonto!', 32: 429437. (1990): Feeding traces and bioturbation by birds on a tidal fiat, Dutch Wadden Sea. - Ichnos, 1: 23-30. (1992): Aeolian transport and leftl right sorting of Mya shells (molusca, bivalvia). - Palaios, 7: 198-202. (in press). Eider, Shelduck, and other predators, the main producers of shell fragments in the Wadden Sea, palaeoecological implications. - Paleontology, (in press). - - - CHRISTIANSEN, C. & MILLER, P. F (1983): Spartina in Mariager Fjord, Denmark: the effect on sediment parameters. Earth Surface Processes and Landforms, 8: 55-62. CREUTZBERG, F. & KUIPERS, B. R. (1984): Cursus Mariene Ecosystemen (praktische gedeelte). - Int. Rep. Neth. Inst. Sea Res.: 40 pp; Texe!. CREUTZBERG, F. & POSTMA, H. (1979): An experimental approach to the distribution of mud in the southern North Sea. - Nether!, J. Sea Res., 13: 99-116. CRISP, D. J. & MEADOWS, P. S. (1962): The chemical bases of gregariousness in cirri pedes. - Roy. Soc. London Proc., B 150: 500-520. DANKERS, N. & KOELEMAIJ, K. (1989): Variations in the mussel population of the Dutch Wadden Sea in relation to monitoring. - Helogolander Meeresunters., 43: 529535. DANKERS, N. & KOELEMAIJ, K. & ZEGERS, J. (1989): De rol van de mossel en de mosse!cultuur in het ecosysteem van de Waddenzee. - RIN-Rep., 89/9: 66 pp. DAVIS, R. A. & Fox, W. T. & HAYES, M. O. & BOOTHROYD, J. C. (1972): Comparison of ridge and runnel sustems in tidal and non-tidal environments. - J. sedim. Petrol., 32: 413-421. DE BOER, M. (1979): Morfologisch onderzoek Ameland. Verslag van het onderzoek op het Amelander Wantij in 1973. - Rijkswaterstaat, Directie Waterhuishouding en Waterbeweging, District kust en zee, Studiedienst Hoorn, Nota WWKZ-79.H005. - - - (1989): Habitats of The Netherlands, German and Danish Wadden Sea, 1: 100.000. - 30 pp.; Texel, Leiden (RIN, Veth Foundn.). DIJKEMA, K. S. & BOSSINADE, ]. H. (1990): Vegetatieclassificatie van Waddenzeekwelders volgens een vast typenstelsel. - RIN, Int. Rep., 90/15: 37 pp. DIJKEMA, K. S. & BOSSINADE, J. H. & BOUWSEMA, P. & DE GLOPPER, R. J. (1990): Salt marshes in the Netherlands Wadden Sea: rising high-tide levels and accretion enhancement. - In: BEUKEMA,].]. & WOLFF, W.J. & BROUNS, J. J. W. M. [Eds.]: Expected Effects of Climatic Change on Marine Coastal Ecosystems: 173-188; Dordrecht (Kluwer). DIJKEMA, K. S. & REINECK, H. E. & WOLFF, W.]. (1980): Geomorphology of the Wadden Sea Area. - Rep. 1 of the Wadden Sea Working Group.: 135 pp; Rotterdam (Balkema). 111 DIJKEMA, K. S. & VAN DEN BERGS, J. & BOSSINADE, J. H. & BOUWSEMA, P. & DE GWPPER, R. J. & VAN MEEGEN, J. W. Th. M. (1988): Effecten van rijzendammen op opslibbing en omvang van de vegetatiezones in de Friese en Groninger landaanwinningswerken. - Rijkswaterstaat, Dir. Groningen, Nota GRAN 1988-2010, RIN-Rep. 88/66, Texel & RlJP-Report 1988-33 Cbw, Lelystad: 119 pp. DORJES, J. (1982): Das Watt als Lebensraum. - In: REINECK, H.-E. [Ed.]: Das Watt, Ablagerungs- und Lebensraum, 3rd Edn.: 107-143; Frankfurt a. M. (Kramer). DRONKERS, J. (1986): Tidal asymmetry and estuarine morphology. - Netherl. J. Sea Res., 20: 117-131. EDELMAN, T. (1964): De historische veranderingen in de natuurlijke gesteldheid van het Nederlandse waddengebied. In: ANDERSON, W. F. & ABRAHAMSE, J. & BUWALDA, J. D. & VAN STRAATEN, L. M. J. U. [Eds.]: Het Waddenboek: 37-65; Zutphen (Thieme). (1974): Bijdrage tot de historische gegrafie van de Nederlandse kuststrook. - Rijkswaterstaat, Dir. Waterhuishouding en waterbeweging, The Hague: 84 pp. EYSINK, W. D. & BIEGEL, E. J. (1992): Impact of sea-level rise on the morphology of the Wadden Sea in the scope of its ecological function. Investigations on empirical morphological relations. - Rijkswaterstaat Dienst Getijdewateren, ISOS*2 Proj., Phase 2: 73 pp. FAVEJEE, J. CH. L. (1960): On the origin of the mud deposits in the Ems-estuary. - In: van Voorthuysen, J. H. & Kuenen, Ph. H. [Eds.]: Das Ems-Estuarium (Nordsee), ein sedimentologisches Symposium. - Verh. kon. Nederl. geol. mijnbouwkdg. Genoot., 19: 147-151. FITZGERALD, D. M. (1988): Shoreline erosional-depositional processes associated with tidal inlets. - In: AUBREY, D. G. & WEISHAR, L. [Eds.]: Hydrodynamics and sediment dynamics of tidal inlets. - Lecture Notes on Coastal and Estuarine Studies, 29: 186-225; New York (Springer). FITZGERALD, D. M. & PENLAND, S. & NUMMEDAL, D. (1984): Control of barrier island shape by inlet sediment bypassing: East Frisian Islands, West Germany. - Mar. Geol., 60: 355-376. FLEMMING, B. W. (1991): Holozane Entwicklung, Morphologie und fazielle Gliederung der Ostfriesischen Inse! SenckenbergSpiekeroog (siidliche Nordsee). am-Meer, Ber. 9113: 51 pp. FLEMMING, B. W. & DAVIS, R. A., Jr. (1992): Holocene evolution, morphodynamics and sedimentology of the Spiekeroog barrier island system (southern North Sea). - Tidal Clastics 92, Wilhelmshaven, Germany, Field GuideBook: 32 pp. & - EHLERS, J. (1988): Morphodynamics of the Wadden Sea. - 397 pp.; Rotterdam (Balkema). EHLERS, J. & KUNZ, H. (1993): Morphology of the Wadden Sea natural processes and human interference. - Coastlines of the southern North Sea, Proc. 8th Symp. on Coastal and Ocean Management: 65-84. EISMA, D. (1981): Supply and deposition of suspended matter in the North Sea. - In: NIO, S. D. & SHUTTENHELM, R. T. E. & VAN WEERING, T. C. E. [Eds.]: Holocene Marine Sedimentation in the North Sea Basin. - Int. Assoc. Sedimentol., Spec. Publ., 5: 415-428. EISMA, D. & BERGER, G. W. & CHEN WEI-YUE & SHEN JIAN (1989): Pb-210 as a tracer for sediment transport and deposition in the Dutch-German Wadden Sea. - Proc. KNGMG Symp. Coastal Lowlands, Geol. and Geotechno!.: 237-253. EISMA, D. & WOLFF, W. J. (1980): The development of the westernmost part of the Wadden Sea in historical time. - In: DIJKEMA, K. S. & REINECK, H.-E. & WOLFF, W. J. [Eds.]: Geomorphology of the Wadden Sea Area. Rep. 1 of the Wadden Sea Working Group: 95-103; Rotterdam (Balkema). EKMAN, M. (1988): The world's longest continued series of sea level observations. - Pure and appl. Geophys., 127: 73-77. EYSINK, W. D. (1979): Morfologie van de Waddenzee, gevolgen van zand- en schelpenwinning, verslag literatuuronderzoek. - Waterl. Lab., Rep. R-1336: 92 pp. (1987): Gaswinning op Ameland-oost, effekten van de bodemdaling, verslag onderzoek. - Water!' Lab. & RIN, Rep. H114: 53 pp. (1992): Impact of sea-level rise on the morphology of the Wadden Sea in the scope of its ecological function. Proposed set-up of a dynamic morphological model for Wadden Sea basins and estuaries based on empirical relations. Vol. 1. - Rijkswaterstaat, Dienst Getijdewateren, ISOS*2 Proj., Phase 3: 38 pp. (1993): Impact of sea-level rise on the morphology of the Wadden Sea within the scope of its ecological function. General considerations on hydraulic conditions, sediment transports, sand balance, bed composition and impact of sea-level rise on tidal flats. - Rijkswaterstaat Dienst Getijdewateren, ISOS*2 Proj., Phase 4: 35 pp. (1994): Holocene evolution, morphodynamics and sedimentology of the Spiekeroog barrier island system (southern North Sea). - Senckenbergiana marit, 24: 117-155. FLEMMING, B. W., DELAFONTAINE, M. T. & BARTHOLOMA, A. (1993): Morphodynamics and mass balance of biodepostion in a backbarrier mussel bank (Spiekeroog, Southern North Sea). - In: AMLER, M. R. W. & TIETZE, K. W. [Eds.]: Geologica et Palaeontologica, Sediment 93, Marburg, Kurzfassungen: 30-31. FLEMMING, B. W. & SCHUBERT, H. & HERTWECK, G. & MULLER, K. (1992): Bioclastic tidal-channel lag deposits: a genetic model. - Senckenbergiana marit., 22: (3/6): 109129. FORMSMA, W. J. (1954): Een reis naar Schiermonnikoog in 1556. - Groningse Volksalmanak 1954: 65-71. (1958): De grens tussen de provincies Groningen en Friesland in de Wadden. - Groningse Volksalmanak 1958: 27-42. GERDES, G. & KRUMBEIN, W. E. & REINECK, H.-E. (1985): Distribution and geological importance of marine microbial mats in the North Sea tidal flats. - Facies, 12: 75-96. GERRITSEN, F. (1990): Morphological stability of inlets and channels of the western Wadden Sea. - Rijkswaterstaat, Dienst Getijdewateren, Nota GWAO-90.019: 86 pp. GERRITSEN, F. & DE JONG, H. (1985): Stabiliteit van doorstroomprofielen in het Waddengebied. - Rijkswaterstaat Adviesdienst Vlissingen, Nota WWKZ-84.v016: 53 pp. GOETHE, F. (1937): Beobachtungen und Untersuchungen zur Biologie der Silbermowe (Larus argentatus PONTOPP.) auf der Vogelinsel Memmertsand. - J. Ornithol., 85: 1-119. (1958): Anhaufungen unversehrter Muscheln durch Silbermowen. - Natur u. Yolk, 88: 181-187. - - - - - - 112 GRIEDE,J. W. (1978): Het ontstaan van Frieslands Noordhoek, een fysisch-geografisch onderzoek naar de holocene ontwikkeling van een zeekleigebied. - Thes. Free Univ. Amsterdam: 186 pp. GROENENDIJK, A. M. (1986): Establishment of Spartina anglica population on a tidal mudflat: a field experiment. - J. Environmental Management, 22: 1-12. HAYES, M. O. (1975): Morphology of sand accumulations in estuaries. - In: CRONIN, L. E. [Ed.]: Estuarine Research, Vo!' 2: Geology and Engineering: 3-22; New York (Academic Press). (1979): Barrier island morfology as a function of tidal and wave regime. - In: LEATHERMAN, S. [Ed.]: Proceedings of the coastal symposium on barrier islands: 1-27; New York (Academic Press). (1980): General morphology and sediment patterns in tidal inlets. - Sed. Geo!., 26: 139-156. LANG, A. W. (1958): Gestaltungswandel des Emsmlindungstrichters, Untersuchungen zur Entwickling der Emsmlindung von der Mitte des 16. bis zum Beginn des 20. Jahrhunderts. - Schriften der wirtschaftswissenschaftlichen Gesellschaft zum Studium Niedersachsens EV., Reihe A, N.F., 58: 153 pp. LEOPOLD, M. F. & MARTEIJN, E. C. L. & SWENNEN, C. (1984): Long distance transport of prey from the intertidal to high tide roosts by the Oystercatcher. - Ardea, 73: 76-82. LIGTENDAG, W. A. (1990): Van IJzer tot Jade, een reconstructie van de zuidelijke Noordzeekust in de jaren 1600 en 1750. - Rijkswaterstaat, Meetkundige Dienst: 55 pp. LUCK, G. (1975): Der Einfluss der Schutzwerke der Ostfriesischen Inseln auf die morphologische Vorgange im bereich der Seegaten und ihrer Einzugsgebiete. - Mitt. Leichtweiss-Inst. Wasserb. Techn. Univ. Braunschweig, 47: 122 pp. (1976): Inlet changes of the East Frisian Islands. Proc. 15th Conf. Coastal Eng., Am. Soc. Civ. Eng., Honolulu, 1938-1957. - - - HERTWECK, G. (1982): Die Bewohner des Wattenmeeres in ihren Auswirkungen auf das Sediment. - In: REINECK, H.-E. [Ed.]: Das Watt, Ablagerungs- und Lebensraum, 3rd Edn.: 145-172; Frankfurt a. M. (Kramer). (1990): Die Zonierung der Lebensspuren im Watt der Jade, Slidliche Nordsee. - Beitrage zur Geologie und Palaontologie Norddeutschlands. - Ber. Fachbereich Geowiss. Univ. Bremen, 10: 118-159. (1992): Zonation of benthos and lebensspuren in the tidal flats of Jade Bay (Southern North Sea). Forschungsinstitut Senckenberg, Abt. Meeresforschung, Wilhelmshaven, Ber. 92/3: 13 pp. - - - MAZURE, J. P. & many others (1974): Rapport van de Waddenzeecommissie; Advies inzake de principiele mogelijkheden en de voor- en nadelen van inpolderingen in de Wad-, denzee. - Rijkswaterstaat, Rep. Wadden Sea Comm., The Hague: 328 pp. MISDORP, R. & STEYAERT, F. & HALLIE, F. & DE RONDE, J. (1990): Climate change, sea-level rise and morphological developments in the Dutch Wadden Sea, a marine wetland. - In: BEUKEMA, J. J. & WOLFF, W. J. & BROUNS, J. J. W. M. [Eds.]: Expected Effects @f Climatic Change on Marine Coastal Ecosystems: 123-131; Dordrecht (Kluwer). NICHOLS, M. J. (1989): Sediment accumulation rates and relative sea-level rise in lagoons. - Mar. Geo!., 88: 201-220. NIEMEYER, H. D. (1986): Ausbreitung und Dampfung des Seegangs im See- und Wattengebiet von Norderney. - Jber. Niedersachs. Landesarnt Wasserwirtschaft, Forschungsstelle Kliste, Norderney, 1985, 37: 49-95. (1990): Morphodynamics of tidal inlets. - In: Coastal Morphology Stichting Postacad. Ond. Civ. Techn. en Bouwtechn., Delft. HEYNIS, H. & BERGER, G. W. & EISMA, D. (1987): Accumulation rates of estuarine sediment in the Dollard area: comparison of Pb-210 and pollen influx methods. Nether!' J. Sea Res., 21: 295-301. HJULSTROM, F. (1935): Studies of the morphological activity of rivers as illustrated by the River Fyris. - Bul!. geo!. Inst. Univ. Uppsala, 25: 221-527. HOUGHTON, J. T. & JENKINS, G. J. & EPHRAUMS, J. J. [Eds.] (1990): Climate Change, the IPCC scientific assessment. - 328 pp.; Cambridge (Cambridge Univ. Press). HUBBARD, D. K. & BARWIS, J. H. & NUMMEDAL, D. (1977): Sediment transport in four South Carolina inlets. - ASCE, Proc. Coastal Sediments '77: 582-601. HUME, T. M. & HERDENDORF, c. E. (1990): Morphologic and hydraulic characteristics of tidal inlets on a headland dominated low littoral drift coast, Northeastern New Zealand. - J. coast. Res., Spec. Iss: Proc. Skagen Symp.: 527-563. IRION. G. (1992): Morphological and sedimentological evolution of the Jade Bay (Southern North Sea). - Tidal Clastics 92, Wilhelmshaven, Germany, Field Guide-Book: 15 pp. JARRETT, J. T. (1976): Tidal prism-inlet area relationships. - GITI, Rep. 3, US Army Eng. Waterways Exp. Stat. Vicksburg. JOHNSON, D. W. (1919): Shore Processes and Shoreline Development. - 584 pp.; New York (Wiley). KAMPS, L. F. (1962): Mud distribution and land reclamation in the eastern wadden shallows. - Rijkswaterstaat Commun., 4: 73 pp.; The Hague. KLOK, B. & SCHALKERS, K. M. (1980): De veranderingen in de Waddenzee ten gevolge van de afsluiting van de Zuiderzee. - Rijkswaterstaat, Rep. 78.H238: 13 pp. KONIG, D. (1948): Spartina townsendii an der Westkliste von Schleswig-Holstein. - Planta, 36: 34-70. NUMMEDAL, D. & FISCHER, 1. A. (1978): Process-response models for depositional shorelines: The German and Georgia bights. - ASCE, Proc. 16th Coastal Engineering Conf.: 1215-1231. NUMMEDAL, D. & OERTEL, G. F. & HUBBARD, D. K. & HINE, A. C. (1977): Tidal inlet variability Cape Hatteras to Cape Canaveral. - ASCE, Proc. Coastal Sediment '77: 543-562. NUMMEDAL, D. & PENLAND, S. (1981): Sediment dispersal in Norderneyer See gate, West Germany. - In: NIO, S. D. & SHUTTENHELM, R. T. E. & VAN WEERING, T. C. E. [Eds.]: Holocene Marine Sedimentation in the North Sea Basin. - Internat. Assoc. Sedimentologists, Spec. Pub!., 5: 187-210. NYBAKKEN, J. W. (1982): Marine Biology, an ecological approach. - 446 pp.; New York (Harper and Row). O'BRIEN, M. P. (1969): Equilibrium flow areas ofinlets on sandy coasts. - J. Waterways and Harbors Div., ASCE, WWI: 43-52. OERTEL, G. F. (1972): Sediment transport on estuary entrance shoals and the for\TIation of swash platforms. - J. sediment. Petro!', 42: 857-863. 113 (1975): Ebb-tidal deltas of Georgia estuaries. - In: CRONIN, L. E. [Ed.]: Estuarine Research, 2: 267-276; New York (Academic Press). OOST, A. P. (in prep. a): Morphodynamic behaviour of islands and inlets: development of the Frisian Inlet in historical times. (in prep. b): The influence of Mytilus edulis on fine grained sedimentation in the Dutch Wadden Sea. (in press): Sedimentological implications of changes in the ebb tidal delta and backbarrier area of the Zoutkamperlaag, The Netherlands, due to decrease of tidal prism. - Proc. Tidal Clastics '92 Symposium, Wilhelmshaven. OOST A. P. & BAAS J. H. (in press): The development of smallscale bedforms in tidal environments: an empirical model for unsteady flow and its applications. Sedimento!', (in press). OOST, A. P. & DI]KEMA, K. S. (1993): EFFECTEN VAN BODEMDALING DOOR GASWINNING IN DE WADDENZEE. - IBN-REP. 025: 133 pp. OOST, A. P. & DE HAAS, H. (1992): Het Friesche Zeegat, morfologisch-sedimentologische veranderingen in de periode 1970-1987, een getijde inlet systeem uit evenwicht, Deel 1 & 2. - Rep. Fac. Earth Sci., Univ. Utrecht, Coastal Genesis: 68 pp. & - (1982b): Rinnen. - In: Reineck, H.-E. [Ed.]: Das Watt, Ablagerungs- und Lebensraum, 3rd Edn.: 89-93; Frankfurt a. M. (Kramer). (1982c): Schichtungsarten und Wiihlgefiige. - In: REINECK, H.-E. [Ed.]: Das Watt, Ablagerungs- und Lebensraum, 3rd Edn.: 69-80; Frankfurt a. M. (Kramer). REMANE, A. (1951): Marine Schillablagerungen im SiiBwasser und aeolischer Hydrobienschil!. - Kieler Meeresforsch., 8: 98-110. SCHOORL, H. (1973): Zeshonderd jaar water en land, bijdrage tot de historische Geo- en Hydrografie van de Kop van Noord-Holland in de periode 1150-1750. - Verh. kon. Neder!. Aardrijkskdg. Genoot., 2: 534 pp.; Groningen (Wolters-Noordhoft). SEED, R. (1976): Ecology. - In: Bayne, B. L. [Ed.]: Marine Mussels: Their Ecology and Physiology. - Internat. bio!. Progr., 10: 13-65; Cambridge (Cambridge Univ. Press). SHA, L. P. (1989a): Cyclic morphologic changes of the ebb-tidal delta, Texel inlet, The Netherlands. - Geo!. en Mijnb., 68: 35-48. (1989b): Sand transport patterns in the ebb-tidal delta off Texel Inlet, Wadden Sea, The Netherlands. - Mar. Geo!., 86: 137-154. (1989c): Variation in ebb-delta morphologies along the West and East Frisian Islands, The Netherlands and Germany. - Mar. Geo!., 89: 11-28. (1989d): Holocene-Pleistocene interface and threedimensional geometry of the ebb-delta complex, Texel Inlet, The Netherlands. - Mar. Geol., 89: 207-228. (1990a): Sedimentological studies of the ebb-tidal deltas along the West Frisian Islands, the Netherlands. Ph.D.-Thesis. - Geologica Ultraiectina. Mededelingen van het Instituut voor Aardwetenschappen der Rijksuniversiteit te Utrecht, 64: 159 pp. (1990b): Surface sediment characteristics in the ebbtidal delta of Texel Inlet, The Netherlands. - Sediment. Geol., 68: 125-141. (1992): Geological Research in the Ebb-Tidal Delta of 'Het Friesche Zeegat', Wadden Sea, The Netherlands. State Geol. Survey-Proj. 40010: 20 pp. SHA, L. P. & DE BOER, P. L. (1991): Ebb-tidal delta deposits along the West Frisian Islands (The Netherlands): processes, facies architecture and preservation. - In: SMITH, D. G. & REINSON, G. E. & ZAITLlN, B. A. & RAHMANI, R. A. [Eds.]: Clastic Tidal Sedimentology. - Canad. Soc. Petrol. Geo!. Mem., 16: 199-218. STEI]N, R. C. (1991): Some Considerations on Tidal inlets. A literature survey on hydrodynamic and morfodynamic characteristics of tidal inlets with special attention to "Het Friesche Zeegat". - Delft Hydraulics, Coastal Genesis, Rep. H840.45.: 109 pp. STlVE, M. J. F. & EYSINK, W. D. (1989): Voorspelling ontwikkeling kustlijn 1990-2020, fase 3, deelrapport 3.1: dynamisch model van het Nederlandse kustsysteem. - Rapport H825, Waterloopkundig Laboratorium, Delft: 66 pp. STREIF, H. (1982a): Geologie des Kiistenraumes. - In: REINECK, H.-E. [Ed.]: Das Watt, Ablagerungs- und Lebensraum, 3rd Edn.: 24-30; Frankfurt a. M. (Kramer). (1982b): Der holozane Meeresspiegelanstieg und die heutige Kiistengestalt. - In: Reineck, H.-E. [Ed.]: Das Watt, Ablagerungs- und Lebensraum, 3rd Edn.: 31-38; Frankfurt a. M. (Kramer). (1993): Het Friesche Zeegat. Morfologisch-Sedimentologische veranderingen in de peri ode 1927-1970, Deel 1 & 2. - Rep. Fac. Earth Sci., Univ. Utrecht, Coastal Genesis: 83 pp. OOST, A. P., & DE HAAS, H. & IJNSEN, F., & VAN DEN BOOGERT, J. M. & DE BOER, P. L. (1993): The 18.6 year nodal cycle and its impact on tidal sedimentation. - Sediment. Geo!', 87: 1-11. OOST, A. P. & LEOPOLD, M. F. (1988): Shell transport by birds. - Internat. Symp. on "Theoretical and Applied Aspects of Coastal and Shelf Evolution, Past and Future", jointly with the inaugural meeting of IGCP Proj. 274 "Quaternary Coastal Evolution; Case Studies, Models and Regional Patterns", Amsterdam, The Netherlands, 19-24 sept. 1988, Supp!. Abstracts: 29-30. POSTMA, H. (1954): Hydrography of the Dutch Wadden Sea. Arch. Neerl. Zoo!', 10: 405-511. (1961): Transport and accumulation of suspended matter in the Dutch Wadden Sea. - Nether!' J. Sea Res., 1: 148-190. (1967): Sediment transport and sedimentation in the estuarine environment. - In: LAUFF, G. A. [Ed.]: Estuaries: 158-179; Washington, D.C. (A mer. Assoc. Advancem. Sci.). POSTMA, H. & DI]KEMA, K. S. (1982): Hydrography of the Dutch Wadden Sea: Movements and properties of water and particulate matter. - Final report on 'Hydrography' of the Wadden Sea Working Group, Rep. 2: 75 pp.; Rotterdam (Balkema). POSTMA, J. T. & VAN PARREEREN, D. (1982): Onderzoek naar de bevaarbaarheid van de wantijen Engelsmanplaat, Lutjewad en Hornhuizerwad. - Rijkswaterstaat, Dir. Groningen, Nota 82-:, 26 pp. REINECK, H.-E. (1982a): Allgemeines, Hydrographie. - In: REINECK, H.-E. [Ed.]: Das Watt, Ablagerungs- und Lebensraum, 3rd Edn.: 12-17; Frankfurt a. M. (Waldemar). 114 THORSON, G. (1966): Some factors influencing the recruitment and establishment of marine benthic communities. Nether!' J. Sea Res., 3: 267-293. VAN DE PLASSCHE, O. (1982): Sea-level change and water-level movements in the Netherlands during the Holocene. PhD-Thes. Free Univ. Amsterdam: 138 pp. VAN DEN BERG, J. H. (1981): Rhythmic seasonal layering in a mesotidal channel fill sequence, Oosterschelde Mouth, the Netherlands. - In: NJO, S. D. & SCHOTTENHELM, R. T. E. & VAN WEERING, T. C. E. [Eds.]: Holocene Marine Sedimentation in the North Sea Basin. - Internat. Assoc. Sedimentologists, spec. Pub!. 5: 147-159. (1986): Aspects of sediment- and morphodynamics of subtidal deposits of the Oosterschelde (The Netherlands). - Rijkswaterstaat Commun., 43; The Hague. (1964): De bodem der Waddenz.ee. - In: Anderson, W. F. & Abrahamse, J. & Buwalda, J. D. & van Straaten, L. M. J. U. [Eds.]: Het Waddenboek: 75-151; Zutphen (Thieme). VAN STRAATEN, L. M. J. u. & KUENEN PH. H. (1958): Tidal action as a cause of clay accumulation. - J. sediment. Petro!', 28: 406-413. VAN VEEN, J. (1936): Onderzoekingen in de Hoofden. - 252 pp.; The Hague (Landsdrukkerij). VAN VOORTHUYSEN, J. H. (1960): Die Foramineren des DollartEms-Estuarium. - In: VAN VOORTHUYSEN, J. H. & KUENEN, PH. H. [Eds.]: Das Ems-Estuarium (Nord see), ein sedimentologisches Symposium. - Verh. kon. Neder!. geo!. mijnbouwkdg. Genoot., 19: 237-269. VAN VOORTHUYSEN, J. & KUENEN, PH. H. (1960): Das EmsEstuarium (Nordsee), ein sedimentologisches Symposium. - Verhandelingen van het K.N.G.M.G., geo!. Ser. 2, 29: 300 pp. VEENSTRA, H. J. & WINKELMOLEN, A. M. (1976): Size, shape and density sorting around two barrier islands along the north coast of Holland. - Geo!. en Mijnb., 55: 87-104. VISSER, M. J. (1980): Neap-spring cycles reflected in Holocene subtidal large-scale bedform depostis: a preliminary note. - Geo!., 8: 543-546. VON WEIHE, K. (1979): Morphologische und okologische Grundlagen der Vorlandsicherung durch Puccinellia maritima (Gramineae). - Helgo!. wiss. Meeresunters, 32: 239-254. VAN DER SPEK, A. J. F. (in press): Reconstruction of tidal inlet and tidal channel dimensions in the Frisian Middelzee, a former tidal basin in the Netherlands Wadden Sea. Proc. Tidal Clastics '92 Symposium, Wilhelmshaven. VAN DER SPER, A. J. F. & BEETS, D. J. (1992): Mid-Holocene evolution of a tidal basin in the western netherlands: a model for future changes in the northern Netherlands under conditions of accelerated sea-level rise? - Sediment. Geo!., 80: 185-197. VAN DIGGELEN, J. (1988): A comparative study on the ecophysiology of salt marsh halophytes. - PhD-Thes., Free Univ. Amsterdam: 208 pp. VAN EERDT, M. M. (1985): The influence of vegetation on erosion and accretion in salt marshes of the Oosterschelde, the Netherlands. - Vegetatio, 62: 367-373. VAN GEMERDEN, H. (1993): Microbial mats: A joint venture. Mar. Geo!., 113: 3-25. VAN HEUVEL, Tj. (1991): Sedimenttransport in het Eems-Dollard estuarium, vol gens de methode McLaren. - Rijkswaterstaat, Dienst Getijdewateren, Rep. GWWS-91.002: 25 pp. VAN KLEEF, A. W. (1991): Empirical relationships for tidal inlets, basins and deltas. - Fac. Geogr. Sci., Univ. Utrecht, Rep. GEOPRO 1991.19: 53 pp. VAN OOSTEN, M. F. (1986): Bodemkaart van Nederland, Schaal 1:50,000, Toelichting op de kaarten van Vlieland, Terschelling, Ameland en Schiermonnikoog. Stichting Bodemkartering Wageningen: 129 pp.; Arnhem (van der Wid). VAN PARREEREN, D. (1980): Waterloopkundige aspecten van de doorbaggering wantij Schiermonnikoog. Rijkswaterstaat, directie Groningen, Nota 80-29: 40 pp. VAN SIjP, D. (1989a): Correcties op gemeten eb- en vloedvolume bij de omrekening naar gemiddeld getij, in het Friese Zeegat. - Rijkswaterstaat, directie Friesland, Notitie ANW 89-02: 4 pp. (1989b): Erosie vooroever Lauwersmeerdijk in relatie met de afsluiting van de Lauwerszee. - Rijkswaterstaat, directie Friesland, Notitie ANW 89.19: 12 pp. VAN STRAATEN, L. M. J. U. (1954): Composition and structure of Recent marine sediments in the Netherlands. - Leidse geo!. Medede!., 19: 1-110. (1960): Transport and composition of sediments. - In: VAN VOORTHUYSEN,J. H. & KUENEN, PH. H. [Eds.]: Das Ems-Estuarium (Nordsee), ein sedimentologisches Symposium. - Verh. kon. Nederl. geo!. mijnbouwkdg. Genoot., 19: 279-292. Vos, P. C. & DE BOER, P. L. & MISDORP, R. (1988): Sediment stabilisation by benthic diatoms. - In: DE BOER, P. L. & VAN GELDER, A. & NIO, S. D. [Eds.]: Tide-influenced sedimentary environments and facies: 511-526; Dordrecht (Reidel). VROOM, M. G. & STEYAERT, F. H. 1. M. & MISDORP, R. & VENEMA, H. & ELGERSHUIZEN, J. H. B. W. & RAKHORST, H. D. & DAMOISEAUX, M. A. (1989): Wadatlas - Ministerie Verkeer en Waterstaat:'95 pp.; Amsterdam (Stadsdrukkerij). . WALTON, T. L. & ADAMS, W. D. (1976): Capacity of inlet bars to store sand. - Proc. 15th Conf. coast. Engng., Am. Soc, Civ. Eng., Honolulu, 1919-1938. WARRICK, R. & OERLEMANS, J. (1990): Sea level rise. - In: HOUGHTON, J. T. & JENKINS, G. J. & EPHRAUMS, J. J. [Eds.]: Climate Change, the IPCC scientific assessm'ent: 259-261; Camb. (Univ. Press). WIGGERS, A. J. (1960): Die Korngrossenverteilung der holozanen Sedimente im Dollart-Ems-estuary. - In: VAN VOORTHUYSEN, J. H. & KUENEN, PH. H. [Eds.]: Das EmsEstuarium (Nordsee), ein sedimentologisches Symposium, Verh. kon. Neder!. geo!. mijnbouwkdg. Genoot., 19: 75-81. WINKELMOLEN, A. M. & VEENSTRA, H. J. (1974): Size and shape sorting in a Dutch tidal inlet. - Sedimento!', 21: 107 -126. WOLFF, W. J. (1986): De Waddenzee: eigenschappen van een dynamisch kustgebied. - Flevobericht 252, RI]P, 11-22. WOODWORTH, P. L. (1990): A search for accelerations in records of European mean sea leve!. - Internat. J. Climato!', 10: 129-143. WUNDERLICH, F. (1967): Die Entstehung von "convolute bedding" an Platenranderr;t. - Senckenbergiana lethaea, 48: 345-349. 115 - - - (1979): Die lnse! Mellum (slidliche Nordsee) Dynamische Prozesse und Sedimentgeflige. 1. Slidwatt, Ubergangszone und Hochflache. - Senckenbergiana marit., 11: 59-113. (1982a): Schichtbanke. - In: Reineck, H.-E. [Ed.]: Das Watt, Ablagerungs- und Lebensraum, 3rd Edn.: 81-88; Frankfurt a. M. (Kramer). (1982b): Marken. - In: REINECK, H.-E. [Ed.]: Das Watt, Ablagerungs- und Lebensraum, 3rd Edn.: 95-105; Frankfurt a. M. (Kramer). ZAGWIJN, W. H. & BEETS, D. J. & VAN DEN BERG, M. & VAN MONTFRANS, H. M. & VAN ROOI]EN, P. (1985): Atlas van Nederland in 20 delen, dee! 13, Geologie: 23 pp.; The Hague (Staatsuitgeverij). ZI]LSTRA, - - - J. J. - - - P. (1994): Sedimentology of the type section of the Late Cretaceous and Early Tertiary (Tuffaceous) Chalk of Northwest Europe. - Geol. Ultraeictina, 119: 192 pp.