Wetlands of Recent Dutch Embankments

Hydrobiologia 265: 179-193, 1993. E. P. H. Best & J. P. Bakker (eds), Netherlands-Wetlands. 179 <D 1993 Kluwer Academic Publishers. Printed in Belgium. Wetlands of recent Dutch embankments W. Joenje 1 & B. Verhoeven 2 1 Agricultural University, Bornsesteeg 69, 6709 PD Wageningen, The Netherlands 2 Waterkant 83, 8307 AZ Ens, The Netherlands Key words: wetland, polder, drainage, desalination, colonization, vegetation-succession, groundwater regimes, meso-series Abstract Wetlands of new embankments are characterized with respect to soil and hydrological regime and the processes of desalination and vegetation succession. Soil- and height gradients, the lack of local drainage and the generally low nutrient content provide conditions for dynamic mesoseries (groundwatertable low in summer, saturated in winter) and locally for stable mesosen·es (relatively high in summer, saturated in winter). Relations between vegetation and herbivores (geese, ducks, cattle) are discussed. Introduction landscapes have not been used as farmland and offer a rich variety of spontaneous, 'natural' de The skill of coastal engineering has evolved in The velopments. Gradually the outstanding value of Nether lands over at least a millennium. The dikes these newly embanked wetlands has been for and dams, sluices and mills which protect against mally recognized~ substantial areas have received flooding of the land drained from sea and swamp, the status of nature reserve and proper nature are backbone structures in the Dutch lowlands, management schemes are being developed with which indeed, are predominantly man-made. increasing input of local and national experiences. Following previous storm floods the dikes have Stimulated by the Dutch examples and tech been repaired and often increased in height. nology, neighbouring countries have also hardly However, the storm surge of 1953 triggered a new left alone any estuary or free discharging river approach: on some places long stretches of dikes along the North Sea coasts. The related losses of were enforced and heightened, but elsewhere new natural habitats are partly counterbalanced by the high dams, huge sluices and sophisticated storm development and careful management of new surge barriers have been constructed in the estu wetlands. Indeed, the whole chain of new habi aries of the southwest and the north. This short tats which has been created along the North Sea ening of the coastline eliminated the most valued coasts presents a major ecological feature. parts of the natural coastal wetlands: the estuar The knowledge of natural succession obtained ies, their mudflats and shallows and gradients in in these new territories over several decades, and salinity and nutrients. experience of habitat construction have promoted However much the extinction of the tidal lands the recent ideas on, and applications of, "nature has been felt as a loss, parts of the newly formed technique'': the habitat construction de novo and 180 the subsequent management of these natural suc creased pressure for the construction of pumping cessions according to formulated aims. Also out installations. side the new embankments these ideas have now A special category of polders consists of re developed to become part of nature conservation claimed lakes ( 443 ·droogmakerijen, (in Dutch), strategies in a much wider sense. comprising 240 former natural lakes and 203 lakes In this chapter the soil- and hydrological pro originating from peat excavation), which cannot cesses in the new wetlands of recent Dutch em be drained without mills (Schultz 1987). bankments are summarized and related to the The excess water in polders and ·droog development of plant species and communities. makerijen' is often lifted to an intermediate res The descriptions and conclusions often hold for ervoir system, (the "boezem') from which it is other areas outside TheN etherlands on the coasts sluiced or pumped into a river or the sea. Figure 1 of the North Sea. shows the principal categories of land protected against flooding in the Netherlands. The polder concept Drainage was always a cumbersome activity, so in most polder-like areas wetlands were com A polder is considered to be an area whose hy mon. Nowadays modern pumping stations allow drology has been made independent from the sur a deep and constant drainage which eliminates rounding watertable. This definition usually is re many wetland conditions. But in the newest stricted to areas which have been reclaimed coastal embankments large wetlands form part of mainly for agricultural purposes. Within this view the project. This text deals only with the latter falls a wide variety of lowlands such as coastal wetlands and derives examples in particular from polders, peat and swamp embankments and re the Lake Grevelingen and the Lauwersmeer claimed lakes. However, in the context of this (Fig. 1). paper, stretches of water with controlled water For a description and ecological survey of the table, such as dammed rivers and reservoirs and effects on the flora and vegetation of embank the holms and watermeadows belonging to them, ments and drainage in the southwest of the Neth can all be included under the heading "polder", erlands, the reader is referred to Beeftink ( 197 5 ). although the polder status of such areas may be quite arbitrary. Coastal embankments The first human inhabitants in the Netherlands lived on high sites, like beach-ridges, creek- and The builders of the oldest coastal polders avoided river levees and outcrops of pleistocene deposits. the embankment of low lying areas: there the tidal Later, man-made mounds safeguard their dwell movement hampered dike construction and ings from flooding. At the time the local popula drainage by gravity of such low patches was dif tion started to protect their grounds on the high ficult. Wetlands formed by the closing of narrow saltmarsh against the sea by means of dikes, the creeks lost their natural value by deeper drainage first coastal polders came into being. During low or disappeared by landfill. However, these polders tide the excess water was drained off through have a variety of wetlands originating from peat flapgates, but the rise of the sea level and the excavation for salt making and clay digging for subsidence of the land increased the need for lift dike repair. ing or pumping devices. Later on the closing of somewhat wider tidal The large and relatively elevated inland peat gaps became feasible and sandy and muddy creek and peat-on-clay areas became also cultivated. beds were transformed into valuable brackish or Small streams as well as the main rivers, provided fresh wetland strips after embankment. In the a certain drainage. But here too the drainage second half of this century this development led caused subsidence (by mineralization and com to the enclosure of large sea arms and estuaries, paction, enhanced by peat digging) and this in- dictated by the greater safety requirement against 181 ; "''ll'""''"-~ . / I' I I I I I I North sea I I ~-----......_- ... _I _,_ I '--, __ ,\_\ I I I I ,_1\ Germany Belgium protected against the sea protected against the rivers - embankments pumped dry IIIIIIHIIIIIIIII embankments drained on low tide Fig. 1. Areas in The Netherlands protected against flooding (after Schultz, 1987). (Key: 1. Braakman; 2. Veerse Meer; 3. Mark iezaatsmeer; 4. Grevelingen Meer; 5. Haringvliet; 6. Brielse Meer; 7. Border Lakes; 8. Oostvaarders Plassen; 9. Flevo polder: 10. IJ sselmeer: 11. Amstelmeer; 12. Lauwersmeer.) stormfloods and the improvement of the drainage the land is often equalled by the water surface and of the hinterland, rather than by the wish to gain it is in these new polders where large areas of new land. wetlands are formed. Unlike in older polders, the In these recently embanked areas the surface of water quality is only partially determined by the 182 precipitation and the drainage water from the i 1500 polder itself, because the bulk of the water re .9- .... ceived in its reservoir lake stems from far larger ! n; catchment areas in the predominantly agricultural 20 E u hinterland. ,... -·- · ," -·-·-ea~ 15 ·~ , ~ The abiotic environment 500 10] ... ~-~-~:.::ryat 5 g ············· ~ Generally the terrestrial part of the new embank o~=···r····_···~--~~--~~--~o······· ~ ment comprises at least two morphologically dif 5 10 15 20 25 30 35 % par tides < 2)1- ferent areas: vegetated land of higher elevation (former saltmarshes, in Dutch 'schorren' or Fig. 2. The relationship between the amount of P:Ps· K:p. 'kwelders') and the void or scarcely vegetated CaC03 • organic matter and the lutum content in marine coastal sediments (Verhoeven. 1952 ). lower areas (tidal fiats, in Dutch 'slikken' or 'wadden'). The higher marshes are usually re claimed for agricultural use; they are too dry to environment (Fig. 2). A fresh silt shows a higher become a wetland. The lower fiats however are pore volume than an older deposit; a vegetated subjected to the natural fluctuations governed by mud contains more organic matter per g of lutum, the new hydrological regime (target-polderlevels, than a bare mud. In samples with the same lutum precipitation and drainage) replacing the tide. content the lime content in Zeeland (southwest) They often remain significant as wetlands, pro is as a rule higher than in the Wadden Sea area. vided that their hydrology meets a wetland defi After drainage the clayey sediments which are nition and the land use does not interfere with no longer or only rarely inundated,

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