The Coastal Plain of Belgium, Joint Product of Natural Processes and Human Activities 19

The Coastal Plain of Belgium, Joint Product of Natural Processes and Human Activities 19 Cecile Baeteman Abstract The coastal plain is a flat low-lying area with hardly any expression of relief at the surface. Its subsurface, however, contains a rich archive of the depositional history that started about 10,000 years ago, and that represents the history of infill of a major pre-Holocene palaeovalley. The infill of the palaeovalley was controlled by several factors, among which relative sea level rise was initially the main driver. In the course of time, the balance between sediment supply and accommodation space took over the control of infill. Toward the end of the infill, humans played a prominent role in the further evolution of the plain, causing changes that ultimately determined its present-day characteristics. The complex interaction of regional and local processes is explained in the geological setting of the area. The development of De Moeren and the Zwin region, two protected areas that experienced a specific evolution, different from the general history of infill, illustrates the effect of local influences in controlling spatial and temporal patterns of sedimentation in response to variations in coastal processes. Keywords Tidal environments Á Sea level rise Á Coastal processes Á Land subsidence Á Human interventions Á Holocene 19.1 Introduction The Holocene coastal plain belongs to the marine-dominated North Sea Lowlands which run from the The Belgian coastal plain is a unique landscape, shaped by Cretaceous marl outcrop at Blanc Nez, near Calais in the sea but modified by humans. The numerous ditches, northern France, to northern Denmark. In Belgium, the plain canals, and sluices attest to the extensive human activities is a 15- to 20-km-wide embanked lowland at altitudes since the mediaeval period. They are essential for the sus- ranging from +2 to +5 m TAW. As the Belgian ordnance tainability of the area, whose high groundwater table is datum (TAW—second general levelling) refers to mean controlled by ingenious drainage systems. For the western spring low water, this corresponds to 2–3 m below spring part of the plain, the drainage systems join all together in high tide (datums for The Netherlands—NAP—and France Nieuwpoort in a complex of sluices, which offer a spectac- —NGF—are 2.33 and 2.29 m higher, respectively). The ular view when the gates are opened at low tide. It is from plain is crossed by a small river, the IJzer, which at present is this site that a part of the plain was voluntarily inundated canalized and debouches into the North Sea at Nieuwpoort during the First World War. and along which the plain extends further south (Fig. 19.1). The river, together with its tributaries, drains a small and relatively low-lying basin to the south and southwest of the & C. Baeteman ( ) coastal plain. Quaternary Environments and Humans, Royal Belgian Institute of Natural Sciences, Jennerstraat 13, 1000 Brussels, Belgium e-mail: [email protected] © Springer International Publishing AG 2018 313 A. Demoulin (ed.), Landscapes and Landforms of Belgium and Luxembourg, World Geomorphological Landscapes, DOI 10.1007/978-3-319-58239-9_19 314 C. Baeteman Fig. 19.1 Map of the coastal plains of northern France, Belgium, and Zeeland with stars indicating the location of two sites presented in detail: De Moeren in the west and the Zwin area in the east (reproduced with permission of Brepols) The plain lies behind a straight coastline. Coastal dunes thickness of about 25–30 m at the coast and thinning out to and a hard seawall in some places protect it from flooding the south, where Late Pleistocene deposits crop out. during storm surges. The dunes, however, are strongly degraded or even destroyed in many places by housing projects for tourism since a few decades. Therefore, parts of 19.2.1 Agents and Controls on Sedimentation: the coastal stretch are often nicknamed ‘Atlantic Wall’. Sea Level and Tides A wide complex of splendid high dunes still exist only near De Panne in the west, close to the French border. The bea- The coast of Belgium is tide-dominated (macro- to mesoti- ches are also being modified continuously for touristic pur- dal), with a tidal range of 5 m in the west decreasing to poses. In many places, beach nourishment is needed now 4.5 m in the east (Fig. 19.2). The shifting play of ebbs and and will become more frequent to cope with the predicted flows has produced a series of sedimentary environments accelerated sea level rise, although human interventions are whose past deposits form the core of the coastal plain. These so far causing more harm than the rising sea level. sedimentary environments are the coastal barrier, tidal channels, mud- and sand flats, salt and freshwater marshes. Understanding the coastal plain development requires 19.2 Geological Setting placing it in the context of dynamic sedimentary environments. An environmental interpretation of the sediment Geologically speaking, the coastal plain is a young area, sequences is essential to document coastal changes because though with a very rich geological history. We thus address each tidal sedimentary environment has its specific relation here its development during the Holocene (last 11.7 ka) to sea level. Besides, their relative positions also show because it explains all notable features of the plain. typical spatial patterns controlled by the location of tidal The characteristics of the present landscapes are inherited channels (Fig. 19.3). These environments form a dynamic from what happened before, in particular in terms of driving system with inland or seaward shifts and overlappings in processes. The coastal plain landscape displays only the response to the following factors: rate of relative sea level terminal geomorphic surface but not the record of the (RSL) rise, sediment supply, and accommodation space. development stages. These stages are preserved in the sub- An extensive network of tidal channels acting as local surface. Because of the presence of a major pre-Holocene sediment suppliers dissects the tidal flat while creeks form valley, the western part of the plain contains the most the drainage system in the supratidal zone (salt marsh). complete sequence of Holocene deposits, reaching a When the sediment supply exceeds the accommodation 19 The Coastal Plain of Belgium, Joint Product of Natural … 315 Fig. 19.2 View of the IJzer estuary with associated mudflat and salt marsh (a) at the occasion of extreme water levels at a spring equinox (Nieuwpoort). (b) salt marshes and mudflat are completely inundated at extreme high water level space created by RSL rise, the area rapidly silts up to high of a freshwater lens in the shallow subsoil. However, at the water level. Consequently, the frequency of tidal inundation same time, intertidal deposition continues in adjoining parts decreases, a part of the drainage network of the area until they in turn experience the same evolution. If no suf- becomes redundant and silts up. Eventually the area ficient time is available for a freshwater lens to form (which becomes out of the reach of the daily tidal inundation and the was the case in the period characterized by a rapid RSL rise), salt marsh encroaches onto the mudflat, followed by peat only vegetation horizons can develop. Consequently, vege- accumulation if sufficient time is available for the formation tation horizons or peat beds are not synchronous and have no 316 C. Baeteman Fig. 19.3 Schematic representation of the different sedimentary environments in a tidal flat in relation to water levels. MHW mean high water spring; MTL mean tidal level; MLW mean low water spring (reproduced with permission of Brepols) stratigraphical meaning, their occurrence depending on 2005a, 2013, 2008b), Baeteman and Van Strydonck (1989), whether sufficient time and sediment were available to allow Baeteman et al. (1999), and Baeteman and Declercq (2002). silting up of a particular part of the tidal flat. Moreover, The development of the coastal plain is controlled by the being related to the position of the tidal channels, the evo- rate of RSL rise, the morphology of the flooded surface, the lution of one environment into another cannot be generalized sediment budget, and the accommodation space (effected by for the entire plain. sediment and peat compaction). During the infill of the area, The coastal barrier forms the boundary between the open initially caused by the RSL rise, the relative importance of sea and the tidal environments. It is a huge sand body the individual factors changed with time. In the last extending below and above sea level, and including the 2000 years, human activities played a prominent role. shoreface, tidal deltas and inlets, washovers, beach and Knowledge of the morphology of the pre-Holocene sur- dunes. Coastal barriers are essentially transitory features in face, i.e. the original topography at the beginning of the dynamic equilibrium with rising sea levels through the postglacial sea level rise, is crucial to the understanding of landward transfer of sand, eroded from the shoreface, to the the sediment successions. In the west, the landscape prior to tidal flat via tidal inlets and overwash processes. This pro- the marine flooding was characterized by four relatively cess nowadays is called coastal erosion. Landward shifting small and shallow rivers joining in the central part of the of the barrier is fairly continuous while sea level is rising western plain and forming a southeast–northwest depression relatively rapidly but may become intermitted when the sea (Fig. 19.4). This depression is the palaeovalley of an ancient level rise slows down. In addition, the rate of inland shifting river IJzer formed during the glacial sea level low stand and is reduced as the accommodation space for sediment in the located several km to the west of the present-day river.

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