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Long-term morphological evolution of the Flemish coast: Holocene, Late Middle Ages to present
Houthuys, R.; Vos, G.; Dan, D.; Verwaest, T. (2021). Long-term morphological evolution of the Flemish coast: Holocene, Late Middle Ages to present. Version 1.0. FHR reports, 14_023_1. Flanders Hydraulics Research: Antwerp. IX, 98 + 5 p. app. pp. https://dx.doi.org/10.48607/44
Deel van: FHR reports. Flanders Hydraulics Research: Antwerp, meer
Long-term morphological evolution of the Flemish coast: Holocene, Late Middle Ages to present

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Documenttype: Projectrapport

Trefwoorden
    Coastal protection > Coastal protection against erosion > Morphodynamics soft coastal defences
    Literature and desktop study
    Flanders, Coastal area
    Zoet water; Terrestrisch
Author keywords
    Coastline shift; Historic maps

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Opdrachtgever: Vlaamse overheid; Beleidsdomein Mobiliteit en Openbare Werken; Vlaams Ministerie van Mobiliteit en Openbare Werken; Agentschap voor Maritieme Dienstverlening en Kust; Afdeling Kust, meer


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Abstract
    This report collects and integrates information about the long-term (Holocene and historic) evolution of the Flemish coast.
    A literature review has been performed about the sedimentological principles of coastline development under a rising relative sea level over a low-gradient land surface, and about the actual formation and movement of the Flemish coast during the Holocene period, mostly based on the study of the sediment layers. The strong interplay between sea-level rise, accommodation space and sediment availability causes the natural coastline at any time to be in an equilibrium location between the inner shelf and the coastal plain. Morphological changes in one domain strongly affect changes in the other, and the location of the coastline. Overall, the movement of the coastline has been retreat.
    From the Roman Time, an increasing influence of human action interfered with the natural evolution. Larger quantities of overland runoff activated the drainage channels which, combined with peat digging, caused compaction of the land surface in the coastal plain. The general evolution, under continuing sea-level rise, is flooding of the coastal plain in separate events or cycles. Each cycle involves marine intrusions, in storm conditions, through existing drainage channels or by new breach(es), followed by flooding of a small or large part of the coastal plain, which will soon be sanding and silting up. The sediment needed to fill the coastal plain to the contemporaneous mean sea level is largely derived from the shoreface and foreshore, causing the coastline to retreat. When around 800 AD, most of the coastal plain had silted up to a level that it could be taken in use for grazing, the first settlements appeared that would be the germ of the present towns and villages. In response to continued breaching events, the first organized embankments reclaimed large areas of the coastal plain. The reclamations were successful, and the coastal retreat slowed down. The present coastline position was reached around 1400 AD along most parts of the Flemish coast, when the first continuous embankment, the Graaf Jansdijk, enclosed the coastal plain from Dunkerque to Terneuzen.
    The analysis of historical maps since the 16th century shows that the major part of the Flemish shoreline has not changed position till now, within an uncertainty margin of a few hundred meters. The relative stability of the last half millennium contrasts with the coastal retreat of the previous one and a half millennium. It is put forward that closure and embanking of the coastal plain has stopped sediment demand, which avoided the shoreface and foreshore to be eroded.
    A remarkable and underexposed aspect of the coastal evolution between about the 9th to the 18th century is dunes growth. Most of the high, mobile and semi-fixed dunes we see today are young and have received the sand incorporated in their present morphology at various deflation stages only during the last millennium. This implies important net sand supply paths were active. It is thought that the sand is marine sourced and was supplied by way of the nearshore and shoreface-connected sandbanks. The link between wide dune belts and the known presence and historic trajectory of some of these banks leaves no doubt: (De Kams-)- Trapegeer-Broer Bank to the wide dunes between the French border and the IJzer and even beyond, to Westende; Stroombank to the dunes between Bredene and Wenduine; Hard Zand to the dunes between Heist and Zwin. This confirms the prominent role of the Flemish shoreface-connected ridges in relation to the coastline development.
    In spite of significant sand supply at least in some areas of the Flemish coast, the coastline has also never prograded more than a few hundred meters during the last half millennium. In this report, the hypothesis is put forward that tidal flow in the channels near the coast may prevent such progradation. At the points where large amounts of sand have entered the subaerial part of the coastal systems, deflation has redistributed it in locally extensive dune belts.
    The natural processes described in this report are still active today. However, they interact with ever increasing human interaction (fixation of the coastline by groins and seawalls, construction of harbour dams, dredging of harbour access channels, artificial beach and shoreface nourishment) and their relative magnitude may change if sea-level rise would accelerate. It is important therefore to further substantiate the postulated processes, especially by elaborating the evidence present in the better quality 19th and 20th century maps.

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