Eastern Scheldt foreshore stability: towards hybrid flood protection strategies in the future -An indicative field study of the Eastern Scheldt, SW- Netherlands-

Abstract

Since the 1970’s the perspective on ideal coastal management has changed in the Netherlands. A shift occurred to more eco-system based management with a focus on the implementation of the so called soft measures. Tidal mudflat and salt marshes are known for their functioning towards enhanced flood protection. In this study research has been conducted on the role of the foreshore in the Scheldt estuary. The foreshore includes mudflats and salt marshes between dike and waterfront. Stability of these systems has mostly been studied for shallow subsurface processes and to a lesser extent for deep subsurface stability. In this master thesis the stability of foreshores in de Eastern Scheldt has been studied coupling both shallow and deep subsurface stability. Subsurface characteristics were studied for 3 locations with different energetic conditions. Characteristics such as lithology, disturbance layers and bulk density were used to assess the stability on 2 different scales; shallow subsurface (0-30 cm) and deeper subsurface (0-max. channel depth~15 m). Additionally, it was tested, whether foreshore stability could relate to an important failure mechanism of water defences shown within the Eastern Scheldt; the occurrence of so-called "flow slides". The stability in both the lateral and vertical stretch of tidal salt marshes and mudflats of all 3 locations in terms of erosion resistance was not to be related directly. The same holds for the probability of flow slides. Erosion resistance was influenced mainly by soil strengthening factors in the top-layer such as sediment density, moisture content and the presence of roots and shells. Based on this study, probability of flow slides is mainly influenced by the degree of consolidation of deeper lain sand layers and the total geometry of the channel to dike foreshore system. Within the scope of a better understanding of foreshore stability, more research is needed on the relation of soil characteristics with stability and erosion resistance. Moreover, boundary conditions for foreshores with different deeper subsurface characteristics should be tested on their sensitivity for flow slides.