Building and raising land: meandering rivers and filling estuaries

Abstract

Tidal floodplain formation fastened land-level rise in experiments and was tested further in a numerical model of the Western Scheldt Estuary (Chapter 6), where transitional polders (in Dutch: wisselpolders) are being considered to raise low-lying land. Four polders were added to the Western Scheldt and tested for different opening sequences and inlet widths. Findings showed land-level rise in all configurations. Transitional polders opened in an upstream-to-downstream sequence resulted in stronger shallowing of the estuary and hence in smaller tidal ranges than a downstream-to-upstream sequence, suggesting the opening sequence can play a part in managing flood risk. Polders opened later in a sequence temporarily experienced a lag in mud deposition, and net sand import was smaller for polders with a wide foreshore due to deeper inlet erosion.This thesis proposes that tidal floodplains (i.e. mudflats and salt marshes) promote the filling of estuaries, where the balance between floodplain formation and destruction determines the steady state of an estuary. A stronger tendency to form floodplains results in a narrower estuary, similarly to how more extensive floodplains transform rivers from braided to meandering. Fast sea-level rise will probably enhance floodplain destruction in estuaries, which asks for effective strategies such as transitional polders to retain sediments, raise land and maintain ecologically valuable intertidal areas.