Building and raising land: meandering rivers and filling estuaries

Abstract

Estuaries form in drowned landscapes where rivers transition into the sea and come in a broad range of shapes and sizes. Previous efforts to categorise these tidal systems used the relative importance of rivers, waves and tides. However, this left differences due to the history of these systems largely unexplained, particularly the degree of estuary filling. Until now, only two kinds of steady-state estuaries are recognised in the literature, namely an unfilled lagoonal estuary with limited sediment supply and dynamics and a completely filled estuary with an ideal upstream-converging shape. A mechanism that could explain the diversity in estuary filling is well-known in rivers: floodplain formation. In this thesis, the parallels of muddy, vegetated floodplain formation in rivers and estuaries are explored. A recurring research question is to what extent these tidal floodplains influence the steady state of entire estuaries. By means of estuary experiments, it was studied to what extent a balance in floodplain formation and destruction also influences the steady states of estuaries. With only sand, the estuary filled sufficiently to form a multi-channel pattern with intertidal bars. Mud filled accommodation in the intertidal-to-supratidal reach. Compared to the experiments without vegetation, the vegetation channelised flow, which increased sediment transport in the upstream direction. Mud and vegetation narrowed the subtidal channels in the upstream direction, where the extent of intertidal and supratidal areas was governed by the balance between floodplain formation and destruction. This suggests that a range of steady states exist between the end-members of an unfilled and a completely filled estuary. Fast sea-level rise was applied in a new filling estuary experiment with otherwise the same conditions and caused sediment deposition closer to its marine and fluvial sources. As a result, the central part of the estuary became sediment-starved and subsequently drowned with fewer locations available for vegetation establishment. Next, four polders were added to the Western Scheldt in a numerical model and tested for different opening sequences and inlet widths. 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 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.