Publication Date:
2017-05-02
Description:
Many deltas are threatened by accelerated soil subsidence, sea level rise, increasing river discharge, and sediment starvation. Effective delta restoration and effective river management require a thorough understanding of the mechanisms of aggradation, erosion, and their controls. Sediment dynamics has been studied at floodplains and marshes, but little is known about the sediment dynamics and budget of newly created wetlands. Here we take advantage of a recently opened tidal freshwater system to study both the mechanisms and controls of aggradation and erosion in newly created wetlands. We quantified both the magnitude and spatial patterns of aggradation and erosion in a former polder area in which water and sediment have been reintroduced since 2008. Based on terrestrial and bathymetric elevation data, supplemented with field observations of the location and height of cut banks and the thickness of the newly deposited layer of sediment, we determined the sediment budget of the study area for the period 2008–2015. Aggradation primarily took place in channels in the central part of the former polder area, whereas channels near the inlet and outlet of the area experienced considerable erosion. At the intertidal flats, sand aggradation especially takes place at low lying locations close to the channels. Mud aggradation typically occurs further away from the channels, but sediment is in general uniformly distributed over the intertidal area, due to the presence of topographic irregularities and micro topographic flow paths. Cut bank retreat does not significantly contribute to the total sediment budget, because wind wave formation is limited by the length of the fetch. Consecutive measurements of channel bathymetry show a decrease in erosion and aggradation rates over time, but the overall result of this study indicate that the area functions as a sediment trap. On average, the area traps approximately 46 % of the sediment delivered to the study area, which is approximately 3 % of the sediment load of the River Rhine at the Dutch-German border. The total sediment budget of the study area amounts to 29.7 × 103 m3 year−1, which corresponds to a net area-averaged aggradation rate of 5.1 mm year−1. This is enough to compensate for the actual rates of sea level rise and soil subsidence in The Netherlands.
Electronic ISSN:
2196-6338
Topics:
Geosciences
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