Establishing a sediment budget in the newly created “Kleine Noordwaard” wetland area in the Rhine–Meuse delta

Abstract

Abstract. 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 sediment deposition, 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 sediment deposition and erosion in newly created wetlands. We quantified both the magnitude and spatial patterns of sedimentation 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. Deposition 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. In the intertidal area, sand deposition especially takes place at low-lying locations close to the channels. Mud deposition 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. Marsh erosion 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 deposition rates over time, but the overall results of this study indicate that the area functions as a sediment trap. The total contemporary sediment budget of the study area amounts to 35.7 × 10 3 m 3 year −1 , which corresponds to a net area-averaged deposition rate of 6.1 mm year −1 . This is enough to compensate for the actual rates of sea-level rise and soil subsidence in the Netherlands.

Factors controlling sediment trapping in two freshwater tidal wetlands in the Biesbosch area, The Netherlands

“…This rate is slightly less than, but in same order as the average net accumulation rate of 5.2 mm y −1 over the entire period since depoldering of the area (Van der Deijl et al, 2017). The slightly lower value found in this study fits well to the observation that accumulation rates have decreased over time since the opening of the polder area ( Van der Deijl et al, 2017).…”

Effects of discharge, wind, and tide on sedimentation in a recently restored tidal freshwater wetland

“…Discharge and water level series were derived from ADCP and diver measurements taken at the locations of the gauging stations. For more information on these time series, we refer to Van der Deijl (). Wind conditions (hourly values of average wind speed and direction during the last 10 min of every hour) for four surrounding stations during the calibration period were obtained from the Royal Dutch Meteorological Institute KNMI (KNMI, ).…”

A global synthesis of the effectiveness of sedimentation-enhancing strategies for river deltas and estuaries

“…This synthesis compares the functioning and effectiveness of sedimentation-enhancing strategies (SES) in deltas globally. Some SES are well-documented (Gain et al, 2017;van der Deijl et al, 2018;Xu et al, 2019), but most existing studies focus on single cases. This synthesis enables intercomparison between SES and helps to further evaluate advantages and drawbacks of each SES.…”

“…As part of the larger "Room for the River" project in the Rhine-Meuse delta, river diversions have been constructed in the depoldered Zuiderklip and Noordwaard areas (RNW & RZK). Dikes were removed to divert river water and sediment into new wetlands (van der Deijl et al, 2017(van der Deijl et al, , 2018. Although the primary goal was nature development and water storage, river floods unintentionally resulted in sedimentation that decreased over time (van der Deijl et al, 2017(van der Deijl et al, , 2018.…”