Inter-annual bar dynamics may vary considerably across sites with very similar environmental settings. In particular, the variability of the bar cycle return period (T r) may differ by a factor of 3 to 4. To date, data studies are only partially successful in explaining differences in T r , establishing at best weak correlations to local environmental characteristics. Here, we use a process-based forward model to investigate the non-linear interactions between the hydrodynamic forcing and the morphodynamic profile response for two sites along the Dutch coast (Noordwijk and Egmond) that despite strong similarity in environmental conditions exhibit distinctly different T r values. Our exploratory modeling enables a consistent investigation of the role of specific parameters at a level of detail that cannot be achieved from observations alone, and provides insights into the mechanisms that govern T r. The results reveal that the bed slope in the barred zone is the most important parameter governing T r. As a bar migrates further offshore, a steeper slope results in a stronger relative increase in the water depth above the bar crest which reduces wave breaking and in turn reduces the offshore migration rate. The deceleration of the offshore migration rate as the bar moves to deeper water-the morphodynamic feedback loop-contrasts with the initial enhanced offshore migration behavior of the bar. The initial behavior is determined by the intense wave breaking associated with the steeper profile slope. This explains the counter-intuitive observations at Egmond where T r is significantly longer than at Noordwijk despite Egmond having the more energetic wave climate which typically reduces T r .

Purpose A thorough understanding of mechanisms controlling sedimentation and erosion is vital for a proper assessment of the effectiveness of delta restoration. Only few field-based studies have been undertaken in freshwater tidal wetlands. Furthermore, studies that measured sediment deposition in newly created wetlands are also sparse. This paper aims to identify the factors controlling the sediment trapping of two newly created freshwater tidal wetlands. Materials and methods Two recently re-opened polder areas in the Biesbosch, The Netherlands are used as study area. Field measurements of water levels, flow velocities, and turbidity at both the in-and outlet of the areas were carried out to determine the sediment budgets and trapping efficiencies under varying conditions of river discharge, tide, and wind in the period 2014-2016. Results and discussion Short-term sediment fluxes of the two study areas varied due to river discharge, tide, and wind. A positive sediment budget and trapping efficiency was found for the first study area, which has a continuing supply of river water and sediment. Sediment was lost from the second study area which lies further from the river and had a lower sediment supply. The daily sediment budget is positively related to upstream river discharge, and in general, export takes place during ebb and import during flood. However, strong wind events overrule this pattern, and trapping efficiencies decrease for increasing wind strengths at mid-range river discharges and for the highest river discharges due to increased shear stress. Conclusions Delta restoration, based on sedimentation to compensate for sea-level rise and soil subsidence, could only be effective when there is a sufficient supply of water and sediment. Management to enhance the trapping efficiency of the incoming sediment should focus on directing sufficient river flow into the wetland, ensuring the supply of water and sediment within the system during a tidal cycle, creating sufficiently large residence time of water within the polder areas for sediment settling, and decreasing wave shear stress by the establishment of vegetation or topographic irregularities.

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.

No 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 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 . This is enough to compensate for the actual rates of sea-level rise and soil subsidence in The Netherlands.

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. 1 Introduction Many deltas in the world cope with drowning and loss of delta land due to flood-protected polders, dams, and embankments of channels, which result in accelerated soil subsidence and sediment starvation (Ibáñez et al., 1997; Syvitski and Saito, 2007; Ibáñez et al., 2014). The urgency of this problem is enhanced by sea-level rise (Syvitski, 2008) or increasing river discharge. Most deltas are valuable and densely populated because of their ideal location for harbors, agriculture, aquaculture, and tourism (Kirwan and Megoni-gal, 2013; Ibáñez et al., 2014). Moreover, they encompass vast wetland areas of great ecological value. Traditional approaches in river management aim at reducing flood risks by constructing dikes and dams. Although such constructions are often effective in reducing flood risks, they disrupt the morphodynamic processes and ecological function...

No abstract