From Ripples to Large-Scale Sand Transport: The Effects of Bedform-Related Roughness on Hydrodynamics and Sediment Transport Patterns in Delft3D

Abstract

Bedform-related roughness affects both water movement and sediment transport, so it is important that it is represented correctly in numerical morphodynamic models. The main objective of the present study is to quantify for the first time the importance of ripple- and megaripple-related roughness for modelled hydrodynamics and sediment transport on the wave- and tide-dominated Ameland ebb-tidal delta in the north of the Netherlands. To do so, a sensitivity analysis was performed, in which several types of bedform-related roughness predictors were evaluated using a Delft3D model. Also, modelled ripple roughness was compared to data of ripple heights observed in a six-week field campaign on the Ameland ebb-tidal delta. The present study improves our understanding of how choices in model set-up influence model results. By comparing the results of the model scenarios, it was found that the ripple and megaripple-related roughness affect the depth-averaged current velocity, mainly over the shallow areas of the delta. The small-scale ripples are also important for the suspended load sediment transport, both indirectly through the affected flow and directly. While the current magnitude changes by 10–20% through changes in bedform roughness, the sediment transport magnitude changes by more than 100%.

The effect of asymmetric dune roughness on tidal asymmetry in the Weser estuary

“…Additional studies on contrasting tidal environments are necessary before arguing a general recommendation to incorporate time‐ and space‐dependent aspects into the calculation of bedform roughness in coastal‐scale numerical models (Brakenhoff et al, 2020a; Brakenhoff et al, 2020b). It is shown to be crucial, however, for coastal settings with large tidal dunes, which preserve their geometry and shape because of an extended morphological response time that is much longer than the semidiurnal tidal cycle.…”

“…Even more than for capturing flow properties, the correct parameterization of the effective bed roughness is crucial for the prediction of sediment transport. Although common in fluvial studies (e.g., Paarlberg et al, 2010), there are only few numerical model studies that apply bedform roughness predictors and deal with the effect of bedform drag on hydro-and morphodynamics at the estuarine and coastal system scales (e.g., Brakenhoff et al, 2020b;Davies & Robins, 2017;Herrling et al, 2017;Villaret et al, 2011Villaret et al, , 2013Wang et al, 2016).…”

Modelling the contribution of wind waves to Cap Ferret's updrift erosion

“…As a result, they are not resolved by the unstructured grid and need to be parameterized. Brakenhoff et al (2020) recently reviewed the different approaches to represent the bed friction in these environments, and how modelled sediment transport rates were sensitive to its parameterization. So, simulation SIM3 was reproduce with four different parameterization of the bottom friction (Table 3, SIM3a-d).…”

Observations of near-bed orbital velocities and small-scale bedforms on the Dutch lower shoreface

“…The Van Rijn 2007 predictor was chosen because it is a generic formula that is commonly used to compute bed roughness in engineering morphological models like Delft3D. Furthermore, it performed best, together with the Soulsby et al (2012) predictor, in comparison to small-scale wave-current ripple data on the Ameland ebb-tidal delta (see Brakenhoff et al, 2020b). Note that the Van Rijn 2007 predictor does not consider the angle between waves and currents.…”