From ripples to large-scale sand transport: the effects of bedform-related roughness on hydrodynamics and sediment transport patterns in Delft3D

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%.