The effects of bedform roughness on hydrodynamics and sediment transport in Delft3D

To contribute to solving scientific and practical questions, numerical morphodynamic models like Delft3D are often used to predict the hydrodynamics, sediment transport processes and morphological development of coastal systems. In such models, many of the processes are parameterized based on a variety of assumptions. One of the parameterized variables is the bedform-related hydraulic roughness ks, which is often assumed to be related to the ripple height. This roughness affects the magnitude and vertical structure of the flow and, consequently, the magnitude of the sediment transport. Yet, their sensitivity to ks is not well understood

Saw-tooth bar dynamics on the Ameland ebb-tidal delta

Ebb-tidal deltas are part of tidal inlet systems, and it is important to understand sediment transport processes and pathways that take place there. Sediment is transported along ebb-tidal deltas as both suspended load and bedload, and bedform migration is one of the processes by which bedload transport can take place. Saw-tooth bars are bedforms that are found regularly on ebb-tidal deltas, but they have not been studied thoroughly yet. The present study shows the characteristics of these bars, like bar height and length. Spatial correlation was used to find their migration speed. It was found that saw-tooth bars are approximately 2m high and have a wavelength of 500-1000m. Both bar height and migration speed appear to change in a cyclic way. The saw-tooth bars contribute substantially to the total sediment transport on the ebb-tidal delta

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%

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%