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

Laura Brakenhoff,Jebbe Van Der Werf,Reinier Schrijvershof,Maarten Van Der Vegt,Bart Grasmeijer,Gerben Ruessink

doi:10.3390/jmse8110892

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%.

Highlights

To contribute to answering scientific and practical questions, numerical morphodynamic models are often used to predict the hydrodynamics, sediment transport processes and morphological developments of coastal systems
Delft3D is a numerical morphodynamic model system that incorporates all parts of the morphodynamic feedback loop: hydrodynamics, sediment transport and morphologic change [20]
The effect of small-scale ripples and megaripples on hydrodynamics and sediment transport was quantified in detail using Delft3D scenarios for a mixed wave-current environment (Ameland ebb-tidal delta)

Summary

Introduction

To contribute to answering scientific and practical questions, numerical morphodynamic models are often used to predict the hydrodynamics, sediment transport processes and morphological developments of coastal systems. In such models, many of the processes are parameterised, based on various assumptions. One of the parameterised variables is the bedform-related hydraulic roughness, which has a direct influence on the magnitude of friction between the bed and the flowing water [1]. Roughness is caused by the individual grains (skin friction), but in many situations bedforms are present, inducing additional drag at the bed (form drag, or bedform-related roughness) [2]. The total effective hydraulic roughness consists of both the grain roughness and the bedform-related roughness. It is important that a parameterisation is chosen in morphodynamic models that correctly represents environmental conditions

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