Modelling the two-way coupling of tidal sand waves and benthic organisms: a linear stability approach

Johan H Damveld,Bas W Borsje,Suzanne J M H Hulscher,Pieter C Roos

doi:10.1007/s10652-019-09673-1

Abstract

We use a linear stability approach to develop a process-based morphodynamic model including a two-way coupling between tidal sand wave dynamics and benthic organisms. With this model we are able to study both the effect of benthic organisms on the hydro- and sediment dynamics, and the effect of spatial and temporal environmental variations on the distribution of these organisms. Specifically, we include two coupling processes: the effect of the biomass of the organisms on the bottom slip parameter, and the effect of shear stress variations on the biological carrying capacity. We discuss the differences and similarities between the methodology used in this work and that from ‘traditional’ (morphodynamics only) stability modelling studies. Here, we end up with a 2times 2 linear eigenvalue problem, which leads to two distinct eigenmodes for each topographic wave number. These eigenmodes control the growth and migration properties of both sand waves and benthic organisms (biomass). Apart from hydrodynamic forcing, the biomass also grows autonomously, which results in a changing fastest growing mode (FGM, i.e. the preferred wavelength) over time. As a result, in contrast to ‘traditional’ stability modelling studies, the FGM for a certain model outcome does not necessarily have to be dominant in the field. Therefore, we also analysed the temporal evolution of an initial bed hump (without perturbing biomass) and of an initial biomass hump (without perturbing topography). It turns out that these local disturbances may trigger the combined growth of sand waves and spatially varying biomass patterns. Moreover, the results reveal that the autonomous benthic growth significantly influences the growth rate of sand waves. Finally, we show that biomass maxima tend to concentrate in the region around the trough and lee side slope of sand waves, which corresponds to observations in the field.

Highlights

In large parts of tide-dominated sandy shelf seas, such as the North Sea, rhythmic bed patterns can be observed [53]
Amongst the various types of marine bed forms, tidal sand waves (Fig. 1a) are often the most relevant type to investigate from an engineering point of view, due to their dimensions and dynamic behaviour
Our model shows that the biomass of benthic organisms and sand waves develop in antiphase, which is supported by observations in the field

Summary

Introduction

In large parts of tide-dominated sandy shelf seas, such as the North Sea, rhythmic bed patterns can be observed [53]. Migration rates can be up to 10 m per year [45], and they form on a time scale of decades [26]. These shallow coastal seas form the habitat for numerous different benthic communities, within which large spatial and temporal variations are observed. These variations are often related to geomorphological patterns of various dimensions [1, 42, 44, 46]. Previous studies have shown that benthos in turn may significantly affect the local hydro- and sediment dynamics, and thereby the morphological development [29, 55]

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Discussion

Conclusion