Field investigation of bedform morphodynamics under combined flow

Abstract

Fieldwork was conducted at the Red Cliff sandbar located in the upper Humber Estuary in order to investigate bedform dynamics under different hydrodynamic conditions related to combinations in tidal and wave-generated currents. A fixed mooring was deployed to obtain current flow and wave properties across the sand bar site at both spring and neap tidal flows and during conditions of high wind generated waves. A terrestrial laser scanner was used to scan the sandbar during low water across the various forcing conditions, acquiring detailed information of ripple geometries. Under spring tide and calm wave conditions, two-dimensional asymmetrical ripples with straight crest lines were generated on the sandbar. Surveys were also conducted during strong winds. Prior to high-wind-waves, two-dimensional symmetrical and washed-out ripples were observed, during spring and neap tidal conditions respectively. The lengths of these ripples were almost the same as those generated under current only conditions, but their heights were relatively smaller. After the strong wind periods, 2D current-induced ripples were replaced by flatbed conditions and 2D symmetrical wash-out ripples, which indicates wave-induced bed shear stress is enhanced by the presence of a tidal current. Most pertinently our study reveals discrepancies between field observations and existing predictions of bed configurations that are largely based on laboratory investigations. This is likely due to a larger grain size distribution in field conditions and due to turbulence dampening by high concentrations of suspended clay particles. The study thus highlights a need to extend a range of field investigations that explore the current deficiencies in our abilities to predict bedforms and bedform dynamics in estuarine systems.