Field measurements of flow fields and sediment transport above mobile bed forms

Abstract

Field measurements of hydrodynamic conditions, bed forms, and suspended sediments, obtained using a suite of approximately colocated sensors above a mobile bed composed of coarse sand (D50 = 1.28 mm) in a strong tidal current with superimposed waves are used to investigate the mean and turbulent flow fields, bed forms, and the resuspension and transport of sediments in suspension and as bed load. While there is no evidence for flow separation, properties of the local turbulent flow field are found to vary spatially in response to the bed morphology. Time‐averaged suspended sediment concentration profiles, c‐profiles, are found to be spatially variable with higher concentrations observed in the trough regions between bed form crests. Well‐known expressions used to predict c‐profiles are found to agree well with measured time‐averaged c‐profiles. The instantaneous suspended sediment concentration field (SSCF) is found to comprise a series of semiperiodic resuspension events interspersed by relatively quiescent periods. Using a modified quadrant analysis scheme, these events are shown to be correlated closely with times when current and wave‐induced flows combine and exceed the threshold for suspension. The effectiveness of a given combined wave‐current flow event in resuspending bed sediments is found to depend upon its location relative to bed morphology. It is considered that local bed response may be influenced by spatial differences in grain size and compaction brought about by the processes of bed load transport over the bed forms. The total mass of sediment in suspension is found to be less than the potential transport capacity of the flow suggesting strongly that the supply of fine sediments is source limited. While measured bed form geometry agrees well with predicted dimensions for ripples, grain size alone suggests only dunes should be present. These were not detected. Bed load transport, Qb, calculated from measured bed form migration speeds and the measured suspended sediment flux, Qs, was found to agree favorably with empirical predictions.