Observations of the boundary layer characteristics and the suspension of sand at a tidal site

Abstract

Measurements of the flow velocity and suspended sediment profiles through one tidal cycle were taken from an observation platform in the Outer Thames Estuary at Maplin Sands. The flow velocity profile during the experiment was characteristic of an oscillatory boundary layer with the height of the bottom logarithmic portion growing through the tide, and with increasing velocity, especially on the accelerating flood tide. The vertical gradients in the sediment concentration and grain size were large on the flood tide and negligible on the ebb tide with the threshold of suspension occurring at a depth averaged flow velocity of about 0.3 m s −1. The size of sediment in suspension varied in a systematic way through the tide in response to the changing magnitude of the flow. The median grain size d 50 in the suspension measured at 0.10 m above the bed increased with increasing flow speed and at the peak of the tide was equal to the d 10 of the bed material. The stratification of the water column by suspended sediment was not important and the profile measurements were used to determine the vertical form of the momentum and sediment diffusivity coefficients K M and K S. An estimate of the ratio β = K S/ K M = 0.65 was obtained.