Examination of reference concentration under waves and currents on the inner shelf

Abstract

[ 1] We examine reference concentration using three different data sets of near-bed suspended sediment concentration observed under combined waves and currents. The data include observations made at 15 and 20 m depth off Dounreay, Scotland, UK, and observations obtained at 13 m depth off Duck, North Carolina, USA. These data accommodate different dynamic conditions ( from wave-dominated conditions at Dounreay to wind-driven, current-dominated conditions at Duck) and sediment properties ( median size of bed sediment ranging from 120 to 350 mm). Near-bed concentration profiles to elevations of about 80 cm were obtained using acoustic backscatter sensors with 1 cm resolution. The reference concentrations ( C-r) at 1 cm were then evaluated by regressing the observed suspended sediment concentrations against a Rouse-type model. Bed shear stresses associated with each estimate of Cr were estimated using the wave-current interaction model of Grant and Madsen. Existing equations for reference concentration based on shear stress alone fail to accommodate all Cr estimates from different environments. We introduce a new empirical relationship between Cr and the product of Shields and inverse Rouse numbers. These dimensionless parameters represent the ratio of bed shear stress and submerged particle weight and the ratio of shear velocity and particle settling velocity, respectively. The new formula adjusts the amount of mobile sediment at the bed ( related to the Shields number) to that available for suspension at the reference height ( related to the inverse Rouse number). The new formula for reference concentration accommodates observations from different environments, suggesting that it may have wide applicability on sandy inner shelves.