NEW APPROACH FOR REFERENCE CONCENTRATION

摘要

We examine reference concentration using three different data sets of near-bedrnsuspended sediment concentration observed under combined waves and currents. The datarninclude observations made at 15 and 20-m depth off Dounreay, Scotland, UK and observationsrnobtained at 13-m depth off Duck, North Carolina, USA. These data accommodate differentrndynamic conditions (from wave-dominated at Dounreay to wind-driven current-dominated atrnDuck) and sediment properties (median size of bed sediment ranging from 120 to 350 μm). Nearbedrnconcentration profiles to elevations of about 80 cm were obtained using Acoustic BackscatterrnSensors with 1-cm resolution. The reference concentrations (Cr) at 1 cm were then evaluated byrnregressing the observed suspended sediment concentrations against a Rouse-type model. Bedrnshear stresses associated with each estimate of Cr were estimated using the wave-and-currentrninteraction model of Grant and Madsen. Existing equations for reference concentrations based onrnshear stress alone fail to accommodate all observations from different environments. Wernintroduce a new, empirical relationship between Cr and the product of Shields and inverse Rousernnumbers. These dimensionless parameters represent, respectively, the ratio of shear stress andrnsubmerged particle weight, and the ratio of shear velocity and particle settling velocity. The newrnformula adjusts the amount of mobile sediment at the bed (embodied in the Shields number) tornthat available for suspension at the reference height (embodied in the Rouse number). The newrnformula for reference concentration accommodates observations from different environments in arnconsistent way, suggesting that it may have wide applicability on sandy inner shelves.