Suspended-sediment reference concentration under waves: field observationsand critical analysis of two predictive models

Two commonly adopted but fundamentally different approaches for predicting time-averaged suspended-sediment reference concentration ((C) over bar(REF) ) under waves are tested against field measurements and compared with each other. The first model relates (C) over bar(REF) to the cube of the non-dim ensional skin friction, whereas the second model adopts a more complex func tion of excess skin friction incorporating the empirical constant gamma(0). The dataset is from the zone of wave shoaling seaward of an open-coast sur fzone and includes measurements of waves, currents, suspended sediment and bedforms. Estimates of (C) over bar(REF) are derived from acoustic backscat ter data, and the seabed and suspension process are described from video fo otage. When waves were energetic, the bed was deformed into large hummocks; during less energetic conditions, the bed was rippled. The time-averaged c oncentration profiles over the ripples were consistent with settling flux b alanced by pure gradient diffusion and a sediment diffusivity that is const ant with elevation above the bed. (C) over bar(REF) in that case is shown t o apply at z = 0, where z is the elevation above the bed. Over the hummocks , there was a sheet flow at the base of the suspension and (C) over bar(REF ) is shown to apply at z = 1 cm. The concentration profiles over the hummoc ks implied sediment diffusivity that varied linearly with elevation within similar to 10 cm of the bed and constant sediment diffusivity above that le vel. For both rippled and hummocky beds, gamma(0) derived from the field da ta was found to be sensitive to the value assumed for critical stress for i nitiation of sediment motion, which could explain the range of values repor ted in the literature for gamma(0). gamma(0) was also found to vary in a co mplex way with skin friction, which suggests that the reference-concentrati on model based on excess skin friction is not correctly formulated. Neverth eless, two functions for gamma(0) (one applying to rippled beds and the oth er to hummocky beds) were contrived to make the model fit the data. The mod el based on non-dimensional skin friction was found to be a good predictor of (C) over bar(REF) when a correction was made for flow contraction over r ipples. The correction was not required for the hummocky bed, where sedimen t was being entrained in a thin sheet flow layer. The model based on non-di mensional skin friction correctly portrayed the relationship between flow a nd sediment response without contrivance and therefore should be the favour ed approach in predicting reference concentration. (C) 1999 Elsevier Scienc e B.V. All rights reserved.