Abstract

Measurements of suspended sand profiles using a 3 MHz pulsed acoustic concentration meter (ACM) and calculations of the quantity of bed load that is mobile under combined waves and steady currents, indicate that bed load transport and suspended-sand concentrations are linked through the bed load concentration. The instantaneous bed load transport _v(t) during each burst, was estimated using the method outlined in Grant and Madsen (1979) to obtain the Shields’ Number ψ(t) for a combined wave current _w(t) and steady current _a, and the empirical equation of Vincent et al. (1981), _v(t) = (0.09±0.03) (ψt) -ψ_TH)(t), where ψ_TH is the critical Shields’ Number for the bed sediment and U(t) is the instantaneous current vector (_w(t) + U_a). This equation can be rewritten as; _v(t) = c^*(t)U(t), where c^*(t) is the volume of material that is mobile per unit area of the bed. The average bed load transport rate _v can be found by integrating over a period that is large compared with the wave period. The transport direction will generally not be the same as that of the steady current (Vincent et al., 1983). The ACM profiles over a single burst show that there is a great variation in sand concentrations in the bottom few decametres of the flow, and that concentrations are coherent with the peaks of the wave packets rather than with the passage of individual waves (Clarke et al., 1982). Suspended sand concentration C_z profiles, temporally averaged over a burst (256 s) and then spacially averaged into 10 cm vertical sections, were found to fit closely to a log-linear profile, C_z = C₁(1-A log_e^z/z,), where C₁ is the concentration at height z₁ (=1 cm) and A was empirically determined as 0.22±0.005 (Vincent et al., 1982). A linear correlation was found between c* and the suspended- sand concentration at 1 cm above the bed, C₁. The correlation coefficient was 0.82 (significant at 1%).