Influence of wave groups on SSC patterns over vortex ripples

Estimates of spatial and temporal variations in suspended sand concentratio ns (SSC) made with a multi-transducer Acoustic Backscatter Sensor (ABS) und er a repeated wave group over a mobile rippled bed in the wave research flu me at the National Hydraulics Laboratory in Ottawa, Canada, reveal an numbe r of complex and intriguing patterns. Ensemble averages of 8 nearly identic al wave groups provided much more robust estimates of SSC and allowed a det ailed examination of the wave group effects. The largest SSC near the bed ( < 0.10 m) occurs in phase with the largest waves in the group. Above approx imately 0.10 m elevation, SSC lags behind the near bed SSC by as much as 2- 3 waves; introducing significant curvature ton a semi-log plot) to the SSC profile. The log linear segments of the SSC profile grow and decay systemat ically on the scale of the wave group. The range in lengths of log-linear p rofile segments (similar to0.03-0.355m) suggest that the boundary layer thi ckness also fluctuates throughout the passage of the wave group, Furthermor e, there are significant variations in the patterns of SSC, which occur und er the largest and smallest waves in the group. Under the largest waves ver tical bands of alternating high and low SSC produce an intra-wave modulatio n in the upper water column (similar to 0.075-0.30 m), The equivalent horiz ontal excursion of these bands scales to the ripple length. Under the small er waves the intra-wave modulation of the SSC disappears and is replaced by temporally homogenous suspension that expands vertically through several i ndividual wave cycles. The former pattern of homogenous suspension appears to be associated with growth of a boundary layer due to the persistent unid irectional horizontal flow during this part of the group together with the persistence of antecedent bed generated turbulence and vorticity which main tains the suspension. The latter pattern of bands of high and low SSC indic ates a strong temporal and spatial constraint on the SSC (phase coupling) i nduced by the presence of the bedforms which may be enhanced bystrong rever sals in both flow and vorticity under the large waves in the group. (C) 200 0 Elsevier Science Ltd. All rights reserved.