Pulsational gravity-driven sediment transport on two energetic shelves

Field observations of shoreface benthic processes made in the Middle Atlantic Bight off Duck, N.C. and on the Eel River shelf off northern California have yielded several types of evidence that gravity-induced downslope transport of suspended sediment may constitute an important, but episodic, mode of across-shelf transport during storms. Observed manifestations of the inferred gravity flows include side scan images showing abrupt interfaces between fine sediment “blankets” and coarser sands, rapid bed accretion during the waning phases of storms and short-lived periods of accelerated seaward sediment flux following groups of higher waves. To explore the possibility that gravity-driven transport may be episodically significant in energetic shoreface and shelf environments, we applied a relatively simple analytical model that considers the effects of wave and current turbulent friction in modulating the downslope response of turbidity-induced gravity currents as well as in suspending sediment to produce negative buoyancy. The theory was applied to data on benthic flows from three phases of a storm event off Duck in October 1994. Results indicate that the gradient Richardson number, Ri, remained appreciably below the critical value of 0.25 during average conditions and during high wave groups. During lulls, however, Ri briefly became supercritical within a few tens of centimeters above the bed implying short-lived turbulence suppression. On the Eel River shelf, direct observations show that, within the thin ((O)10 cm) muddy layer, Ri values also fluctuated between subcritical (<1/4) during groups and supercritical (>1/4) during lulls again implying that stratification suppressed production of turbulence during lulls. Downslope transport was observed during both high and low wave packets but accelerated during lulls as in the Duck case. We conclude that gravity-induced downslope transport of sediment suspended by waves is likely to be an important mode of across-shelf transport on the inner shelf of energetic sandy environments and the mid-shelf of energetic muddy environments. Such transports are most likely pulsational, with high turbidities and buoyancy anomalies being created by the high stresses associated with groups of large waves and moderately rapid but brief downslope movement occurring when bed stresses are temporarily relaxed during the lulls between high groups. This mechanism operates out of phase with group-forced long waves. [Copyright &y& Elsevier]