Fluvial backwater zones as filters on source to sink sediment transport (Invited)

Sediment flux from rivers to oceans is the fundamental driver of fluvio-delatic morphodynamics and continental-margin sedimentation, yet sediment transport across the river to marine boundary remains poorly understood. Rivers near their mouths typically are affected by backwater, a zone of spatial decelerating flow and concave water-surface profile that is transitional between normal flow upstream and static water beyond the shoreline. Deceleration in the backwater zone, as well as spreading of the offshore plume should render rivers highly depositional near their mouths. Field observations have shown, however, that the riverbed within the backwater zones of some rivers (e.g., Mississippi River) show flutes and potholes along the riverbed indicating scour into the underlying substrate. We hypothesize that zones of backwater can transition to zones of drawdown and erosion at high flows. Numerical modeling results show that such a scenario is possible where river plumes spread laterally beyond the shoreline. Model results compare favorably with measurements of river water-surface elevation and velocity over a range of discharges for the lower Mississippi River. Our results suggest that that fluvial sediment export to marine environments can be pulse like, where backwater zones retard sediment export at low flows and enhance sediment export at high flows.