Backwater and River-Plume Controls on Scour Upstream of River Mouths: Implications for Fluvio-Deltaic Morphodynamics and Stratigraphy

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 is poorly understood. Coastal rivers typically are affected by backwater, a zone of spatially decelerating flow that is transitional between normal flow upstream and the offshore river plume. Flow deceleration in the backwater zone, as well as spreading of the offshore plume, should render rivers highly depositional near their mouths, leading to sedimentation and eventual elimination of the backwater zone at steady state. This reasoning is counter to observations of riverbed scour, erosional bedforms, and long-lived backwater zones near the mouths of some coastal rivers (e.g., Mississippi River). To explain these observations, we present a quasi-2D model of a coupled fluvial backwater and offshore river-plume system for the case of the Mississippi River. Results show that during high-discharge events the normal-flow depth can become larger than the water depth at the river mouth resulting in drawdown of the water surface, spatial acceleration of flow, and surprisingly erosion of the riverbed. Furthermore, it is the transient adjustment of the river to low flow and high flow events that allows a persistent backwater/drawdown zone. This zone in turn act as a filter on sediment transfer to marine environments whereby sediment flux from low-discharge events is muted and sediment flux from high-discharge events is enhanced from what would be expected from normal flow alone. Backwater dynamics and the potential for scour are rarely accounted for in fluvio-deltaic models, but they could have a significant impact morphodynamics and stratigraphy. For example, we show that backwater combined with variable discharges leads to a preferential deposition zone, which may explain why rivers tend to avulse about a persistent node setting the fundamental lengthscale of deltas. Furthermore, scour events in coastal rivers and distributary channels likely leave unconformities in fluvio-deltaic stratigraphy, which may appear similar to those previously interpreted to be a result of relative sea level changes or other allogenic forcings. Results are compared to flume experiments presented in a companion study by Chatanantavet et al.
(2011).

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California