Backwater and river plume controls on scour upstream of river mouths: Implications for fluvio-deltaic morphodynamics

• Published in: Journal of Geophysical Research: Earth Surface Vol. 117; no. F1
• Main Authors: Lamb, Michael P., Nittrouer, Jeffrey A., Mohrig, David, Shaw, John
• Format: Journal Article
• Language: English
• Published: Washington, DC Blackwell Publishing Ltd 01.03.2012; American Geophysical Union
• Subjects: Accelerated flow; backwater; Backwaters; Continental margins; Drawdown; Earth sciences; Earth, ocean, space; erosion; Exact sciences and technology; Fluvial sediments; Geomorphology; High flow; Hydrology; Oceans; Offshore; River beds; River mouth; river plume; River plumes; Rivers; Scour; Sediment transport; Sedimentary structures; Sedimentation; Sediments; Upstream; Water depth; United States; Mississippi River; North America, Mississippi R; sediment sources; Deceleration; plumes; North America; offshore; bedforms; continental margin sedimentation; acceleration; coastal zone; dynamics; scours; depth; sediment transport; discharge; erosion; two-dimensional models; estuaries; flow
• ISSN: 0148-0227; 2169-9003; 2156-2202; 2169-9011
• DOI: 10.1029/2011JF002079

Sediment flux from rivers to oceans is the fundamental driver of fluvio‐deltaic 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 bed forms, and long‐lived backwater zones near the mouths of some coastal rivers (e.g., Mississippi River, United States). To explain these observations, we present a quasi‐2‐D model of a coupled fluvial backwater and offshore river plume system and apply it to 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 erosion of the riverbed. As proposed by Lane (1957), the transition to drawdown and erosion is ultimately forced by spreading of the offshore river plume. This points to the need to model coupled river and river plume systems with a dynamic backwater zone under a suite of discharges to accurately capture fluvio‐deltaic morphodynamics and connectivity between fluvial sediment sources and marine depositional sinks. Key Points Rivers are affected by backwater dynamics near their mouths Drawdown during high flow causes scour upstream of river mouths Persistent backwater zones occur due to variable discharges and divergent plumes