Reports: DNI850248-DNI8: Backwater Controls on Fluvial Morphodynamics near River Mouths

Michael Lamb, PhD , California Institute of Technology

Research Problem: 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 spatial 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).

Numerical Modeling: To explain these observations, we have developed a quasi-2D numerical model of a coupled fluvial backwater and offshore river-plume system for the case of the Mississippi River.  Model results (Lamb et al., in review) 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.  The transition to drawdown and erosion is ultimately forced by spreading of the offshore river plume.  This in turn may affect river avulsion processes and delta building (Chatanantavet et al., in review)

Field Application: Application of the model suggests that the lower 500 km of the Mississippi River experiences erosion for flood events with a recurrence interval larger than ~2 years (Nittrouer et al., in press).  More frequent, smaller discharge floods have a backwater zone that can act as a filter forcing deposition and retarding source-to-sink sediment transfer.  Large flood events with divergent offshore plumes, however, can eliminate the backwater zone resulting in fluvial erosion and enhanced sediment delivery basinward. 

Flume Experiments: To further test these ideas, flume experiments are underway in the Earth Surface Dynamics Laboratory at Caltech.  Using a 5-m long river flume connected to an “ocean” basin we have simulated river bed morphodynamics and delta growth under a wide range of scenarios.  Preliminary experimental results suggest that erosion and deposition in backwater zones may significantly affect levee growth, delta morphology and stratigraphy.

 An unexpected consequence of scaling ripple bedforms in the flume experiments has led to the development of a new bedform stability diagram for sandy beds with application to Earth and Mars (Lamb et al., in press).

 Publications: To date, this work has resulted in two manuscripts in press, two manuscripts in revision and six conference presentations (American Geophysical Union Fall Meeting, American Geophysical Union Chapman Conference, American Association of Petroleum Geologists Meeting, Geological Society of America Meeting).

 Research Team: To date, this grant has supported three summer undergraduate fellows who assisted with the experiments in 2011 and a postdoctoral scholar (Phairot Chatanantavet).

Papers in press

Lamb, M.P., Grotzinger, J., Southard, J.B., Tosca, N., in press, Were ripples on Mars formed by flowing brines?  in J. Grotzinger and R. Milliken (eds.), Sedimentary Geology on Mars, SEPM Special Publication

Nittrouer, J.A., Shaw, J., Lamb, M.P., Mohrig, D., in press, Spatial and temporal trends for water-flow velocity and bed-material transport in the lower Mississippi River.  Geological Society of America Bulletin.

Papers in review

Chatanantavet, P., Lamb, M.P., and Nittrouer, J.A., in revision, Backwater controls on avulsion location on deltas, Geophysical Research Letters.

Lamb, M.P., J. Nittrouer, D. Mohrig, J. Shaw, in review, Backwater and river-plume controls on scour upstream of river mouths. Journal of Geophysical Research – Earth Surface.

Conference Abstracts

Nittrouer, J., Petter, A., Mohrig, D., Chatanantavet, P., Lamb, M.P., 2011, Backwater flow dynamics in lowland rivers, influence on channel avulsions and the development of fluvial-deltaic stratigraphic architecture, GSA Abstracts with Programs Vol. 43, No. 5.

Mohrig, D., Lamb, M.P., Nittrouer, J.A., 2011, Direct transfer of sand from shelf-edge deltas to the continental slope, American Association of Petroleum Geologists, Search and Discovery Article 90124. **Certificate of Recognition for outstanding presentation.

Nittrouer, J. A.; M. Lamb; J. Shaw; D. Mohrig, 2011, Predicting the time and space properties of bed-material transport in the normal-flow to backwater transition of the lowermost Mississippi River. Chapman Conference, AGU.

Lamb, M. P.; J. Nittrouer; P. Chatanantavet; B. McElroy; B. Kopriva; D. Mohrig; J. Shaw, 2011, The influence of fluvial-backwater and plunge-point dynamics on hyperpycnal plume generation, Chapman Conference, AGU.

Lamb, M.P., Nittrouer, J., Mohrig, D., McElroy, B., and Shaw, J., 2010, Fluvial backwater zones as filters on source to sink sediment transport. American Geophysical Union. Invited.

Nittrouer, J., Shaw, J., Lamb, M., Mohrig, D., 2010, Modeling Water-flow Velocity and Bed-material Sediment Transport in the Normal-flow to Backwater Transition of the Lowermost Mississippi River, EOS Trans. AGU.

Lamb, M. P., D. C. Mohrig, B. J. McElroy, B. Kopriva, J. Shaw, 2010, Source-to-Sink: Connecting Hyperpycnal-Flow Deposits to River-Flood Dynamics. American Association of Petroleum Geologists Annual Convention.

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