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,which should render rivers highly depositional near their mouths, leading to sedimentation.  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., 2012a) 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.  This in turn may affect river avulsion processes and delta building (Chatanantavet et al., 2012).

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 (Lamb et al., 2012a; Nittrouer et al., 2012).  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 were performed in the new Earth Surface Dynamics Laboratory at Caltech.  Using a 5-m long river flume connected to an “ocean” basin we have simulated river bed morphodynamics under a wide range of scenarios.  Experimental results indicate that erosion and deposition in backwater zones can significantly affect the river long profile and stratigraphy (Chatanantavet and Lamb, in prep).

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., 2012b), as well as an analysis of wave ripples in Neoproterozoic rocks that relate to the snowball-earth hypothesis (Lamb et al., 2012c).

Overall Scientific Impact:  Research from this grant has allowed the development of a new framework for the way in which rivers behave near coasts – regions where much of the human population resides, and substantial deposits are constructed of interest for hydrocarbon exploration. Ultimately a better understanding of the controls on the patterns of erosion and deposition in coastal rivers will lead to better models for flood hazards, land restoration, and reservoir quality.

Research Team and Impact: This grant supported four summer undergraduate fellows who assisted with the experiments in 2011 and 2012.  Students learned how to conduct a scientific investigation, collect data and report results.  Most of the funding of this project went to support a postdoctoral scholar, Phairot Chatanantavet.  This project was the first funded project of a new PI, Michael Lamb, at Caltech.  It helped launch his program in coastal river science which now includes a state-of-the-art modeling facility.  Furthermore, the work performed under this award was used as a springboard for a larger research effort in coastal rivers, which was funded in 2012 by the National Science Foundation.


Chatanantavet, P., M. P. Lamb, and J. A. Nittrouer, 2012, Backwater controls on avulsion location on deltas, Geophys. Res. Lett., doi:10.1029/2011GL050197.

Lamb, M.P., J. Nittrouer, D. Mohrig, J. Shaw, 2012a, Backwater and river-plume controls on scour upstream of river mouths: Implications for fluvio-deltaic morphodynamics. Journal of Geophysical Research Earth Surface, v. 117, F01002, doi:10.1029/2011JF002079

Lamb, M.P., Grotzinger, J., Southard, J.B., Tosca, N., 2012b, Were ripples on Mars formed by flowing brines?  in J. Grotzinger and R. Milliken (eds.), Sedimentary Geology on Mars, SEPM Special Publication.

Lamb, M.P., Fischer, W.W., Raub, T.D., Perron, J.T., Myrow, P.M., 2012c, Origin of giant wave ripples in Snowball Earth cap carbonates, Geology, v. 40(9), p. 827-830, doi:10.1130/G33093.1.

Nittrouer, J.A., Shaw, J., Lamb, M.P., Mohrig, D., 2012, Spatial and temporal trends for water-flow velocity and bed-material transport in the lower Mississippi River.  Geological Society of America Bulletin, doi: 10.1130/B30497.1

Chatanantavet, P. and M. P. Lamb, in prep., Experiments on a coupled river and river plume system.

Chatanantavet, P.E., Lamb, M.P., 2012, Experimental Study of a Coupled River and River-Plume System: Backwater Controls on Source-to-Sink Sediment Transfer and Channelization on Deltas, AAPG Search and Discovery Article 90142.

Lamb, M. P., *P. Chatanantavet, J. Nittrouer, D. Mohrig, J. Shaw, 2012, Backwater and River-Plume Controls on Scour Upstream of River Mouths: Implications for Fluvio-Deltaic Morphodynamics and Stratigraphy, AAPG Search and Discovery Article 90142.

Chatanantavet, P.E., Lamb, M.P., and *Nittrouer, J., 2011, Backwater controls on avulsion location on deltas, EOS Trans. AGU EP21A-0657.

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 ProgramsVol. 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.