Geological Society of America

20 Transport and Sediment Dynamics in Lowland Rivers

Tuesday, 29 March 2011
3:00 PM - 5:30 PM
D.H. Holmes A & B

THREE-DIMENSIONAL MODELING OF DIVERSIONS FROM THE LOWER MISSISSIPPI RIVER

Paper number: 20-9
Program: Theme Sessions
Time: 5:00 PM – 5:15 PM

Authors

Joao F. Pereira Presenting Author
University of New Orleans, New Orleans, LA
Mallory A. Davis
University of New Orleans, New Orleans, LA
John A. McCorquodale
University of New Orleans, New Orleans, LA
Ioannis Y. Georgiou
University of New Orleans, New Orleans, LA
Ehab A. Meselhe
University of Louisiana at Lafayette, Lafayette, LA
Mead A. Allison
Jackson School of Geosciences, Austin, TX
John A. Lopez
Lake Pontchartrain Basin Foundation, New Orleans, LA

Abstract

The presence of man-made levees along the Lower Mississippi River (MR) has significantly reduced the River sediment input to the wetlands. Much of the River’s sediment is now lost to the Gulf of Mexico. Historically, this input maintained a closer balance between the subsidence and deposition rates throughout the delta, when compared to modern rates. In addition, to the construction of levees, the sediment load has been decreased by dams and river revetments along the Upper MR. Freshwater and sediment diversions are possible options to help combat land loss. Numerical modeling of hydrodynamics and sediment transport of the MR is a useful tool to evaluate restoration projects and to improve our understanding of the resulting River response. The emphasis of this study is on the fate of sand in the river and the distributaries.

In 2010, a 1-D HEC-RAS (USACE 2008) unsteady flow hydrodynamics model was developed for the Lower MR from Tarbert Landing (RM 306) to the Gulf of Mexico, including distributaries and existing diversions. The effects resulting from existing and proposed diversions were modeled. A 3-D unsteady flow mobile-bed model (ECOMSED; HydroQual 2002) of the Lower MR reach between Belle Chasse (RM 76) and Main Pass (RM 3) was calibrated using field sediment data from 2008 – 2010 (Allison, 2008;2010). The model was used to simulate River currents, diversion sand capture efficiency, erosional and depositional patterns with and without diversions. The introduction of new diversions at different locations, e.g., Myrtle Grove (RM 59) and Belair (RM 65), with different geometries and with different outflows was studied.

The simulated diversions varied from 1,000 cfs to 200,000 cfs for river flows up to 1.2x10⁶ cfs. The model showed that smaller diversions (less than 30,000 cfs) had little impact on the downstream sand transport. However, the diversion of more than 10% of the river flow had the following effects: 1) reduction in the slope of the hydraulic grade line; 2) reduction in the available energy for transport of sand along distributary channels; 3) reduced sand transport capacity in the main channel downstream of the diversion; 4) increased shoaling downstream of the diversion; and 5) a tendency for erosion and possible head-cutting upstream of the diversion.