Mississippi Valley Division

Engineer Research and Development Center

Southwest Pass Outlets Bathymetry and Flow 

Distribution Assessment

Mississippi River Geomorphology and Potamology Report No. 5

The Southwest Pass distributary

of the Mississippi River is the 

primary navigable channel 

connecting the Gulf of Mexico 

and the Mississippi River. The 

average expenditure of the U.S. 

Government on Southwest Pass 

and the lower Mississippi River 

dredging has averaged 

approximately $55 million/year 

for the previous 5 years. 

Numerical modeling tools 

covering this important 

navigational route are being 

developed to provide a better 

understanding of the impact of 

proposed Mississippi River system changes. Understanding the flow exchange between the 

Southwest Pass distributary and the surrounding Gulf of Mexico under various river and 

tide conditions is essential to the development of these analytic modeling tools to be used 

to predict system response to proposed upriver diversions and dredging templates resulting 

from navigable channel deepening proposals. The ongoing development of these 

numerical modeling tools has revealed a data need for bathymetry and velocity 

information for the four major lateral outlets of the Southwest Pass. Multi-beam channel 

bed surveys and Acoustic Doppler Current Profiler data were collected over 3 days. The 

collected data will be used to calibrate the flow exchange and to improve the predictive 

ability of the multidimensional numerical models and the resulting water/sediment ratios at 

the four lateral outlets.

Multi-Beam and Single-Beam Surveys

Survey data were collected between 30 June and 17 July 2014 by Chustz Surveying Inc. 

Topographic data were collected using a Trimble TSC 3, Trimble R8, and Samsung 

Galaxy Rugby. The hydrographic data were collected using an Odom Hydrotrac Echo 

Sounder and a Teledyne RESON SeaBat 7101 multi-beam system. All of the investigated 

Southwest Pass outlets displayed scour holes of varying degree in size and depth just 

downstream of their entrances. Burrwood Bayou is unique in that it has two scour holes, 

the largest of which is on the Gulf side of the emergency closure structure, which would 

indicate that the flow through Burrwood Bayou is controlled by the emergency closure 

structure as intended.
Points of Contact

Steven K. Ayres

Author

USACE MVN

Steven.K.Ayres@usace.army.mil

Freddie Pinkard

MRG&P, Sr. Program Manager

USACE MVK

Freddie.Pinkard@usace.army.mil

Barb Kleiss, Ph.D.

MRG&P Technical Director

USACE MVD

Barbara.A.Kleiss@usace.army.mil

Acoustic Doppler Current Profiler Data

Acoustic Doppler Current Profiler measurements were taken at each site, including measurements in the outflow 

channels, and upstream and downstream of the outflow channel in Southwest Pass. Each location included 

measurements conducted under a variety of tide conditions. The measurements were conducted on 13–15 May 2014 

using two RDI 600 MHz WorkHorse Rio Grande Broadband Acoustic Doppler Current Profiler.

For More Information

To access the full version of Southwest Pass Outlets Bathymetry and Flow Distribution Assessment report, click the 

following link: http://acwc.sdp.sirsi.net/client/en_US/search/asset/1044567.

Additional MRG&P reports, historic reports, and MRG&P information can be accessed from the technology transfer 

section link at the MVD Historic Studies website:

http://www.mvd.usace.army.mil/Missions/MississippiRiverScienceTechnology/MSRiverGeomorphologyPotamology/

FieldData/HistoricStudies.aspx.

Summary of Significant Findings

 Given certain tidal conditions and Mississippi River 

discharge, the four lateral outlets along Southwest Pass 

are capable of diverting almost half of the flow entering 

Southwest Pass from the lower Mississippi River 

channel.

 The impact of these lateral outlets on Southwest Pass 

flow and sedimentation dynamics must be considered in 

any model study that includes Southwest Pass in its 

domain. 

 The loss of stream power along the channel due to the 

diversion of water through the outlets is likely a primary 

driver of resulting shoaling patterns in Southwest Pass. 

 The flow stratification evident in the velocity profiles 

indicates that flocculation due to salinity contributes to 

the sedimentation characteristics observed in Southwest 

Pass.