U.S. Army Corps of Engineers

New Orleans District

Hydraulic Assessment of the
Vermilion River
Operations and Maintenance
Program

February 2020

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 Hydraulic Assessment of the
Vermilion River
Operations and Maintenance Program

Contents
1.0 Summary
2.0 Assessment Overview
3.0 Modeling
4.0 Results
4.1 Partial Dredging Evaluation
4.2 Full Dredging Evaluation
5.0 Conclusion
5.1 Considerations
5.2 Path Forward

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 Hydraulic Assessment of the
Vermilion River
Operations and Maintenance Program
1.0 Summary
A 2-day low level pressure system brought
heavy rainfall to parts of the Acadiana parishes
of Lafayette, Iberia, St. Martin and Vermilion
in August 2016. During the event, the
Vermilion River in Lafayette experienced a
rise from 6 feet at the Surrey Street gage on
August 12 to a record 17.62 feet on August 15,
2016. Overall, more than 7,000 structures
located within these four parishes experienced
flood damage as a result of this historic
weather event.
Subsequent to this event, the U.S.
Army Corps of Engineers modeled the
Vermilion River to determine if dredging were
to occur, would there be any associated
USGS 07386880: Vermilion River at Surrey St. at Lafayette, LA . Gage reading from
impacts, both positive or negative, upstream
August 10 to August 16, 2016. Source: http//nwis.waterdata.usgs.gov/usa/nwis/uv/
and downstream along the Vermilion River.
Once the New Orleans District became aware
of an existing modeling effort that could support the District’s own assessment, the U.S. Army Corps of Engineers
contracted the University of Louisiana-Lafayette and entered into a contract to conduct the hydraulic assessment.

2.0 Assessment Overview
Authorized by the Flood Control Act of 18 August 1941, the Bayou Teche and Vermilion River Operations and
Maintenance project provides for a –8 ft by 80 ft channel from the Vermilion Bay to the Gulf Intracoastal Waterway
(GIWW); a –9 ft by 100 ft channel from the GIWW to mile 52 in Lafayette, La.; for navigation and flood control
improvement of the non-navigable channels of Vermilion River and Bayou Fusilier from Lafayette, La. to Bayou Teche at
mile 79; and channel enlargement in Bayou Teche from two miles betow Arnaudville to Port Barre, La.
The last funded dredging effort for this project took place in 1998. In the aftermath of the 2016 floods, U.S. Army
Corps of Engineers received numerous requests both to dredge the Vermilion and requests to not dredge the Vermilion due
to perceived benefits or impacts. In 2019, the U.S. Army Corps of Engineers began to model the Vermilion river to
determine whether there were positive or negative impacts associated with any dredging action. The modeling effort would
evaluate two approaches to dredging:
1. Dredge the Vermilion River to project dimensions as authorized by the 1941 Flood Control Act.
2. Undertake a partial dredging approach to address specific sites, to include 15 miles of river located within Lafayette.
For the purpose of this evaluation, these approaches would be compared to the existing conditions using two
weather events: a heavy rainfall modeled from the August 2016 event (large-scale) and a smaller, more frequent rain event
(10-year) modeled after rainfall that occurred in 2014.

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 Hydraulic Assessment of the
Vermilion River
Operations and Maintenance Program
3.0 Modeling
The authorized purpose of this hydraulic assessment was to evaluate what stage lowerings, if any, could result from either
of the dredging scenarios referenced in section 2.0. The modeling effort was undertaken by the University of LouisianaLafayette (ULL) in close partnership with the U.S. Army Corps of Engineers, New Orleans District.
The model is a Hydrologic Engineering Center's (CEIWR-HEC) River Analysis System (HEC-RAS). HEC-RAS is
designed to perform one and two-dimensional hydraulic calculations for a full
network of natural and constructed channels. For this particular model, a drainage area of 601 miles2 was modeled. The total river length modeled was approximately 69.2 miles. An interesting feature of the topography of the Vermilion
Watershed is the relative flatness of the slope of the Vermilion river, which has
an average slope of 1 foot for every 10 river miles.
The model utilizes 2,168 surveyed river cross section and over 500,000
individually mapped points along both the Vermilion River and its tributaries.
ULL utilized bathymetry and survey maps provided by the New Orleans District to aid in the development of the model grid. In addition to a well configured channel geometry, the model also includes a number of key hydraulic
structures such as Fusilier Weir and Ruth Canal Structure, as well as 12 major
bridge crossings along the Vermilion River. The model also includes the latest
LiDAR topographical data from the State of Louisiana with 5m resolution.
The model is fully calibrated and validated to a number of observed
events. The model was able to effectively recreate observed phenomena on the
Vermilion such as reverse flow and even flow into and out of the adjacent Bayou Tortue swamp area. The results of the validation indicate this is a highly
credible model that can accurately represent changes along the Vermilion river
under multiple scenarios.
In conjunction with the New Orleans District, ULL ran a number of
scenarios including a partially dredged Vermilion channel, mainly through
Vermilion Watershed captured in ULL’s HEC-RAS model
downtown Lafayette, and another scenario where the channel was returned to its
fully authorized Congressional dimensions (9’ x 100’ from river mile 52 to the GIWW).

Model calibration/validation from ULL’s model

Model output data along the Vermilion for multiple scenarios

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 Hydraulic Assessment of the
Vermilion River
Operations and Maintenance Program
4.0 Results
4.1 Partial Dredging
The hydraulic modeling indicated
that the partial dredging approach
would require removal of approximately 1 to 1.5 million cubic yards
of sediment at an estimated cost of
approximately $75 million. These
costs were derived from similar
work in the New Orleans District
and are rough orders of magnitude. This work would result in an
average stage reduction between
two to three inches with a maximum of roughly a half foot in a
relatively small portion of the Vermilion River. However, the modOutput of stage lowering along Coulee Des Poches from the RAS Model; a very limited effect upstream occurs
eling also indicates that the partial
dredging approach could result in a stage increase of between two and three inches along the lower river in Vermilion Parish during a higher frequency event such as one that occurred in 2014 (10 year event). While it was outside the scope of
this assessment, a very preliminary GIS analysis determined that less than 100 structures were removed from flooding via
dredging activities.

A very limited stage lowering occurs on the Vermilion River during the modeled 2016 event with partial dredging

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 Hydraulic Assessment of the
Vermilion River
Operations and Maintenance Program
4.2 Fully Dredging the River to Authorized Project Dimensions
Fully restoring the Vermilion River to its 1941 authorized dimensions would require a significant investment on behalf of
both federal and local partners. Approximately 4 million cubic yards of sediment would have to be removed to reach project dimensions. This effort is estimated to cost approximately $100-150 million and would require substantial environmentally approved-disposal areas. Again, these costs were derived from similar work in the New Orleans District and are
rough orders of magnitude. A very preliminary GIS analysis suggests that approximately 250 structures would be removed
from flooding were the channel returned to authorized dimensions.
Illustrated
comparison of
structural
impacts related
to the 2016 rain
event under
existing
conditions (Left)
and with a fully
dredged river
(Right).

Above: Similar to the partial dredge scenario, limited
upstream effects were observed for the full dredging
scenario (green line in the blue shaded region above)

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 Hydraulic Assessment of the
Vermilion River
Operations and Maintenance Program
5.0 Conclusion
5.1 Considerations
In the aftermath of 2016, there was a great call to dredge the Vermilion. As the agency authorized to dredge the River under its operations and maintenance program, it was important for the U.S. Army Corps of Engineers to conduct this hydraulic assessment to ascertain whether or not there would be any negative impacts either upstream or downstream. The
results of this analysis show that there are limited positive and negative impacts on water stages. Additionally, while not part
of the original scope of the hydraulic assessment, a cursory investigation into what effect dredging either a partial or full
portion of the authorized Vermilion River was also completed. The results of that limited analysis is that very few structures would be removed from flooding by dredging, approximately 100 due to the partial dredging and approximately 250
structures due to the full dredging of the river.

5.2 Path Forward
The process for evaluating, identifying and implementing flood risk management strategies in South Louisiana must be undertaken using the best science, data and engineering available. This USACE-ULL modeling
effort was conducted with a very specific purpose: does dredging the Vermilion create any positive or negative
impacts upstream or downstream? The results of this analysis are clear: there is very limited stage reductions
associated with dredging the Vermilion River and thus there are limited impacts associated with dredging.
However, despite not yielding the results anticipated or hoped for by many citizens, the data and lessons
learned from this work has real value in helping move forward the overall local, state and federal mission of
identifying the best approach, or combination of approaches, for delivering much needed flood damage risk reduction to the people of Lafayette, Iberia, St. Martin and Vermilion parishes.
The U.S. Army Corps of Engineers is committed to working with its local and state partners in seeking
floodplain management alternatives for this region. We are moving to enter into a Memorandum of Understanding (MOU) with the Louisiana Watershed Initiative (LWI), a task-force convened by Governor John BelEdwards to serve as the program through which floodplain management responsibilities are coordinated across
federal, state and local agencies. In addition, LWI is intended to be supported by experts who serve as advisors
in building a foundation of data, projects, policies, standards and guidance. The MOU that the New Orleans
District intends to enter into will codify our involvement in future modeling and planning activities that the State
undertakes in the coming years as regional level watershed models are developed and utilized.
The results of the analysis will be shared with Acadiana Planning Commission (APC) and LWI to advance future efforts at a State/Regional level with support from the U.S. Army Corps of Engineers. In the next
near term, APC will be convening a public meeting where the results of ULL and U.S. Army Corps of Engineers
modeling efforts on the Vermilion will be discussed. Additionally, APC intends to invite modelers involved with
the LWI-led state-wide regional modeling initiative. More information will be forthcoming on this public meeting in the coming weeks.

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