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US Army Corps
of EngineersCoastal And Hydraulics Laboratory
Engineer Research & Development Center
Numerical Modeling of a Floating
Marsh System
US Army Engineer Research and Development Center
Tate O. McAlpin, USACE Coastal and Hydraulics Laboratory
Joseph V. Letter, USACE Coastal and Hydraulics Laboratory
S. Keith Martin, USACE Coastal and Hydraulics Laboratory
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US Army Corps
of EngineersCoastal And Hydraulics Laboratory
Engineer Research & Development Center
Outline of this Presentation
1) Flotant Marsh Representation in RMA2
2) Frictional Effects in RMA2
3) Davis Pond Project Overview
4) Model Domain, Bathymetry, and Boundary Conditions
5) Model Validation
6) Conclusions
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US Army Corps
of EngineersCoastal And Hydraulics Laboratory
Engineer Research & Development Center
How to Model Hydrodynamic Conditions in
a Floating Marsh?
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US Army Corps
of EngineersCoastal And Hydraulics Laboratory
Engineer Research & Development Center
RMA2’s Method of Dealing with complex
topography/bathymetry
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US Army Corps
of EngineersCoastal And Hydraulics Laboratory
Engineer Research & Development Center
Marsh porosity = Deal with it statistically
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US Army Corps
of EngineersCoastal And Hydraulics Laboratory
Engineer Research & Development Center
Development of effective marsh porosity
0
h
eff
hK K dz
h h
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US Army Corps
of EngineersCoastal And Hydraulics Laboratory
Engineer Research & Development Center
Floatant Marsh Representation using
Marsh Porosity
Case 1a
Case 1d
Case 1cCase 1b
Case 2a Case 2b
Case 2c Case 3a Case 3b
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US Army Corps
of EngineersCoastal And Hydraulics Laboratory
Engineer Research & Development Center
Approximation of nonlinear effects of floating mat
Based on peak
water levels
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US Army Corps
of EngineersCoastal And Hydraulics Laboratory
Engineer Research & Development Center
Marsh Porosity approach
• Because of uncertainty in flotant mat characteristics
• And anticipated non-uniformity in the spatial variation
• Use the existing formulation with calibrated coefficients
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US Army Corps
of EngineersCoastal And Hydraulics Laboratory
Engineer Research & Development Center
Frictional Specification
N-VALUE = RDR0 / (DEPRDCOEF) + (RDRM * e-DEP/RDD0)
RDR0 = Maximum Manning’s n-value for non-vegetated water
RDD0 = Depth at which vegetation effects roughness
RDRM = Manning’s n-value for vegetated water
RDCOEF = Roughness by depth coefficient
DEP = Depth
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US Army Corps
of EngineersCoastal And Hydraulics Laboratory
Engineer Research & Development Center
Inputted Frictional Values
RDR0 RDD0 RDRM RDCOEF
1 (Channel) 0.06 1.00 0.10 0.05
2 (Channel) 0.06 1.25 0.40 0.20
3 (Channel) 0.06 1.00 0.40 0.20
4 (Non-Channel) 0.12 2.00 0.82 0.41
5 (Non-Channel) 0.12 2.50 0.82 0.41
6 (Channel) 0.06 1.25 0.40 0.20
7 (Non-Channel) 0.12 2.25 0.82 0.41
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US Army Corps
of EngineersCoastal And Hydraulics Laboratory
Engineer Research & Development Center
Manning’s Value as a Function of Depth
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US Army Corps
of EngineersCoastal And Hydraulics Laboratory
Engineer Research & Development Center
The Davis Pond diversion
was dedicated in March
2002 as the world’s largest
freshwater diversion. The
project was designed to
divert freshwater flows of
up to 10,650 cfs from the
Mississippi River into the
Barataria Basin by way of
Davis Pond and Lake
Cataouatche. The total
estimated cost of the
project was $106 million.
Davis Pond Diversion Structure
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US Army Corps
of EngineersCoastal And Hydraulics Laboratory
Engineer Research & Development Center
The freshwater inflow reduced
saltwater intrusion and established
improved salinity conditions,
resulting in enhanced vegetation
growth for a healthier estuarine
ecosystem in the Barataria Bay
Estuary. At optimum production the
Davis Pond diversion would preserve
33,000 acres of wetlands and benefit
an additional 777,000 acres of
marshes and bays. The cost benefits
are estimated at $15 million for fish
and wildlife with an additional
$300,000 for recreation.
Project Benefits
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US Army Corps
of EngineersCoastal And Hydraulics Laboratory
Engineer Research & Development Center
Flows in excess of
approximately 40
percent of the design
flow would overwhelm
the projects levees
and spill into healthy
swampland and
adjacent populated
areas. The goal of this
modeling effort was to
determine the best
means of eliminating
this flooding.
Numerical Modeling Goal
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US Army Corps
of EngineersCoastal And Hydraulics Laboratory
Engineer Research & Development Center
Model Domain
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US Army Corps
of EngineersCoastal And Hydraulics Laboratory
Engineer Research & Development Center
Model Bathymetry
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US Army Corps
of EngineersCoastal And Hydraulics Laboratory
Engineer Research & Development Center
Boundary Conditions
These measurements
were taken every 15
minutes with the
model being run
using 30 minute time
steps.
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US Army Corps
of EngineersCoastal And Hydraulics Laboratory
Engineer Research & Development Center
Hydrodynamic Model Validation
Water Surface Elevation
gages used to validated
the model.
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US Army Corps
of EngineersCoastal And Hydraulics Laboratory
Engineer Research & Development Center
Highway 90 Comparison
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US Army Corps
of EngineersCoastal And Hydraulics Laboratory
Engineer Research & Development Center
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US Army Corps
of EngineersCoastal And Hydraulics Laboratory
Engineer Research & Development Center
Gage 23 Comparison
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US Army Corps
of EngineersCoastal And Hydraulics Laboratory
Engineer Research & Development Center
Conclusions
1. Accurate modeling of the hydrodynamics of a floating marsh
system can be achieved through the use of RMA2’s Marsh Porosity
capability
in conjunction with frictional specification by depth.
2. RMA2 was used in the Davis Pond project illustrate the
applicability of this capability. RMA2 was used to determine the
effects of possible
system alternatives with the results being used to determine the
most
effective way to increase the efficiency of the Davis Pond
diversion.
Acknowledgement: The experiments described and the results
presented were obtained from research sponsored by the US
Army Corps of Engineers District, New Orleans and the U.S. Army
Engineer Research and Development Center, Coastal and
Hydraulics Laboratory. Permission was granted by the Chief of
Engineers to publish this information.
