MISSISSIPPI RIVER WATER 

REINTRODUCTION INTO BAYOU 

LAFOURCHE HYDRODYNAMIC MODELING 

March 9, 2006 
MISSISSIPPI RIVER WATER REINTRODUCTION INTO 

BAYOU LAFOURCHE HYDRODYNAMIC MODELING 

Prepared for the 

Louisiana Department of Natural Resources, 

Coastal Restoration Division 

under subcontract with 

CH2M Hill 

1515 Poydras, Suite 2110 

New Orleans, LA 70112 

Prepared by 

FTN Associates, Ltd. 

3 Innwood Circle, Suite 220 

Little Rock, AR 72211 

March 9, 2006 
March 9, 2006 

i 

EXECUTIVE SUMMARY 

The areas benefited by the Bayou Lafourche Division project as identified in “Evaluation 

of Bayou Lafourche Wetlands Restoration Project” (United States Environmental Protection 

Agency (USEPA) 1998) (Figure 1.5) were re-evaluated to compensate for limitations of the 

previous modeling. Marsh surveys required for accurate determination of salinity reductions for 

the identified benefit areas would have been cost prohibitive. Consequently, a new model that 

more accurately represented the forces driving flows in the Terrebonne and Barataria Basins was 

developed to determine the benefit areas. This work was done using a two-phase approach that 

allowed project funds to be used judiciously. 

In Phase 1 of the study, it was shown that tides peak higher and earlier from west to east 

along the Terrebonne and Barataria Basin coastline (Figures 1.2 and 1.3). This causes flow in the 

Gulf Intracoastal Waterway (GIWW) to flow from west to east, except during storms when the 

normal tidal forcing can be reversed. During the 13-month period over which hourly data was 

collected, the flow was in an easterly direction more than 90% of the time (Figure 1.4). This 

observation coupled with the flow, stage, and salinity record available at the only connection 

between the two basins (intersection of the GIWW and Bayou Lafourche) allowed the 

computational mesh to be reduced to represent only the Terrebonne basin, thus reducing 

simulation times from weeks to days. This allowed the model to be quickly calibrated to 

observed flow, water surface elevation (WSEL), and salinity data from May to June 2004. 

Several tracer simulations were run in Phase 1 that showed the movement of Bayou 

Lafourche water through the Terrebonne Basin. These simulations showed that very little of the 

tracer moved south of the GIWW. Flow moving south down Company Canal or Bayou 

Lafourche is largely captured by the GIWW and moved eastward due to the larger conveyance 

capacity and eastward momentum of the GIWW. 

In Phase 2, the model was expanded to include the Barataria Basin so that it could be 

used to evaluate the effects of 15 operational and channel improvement scenarios on salinities, 

water surface elevations, and benefit areas (Table 7.2). The model was improved by collecting 

additional survey data, refining portions of the grid, and re-calibrating model coefficients. 
March 9, 2006 

ii 

The channel improvement scenarios that were simulated included five different dredging 

plans in Bayou Lafourche; the widening of Company Canal between the GIWW and Bayou 

Terrebonne; and the widening of the end of Bayou L’eau Bleu south of the GIWW that connects 

to Grand Bayou. The results from these simulations show that: 

• Bayou Lafourche Alternative 47 (8-ft average dredging upstream of Lockport 

with a 2,000 cfs diversion) (Run 10) results in the largest reduction of salinities of 

all the Bayou Lafourche dredging alternatives, but primarily in the upper 

Barataria Basin and with almost no benefit to the Terrebonne Basin south of the 

GIWW. 

• Widening Company Canal between Bayou Terrebonne and the GIWW in 

conjunction with Bayou Lafourche Alternative 47 (Run 14) increases freshwater 

flows only marginally in southern Bayou Terrebonne and allows saline Gulf water 

to be more readily transported northward and into the GIWW. This alternative 

would also require replacing the constricting Highway 24 bridge crossing. 

• Increasing flows to Grand Bayou via widening Bayou L’eau Bleu south of the 

GIWW and Bayou Lafourche Alternative 47 (Run 12) increases freshwater flows 

significantly in all channels modeled south of the GIWW in the Terrebonne basin 

(the Houma Navigation Canal, Bayou Terrebonne, Bayou Lafourche, and Grand 

Bayou). 
March 9, 2006 

iii 

TABLE OF CONTENTS 

1.0 INTRODUCTION ........................................................................................................... 1-1 

1.1 Description of the Project Area............................................................................ 1-1 

1.2 Study Approach ................................................................................................... 1-3 

1.3 Report Content Overview and Organization........................................................ 1-9 

2.0 MODEL DESCRIPTION ................................................................................................ 2-1 

2.1 Model Selection ................................................................................................... 2-1 

2.2 Assumptions and Limitations .............................................................................. 2-1 

3.0 MODEL INPUT............................................................................................................... 3-1 

3.1 Finite Element Mesh............................................................................................ 3-1 

3.2 System Topography ............................................................................................. 3-3 

3.3 Boundary Conditions ........................................................................................... 3-4 

3.4 Model Coefficients............................................................................................... 3-5 

3.5 Model Options ..................................................................................................... 3-5 

3.5.1 1D Channels............................................................................................. 3-5 

3.5.2 Channel Junctions .................................................................................... 3-8 

3.5.3 Channel and Marsh/Swamp Interconnections ......................................... 3-8 

3.5.4 Mesh Wetting/Drying .............................................................................. 3-8 

4.0 MODEL TESTING AND SENSITIVITY ANALYSES................................................. 4-1 

4.1 Model Testing ...................................................................................................... 4-1 

4.1.1 Convergence ............................................................................................ 4-1 

4.1.2 Channel Geometry Representation .......................................................... 4-1 

4.1.3 1D Junction Formulation ......................................................................... 4-2 

4.1.4 Lateral Weir 1D-2D Connections ............................................................ 4-3 

4.1.5 Continuity ................................................................................................ 4-4 

4.2 Sensitivity Analyses............................................................................................. 4-5 

4.2.1 Computational Time Step ........................................................................ 4-6 

4.2.2 Terrebonne Basin Marsh Elevations........................................................ 4-6 

4.2.3 1D Junction Nodes................................................................................... 4-6 

5.0 MODEL MESH MODIFICATION................................................................................. 5-1 

5.1 Phase 1 Mesh Refinement.................................................................................... 5-1 

5.2 Mesh Truncation / Addition................................................................................. 5-1 

5.2.1 Upper Bayou Lafourche Truncation ........................................................ 5-1 
March 9, 2006 

TABLE OF CONTENTS (CONTINUED) 

iv 

5.2.2 Phase 1 Barataria Basin Truncation......................................................... 5-1 

5.2.3 Phase 2 Barataria Basin Addition ............................................................ 5-1 

5.3 Phase 2 Mesh Refinement.................................................................................... 5-1 

6.0 MODEL RE-CALIBRATION AND VERIFICATION .................................................. 6-1 

6.1 RMA-2 Calibration and Verification................................................................... 6-1 

6.2 RMA-11 Calibration and Verification................................................................. 6-8 

7.0 MODEL APPLICATION ................................................................................................ 7-1 

7.1 Phase 1 Truncated Model Preliminary Simulations............................................. 7-1 

7.1.1 RMA-2 Simulations................................................................................. 7-1 

7.1.2 RMA-11 Simulations............................................................................... 7-5 

7.2 Phase 2 Simulations ............................................................................................. 7-6 

7.2.1 RMA-2 Simulations................................................................................. 7-6 

7.2.2 RMA-11 Simulations............................................................................. 7-18 

8.0 CONCLUSIONS.............................................................................................................. 8-1 

8.1 Bayou Des Allemands Boundary Condition Investigation.................................. 8-1 

8.1.1 Boundary Condition Development .......................................................... 8-4 

8.1.2 Discussion................................................................................................ 8-4 

8.2 Future Mesh Improvements ................................................................................. 8-9 

9.0 REFERENCES ................................................................................................................ 9-1 

LIST OF ATTACHMENTS 

ATTACHMENT 1: RMA-2 Calibration and Verification Simulations Results 

ATTACHMENT 2: RMA-2 Observed vs. Predicted Water Surface and Velocity Data 

ATTACHMENT 3: RMA-11 Calibration and Verification Simulation Results 

ATTACHMENT 4: RMA-11 Observed vs. Predicted Salinity Data 

ATTACHMENT 5: Week 6 Salinity Isohalines 
March 9, 2006 

v 

LIST OF TABLES 

Table 3.1 Data collection stations and their sampling durations and intervals....................3-7 

Table 6.1 Final RMA-2 calibration parameters ................................................................... 6-6 

Table 6.2 Error Statistics for RMA-2 Water Surface Elevation Calibration ....................... 6-7 

Table 6.3 Error Statistics for RMA-2 Velocity Calibration................................................. 6-7 

Table 6.4 Error Statistics for RMA-2 Water Surface Elevation Verification......................6-9 

Table 6.5 Error Statistics for RMA-2 Velocity Verification ............................................... 6-9 

Table 6.6 Final RMA-11 calibration parameters ............................................................... 6-11 

Table 6.7 Error statistics for RMA-11 salinity calibration ................................................6-13 

Table 6.8 Error statistics for RMA-11 salinity verification...............................................6-13 

Table 7.1 Tracer simulation conditions................................................................................ 7-5 

Table 7.2 Dredging and operational scenarios modeled......................................................7-7 

Table 7.3 Average flows at key intersections over the 9-week simulation period .............. 7-8 

LIST OF FIGURES 

Figure 1.1 Vicinity map showing major waterways within the system modeled.................. 1-2 

Figure 1.2 Tidal gage locations ............................................................................................. 1-4

Figure 1.3 Tidal cycles with peaks occurring earlier and higher at the western 

tidal stations ......................................................................................................... 1-5 

Figure 1.4 Cumulative frequency distribution of hourly flows in the GIWW at Larose 

from January 2004 through January 2005 ........................................................... 1-6 

Figure 1.5 Benefit areas from USEPA (1998) ...................................................................... 1-8 

Figure 3.1 Initial finite element mesh for the study area....................................................... 3-2 

Figure 3.2 Data collection locations...................................................................................... 3-6 

Figure 5.1 Extent of type 999 elements used in model mesh................................................ 5-2 

Figure 5.2.a Phase 1 truncated model mesh............................................................................. 5-3 

Figure 5.2.b Detail from the dense mesh area shown in Figure 5.1.a ...................................... 5-4 

Figure 5.3 Boundary condition locations for Phase 1 truncated model mesh....................... 5-5 

Figure 5.4 Final Phase 2 finite element mesh for the study area........................................... 5-6 

Figure 5.5 Boundary condition locations for Phase 2 model mesh....................................... 5-7 

Figure 5.6 Final Phase 2 model mesh bathymetry ................................................................ 5-8 
March 9, 2006 

LIST OF FIGURES (CONTINUED) 

vi 

Figure 6.1 25-hour average of observed water surface elevations from May-June 2004 ..... 6-2 

Figure 6.2 Location of RMA-2 element types in the mesh................................................... 6-5 

Figure 6.3 Location of RMA-11 element types in the mesh............................................... 6-10 

Figure 7.1 Average flow distribution resulting from 1,000 cfs Bayou Lafourche tracer 

simulations ........................................................................................................... 7-3 

Figure 7.2 Average flow distribution resulting from 2,000 cfs Bayou Lafourche 

diversion flow tracer simulations......................................................................... 7-4 

Figure 7.3 Average flows in Bayou Lafourche, Company Canal, Bayou L’eau Bleu, 

and the GIWW for key simulations ................................................................... 7-10 

Figure 7.4 Average flows in Company Canal, Bayou Terrebonne, St. Louis Canal, and 

Humble Canal for key simulations .................................................................... 7-11 

Figure 7.5 Average flows in the GIWW and Grand Bayou for key simulations ................ 7-12 

Figure 7.6 Average flows in the GIWW and the Houma Navigation Canal for key 

simulations ......................................................................................................... 7-13 

Figure 7.7 Location and cross section of Company Canal improvement ........................... 7-15 

Figure 7.8 Computed flows in Company Canal north of junction with Bayou 

Terrebonne for Run 5 and Run 14 ..................................................................... 7-16 

Figure 7.9 Location and cross section of Grand Bayou improvement................................ 7-17 

Figure 8.1 Predicted flow and stage at Bayou des Allemands boundary.............................. 8-2 

Figure 8.2 Period-averaged estimated and predicted flows through Bayou des Allemands 

boundary .............................................................................................................. 8-7 

Figure 8.3 Predicted and observed flow in the GIWW at Larose ......................................... 8-8 
March 9, 2006 

1-1 

1.0 INTRODUCTION 

The Coastal Wetlands Planning Protection and Restoration Act (CWPPRA) Program is 

assessing the benefit of diverting Mississippi River flows into Bayou Lafourche while 

maintaining or potentially increasing current water levels in the bayou. Numerical modeling was 

used to evaluate the different alternatives for this project, the Mississippi River Water 

Reintroduction into Bayou Lafourche Project (Louisiana Department of Natural Resources 

(LDNR) “Project”). This report describes that modeling effort. 

1.1 Description of the Project Area 

Bayou Lafourche divides the portions of the two basins that comprise the study area: the 

Barataria and Terrebonne basins (Figure 1.1). The Barataria Basin lies to the east of Bayou 

Lafourche with the Terrebonne Basin to the west. The study area boundaries are roughly the 

Mississippi River to the north and east, Bayou duLarge to the west, and the Gulf of Mexico to 

the south. The only hydrologic interconnections between the Terrebonne and Barataria basins are 

at the Gulf Intracoastal Waterway (GIWW) and Company Canal. 

The forces that drive flow within the system are: 

• Gulf tides, 

• Mississippi River diversion flows at the Davis Pond Diversion Structure, 

• Flow that is pumped into the headwater of Bayou Lafourche, 

• Wind, 

• Precipitation, and 

• Atchafalaya River flows that move into the system via the GIWW. 

Direct precipitation was included for model calibration; runoff from areas adjacent to the 

mesh is negligible and was not included. 
1-2 

Figure 1.1. Vicinity map showing major waterways within the system modeled. 
March 9, 2006 

1-3 

Examination of the tide records for stations in the area (Figure 1.2) showed that tides 

peak higher and earlier in the western part of the study area than in the east (Figure 1.3). This 

causes flow in the GIWW to flow from west to east, except during storms when the normal tidal 

forcing may be reversed. During the 13-month period over which hourly data was collected 

(January 2004 through January 2005) the flow in the GIWW was in an easterly direction more 

than 90% of the time (Figure 1.4). 

The GIWW acts as an interceptor for the five main waterways that convey flow north and 

south within the Terrebonne basin (the Houma Navigation Canal (HNC), Grand Bayou, Bayou 

Petit Caillou, Bayou Grand Caillou, and Bayou Terrebonne). 

Forced drainage areas (leveed areas under pump) are common in the Terrebonne basin 

and are generally attached to the natural levees along the bayous and extend into the adjacent 

marsh. As such, they are hydraulically isolated and were not included in the study. 

Relatively shallow marshes and channels dominate both basins. The marshes exhibit little 

vertical variation in velocity, density and salinity and can be considered two-dimensional (2D). 

The channels in the Terrebonne basin generally can be considered one-dimensional (1D) since 

no significant vertical or lateral variation occurs hydrodynamically except in their southern-most 

reaches where salt wedges move upstream in deeper channels (e.g., the HNC) causing a 

vertically stratified flow regime. In the Barataria basin the main north-south flow path is a series 

of wide interconnected lakes and generally can be considered 2D with only small variations 

vertically. 

1.2 Study Approach 

The original LDNR Scope of Services for the Project was based on the previous 

modeling which showed that some of the benefit areas were in the southernmost portions of the 

Barataria and Terrebonne basins. The survey effort to develop the detailed bathymetry required 

to accurately model an area that large would have been cost-prohibitive. 
1-4 

Figure 1.2. Tidal gage locations. 
1-5 

Figure 1.3. Tidal cycles with peaks occurring earlier and higher at the western tidal stations. 
1-6 

Figure 1.4. Cumulative frequency distribution of hourly flows in the GIWW at Larose from January 2004 through January 2005. 
March 9, 2006 

1-7 

In addition, a review of the modeling upon which the benefits were based (LDNR 1999) 

revealed that the 1D dynamic wave model UNET (HEC 2001) simulations were made in steady 

state mode without any representation of the interaction between the channels and marsh areas 

and without any water quality transport. By running only steady state flows and constant 

boundary elevations in the model, the effect of tides that move water in and out of the system on 

a diurnal basis were not accounted for. The model also excluded complex interactions of flow 

between the channels and marshes in the area that affect the distribution of flows in the 

interconnecting canals. The net effect of not including these components in the model was an 

inaccurate representation of the movement of the freshwater in the channels that biased the 

interpretation of benefit areas. The benefit areas identified by the previous modeling (United 

States Environmental Protection Agency (USEPA) 1998) are shown in Figure 1.5. 

Without including water quality transport, benefits were assumed to occur wherever flow 

increases were predicted. However, flow increases could be due to displaced ambient or Gulf 

water, not to the transport of freshwater. The previous modeling did not show where the 

freshwater was transported. 

Consequently, an alternative Scope of Services was developed. The modeling was 

divided into two phases in order to “creep up on the solution” - i.e., gain insight into the system 

behavior with a model where complexity is gradually added. In Phase 1, only the Terrebonne 

basin was modeled because the simulation times for the entire Terrebonne–Barataria system 

proved to be about one-fourth of the prototype time. By removing the Barataria basin, the 

simulation times were reduced by more than 50%. Reducing the simulation times allowed for 

many more simulations to be made to learn where mesh refinements were needed, what 

coefficients were appropriate, where new surveys might be required, and to gain insight into the 

system in general. What made the reduction in simulation area possible was the fact that flow 

into the Barataria Basin is predominantly one-way in an eastward direction (Section 1.1). This 

flow passes through the Bayou Lafourche ridge near Larose, LA where a recording water surface 

and salinity gage were placed. The gage provided boundary condition data for calibration and 

verification of the truncated model. 
1-8 

Figure 1.2. ? 

Figure 1.5. Benefit areas from USEPA (1998). 
March 9, 2006 

1-9 

The limitation of the smaller Terrebonne basin model was that a boundary condition 

would need to be synthesized at Larose when it came time to simulate conditions associated with 

proposed management alternatives; however, any synthesized boundary condition there would 

have a tremendous influence on the model results due to its close proximity to the study area. 

Consequently, in Phase 2, the Barataria Basin was added back to the model to eliminate the 

Larose boundary condition and capture the true interaction of flow between the basins. 

The Phase 2 modeling effort also involved refinement of the model and its application to 

evaluate several proposed dredging and operational scenarios. The refinements included the 

addition of new survey data to the model, refinement of the mesh, and verification of the mesh 

boundaries against more current digital ortho-quarter-quadrangle images (DOQQs). A total of 15 

operational and channel improvement scenarios were simulated to determine their effects on 

salinities, water surface elevations, and benefit areas. 

1.3 Report Content Overview and Organization 

This report is an update to “Phase 1 Modeling Preliminary Results”, which was included 

as Appendix 1 in the Phase 1 Design Report (CH2M HILL 2004). Sections of that Appendix 1 

are repeated herein for the convenience of the reader but it should be noted that many of these 

sections have been revised to reflect the additional data and insight gained in Phase 2. The 

description of the calibration and verification efforts for the Phase 1 study is not repeated since 

the model was re-calibrated in Phase 2 because the mesh was increased to include the Barataria 

basin. The description of the tracer simulation results from the Phase 1 modeling effort are only 

summarized in Section 7.1; for a complete description of tracer simulation results from the 

Phase 1 modeling effort, the reader is referred to Section 7.0 of Appendix 1 in the Phase 1 

Design Report. 

This report is organized in the following sections: 

• Section 1 – Introduction 

• Section 2 – Model Description 

• Section 3 – Model Input 

• Section 4 – Model Testing and Sensitivity Analysis 

• Section 5 – Model Mesh Modification 
March 9, 2006 

1-10 

• Section 6 – Model Re-Calibration and Verification 

• Section 7 – Model Application 

• Section 8 – Conclusions and Future Mesh Improvements 

• Section 9 – References 
March 9, 2006 

2-1 

2.0 MODEL DESCRIPTION 

2.1 Model Selection 

A 2D vertically-averaged hydrodynamic transport model was deemed appropriate for 

representing the Terrebonne and Barataria Basins (Section 1.2). Although, at times, portions of 

the system may be three-dimensional (3D), they are the exception rather than the rule and are not 

representative of the conditions to be modeled. 

Modeling was initiated using the TABS-MD modeling system (RMA-2 and RMA-4) (US 

Army Corps of Engineers (USACE) Engineering Research and Development Center 

(ERDC) 2000) and the Surface Water Modeling System (SMS) (Brigham Young 

University 2004). TABS-MD is distributed by the ERDC (formerly the Waterways Experiment 

Station) of the USACE. SMS is a pre/post-processing interface developed by ERDC and the 

Environmental Modeling Research Laboratory at Brigham Young University. However, this 

study required that several unique features be added that were not in the ERDC version of 

TABS-MD. Consequently, Dr. Ian King’s version of the RMA modeling suite (RMA-2 

hydrodynamic and RMA-11 water quality models) (King 2005) was used. Dr. King was coauthor of the code the ERDC version of TABS-MD is based on. His latest version was selected 

because: 

• Dr. King was available to add the necessary project-specific features (Section 3.2) 

to the model code in a short time frame (he is one of the original model’s 

developers), and 

• Dr. King’s version had other enhancements that promoted model stability and 

allowed greater flexibility in modeling the system (Sections 3.5.3 and 3.5.4). 

2.2 Assumptions and Limitations 

RMA-2/RMA-11 is a finite element model designed for simulation of flow and water 

quality in rivers, estuaries, and coastal systems. RMA-2 solves the vertically averaged equations 

of continuity and momentum using the Galerkin Method of weighted residuals to calculate timevarying velocities, elevations, and flows. A 1D representation of system geometry is also 

accommodated using the area-integrated 1D shallow water equations. Flow is assumed to be 
March 9, 2006 

2-2 

subcritical with a free water surface, and vertical accelerations are assumed negligible. The 

model incorporates a fully non-linear solution of the governing equations including terms for bed 

friction, turbulent viscosity, wind stresses, pressure gradient, and Coriolis forces. 

This means that the model cannot represent: 

• Stratified flows (e.g., salt wedge movement), 

• Supercritical flows (high velocity, shallow depth flows that may occur near 

spillway outlets, in steep sloped channels, or in some areas where large 

constrictions in the channel may occur), 

• Flows in long culverts or tunnels flowing full, or 

• Flows in the immediate vicinity (near-field) of a pump station intake as well as 

other situations where the flow varies significantly with depth. 

It also means that observed water surface elevation, velocity, and salinity data are 

required to calibrate the various model coefficients. 

The finite element solution method allows the user to specify an irregular mesh of 

quadrilateral and triangular elements of varying sizes over the 2D areas of the system so spatial 

resolution may vary. In contrast, a finite difference grid for the same system would have many 

more elements to achieve the same resolution in the area of interest by the requirement that the 

grid be of uniform size, which results in a larger computational burden and longer simulation 

times. 

One-dimensional elements (usually channels) may be connected to 2D elements to allow 

a transition from riverine to estuarine environments and to allow the representation of overbank 

flooding into areas where depth-averaged 2D circulation patterns are needed. 

In this application, the following assumptions were also made: 

• Winds are accounted for implicitly in the boundary conditions. 

• The calibration data collected (Section 3.3) is representative of conditions to be 

evaluated in future simulations. 

• The assumed bathymetry for the marsh areas in the Terrebonne basin is 

representative (Section 3.2). 
March 9, 2006 

3-1 

3.0 MODEL INPUT 

The data required for model input are the finite element mesh, topography of the study 

area, salinities, flows, or water surface elevations at the edges of the study area (boundary 

condition data), and model coefficients. In addition, several model options must be specified to 

properly represent the system. 

3.1 Finite Element Mesh 

The finite element mesh defines where results will be calculated and reported spatially 

within the system. The finite element mesh used for the project area is shown in Figure 3.1. This 

is the largest and most complex TABS model that has ever been constructed3

. 

The finite element mesh for the Barataria basin was obtained from the New Orleans 

District (NOD) office of the USACE. It was developed for a TABS-MD model to evaluate the 

Myrtle Grove Siphon Project. 

The finite element mesh for the Terrebonne basin was developed by FTN Associates, 

Ltd. (FTN) using 1998 georeferenced DOQQ aerial photos as a guide to marsh boundaries and 

channel centerlines. It should be noted that numerous large forced drainage areas are excluded 

from the mesh because they are separated from the marsh by levees and are drained by pump 

stations (Section 1.2). 

 

3

 Personal Communication, Ian King, PhD, author of RMA-2 and RMA-11, based on number of computational 

nodes, degree of wetting and drying of mesh, and length of marsh-channel exchange zones. 
3-2 

Figure 3.1. Initial finite element mesh for the study area. 
March 9, 2006 

3-3 

3.2 System Topography 

In terms of topography, the system has two distinct features, namely canals and marshes. 

In the model the canals are represented using over 130 newly surveyed cross sections for Bayou 

Lafourche, Company Canal, the GIWW, and portions of Grand Bayou. Topography for the rest 

of the channels was represented using 740 surveyed cross sections from the USACEs’ Morganza 

to the Gulf Feasibility Study (USACE NOD 2000) UNET model. These sections from the UNET 

model were adjusted vertically as necessary to tie them into the Louisiana Coastal Zone Primary 

GPS network. 

Previously in RMA-2, 1D channels could only be represented by certain regular shapes 

(e.g., trapezoidal, triangular, rectangular). In this study, however, preliminary testing 

(Section 4.1.2) showed that improved computed water surface elevations and flows would result 

from refined representation of the channels. Representing the channels with 2D elements would 

add considerable computational overhead and stability problems. Therefore, RMA-2 was 

modified to read in tables of elevation versus cross-sectional area and top width that were 

computed using detailed cross section points for 1D elements. The tables were computed using 

the same algorithms used in HEC-RAS (HEC 2002) and in the UNET (HEC 2001) preprocessor 

program CSECT. 

The only available useful topographic data for the marsh areas was in the Barataria Basin. 

This consisted of bathymetry from another RMA-2 model created by the NOD to evaluate the 

Myrtle Grove Siphon Project. LDNR collected eight transects in the large open water areas of the 

basin for the Barataria Basin Feasibility Study and the NOD Barataria basin model bathymetry 

was updated in these areas. The only available topographic data for the Terrebonne basin marsh 

areas was the National Oceanic and Atmospheric Administration (NOAA) National Geophysical 

Data Center’s (NGDC) compendium of various hydrographic, topographic, and other surveys 

that proved to be inaccurate in many areas. This was not unexpected since none of the data were 

tied to a common reference datum. Consequently, marsh topography had to be assumed. A 

sensitivity analysis was performed to determine the effect of this assumption on model results 

(Section 4.2.2). 
March 9, 2006 

3-4 

3.3 Boundary Conditions 

The hydrodynamics and the salinity transport in a modeled system are driven by the 

forces applied to the system via flow and salinity entering or leaving it at the boundaries. 

Continuous readings of water surface elevations and salinity data were collected over a period of 

13 months at several locations in and around the study area. Figure 3.2 shows the locations and 

station identifiers of the gages where data was collected. Table 3.1 summarizes characteristics of 

the collected data sets. 

The recorded data from the boundary gages (10, 11, and 19) were used to provide 

boundary input to the model. Water surface elevations at the GIWW at Minor’s Canal gage 

(Gage 18) initially was used as a boundary condition, but during model calibration (Section 6.1), 

the data was found to be questionable, and the GIWW at Houma gage data (Gage 17) was 

applied instead. Data recorded at the internal gages (1 through 8, 12 through 17, and 20 through 

21) were not part of the input to the model but were used to compare against simulated values 

produced by the model. Most of the newly installed gages were active during the period from 

February 2004 through January 2005. The remainder were active during the whole period except 

for a few days or hours. With exception to the upstream end of Bayou Lafourche where the 

diversion flow was applied, the upstream ends of all other 1D channels were assumed to have 

zero flow because of little runoff contribution from upland areas. 

Before processing the observed data into the format readable by the RMA-2 modeling 

program, it was visually inspected to identify any artificial or erroneous values, revealed in most 

cases by spikes in plots of the data. Any spikes that were unrealistic were smoothed out. 

Daily rainfall data were obtained from the Louisiana State University (LSU) Agricultural 

Center in Houma, Louisiana. These data were used as time-varying input to the model. 

The original tidal gage locations were at Cocodrie, Dulac, and Grand Isle. However, it 

was determined that the recorded water surface elevations at the Cocodrie and Dulac gages (data 

collection location 15 and 16 respectively) did not accurately represent the tidal fluctuations 

along the Gulf of Mexico boundary in the Terrebonne Basin. After investigating potential new 

sources of tidal gage data, gage data at Isle Dernieres and Port Fourchon provided by the LSU 

Coastal Studies Institute and the NOAA National Data Buoy Center, respectively, were obtained 
March 9, 2006 

3-5 

for use in the model instead. The locations of the tidal stations used for the model are shown in 

Figure 1.2. The Isle Dernieres elevations were applied at the westernmost point of the southern 

boundary. Port Fourchon data were applied near the outlet of Bayou Lafourche. The Grand Isle 

data were applied at the easternmost point. RMA-2 linearly interpolated between the three 

locations along the southern boundary. These interpolated data proved to provide the best 

available representation of tidal forcing along the southern boundary. 

Figure 1.3 shows that the tides peak earlier and higher from west to east. This differential 

drives flows in the GIWW from west to east, and is the reason why more than 90% of the 

observed hourly data over the 13-month data collection period shows eastward flows in the 

GIWW (Figure 1.4). 

3.4 Model Coefficients 

Three important coefficients must be specified that affect model results. First, bottom and 

bank friction within the prototype is represented using Mannings ‘n’ roughness coefficients. 

Second, eddy viscosity represents the resistance of flow to changes in applied force. Third, the 

dispersion coefficient defines the spreading of a constituent due to differential shear within the 

water column. Values for these coefficients vary spatially and were assigned initially based on 

engineering judgment and adjusted through the calibration process. 

3.5 Model Options 

3.5.1 1D Channels 

As described in Section 3.2, a new capability was added to RMA-2 that used irregular 

cross sections instead of trapezoidal approximations to represent channel geometry. This allowed 

more accurate water surface profiles and flow distributions to be calculated. 
3-6 

Figure 3.2. Data collection locations. 
durations and intervals. ations and their sampling Table 3.1. Data collection st

Station 

Station Description No 

Starting 

Date 

Ending 

Date 

Sampling 

Interval 

Collected 

Parameter3 Agency 

l 02/19/04 01/10/05 15 min ZSV New in

1 Bayou Lafourche north of Company Cana2 Company Canal at Highway 1 near Lockport 01/01/04 01/30/05 15 min ZS USGS 

15 min/1hr ZS New in

3 Lake Fields 02/16/04 01/10/05 

 New in

02/15/04 01/11/05 15 min ZSV 4 GIWW east of Bayou Lafourche at Larose 

04 01/11/05 15 min ZSV New in

5 Bayou Lafourche south of GIWW 02/12/6 GIWW west of Bayou Lafourche at Larose 01/01/04 01/30/05 15 min ZSVQ USGS 

10/05 15 min ZS New in

7 Grand Bayou Marsh 02/16/04 01/

13/04 01/11/05 15 min ZSV New in

8 Bayou Terrebonne southeast of Houma 02/

01/01/04 10/01/04 15 min ZSVQ USGS 

9 Bayou Lafourche at Donaldsonville 

10 Bayou Lafourche at Thibodaux 01/01/04 01/30/05 15 min ZVQ USGS 11 Davis Pond Freshwater Diversion 01/01/04 01/30/05 15 min ZSVQ USGS 

30/05 15 min S USGS 

12 Lake Cataouatche 01/01/04 01/13 Lake Salvador 01/26/04 01/30/05 15 min ZS USGS 

01/01/04 01/30/05 15 min ZSQ USGS 

14 Barataria Pass east of Grand Isle 

01/04 10/01/04 15 min ZSV USGS 

15 Houma Navigation Canal at Dulac 01/

 01/01/04 01/31/05 1 hr ZSV LUMC

16 Bayou Petit Caillou at Cocodrie17 GIWW at Houma 01/01/04 01/30/05 15 min ZSVQ USGS 18 GIWW at Minors Canal 02/19/04 01/25/05 15 min/1 hr ZS USACE 

01/01/04 06/28/04 15 min ZS LDNR 

19 Bayou des Allemands at Highway 90 

ke 01/01/04 09/30/04 1 hr ZSV DNR ID 

20 Barataria Bay Waterway north of Mud La

04 07/07/04 1 hr ZS DNR ID 

21 South end of Little Lake 01/01/

3Key: Z=Water Surface Elevation 

 S=Salinity Concentration  V=Velocity  Q=Discharge 

3-7
March 9, 2006 

3-8 

3.5.2 Channel Junctions 

At channel intersections, junction elements were coded to use matching water surface 

elevations (element type 901) since the momentum conservation method (element type 903) 

proved to produce unrealistically large water surface elevation and flow differences at the 

intersections. This was due to the tendency for channel flows to change directions in response to 

the tidal influence of the Gulf. 

3.5.3 Channel and Marsh/Swamp Interconnections 

In the ERDC version of RMA-2, connections between 1D (channel) and 2D (marsh) 

elements are modeled using special “Transition Elements.” For the dozens of miles of 

connectivity within this system, these elements proved to be time-consuming to code and 

introduced stability and mass conservation problems in preliminary simulations. 

Consequently, for this study, a new type of 1D to 2D connection technique was used in 

RMA-2 as described in King and Williams (1999). This technique forces water surface 

elevations to match along the line of the 1D channel nodes and then treats these water levels as 

water surface elevation boundary conditions for the 2D portion of the mesh. The advantages of 

this technique are that the bounding 1D nodes are connected to the 2D nodes with a new 

quadrilateral element that: 

• Significantly reduces the mass conservation problems that can occur with 

Transition Elements (sharp element angles), and 

• Allows the full length of the overflow area to be represented. 

3.5.4 Mesh Wetting/Drying 

The Marsh Porosity option was used for handling element wetting and drying issues. This 

is a technique that allows a smooth transition computationally when shoreline elements vary 

between wet and dry states. This was used instead of the Element Elimination Method that 

requires much more refinement of the mesh in the shoreline areas in order for the solution to 

converge, which would result in significantly longer simulation times. 
March 9, 2006 

4-1 

4.0 MODEL TESTING AND SENSITIVITY ANALYSES 

4.1 Model Testing 

Model testing has been an integral and continuous part of the model development, 

starting with small simplified test cases during the initial model development and continuing 

through to initial calibration simulations with the full model mesh. Just a few of the many 

purposes for model testing are: 

1. Checking model convergence, 

2. Evaluating the implications of representing the 1D channel geometry with a 

simplified shape, 

3. Testing newly added model capabilities, 

4. Comparing different modeling methodologies or formulations, and 

5. Revealing problems with the mesh that cause mass not to be conserved locally. 

4.1.1 Convergence 

Since the model uses the Newton-Raphson iterative solution technique, convergence 

criteria must be specified that define when to stop the iterations and move to the next 

computational time step (i.e., the maximum acceptable change between computed values of 

depth and velocity components between iterations). For all the model simulations, convergence 

criteria of 0.005 ft were used for depth and 0.001 ft/sec for both the x- and y-velocity 

components. Using a 15-minute timestep, the solution usually converged within 20 iterations 

initially and then within 8 to 10 iterations as the simulations progressed. 

4.1.2 Channel Geometry Representation 

Some of the earliest model testing conducted for this project involved testing the effects 

of the trapezoidal shape to approximate the channel cross sections in RMA-2. A simplified test 

case was developed consisting of a small network of 1D channels and interconnected storage 

areas. The UNET model was used for comparing flows and water surface elevations using 

irregular cross section geometry from Bayou Lafourche along with vertical-walled storage areas. 
March 9, 2006 

4-2 

Lateral weir (UNET “LA” and “WD”) records were used to represent the connections between 

the 1D channels and the storage areas with UNET. 

For RMA-2, the geometry included 1D trapezoidal representations of the irregular cross 

sections for the channels and simplified 2D meshes representing the same storage areas defined 

in the UNET input test case. Transition Elements were first used to connect the 1D channels to 

the 2D mesh to allow for the interaction of flow between the channels and storage areas; 

however, these proved problematic and a new method for connecting the channels and marsh 

was used (Section 4.1.4). 

The same boundary conditions were specified for both models. Various runs were 

performed with both steady state and dynamic boundary conditions, and the results from the two 

models were compared at selected locations. Computed water surface elevations typically were 

within 0.5 ft or less, and computed flows were within 20%. However, this was not deemed 

accurate enough, which gave rise to the need for the representation of irregular channels in 

RMA-2 (Section 3.2). 

After adding the irregular channel capability to RMA-2, RMA-2 computed water surface 

elevations matched those from UNET to within 0.2 ft with flows matching within 5%. 

4.1.3 1D Junction Formulation 

Another example of model testing conducted with a simple test case involved the 

comparison of available 1D junction node types in RMA-2. RMA-2 includes the following three 

types of junction nodes: 

• Type 901 - Forces equal water surface elevation at the ends of all the reaches 

forming the junction; 

• Type 902 - Matches total head (water surface elevation + velocity head) at the 

ends of all the reaches forming the junction; and 

• Type 903 - Conserves momentum at the junction. 

For the type 903 junction, it is necessary to specify which reaches represent the “main 

channel” at the junction. It seems reasonable to assume that any differences between these 

formulations would be highly dependent upon the magnitudes of flow velocities in the reaches at 
March 9, 2006 

4-3 

the junction and upon the differences in velocity among the reaches forming the junction. These 

expectations were confirmed with a small test case consisting of three 1D reaches creating a 

simple T-shaped junction. The different formulations were tested by comparing the flow 

distribution and computed water surface elevations at the junction. 

When velocities in the reaches were low, there was practically no difference between the 

three formulations. For higher velocities, more pronounced differences were observed. These 

results confirmed that the momentum conservation method (type 903) produced unrealistic 

results under some flow conditions. 

4.1.4 Lateral Weir 1D-2D Connections 

A fairly recent addition to King’s version of RMA-2 is the type 999 lateral connection 

element between 1D and 2D elements (King and Williams 1999). This capability was added to 

the RMA-2 model to simulate the lateral flow exchange between a river channel and leveed 

overbank areas during flood events. Conceptually, the same type of flow exchange occurs 

between the channels and adjacent marsh areas in the project area. 

To use the type 999 lateral flow exchange feature, the 2D elements along the edge of a 

mesh that abuts a 1D channel reach where flow exchange can occur must be aligned and nodes 

spaced to be adjacent to end nodes of 1D elements on the channel. Internally, the model matches 

water surface elevation in the channel to the adjacent 2D element and matches flow entering or 

leaving a 1D element as inflow to or outflow from the adjacent 2D element. In effect, the 1D 

channel becomes a boundary condition for the 2D mesh. This formulation allows for the 

exchange of flow between 1D and 2D elements in either direction. The connection between a 1D 

and 2D element is specified by creating a quadrilateral element from the corner nodes of adjacent 

1D and 2D elements with the element type 999. 

Because accurate representation of the interaction between the channels and adjacent 

marsh areas is such a critical component of this project, it was decided to prepare a simple test 

case to test the type 999 lateral flow exchange formulation. The purpose of this test was to 

determine the effect on computation times. According to Dr. Ian King, the primary drawback of 

the type 999 lateral connections is increased computation times. Because manageable simulation 
March 9, 2006 

4-4 

times are critical to meeting project deadlines, it was essential to know how the type 999 

elements would affect simulation times to decide whether or not to use them. 

Two simple test cases were set up with a 1D channel reach connected to a 2D mesh; one 

with traditional Transition Elements and one with the type 999 elements. While both 

formulations gave similar results, the runs with the type 999 elements took about twice as long. 

The decision was initially made not to use the type 999 lateral connections, and to code all lateral 

connections with traditional Transition Elements based on these test results. 

However, it was later decided that the best possible formulation for the type of 

continuous connection between the channels and marshes in the project area was the type 999 

element. At issue was the preservation of lateral weir length along the channel that controlled the 

exchange between the channel and adjacent marsh areas (too short a length meant that flows into 

or out of the marsh would be underestimated). With the Transition Elements it was difficult to 

conserve the weir length due to the discrete nature of the connection from a single 1D node to a 

2D element. At best, the Transition Elements could give only a rough, discretized approximation 

of the weir length. When tests were run with type 999 elements implemented in the full model 

network, it was found that their impact on computation times was minimal. 

The earlier simplified tests with the type 999 elements led to a biased conclusion about 

simulation times due to the scale of the original test. In the original test, only one reach and two 

adjacent marsh areas were tested. Due to the small number of nodes and elements in the test 

network, the type 999 elements had a substantial influence on the simulation times. The full 

model simulation times are more heavily influenced by the very large total number of elements, 

so the type 999 elements have much less influence. For this particular case, the original model 

testing led to a misleading conclusion. 

4.1.5 Continuity 

One of the fundamental tests of acceptability for any model is adequate continuity and 

conservation of mass. For a model mesh this large, it is critical to ascertain whether mass is 

being created or lost in the model. Therefore, continuity checks are one of the most important 
March 9, 2006 

4-5 

kinds of model testing. In fact, the goal of having acceptable continuity errors has driven the 

development and refinement of this model more than any other single factor. 

4.1.5.1 1D Reach Continuity 

Due to the density and sheer numbers of 1D nodes in this model, it was originally 

difficult to quantify continuity in the 1D reaches. The majority of visualization tools for 

interpreting output data from RMA-2 are specific to 2D elements (e.g., contouring). To eliminate 

the laborious task of tracking flows by 1D node numbers in the model output file, Dr. Ian King 

developed a tool for the RMAPLT program that graphically displays flows at 1D nodes. This 

tool proved invaluable in finding areas of poor continuity in the 1D reaches. These areas 

typically occurred around bridges or other significant changes in the channel geometry (e.g., 

cross-sectional area, invert elevation). The solution for these continuity errors in most cases was 

simply to refine the spacing of the 1D nodes. The addition of 1D nodes had a negligible effect on 

computation times, so the 1D elements could be made very dense at locations where extra 

resolution was needed. 

4.1.5.2 2D Continuity – Flow Through a Marsh 

Two-dimensional continuity was tested by establishing continuity lines at inflow, 

outflow, and interior points of the 2D mesh. The total inflow versus outflow could then be 

tracked, giving an estimate of the mass created or lost in the mesh. Continuity was tested for 

small marsh areas as well as the entire Barataria Basin. Under steady state conditions, the entire 

mesh showed good continuity at all continuity lines, with no more than 10% losses at any given 

location. 

4.2 Sensitivity Analyses 

Several sensitivity analyses have been performed on the full model. These include 

investigation of the effects of computational time step, the marsh elevations in the Terrebonne 

Basin, and the 1D junction node types. 
March 9, 2006 

4-6 

4.2.1 Computational Time Step 

In Phase 1, to test the effect of the computational time step for dynamic simulations, three 

simulations were performed, one with a one hour time step, one with a 30-minute time step, and 

another with a 15-minute time step. While the longest time step simulation ran faster, results 

differed from shorter time step simulations during periods of large tidal changes. The largest 

impact of computational time step was observed in the RMA-11 simulations where negative 

salinity concentrations were computed on occasion with time steps larger than 15 minutes. 

Consequently, all simulations in Phase 1 were made using a 15-minute time step. 

In Phase 2, with the addition of the Barataria Basin, model run times doubled in duration. 

In order to meet deadlines, the model was run with a 30-minute time step. Testing showed no 

changes in the results between using a 15-minute and 30-minute time step. 

4.2.2 Terrebonne Basin Marsh Elevations 

Because of the lack of bathymetry data in the Terrebonne Basin, all marsh invert 

elevations north of the Gulf were initially set at –5 ft NAVD88 based on local knowledge. 

Because of concerns about artificially deep depths affecting flows and velocities in the marshes, 

a simulation was performed with all marsh bottom elevations increased to –2 ft NAVD88. 

Because this change did not have a significant impact on the computed marsh velocities, it was 

decided to keep the marsh elevations at –5 ft NAVD88. 

4.2.3 1D Junction Nodes 

As described in Section 3.5.2, RMA-2 allows a choice of 1D junction node types. The 

model for this project was initially developed using the default junction node type (type 901). In 

response to questions about the flow distribution at 1D nodes, the junction nodes at the Bayou 

LaFourche/Company Canal and Bayou LaFourche/GIWW junctions were changed to a type that 

conserved momentum (type 903). This change had a negligible impact on the flow distributions 

at these junctions due to the low velocities in the channels. Thus, the flow distribution was more 

significantly impacted by head differential than momentum for the conditions tested. Later 

simulations showed, however, that the momentum junction (type 903) produced unrealistically 
March 9, 2006 

4-7 

large head and flow differentials at the junctions due to the tidally-induced reversing flows. 

Consequently, all junctions were changed back to equal elevation (type 901) junctions. 
March 9, 2006 

5-1 

5.0 MODEL MESH MODIFICATION 

After the model testing phase, several sets of modifications were made to the geometry to 

“creep up on the solution” – i.e., starting with a simplified model and adding complexity as 

needed. 

5.1 Phase 1 Mesh Refinement 

As stated in Section 3.5.3 (Channel and Marsh/Swamp Interconnections), the connections 

between 1D (channel) and 2D (marsh) elements were modeled using a new quadrilateral element 

type 999 instead of Transition Elements. The nodal elevations for all 2D elements connected to 

type 999 elements were held constant so that there was no slope across the 2D elements. 

Additionally, a second tier of 2D elements adjacent to the weir connections was coded with a 

constant elevation to create a flat “shelf” at the overbank weir elevation. These shelves were 

coded to help smooth the transition of flow between the 1D channels and 2D marsh areas. As the 

model development progressed, the 2D marsh areas to which the type 999 elements are 

connected were refined further to more accurately compute the flow exchange between 1D 

channels and 2D marsh. For marshes connected to 1D channels, nodal elevations for perimeter 

nodes not included in a weir connection were increased to +5.0 ft NAVD88. Figure 5.1 shows 

the extent of the type 999 elements in the model. 

5.2 Mesh Truncation / Addition 

5.2.1 Upper Bayou Lafourche Truncation 

The representation of the entire length of Bayou Lafourche proved problematic. First, the 

flow data record for Bayou Lafourche at Donaldsonville had periods of missing and invalid data. 

Second, initial simulations showed that the transport travel time from Donaldsonville to 

Thibodaux was approximately 1 week. Therefore, the northern end of Bayou Lafourche was 

eliminated from the mesh and the model boundary was instituted at Thibodaux instead. The 

measured flow data for Bayou Lafourche at Thibodaux was then used in, lieu of the 

Donaldsonville data, as the upstream flow boundary condition for Bayou Lafourche.
5-2 

Figure 5.1. Extent of type 999 elements(shown in red) used in model mesh. 
March 9, 2006 

5-1 

5.2.2 Phase 1 Barataria Basin Truncation 

Initial simulations with the complete Terrebonne - Barataria Basin mesh were 

unacceptably slow (approximately one-fourth of the prototype time) given the number of 

simulations that were known to be required to gain insight into how this complex system worked. 

Consequently, it was decided to reduce the size of the mesh by eliminating the Barataria Basin 

for the Phase 1 effort (Figures 5.2.a and 5.2.b). The result of using just the Terrebonne Basin 

mesh was that more than double the number of simulations could be made, allowing significant 

insight into the system behavior with a less complex model. The model with only the Terrebonne 

Basin mesh was considered an allowable approximation because the observed data showed flow 

in the GIWW at Bayou Lafourche moved eastward more than 90% of the time for the 13-month 

period over which data was collected (Figure 1.4) - i.e, the Barataria Basin received flow more 

than 90% of the time and contributed flow into the Terrebonne Basin less than 10% of the time. 

The GIWW gage near Larose served as the eastern boundary for the model with only the 

Terrebonne Basin mesh. 

Consequently, only the Isle Dernieres and Port Fourchon tidal records were used with the 

truncated mesh in Phase 1. Figure 5.3 shows the location of the final boundary condition 

locations used in the Phase 1 truncated model. 

5.2.3 Phase 2 Barataria Basin Addition 

After the Phase 1 modeling effort was completed and insight was gained into how the 

Terrebonne Basin system worked, the model was modified to add the Barataria Basin back in so 

that it could be used to evaluate the effects of 15 operational and channel improvement scenarios 

on salinities, water surface elevations, and benefit areas. 

5.3 Phase 2 Mesh Refinement 

In addition to adding the Barataria Basin mesh, the Phase 2 mesh was improved in the 

following ways: 

• Grand Bayou was modified to better reflect its connectivity to the adjacent marsh 

areas and the open water area at its southern end. 
March 9, 2006 

5-2 

• New cross sections and bank elevations that were surveyed in the GIWW east of 

Bayou Lafourche were incorporated into the model. 

• Bush Canal was connected to the marsh on its southern side via type 999 

elements. 

• Mesh boundaries and elevations were refined to reflect the location of features 

such as forced drainage areas, and other areas that do not flood that were 

identified using the 2004 DOQQ imagery. 

• Even though the NGDC dataset was not used based on calibration results 

(Section 6.1), ground elevations from the NGDC bathymetry data was used to 

represent several barrier islands that are present in this area could not be refined to 

the level of detail necessary. This one area where the model could be improved 

upon in the future. 

The resulting final mesh is shown in Figure 5.4. Figure 5.5 shows the location of the final 

boundary conditions used in the Phase 2 model. Figure 5.6 shows the final bathymetry used in 

the mesh. 
5-3 

Figure 5.2.a. Phase 1 truncated model mesh. 
5-4 

Figure 5.2.b. Detail from the dense mesh area shown in Figure 5.2.a. 
5-5 

Figure 5.3. Boundary condition locations (shown in yellow) for Phase 1 truncated model mesh. 
5-6 

Figure 5.4. Final Phase 2 finite element mesh for the study area. 
5-7 

Figure 5.5. Boundary condition locations for Phase 2 model mesh. 
5-8 

Figure 5.6. Final Phase 2 model mesh bathymetry. 
March 9, 2006 

6-1 

6.0 MODEL RE-CALIBRATION AND VERIFICATION 

Calibration is a process where model coefficients are systematically varied through a 

series of simulations to reproduce observed data. Doing this offers more confidence in the model 

when it is used in predicting data under different scenarios. In Phase 1, the model was calibrated, 

and the initial Phase 2 calibration was performed using the Phase 1 coefficients. Verification is 

the process where a single simulation is performed using the model coefficients selected during 

calibration to reproduce values during a different period to give further confidence in the model. 

6.1 RMA-2 Calibration and Verification 

Water surface elevation, flows, velocities, and salinity were calibrated to observed gage 

data. An initial RMA-2 simulation was run for a 2-week period to establish a baseline condition 

to which other calibration simulations could be compared. After the baseline results were 

examined, a series of runs were executed using different boundary conditions at the western 

GIWW boundary at Minor’s Canal in combination with different geometry modifications 

because the results did not show a good correlation with the observed data. 

The boundary conditions were varied because the observed data in the GIWW showed 

consistently higher water surface elevations at Houma than at Minor’s Canal or at Larose 

(Figure 6.1). Based on the fact that the observed flow data showed eastward flow in the GIWW 

90% of the time, the differences in the observed water surface elevations seemed to indicate that 

there was a datum problem associated with one or more of the gages. Consequently, the model 

was run with modified boundary conditions at the Minor’s Canal boundary. Modification 

methods used included increasing the water surface elevations there by a fixed incremental 

value, and applying the GIWW at Houma gage elevation data at Minor’s Canal. 

The geometry modifications used for calibration included using the bathymetric sounding 

data from the NGDC in the Terrebonne marsh, adding constrictions at floodgate locations, and 

adding 1D channel – 2D marsh connections to represent the known flow exchange between the 

channel and adjacent marsh.
6-2 

Figure 6.1. 25-hour average of observed water surface elevations from May-June 2004. 
March 9, 2006 

6-3 

A series of runs were executed for a 4-week period, using different combinations of 

geometries (NGDC Terrebonne basin bathymetry versus original synthesized Terrebonne basin 

bathymetry), boundary conditions at Minor’s Canal (GIWW at Houma versus GIWW at Minor’s 

Canal elevation plus 1.0 ft), and Manning’s ‘n’ values for the 1D channels, 2D open water 

elements, and 2D marsh elements. All the simulations were carried out with a computational 

time step of 30 minutes. 

The NGDC Terrebonne basin bathymetry proved problematic. Although it intuitively 

looked more realistic than the synthesized bathymetry, it proved inaccurate in most but not all 

areas where survey data or local knowledge were available for verification. Given that the 

NGDC data is a compendium of a number of sources, many of which were never tied to a known 

datum, this was not surprising. Setting Manning’s ‘n’ values for the different areas yielded little 

improvement in results. Consequently, the synthesized data was kept in the model. 

In testing the different Minor’s Canal boundary conditions, the results showed that using 

the GIWW at Houma gage elevation data at the Minor’s Canal location produced the best results. 

Of all the locations where computed elevations matched observed, the poorest matches 

were found at HNC at Dulac (Station 15), and Lake Cataouache (Station 12). This may be due to 

problems with the datum for the gages. This is indicated by the fact that the computed elevations 

at Dulac match up quite well with the observed elevations at the Bayou Petit Caillou at Cocodrie 

gage (Station 16), which is located relatively close to the gage on the HNC. 

The observed water surface elevations for the gages in Lake Cataouache and Lake 

Salvador did not match well (Figure 6.1). While the computed data matched up fairly closely 

with the Lake Salvador gage data, the values for the observed Lake Cataouache were 

consistently higher than the computed values. This indicates a problem with the Lake 

Cataouache gage datum because both lakes are connected and both gages are within a short 

distance of each other. 

The water surface elevation and velocity results from the final calibration run for the 

various gages are shown in Figures A.1 through A.24 in Attachment 1, and the final calibration 

parameters are shown in Table 6.1 and Figure 6.2. The computed water surface elevations and 

velocities showed good agreement with the observed data for most of the observed gage 
March 9, 2006 

6-4 

locations. In instances where values were not matching in magnitude, the computed results 

reproduced overall observed trends. 

When assessing the quality of model calibration, it is a good practice to look at the results 

in a variety of ways, instead of focusing merely on matching observed and predicted time series 

values. This is especially true in systems such as the one studied here, where topography, 

hydrodynamics, and collection of representative monitoring data have been complex tasks. Other 

calibration assessment methods used included looking at scatter plots and error statistics, and 

how well the model captures overall trends. 

Figures B.1 through B.17 (Attachment 2) show scatter plots where observed and 

predicted water surface elevations are compared, and Figures B.18 through B.27 (Attachment 2) 

show plots where observed and predicted velocities are compared. Some of the statistical 

parameters that quantify “reasonable fit” are shown in Tables 6.2 and 6.3. It should be noted that 

the uncertainty associated with the measurements should be kept in mind in such an assessment 

of simulation errors. The equations for these statistics are shown below. 

Average Error = ∑=

= −

N

i 1

a i i (0 S ) N

1 E 

Oi = Observed value, and 

Si = Simulated value 

Standard Error RMSE = 

N

(O S )

N

i 1

2

∑ i i

=

−

R squared N O O N S S

N OS O S

i i ii

ii i i

i

N

−= − −

−

∑ ∑ ∑ ∑

∑ ∑ ∑ =

[ ( ) ][ ( ) ]

( ) ( )( )

2 2 2 2

1
6-5 

Figure 6.2. Location of RMA-2 element types in the mesh. 
Table 6.1. Final RMA-2 calibration parameters. 

Element Type Water Body

EV-x 

u-vel 

(lb-sec/ft2) 

EV-y 

u-vel 

(lb-sec/ft2)

EV-x 

v-vel 

(lb-sec/ft2)

EV-y 

v-vel 

(lb-sec/ft2 

Bayou Lafourche 1 20.0 20.0 20.0 20.0 Bayou Terrebonne 8 20.0 20.0 20.0 20.0 Bayou Petit Caillou 12 20.0 20.0 20.0 20.0 Houma Navigation Canal 17 20.0 20.0 20.0 20.0 GIWW East of Bayou Lafourche 43 20.0 20.0 20.0 20.0 All remaining 1D Channels 2-8,9-11,13-16,18-28 20.0 20.0 20.0 20.0 

 Scaled Scaled 

Overbanks 29 Scaled Scaled

42 Scaled Scaled Scaled Scaled 

Marsh areas connected to Gulf 30,32,35,

 Scaled Scaled 

33, 34,36 Scaled Scaled Isolated marsh areas with connections to channels

caled Scaled Scaled 

Gulf of Mexico 41 Scaled S

6-6 
Table 6.2. Error Statistics for RMA-2 Water Surface Elevation Calibration. 

Statistical Parameter Gage 1 Gage 2 Gage 3 Gage 4 Gage 5 Gage 6 Gage 7 Gage 8 Gage 10 Gage 12 Gage 13 Gage 

Average Error (ft) 0.01 -0.03 0.04 -0.10 -0.06 -0.11 -0.13 0.11 2.25 -0.51 0.17 0.0RMSE (ft) 0.14 0.14 0.16 0.19 0.18 0.19 0.23 0.22 2.26 0.55 0.25 0.1R-squared 0.80 0.79 0.73 0.75 0.22 0.78 0.51 0.74 0.31 0.71 0.73 0.9

Table 6.3. Error Statistics for RMA-2 Velocity Calibration. 

Statistical Parameter Gage 1 Gage 2 Gage 4 Gage 5 Gage 6 Gage 8 Gage 10 

Average Error (ft/s) -0.03 0.15 0.01 0.43 -0.06 0.11 0.02 RMSE (ft/s) 0.07 0.24 0.27 0.86 0.37 0.36 0.03 R-squared 0.19 0.15 0.32 0.09 0.01 0.66 0.98 
March 9, 2006 

6-8 

Average error is a measure of model accuracy, bias or systematic error. Positive values 

indicate systematic oversimulations while negative values indicate a systematic undersimulation. 

The tolerances within which the models are judged to be free of such systematic errors are not 

well defined. Standard Error, RMSE, determines the precision of a simulation. R-squared is the 

coefficient of determination, which is a statistical measure of how well predicted values 

approximate observed data points; an r-squared of 1.0 (100%) indicates a perfect fit of predicted 

values to observed data. 

After calibration, the next step is model verification. Verification is the process where a 

single simulation is performed using the model coefficients selected during calibration. 

Verification of the model was carried out using observed data for October 2004 through 

December 2004. Results are shown in Figures A.25 through A.38. The observed values are 

plotted against the simulated in Figures B.28 through B.42. The verification comparison statistics 

are shown in Tables 6.4 and 6.5. 

Based on experience from several modeling studies, the statistical parameters tabulated 

above indicate satisfactory verification. 

6.2 RMA-11 Calibration and Verification 

Several simulations were carried out to calibrate the salinity data at various monitoring 

gages by systematically varying dispersion coefficients. 

As a first step, specific portions of the southern mesh were assigned different element 

types to account for the variation in mixing characteristics of the various marsh regions. The 

southern segments of the canals were also assigned different element types than those of the 

northern portion. This allowed higher dispersion coefficients to be set in the southern areas 

where the primary mechanism of transport is mixing and dispersion. Figure 6.2 shows the 

various element types of the mesh. 
2 Water Surface Elevation Verification. Table 6.4. Error Statistics for RMA

Gage 

Gage 12 Gage 10 Gage 8 Gage 7 Gage 6 Gage 4 Gage 3 Gage 2 Gage 1 Statistical Parameter 

0.96 0.04 0.60 0.75 -0.02 0.12 0.98 2.40 2.77 -0.0

Average Error (ft) 

1.93 0.21 0.63 0.77 0.15 0.28 1.01 2.42 2.79 0.1

RMSE (ft) 

0.94 0.93 0.92 0.91 0.81 0.78 0.81 0.62 0.81 0.9

R-squared 

r RMA-2 Velocity Verification. Table 6.5. Error Statistics fo

Gage 16 Gage 8 Gage 5 Gage 4 Gage 1 Statistical Parameter 

Average Error (ft/s) -0.08 0.05 0.30 0.09 -0.03 RMSE (ft/s) 0.12 0.41 0.82 0.43 0.47 R-squared 0.72 0.09 0.11 0.66 0.02 

6-9 
6-10 

Figure 6.3. Location of RMA-11 element types in the mesh. 
March 9, 2006 

6-11 

In RMA-11, one can either specify the absolute value of the dispersion coefficient or a 

dispersion coefficient scaling parameter for a given element type. When a scaling parameter is 

specified, the program computes dispersion coefficients based on the size of the element and the 

average velocity at that element. The scaling parameter approach was used for the present 

calibration; the final values are tabulated in Table 6.6. 

Table 6.6. Final RMA-11 calibration parameters. 

Water Body 

Element 

Type 

Dispersion 

scale factor in 

X-direction 

Dispersion 

scale factor in 

Y-direction 

Minimum 

Dispersion in 

X-direction 

(m2/s) 

Minimum 

Dispersion in 

Y-direction 

(m2/s) 

Bayou Terrebonne 8 5 5 0.5 0.5 

Bayou Petit Caillou 12 5 5 0.5 0.5 

Grand Bayou 28 5 5 0.5 0.5 

Marsh south HNC-Bayou 

Dularge junction 30 10 10 0.5 0.5 

Barataria Bay near inlet 31 500 500 0.5 0.5 

Marsh south Point-auxChenes canal 32 10 10 0.5 0.5 

Marsh to the east of Grand 

Bayou 34 20 20 0.5 0.5 

Western part of Barataria 

Bay 35 100 100 0.5 0.5 

Rigollets, Little Lake area 37 10 10 0.5 0.5 

Upper Terrebonne Bay 38 10 10 0.5 0.5 

Terrebonne Bay 39 10 10 0.5 0.5 

Gulf of Mexico 41 500 500 0.5 0.5 

Open Water in Barataria Bay 42 2 2 0.5 0.5 

Southern segment of Bayou 

Dularge 44 100 100 0.5 0.5 

HNC between Cocodrie and 

B. Grand Caillou 45 100 100 0.5 0.5 

HNC south of Cocodrie 46 100 100 0.5 0.5 

Lower Bayou Terrebonne 47 100 100 0.5 0.5 

Southern part of Pointe-auChenes 48 10 10 0.5 0.5 

Lower Grand Bayou 49 10 10 0.5 0.5 

Barataria Waterway 50 2 2 0.5 0.5 

Southern 2D portions of 

canals 51-57 500 500 0.5 0.5 

All other marshes and 

channels 

1-7, 9-11, 

12-27, 33-

36,43 

1 1 0.5 0.5