LWI Region 3 Modeling Overview

BOEUF

MACON TENSAS

COCODRIE

3.6.3 Tensas a. Hydraulic Model (HEC-RAS) Topographic and Bathymetric Survey, Channel and Lake Bathymetry

LiDAR data cannot penetrate water surfaces, leading to missing or inaccurate channel and lakebed elevations and resulting in elevation spikes in the raw DEM that distorted flow and storage representation. These issues were corrected through DEM modifications, including channel burning and hydro-flattening. Where available, survey data was used to adjust DEM shapes; otherwise, aerial imagery and land use data were used to estimate channel widths and lake bathymetry, especially in oxbow lakes where storage plays a key role. Accurate bathymetry and terrain representation were essential for calibrating the Tensas River Watershed model, ensuring peak stage timing at gage locations matched observed data. Figure 3-34 and Figure 3-35 show Boggy Lake before and after bathymetric updates.

Figure 3-34: Terrain at Boggy Lake before lake bathymetry update.

Figure 3-35: Terrain at Boggy Lake After Lake Bathymetry Update

b. HEC-RAS Boundary Options Outflows The Tensas Watershed discharges near the confluence with the Ouachita River, which causes significant backwater effects that impact flooding just as much as local rainfall. To model this influence, a stage hydrograph derived from observed data at Clayton, Harrisonburg, and Jonesville gages was applied at the watershed outlet. Figure 3-36 illustrates the downstream confluence.

c. HEC-RAS Model Calibration and Validation Calibration was performed using historical storm event data from USGS and USACE gages across the watershed (e.g., Transylvania, Tendal, Newlight, and Clayton). Model adjustments were made to align simulated results with observed data. Accurate calibration depended on realistic terrain storage, boundary conditions, and hydraulic structures like weirs. Figure 3-37 shows a calibration example at Southeast of Tendal; Table 3-7 details Fall 2008 storm metrics.

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LWI REGION 3 TRAINING

LOUISIANA WATERSHED INITIATIVE