LWI Region 3 Modeling Overview

BOEUF

MACON TENSAS

COCODRIE

b. Hydraulic Model (HEC-RAS) Model Overview and 2D Mesh Design { The hydraulic modeling of the Bayou Macon Watershed was performed using the HEC-RAS 2D ROM approach due to its flat topography and complex flow patterns throughout the basin. { Starting at Eudora, Arkansas, the model domain covers approximately 42 sq. mi. in Arkansas and 481 sq. mi. in Louisiana. { Mesh resolution was kept coarse at 1,000 ft in rural and flat areas and refined to 100 ft in urban areas like Delhi, Epps, and Oak Grove. { Breaklines were used extensively along roads, channels, dams, and high ground to preserve important hydraulic features and realistic flow paths. Survey Data Incorporation Hydraulic Structures { A total of 55 bridges, 77 culverts, 12 weirs, 20 control structures, and 105 cross-sections were surveyed along priority streams. { The number of structures surveyed differed from the actual modeled structures due to boundary updates and HEC-RAS model refinements during calibration. { Structures were modeled using Storage Area/2D connections with surveyed data, including deck elevations, culvert diameters, and spillway profiles, where available. Figure 3-30 shows the modeled structures in the Macon HEC-RAS model. LEARN MORE: M, 2.7 & M, 2.7.1.2.3

Terrain Modifications { LiDAR data received during model setup was flown during high water levels, causing inaccurate bathymetry in some cases. { Survey data was used to interpolate channel depth, width, and slopes between structures and cross sections. Figure 3-31 shows channel bathymetry before and after survey incorporation. { Terrain was refined through several iterations to best represent conveyance and storage during several storm events. { Changes in terrain data were evaluated across multiple events to verify accuracy. { Buildings on terrain were raised 20 ft, and hydroconnectors were used to create openings for unsurveyed structures. Boundary Conditions { The model's northern boundary at Eudora, Arkansas, uses a flow hydrograph from the USGS gage to account for inflows from the Arkansas portion of the Macon Watershed. { The downstream boundary at the Macon and Tensas River outlet uses the Fool River Stage hydrograph to represent significant backwater impacts during historical events. { Normal depth is set at the Joes Bayou outlet below Swan Lake. { The remaining boundary condition lines use rating curves characterized using results from the downstream Tensas Watershed RAS model. Figure 3-32 shows the rating curve developed for Sutt Bayou. LEARN MORE: M, 2.4.2.3 +

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LWI REGION 3 TRAINING

LOUISIANA WATERSHED INITIATIVE