LWI Region 3 Modeling Overview

BOEUF

MACON TENSAS

COCODRIE

4.2 WATERSHED-SPECIFIC DESCRIPTION 4.2.1 Boeuf a. Hydraulic Model - Design Storm Analysis

A total of nine gages were reviewed throughout the Boeuf Watershed to support frequency analysis and model calibration. Bulletin 17C analysis at Eudora produced a 1% Annual Exceedance Probability (AEP) discharge of 20,800 cfs, which closely aligned with HEC-HMS model results from upstream Arkansas basins. Conservative 100% saturation assumptions were used for design storms, while varied saturation scenarios supported calibration flexibility. The agreement between statistical gage results and hydrologic modeling confirmed the validity of inflows applied in the hydraulic model and ensured robust peak flow estimates moving forward.

BOUNDARY CONDITION UPDATES

OUACHITA BACKWATER: The downstream boundary at the Boeuf–Ouachita confluence was updated using a coincidence frequency analysis to capture compound flooding scenarios. INITIAL WSE (2YR-24HR): Pre-storm water levels for the 2-year, 24-hour event were set using steady-state simulations to reflect normal antecedent flow conditions. BAYOU BARTHOLOMEW (REGION 2): A lateral inflow boundary was added to represent levee overtopping from Region 2 during major events, improving calibration in the upper Boeuf.

MODEL SPLIT (CITY OF MONROE): A rating curve at the boundary between the City of Monroe and the rest of the watershed was applied. This segmentation improved runtime and model flexibility while preserving hydraulic continuity. See LWI Region 3 Map Book for the boundary of City of Monroe. BAYOU DESIARD INFLOWS: Region 2 inflows from Bayou DeSiard were also introduced as lateral boundaries, ensuring better volume accounting in Black Bayou and Monroe.

LEARN MORE: (B_4.1.1)

Antecedent Saturation for Design Storms { Design storm simulations assumed fully saturated soils across the basin, providing a conservative approach for estimating flood extents and depths. { The initial water surface elevation for all design storm events in the Boeuf Watershed was established by running the 2-year, 24-hour storm and using its resulting water surface as the restart condition. b. Rating Curve Development { Rating curves were created at key internal and boundary locations using rising limbs of hydrographs from model results. { Each rating curve was fit with a third or fourth degree polynomial of the form:

{ Sites included: Boeuf at Alto, Bayou Lafourche at Alto and Crew Lake, and Big Colewa Bayou near Oak Grove. { R² values ranged from 0.963 to 0.987, indicating strong fits, as demonstrated in Figure 4-9 .

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LOUISIANA WATERSHED INITIATIVE