LWI Region 3 Modeling Overview

BOEUF

MACON TENSAS

COCODRIE

3.6.4 Cocodrie a. Hydraulic Model (HEC-RAS) Introduction

upstream of the Lower Gravity Drainage Structure aided interpolation, using values between the Acme and Old River gages. For events with closed gates due to high Riverside stages, starting elevations were assumed. LEARN MORE: C_4, C_4.4.1, & C_4.4.5 + PUMP STATION The Tensas-Cocodrie Pumping Plant Project ( Figure 3-39 ), authorized by the Flood Control Act of 1965, aims to mitigate flooding in Bayou Cocodrie. The project includes channel excavation, a pumping plant with a 4000 cfs capacity, and various levee gates. LEARN MORE: C_4.3.6 + Rules were defined to control the Tensas-Cocodrie Pumping Station, which operates only when the Upper Gravity Drainage Structure gates are closed. The pump station activates when Riverside and Landside stages exceed 35 ft, with a 0.1 ft buffer for stability. During calibration, recorded pumping did not match the Water Control Manual's procedure, so existing rules were disabled, and a lookup table was created to apply recorded flows. LEARN MORE: C_4.4.4 + LEVEE OUTLET STRUCTURES Rules were set for operating the Upper and Lower Gravity Drainage Structures. Upper gates close when the Landside stage exceeds 35 ft or when Riverside is higher than Landside and reopen when Riverside is below 35 ft and falling. Lower gates close when Riverside exceeds 30 ft or is higher than Landside and reopen when Riverside drops below 30 ft and is lower than Landside. LEARN MORE: C_4.4.2 & C_4.4.3 +

Bayou Cocodrie Watershed (≈590 sq. mi.) is fully leveed, with the Tensas-Cocodrie Pumping Plant as the only outlet when gates are closed. The terrain includes oxbow lakes and low-lying areas, making overbank areas key for flood conveyance and storage, as shown in Figure 3-38. LEARN MORE: C_1.1 + Data Collection Structures and cross-sections (93 culverts, 32 bridges, 1 weir, 21 outlets structures and 127 cross-sections) were surveyed along priority streams to improve model accuracy, especially in areas where LiDAR missed channel bottoms. AORC rainfall data (hourly, 4 kilometer resolution) was used for precipitation inputs, with events like March 2016 and September 2020 selected for validation due to contrasting hydrologic conditions. Land cover was derived from NAIP 1-meter aerial imagery, with cultivated areas and forests as the dominant types. These were applied as spatially varying Manning’s roughness values. Soils data came from the gSSURGO database. The Deficit and Constant Loss method was used to estimate infiltration and use parameters like initial deficit (2 in), max deficit. MORE INFORMATION: C_2, C _3.1-3 + During calibration, March 2016 flows showed a 60 cubic ft per second (cfs) peak inflow from the Tensas River into Bayou Cocodrie, causing less than a 0.1% rise in Lake St. John. Outlet flows stayed unchanged, so Tensas River inflows were found to have minimal impact and were excluded from model calibration. INITIAL CONDITION POINTS IC Points set starting water elevations at key watershed locations, including gages and Riverside outflows from the Cocodrie basin. An extra point Hydraulic Modeling INFLOWS

Figure 3-39: Tensas-Cocodrie Pumping System.

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LWI REGION 3 TRAINING

LOUISIANA WATERSHED INITIATIVE