LWI Region 3 Modeling Overview

7 Future Use of Region 3 Models

7.1 BREAKOUT MODEL DEVELOPMENT Goal: Create a stand-alone model from a larger HUC8 watershed for focused simulations, as a breakout model (Figure 7-1).

1. Determine Subbasin Type { Fringe: no upstream inflow { Interior: receives inflow from the larger model

3. Fringe Subbasin Setup { Downstream BCs: use Normal Depth, Free Outfall, or Stage vs. Time { Validation: run model and compare runoff/timing with original 4. Interior Subbasin Setup { Upstream Inflow: extract flow or stage hydrographs from parent model and apply at boundaries { Downstream BCs: same as fringe { Lateral Inflows: include any local inflows or point sources from the original model as lateral BCs 5. Final Checks { Validate hydrographs and mass balance { Match or adjust time steps and solver settings from the original model

2. Common Setup Steps { Delineate Subbasin: use GIS or HEC-RAS tools; leverage depth grids to define boundaries, especially in flat or complex terrain { Clip and Modify Mesh: copy geometry in RAS Mapper → import subbasin shapefile as perimeter → remove external features; or, create a new 2D Flow Area using the polygon as a guide { Refine Mesh: use finer cells in channels/areas of interest, coarser elsewhere; reassign terrain, elevation, and Manning’s n as needed { Apply ROM: use original model’s precipitation raster; for basins <100 mi,² use grids without ARF { Modify Terrain (if applicable): clone terrain and add a Terrain Modification Layer for new/altered channels or added storage

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LWI REGION 3 TRAINING

LOUISIANA WATERSHED INITIATIVE