HEC-RAS Model Output

HEC-FIA can use either cross section or gridded output for damage computes. Though not a requirement, it is strongly recommended that depth grids are utilized for FDR computes. There are several reasons grids are preferred over cross section output. Gridded output allows for the flexibility of future enhancements to HEC-RAS models and makes the without-levee compute in HEC-FIA much more straightforward. Additionally, any HEC-RAS model utilizing 2D flow areas can only produce gridded output for HEC-FIA.

Once each of the HEC-RAS plans are run, you can layer the inundation results to get a better picture of what to expect in the damage allocation step. Figure 1 shows an example of a watershed with multiple levees and the inundation results for the four different HEC-RAS plans. 



By layering the four inundation profiles in the order shown in the figure above, several assumptions can be made. First, if the regulated (i.e., observed) does not overtop the leveed area, it is safe to assume there is likely FDR that should be allocated to the levees. This is verified by seeing the With-Reservoir and Without-Levee inundation, which will give an indication of the magnitude of damages reduced by the levee. If the Without-Reservoir and With-Levee inundation does not show inundation within the leveed area, it is safe to assume the proportion of FDR allocated to the reservoir(s) will be very low, if not zero. The levee could have held back the flood event without the reservoirs, and therefore the vast majority of FDR should be allocated to the levee. Conversely, the more inundation shown in the leveed area for the Without-Reservoir and With-Levee scenario would likely attribute more of the FDR percentage to the reservoirs. This is an oversimplification of FDR, since it depends upon the density, location, and values of the structures throughout the leveed area and also the density, location, and time of year of the agricultural inventory within the leveed area. Much more detail is covered on the FDR allocations for reservoirs and levees in the HEC-FIA Model Setup, but the visual will help you understand (and verify) what to expect from the HEC-FIA computes.

Though the ultimate goal of FDR is to quantify the economic benefits of USACE projects, the gridded output from HEC-RAS provides a visual that can be included in annual reports to Congress and used for communication with the local stakeholders within the watershed, especially downstream of USACE projects.

Figure 2 shows a stark comparison of regulated versus unregulated inundation depth grids for a headwater reservoir watershed. In this figure, the black dots are the structures defined in the structure inventory. This image demonstrates the impacts the reservoir had on reducing downstream flooding with the red inundation area (without-reservoir condition) impacting substantially more structures than the blue inundation area (regulated reservoir condition).