Interpreting Results for the HEC-FIA Holdouts Methodology

The HEC-FIA holdouts method computes FDR results within the model run. A majority of the effort involves verifying the total damages results make sense and understanding how FDR results are compiled.

Reviewing Total Damage Results

Since FDR results are generated from comparing individual damage computes, interpreting the FDR results starts with reviewing and verifying those damage computes. This review confirms total damage results from each compute are reasonable given the inundation data for the event. This review leads to uncovering errors not only confined to the HEC-FIA setup, but also errors in other models setup (i.e., bad boundary conditions and incorrect model linking).

The Flood Damage Reduction Aggregated Report in HEC-FIA (Figure 1) is a good place to review the results. This report displays the total damages and the total damages reduced for each of the four individual computes. By default, the results in this report are aggregated by holdout area.  However, any shapefile boundary (state, county, USACE district, etc.) can be used to aggregate results. The FDR results in the right side of the table are dependent on the precedence selected and are discussed more below.



Reviewing the total damage results for each compute reveals problem areas in the modeling domain. For example, the report may reveal unexpectedly high or low damages, regulated damages exceeding unregulated damages (negative FDR results) or suspect without-levee damage results in a particular area of the watershed.

You can investigate the causes of the potential errors by reviewing the damage results spatially.

Each individual compute has a results shapefile with prefix identifiers of:

• Reg_ – Regulated
• Unreg_ – Unregulated
• UnregWOL_ – Unregulated Without Levee
• RegWOL_ – Regulated Without Levee

You can view the results files and scrutinize their attribute tables by using GIS software. This can reveal where damages occur spatially, the depths used to compute the damages, and the source of the damage (cars, content, structure, etc.).

Going further, HEC-FIA outputs a hydraulic inputs shapefile that includes the computed WSEs and terrain elevations for each structure. Overlaying the damage results and hydraulic inputs ("Hinputs") shapefile with the inundation grids from HEC-RAS is an excellent way to confirm the HEC-FIA results are reasonable. Figure 2 demonstrates how you can use HEC-RAS Mapper to compare damage results against inundation results.

Interpreting Flood Damage Reduced Aggregate Results

Once the total damages from each of the four computes is determined to be satisfactory, you can interpret the FDR results from both the Aggregate Flood Damage Reduced Report and the Flood Damage Reduced by Project Report.

The HEC-FIA holdouts approach for FDR apportions damages from a structure to reservoir and levee projects based on the structure's location. The holdout area in which the structure resides determines how its damage is divided among reservoir projects based on the holdout flows associated with the holdout area. If the structure also resides in a leveed area, damages could be awarded to that levee as well as the appropriate reservoir projects.

The total damage reduced by all projects is computed by subtracting the regulated scenario (observed event) from the unregulated without-levee scenario. This subtracts the damages of least damaging scenario from the greatest damaging scenario. These results are found in the far right column of the aggregated report, with the combined total FDR for the entire modeling domain shown at the bottom of the column.

However, splitting the total damages reduced between levees and reservoirs creates a dilemma, because protection within a leveed area can be influenced by both the levee and the reservoir regulation. Currently, HEC-FIA allows you to select (via drop-down menu in the Aggregate Flood Damage Reduced Report) whether the reservoirs or levees takes precedence when splitting up the damages reduced between reservoirs and levees (Figure 3).



For the project given precedence (levee or reservoir), the benefit is calculated assuming the absence of the other project. For example, reservoir benefits are computed for reservoir precedence by subtracting the unregulated without-levee damage from the regulated without-levee damage. The Tables below show how the levee benefits and reservoir benefits are computed for the two different types of precedence.

HEC-FIA Flood Damages Reduced Computations with Reservoir Precedence

HEC-FIA Flood Damages Reduced Computations with Levee Precedence

Future enhancements in HEC-FIA will include a new option for more evenly apportioning damages reduced among reservoirs and levees using a weighted average instead of project precedence.

Interpreting Flood Damage Reduced by Project Results

The Flood Damage Reduction Project Report displays the damages reduced by each reservoir or levee project in the modeling domain. This report, shown in Figure 4, contains the results of most interest for reporting AFDR by project. This report is generated by apportioning the total reservoir damages and total levee damages to the individual levees and reservoirs in the watershed. For levees, the levee benefit for the selected precedence is computed as described in the previous section for each individual levee. For reservoirs, the totals are apportioned to the individual reservoirs by holdout flows or overrides.



When multiple reservoirs reduce damages at downstream locations, the total damages reduced by reservoirs must be divided among the individual contributing reservoirs. HEC-FIA provides two ways to accomplish this. The first allows you to manually set the percentage of damages reduced for each reservoir in each holdout area. This is useful if districts have legacy methods for allocating damages to reservoirs already defined and can be done in the HEC-FIA Alternative Editor, as shown in Figure 5.



The alternative method utilizes routed flows held out by each reservoir to characterize the relative impact the reservoir had in reducing flows at downstream locations. The damages reduced for all structures in the holdout area are apportioned to reservoirs based on the holdout flows at the CCPs. A reservoir's allocation percentage is calculated by taking the reservoir's holdout flow at a CCP and dividing by the sum of all holdout flows at the CCP for the peak flow. The results of the process are percentages used to allocate damages to each reservoir and can be viewed in the Reservoir Allocation Table, as shown in Figure 6.

You can also aggregate damages by shapefile from the Flood Damage Reduced Project Report in HEC-FIA. Since AFDR for projects are often reported by state, it is helpful to utilize a state boundary shapefile for reporting purposes. Figure 7 demonstrates how you can view results by state within the Flood Damage Reduction Project Report in HEC-FIA.