Computing Damages Reduced for the Detailed Approach

This method, while still valid, has been largely replaced by advances in HEC-FIA, and new users are encouraged to follow the steps outlined in the HEC-FIA Holdout RAS HDF Compute. However, for the sake of completeness, we'll cover this approach here. It requires more data manipulation than the RAS Holdout HDF Compute, but it remains a feasible option.

Summary of Key Steps

Extract damage results from individual damage computes
Validate results against logical constraints
Compute reservoir and levee weights for each levee
Compute weighted average FDR for each levee
Allocate total FDR between levees and reservoirs using weighted averages
Distribute reservoir FDR among individual reservoirs

Required Damage Scenarios

You’ll need results from four HEC-FIA computes, representing all combinations of levee and reservoir presence:

$\begin{array}{l}\displaystyle D_{OBS}=\textsf{total damage with both reservoirs and levees (regulated or observed)}\end{array}$

$\begin{array}{l}\displaystyle D_{WOL}=\textsf{total damage with reservoirs but no levees (regulated without levees)}\end{array}$

$\begin{array}{l}\displaystyle D_{WOR}=\textsf{total damage with levees but no reservoirs (unregulated)}\end{array}$

$\begin{array}{l}\displaystyle D_{WOP}=\textsf{total damage without reservoirs or levees (unregulated without levees)}\end{array}$

These results are available in the Aggregated Consequences Report in HEC-FIA.

Only areas with damages are listed in the report. If a leveed area experiences no damage in a particular scenario, it won’t appear—but it must still be reported to Congress. Ensure all federal leveed areas are included in your final AFDR dataset, regardless of whether they saw damage.

Example of Aggregated Consequence Reports from Individual Computes in HEC-FIA

Validating the Results Against Logical Constraints

Before proceeding, ensure the following constraints are satisfied:

$\begin{array}{l}\displaystyle D_{OBS} \leq D_{WOP}\end{array}$

$\begin{array}{l}\displaystyle D_{OBS} \leq D_{WOR} \leq D_{WOP}\end{array}$

$\begin{array}{l}\displaystyle D_{OBS} \leq D_{WOL} \leq D_{WOP}\end{array}$

If any constraints are violated:

Start troubleshooting from the HEC-HMS alternative and continue through HEC-RAS.
Check for instabilities in flow, bridge modeling, model linking, or lateral structures.
Once corrected, recompute all four HEC-FIA alternatives and validate again.

If errors persist and you're confident in the regulated (observed) damage compute, incrementally adjust the other scenarios, assuming observed damages is correct.

Allocating FDR Between Levees and Reservoirs (Weighted Method)

Once your damage results are validated, allocate the total leveed area FDR between the levee and upstream reservoir(s).

Calculate Key Variables

The following equations represent how much each component (levee or reservoir) reduces damage, depending on whether it was assumed to be installed first or last.

Equation 1:

$\begin{array}{l}\displaystyle R_{1}=D_{WOP}-D_{WOL} \quad \textsf{and} \quad R_{2}=D_{WOR}-D_{OBS}\end{array}$

$\begin{array}{l}\displaystyle L_{1}=D_{WOP}-D_{WOR} \quad \textsf{and} \quad L_{2}=D_{WOL}-D_{OBS}\end{array}$

Where:

$\begin{array}{l}\displaystyle R_{1}=\textsf{reservoir(s) in place first}\end{array}$

$\begin{array}{l}\displaystyle R_{2}=\textsf{reservoir(s) in place last}\end{array}$

$\begin{array}{l}\displaystyle L_{1}=\textsf{levee in place first}\end{array}$

$\begin{array}{l}\displaystyle L_{2}=\textsf{levee in place last}\end{array}$

Compute Weighted Allocation Percentages

Use the values from above in Equation 2 to compute allocation weights for each levee.

Equation 2:

$\begin{array}{l}\displaystyle W_{R}=\frac{R_{1}+R_{2}}{R_{1}+R_{2}+L_{1}+L_{2}} \quad \textsf{and} \quad W_{L}=\frac{L_{1}+L_{2}}{R_{1}+R_{2}+L_{1}+L_{2}}\end{array}$

Where:

$\begin{array}{l}\displaystyle W_{R}=\textsf{percentage of leveed area FDR allocated to all reservoirs}\end{array}$

$\begin{array}{l}\displaystyle W_{L}=\textsf{percentage of leveed area FDR allocated to the levee}\end{array}$

Why this matters: Unlike the "precedence" approach, the weighted method provides a more equitable distribution by treating reservoirs and levees as a system working together.

Calculate Final FDR Allocations

Once the weighted allocation percentages are computed, allocate the the total FDR per leveed area using the following equations.

Equation 3:

$\begin{array}{l}\displaystyle FDR_{T}=D_{WOP}-D_{OBS} \quad \textsf{and} \quad FDR_{R}=W_{R}*FDR_{T} \quad \textsf{and} \quad FDR_{L}=W_{L}*FDR_{T}\end{array}$

Where:

$\begin{array}{l}\displaystyle FDR_{T}=\textsf{total leveed area FDR to be allocated between reservoir(s) and the levee}\end{array}$

$\begin{array}{l}\displaystyle FDR_{R}=\textsf{total leveed area FDR to be allocated to all reservoirs}\end{array}$

$\begin{array}{l}\displaystyle FDR_{L}=\textsf{total leveed area FDR to be allocated to the levee}\end{array}$

This should be done for each leveed area in the modeling domain. Even if the reservoirs weren’t designed specifically to protect a given leveed area, any incidental benefits they provide should still be included.

System-Wide Allocation in Large Watersheds

Ideally, reservoir holdouts should be routed all the way downstream through connected watersheds—even across district and division boundaries.

Coastal watersheds may already include this routing in their setups.
Inland systems may require coordination between districts to ensure complete attribution.

The table below illustrates how upstream reservoirs (e.g., from the Missouri and Upper Mississippi Rivers) contribute to FDR in leveed areas on the Middle Mississippi River.

Leveed Area	Damage Reach	State	Overtopping		FDR Ratio (%)		FDR for Structures
Leveed Area	Damage Reach	State	OBS	WOR	Levee	Reservoirs	Levee	Reservoirs
Wood River	St. Louis	IL	No	No	100	0	$701,791,323	$0
Metro East		IL	No	No	41	59	$11,591,977,427	$0
St. Louis		MO	No	No	83	17	$193,485,623	$0
Prairie Du Pont		IL	No	No	59	41	$579,635,484	$0
Columbia		IL	No	Yes	95	5	$9,070,609	$12,833,648
Harrisonville		IL	No	Yes	100	0	$63,370,286	$43,139,736
Festus–Crystal City		MO	No	No	100	0	$105,204,862	$0
Prairie Du Rocher	Chester	IL	No	Yes	65	35	$21,234,347	$11,452,269
Ste. Genevieve		MO	No	No	100	0	$8,514,816	$0
Kaskaskia Island		IL	No	Yes	82	18	$10,167,674	$502,841
Bois Brule		MO	No	Yes	100	0	$56,310,769	$12,524,954
Grand Tower	Thebes	IL	No	Yes	100	0	$85,443,891	$18,054,731
Big 5 (East Cape)		IL	No	No	100	0	$175,018,154	$0
Cape Girardeau		MO	No	No	100	0	$45,783,854	$0

Allocating FDR Among Multiple Reservoirs

After determining how much total FDR is allocated to reservoirs, you must assign those benefits to individual upstream reservoirs.

Flow-Based Holdout Method (Recommended)

Correlate the peak stage date and time from the unregulated HEC-RAS simulation to the holdouts from the HEC-ResSim simulation for the respective damage reach gage locations. Then use the time slice (time of peak flow) of holdouts per damage reach to determine the proportion of the reservoir FDR for each damage reach. The figure below is an example plot showing peak stages plotted with holdouts from several upstream reservoirs.

Example Holdout Flows for Multiple Reservoirs

Use the following equation to compute percentage contributions for each reservoir.

Equation 4:

$\begin{array}{l}\displaystyle h_{A} = \frac{H_{A}}{\Sigma H}\end{array}$

Where:

$\begin{array}{l}\displaystyle h_{A}=\textsf{holdout for reservoir A as a percentage of the total holdouts of all reservoirs}\end{array}$

$\begin{array}{l}\displaystyle H_{A}=\textsf{holdout for reservoir A as a flow (cfs)}\end{array}$

$\begin{array}{l}\displaystyle \Sigma H=\textsf{summation of all reservoir holdouts for the damage reach area}\end{array}$

Set negative holdouts to zero. Reservoirs that were evacuating storage do not receive FDR credit.

Alternative Method: Manual Allocation (Legacy Method)

If your district has predefined splits for reservoirs, those percentages can be applied instead. However, using real-time holdout data from HEC-ResSim is the preferred engineering method. If you're still relying on legacy methods, it's strongly recommended to transition to the holdout-based approach.