Computing holdouts using HEC-FIA allows a user to compute FDR in a single HEC-FIA alternative from a CWMS forecast and apportion FDR to reservoirs and levees. As described in Selecting an Approach and CAVI Setup, the holdouts compute utilizes inundation data from two HEC-RAS alternatives representing the regulated and unregulated event. Additionally, it can utilize the holdouts computed with HEC-ResSim to apportion FDR among multiple reservoirs.

A HEC-FIA alternative used to compute FDR requires additional steps that go beyond the setup for CWMS implementation. This section covers the basic workflow for setting up the HEC-FIA alternative using the Grid Holdouts Inundation Configuration. The inundation configuration (IC) does not require the import or use of HEC-RAS cross sections in HEC-FIA, simplifying the setup significantly.

The naming of various model elements and DSS records is very important and should be consistent for all the models. In some cases, the naming alone is what relates model inputs to spatial elements to include items such as common computation points and reservoirs. The following sections will discuss this in more detail.

Boundaries

The national FDR database requires reservoir benefits to be reported by state. That said, if the watershed contains more than one state, a state boundary shapefile should be added to the HEC-FIA model. This allows the resulting FDR for each project to be broken down by state.

Storage Areas

In the context of gridded HEC-FIA computes, the storage areas in a study are only used to identify the areas protected by levees, and, therefore, what structures lie within levees. If there are existing storage areas in HEC-FIA, it is important to ensure they accurately represent the extent of the area protected by the levees. The storage areas can then be identified as leveed areas in the Storage Area Editor.

Start by exporting the storage area elements (1D storage areas and 2D flow areas) from the HEC-RAS model geometry using HEC-RAS Mapper.

You can then import storage areas into HEC-FIA from the Storage Areas node in the project tree. The Import Storage Areas dialog allows you to define which storage areas represent leveed areas. Note the RiverName field is optional. Only the areas that you identify as leveed areas will be included in the without levee compute. The Corps Project (CorpsPrj) attribute is optional as well, and is only used to distinguish results in reports.

The Storage Areas table can be edited so that the HEC-RAS storage area names are easy to understand.

The RiverName attribute can also be populated in the Storage Area editor. In a without-levee compute, HEC-FIA will determine the stream alignment from which to query in-channel WSEs by finding the closest stream alignment to the centroid of the storage area. In sinuous systems with adjacent levees, HEC-FIA may not determine that correctly. In that case, you should specify the stream alignment in the RiverName column, as shown in Figure 1 below.

 HEC-FIA Storage Area Editor

Reservoirs

For HEC-FIA to allocate damages to reservoirs, a reservoir set must be introduced to the study. You can import a polygon shapefile of the reservoirs into the HEC-FIA model, and one field in the shapefile should contain the names of the reservoir elements as they appear in the HEC-ResSim model. The names of these reservoirs relate flow holdout records at each common computation point (CCP) back to each reservoir. The polygons themselves are only for display purposes. A new reservoir shapefile can be created, or an existing shapefile can be edited using any GIS software application.

Figure 2 depicts the Import Reservoirs menu in HEC-FIA. "Jennings Randolph" and "Savage" are the reservoir names from the HEC-ResSim model in this example.

HEC-FIA Import Reservoirs Menu

Figure 3 shows an example of two holdout records for the "Alexandria" CCP. The HEC-ResSim computed flow holdout time series for each reservoir at the CCP has a C-Part of "FLOW-HOLDOUT-<Reservoir Name>".

Example Flow Holdout Records for Two Reservoirs

Stream Alignment

The stream alignment plays an important role in the without-levee compute, serving to determine where structures within levees query their in-channel depths. If starting from an existing HEC-FIA model, it is likely that a stream alignment exists. In this case, it is good practice to verify the stream alignment is properly situated in the center of the channel. If no stream alignment is defined, the centerline from the HEC-RAS model should be imported.

Holdout Distribution Areas

Holdout distribution areas, or holdout areas (HOAs), are shapefile polygons used to apportion damages reduced at structures to reservoirs throughout the system. HOAs define the geographic extent of the influence of the reservoirs for the associated CCP location. The HOAs group structures together so the damage reduced at every structure in a particular HOA is divided among the reservoirs using the same ratio. In Figure 4 below, the damages reduced at structures in HOA 1 are awarded based on the flow holdouts computed in HEC-ResSim at CCP 1. Therefore, RES A is awarded all the damages reduced. Conversely, damages reduced at structures in HOA 2 is divided among both reservoirs, based on the flow holdouts at CCP 2.

Example Schematic - Holdout Areas and Common Computation Points for a Two-Reservoir System

The workflow to introduce HOAs into the HEC-FIA study starts with exporting the bounding polygon that encompasses all the model elements from the HEC-RAS geometry using HEC-RAS Mapper. You can use GIS software to split the polygons into smaller reaches, so that all damages reduced at structures within the HOA are divided in the same way.

Note
This is a subjective process where engineering judgement should be used. Some examples of good places to split HOAs are 1) just upstream of gage locations and 2) confluences where reservoir outflows enter the system.

Reviewing hydrograph attenuation through the system may be helpful for you during this process. If gages are close together and holdout flow hydrographs don't attenuate much in between, larger HOAs are warranted. In general, fewer HOAs result in a less complicated model setup, and, in many systems, only a handful of HOAs are required. In a single reservoir watershed, only one HOA is required.

Figure 5 is an example of how HOAs can be set up for a small two-reservoir system. The HOAs shown in green each have an associated CCP shown in red. The two upstream reservoirs each have a CCP and associated holdout area to which only they contribute. Moving downstream, HOAs are broken up where changes in the system occur that impacts routing. 

Example of Common Computation Point and Holdout Area Placement in HEC-FIA for a Two-Reservoir System


You will then have to associate each HOA to a CCP from the HEC-ResSim model, making naming the HOA features important. A good practice is to name the HOAs based on the associated gage location name or HEC-ResSim junction name. Each HOA must be associated with a CCP.

Common Computation Points

A CCP is a point shapefile used with an HOA to apportion damages reduced to reservoirs throughout the system. Specifically, the HEC-ResSim flow holdouts records at CCPs are used to calculate a percentage of total damages reduced to award each contributing reservoir.

To import CCPs for a FDR compute, the HOAs shapefile must first be defined and the reservoir set must be imported.

Note:
This could be an iterative process as the HOA delineation is refined.

The first step in the process is exporting the CCPs from the HEC-ResSim model to a shapefile from the HEC-ResSim watershed module. You should use GIS software to thin out the point shapefile so that it only includes the locations associated with HOAs. Only one CCP is associated with every holdout area. These locations should be downstream of at least one reservoir, so that holdout records are available at the CCPs.

The CCP shapefile only requires a name field that matches the name of the HEC-ResSim junctions. This name is important, as it is used to query the forecast.dss file for holdout records. Once the CCPs, reservoirs, and HOAs are defined, you can import the CCPs into the study.

Within the Import Computation Points dialog window (Figure 6), select the appropriate shapefile for the CCPs and HOAs and the field that contains the name attribute for both. The A-Part field is typically left blank and the B-part should be the name of the CCP. You should then select the stream alignment and reservoir set the CCPs are associated with. The last step is to link each HOA with the appropriate CCP.

Example of the Import Computation Points Window in HEC-FIA 

Watershed Configuration

To combine all the previous setup elements into the HEC-FIA study, update the existing watershed configuration or create a new watershed configuration to contain the new reservoirs, stream alignment, and CCPs.

Inundation Configuration

A new Inundation Configuration type called Holdout Grids is available within HEC-FIA to compute FDR. The IC specifies what type of input data is used for the the computations. The Holdout Grids IC uses maximum depth rasters from HEC-RAS and flow holdout records from HEC-ResSim. You should create a new Holdout Grids IC and specify the appropriate watershed configuration.

First, you need to define a new Event for the IC, select the appropriate HEC-RAS and HEC-ResSim output files, and point to the resulting maximum depth grids for both the regulated and unregulated HEC-RAS simulations for the event. You should select the ".vrt" file that HEC-RAS outputs - if there are multiple terrain tiles, the ".vrt" references the results from all of them when depths are queried in HEC-FIA. Select the HEC-ResSim flow holdout data from a DSS file by selecting the appropriate file path, E Part, and F Part. If using overrides instead of HEC-ResSim data, uncheck the box next to "Use HEC-ResSim Calculated Holdout Time Series" shown in Figure 7 and leave the E Part and F Part empty.

Defining Event Data for Holdouts in HEC-FIA

Alternative and Simulation

The final steps for the HEC-FIA model setup are the creation of a new HEC-FIA alternative, time window, and simulation using the new model elements created in the previous step. Once the new simulation is computed, you can review results right-clicking the simulation's time window and selecting one of the two Flood Damage Reduction Report options. More information regarding the interpretation of these results can be found in Interpreting Consequence Results.