What is an Aggregated Stage-Damage Function?

An aggregated stage-damage function is the relationship between the stage at the location of the hydrologic and hydraulic input relationships and the coinciding level of damage in the floodplain. The aggregated stage-damage function is aggregated at the impact area-damage category-asset category level for a given set of hydraulics modeling and H&H input functions (e.g. flow-frequency and stage-flow or just stage-frequency). 

What is the Purpose of an Aggregated Stage-Damage Function?

An aggregated stage-damage function reflects the consequences piece of the risk equation. When an aggregated stage-damage function is linked to a stage-frequency function, we get a damage-frequency function, the relationship that is integrated to calculate expected annual damage.

Where Do I Get an Aggregated Stage-Damage Function?

An aggregated stage-damage function with uncertainty is computed within HEC-FDA using the structure inventory, a set of hydraulic profiles, and H&H input functions (flow-frequency and stage-flow or just stage-frequency). Alternatively, for unique situations, an aggregated stage-damage relationship may be manually entered.

Working with Aggregated Stage-Damage Relationships in HEC-FDA

Typically, you will compute aggregated stage-damage relationships in HEC-FDA. You will need to have imported a set of hydraulics and a structure inventory as well as have defined for each impact area the flow-frequency and stage-flow functions or just stage-frequency function to compute the aggregated stage-damage relationships.

Stage-Damage Model Concordance 

A computed model of aggregated stage-damage (a function) depends on several inputs which are themselves models: hazard curves (e.g. flow-frequency, stage-frequency), transform curves (unregulated-regulated, stage-flow), hydraulics (water surface elevation grids), structure inventories, and occupancy types (including depth-percent damage functions, the model of structure damage). All of the modeled conditions reflected in each of the input models must concord with the intended condition of the model of aggregated-stage damage. For example, if the intention is to compute aggregated stage-damage functions for the base year of the without-project condition, then the input hazard curves (one for each impact area), transform curves (one for each impact area), hydraulics model, structure inventory, occupancy type data set should all represent the expected conditions in the river and floodplain in the base year of the without-project condition. Likewise, if modeling a with-project condition with a proposed levee, then the same set of input models must reflect a levee, which can mean revised modeling for the hazard curves, transform curves, and hydraulics. In this example, hydraulic modeling without a levee should not be used to compute levee condition aggregated stage-damage functions because the stage in a leveed river may not cause the same amount of damage as the same stage in the same river without a levee. Related, if there are reasonable projections of economic development in the study area independent of any flood risk management actions, then future condition stage-damage functions should be computed with a structure inventory that models the projected economic development. This concordance of modeled conditions is critical for proper counterfactual analysis with a well-defined intervention and because the aggregated stage-damage math depends on concordance. 

Differences Between HEC-FDA Versions 1 and 2

  • In HEC-FDA Version 1.4.3, stage-direct damage functions were entered into study data through the structure inventory, and were saved, imported from/exported to tab delimited within the occupancy types. In HEC-FDA Version 2.0, a stage-direct damage function for a particular structure should be entered using a dedicated occupancy type that has a reverse-engineered depth-percent damage function that replicates the stage-direct damage function. An alternative is to develop a separate set of aggregated aggregated-stage damage functions which are manually entered rather than computed internally. A separate scenario will need to be generated. Please note that stage-direct damage functions will not import into HEC-FDA Version 2.0 from HEC-FDA Version 1.4.3.
  • Index points are no longer required for a stage-damage compute. The aggregation stages are identified using the H&H input functions. 

Steps to Compute Aggregated Stage-Damage Relationships

Special Geospatial Data Requirements

Ensure that the coordinate reference system is either set to be the same across all geospatial data sets, or that the study coordinate reference system is set by selecting the study projection from the study properties editor under the File menu. 

The following items must be selected for configuration of an aggregated stage-damage compute: 

  1. Discharge-frequency function and discharge-stage function or just a stage-frequency function 
  2. Hydraulic data set
  3. Structure inventory 
  4. Occupancy types 

To create a new aggregated stage-damage function:

  1. Right-click on Aggregated Stage-Damage Functions under Economics within the Study Tree and select the create new option.
    Aggregated Stage-Damage Functions - Create and Import shortcut menu commands.
  2. Define a useful Name and Description.
  3. Select Computed.
  4. Enter the Analysis Year (e.g., base year is 2024 in the example).
  5. In the Configure and Compute panel, select the the appropriate hydraulic dataset (e.g., Steady-State_RASdata) and structure inventory (e.g., BaseWO). Then for each impact area select the appropriate Frequency and Stage Discharge functions. 
  6. Optionally, select Write Details to CSV to obtain structure-level stage-damage details (selected by default). The CSV file for the created function will be saved to you study directory in a new folder named StuctureStageDamageDetails.
  7. Click Compute Curves.

    Aggregated Stage-Damage Function FDA icon An aggregated stage-damage function is computed for each impact area, damage category, asset category combination.

     
    Example Aggregated Stage-Damage Functions dialog.
  8. Click Save, and Close.