The HEC-FDA software provides the capability to perform an integrated hydrologic engineering and economic analysis during the formulation and evaluation of flood risk management plans. The software follows functional elements of a study involving coordinated study layout and configuration, hydrologic engineering analyses, economic analyses, and plan formulation and evaluation. The model will be used continuously throughout the planning process as the study evolves from the base year without- project condition analysis through the analyses of alternative plans over their project life. Hydrologic engineering and economics (flood inundation damage analyses) are performed separately, in a coordinated manner after specifying the study configuration and layout, and merged for the formulation and evaluation of the potential flood risk management plans.

USACE requires the use of risk analysis procedures for formulating and evaluating flood risk management measures (EM 1110-2-1619, ER 1105-2-101). These documents describe how to quantify uncertainty in discharge-exceedance probability, stage-discharge, stage-damage functions, system response curves, and incorporate it into economic and engineering performance analyses of alternatives. The process applies Monte Carlo simulation, a numerical analysis procedure that computes the expected value of damage while explicitly accounting for the uncertainty in the basic parameters used to determine flood inundation damage. HEC has developed the HEC-FDA software to assist in analyzing flood risk management plans using these procedures. Expected and/or equivalent annual damage are computed in the evaluation portion of the program.

For more details about the methodology of HEC-FDA, see the HEC-FDA Users Manual

HEC-FDA requires a significant amount of data from external sources, and the input data requirements vary according to the size of a study. The following provides a basic outline of the data required by HEC-FDA Version 2.0:

• Impact Areas – An impact area consists of a boundary around a given area within the study area. A single set of impact areas is imported into HEC-FDA as a polygon shapefile. Previously known as damage reach.
• Hydraulic Modeling – Contains data on the modeled water surface elevations of flooding throughout a floodplain - this is the same information contained in HEC-FDA Version 1 Water Surface Profiles, except that this information needs to be georeferenced in HEC-FDA Version 2. Hydraulic modeling is used in HEC-FDA to identify the water surface elevation at each structure for at least 8 unique events. This information is used in calculating the stage-damage functions for each structure which are then aggregated by frequency to the stage-frequency relationship for that impact area, which is defined through the combination of the input summary relationships. The hydraulics data can also be used to develop a graphical stage-exceedance probability functions. This data should be provided in HDF, or as gridded data if an HDF is not possible.
• Frequency Functions – This function (also known as, exceedance probability functions) is the relationship between annual peak flood magnitude (flow or stage) and the probability of that flood magnitude being exceeded. The frequency function is a cumulative distribution function. A frequency function is part of the description of the hazard. In the risk computation algorithm, the level of damage in an impact area is identified using the flood magnitude. With a relationship between the flood magnitude and the probability of that flood magnitude being exceeded, we can identify the probability that a level of damage is exceeded, giving us the function that we need to calculate expected annual damage.
• Regulated-Unregulated Transform Functions – This relationship defines the flow transformation between unregulated flows into regulated flows. An example is an inflow-outflow function. An inflow-outflow function defines the outflow (flow) rate from a reservoir for a given inflow into the reservoir. A regulated-unregulated flow function is used to model the way that a structure such as a dam reduces the flow rate, and represents a part of the hazard piece of the risk equation. When dams are used to reduce peak flow rates, lower flows mean lower stages and therefore lower damages.
• Stage-Discharge Functions – A stage-discharge function is also known as a stage-flow function and under certain circumstances, a rating curve. This function represents the relationship between a given discharge (flow) and the expected distribution of stages at the index location of a given impact area. The stage-discharge function is used to transform the discharge into stage (and subsequently damage) for each probability. This function is required unless a stage-frequency function is used as the defining frequency relationship.
• Lateral Structure Data – A lateral structure is a barrier against the hazard which is built between the channel and the people and assets exposed in a floodplain. Lateral structure is the general term for flood risk management measures like levees. At minimum the top of levee elevation is required; however, a failure function (or system response curve), is desirable. This failure function represents the probability of breach for each stage up to the top of levee elevation.
• Structure Occupancy Type Data – This data is used to help link the structure inventory to the hazard, describe the vulnerability to the exposed assets, and reflect the uncertainty in consequence calculations. Occupancy type data includes asset depth-percent damage functions (structure, content, vehicle and other); content-to-structure value ratio; and, the uncertainties in the first floor elevation, depreciated replacement values, depreciated replacement value ratios, and the damage in the depth-damage functions.
• Structure Inventory Data – A structure inventory is a record of the attributes of unique or groups of structures in the floodplain relevant to flood damage analysis. For a given impact area, structure inventory data is used to compute an aggregated stage-damage function by damage and asset category. This data is imported as a point shapefile.
• Stage-Damage Functions – An aggregated stage-damage function is the relationship between the stage at the location in the river of the hydrologic and hydraulic input relationships and the coinciding level of damage in the floodplain. An aggregated stage-damage function with uncertainty is computed within HEC-FDA using the structure inventory, occupancy type data, a set of hydraulic profiles, and H&H input functions (flow-frequency and stage-flow or just stage-frequency) for each analysis year. Alternatively, for unique situations, an aggregated stage-damage relationship may be manually entered. 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.

HEC-FDA has several different types of output; most of this output is stored in database files and accessed through the user interface.Some files are saved to CSV (e.g., errors, and structure-level stage-damage details). For most of the input data, there is some form of output that is generated, since the model can generate a certain number of the functions (exceedance probability, stage-discharge, functions associated with a levee, and stage-damage functions). The output is displayed visually in the form of either plots or in a tabular format. The model also has output that is related to the results from the computations. Scenarios provide results on expected annual damage and performance, alternatives provide results on equivalent annual damage, and alternative comparison reports provide results on damage reduced (benefits). These reports are consistent with requirements of USACE planning regulations for formulation and evaluation of flood risk management. Display of model results are consistent with technical procedures described in EM 1110-2-1619.