Philosophy

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, geotechnical probability of failure relationship, 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.

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:

      ●  Study Configuration Data – the basic data defined for a study area; the physical stream locations (streams, damage reaches) and specific plans (analysis years, plans).  This data is common for all analyses, and is required for an assignment in HEC-FDA which is an integral part of the model.

      ●  Water Surface Profiles– a water surface profile set must consist of eight flood events and can be discharge- or stage-based for each stream in the study area.  Water surface profile data may be used to develop discharge-probability functions, stage-discharge functions, and stage-damage functions.

      ●  Exceedance Probability Functions – for economic and performance analyses an exceedance probability function is required.  An exceedance probability function is the relationship between flood magnitude and the probability of exceeding that magnitude.  This data maybe defined in terms of discharge (flow) or stage.  This relationship can be defined through statistical or hydrologic analyses.

      ●  Regulation Inflow-Outflow Functions – for reservoir operation and modification to unregulated exceedance probability function (if using flow).  In HEC-FDA this is referred to as the transform flow relationship and is entered with a defined exceedance probability function.  This function is used to define a relationship between unregulated and regulated flow, inflow and outflow, or another relationship to transform the flow defined by the exceedance probability function.

      ●  Stage-Discharge Functions – stage-discharge functions are required when an exceedance probability function is defined in terms of discharge.  The stage discharge function is used to transform the discharge into stage (and subsequently damage) for each probability.  A stage-discharge function is the relationship between discharge (flow) at a river cross-section and the stage (depth) produced by that discharge.  This relationship can be defined through a gage or hydraulic analysis.

      ●  Levee Data – levee data includes the top of levee stage, failure characteristics, interior versus exterior stage relationships associated with the levee, or wave overtopping criteria.

      ●  Damage Categories – damage category data includes a name, description, and price index (updates the monetary values of the structure that will be assigned to a damage category).

      ●  Structure Occupancy Type Data – structure occupancy type data includes depth-percent damage functions (structure, content, and other); content-to-structure value ratio; and, the uncertainties in the first floor elevation, value ratios, and the damage in the depth-damage functions.

      ●  Structure Modules – structure module data includes a name, a description, and an assignment to a plan and analysis year.

      ●  Structure Inventory Data – a structure inventory is a record of the attributes of unique or groups of structures relevant to flood damage analysis.  Structure inventory data is used to compute an aggregated stage-damage function by damage category at the damage reach index location station.

      ●  Stage-Damage Functions – stage-damage functions are the relationship of direct economic costs caused by flood inundation to a range of flood stages for a given stream or damage reach.  The model can compute stage-damage functions, if depth-percent damage functions, water surface profiles, exceedance probability functions, stage-discharge functions, first floor elevations, structure and content values are provided.

HEC-FDA has several different types of output; most of this output is stored in database files, with some being saved to ASCII text files.  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, these results are – Damage by Analysis Years (expected annual damage), Equivalent Annual Damage Analysis, and Project Performance.  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.