Risk assessments provides supporting evidence for plan formulation in the context of civil works water resources development. It is done to gain an understanding of flood risks and to measure and describe them to the fullest extent possible. Risk is broadly defined as a situation or event where something of value is at stake and its gain or loss is uncertain. Flood risk can be conceptualized as a function of the hazard, performance, and consequences, where consequences are a function of exposure and vulnerability as depicted in the figure below (USACE, ER 1105-2-101).

Each of the risk determinants has a specific implementation in HEC-FDA:

  • The hazard is the object that causes the harm: in this case, a flood. In HEC-FDA, the flood hazard is described in terms of frequency, flow, stage, extent, and depth. The hazard for the primary compute of risk (EAD) is modeled using an appropriate combination of the hydrologic and hydraulic input functions: discharge-frequency, regulated-unregulated flow transform, stage-discharge, interior-exterior, and stage-frequency. For example, the hazard of flooding from an undeveloped free-flowing stream would be modeled using an analytical discharge-frequency function and a stage-discharge function. For the purposes of aligning the hazard with the consequences (the stage-damage compute), the hazard is also modeled using geo-referenced hydraulic modeling (formerly known as water surface profiles), which describes stage and extent. 
  • Performance is the system’s reaction to the hazard. In the above figure, performance refers to the system features and the uncertain capability to contain/manage the flood hazard for the full range of possible events and for a specific event or load. We include performance in an HEC-FDA study as a top of levee elevation and a system response curve - a relationship between the hazard loading (stage) and the probability of system failure. 
  • Consequence is the harm that results from a single occurrence of the hazard. In HEC-FDA, consequences are typically measured in terms of economic damage for a given stage. Consequences are modeled using aggregated stage-damage functions (damage from a single occurrence of the hazard, for the entire range of potential hazard magnitudes). The aggregated stage-damage function is calculated by connecting the range of potential hazard magnitudes with exposure and vulnerability. We calculate depth above the first finished floor, find the percent damage for that depth, and multiply percent damage by structure value. 
    • Exposure describes who and what may be harmed by the flood hazard. Tools such as flood inundation maps provide information on the extent of flooding, which is used to define a boundary around who and what may be harmed. In HEC-FDA, we typically represent exposure with the use of a structure inventory which provides information on the population and property that may be affected by the flood hazard.
    • Vulnerability describes the susceptibility to harm of human beings, property, and the environment exposed to the hazard. In HEC-FDA, we represent vulnerability using depth-percent damage functions and structure characteristics like the elevation of the first finished floor. 

Flood risk explicitly considers the probability and consequences of subjecting people and property to the entire range of likely flood events, with consideration of risk management possibilities provided by structural or nonstructural measures. Using the HEC-FDA software, both economic flood damage and hydrologic engineering analyses are performed to characterize risk and evaluate plans. To assess the risks associated with flooding, the FDA program integrates information from a range of likely hazard events, evaluates the performance of defense strategies, and calculates the consequences to structures and content that remain exposed.

One commonly used metric of economic risk is expected annual damage (EAD) or average annual equivalent damage when computed on an annual basis over the period of analysis. EAD expresses risk as a combination of the likelihood and consequence of an event. Expected annual damages are the most tangible measure of the severity of the existing flood problem.

Expected annual damages is derived by combining the information from three basic relationships: the stage-frequency relationship, the stage-damage relationship and the damage-frequency curve. HEC-FDA combines stage frequency relationship curves with stage damage curves to develop the damage frequency curve which is then integrated to determine the area under the curve aka EAD.

The FDA program also produces engineering performance metrics to describe the capability of a system to accommodate the flood hazard as a single event or the full range of events. Metrics include expected annual exceedance probability (AEP), long-term risk (LTEP) and conditional probability (CNP). AEP is a measure of the likelihood of exceeding a specified target in any year. LTEP characterizes the likelihood of one or more exceedances of a selected target or capacity in a specified duration. CNP is the likelihood that a specified target will not be exceeded, given the occurrence of a hydro-meteorological event.