For each impact area, there are two sets of results that come out of a scenario compute: expected annual damage (EAD) outcomes and system performance statistics outcomes. Detailed results are available for each scenario by right-clicking on the scenario of interest. Summary damage and performance results for all scenarios can be viewed by right-clicking on Scenarios and selecting View Summary Results. All compute algorithms in HEC-FDA Version 2.0 are the same as those found in HEC-FDA Version 1.4.3, except those identified in the HEC-FDA Version 2.0 Release Notes.

Damage Outcomes

Two reports are available for damage outcomes: Damage with Uncertainty and Damage by Damage Category.

Damage with Uncertainty 

During the compute, HEC-FDA collects each expected annual damage (EAD) realization in a histogram (empirical distribution). The damage with uncertainty report provides information on that empirical distribution of expected annual damage. 

  • On the left-hand side of the results window, a table with the first, second, and third quartiles of the expected annual damage (EAD) distribution is provided. Implicitly, each quartile is associated with an exceedance probability. For example, the first quartile (25th percentile) in the image below is $735,186.69. Thus, there is a 75% chance that expected annual damage exceeds $735,186.69.  
  • A plot of the expected annual damage distribution is displayed on the right-hand side. The y-axis reflects the exceedance probability and the x-axis reflects the level of expected annual damage ($). This plot illustrates the expected annual damage distribution as a relationship between risk and probability of exceedance. This plot does NOT model a damage-exceedance probability function, a relationship between consequences and probability of exceedance, developed through the combination of flow-frequency, stage-flow, and stage-damage functions. Use the Scenarios Preview Compute option to inspect the damage-exceedance probability functions. 

Expected Annual Damage Distribution Scenario Damage Results

Damage by Damage Category

Damage by damage category displays the mean expected annual damage for each damage category found in the selected impact area. For the example scenario results included below, the mean expected annual damage is $465,559.85 for residential structures. This expected annual damage metric includes damage for all asset categories that were found in the occupancy types with which the pertinent stage-damage functions were computed. The asset categories include structure, content, other, and vehicle.

System Performance Statistics 

Three reports are available for performance outcomes: annual exceedance probability, long-term exceedance probability, and assurance of threshold (formerly known as performance target in Version 1). These reports are available for each impact area and optionally for each threshold. A default threshold should be calculated for the without-project conditions and is calculated the same way as in HEC-FDA Version 1.4.3. As a friendly reminder, there are four possibilities:

  1. For impact areas without levees, and if the scenario reflects the without-project condition, the user should have selected default threshold stage which the software calculates as the stage at which 5% of the damage of the 1% event occurs. This is considered to be the stage at which significant damage begins. 
  2. For impact areas without levees, and if the scenario reflects the with-project condition, then the user must have entered the default threshold stage from the without-project condition. With-project system performance then reflect the stage at which significant damage begins in the without-project condition. 
  3. For impact areas with levees but without a system response curve, the software calculates the threshold as the stage equal to the top elevation of the levee. 
  4. For impact areas with levees and with system response curves, performance does not reflect a fixed threshold. Instead, the software calculates performance as the joint probability of hazard loading (e.g. stage) and levee breach for the entire range of stages for which there is a non-zero probability of breach.

Annual Exceedance Probability 

In situations where structural performance (i.e., system response curve) is not part of the assessment, annual exceedance probability is the probability that a specific threshold is exceeded at a given location in any given year. In the example below, the threshold has been identified as a stage of 945.34 feet. The annual exceedance probability is then the frequency of 945.34 feet on the stage-frequency curve and the annual exceedance probability distribution reflects the distribution of frequencies of 945.34 feet from the set of stage-frequency curves calculated as part risk compute with uncertainty. 

In situations where structural performance is considered, annual exceedance probability includes the probability of failure as described by the system response curve. Annual exceedance probability as a function of system performance is the joint probability of hazard loading specified by the stage-frequency function and structural failure specified by the system response curve. During a risk compute, many stage-frequency functions are generated, resulting in many different annual exceedance probabilities. 

HEC-FDA collects each annual exceedance probability realization in a histogram (empirical distribution) during a risk compute. The annual exceedance probability report provides a summary of the empirical distribution of annual exceedance probabilities with three pieces of information:

  • On the top left-hand side of the report, the mean and median annual exceedance probabilities are reported. The AEP that has 90% assurance is also reported. 
  • Under the mean and median is a table that displays assurance of AEP. Observe in the table that there is a 75.08% chance that AEP will be less than 0.1. 
  • A histogram of the annual exceedance probabilities is displayed on the right-hand side of the report. This histogram displays the relative frequencies of binned annual exceedance probability realizations. 

Long-Term Exceedance Probability

Long-term exceedance probability reflects the probability that the threshold is exceeded at least once within a given interval of time. Three intervals are reported for the long-term exceedance probability report: 10yrs, 30yrs, and 50yrs. In the example below, the there is a 90.63% chance that the stage of 945.34 feet is exceeded at least once in 30 years. 

Threshold Assurance 

In cases where a system response curve is excluded, assurance reflects the chance that a random stage realization of a given flood event is less than the target stage. In the example of a levee, assurance is the chance that a random realization of channel’s water surface elevation does not exceed (overtop) the levee for a given flood event. When a system response curve is included in the assessment, then assurance reflects the chance that water does not move past the levee and inundate the floodplain, given the joint probability of hazard loading and structural failure or exceedance. Observe in the example below that there is a 71% chance that the threshold stage of 945.34 feet will not be exceeded among the 0.10 AEP flood events. Assurance is the term selected to replace “conditional non-exceedance probability” (CNP).


View Scenario Summary Results

To view summary results for more than one scenario at a time, right click on Scenarios and select View Summary Results. By default all computed scenarios are automatically selected. Uncheck the box to view only specific scenario results. There are four tables provided in the summary results: (1) Expected Annual Damage Distribution Summary, (2) Expected Annual Damage by Damage and Asset Category, (3) System Performance Metrics: Annual Exceedance Probability, Long-term Exceedance Probability, and Assurance of Threshold (Table 1, ER 1105-2-101), and (4) System Performance Metrics: AEP with 90% Assurance and Assurance of AEP. 

The top table summarizes the damage with uncertainty (mean and quartiles of the expected annual damage (EAD) distribution) outcomes for all scenarios. The second table in the summarized results provides the mean EAD for each damage and asset category found in the pertinent occupancy type data. The third table (from the top of the summary results window) combines the performance outcomes for the annual and long-term exceedance probability reports for the selected scenarios. The last summary table combines the annual exceedance probability and assurance of threshold reports for the selected scenarios.