This is an application guide for crafting reasonable assumptions for addressing modeled high-frequency damage using HEC-FDA. It is a best practice to check models for high-frequency flooding during creation, DQC, and ATR. 

What is modeled high-frequency damage? 

Modeled high-frequency damage occurs when a model demonstrates positive damage with an annual exceedance probability more frequent than the 0.1 AEP event. In other words, high-frequency damage occurs when the software calculates damage at structures at or more frequently than every ten years. 

Why is modeled high-frequency damage a problem? 

High-frequency damage is not likely to play out in reality the way that models often reflect. The gravest problems are for the most likely future condition, but this issue can show up in the existing condition, too. Additionally, high-frequency flooding in the base year has a large impact on the annual damages that are computed, so they require justification from a policy aspect. Proof of compliance with Section 308 of WRDA 1990, modeling the most likely future without a project in place, and historical data showing repetitive flooding will likely be required.  Unexpected high-frequency flooding in the model may indicate issues with either the engineering data or the structure inventory. 

Why wouldn’t high-frequency damage play out in reality? 

Homeowners are unlikely to allow frequent flooding resulting in significant damage. The most likely future without intervention is that owner engaging some sort of mitigation, which is usually a nonstructural solution such as raising the structure.

How can I find modeled high-frequency flooding? 

When you compute stage damage, click the box that says “write details to file.” A structure damage detail report will be written to the study directory. When opened in Excel, the structure detail output file generated by HEC-FDA 1.4.3 places the depths relative to first floor elevation and total damages by frequency event in columns AS through BH. Using the filter function on row 11 will allow each of these columns to be sorted largest to smallest, which will allow the user to identify which structures are receiving damage at frequent events. Note that there is a known error in this output file that places the generic AEP events (0.5, 0.2, 0.1, 0.04, 0.02, 0.01, 0.005, 0.002) into row 11 instead of the actual events that were given to the model. The data does tie to the frequency events in the water surface profile, not the generic events.

How can I address modeled high-frequency flooding? 

When we think our models are not good reflections of reality, then we need to revisit the assumptions that went into building the model. We need to do everything we can to model good assumptions before resorting to truncating or zeroing out or other result manipulation. Below are some assumptions to consider when evaluating high-frequency damage in your model of risk. 

Homeowner raises structure in most likely future condition.

  • If a homeowner experiences significant damage in the existing condition and the stage of inundation with an approximate 0.01 annual exceedance probability (AEP) is above the structure’s first floor elevation, then the homeowner is likely to elevate the structure to the 0.01 AEP stage at their own expense. 
  • This can be done by creating two separate inventories: one for the existing condition and one for the future condition. Raise the first floor elevation to the 0.01 AEP stage for each structure of interest. 
  • When this assumption is applied to structures for which nonstructural actions are being considered, then flood risk reduction benefits would only occur in the existing condition. Costs avoided are only relevant to the extent that the government can do the work cheaper than a homeowner, and must be appropriately discounted and amortized. 

Homeowner uses temporary measures to reduce damage from nuisance flooding. 

  • If frequent depths of flooding are below the first floor, a reasonable assumption is that homeowners will use temporary actions to reduce the change of damage, such as placing sandbags at low openings. 
  • This can be done by using a beginning damage depth equal to the distance between the first floor elevation and the elevation of the temporary protective measures. For example, a structure with a basement might have a low opening at 2.5 feet below the first floor elevation, where the first floor elevation is 105 feet, ground elevation is 102 feet, and the 0.5 AEP stage is 102.7 so that the 0.5 AEP stage fills the basement. The homeowner would sand bag up to 103 feet, so that beginning damage is now 2 feet below the first floor elevation. A homeowner with a first floor elevation equal to the ground elevation might equivalently sandbag in a way that beginning damage depth is 1 foot above the first floor elevation.  
  • This assumption implies that emergency costs will be incurred and can be reasonably included the model of risk. 
  • When this assumption is applied to structures for which nonstructural actions are being considered, emergency costs are avoided, and risk reduction benefits are accrued to the extent of the difference between the homeowner’s temporary actions and the government’s permanent actions.  

Homeowner does not have means to arm structure against flooding. 

  • In this case, the homeowner is also unlikely to be able to repeatedly rebuild because insurance is unlikely to provide a complete compensating differential. 
  • This assumption can be modeled using a reasonable estimate of the most likely future condition structure value that would be commensurate with the reduction in quality that would occur between the existing condition and mostly likely future condition after 30-50 years of high-frequency damage. Teams could use RS MEANS to study the variation in values per square foot that correspond to variation in structure quality to identify a reasonable structure value discount for the reduction in quality. 
  • When this assumption is applied to structures for which nonstructural actions are being considered, risk reduction benefits include the quality loss avoided in the future condition.