Rural Iowa towns often lack flood mitigation because of a lack of resources and inability to compete for federal assistance funds. The Federal Emergency Management Agency relies on a benefit-cost analysis which produces benefit-cost ratios (BCRs) for proposed projects to determine which communities receive funding, with an emphasis on the economic BCR, which compares potential future benefits with estimated capital costs. The FEMA requirement for an economic BCR is at least 0.75. The economic BCRs for mitigation projects in rural towns are often lower compared to those in urban centers due lower potential future benefits from lower building count and potential exposure. Here we use a multijurisdictional approach which analyzes flood mitigation at a watershed scale to join upstream agricultural potential future benefits with downstream potential avoided benefits in rural towns. We predicted BCRs of simulated flood retention wetlands using HAZUS-MH to find the potential future benefits a range of estimated capital costs via a percent reduction approach and a targeted peak flow approach to calculating wetland effects on peak flow.
The percent reduction approach generated BCRs of over 0.75 in the Mud Creek watershed for estimated capital costs per wetland up to $177,400. However, the simulated flood retention wetlands did not generate BCRs high enough to meet the minimum requirement in the Hinkle Creek watershed by itself. However, a multijurisdictional approach is not limited to each watershed individually. When the simulated flood retention wetland projects in each watershed were combined, the BCRs were high enough to meet the FEMA requirement. The combined BCRs were over 0.75 for estimated capital costs up to $143,300.
The targeted peak flow approach included BCRs which account for dry and wet antecedent soil moisture conditions and minimum, maximum and average peak flow change scenarios. The scenarios with dry antecedent soil moisture conditions created BCRs higher than wet antecedent soil moisture conditions. Further, the maximum peak change scenarios generated BCRs higher than average peak change scenarios, which in turn generated higher BCRs than the minimum peak change scenarios. In the Mud Creek watershed, the only scenario to generate BCRs above 0.75 for any part of the range of estimated capital costs was the maximum peak change scenario under dry antecedent soil moisture conditions. However, the maximum and average peak change scenarios under dry antecedent soil moisture conditions and the maximum peak change scenario under wet soil moisture conditions generated BCRs over 0.75 in the Hinkle Creek watershed. When the simulated flood retention wetland projects for both watersheds were combined, only the maximum peak change scenario under dry antecedent soil moisture conditions generated BCRs above 0.75. We found that a multijurisdictional approach is a viable method for rural watersheds to analyze potential flood mitigation projects to help increase their BCRs.