Assessing Illinois’s flood vulnerability using Hazus-MH

In this study, we developed a flood vulnerability index to help planners screen the relative flood vulnerability across the entire state of Illinois at the county, jurisdictional, and census block scales. Our flood vulnerability index was comprised of a deterministic flood loss assessment using the Federal Emergency Management Agency’s Hazus-MH multi-hazard loss estimation software, coupled with a parametric social vulnerability index developed from US Census data. The flood-vulnerability screening revealed that approximately half (46 %; 8500 km2) of Illinois’s 18,500 km2 special flood hazard areas (i.e., 100-year floodplain) had low flood vulnerability (i.e., few people affected, with little or no flood losses). This finding substantially reduces the area that Illinois planners may need to focus their mitigation efforts. The relative flood vulnerability across the three spatial scales evaluated in this study generally mirrored each other (i.e., counties with high flood vulnerability had a substantial number of its jurisdictions and census blocks with high flood vulnerability). However, the census block-level analysis revealed that counties and a substantial number of jurisdictions with moderate-to-low relative flood vulnerability often had pockets (one to a few census blocks) of high relative flood vulnerability. This suggests flood-vulnerability screening should be performed to at least the census block scale to ensure pockets of vulnerability are not overlooked. Jurisdictional flood loss ratios (flood losses proportional to total floodplain exposure) in Illinois were generally largest in rural and relatively unprotected floodplain communities located along the state’s large rivers. This suggests the economic impacts related to riverine flooding would be more severe in these rural jurisdictions, relative to their economic base, and likely exceed the economic resources these communities could assemble for flood recovery. The jurisdiction flood-vulnerability screening results indicated the primary driver of flood vulnerability was different between urban and rural communities. In urban jurisdictions, social vulnerability was the main driver in flood vulnerability where in the rural jurisdictions flood losses tended to be the primary driver. This suggests different mitigation strategies will likely need to be employed in urban versus rural jurisdictions in order to reduce flood vulnerability.