Urban Floodplain Development, Building Damage Exposure, and the Economic Feasibility of Flood Mitigation

Monetary flood damage exposure to 2,255 buildings in an urban watershed in Omaha, Nebraska, subject to pluvial stream flooding, was used to quantify the characteristics of exposure and the impact of recent floodplain development on the economic feasibility of a pending $75 million USACE mitigation effort consisting of a dry dam combined with a floodwall and levee improvements. Total annual expected damage to building and contents was $9.8 million, and was concentrated among a relatively small number of recent structures. Over the 2012–2019 period, 157 buildings were constructed in the 500-year floodplain study area, representing only 7% of all buildings yet 24% of exposure. The majority (60%) of these buildings were commercial properties, and if they had not been built, the marginally low 1.21 benefit–cost ratio of the mitigation would be 0.78, making it economically infeasible and ineligible for federal funding. Without publicly owned and/or tax increment–financed development, the ratio would be 0.83 (also infeasible). Local water resource managers and urban planners should more carefully evaluate the merits of future development within the 500-year floodplain areas of the Papillion Creek Watershed, with particular attention to publicly owned buildings and subsidized development. More emphasis should be placed on educating property owners and builders on floodplain risk rather than trying to reduce regulatory flood risk boundaries while promoting economically infeasible mitigation. The USACE should more transparently report the characteristics of exposure, confirm the validity of decades-old depth-damage functions for estimating modern construction exposure, and consider excluding recently created exposure when evaluating the economic feasibility of mitigation.