ASSESSING LEVEE IMPACT ON FLOOD HAZARD WITH FLOOD-LOSS MODELING AND RETRO MODELING

By combining FEMA’s HAZUS-MH (Hazards U.S. Multi-Hazard) flood-loss estimation software and the HEC-RAS hydraulic modeling package, we are able to quantify potential beneficial and adverse impacts of levee configuration along the Middle Mississippi River (MMR). The MMR runs from the Mississippi-Missouri River confluence south to Thebes, IL. We are assessing changes in flood levels and associated flood losses for four scenarios for the 100- and 500- year floods: 1) current MMR levee configuration (levee protecting for ≤50-year flood), 2) no-levee scenario, 3) increasing the height of all MMR levees to the 500-year flood level, and 4) increasing the height of levees and floodwalls in metropolitan St. Louis to protect urban areas at the 500-year flood level, while simultaneously removing all agricultural levees downstream.

Assessment of total potential flood losses along the MMR was completed using HAZUS-MH national-level data. It was updated by adjusting the depreciation rate in HAZUS-MH to better represent the cost of structures in the floodplain. A raster layer representing the water surface elevation for each of the four scenarios was subtracted from a high-resolution DEM for the study area to produce a flood inundation layer.

Comparison of the scenario for current conditions and the other three scenarios allows for quantitative assessment of the impacts of inundation and inundation depths, associated flood losses, and levees, on flood hazard. Preliminary results show that the average flood levels for the no-levee scenario decreased by 1.9 meters for a 100-year flood and 2.5 meters for a 500-year flood compared to the current levee configuration scenario. Counterintuitively, building losses for a 100-year flood are estimated to be 25% lower for the no-levee scenario than for the current configuration. These preliminary results suggest that levees along the MMR may cause more damage than they prevent. New levees and levee systems continue to be planned and built nationwide, for example $4 billion in new 500-year levees proposed under the Upper Mississippi River Comprehensive Plan. Such plans need to balance the benefits of local protection with the potential harm from regionally elevated flood levels and increased floodplain development.