Using HAZUS-MH flood model as a floodplain management tool: Evaluation of river engineering effects on flood losses for the Middle Mississippi River

Author

Megan L. Carlson, Southern Illinois University CarbondaleFollow

Date of Award

12-1-2010

Degree Name

Master of Science

Department

Geology

First Advisor

Pinter, Nicholas

Abstract

By combining FEMA's HAZUS-MH (Hazards U.S. Multi-Hazard) flood-loss estimation software and the HEC-RAS hydraulic modeling package, this study was able to quantify potential beneficial and adverse impacts of flood-control and navigational structures along the Middle Mississippi River (MMR; between Mississippi-Missouri River confluence and Thebes, IL). The goal of this investigation was to assess changes in water-surface elevations and associated flood losses to: 1) quantify the potential exposure of flooding under different flood-control configurations along the Middle Mississippi River (MMR), and 2) assess the relative contributions of various engineered structures and flood-loss strategies to potential flood losses. Assessment of the impact of engineering structures was accomplished by modeling five scenarios for the 100- and 500- year floods: 1) current MMR levee configuration (levee protecting for ≤50-year flood); 2) removal of all flood-control structures on the MMR; 3) increasing the height of levees and floodwalls in metropolitan St. Louis to protect urban areas to the 500-year flood level while simultaneously removing all agricultural levees downstream; 4A) a less engineered MMR channel and floodplain with fewer flood control and navigation structures, simulating conditions from 65 years ago (1942-1947) with 1940's levees; and 4B) a less engineered MMR channel and floodplain with fewer flood control and navigation structures, simulating conditions from 65 years ago (1942-1947) with current levee configuration. Comparison of scenarios 2 and 3 relative to scenario 1 allows for quantitative assessment of the flood-control structures on stages and flood losses. Results from scenario 2 revealed that removing all levees along the MMR reduces the average stages from 2.2 m (100-year) to 2.5 m (500-year, but also increased economic and social impacts relative to scenario 1. Scenario 3 revealed that removing agricultural levees downstream of St. Louis on the MMR decreased stages by 1.4 m (100- and 500-year); however, flood losses for the 100-year flood were increased. Flood losses for the 500-year flood were decreased relative to scenario 1. These results suggest that agricultural levees along the MMR protect against medium size floods (50- or 100-year flood) but cause more damage than they prevent during large floods such as the 500-year flood. Comparison of scenarios 4A and 4B relative to scenario 1 allows for a quantitative assessment of river engineering structures and modern buildings constructed over the last 65 years. In scenarios 4A and 4B, a less engineered river decreased stages by 1.2 m (for the 100-year flood) relative to scenario 1. In scenario 4A, the 1940's levees expose modern buildings in the floodplain to flooding, causing economic building losses to increase; however, in scenario 4B, current levee configuration protects modern buildings in the floodplain from flooding causing, economic building losses to decrease. If the current flood-control structures were not built, it is likely that the land in the floodplain for scenarios 4A and 4B would not be developed and the land used would be more flood-tolerant. Sensitivity analyses were run to assess the impact of using the default HAZUS-MH national-level data; this was done by comparing results produced by using aggregate analysis (coarse data) versus results using UDF analysis (detailed data). The aggregate analysis estimated 51% fewer buildings damaged than the UDF analysis. Conversely, the aggregate analysis increased the economic building losses by 51% relative to the UDF analysis. Although collecting local data for a study is not always feasible, the large differences documented here need to be considered when discussing HAZUS-MH results. Overall, this project shows implications for historic and future flood-control and navigational structure projects on the MMR and other rivers. It also emphasizes the importance of studying the impact future engineering structures will have on water-surface elevations and flood losses before implementing them.