INTEGRATED HYDRAULIC AND ECONOMIC MODELING OF FLOOD INUNDATION AND LEVEE IMPACTS, UPPER MISSISSIPPI RIVER, USA

Multidisciplinary modeling of levee-related impacts along a 134-km reach of the Upper Mississippi River quantifies the hydraulic impacts of levees as well as the economic benefits (flood damages prevented) across the spectrum of flood magnitudes.

Levee protection is known to involve tradeoffs: (1) protecting floodplain infrastructure, but also (2) altering rivers' natural geomorphic, ecological, and hydraulic response to flood flows. Most tangibly, by excluding floodplain inundation, levees increase stages, although surcharge estimates differ, and few studies have adequately quantified the long-term costs of levee protection.

This study modeled the Sny Island levee district, ~465 km² along the Upper Mississippi River. Hydraulic modeling included a 1D channel flow module and a high-resolution 2D overland flow module. Five flood magnitudes were modeled: the 2-, 5-, 100-, and 500-year events, with several scenarios each: current conditions (with levees), no Sny levees present, and several breach scenarios using levee-failure probability functions. Economic damages were modeled using FEMA's Hazus-MH, applied to a structure-by-structure database of residential, commercial, and industrial structures in the floodplain as well as utilities and crops. The annualized benefit of the levee system equals (1) flood damages with no levees minus (2) flood damages with the levees present, integrated across the range of event magnitudes/probabilities.

Many individual properties, and some property classes, derive a negative net benefit from their levees, due to the flood-stage surcharge from these levees. For example, average water levels on the Mississippi for the 100-year event are 1.2-1.5 m higher than without levees, and locally up to 2.1-2.4 m higher. Structures with negative benefits were located at higher elevations at the inland margins of the floodplain, with little and only very rare inundation prior to levee construction. This increased exposure and risk is manifested during levee breach and overtopping events. Elevated flood risk is also exported upstream, downstream, and to neighboring areas outside the levee system. As climate and anthropogenic factors increase flood frequencies on many rivers, the US needs to rigorously assess the hydraulics and economics of its structural flood control systems.