Hydraulic analysis for strategic management of flood risks along the Illinois River

Abstract

Causes of flooding in the Illinois River were due to heavy rains in the Illinois River Basin or the backwater effect from the Mississippi River, but most often due to flooding from the Illinois River Basin combined with the raised water level in the Mississippi River at Grafton. During the 2013 flood, two of The Nature Conservancy ecosystem restoration levee and drainage districts (Spunky Bottoms and Emiquon) were flooded by levee breach and overtopping, respectively. Simulation for levee breach at the Spunky Bottom LDD (levee and drainage district) indicated that the water surface elevation at the breach location in the Illinois River was around 446 ft and levee breach elevation started at around 445 ft, which was consistent with the field data. The impact to flood peak reduction using a single LDD as flow diversion storage has been analyzed for the 50- and 100-year frequency floods on the Lower Illinois River to provide information for strategic management of LDD for flood risk reduction. Thousands of UNET model simulations have been performed for this analysis. This study showed that the maximum flood stage reduction can be provided using a single LDD in the La Grange Pool are mostly less than 0.1 m for both the 50 and 100-year floods. LDDs in the Alton Pool below the La Grange L&D can provide more flood stage reduction. The reductions from the Scott County and the Big Swan LDDs can be as high as 1 m. Although LDDs below hillview can provide even higher stage reduction, LDDs would have been flooded by the 50 and 100-year floods thus would not have much impact as flood diversion storages. This study has shown floodplain storages can be used to reduce flood peaks. However, the effectiveness of each storage area needs to be strategically managed to achieve the best solution, due to the dynamic features of flooding conditions.