Management Strategies for Flood Protection in the Lower Illinois River Phase I: Development of the Lower Illinois River - Pool 26 UNET Model

One of the main concerns was the ability to specify proper stage hydrographs at the downstream boundary of the Lower Illinois River for hydraulic design and analysis. We found that a unique stage-discharge rating relationship does not exist at the lower boundary of the Lower Illinois River at Grafton because the backwater effects from the Upper Mississippi River. Management options and results for managed storage and emergency activities need to be analyzed under more comprehensive design of flooding conditions. To improve the capability of UNET for modeling backwater effects for the Lower Illinois River, an extended model including Pool 26 of the Upper Mississippi River was developed. The downstream stations of the model are at the tail of Lock and Dam 25 and the Mel Price Lock and Dam Pool, where stage readings are available. The model was calibrated with a 1979 flood and verified with a 1983 flood. Discharge and stage frequency analysis have also been performed for stations at Troy on Cuivre River, Lock and Dam 25 tail, Lock and Dam 26 pool, and Lock and Dam at Mel Price on the Mississippi River. Introduction Morphologic features of the Lower Illinois River from Peoria to Grafton include wide river valleys, steep bluffs, and flat channel slopes. Most of the floodplains have been reclaimed and included in organized Levee Drainage Districts (LDDs) for agricultural purposes (Illinois State Planning Commission, 1940). Construction of the LDDs was also mostly completed between 1879 and 1916 (Thompson, 1989), and levee heights in practically all districts had conformed to a grade line, at least 4 feet above the 1926 flood level, approved by the federal government (Illinois State Planning Commission, 1940). Figure 1 shows the locations of currently active LDDs along the Lower Illinois River. Reported levee heights could afford protection at a level approximating 20to 50-year return intervals (Singh, 1996; USACOE 1994). With existing buildings on the floodplain along the main channel, these levees apparently could not meet the floodplain encroachment regulations later specified by the Federal Emergency Management Agency or FEMA (1987). That agency defined the Regulatory Floodway as that portion of the floodplain that must be reserved from encroachment in order to pass a 100-year flood without increasing the water-surface elevation more than 1 foot, providing hazardous velocities are not produced. Studies have shown that the alignments of levees could affect the flood elevations at various magnitudes (Hall, 1991); there are real concerns about the actual levels of protection the levee system can provide. Changes in estimated flood heights also affect the level of protection a levee system can provide. There are indications of increased flood heights since 1970 due to a trend of increasing precipitation in the upper half of the Illinois River basin (Singh and Ramamurthy, 1990). The