GEOLOGIC INFLUENCE ON 2008 MIDWEST LEVEE FAILURES

Severe precipitation befell the upper Mississippi River Basin during the summer months of 2008 flooded the upper Mississippi River and its tributaries in Illinois, Iowa, and Missouri. The Indian Graves Levee District near Quincy, IL was one of the areas inundated. The first earth core levee was constructed in 1879 and was heightened by adding 1.7 m of dredged sand fill, after 1965. This sand shell levee was approximately 5.6 m high and had a 50 year flood rating. Levees in this district failed previously, in 1947, 1965, 1965, and 1993. All of these breaches involved excessive underseepage when flood waters rose to within 0.35 to 0.75 m of the levee crests. The 1965 and 1993 breaches occurred where the dike crosses an old unnamed watercourse. The 2008 breach coincides with the location of previous break in 1888. A cursory review of historic aerial photos and NRCS soil surveys revealed the presence of old slough channels (commonly filled with sand) running beneath the levee at each of the historic breach sites, suggesting a preferential seepage path. It does not appear that these geomorphic factors were considered during construction of these levees. The historic levee breaches and their associated scour holes are repaired by infilling scour holes with dredged sand, without replacing the earthen cores. In these cases, both the levee and its foundation are comprised of clean, pervious sands, which are highly erodible, if overtopped. These limitations likely account for the recurrence frequency of failures along the dikes protecting the south basin (which are considerably higher than those protecting adjacent basins within the same district). These issues are of concern for agricultural levees throughout the USA. If the underlying geology is ignored it usually leads to poor levee performance. Many of these levees are quasi ‘legacy structures,’ whose cores were placed using non-engineered methods over 100 years ago. Subsequent improvements and heightening have been undertaken using standardized approaches, which employ idealized assumptions about foundation conditions and under seepage. Whenever these idealized assumptions are invalid, problems inevitably occur, leading to seepage-triggered failures, before the embankments are overtopped.