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BACKGROUND: Underseepage is a phenomenon that normally occurs during floods and is only associated with levees constructed on a surficial clay blanket that covers a sandy or gravelly aquifer that is adjacent to a river channel which provides a hydraulic connection to the aquifer. Much of the Wood River Levee District meets these requirements with one area in particular that represents a serious threat to the system’s integrity (described below).

The Corps evaluates the underseepage potential of every flood protection system during the design phase. Excessive underseepage will manifest itself with sand boils and, if left uncontrolled, can soften the landside blanket and potentially lead to levee failure. If the Corps’ analysis predicts that the underseepage potential will exceed the Corps’ established factor of safety for underseepage, one of several methods can be employed to control the underseepage.

A serious condition exists in the low, marshy area, located landside of the levee extending about 3,500-feet upstream from the centerline of the Melvin Price Locks and Dam. This part of the levee contains Pool 26 from the dam’s centerline upstream to Alton Illinois. This 3,500 foot long area is very low (elevation 404) and, charged only by normal pool (water to elevation 419), exhibits heavy seepage and very soft conditions.

Timeline of Recent Events:
Aug 2009 Identified Uncontrolled Seepage
Nov 2009 Observed Movement of Levee Material during high water event
Nov 2009 Developed Risk Mitigation Measures
Nov 2009 High Water Forecast (Wettest Oct on Record)
Dec 2009 Public Briefing/Requested Funding
Mar 2010 High Water event triggered immediate implementation
     of Risk Management Plan
27 Mar 2010 Emergency Declaration
29 Mar 2010 Funding Received/Initiated Risk Mitigation Measures