Flood risk in City of Des Moines Iowa is reduced by 16 miles of earthen levee and concrete floodwall that make up the Des Moines Downtown Levee Local Flood Protection Project (LFPP). The levees were federally authorized and were designed and constructed by the U.S. Army Corps of Engineers (USACE) between 1963 and 1972. The levees are operated and maintained by the City.
Following the 2008 Midwest Flood, updated flow frequency estimates increased over previous values and suggested that floods like the 1993 and 2008 events are more likely to occur than previously estimated. In addition to identifying the higher risk of flooding, it showed that the existing levees in downtown Des Moines do not meet FEMA freeboard requirements, thus affecting accreditation of the levees and impacting FEMA flood hazard mapping.
Stantec Consulting Services Inc. (Stantec) and the City began working in 2011 to identify measures to reduce flood risk in the City and bring the levees into compliance with FEMA requirements. Stantec identified potential alternatives to lower water surface elevations, and increase the levee height to meet these requirements, as well as considering alternative criteria to increase system resiliency to future flood frequency studies. The team also performed engineering analyses on closure structures, embankment protection, embankment seepage and stability, settlement and interior drainage to evaluate additional FEMA requirements listed in CFR 65.10. Additional projects were identified, including geotechnical and structural mitigation measures, pump station improvements and closure structure modifications. Stantec and the City developed an implementation plan in 2017 consisting of eight phases for design and construction of these alterations.
Because the levees were federally authorized and constructed, Section 408 permission was required to modify the levees. Stantec and the City worked closely with the USACE Rock Island District to navigate the Section 408 requirements and secure approval in 2018 for construction of the alterations. Design and construction of the first two phases was completed from 2020 to to 2023.
Curerntly, the City and Stantec are supporting USACE in a semi-quantitative risk assessment of the system given industry advancements in analyzing levee systems from a risk-based approach.
This presentation will focus on the overall approach and lessons learned in implementation of programmatic alterations of a federal levee system from a local sponsor perspective.
