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Litchfield, Illinois
2020-2023

Working under an IDIQ/MATOC contract with USDA NRCS, WRS is managing a joint-venture project to perform all work required to complete a PL 83-566 watershed project supplemental plan and environmental assessment to construct improvements to western Illinois' Lake Lou Yaeger dam and spillway to accommodate a dam-hazard classification change. The 44,430-acre/feet flood-protection reservoir also supplies water to about 10,000 citizens of nearby Litchfield, as well as supporting a major recreational area and park.

The joint venture is currently performing the 770-day contract to complete preliminary designs of alternative solutions, inventory, survey, investigate, coordinate, and plan all necessary functions to complete a supplemental watershed plan and environmental assessment acceptable to Illinois NRCS and the City of Litchfield under NRCS technical guidelines and criteria, regulations, policy, and conservation standards. Some unique challenges in meeting the goals of the sponsor and NRCS include the mixed-use nature of the lake and its proximity to the metropolis of Saint Louis, sedimentation of the lake relative to the presence of some of the nation's most productive farmland in the upper watershed, and competing interests among public participants in terms of urban vs. rural usage priorities.

WRS is specifically responsible for the H&H analysis of the existing dam and improvement alternatives. Ad Astra has completed the hydrologic analysis, including:

  • Delineating the drainage areas within the Shoal Creek basin

  • Quantifying current and future land uses

  • Developing weighted NRCS curve numbers

  • Identifying precipitation depths

  • Computing the time of concentration

The results of the hydrologic analysis provided the flow hydrographs used in the hydraulic models for the breach analysis and frequency- based floods routing. Land use maps and hydrologic soil group maps for each drainage basin were prepared, as well.