Western Slopes Drainage Study and Design

CHALLENGE: The 2017 flood event devastated the Village of Mundelein with flooding of residents and businesses in its downtown area in the vicinity of Division Street and US 45 (Lake Street). The Village hired HR Green to investigate reasons for flooding and prepare the Western Slopes drainage study and design. The upstream watershed is comprised of approximately 1,200 acres of urbanized watershed that converged to a single 54” storm sewer, which had insufficient capacity to convey even a 2-year storm event. This led to severe residential flooding, infrastructure damage, and roadway closures. Seavey Ditch is the ultimate outlet of the project and is a regulatory floodway. Increasing the flow in Seavey Ditch would cause downstream community impacts in Vernon Hills. Therefore, the project had to provide flood relief to the impacted area, while maintaining or reducing flows downstream.

SOLUTION: HR Green prepared a drainage study and developed four alternatives. The preferred alternative utilized a combination of strategically placed detention and conveyance improvements. HR Green worked with the Village to determine the appropriate balance between cost and level of service (LOS). One of the major accomplishments of this project was the adaptive reuse of the existing 54” storm sewer along Division Street, which was in good condition. An additional 72” pipe (in lieu of the required 96” pipe) was proposed in parallel to the 54” pipe with equalizing cross-connections to reduce costs while providing the desired LOS for flood reduction. To mitigate increases in flow, a 38-acre-feet wetland detention basin was proposed on a brownfield site adjacent to the waterway. The basin also provides water quality and recreational and educational benefits in the watershed.

HR Green then developed the preliminary and final engineering plans for the preferred alternative. HR Green navigated critical design aspects during the preliminary design stage and met with IDOT, IDNR-OWR, LCSMC, and Park District and coordinated geotechnical and utility conflicts. As a result, HR Green also assisted the Village in securing $500,000 in cost participation from IDOT for improvements within the US 45 ROW. In addition to the 72” RCP storm sewer along Division Street and the detention basin, the project includes three pre-cast box culverts and associated end sections; new storm sewer structures and junction chambers, and water main and sanitary sewer adjustments, including service connections/reconnections. Surface improvements included sidewalk, curb and gutter, pavement patching, pavement replacement and resurfacing. Maintenance of Traffic Plans was completed for US Route 45.

BENEFIT: The Western Slopes drainage study and design were bid with the lowest responsible bid approximately 10% below the engineer’s estimate. The project provides regional benefits by attenuating peak flows in the 38-acre-ft detention basin and water quality benefits by reducing sediment and pollutant transport downstream.