Optimizing Davenport's Sanitary Sewer Capacity to Support Community Growth

Over the course of two decades, the City of Davenport, Iowa, has sought to develop an overall hydraulic strategy and hydraulic model for its major trunk sewer networks to determine anticipated peak-hourly wet-weather flows and identify conveyance capacity deficiencies. In 2016, field work verifications and a flow monitoring study were completed, which informed the development of a desktop hydraulic model for the Duck Creek sewershed. The hydraulic model indicated the Duck Creek North trunk sewer was severely hydraulically overloaded, leading to periodic sanitary sewer overflows and basement backups, while the West Diversion Tunnel (a deep tunnel interceptor sewer up to 96' diameter that bisects the Duck Creek North trunk sewer and was constructed to relieve overloading some two decades prior), was significantly underutilized and had excess capacity during the same events. During this same period, the City had been experiencing growth in industrial areas upstream of the Duck Creek North trunk sewer and sought a methodology to optimize its sanitary sewer capacity to eliminate backups and overflows, while providing additional capacity for economic development and growth. McClure completed a systemwide hydraulic strategy study and recommended low-cost modifications to the West Diversion Tunnel to relieve pressure on the Duck Creek North Sewer. Additionally, upstream sanitary sewer improvements were recommended to provide additional capacity to growth areas north and west of the City's existing collection system, which had the added benefit of connecting to a small, satellite controlled-discharge lagoon treatment facility the City has been operating. The primary project challenge was accomplishing these goals while minimizing impacts to other City amenities, including a popular recreational trail, as well as impacts to private landowners, given limited available financial resources. The Duck Creek Sewer Interceptor Extension project consists of approximately 26,600 LF of sanitary sewer, ranging in diameter from 54' to 18' at depths up to 35 feet below grade. Approximately 21,700 LF is 36' diameter or greater having an average depth of 25 feet. The project extends from the terminus of the West Diversion Tunnel, west along Duck Creek to Interstate 280, and south to the West Locust Industrial Park. Following completion, the satellite lagoon treatment facility will be subsequently abandoned and decommissioned. Along the alignment, there are seven creek crossings which were planned to be constructed using open-cut construction and temporary cofferdams. Additionally, the proposed sewer route also crosses the Iowa Interstate Railroad, two arterial streets critical for traffic flow in western Davenport, as well as Interstate 280. Due to sandy soils, persistently high groundwater, precise grade requirements, and complex regulatory permitting requirements, these crossings were designed to utilize closed-face slurry microtunneling as the allowable method, a highly specialized trenchless construction technology typically performed only by regional or nationwide specialty contractors and uncommon in Iowa. After 18 months of design and permitting, the project was bid and awarded in Fall 2023. The estimated construction cost of $16.5 million was funded by local American Rescue Plan Act (ARPA) funds and without the need for additional sewer enterprise revenue debt. The project is currently on track to be completed by the end of 2026. This presentation will focus on the technical challenges experienced during the design process, including deep open-cut construction and long trenchless crossings, as well as value engineering opportunities in construction specifically related to trenchless construction, the process by which those opportunities were evaluated, and how such alternative methods that have provided significant value to the City. Lastly, the presentation will conclude with lessons learned that can be applied to future projects in the Upper Midwest.