Sewer diving construction delivers cost-effective storage capacity
Gregory A. Weeks, P.E.
Modifying existing infrastructure to maximize performance rather than building a new system can be well worth the effort. The City of Garden City, Michigan, embarked on an effort to increase their in-system storage capacity by modifying their existing system when faced with capacity issues. The project needed to demonstrate its merit to regulators and cost-effectively modify the sewer to maximize available excess capacity. Using an alternative construction method, commercial divers constructed in-line storage modifications inside an active sanitary sewer. The project was completed at a cost 42% lower than the initial bid based on traditional construction methods using bypass pumping.
After Garden City separated its storm sewer flow from its sanitary sewer system in the 1990s, structures were installed in the sanitary sewer to take advantage of available storage capacity and help prevent the City from exceeding its contract capacity to the Wayne County North Huron Valley/Rouge Valley (NHV/RV) Interceptor System. Despite these changes, flow monitoring revealed that the system was hydraulically deficient and provided minimal in-line storage benefit. Dry weather flow backups were occurring behind the in-line storage devices and causing significant sediment buildup. Upstream portions of the sewer system were also experiencing elevated levels during major storm events, creating the potential for basement flooding and the need for occasional bypass pumping.
As part of an ongoing Corrective Action Plan to reduce flows, the City embarked on Phase I of a multi-phased project to evaluate and modify its in-line storage system. Modifications included removal, relocation or resizing of weir and vortex devices. The goal was to maximize the in-system storage capacity and reduce the potential for wet weather sanitary sewer overflows (SSO), hydraulic surcharges and basement flooding. The project was partially funded with a Rouge River National Wet Weather Demonstration Project grant.
Sewer model demonstrates project viability
Initially, the Michigan Department of Environmental Quality (MDEQ) had discouraged Garden City's in-line system modifications project due to the poor performance of their current in-line system. To help demonstrate how in-system modifications could alleviate the problems, a dynamic Storm Water Management Model (SWMM) of the City's sanitary sewer system was developed. Dynamic system modeling demonstrated that in-system storage could be gained without adversely impacting the system's hydraulic gradelines. Modeling also helped determine the optimum configuration for the sewer and what in-system modifications were needed to maximize storage.
Sediment buildup in the sewers made model calibration difficult. Initially, the storage volume taken up by the sediment was not accounted for in the model. Field measurements were taken to estimate probable sediment quantities and determine a more accurate sewer storage volume. Once the sediment volume was accounted for in the model, good calibration was achieved.
Contractor negotiations prove worthy
Gaining contractor acceptance of the diver construction method was critical to meeting the City's budget. Initially, no contractors bid on the project. After lengthy discussions with the City, County and prospective bidders about why bids were not made, the bid date was extended and a pre-bid meeting was held with five prospective bidders. The idea of using commercial divers and working "in the wet" as an alternative to traditional bypass pumping approaches was presented at the meeting. Three construction bids were subsequently submitted. However, the lowest bid of $907,250 was nearly double the amount budgeted and still based on traditional construction methods. Further discussions were held with the low bidder about using commercial divers. After researching the idea and available diving firms, the contractor agreed to negotiate a shared risk (modified time and materials) change order with the City to use the diver approach.
Traditional construction would have required above-ground activities including installation of upstream dams and surface-installed bypass pumping around each of the 13 project sites to allow modifications to be made in dry sewer conditions. With 10 of the 13 sites located in major, five-lane roadways, bypass pumping could have required closing major portions or entire roadways for a significant time period, as well as higher costs and a longer construction schedule. Bypass pumping also could have resulted in potential risk for sewer backups and flooding during wet weather events if pumping capacities were inadequate.
Divers performed all in-system storage device modifications and installations. Eleven vortex regulators and three in-line weir plate assemblies were removed. Modifications were made at eight sites, and weir assemblies were constructed at two new sites. In addition to the modifications at the 13 sites, approximately 32,500 feet of 36-inch through 72-inch diameter sewers were cleaned and televised. Cleaning was conducted using traditional jet-vactor equipment specially outfitted for long runs of larger diameter sewers.
Removing sediment buildup to construct the modifications and further enhance available in-line storage capacity was challenging. Careful coordination was required to remove the old in-system restrictions and dewater backed-up flows to facilitate cleaning. Divers removed the existing devices one at a time, starting at the downstream end. After each was removed, the system was partially dewatered and the next device upstream was addressed. Once all devices were removed, localized sewer cleaning necessary for completing the in-line storage device modifications was conducted. After the modifications were completed, the remainder of the sewer was cleaned.
Constructing sewer modifications with commercial divers proved to be highly beneficial in this metropolitan area. Disruption to the traveling public was limited to intermittent lane closures around manhole entries into the sewer system. The method also kept the project on schedule. Although time lost in rebidding the project and negotiating an agreement with the contractor delayed the start of construction by nearly eight weeks, this alternative approach allowed all but about ten days of the delay to be made up during construction.
In-system capacity realized
Post-construction evaluation of the sewer system's performance indicates the in-line storage modifications have accomplished what the sewer system model predicted. Data collected from two permanent ADS flow meters that record the sanitary flow from the City's connections to the Wayne County Interceptor indicates that the City's peak 10-year, 1-hour flows have been reduced to approximately 50 cfs. Garden City's ability to more effectively use their existing storage capacity will further reduce the potential for wet weather SSO, hydraulic surcharges and basement flooding. Dry weather flow backups at the previous in-line storage sites have been eliminated reducing the amount of solids deposition and accumulation upstream of the devices. With less accumulated sediment being flushed out during major storm events, a lower quantity of sediment will be discharged to the Rouge River in the event of overflows.
This cost-effective solution advanced the City's Corrective Action Plan activities with the MDEQ. It positions the City to participate in a potential regional solution to control SSO and improve water quality in southeast Michigan at a significant cost savings than addressing flows on a strictly local basis. It also demonstrates that divers can be a cost-effective, fast and unobtrusive method to construct in-system sewer modifications given the proper sewer and project conditions.
Gregory A. Weeks, P.E, has 26 years of experience in consulting engineering focusing on stormwater, wet weather and wastewater facilities. He can be reached at (734) 947-9700 or firstname.lastname@example.org.