Planning, Design, and Construction of the Gravois Trunk Sanitary Storage Facility in St. Louis, Missouri

INTRODUCTION The Metropolitan St. Louis Sewer District (MSD) is undertaking a long-term program called 'MSD Project Clear' to improve water quality for the St. Louis region by reducing the occurrence of basement backups and overflows caused by excessive wet weather sanitary and combined sewage flows. One of the approaches MSD is taking to meet this goal is constructing new infrastructure to increase the capacity of their existing sewer collection system, including new relief sewers, wet weather storage facilities, and tunnels to store and transport wet weather sanitary and combined sewer flows. This presentation will describe how wet weather storage technology was applied to attenuate excess flows and eliminate wet weather sanitary sewer overflows (SSOs) and backups in a portion of MSD's collection system. This approach has been used elsewhere in MSD's system and has potential application to other similar wastewater systems through the United States. PROJECT BACKGROUND In 2014, MSD, in association with engineering consultants Burns & McDonnell Engineering Company, Inc. and Wade Trim, Inc. began planning the Gravois Trunk Sanitary Storage Facility in Crestwood, Missouri. This facility is located along the 54-inch diameter Gravois Trunk Sewer, within the Gravois Creek watershed and the sanitary sewer service area of MSD's Lemay Wastewater Treatment Facility. Multiple sites were reviewed for location of the storage tank(s) based on a series of criteria, including operational flexibility, land needed and availability, location within the watershed and ability to meet hydraulic and flow reduction requirements, maintainability, reliability, operability, constructability, and environmental considerations. All sites evaluated were required to accommodate the preliminary storage requirement of 9.25 million gallons (MG). The site of an existing City of Crestwood public works maintenance facility was selected, which is located on Pardee Lane adjacent to the Gravois Trunk Sewer and Gravois Creek. HYDRAULIC MODELING Hydraulic modeling evaluations were performed to assess the capability of the facility to achieve two (2) goals: 1) the removal of two (2) Constructed Sanitary Sewer Overflow (SSO) Outfalls; and 2) limiting the hydraulic grade line (HGL) within the Gravois Trunk Sewer to a minimum of 8 feet below grade for a 10-year return period, except in areas where the sewer is less than 8 feet deep or in areas where basements will not be impacted. Meeting these goals will provide a 10-year Level of Service (LOS) for the Gravois Trunk Sewer. A calibrated XPSWMM model of the Gravois Creek watershed was used to verify that the required 10-year LOS is being provided for three different conditions, including for the 10-year 24-hour synoptic and 10-year 3-hour cloudburst design storms, as well as for a continuous simulation using 64 years of historical rainfall data to evaluate the long-term performance using real rainfall events. The 3-hour cloudburst event represents a high-intensity lower volume rainfall event, in comparison to the lower intensity, higher volume 24-hour synoptic rainfall event. The required storage volume to provide the 10-year LOS was determined using two methods: 1) Log Person method to determine probability of recurrence of exceeding required volume, and 2) meeting HGL limitation criteria for the downstream trunk sewer. The model was also run for the full rainfall data for the years 2000 and 2008 ('Typical' and 'Wettest' years, respectively) to evaluate the expected frequency that the storage facility would be in operation. Final design sizing concluded that the following minimum storage volumes would be required to provide a 10-year LOS: Synoptic: 7.97-MG; Cloudburst: 4-MG, and Continuous Simulations: 7.85-MG. Based on these results, a total storage volume of 8.0-MG was used for design. Modeling efforts also included development of a HEC RAS model to evaluate hydraulic conditions in the sewer downstream of the diversion structure for the purpose of optimally locating level measurement instrumentation to be used in process control of the facility. DESIGN CONFIGURATION Due to the site configuration, two (2) sanitary storage tanks with a storage volume of 4.0-MG each were planned rather than a single 8-0-MG storage tank. Above-ground storage tanks were designed using the AWWA D-110 Type III standard for Wire- and Strand-Wound, Circular, Prestressed Concrete Water Tanks. Tanks are 150-feet in interior diameter, with a maximum height above ground of 45-feet, in compliance with local ordinance. Tank floors are sloped to the center at a depth of approximately 8 feet below grade, and drained by gravity back to the trunk sewer. An aesthetics committee comprised of representatives of MSD, the design team, and public stakeholders was convened during design for the purpose of soliciting feedback on such features as tank façade, landscaping, and fencing. Other facility features included: - Non-staffed and fully automated - Diversion structure - Consolidation sewers - 54-MGD influent submersible pumping station - Granular activated carbon odor control system - Vertical turbine pumping system and rainwater harvesting system - Tank and wet well cleaning systems - Gravity dewatering sewers and control valves - Site paving, lighting, and landscaping including stormwater BMPs - Instrumentation and controls - SCADA system for centralized remote control - Control Building - Electrical Primary and Secondary Power - Miscellaneous Improvements POST-DESIGN ASSESSMENT Design was completed in 2018, and a construction contract in the amount of approximately $24 Million was awarded, after competitive bidding with four responsive bids received, to Plocher Construction Co. Inc. of Highland, Illinois. Construction began in 2019 and was completed in 2021. The Gravois Trunk Sanitary Storage Facility has been in successful operation by MSD since January 2021. CONCLUSIONS - AWWA D-110 tanks provide cost-effective above-ground storage - Above-ground wet weather sanitary storage requires extensive public outreach - Hydraulic analysis needs to consider future pipe lining, as well as potential for rapidly varied flow and/or backwater conditions in locations used for flow or level measurement - Need to build flexibility and operational feedback into facility design and process flow control strategies Refer to Picture 1 below for an aerial photo taken of the completed facility.