Agenda - Public Works Committee - 02/21/2023

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2019 Municipal Plaza and Center Street Concept Stormwater Reuse 06/26/2019 Page: 2 Preliminary Roadway Runoff Treatment The majority of the stormwater runoff will be generated from Center Street NW and the sidewalks on either side of the road. Stormwater runoff is proposed to be collected and treated by a combination of tree vaults and rain gardens located in the boulevard and on either side of the road. It is assumed that the runoff would drain directly into each of the raingardens and tree vaults through surface capture systems (i.e. curb cuts, ribbon curb, pretreatment structures, et). Granular filter media will be used for treatment and a perforated underdrain will connect the tree vaults and raingardens and route all the stormwater to the storage tanks. Preliminary Stormwater Reuse System Water collected and stored within the storage tanks will discharge via gravity flow to the pump house wet well, where it will be filtered and treated. The reuse water will be disinfected using UV treatment to address any fecal coliform, pathogens or other health concerns related to use of reuse water for irrigation. The assumed operating season for the irrigation systems is a 26-week period; beginning April 1st and extending through September 30th. The Metropolitan Council Stormwater Reuse Guide Water Balance Tool for Constant Irrigation Demand was used to determine the percent of time over the assumed 26-week irrigation season that the stormwater reuse system would be functional while operating within the assumed conditions. The tool assumes 1" of water over the irrigable area per week is required for adequate vegetation growth and health. The method uses historic rainfall data spanning 31 years to determine the functionality of the stormwater reuse system within the 26-week pumping period. Exhibit 1 summarizes the preliminary reuse system sizing. The "Irrigation Season" is assumed to be weeks 13 to 39 of the year, or approximately April 1 to September 30. In the calculator, these fields are used to look up weekly average rainfall to estimate the total potential volume of runoff that could be used for irrigation and how often we get 1 inch or more rain. Narrowing or widening the irrigation season changes the irrigation demand. The "Weeks that Flows Must be Augmented" and "Weeks that Reuse System will Meet Demand" cells cover the total period of record (837 weeks, or 16 years). So, 10% of the weeks (85 weeks) during the entire period of record result in a tank volume and rainfall volume less than the demand for that week. According to the reuse calculator, there will be enough captured stormwater water in approximately 90% of the pumping season weeks to apply 1-inch per week of water over the assumed 1.31-acre turf irrigation area (57,206 sq. ft.). The calculator provides a preliminary need of 454,00 gallons required to apply 1-inch per week during the irrigation season. Since rainfall contributes to that total, as well as stored water in that tank, the total required volume per year does not translate to the size of the tank required. Instead, the volume of the tank can be modified to target an irrigation efficiency. Therefore, the storage tank or tanks should be approximately 150,000 gallons in size to irrigate with stormwater for 90% of the year. Additional irrigation to meet the 1-inch demand, should the City choose to irrigation at that rate, would need to be supplemented with potable sources.