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2018-10-09 Triton GUL Page 1 of 2 October 9, 2018 Triton Stormwater Solutions Attn: James Maier, Triton Chief Engineer RE: Triton Stormwater Solution Chambers and Triton Main Header Row Dear Mr. Maier: MSD grants approval of the Triton Stormwater Solution M6, C10, and S29 chamber systems and Main Header Row for use as a standalone post construction stormwater Best Management Practice. These chambers may be used on new development, redevelopment and highway/roadway improvement projects of any size under the following conditions: Properly sized sediment row chambers are provided as a measure of pretreatment at each location where flow is injected into the Triton system. The sizing of the sediment rows shall be based upon the Peak Discharge for the Water Quality Storm (Appendix D.10 of the 2000 Maryland Stormwater Design Manual) tributary to each point of injection, and Table 1: Chamber Model Max Allowable Peak Discharge for the Water Quality Storm (cfs) per Chamber M6 0.030 cfs C10 0.037 cfs S29 0.061 cfs Table 1: Main Header Row – Flow Rates per Chamber At a minimum, an equal number of non-sediment row chambers are required for each installation. The Triton outlet shall be perched a minimum 6” and the infiltration bed area sized in accordance with Figure 1 noted in an approval letter granted to a similar product dated September 20, 2016. With a perched outlet, and when sized in accordance with Table 1 and the referenced Figure 1, the Main Header Row and Triton Chambers provide sufficient water quality treatment for its tributary area. A runoff volume reduction credit of 0.45 may be entered into “11. Site Specific BMP” in the MSD MEP spreadsheets. It is noted that Figure 1 was derived using an assumed underlying subgrade saturated hydraulic conductivity of 0.05 in/hr. This may be the default assumption for all sites utilizing a 6” perched outlet. When configured with a greater than 6” but less than 18” perched outlet height, MSD will require the following from a Professional Geotechnical Engineer licensed in Missouri: a) Onsite geotechnical Page 2 of 2 investigation to determine the actual saturated hydraulic conductivity of the underlying soils beneath the Triton/Main Header Row system, b) assessment and recommendation for pavement structural stability, c) assessment of possible impacts to adjacent structures due to seepage, including recommended setback distances between those features and the Triton/Main Header Row system. In all instances, a minimum 2 feet separation is required between the bottom of the infiltration bed and the seasonal high water table. Integration with Channel Protection and Detention Design Channel Protection storage, 2yr-24 hour and 100yr-24 hour detention storage may be managed within the same system, with a properly configured, sized, maintenance-accessible, reinforced concrete control structure. The footprint of the infiltration bed/chamber system shall be sized at a minimum to manage Water Quality, and expanded as necessary to manage these additional volumes as required on sites. MSD reserves the ability to withdraw or modify this approval based on subsequent information, including information indicating that the Triton Stormwater Solutions Chambers and Main Header Row do not satisfy MSD rules, requirements, or construction and material specifications. Sincerely, Kaleena Menke, PE BMP Committee Chair Engineering / Development Review