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Prinsco 4-13-18 1 April 13, 2018 Prinsco Attn: Mr. Matt Walter 7218 S. Ridgetop Ct. Dardenne Prairie, MO 63368 RE: Prinsco HydroStor and Sediment Row Chambers Dear Mr. W alter: MSD grants approval of the Prinsco HydroStor HS75 and HS180 chamber system and sediment 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 HydroStor 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: Table 1 Sediment Row – Flow Rates per Chamber At a minimum, an equal number of non-sediment row chambers are required for each installation. The HydroStor 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 sediment row and HydroStor Chambers provide sufficient water quality treatment for its tributary area. A runoff volume reduction credit of 0.45 may be entered into “Item 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 investigation to determine the actual saturated hydraulic conductivity of the underlying soils beneath the HydroStor/sediment row system, b) assessment and Chamber Model Max Allowable Peak Discharge for the Water Quality Storm / (cfs) per Chamber HS75 0.15 HS180 0.24 2 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 HydroStor Chambers/sediment 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 HydroStor Chambers and sediment row do not satisfy MSD rules, requirements, or construction and material specifications. Sincerely, Jason Peterein, P.E. Principal Engineer (Proprietary BMP Committee Chair) Engineering/Planning-Development Review