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ADS Stormtech and Isolator Row 9-20-16Metropolitan St. Louis Sewer District 2350 Market Street St. Louis, MO 63103-2555 (314) 768-6200 September 20, 2016 ADS Attn: Mr. Mark Joersz 605 Dartmouth Crest Dr. St. Louis, MO 63011 RE: Stormtech and Isolator Row Dear Mr. Joersz, The Metropolitan St. Louis Sewer District (MSD) has reviewed your July 5, 2016 analysis of the ADS Stormtech/Isolator Row system as a standalone post -construction stormwater Best Management Practice. MSD hereby grants approval of the Stormtech/Isolator Row system for use on new development, redevelopment and highway/roadway improvement projects of any size under the following conditions: Properly sized Isolator Row chambers are provided as a measure of pretreatment at each location where flow is injected into the Stormtech system. The sizing of the Isolator 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 Isolator Row — Flow Rates per Chamber Chamber Model Max Allowable Peak Discharge for the Water Quality Storm / (cfs) per Chamber SC-160 0.06 SC-310 0.10 SC-740 0.15 MC-3500 0.24 MC-4500 0.16 At a minimum, an equal number of non -Isolator Row chambers are required for each installation. The Stormtech outlet shall be perched a minimum 6" and the infiltration bed area sized in accordance with the attached Figure 1. With a perched outlet, and when sized in accordance with Table 1 and Figure 1, the Isolator Row and Stormtech 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 Stormtech/Isolator 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 Stormtech/Isolator 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 Stormtech and Isolator Row do not satisfy MSD rules, requirements, or construction and material specifications. Sincerely, Jason T. Peterein, P.E. Principal Engineer (BMP Committee Chairman) Engineering / Planning - Development Review Metropolitan St. Louis Sewer District Attachment: Figure 1 — Stormtech Underground Infiltration Bed Area Sizing Graph Infiltration Bed Area as Percentrage of Tributary Area (%) 50% 40% 30% 20% 10% 0% Figure 1 - StormTech Underground Infiltration Bed Area Sizing Graph 0 rtm6" Underdrain Height t12" Underdrain Height 41•8•018" Underdrain Height 10 20 30 40 50 60 Contributing Watershed Percent Impervious 70 80 90 100