Loading...
Pace ConstructionMetropolitan St. Louis Sewer District 2350 Market Street St. Louis, MO 63103-2555 (314) 768-6200 October 27, 2009 Mr. Phil Hocher, President Pace Construction, Inc. 1620 Woodson Road St. Louis, Missouri 63114 RE: Porous Asphalt — Proprietary BMP Application Dear Mr. Hocher, The Metropolitan St. Louis Sewer District (MSD) has reviewed your application of Porous Asphalt that is a proprietary pervious pavement type of BMP. MSD hereby grants Provisional Use Level approval for the Porous Asphalt with the storage bed underneath. MSD understands that the Porous Asphalt enhances surface permeability with the porous openings in the asphalt. The open graded stone storage bed underlying the Porous Asphalt provides temporary storage for the runoff. MSD has deteititined that Porous Asphalt with open graded stone storage bed system may be used under the following conditions: 1. Proposed uses and designs of the product must be in general confoitiiance with the infoiiiiation and methodologies provided in the Pace Construction Inc. Porous Asphalt Pavement application dated August 22, 2008. 2. Channel Protection Volume (CPv) storage may be provided in the storage bed underneath the pavement. 3. With a minimum gravel storage bed of 12" underneath the pavement and a designed underdrain, the post -developed curve number (CN) may be reduced. Except for type A soils, the CN number that should be used should be reflective of Open Space Good Condition areas and the soil type of the underlying native soil. For convenience, the CN numbers are provided below: Soil A B C D CN 61 61 74 80 4. The imperviousness used in calculating the Water Quality Volume (WQv) may be reduced. The area of porous pavement may be considered as pervious area for the purpose of calculating Rv, and accordingly WQv. 5. Porous Asphalt may be used to provide WQv treatment, when used in conjunction with a properly designed infiltration basin/trench or sand filter per the 2000 Maryland Stoiruwater Design Manual. However, Porous Asphalt is not to be used as a stand-alone WQv treatment BMP. The Porous Asphalt pavement must be designed in conjunction with an infiltration basin/trench, sand filter, or other supplemental BMP in order to meet MSD WQv treatment criteria. It is noted that the supplemental BMP shall not be located underneath the Porous Asphalt or the base system. 1 of 2 6. Porous Asphalt can be considered as pervious area for the purpose of calculating Water Quality Volume (WQv). It can be considered as 5% impervious when calculating the differential runoff with PI factors. However, the Porous Asphalt will be considered as 100% impervious area for the purpose of calculating pipe sizes downstream. 7. While Porous Asphalt is approved for provisional use, its application shall be limited to low traffic areas. 8. Project specific designed calculations and maintenance plans must be included within the project's "Stormwater Management Facilities Report" prepared by the consulting Engineer. MSD reserves the ability to withdraw or modify this approval based on subsequent infoirnation, including infoiniation indicating that Porous Asphalt with storage bed does not satisfy MSD rules, requirements, or construction specifications. Please provide three pavement samples to MSD to serve as representative samples of the pavement layer. The three samples shall be minimum 4 inches in diameter. Sincerely, micitafa r Michael T. Buechter, P.E. Principal Engineer (BMP Committee Chairman Engineering/Design Metropolitan St. Louis Sewer District 2 of 2