HomeMy Public PortalAboutPace 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.
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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
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