HomeMy Public PortalAbout430880M5EI
Metropolitan
St. Louis Sewer
District
2350 Market Street
St. Louis, MO 63103-2555
(314) 768-6200
March 16, 2012
RE: Permeable Pavement Applicability as a Stand Alone Water Quality BMP
To Whom It May Concern:
The purpose of this letter is to convey a Metropolitan St. Louis Sewer District (MSD) policy change
regarding pervious pavement as a stand-alone water quality post -construction best management practice
(BMP).
Policy Change Summary
MSD will recognize permeable pavement as a stand-alone water quality BMP, when designed and
constructed to comply with MSD permeable pavement guidance described herein.
As a result of this change, MSD will no longer require that porous asphalt, permeable concrete, and
permeable interlocking concrete pavement (PICP) be provided with an infiltration basin/trench, sand
filter, or other supplemental BMP in order to meet MSD stormwater quality criteria.
Design Performance
For new and redevelopment sites, to be considered an effective stand-alone water quality BMP, BMP
designs shall be capable of the following.'
1. Capture and treat the required water quality volume (WQ,,). (This is equivalent to capturing and
treating 90% of annual rainfall.)
2. Remove 80% of the TSS.
3. Have an acceptable longevity rate in the field.
Additionally, on new development sites, BMPs performance includes "mimicking the pre -construction
runoff condition", to the maximum extent practicable. One objective of this criterion is to utilize BMPs
that help reduce runoff volume to its pre -development condition.2
For both new and redevelopment sites, MSD will presume permeable pavement to meet these criteria
when the BMP's saturated soil hydraulic conductivity, infiltration bed thickness, and ratio of impervious
pavement to pervious pavement satisfy the parameters listed in Table 1.
Table 1
Stand -Alone Water Quality Permeable Pavement Design Parameters
Subgrade Saturated
Hydraulic Conductivity
(in./hr.)
Infiltration Bed
Gravel Thickness (in.)
Maximum Ratio of Drainage Area
to Pervious Pavement Area
0.05
6
2:1
0.1
12
3:1
0.15
18
4:1
See Section 4.080.05 of MSD Rules and Regulations.
2 See Amendment 3 of Section 4.080.01 of MSD Rules and Regulations.
As part of permeable pavement design, MSD recommends a geotechnical assessment of soil conditions.
MSD will accept 0.05 in./hr as the subgrade saturated hydraulic conductivity without site specific soil
testing (i.e., this is a "default" subgrade saturated hydraulic conductivity). To utilize rates greater than
0.05 in./hr, on -site soil testing will be required.
The minimum thickness of the gravel storage bed beneath the permeable pavement remains 12 inches.
Values listed as "infiltration bed gravel thickness" refer to the portion of the gravel storage bed which
infiltrates water into the subgrade. Infiltration is typically accomplished using a berm, weir, or underdrain
configuration.
The ratio of upgradient drainage area to permeable pavement area affects the percentage of storms
captured and BMP longevity. In no case should this ratio exceed 4:1. This is necessary to ensure an
acceptable longevity rate in the field.
Table 1 is based on continuous simulation modeling and some conservative assumptions. (For example,
the tributary area draining to the pervious pavement is assumed to be 100 percent impervious.) MSD will
accept site specific continuous simulation analysis that demonstrates the required BMP performance
criteria are achieved.
Proposed uses and designs should still comply with current MSD Rules, Regulations, and Design
Requirements, and all other applicable items outlined in the permeable pavement's MSD approval letter
and design guide found at the following link:
http://www. stlmsd. com/engineering/planrev iew/P lanRevi ewI nformation/ProprietaryB MPs#pervious
Please direct any questions regarding this policy to my attention, at (314) 768-2709.
Sincerely,
kins, PE
Principal Engineer
Engineering/Planning — Development Review
Metropolitan St. Louis Sewer District
cc: Phil Hocher, Pace Construction
David B. Marshall, Fred Weber, Inc.
Evan Bolesta, Concrete Council
David Mudd, Kirchner Block & Brick
Aaron Rauls, Building Products Corp.