Permeable Pavement Performance Criteria
For new and redevelopment sites, to be considered an effective stand-alone water quality BMP, permeable pavement designs shall be capable of the following:
- Capture and treat the required water quality volume (WQv). (This is considered to be equivalent to capturing and treating 90% of annual rainfall.)
- Remove 80% of the TSS.
- 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.
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 drainage area to pervious pavement area satisfy the parameters listed in Table 1.
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 total thickness of the gravel storage bed beneath the permeable pavement is 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 that retains a portion of the runoff.
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 (upgradient area : permeable pavement area). 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 for each type of permeable pavement: