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Ammended Soils Performance Criteria

For both new and redevelopment sites, MSD will presume amended soil disconnection to meet the performance criteria for a standalone water quality BMP when the impervious area treated is designed to drain into the disconnection area and the design satisfies the parameters listed in Table 1.

Table 1. Amended Soils Disconnection Design Criteria

Design FactorDisconnection Criteria
Maximum impervious area treated1,000 square feet
Minimum area of amended soilsEqual to area of impervious disconnect
Minimum width of amended soils10 feet
Longest flow path of impervious area50 feet
Disconnection lengthEqual to the longest flow path of impervious area. No less than 20 feet for downspout disconnections.
Disconnection slope5% maximum

Rooftop disconnection downspout discharge points should be located to drain away from the structure and to avoid potential erosion problems. Proposed uses and designs should also comply with current MSD Rules, Regulations, and Design Requirements.

Refer to Specifications for compost soil amendment description, procedures and material requirements.

Examples of a rooftop disconnection Example: Rooftop Amended Soils Disconnection and a trail disconnection Example: Linear Pavement Amended Soils Disconnection are provided to illustrate the BMP application and design criteria.