Stormwater Quality

The St. Louis County Phase II permit applies to MSD, 59 municipalities and St. Louis County. While MSD is the coordinating authority for the permit, MSD does not have authority for all aspects of permit compliance. The authority of planning and zoning, grading, plumbing, and other items that affect stormwater quality lies with the 59 municipalities and St. Louis County.

The Site Design Guidance provides a process for planning stormwater quality strategies into site developments; the municipalities within MSD are required to use this process, or an equivalent one, to comply with the Phase II permit. The first step in this planning process is the investigation of the site for natural resource considerations, such as karst features (e.g., sinkholes), streams, wetlands, and erodible soils. These types of areas should be preserved and protected, and frequently they are provided a buffer.

The next step is to investigate and identify potential areas of the site where BMP placement would be optimal. Thorough up-front site development planning is crucial to deploying preferred stormwater quality strategies. Examples of preferred BMPs include:

Examples of areas where BMP placement is optimal include areas of hydrologic soil group (HSG) A or B soils, where infiltration can be maximized. Also, grassland areas and forests can provide buffer areas that filter and slow stormwater release.

It is also necessary to identify areas with utilities or soil and groundwater contamination, as these may be areas BMPs should avoid.

Buildings, roads, and other site features are best laid out after environmentally sensitive areas and potential BMP locations are identified. Keep in mind that minimizing disturbance and impervious area reduces the amount (and cost) of stormwater runoff controls.

Look for ways to distribute BMPs throughout the site, utilizing site greenspace (such as parking islands) for bioretention areas. Promote sheetflow of stormwater into vegetative buffers, effectively disconnecting impervious area from downstream streams and sewers. Consider using porous pavement and green roofs to further reduce impervious area.

Minimizing disturbance, minimizing impervious area, disconnecting impervious area, and distributing BMPs into other site features reduces and potentially eliminates the need for additional land and/or underground water quality and detention facilities.

If after this due diligence effort, it is apparent that volume-reducing BMPs are not appropriate for a site, then other types of BMPs may be accepted. Examples of these types of water quality BMPs are proprietary (manufactured) filters, sand filters, and wet ponds.

MSD requires that structural BMPs capture and “treat” the 90th percentile daily rainfall event (P=1.14 inches). (The caveat to this requirement is rainwater harvesting, which herein is discussed in further detail.) To comply with the region’s Phase II stormwater permit, BMPs installed on new development projects are required to reduce the volume of discharged rainfall to the site’s pre-construction condition to the maximum extent practicable.

Redevelopment projects are not required to mimic pre-development or pre-construction runoff conditions, but are required to utilize the best performing technologies and water quality strategies, including those that reduce runoff volume.