Phase II Stormwater Permit Questions
Bioretention is the most common type of post-construction BMP within MSD. However, answering which BMP is best for a particular site depends on many factors. Is the site required to control runoff volume? What is the size of the drainage area to the BMP? What are the existing site conditions, like soil type? Who is going to maintain the BMP? How difficult or costly is BMP maintenance? Site development questions and considerations, including MSD preferred BMPs, are detailed in the Site Design Guidance (pdf)
As part of Minimum Control Measure (MCM) 5, the St. Louis County Phase II permit requires post-construction BMPs on development projects that disturb greater than one acre. On the other hand, maintenance activities are not required by MCM 5 to provide post-construction stormwater BMPs. If a street or parking lot maintenance activity disturbs earth but maintains the pre-construction runoff flow direction, quantity, and velocity, then MSD typically considers this type of activity “maintenance”. Examples of maintenance activities are street subgrade remediation projects, where the street profile is unchanged; pavement milling and overlaying; and in-kind utility replacement.
On the other hand, street projects that replace open ditches with curbs, gutters, and/or inlets are considered development because the flow direction changes and rate of runoff increases. Likewise, parking projects that change the direction and flow of runoff, increase the amount of imperviousness, or change the surface material such that runoff rate or volume increases (e.g., conversion of gravel to asphalt or concrete) are considered development. These types of projects are required to comply with MCM 5 development requirements for post-construction BMPs.
Yes. The small MS4 stormwater permit requires its permittees to provide post-construction BMPs as part of municipal operation and maintenance activities. Sometimes the type of BMP or BMP performance is different from development projects. For example, site conditions simply may not allow complete treatment of the water quality volume, and partial treatment may be considered the maximum extent practicable for maintenance activities.
MSD classifies projects with a pre-construction impervious footprint of less than 20 percent of total site area as new development. (Note that “site” is the whole site, not only the disturbed area.) A distinction is needed because new development sites are required by the Phase II permit to reasonably mimic the pre-construction runoff condition, necessitating controls and practices that reduce runoff volume through infiltration, evapotranspiration, and/or stormwater harvesting. Given that redevelopment projects with BMPs improve the pre-construction runoff condition, the “mimicking pre-construction conditions” component of the Phase II permit is not applicable to redevelopment sites. Therefore, redevelopment projects are required to use controls to the maximum extent practicable (e.g., as site conditions allow), but a target condition is not set.
Stormwater discharges to combined sewers and are not subject to the Phase II stormwater permit. Thus, the City of St. Louis and some inner-suburb municipalities are not Phase II stormwater co-permittees. However, some stormwater discharges in the combined sewer area are regulated by the Phase II permit. (An example of this exception is the River Des Peres, which is a listed stormwater outlet in the Phase II permit. Stormwater from development projects that discharge directly into the River Des Peres is subject to post-construction BMP requirements.)
While there is not a “Phase II” requirement for discharges to the combined sewer area, MSD sometimes requires BMPs that reduce and detain runoff as mitigation for increases in runoff to overcharged sewers. Because some of the BMPs installed to protect stormwater quality, like bioretention, also reduce and detain stormwater runoff, similar practices are sometimes used in the combined and separate sewer areas. Thus, stormwater BMPs are sometimes constructed in the combined sewer area.
MSD does not have “streambank buffer” rules. However, MSD led a workgroup that developed a model stream buffer protection ordinance for co-permittees’ benefit. Most of MSD’s municipal co-permittees have a streambank buffer ordinance, and MSD coordinates with them to ensure streambank buffer rules are followed. Designers should contact the project’s zoning authority for specific requirements.
MoDOT is the holder of a (separate) Phase II stormwater permit; this permit applies to work performed on MoDOT ROW by MoDOT or its contractors. MoDOT’s permit does not authorize work by public or private entities that perform work as part of development. Therefore, development projects that discharge into MoDOT maintained sewers shall provide BMPs as required to meet MSD rules and regulations.
The Partnership for Tomorrow, a coalition of the construction and development industries comprised of the following organizations: the Associated General Contractors of St. Louis, the Home Builders Association of St. Louis and Eastern Missouri, the Missouri Growth Association, the St. Louis Association of REALTORS©, the St. Louis Regional Chamber and Growth Association, and the St. Louis Council of Construction Consumers; and MSD collaborated to prepare an informational brochure to promote design tips and design options, called Industry Update: Stormwater Regulations.
Post-construction stormwater BMPs are natural and engineered systems that control the quality, rate, and volume of stormwater runoff from developed areas. Examples of BMPs systems include rain gardens (bioretention), pervious pavement, green roofs, stormwater harvesting systems, wet ponds, and constructed wetlands.
All development projects, public and private, that are not served by combined sewers and disturb greater than one acre will require post-construction BMPs.
Development projects that are less than one acre may also be required to provide post-construction BMPs to address nearby downstream flooding problems. On a project by project basis, MSD engineering staff investigates the presence of problems after plans are initially submitted to the district, or as part of a (paid) conceptual review. MSD will notify the design engineer if these special circumstances require BMPs on projects less than one acre.