Requirements for post-construction stormwater management during new and re-development
By Richard Haimann
Most cities, towns, municipalities, incorporated areas, and some counties that have Municipal Separate Storm Sewer Systems (MS4s) will have National Pollutant Discharge and Elimination System (NPDES) permits for their MS4 systems. These fall into two categories: Phase I and Phase II. Phase I was established in the early 90s and applied to incorporated areas greater than 250,000 in population in most states and incorporated areas greater than 100,000 in population in a few states.
Each incorporated area received a permit from its state or, where the state did not have primacy to regulate under the Clean Water Act (CWA), the United States Environmental Protection Agency (EPA). In 2003, EPA moved forward with Phase II of the program, which developed general permits for all incorporated areas with MS4s smaller than those in the Phase I program. Each state that had primacy under the CWA developed a general permit for phase II communities and EPA developed a phase II permit for those remaining states. The phase II program also includes some non- incorporated entities, such as school campuses, transit organizations, or other entities that control land with storm drainage, which, in turn, discharge into either water of the U.S. or another MS4.
Under both the Phase I and II permits, one element implemented by all permitted entities that affect builders during planning and design is the Post-Construction Storm Water Control element, sometimes referred to as the New and Redevelopment requirements. Under this element, all new and re-development that results in greater than a threshold of land disturbance or impervious surface are required to put in structural treatment control Best Management Practices (BMPs). The specific requirements vary by permit and permitted entity. These thresholds vary. In some jurisdictions, they are as small as 5,000 square feet. In other jurisdictions, they may be 1 acre.
Some permits have language that prioritizes the use of Green Stormwater Infrastructure (GSI), otherwise referred to as Low Impact Development (LID). Under these permits, new and re- development is required to evaluate the feasibility of capturing, retaining, and infiltrating and/or harvesting for using some quantity of water, typically the 85th percentile storm for that location. The volume varies by permit and, sometimes, by permitted entity. Some permitted entities have opted to prioritize GSI through an ordinance, such as the City of Los Angeles.
When retention and infiltration or use is shown to be infeasible, some permits then require that biotreatment be evaluated. Biotreatment is the flow of stormwater through a vegetated system. Specifications for allowable types of biotreatment systems vary per permitted entity. In these permits, if biotreatment is infeasible, then other types of flow- through structural treatment control BMPs can be selected.
Not all MS4 permits require that GSI be evaluated before selecting other types of treatment control BMPs.
Some jurisdictions have been evaluating and, in at least one instance, implementing a system for trading of credits to allow new and re-development to move forward with purchase of credits from a regional retention system in lieu of constructing retention or biotreatment on a development parcel. This has several advantages for both water quality and new and re-development:
- Regional retention can capture and infiltrate water more cost-effectively than distributed systems.
- Regional retention systems can be optimized to be in areas where greater infiltration rates can be taken advantage of to supplement local groundwater supplies.
- Selling credits in regional retention systems provide a stream of funding for ongoing operation and maintenance of those regional retention systems.
- New and re-development can better optimize the value-creation use of their parcels rather than setting aside parcel sections for onsite retention or biotreatment.
San Diego County, Calif. is the first jurisdiction to establish an approved program for entities to build regional retention projects and sell credits to new and re-development within a watershed. The county developed a guidance document to provide formulas to be used for credit calculation and transfer that takes into account pollutant capture differences between regional retention and new and re-development to establish pollutant removal equivalency, among other factors. The program has recently been approved by the State and its implementation is being worked on in the region.
Since requirements vary by state and, within states, by county, and within counties, by incorporated or permitted entity, a builder needs to review local requirements during their planning stage to determine what will be required on the project they are planning.
Richard Haimann has over 30 years of experience in a broad range of environmental engineering practices. He has focused on storm- water compliance for municipalities, developers, and industrial clients for the past 20 years and is licensed as a professional engineer in California, Texas, and Washington.