Stormwater credit is a tool for local stormwater authorities who are interested in
There is no universal definition for the term stormwater credit. As used in this manual, credit refers to the stormwater runoff volume or pollutant reduction achieved toward meeting a runoff volume or water quality goal. Examples of goals include meeting the 1 inch volume reduction requirement in the Construction Stormwater General Permit and meeting TMDL pollutant reduction requirements. Credits can be achieved either by an individual BMP or cumulatively with multiple BMPs. Examples include the following.
Credits apply to a single pollutant or to runoff reduction. A BMP may thus generate credits for more than one pollutant. Total credit for a specific pollutant or for runoff reduction must therefore be computed individually for each pollutant or volume of runoff reduced. In the example above, the rain garden generates credits for phosphorus reduction and for volume reduction.
Ideally, stormwater credits are simple to calculate, easy to review and delineate on site plans, and quickly verified in the field.
Stormwater credits may be generated for a number of reasons. Credits can be used for the following situations.
Although not explicitly allowed under the current MPCA CGP, there are situations where a local authority could create a water quality credit system which does not conflict with the CGP. For example, a local authority that requires a water quality volume that is greater than the CGP water quality volume, could apply credits against the difference between the two volumes. Another situation appropriate for credits could be retrofit projects that do not create new impervious surfaces. These projects are not subject to permanent stormwater management requirements of the CGP.
One concern with credits is that two different entities may calculate a credit for the same BMP in different ways or using different assumptions or values. It is important that credits be calculated consistently from one entity to the next. For example, one entity may assume a phosphorus removal efficiency of 50 percent for a wet pond, while another entity may assume a removal efficiency of 60 percent, even though the two ponds may be similar in design. To minimize inconsistencies in calculating credits, this manual provides recommended values to be used for calculating credits for BMPs. These recommended values assume the BMPs are properly designed, constructed and maintained. Design, construction, maintenance, and performance assessment are discussed for each BMP in this manual. Design sections include design specifications where applicable.
Experience in other states has shown that it can take a while for both local plan reviewers and engineering consultants to understand and effectively use credits during stormwater design. Adoption of credits by a local regulator is particularly difficult in communities where stormwater design occurs long after final site layout, giving designers or plan reviewers little chance to apply the better site design techniques at the heart of the credit system.
The following four ingredients appear to be important in establishing an effective local credit system:
If a community feels it has many of these ingredients in place, it is ready to decide whether to offer some or all of the credits described in this chapter. The first step in the adoption process is to review each stormwater credit to ensure whether it is appropriate given local conditions and review capability. Plan reviewers should pay close attention to how credit conditions and restrictions will be defined. It may be advisable to establish a team of local consulting engineers, plan reviewers and contractors to test out the proposed credits on some recently submitted site plans to make sure they are workable. Future plan review conflicts can be avoided when designers and plan reviewers agree on how credits will be handled in the local development review process.
Stormwater credits need to be explicitly addressed during three stages of the local development review process, as shown below:
The first stage where credits are considered is during initial stormwater concept design prior to site layout. The designer should examine topography and flow patterns to get a sense for how stormwater can be distributed and disconnected across the site, and explore opportunities to orient lots, grading or conveyance to maximize use of better site design techniques in the proposed site plan. While stormwater credits can be applied to any kind of site, they are ideally suited for low density residential development, particularly when open space or conservation designs are planned. Communities may also elect to offer additional stormwater credits to promote adoption of innovative practices such as green rooftops, soil compost amendments, permeable pavements, and stormwater planters.
Once better site design techniques are incorporated into the site plan, the designer can delineate the approximate areas at the site that are potentially eligible for stormwater credits, making sure that credit areas do not overlap. Ideally, proposed credit areas are drawn directly on the stormwater element of the site plan. Next, the adjusted Vwq is computed, and the remaining elements of the BMP treatment system are sized and located. The local review authority then checks both the credit delineations and computations as part of the review of the stormwater concept plan.
The credits are reviewed a second time during final design to confirm whether they meet the site-specific conditions outlined earlier in this chapter (e.g., slope, contributing drainage area, flow path lengths, etc). The designer should be able to justify the precise boundaries of each credit area on the plan, and indicate in the submittal whether any grading or other site preparation are needed to attain credit conditions (this is particularly important for rooftop disconnection and grass channel credits). Designers should be encouraged to use as many credits as they can on different portions of the site, but plan reviewers should make sure that two or more credits are not claimed for the same site area (i.e., no double counting). Reviewers should carefully check the delineation of all credit areas, make sure flow paths are realistic, and then approve the adjusted Vwq for the site. In addition, the plan reviewer should check to make sure that any required easements or management plans associated with the credit have been secured prior to approval.
Field inspection is essential to verify that better site design techniques used to get the stormwater credits actually exist on the site and were installed properly. This is normally done as a site walk through as part of the final stormwater inspection at the end of construction. To ensure compliance, communities may want to set the value of performance bond for the stormwater system based on the unadjusted Vwq for the site (pre-credit) to ensure better site design techniques are installed properly.