Overview of stormwater credits

Example schematic showing how pollutant load reductions are calculated for a bioretention basin with an underdrain. In this case, total volume, TSS and phosphorus credits are 100 percent for infiltrated water, while credits for filtered water (water captured by the underdrain) are 60 percent for TSS and are dependent on the media for phosphorus.

Stormwater credit is a tool for local stormwater authorities who are interested in

• providing incentives to site developers to encourage the preservation of natural areas and the reduction of the volume of stormwater runoff being conveyed to a best management practice (BMP);
• complying with antidegradation requirements, including meeting the MIDS performance goal; or
• meeting or complying with water quality objectives, including total maximum daily load (TMDL) wasteload allocation (WLAs). If interested in information on MS4 annual reporting see here.

Definition of credit

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.

• A rain garden infiltrates 50,000 cubic feet of water per year. The rain garden receives a credit of 50,000 cubic feet to be applied toward runoff reduction (volume control).
• A rain garden results in the removal of 10 pounds of phosphorus per year from stormwater runoff. The rain garden receives a credit of 10 pounds to be applied toward pollutant load reduction.
• Three rain gardens each remove 10 pounds of phosphorus per year. Each rain garden receives a credit of 10 pounds, resulting in a total credit of 30 pounds.

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.

Applicability of credits

Stormwater credits may be generated for a number of reasons. Credits can be used for the following situations.

• To meet a TMDL WLA or other water quality goal. In this situation, BMPs are implemented to reduce pollutant loads. Each BMP results in a specific reduction in loading, which equates with the credit for that BMP. The cumulative reduction in loading achieved with all BMPs is the cumulative or total credit.
• To meet the Minimal Impact Design Standards performance goal. The MIDS performance goal is intended to achieve pre-development (pre-settlement) conditions, thereby resulting in compliance with antidegradation requirements. As with the TMDL example, each BMP receives a credit and the cumulative credit is the sum of individual BMP credits.
• To provide incentives to site developers to encourage the preservation of natural areas and the reduction of the volume of stormwater runoff being conveyed to a best management practice (BMP).
• To reduce costs associated with structural stormwater BMPs. For example, sizing requirements for a wet pond may be reduced if volume reduction credits are generated upstream of the pond. An example is increasing forested area upstream of the pond, which results in decreased runoff amounts and reduced sizing requirements.
• To supplement the Minnesota Pollution Control Agency Construction General Permit (CGP) or be used for projects not covered under the CGP.
• As part of the financial evaluation under a local stormwater utility program, similar to the Minneapolis approach.

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.

Consistency in calculating credits

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.

Concerns/disadvantages of crediting

Stormwater credits provide a relatively simple process for estimating progress toward reducing stormwater volume and pollutant loads. There are some drawbacks to the credit process however, including but not limited to the following.

• Credits must be applied consistently. Specific values for the effectiveness of stormwater bmps vary widely in the literature.
• Interpretation of credits can be challenging, particularly when treatment trains are employed
• Credits vary with time. This includes event, seasonal, and long-term variability. For example, the effectiveness of bmps diminishes over time if the bmps are not properly maintained.
• Credits do not always align with water quality objectives. For example, if a pollutant reduces total phosphorus (TP) by 50% but TP in the influent to a bmp is 0.50 mg/L, the effluent of 0.25 mg/L is still well above surface water standards (typically 0.04-0.090 mg/L).
• Conversely to the previous bullet, bmp credits will be overestimated when influent pollutant concentrations are very low, since bmps cannot effectively remove pollutants below a certain concentration. For example, if influent TP concentration is 0.05 mg/L, applying a 50 percent reduction gives an effluent of 0.025 mg/L, which is not achievable for most bmps.

Is my community ready for credits?

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:

• Strong interest and some experience in the use of better site design techniques
• A development review process that emphasizes early stormwater design consultations during and prior to initial site layout
• Effective working relationships between plan reviewers and design consultants
• A commitment by both parties to field verification to ensure that credits are not a paper exercise.

Adapting credits for local use

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.

Integrating credits into the local development review process

Stormwater credits need to be explicitly addressed during three stages of the local development review process, as shown below:

• Feasibility during concept design
• Confirmation in final design
• Verification at final construction inspection

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 V_wq 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.