Stormwater Credits
A stormwater credit system provides a direct incentive for developers and designers to incorporate Better Site Design techniques into development projects. By implementing recognized design practices that reduce runoff or improve water quality at the source, a project can earn credits that reduce the required volume of stormwater to be managed by structural practices. This direct translation to smaller storage facilities and conveyance systems can result in significant cost savings while promoting development patterns that are less impactful to aquatic resources.
Stormwater credits are a flexible tool that can be adapted to local resource protection goals. A community can select which credits to offer and define the specific criteria for each based on its unique environmental, economic, and social needs. This section outlines common stormwater credits and provides guidance on their calculation and application.
How Credits Interact with Sizing Criteria
Stormwater credits are directly linked to the unified stormwater sizing criteria that define the volumes to be managed for a given site. Credits function by reducing the total runoff volume that must be treated or detained to meet one or more of the four primary management objectives:
- Recharge volume (Rev): Credits often reduce the volume needed to replicate pre-development groundwater recharge.
- Water Quality Volume (WQv): Many credits reduce the volume of runoff requiring treatment by preventing its generation in the first place (e.g., conserving natural areas) or by providing effective pre-treatment (e.g., sheet flow over a buffer).
- Channel Protection Volume (Cpv): Credits for quantity control are typically applied by adjusting the runoff curve number (CN) of the credited area to reflect a more natural hydrologic condition, thereby reducing the calculated runoff volume and discharge for the channel protection storm event.
- Overbank and Extreme Flood Volume (Qp and Qf): Similar to channel protection, credits are applied by using a lower CN for the credited area, which reduces the calculated peak discharge for larger storm events.
The availability of a specific credit is contingent on site conditions. For example, a stream buffer credit is only applicable to sites with an intermittent or perennial stream, while a green rooftop credit can be applied to most building projects.
Site Design Credits
Site design credits are incentives that directly reduce required on-site storage volumes when specific Better Site Design techniques are used. These credits reward practices that preserve natural hydrology and minimize the generation of stormwater runoff.
Natural Area Conservation
This credit encourages the preservation of valuable natural areas within a development site, such as forests, wetlands, and other critical habitats. By protecting these areas, their pre-development hydrologic and water quality functions are maintained. To qualify, these areas must be legally protected from future development and disturbance through a permanent conservation easement or similar recorded instrument, consistent with an open space development ordinance.
| Stormwater Objective | Credit Application |
|---|---|
| Water Quality & Recharge | The credited conservation area is subtracted from the total site area when calculating the required water quality volume (WQv) and recharge volume (Rev). |
| Channel Protection & Flood Control | The conservation area is modeled hydrologically using a curve number for “woods in good condition.” |
The credit toward the Water Quality Volume is calculated based on the proportion of the site that is conserved:
CWQ = (ANA / A) × WQv
Where:
CWQ = Natural Area Credit for Water Quality (ac-ft)
ANA = Natural Conservation Area (acres)
A = Total Site Area (acres)
WQv = Original Water Quality Volume (ac-ft)
The credit for the Recharge Volume is then expressed as a fraction of the water quality credit:
CR = CWQ × S
Where:
CR = Credit for Recharge (ac-ft)
S = Recharge fraction (ratio of the total required Rev to the total required WQv)
The required WQv and Rev for the site are then reduced by the calculated credit amounts, CWQ and CR.
Stream Buffers
This credit incentivizes the use of legally protected, undisturbed stream buffers or vegetated filter strips to treat stormwater runoff from adjacent developed areas. Runoff must be directed to the buffer as sheet flow.
| Stormwater Objective | Credit Application |
|---|---|
| Water Quality & Recharge | The area draining to the buffer is subtracted from the site area when calculating the required WQv and Rev. |
| Channel Protection & Flood Control | The area draining to the buffer is modeled hydrologically using a curve number for “woods in good condition.” |
To be eligible for this credit, the buffer and contributing drainage area must meet several criteria:
- The minimum undisturbed buffer width must be 50 feet.
- The buffer must be protected by a conservation easement and meet all design and maintenance requirements of the local stream buffer ordinance.
- The maximum contributing overland flow length to the buffer is 150 feet for residential areas and 75 feet for commercial areas.
- The average slope of the contributing area must be 3% or less.
- A level spreader or similar device is required to establish sheet flow if the overland flow path exceeds 50 feet.
The credit for both water quality and recharge volumes is calculated based on the fraction of the site draining to the buffer:
C = (AB / A) × WQv
Where:
C = Buffer Credit (ac-ft)
AB = Area Draining to the Buffer (acres)
A = Total Site Area (acres)
WQv = Original Water Quality Volume (ac-ft)
Both the required WQv and Rev are then reduced by the calculated credit amount, C.
Green Rooftops
This credit is for vegetated roof systems that detain, filter, and evaporate stormwater runoff directly on the rooftop. These systems, also known as eco-roofs, reduce the volume and peak rate of runoff from buildings.
| Stormwater Objective | Credit Application |
|---|---|
| Water Quality & Recharge | The total green rooftop area is subtracted from the site’s impervious cover when calculating required WQv and Rv. |
| Channel Protection & Flood Control | The green rooftop area is modeled hydrologically using a curve number for “woods in good condition.” |
To qualify for the credit, a green roof must meet specific design and maintenance standards, including:
- A soil bed of at least 6 inches with a silt loam texture.
- A 2-inch gravel drainage layer beneath the soil, separated by filter fabric.
- An impermeable membrane protecting the roof structure.
- A roof slope not exceeding 25%.
- Structural design that can support an additional 25 lbs/sf beyond standard regional requirements.
- Vegetation that requires minimal fertilization and irrigation, established within two growing seasons.
- A recorded maintenance agreement ensuring quarterly inspection and upkeep.
The water quality and recharge credit is calculated as follows:
C = (AGR / AI) × WQv
Where:
C = Green Rooftop Credit (ac-ft)
AGR = Green Rooftop Area (acres)
AI = Total Site Impervious Area (acres)
WQv = Original Water Quality Volume (ac-ft)
The required WQv and Rev are then reduced by the calculated credit amount, C.
Disconnection of Non-Rooftop Impervious Runoff
This credit applies to the practice of directing runoff from small impervious surfaces—such as driveways, walkways, and patios—to adjacent vegetated areas for infiltration and filtration, rather than conveying it directly to the storm drain system.
| Stormwater Objective | Credit Application |
|---|---|
| Water Quality & Recharge | The total disconnected impervious area is subtracted from the site’s total impervious cover when calculating required WQv and Rv. |
| Channel Protection & Flood Control | The disconnected impervious area is modeled hydrologically using a curve number for “woods in good condition.” |
To receive this credit, disconnections must meet the following criteria:
- The maximum contributing impervious flow path length is 75 feet.
- Runoff cannot originate from a designated pollution hotspot.
- The vegetated disconnection area must be on a slope of 3% or less.
- The length of the vegetated disconnection must be equal to or greater than the contributing impervious length.
- The impervious area draining to any single discharge location cannot exceed 1,000 ft2.
- For sites with less permeable soils (Hydrologic Soil Groups C and D), a geotechnical evaluation may be required to determine if an engineered level spreader is needed.
The water quality and recharge credit is calculated with the following equation:
C = (AD / AI) × WQv
Where:
C = Non-Rooftop Disconnection Credit (ac-ft)
AD = Disconnected Impervious Area (acres)
AI = Total Site Impervious Area (acres)
WQv = Original Water Quality Volume (ac-ft)
Both the required WQv and Rev are reduced by the calculated credit amount, C.
Watershed Credits
In addition to site-level design credits, some communities offer watershed-based credits. These are typically reductions or exemptions from standard stormwater management requirements designed to support broader watershed restoration goals. Examples include offering streamlined requirements or reduced sizing criteria for redevelopment projects in designated revitalization areas or for projects that conform to a specific watershed-based zoning plan. These credits encourage growth in areas where infrastructure already exists, which can help reduce development pressure on sensitive headwater streams and natural lands elsewhere in the watershed.
Implementing a Credit System Locally
For a stormwater credit system to be effective and enforceable, it must be supported by clear local procedures. Key elements of implementation include:
- Credit Caps: Most communities cap the total percentage of stormwater requirements that can be met with credits (e.g., a maximum of 50% of the WQv). This ensures that every site provides a baseline level of structural stormwater treatment.
- Plan Documentation: All proposed credits must be clearly documented on stormwater management plans. This includes detailed calculations, the location and dimensions of all credited features, and specifications for their construction.
- Inspection and Verification: The installation and final condition of credited features must be verified through site inspections and as-built surveys as part of the local review process. This confirms that the features were built according to the approved design.
- Permanent Protection: Any site feature that receives a credit, such as a conservation area or stream buffer, must be permanently protected from future disturbance. This is typically accomplished through a recorded conservation easement, deed restriction, or protective covenant.
Frequently Asked Questions
What is the primary purpose of a stormwater credit system?
The main purpose is to provide a financial and design incentive for developers to use Better Site Design techniques. These techniques reduce the generation of stormwater runoff and pollutant loads at the source, which can lead to smaller, less expensive structural stormwater practices while improving environmental outcomes.
Can credits completely eliminate the need for stormwater management facilities?
Typically, no. Most local ordinances that include a credit system also establish a “cap,” or a maximum percentage of the stormwater requirement that can be met with credits. This ensures that all development projects provide a minimum level of engineered stormwater treatment.
How is the credit for a stream buffer calculated?
The credit is based on the area of the site that drains as sheet flow to a qualifying, protected buffer. This contributing drainage area is used to calculate a proportional reduction in the required water quality and recharge volumes for the entire site.
What does it mean to “disconnect” impervious surfaces?
Disconnection is the practice of directing runoff from small impervious areas like driveways or patios onto an adjacent vegetated area (such as a lawn or filter strip) instead of routing it directly into a pipe or storm drain. To qualify for a credit, the receiving vegetated area must be large enough and sloped correctly to slow and filter the runoff.
What are the key requirements for a green roof to earn stormwater credits?
A green roof must meet specific design criteria related to its physical composition, including minimum soil and gravel depth, structural load capacity, and appropriate, low-maintenance vegetation. A legally binding, long-term maintenance agreement is also required to ensure its continued performance.
How are credited conservation areas protected in the long term?
Areas that receive credits for natural conservation or stream buffers must be permanently protected from future disturbance or development. This is accomplished using a legally binding instrument, such as a conservation easement or deed restriction, that is recorded with the property deed.
Do credits apply equally to water quality and flood control?
Not exactly. For water quality and recharge, credits typically provide a direct reduction in the required treatment or infiltration volume. For channel protection and flood control, credits are usually applied by adjusting the runoff curve number (CN) for the credited area to that of a natural condition, like “woods in good condition.” This lowers the calculated peak discharge rate used to size detention facilities.