Credits for Better Site design

Schematic showing better site design priciples

The six better site design approaches that could be eligible for water quality volume reduction stormwater credits include:

• Natural Area Conservation
• Site Reforestation or Prairie Restoration
• Drainage to Stream or Shoreline Buffers
• Surface Impervious Cover Disconnection
• Rooftop Disconnection
• Use of Grass Channels

For consistency with other sections of this Manual, the formulas for computing water quality volumes and related credits are based on the requirements contained in the MPCA Construction General Permit (CGP). These formulas provide just one option for local authorities to consider. The approach used in these examples subtracts the credit volume from the water quality volume (V_wq); the volume of a permanent pool (V_pp) in a stormwater pond or wetland is not adjusted. Other options that could be considered include applying credits to V_pp, or proportional application of a credit to both V_pp and V_wq.

It is not the intention of this system of credits to eliminate the need for a water quality volume. It is possible that the area proposed for conservation exceeds the area proposed as impervious surface. In this circumstance, it is recommended that a minimum water quality volume equal to 0.2 watershed inches be maintained (see Unified sizing criteria).

Local authorities should keep in mind that the current MPCA CGP, which expires in 2008, does not incorporate a technique for application of these credits. This CGP does allow up to a maximum of 1% of a site, up to three acres to drain untreated to natural areas, in sites where any BMP is not feasible.

Although stormwater credits are not currently used under the Minnesota Pollution Control Agency Construction General Permit (CGP), they can be applied at the local and watershed levels to supplement the CGP or be used for projects not covered under the CGP. Credits can also be used as part of the financial evaluation under a local stormwater utility program, similar to the Minneapolis approach. Further evaluation of the use of credits in the CGP process will occur as part of the permit update process over the next year.

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. Local authorities interested in establishing a credit system prior to expiration of the CGP in 2008, are encouraged to contact the MPCA to explore if the local proposal is compatible with the CGP.

The last section presents tips on how to establish and administer a local stormwater credit system, with an emphasis on review and verification during concept design, final design and construction.

Better site design and stormwater credits

There are more than a dozen better site design techniques that can be applied at development sites. When applied early in the design process, these techniques can dramatically reduce stormwater runoff and pollutants generated from development sites (CWP, 1999). In recent years, several states have sought to encourage greater use of better site design techniques by allowing for computation of stormwater credits that reduce the required water quality volume that must be provided at a development site.

Agencies that utilize stormwater credits can sharply reduce water quality and stormwater management BMP size requirements and recommendations. This translates directly into cost savings for developers since the size and cost of stormwater conveyance and treatment systems needed for the site are reduced, and less land area is needed for BMPs. The use of credits by developers is strictly voluntary, although they do offer a meaningful incentive to reduce the cost of stormwater compliance.

Information: The decision to offer some, all or no stormwater credits solely rests with the local reviewing authority and should reflect local stormwater management goals and design review capability

Stormwater credits are tied directly to the water quality volume requirements (V_wq and V_pp). In addition, credits can be used to reduce the storage volumes needed to manage larger storm events, such as any locally-required channel protection and overbank floods (by increasing times of concentrations and reducing curve numbers in post-development hydrological modeling). Not all credits will be available for each site, and certain site-specific conditions should be met to receive each credit. These minimum conditions include site factors such as maximum flow length or contributing area that avoid situations that could lead to runoff concentration and erosion. Stormwater credits do not relieve designers from the normal standard of engineering practice of safe conveyance and drainage design. Multiple credits can be used at a development site, although two credits cannot be taken for the same physical area of the site.

Summary of stormwater credits function illustrating how to calculate water quality volume, channel protection and overbank storm volumes for different Better Site Design credits.
Link to this table.

Stormwater credit	Adjusted water quality volume	Channel protection and overbank storms
Natural area conservation	Subtract CA from site IC when computing Vwq	Adjust CN for CA to woods in good condition
Site reforestation/prairie restoration	Subtract 1/2 RA from site IC when computing Vwq	Adjust CN for RA to woods or prairie in fair condition
Streams and shoreline buffers	Subtract ADB from site IC when computing Vwq	Adjust CN for ADB to woods in good condition
Surface impervious cover and disconnection	Subtract DIA from site IC when computing Vwq	Adjust CN for DIA to grass in good condition. Adjust Tc
Rooftop disconnection	Subtract DRA from site IC when computing Vwq	Adjust CN for DIA to grass in good condition. Adjust Tc
Grass channels	Subtract GA from site IC when computing Vwq	Adjust Tc

Note: Unless otherwise noted, all units below are measured in acres

CA = combined area of all natural areas conserved at site

RA – Total area of site reforestation or prairie restoration

ADB – Total area draining to buffer with appropriate flow path distance

DIA – Total area of surface impervious cover that can be effectively disconnected

DRA – Aggregate rooftop area that can be effectively disconnected

GA - total non-roadway area draining to swale (rooftop, yard and driveway)

CN – Runoff curve number for area (units: dimensionless) (see Ch.8 and App. B)

Tc – Time of Concentration (units: time)
V_wq – Water quality volume, as defined by relevant MPCA sizing rule

IC - Impervious area of site (acres)

Stormwater credit categories

Six credit categories are addressed in this section.

1. Natural Area Conservation Credit
2. Site Reforestation or Prairie Restoration Credit
3. Drainage to Stream, Wetland or Shoreline Buffer Credit
4. Surface Impervious Cover Disconnection Credit
5. Rooftop disconnection credit
6. Grass channel credit

Example schematic illustrating credit calculation for natural area conservation. (see NOTES)

Natural area conservation credit

Natural area conservation protects natural resources and environmental features that help maintain the pre-development hydrology of a site by reducing runoff, promoting infiltration and preventing soil erosion. Natural areas should be eligible for stormwater credit if they remain undisturbed during construction and are protected by a permanent conservation easement prescribing allowable uses and activities on the parcel and preventing future development. Examples of conservation areas include any areas of undisturbed vegetation preserved at the development site, such as forests, prairies (native grasslands), floodplains and riparian areas, ridge tops and steep slopes, and stream, wetland and shoreline buffers. Floodplain credits should not be issued to areas that cannot be developed due to existing floodplain ordinance restrictions.

The undisturbed soils and native vegetation of conservation areas promote rainfall interception and storage, infiltration, runoff filtering and direct uptake of pollutants. Portions of the site devoted to natural area conservation are eligible for two credits, such as the addition of a buffer credit.

Water quality

The total combined area of all conservation areas can be subtracted from total site area when computing the water quality volume (Vwq) portion of the total storage volume. In the context of the four MPCA V_wq [[Unified sizing criteria|sizing rules, the credit is numerically expressed as:

Rule 1 (Stormwater Ponds and Constructed Wetlands) and Rule 3 (Non-Ponds):

\(V_{wq} = 1815 (IC - CA)\)

where:

V_wq = required water quality volume in cubic feet;

IC = new site impervious cover, in acres;

CA = total natural area conserved at site, in acres; and

1815 = conversion factor (to cubic feet)

Rule 2 (Stormwater Ponds and Constructed Wetlands Draining to Special Waters) and Rule 4 (Non-Ponds Draining to Special Waters):

\(V_{wq} = 3630 (IC - CA)\)

where:

V_wq = required water quality volume in cubic feet;

IC = new site impervious cover, in acres;

CA = total natural area conserved at site, in acres; and

3630 = conversion factor (to cubic feet)

Larger storm events

The post-development curve number (CN) used to compute the V_cp, V_p10, and V_p100 for all natural conservation areas can be assumed to be “woods or prairie in good condition” when calculating the total site CN.

Conditions for credit

Information: It is HIGHLY RECOMMENDED that proposed conservation areas meet all of the conditions outlined below to be eligible for credit:

• The minimum combined area of all natural areas conserved at the site must exceed one acre. Full ecological function for natural grassland (prairie) begins at 5 acres and for forested land starts at 20-40 acres. Credits could be increased beyond 1:1 for acreages that approach and exceed these values.
• No disturbance may occur in the conservation area during or after construction (i.e., no clearing or grading except for temporary disturbances associated with incidental utility construction or restoration operations, or removal of nuisance vegetation).
• The limits of disturbance around each conservation area should be clearly shown on all construction drawings.
• A long-term vegetation management plan must be prepared to maintain the conservation area in a natural vegetative condition. Managed turf is not considered an acceptable form of vegetation management, and only the passive recreational areas of dedicated parkland are eligible for the credit (e.g., ball fields and golf courses are not eligible).
• The conservation area must be protected by a perpetual easement that clearly specifies that no future development or disturbance can occur within the area.
• The credit cannot be granted for natural areas already protected by existing federal, state, or local law.
• Conservation areas should be preserved to maximize contiguous area and avoid habitat fragmentation.
• Credits should be considered for establishing native plant community corridors or naturally vegetated connections between sites.

Site reforestation or prairie restoration credit

[[File:Site reforestation credit example.png|thumb|300px|alt=schematic showing an example for calculating credits for site reforestation|Schematic showing an example for calculating credits for site reforestation. see NOTES⁼⁼

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