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Stormwater Management/Impervious Surface Mitigation Standards
This information was adapted from the 'Shiawassee & Huron Headwaters Preservation Project' developed by Oakland County Planning and Carlisle Wortman & Associates.

The following is sample language relative to stormwater management that may be adopted as part of either the Zoning Ordinance or community design standards. Most communities have adopted standards that limit the rate of runoff from sites, but do little to encourage either creative means of managing stormwater on-site or reducing the amount of impervious surface. Frequently, local ordinances contain provisions that result in significant amounts of impervious surface from streets, sidewalks, and parking lots and do not promote environmentally compatible design. Single purpose solutions are proposed that may efficiently remove stormwater but do not promote infiltration, improve water quality, or enhance integrated stormwater management as an integral component of aesthetic site design.



Section _________. Stormwater Management/Impervious Surface Mitigation

(a.) Purpose. The (community) recognizes that stormwater runoff has been traditionally treated as a by-product of development to be disposed of as quickly and efficiently as possible. The result has often been increased flooding, degradation of water quality, soil erosion and sedimentation, and a failure to capitalize on the benefit of creative stormwater management. It is also recognized that certain community development standards may contribute to decreased pervious surface and increased stormwater runoff.

It is the intent of this Ordinance to encourage the use of Best Management Practices (BMPs) which are structural, vegetative, or managerial practices designed to treat, prevent, or reduce degradation of water quality due to stormwater runoff. All development projects subject to review under the requirements of this Ordinance shall be designed, constructed, and maintained using BMPs to prevent flooding, protect water quality, reduce soil erosion, maintain and improve wildlife habitat, and contribute to the aesthetic values of the project. The particular facilities and measures required on-site shall reflect and incorporate existing grade, natural features, wetlands, and watercourses on the site, to the maximum extent feasible.

(b.) Stormwater Drainage/Erosion Control. All stormwater drainage and erosion control plans shall meet the standards adopted by the (community) for design and construction and shall, to the maximum extent feasible, utilize nonstructural control techniques, including but not limited to:

(1) limitation of land disturbance and grading;

(2) maintenance of vegetated buffers and natural vegetation;

(3) minimization of impervious surfaces;

(4) use of terraces, contoured landscapes, runoff spreaders, grass or rock-lined swales;

(5) use of infiltration devices;

(c.) General Standards.
(1) Stormwater management systems shall be designed to prevent flooding and the degradation of water quality related to stormwater runoff and soil erosion from proposed development.

(2) All properties which are subject to this ordinance shall provide for on-site storage of stormwater. Facilities shall be designed to provide a volume of storage and discharge rate which meets the Oakland County Drain Commissioner's standards or the standards of (community), whichever are stricter.

(3) Priority shall be placed on site design that maintains natural drainage patterns and watercourses. Alterations to natural drainage patterns shall not create flooding or degradation in water quality for adjacent or downstream property owners.

(4) The use of swales and buffer strips vegetated with native materials is encouraged as a method of stormwater conveyance so as to decrease runoff velocity, allow for biofiltration, allow suspended sediment particles to settle, and to remove pollutants.

(5) Drainage systems shall be designed to be visually attractive. The integration of stormwater conveyance systems and retention and detention ponds in the overall landscape concept is recommended. Ponds with a naturally contoured, rather than square or rectangular design and appearance shall be encouraged.

(d.) Use of Wetlands. Wetlands may be used for stormwater management if all the following conditions are met:
(1) Direct discharge of untreated stormwater to a natural wetland is prohibited. All runoff from the development will be pre-treated to remove sediment and other pollutants prior to discharge to a wetland. Such treatment facilities shall be constructed before property grading begins. Stormwater runoff discharged to wetlands must be diffused to non-erosive velocities before it reaches the wetland.

(2) Wildlife, fish, or other beneficial aquatic organisms and their habitat within the wetland will not be impaired

(3) The wetland has sufficient holding capacity for stormwater, based upon calculations prepared by the proprietor and reviewed and approved by the township engineer.

(4) Adequate on-site erosion control is provided to protect the natural functioning of the wetland.

(5) Adequate private restrictions are established so as to insure that the wetland is not disturbed or impaired in the future relative to the needed storage capacity.

(6) Applicable permits from the Michigan Department of Environmental Quality are obtained.


(e.) Impervious Surface Reduction/Infiltration Enhancement. The (community) recognizes that, due to the specific requirements of any given development, inflexible application of the design standards may result in development in excessive paving and stormwater runoff and a waste of space, which could be left as open space.

Either through procedures prescribed by Ordinance or creative land development techniques permitted by Ordinance, the (community) may permit deviations from requirements allowing for reduction in impervious surfaces whenever it finds that such deviations are more likely to meet the intent and standards of this Ordinance and accommodate the specific characteristics of the use in question.

The (community) may attach conditions to the approval of a deviation that bind such approval to the specific use in question. Measures that reduce impervious surface and increase infiltration may include, but are not limited to, the following:

(1) Streets and Access.
a. Residential streets designed with the minimum required pavement width needed to support travel lanes; on-street parking; and emergency, maintenance, service vehicle access, and function based on traffic volumes.

b. The total length of residential streets reduced by examining alternative street layouts to determine the best option for increasing the number of homes per unit length.

c. Street right-of-way widths designed to reflect the minimum required to accommodate the travel-way, the sidewalk, and vegetated open channels.

d. Minimize the number of street cul-de-sacs and reduce the radius of cul-de-sacs to the minimum required to accommodate emergency and maintenance vehicles. Alternative turnarounds shall be considered, including the use of mountable curbing and grass shoulders for the occasional event of access by fire trucks and other large commercial trucks. Where cul-de-sacs do exist, provide landscape center islands.

e. Where density, topography, soils, and slope permit, use of vegetated open channels in the street right-of-way to convey and treat stormwater runoff.

f. Use of alternative driveway surfaces and shared driveways that connect two or more uses.

g. Promote more flexible design standards for residential subdivision sidewalks. Where practical, consider locating sidewalks on only one side of the street and providing common walkways linking pedestrian areas.



(2) Parking
a. Base parking requirements on the specific characteristics of the use, and landbank in open space, parking that is required to satisfy Ordinance requirements.
b. Reduce the overall imperviousness associated with parking lots by providing compact car spaces, minimizing stall dimensions, incorporating efficient parking lanes, and using pervious materials in the spillover parking areas where possible.

c. Encourage shared parking between compatible users.

(3) Site Design
a. Relax side yard setbacks and allow narrower frontages to reduce total road length in the community and overall site imperviousness. Relax front set back requirements to minimize driveway lengths and reduce overall lot imperviousness.

b. Direct rooftop runoff to pervious areas such as yards, open channels, or vegetated areas and avoid routing rooftop runoff to the roadway and the stormwater conveyance system.

c. Create a variable width, naturally vegetated buffer system along all drainageways that also encompasses critical environmental features such as the 100-year floodplain, steep slopes, and wetlands.

d. Minimize clearing and grading of woodlands and native vegetation to the minimum amount needed to build lots, allow access, and provide fire protection.

e. Conserve trees and other vegetation at each site by planting additional vegetation, clustering tree areas, and promoting the use of native plants.

(f.) Maintenance. Whenever a landowner is required to provide on-site storm water retention and/or surface drainage to wetland, or whenever other protective environmental measures including monitoring devices are required, such measures or facilities shall be provided and maintained at the landowner's expense. The landowner shall provide satisfactory assurance to the Township whether by written agreement or otherwise, that the landowner will bear the responsibility for providing and maintaining such methods or facilities.
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