Township of Wilkins, PA Stormwater Management Standards

The measurement of impervious areas shall include all of the impervious areas in the total proposed development even if development is to take place in stages.

For development taking place in stages, the entire development plan must be used in determining conformance with this chapter.

For projects that add impervious area to a parcel, the total impervious area on the parcel is subject to the requirements of this chapter, except that the volume controls in § 374-16 and the peak rate controls of § 374-17 do not need to be retrofitted to existing impervious areas that are not being altered by the proposed regulated activity.

(4)

Gravel surfaces in the existing (predevelopment) condition are to be considered pervious.

(5)

Gravel surfaces in the proposed (post-development) condition are to be considered impervious.

Stormwater flows onto adjacent property shall not be created, increased, decreased, relocated, or otherwise altered without written notification of the adjacent property owner(s) by the developer. Notification shall include a description of the proposed development and the stormwater flows that are being created, increased, decreased, relocated, impeded, or otherwise altered. Adjacent property shall, at a minimum, include any property having a shared boundary with the subject property of the SWM site plan, however, if in the judgement of the designated plan reviewer additional properties are being affected, additional notifications may be required. Proof of notification (for example, signed postal receipt) shall be included as part of the SWM plan submission to the municipality. Such stormwater flows shall be subject to the requirements of this chapter.

All regulated activities shall include such measures as necessary to:

Protect health, safety, and property.

Meet the water quality goals of this chapter by implementing measures to:

Minimize disturbance to floodplains, wetlands, and wooded areas.

Maintain or extend riparian buffers.

Avoid erosive flow conditions in natural flow pathways.

Minimize thermal impacts to waters of this commonwealth.

(e)

Disconnect impervious surfaces by directing runoff to pervious areas, wherever possible.

To the maximum extent practicable, incorporate the techniques for low-impact development practices described in the Pennsylvania Stormwater Best Management Practices Manual (BMP Manual).[3] If methods other than green infrastructure and LID methods are proposed to achieve the volume and rate controls required under this chapter, the SWM site plan must include a detailed justification, acceptable to the designated plan reviewer, demonstrating that the use of LID and green infrastructure is not practicable.

[3]

Editor's Note: See § 374-42A.

The design of all facilities over karst shall include an evaluation of measures to minimize adverse effects.

Infiltration BMPs should be spread out, made as shallow as practicable, and located to maximize use of natural on-site infiltration features while still meeting the other requirements of this chapter.

Normally dry, open-top storage facilities should completely drain both the volume control and rate control capacities over a period of time not less than 24 and not more than 72 hours from the end of the design storm.

Design storm volumes.

The design storm volumes to be used in the analysis of peak rates of discharge shall be as follows:

One-year, twenty-four-hour storm: two inches.

Two-year, twenty-four-hour storm: 2.50 inches.

Ten-year, twenty-four-hour storm: 3.61 inches.

Twenty-five-year, twenty-four-hour storm: 4.31 inches.

(e)

One-hundred-year, twenty-four-hour storm: 5.71 inches.

Other storm volumes and intensities should be obtained from the Precipitation-Frequency Atlas of the United States, Atlas 14, Volume 2, Version 3.0, U.S. Department of Commerce, National Oceanic and Atmospheric Administration (NOAA), National Weather Service, Hydrometeorological Design Studies Center, Silver Spring, Maryland. NOAA's Atlas 14 can be accessed at: http://hdsc.nws.noaa.gov/hdsc/pfds/.[4]

[4]

Editor's Note: See § 375-42E.

For all regulated activities, SWM BMPs shall be designed, implemented, operated, and maintained to meet the purposes and requirements of this chapter and to meet all requirements under Title 25 of the Pennsylvania Code, the Clean Streams Law, and the Storm Water Management Act.

Various BMPs and their design standards are listed in the BMP Manual, subject to any requirements of this chapter.[5]

[5]

Editor's Note: See § 374-42A.

§ 374-15 Exemptions.

Some activities may be exempted from the full requirements of this chapter as detailed below. Unless specifically noted as automatic, exemption requests must be submitted to and approved by the Township. Any exemption granted shall apply only to the portions of this chapter specifically noted. All other stormwater management design elements, such as a storm sewer system, culverts, erosion and sedimentation controls, etc., shall still be required.

There shall be no exemptions for activities resulting in cumulative earth disturbances of one acre or greater.

Activities that do not require a site plan or land development plan approval from the Township may be exempt, subject to the review of the exemption request by the Township.

Individual detached single-family homes that are not part of an existing or planned subdivision with an approved SWM site plan shall be exempt from the requirement to submit a complete SWM site plan.

The subdivision of one single-family detached residential lot into two such lots, or a subdivision including only the revision of the lot line(s) between two such lots, shall be exempt from the requirement to submit a complete SWM site plan. Should the Township suspect that repeated such subdivisions are being used to circumvent the planning requirement, the Township reserves the right to revoke the exemption.

The work required to return a property to its previous condition following a loss resulting from an act of nature, fire, or accident, etc., shall be exempt from the requirement to submit a complete SWM site plan.

Emergency maintenance work performed for the protection of public health, safety, and welfare may be exempted from the requirements in this chapter to obtain approval for a SWM site plan prior to the commencement of the activity. A written description of the scope and extent of such work shall be submitted to the Township within two calendar days of the commencement of the activity. If the Township deems that the work does not constitute an emergency, then the work shall cease immediately and not resume until a SWM site plan is submitted to and approved by the Township. The purpose of this exemption is to allow emergency work to proceed immediately only. All work must otherwise comply with all applicable provisions of this chapter.

Maintenance work performed on an existing stormwater management facility that was installed in accordance with plans and specifications approved by the Township may be exempted from the requirements of this chapter. The facility owner shall notify the Township a minimum of three working days in advance, with such notice to include a description of the work to be performed. All work should be completed in accordance with the approved operation and maintenance plan for the facility and shall only be performed to bring the facility back into conformance with the original approved plan. Any work that would alter the configuration, capacities, or operation of the facility must be submitted to the Township for approval.

Activities related solely to property maintenance, such as resurfacing an existing parking lot, replacement of existing sidewalks, roof replacement, building exterior repairs, etc., shall be automatically exempt from the requirements of this chapter if no additional impervious area is added as a result of the activities.

The use of land for home gardening for home consumption shall be automatically exempt from the requirements of this chapter.

Agricultural activity is exempt from the SWM site plan preparation requirements of this chapter, provided the activities are performed according to the requirements of 25 Pa. Code Chapter 102.

Forest management and timber operations are exempt from the SWM site plan preparation requirements of this chapter, provided the activities are performed according to the requirements of 25 Pa. Code Chapter 102.

Exemptions from any provisions of this chapter shall not relieve the applicant from the requirements in § 374-14D through K.

The following activities may be exempt from the requirements to submit a complete SWM site plan. In all cases, however, appropriate stormwater quality BMPs must be implemented on the site where required. Exemptions will not be granted if the activity is found to contribute pollution to the waters of the commonwealth, or if the runoff from the activity impacts an adjacent property:

Minor impact activities.

This shall apply to an activity not classified as a single-family detached home on a single lot.

An activity shall be classified as a "minor impact" if it involves less than 2,000 square feet (< 2,000 square feet) of impervious area, to include both newly created impervious area and any reconfiguration of existing impervious area, and results in a total earth disturbance of less than 5,000 square feet (< 5,000 square feet).

This exemption can only be applied once for each parcel. Subsequent, or phased, regulated activities on the same or contiguous properties which result in an overall total of 2,000 square feet or greater (> 2,000 square feet) of impervious surface, or 5,000 square feet or greater (> 5,000 square feet) of earth disturbance, shall be subject to the full requirements of this chapter.

The area of any existing impervious surfaces that may be permanently replaced with pervious surfaces on a project site may not be deducted from the area of impervious surface created/reconfigured to qualify for this exemption.

(e)

The applicant shall submit a narrative describing the project, documentation of the total proposed impervious area and disturbed area, proposed use of any BMPs on the site, including any calculations or justification for their selection, as well as the operation and maintenance plan and agreement for the facilities.

Vertical extensions.

This shall apply to an activity not classified as a single-family detached home on a single lot.

Such activities must involve only the vertical extension of a structure or portions thereof, with an addition of the same size and shape as the structure or portion thereof directly beneath the addition, and which require no earth disturbance activities other than for the construction of said addition. Appropriate erosion and sedimentation control measures must be employed during any construction activities.

Small projects. Regulated activities involving between 2,000 square feet and 5,000 square feet of new or reconfigured impervious area and less than one acre of earth disturbance shall be considered a "small project" and must submit a SWM site plan to the Township, which shall consist of the following items and related supportive material needed to determine compliance with §§ 374-16 through 374-18. The applicant can use the protocols in the "Small Project Stormwater Management Site Plan" included in Appendix C.[1]

General description of proposed stormwater management techniques, including construction specifications of the materials to be used for stormwater management facilities.

An erosion and sediment control plan, including all reviews and letters of adequacy from the Conservation District where appropriate.

Limits of earth disturbance, including the type and amount of impervious area that is proposed; proposed structures, roads, paved areas, and buildings; and a statement, signed by the applicant, acknowledging that any revision to the approved drainage plan must be approved by the Township, and that a revised erosion and sediment control plan must be submitted to the Township or Conservation District for approval.

All stormwater management facilities must be located on a plan and described in detail, and all calculations, assumptions, and criteria used in the design of the stormwater management facilities must be shown.

[1]

Editor's Note: Appendix C is included as an attachment to this chapter.

Roadway resurfacing and maintenance projects, which do not increase impervious area, and underground infrastructure projects are exempt from the provisions of this chapter, provided the activities meet the requirements of all other municipal, state and federal requirements.

The Township may deny or revoke any exemption pursuant to this section at any time for any project that the Township believes may pose a threat to public health and safety or the environment.

Voluntary green stormwater infrastructure (GSI) retrofit projects that are solely intended to better manage runoff from existing development and are not part of new development or redevelopment are exempt from the stormwater management provisions of this chapter. This does not exempt such projects from any other municipal, state, or federal regulation.

§ 374-16 Volume controls.

The green infrastructure and low-impact development practices provided in the BMP Manual shall be utilized for all regulated activities to the maximum extent practicable.[1] Water volume controls shall be implemented using the Design Storm Method in Subsection A or the Simplified Method in Subsection B below. For regulated activity areas equal to or less than one acre that do not require hydrologic routing to design the stormwater facilities, this chapter establishes no preference for either methodology; therefore, the applicant may select either methodology on the basis of economic considerations, the intrinsic limitations on applicability of the analytical procedures associated with each methodology, and other factors.

The Design Storm Method (CG-1 in the BMP Manual) is applicable to any size of regulated activity.[2] This method requires detailed modeling based on site conditions.

Do not increase the post-development total runoff volume for all storms equal to or less than the two-year, twenty-four-hour duration precipitation.

At least the first one inch of runoff from the net increase in impervious surfaces shall be permanently removed from the runoff flow, i.e., it shall not be released into the surface waters of this commonwealth. Removal options include reuse, evaporation, transpiration, and infiltration. If the developer provides justification that the listed removal options are not feasible, and the designated plan reviewer agrees, runoff shall be detained in a facility designed for a twenty-four- to seventy-two-hour dewatering time in an area with a dedicated stormwater system (not contributory to a combined sewer system) and shall be detained in a facility designed for a seventy-two-hour dewatering time in an area contributory to a combined sewer system before discharge to local stormwater systems or the environment.

For modeling purposes:

Existing (predevelopment) nonforested pervious areas must be considered meadow in good condition.

Twenty percent of existing impervious area, when present, shall be considered meadow in good condition in the model for existing conditions.

[2]

Editor's Note: See § 374-42A.

The Simplified Method (CG-2 in the BMP Manual) provided below is independent of site conditions and should be used if the Design Storm Method is not followed.[3] This method is not applicable to regulated activities greater than one acre or for projects that require design of stormwater storage facilities. For new impervious surfaces:

Stormwater facilities shall capture at least the first two inches of runoff from all new impervious surfaces.

At least the first one inch of runoff from new impervious surfaces shall be permanently removed from the runoff flow, i.e., it shall not be released into the surface waters of this commonwealth. Removal options include reuse, evaporation, transpiration, and infiltration. If the developer provides justification that the listed removal options are not feasible, and the designated plan reviewer agrees, runoff shall be detained in a facility designed for a twenty-four-hour dewatering time in an area with a dedicated stormwater system (not contributory to a combined sewer system) and shall be detained in a facility designed for a seventy-two-hour dewatering time in an area contributory to a combined sewer system before discharge to local stormwater systems or the environment.

Wherever possible, infiltration facilities should be designed to accommodate infiltration of the entire permanently removed runoff; however, in all cases at least the first 0.5 inch of the permanently removed runoff should be infiltrated.

(4)

This method is exempt from the requirements of § 374-17, Rate controls.

[3]

Editor's Note: See § 374-42A.

[1]

Editor's Note: See § 374-42A.

§ 374-17 Rate controls.

Areas covered by a release rate map from the Turtle Creek Watershed Act 167 Storm Water Management Plan and the 2018 Allegheny County Act 167 Storm Water Management Plan for the Thompson Run Watershed.

For the one-, two-, five-, ten-, twenty-five-, and 100-year storms, the post-development peak discharge rates will follow the applicable approved release rate maps. For any areas not shown on the release rate maps, the post-development discharge rates shall not exceed the predevelopment discharge rates.

§ 374-18 Runoff calculation methodology.

All calculations shall be consistent with the guidelines set forth in the BMP Manual, as amended herein.

Stormwater calculation methods shall be selected by the design professional based upon the requirements stated in this chapter and suitability of each method for a particular site. The selected methodology shall be acceptable to the designated plan reviewer.

Calculation of runoff volume.

The Rational Method shall not be used to calculate runoff volume.

The NRCS Rainfall-Runoff Methodology shall be used to estimate the change in volume due to regulated activities.

The applicant shall submit all relevant worksheets from the BMP Manual to demonstrate the change in runoff for a two-year storm event.

Calculation of peak runoff rate.

The Rational Method may be used for the design of conveyance facilities only. Caution should be used by the design professional if the watershed has more than one main drainage channel, if the watershed is divided such that hydrologic conditions are significantly different from one area to the other, if the time of concentration exceeds 60 minutes, or if stormwater runoff volume is an important factor. The combination of Rational Method hydrographs based upon timing shall be prohibited.

The NRCS Rainfall-Runoff methods are recommended for design of stormwater management facilities and where stormwater runoff volume must be taken into consideration.

Other methods as submitted to and preapproved by the designated plan reviewer.

Runoff coefficients.

The applicant shall use accepted land cover coefficients according to the selected runoff calculation method.

Combining runoff coefficients for land areas proposed for development with coefficients for areas unaffected by the proposed development into a single weighted coefficient is not acceptable.

Time of concentration.

The time of concentration (Tc) is to represent the average condition that best reflects the longest hydrologic route to the point of interest. The applicant shall delineate the flow paths used for calculating the Tc for both the predeveloped and post-developed conditions.

The post-development Tc shall never be greater than the predevelopment Tc for any watershed or subwatershed. This includes situations when the designer has specifically used swales or other measures to reduce flow velocities. In the event that the designer believes that the post-development Tc is greater, it will still be set by default to the predevelopment Tc for modeling purposes.

The minimum Tc for any site shall be five minutes.

(4)

The designer may choose to assume a five-minute Tc for post-development conditions without providing any computations.

(5)

The designer must provide computations for all predevelopment Tc paths. A five-minute Tc cannot be assumed for predevelopment conditions.

For comparison of peak flow rates, flows shall be rounded to a tenth of a cubic foot per second (cfs).

The stormwater management and conveyance system shall be designed to safely convey the post-development 100-year storm event to stormwater detention facilities, for the purpose of meeting peak rate control.

§ 374-19 Riparian buffers.

In order to protect and improve water quality, a riparian buffer easement shall be created and recorded as part of any subdivision or land development that encompasses a riparian buffer. The intent of this chapter in establishing a riparian buffer is to protect and improve stream water quality. The riparian buffer is intended to slow overland flow to the stream through the presence of native grasses, trees and shrubs, allowing infiltration/groundwater recharge; causing deposition of sediment, nutrients, pesticides, and other pollutants in the buffer rather than in the stream; and reducing erosion by providing stream bank stabilization. The trees provide shade for streams; keeping waters cooler and reducing evaporation.

The riparian buffer easement shall be required for all streams (as defined in Article II) with a contributing watershed area of greater than 10 acres. The riparian buffer easement shall be measured to be a minimum of 35 feet from the top of the streambank (on each side).

Minimum management requirements for riparian buffers.

No use or construction within the riparian buffer shall be permitted that is inconsistent with the intent of the riparian buffer as described in § 374-19A.

Existing native vegetation shall be protected and maintained within the riparian buffer easement.

Whenever practicable, invasive vegetation shall be actively removed and the riparian buffer easement shall be planted with native trees, shrubs and other vegetation to create a diverse native plant community appropriate to the intended ecological context of the site.

The riparian buffer easement shall be enforceable by the municipality and shall be recorded in the appropriate county recorder of deeds office so that it shall run with the land and shall limit the use of the property located therein. The easement shall allow for the continued private ownership and shall count toward the minimum lot area required by zoning, unless otherwise specified in Chapter 450, Zoning.

Any permitted use within the riparian buffer easement shall be conducted in a manner that will maintain the extent of the existing 100-year floodplain, improve or maintain the stream stability, and preserve and protect the ecological function of the floodplain.

Stormwater drainage pipes shall be permitted within the riparian buffer easement, but they shall cross the easement in the shortest practical distance. Other structural stormwater management facilities are not permitted within the riparian buffer easement.

The following conditions shall apply when public and/or private recreation trails are permitted by the municipality within riparian buffers:

It is preferred that trails be designed to be permeable and for nonmotorized use only; however, impermeable trails are permitted provided they have adequate drainage.