Borough of Shippensburg, PA Stormwater Maintenance 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 § 136-17 and the peak rate controls of § 136-18 do not need to be retrofitted to existing impervious areas that are not being altered by the proposed regulated activity.

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:

(a)

Minimize disturbance to floodplains, wetlands, and wooded areas.

(b)

Maintain or extend riparian buffers.

(c)

Avoid erosive flow conditions in natural flow pathways.

(d)

Minimize thermal impacts to waters of this Commonwealth.

(e)

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

Incorporate methods described in the Pennsylvania Stormwater Best Management Practices Manual (BMP Manual). Where practicable, green infrastructure and LID methods should be employed to achieve the volume and rate controls required by this chapter.

Various BMPs and their design standards are listed in the BMP Manual.

§ 136-15 Exemptions.

Regulated activities that result in cumulative earth disturbances less than one acre are exempt from the requirements in § 136-17, § 136-18, and Article IV 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 § 136-14D.

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

§ 136-16 General design standards.

The design of all stormwater management facilities shall incorporate sound engineering judgment, principles and practices. The Borough reserves the right to disapprove any design that would result in the creation or continuation adverse hydrologic or hydraulic conditions. The following general requirements shall apply to all regulated activities within the Borough:

Impervious area.

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 phases.

For development taking place in phases, the total proposed impervious area within the SWM site plan must be used in determining conformance with this chapter.

All calculations, assumptions, criteria and references used in the design of the SWM site plan shall be provided.

Computations for determining stormwater runoff, and for the design of stormwater management facilities, shall be based upon the NRCS Soil-Cover-Complex Method (Technical Release 554 or Technical Release 20, as amended), or the Modified Rational Method. NRCS methods shall be acceptable for watershed analyses of all sizes. The Modified Rational Method shall be acceptable for watersheds up to 25 acres in size.

Precipitation intensities utilized for Rational Method runoff computations shall be based on mean precipitation frequency estimates (Duration equals 60 minutes) published in the Precipitation-Frequency Atlas of the United States, Atlas 14, Volume 2, U.S. Department of Commerce, National Oceanic and Atmospheric Administration (NOAA), National Weather Service, Hydrometeorological Design Studies Center, Silver Spring, Maryland 20910. NOAA's Atlas 145 can be accessed at http://hdsc.nws.noaa.gov/hdsc/pfds/.

Precipitation depths utilized for NRCS Method runoff computations shall be based on mean precipitation frequency estimates (Duration equals 24 hours) published in the Precipitation-Frequency Atlas of the United States, Atlas 14, Volume 2, U.S. Department of Commerce, National Oceanic and Atmospheric Administration (NOAA), National Weather Service, Hydrometeorological Design Studies Center, Silver Spring, Maryland, 20910. NOAA's Atlas 145 can be accessed at http://hdsc.nws.noaa.gov/hdsc/pfds/.

Runoff Curve Numbers (CN) utilized for runoff computations based on the Soil-Cover-Complex Method shall be obtained from the NRCS TR-55 Manual 4.

Runoff coefficients (c) utilized for runoff computations based on the Modified Rational Method shall be obtained from a source that is widely accepted within the industry.

The stormwater management plan shall consider and provide provisions for management of all stormwater runoff affecting the development site, including safe conveyance of the 100-year design storm runoff from offsite watersheds, if any.

Storm sewers, swales and other stormwater conveyance structures shall be designed to convey post-development runoff from a twenty-five-year design storm with adequate freeboard. Storm sewers shall not surcharge above inlet grates. Six inches of freeboard shall be provided in swales carrying less than 15 cfs. One foot of freeboard shall be provided for in drainageways designed to carry flows of 15 cfs or greater.

The minimum pipe diameter permitted for use in storm sewer systems and/or stormwater carrying culverts dedicated to the Borough shall be 15 inches.

Stormwater drainage systems shall be provided in order to permit unimpeded flow along natural watercourses.

Stormwater flows onto adjacent property shall not be created, increased, concentrated or relocated without written consent by the adjacent property owner(s). Such stormwater flows shall be subject to the requirements of this chapter.

If existing diffused drainage discharge will be concentrated in the post development condition and discharged onto an adjacent property, the applicant must document that adequate downstream conveyance facilities exist to safely transport the concentrated discharge, or otherwise prove that no erosion, sedimentation, flooding or other harm will result from the concentrated discharge.

Any stormwater management facilities regulated by this chapter that would be located in or adjacent to waters of this Commonwealth or wetlands shall be subject to approval by DEP and, if applicable, the U.S. Army Corps of Engineers.

When there is a question whether wetlands or other regulated resources may exist, it is the responsibility of the applicant or his agent to demonstrate to the applicable regulatory agencies that the lands or features in question cannot be classified as wetlands or other regulated resources.

Any stormwater management facilities regulated by this chapter that would be located in state highway rights-of-way shall be subject to approval by the Pennsylvania Department of Transportation.

A manhole, inlet or cleanout structure shall be provided at all horizontal and vertical directional changes in storm sewer piping. They shall install intermediate cleanout structures as required at spacing intervals that will ease adequate maintenance of all storm sewer runs.

The stormwater management plan shall include calculations indicating velocities of flow, grades, sizes and capacities of water carrying structures, and retention and detention structures, as well as sufficient design information to construct such facilities.

Proposed lots or buildings adjacent to stormwater management basins and channels designed to convey a 100-year design storm shall have a finished grade elevation or first floor elevation two feet above the calculated 100-year storm event water surface elevation. First floor and finished grade elevations shall be specified on the plan.

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, Pa.Code, the Clean Streams Law, 35 P.S. § 691.1 et seq., and the Storm Water Management Act, 32 P.S. § 680.1 et seq. Special provisions may be required for projects draining to, or located in, special protection waters, which are high quality (HQ) or exceptional value (EV) by designation in 25 Pa.Code, Chapter 93, or by statewide existing use as dictated by DEP.

§ 136-17 Volume controls.

Green infrastructure and low impact development practices provided in the BMP Manual should be utilized for all regulated activities wherever practicable.

Volume controls shall be designed to meet the requirements of Control Guideline 1 (CG-1) in the BMP Manual.

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.

For modeling purposes, all existing (predevelopment) nonforested pervious areas must be considered meadow in good condition.

Volume control facilities.

A detailed soils evaluation of the project site shall be required to decide the suitability of infiltration facilities. A qualified design professional shall perform the evaluation, and at a minimum, address soil permeability, depth to bedrock and normal or a seasonal high water table, susceptibility to sinkhole formation, and subgrade stability.

Provide field tests to determine the presence of appropriate soil infiltration/percolation rates between 0.10 and 10.0 inch per hour. Testing shall be performed at the location of each proposed infiltration BMP at the bottom elevation of the proposed facility. A plan including the frequency and locations of soil tests shall be submitted to the Borough for review and approval.

Infiltration facilities shall be capable of completely dewatering the infiltration volume within 72 hours from the end of the design storm.

(4)

The bottom (infiltration surface) of infiltration facilities shall be separated by a minimum 24 inches vertically from the static or seasonal high water table and bedrock layer, as documented by soil testing at the location of the facility.

(5)

Specific requirements shall be included on the plans to protect infiltration facilities from compaction by equipment and to prevent sediment from entering infiltration facilities during construction.

(6)

Infiltration facilities shall be at least 15 feet down-gradient, or 50 feet up-gradient from on-site and/or offsite building foundations; except that infiltration facilities that collect runoff from house roof drains, having a roof area contributory to the infiltration facility equal to 1,500 square feet or less, shall be at least 15 feet down-gradient, or 15 feet up-gradient from on-site or off-site building foundations.

(7)

Infiltration facilities shall be a minimum of 100 feet from any drinking water supply well.

(8)

Infiltration facilities shall not be located within 50 feet of septic systems and/or septic system drain fields.

(9)

The bottom (infiltration surface) of infiltration facilities shall not be constructed in a compacted fill layer.

(10)

The bottom (infiltration surface) of infiltration facilities shall be constructed at a slope of 1% or flatter.

(11)

All infiltration facilities shall incorporate a conveyance and control for overflow runoff.

(12)

In carbonate areas, infiltration facilities shall be designed to disperse stormwater over the largest area feasible so as not to significantly modify the natural hydrologic regime. The use of widely distributed infiltration facilities (as opposed to centralized infiltration facilities) is highly recommended.

§ 136-18 Rate controls.

For areas not covered by a release rate map from an approved Act 167 stormwater management plan post-development discharge rates shall not exceed the pre-development discharge rates for the one-, two-, five-, ten-, twenty-five-, fifty-, and 100-year, twenty-four-hour storm events. If it is shown that the peak rates of discharge indicated by the post-development analysis are less than or equal to the peak rates of discharge indicated by the predevelopment analysis for one-, two-, five-, ten-, twenty-five-, fifty-, and 100-year, twenty-four-hour storms, then the requirements of this section have been met. Otherwise, the applicant shall provide additional controls as necessary to satisfy the peak rate of discharge requirement.

For areas covered by a release rate map from an approved Act 167 stormwater management plan for the one-, two-, five-, ten-, twenty-five-, fifty-, and 100-year, twenty-four-hour storm events, 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.

Rate control facilities:

Embankment material used for earthen basin construction shall be comprised of either on-site or imported fill which meets the following criteria:

(a)

Free of organic material, ash, cinders and demolition debris.

(b)

Particle size distribution that is well graded.

(c)

Less than 15% by weight rock fragments larger than three inches, less than 30% by weight larger than 3/4 inch and less than 30% smaller than No. 200 sieve.

Embankment slopes shall not be steeper than one foot vertical in three feet horizontal.

The top width of basin embankments shall not be less than six feet.

(4)

Emergency spillways in berms or earthen embankments shall be designed to convey the peak discharge from a 100-year design storm event while maintaining a minimum one foot of freeboard, assuming that the principal outlet structure is completely blocked.

(5)

Where possible, emergency spillways shall be constructed in virgin material, not in fill. Proper erosion control measures shall be provided to protect the spillway and embankment against the erosive effects of accelerated discharge. Calculations for erosion protection shall be provided.

(6)

All embankments shall incorporate a compacted clay core and cutoff key trench meeting the criteria for embankment material as well as the following additional criteria: soil shall be relatively impermeable and meet the following USCS classification groups as determined by ASTM D2487/D2488 CL and/or CL-ML. Top width of compacted clay core shall be a minimum of two feet with a top elevation equal to the twenty-five-year storm peak water storage elevation or higher. The bottom width of clay key trench shall be a minimum of four feet and the key shall extend a minimum of two feet into virgin, undisturbed soil below the topsoil layer.

(7)

Anti-seep collars shall be installed on all basin outlet pipes. The required size and spacing of the collars shall be confirmed through calculations.

(8)

Basins designed to provide extended detention times as a water quality control shall dewater in not less than 48 hours and not more than 96 hours.

(9)

Basins shall be kept in a maintainable condition with a minimum bottom slope of 1% toward the principal outlet. This requirement shall not apply to basins that do not dewater the entire storage volume through the principal outlet (i.e., infiltration basin, wet basin, retention basin, etc.).

§ 136-19 Riparian buffers.

In order to protect and improve water quality, the creation of a riparian buffer easement shall be created and recorded as part of any subdivision or land development proposed on previously undeveloped lands through which surface water flows.

Riparian buffers are strongly encouraged, but not required, on previously developed properties on which redevelopment is proposed.

Except as required by Chapter 102, the riparian buffer easement shall be measured to be 35 feet from the top of the streambank on each side of the watercourse.

Minimum management requirements for riparian buffers.

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 Borough 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 a required by zoning, unless otherwise specified in the municipal Zoning Ordinance.[1]

[1]

Editor's Note: See Ch. 150, 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.

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

Trails shall be for nonmotorized use only (except for normal maintenance).