Township of Dickinson, PA Article III Stormwater Management Standards

§ 170-13 General requirements.

A. For all regulated activities, unless preparation of an SWM site plan is specifically exempted in § 170-14:

(1) All regulated activities within the Township which do not fall under the exemption criteria listed in § 170-14 shall submit an SWM site plan to the Township for review.

(2) Regulated activities shall not commence until the Township issues written approval of an SWM site plan, which demonstrates compliance with the requirements of this chapter.

B. SWM site plans approved by the Township, in accordance with this chapter, shall be on site throughout the duration of the regulated activity.

C. For all regulated earth-disturbance activities, erosion and sediment control BMPs shall be designed, implemented, operated, and maintained during the regulated earth-disturbance activities (e.g., during construction) to meet the purposes and requirements of this chapter. An approval letter from the Cumberland County Conservation District shall accompany all stormwater management plans requiring County Conservation District submittal and approval as determined by 25 Pa. Code Chapter 102.

D. For all regulated activities not exempted by § 170-14, implementation of the volume controls in § 170-15 is required.

E. Impervious areas.

(1) 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. Similarly, for development taking place in stages, the entire proposed new development plan must be used in determining conformance with this chapter.

(2) 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 § 170-15 and the peak rate controls of § 170-16 do not need to be retrofitted to existing impervious areas that are not being altered by the proposed regulated activity.

F. Stormwater flows onto adjacent property shall not be created, increased, decreased, relocated, concentrated or otherwise altered.

G. The design of all facilities within an area of karst topography and or carbonate geology shall include an evaluation of measures to minimize adverse effects in accordance with the procedures outlined in Section 7.4 (Special Management Areas — Karst Areas) of the most current version of the BMP Manual.

H. Storage facilities should completely drain both the volume control and rate control capacities within 72 hours from the end of the design storm, subject to site conditions.

I. Individual residential underground infiltration facilities shall be a minimum of 10 feet from the property line. Discharge of controlled flows can be no closer to an adjacent property than two times the length of the required discharge riprap apron. This requirement applies to discharge aprons that do not outlet to a defined waterway or an existing storm sewer. The minimum distance is 10 feet.

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

K. Roof drains and sump pumps shall discharge to infiltration or vegetative BMPs and to the maximum extent practicable to satisfy the criteria for disconnected impervious areas (DIAs).

§ 170-14 Exemptions.

A. Regulated activities resulting in less than or equal to 1,000 square feet of new impervious surface are exempt from implementing stormwater controls and SWM site plan preparation requirements.

B. Regulated activities greater than 1,000 square feet and less than or equal to 5,000 square feet of new impervious area may be exempt from the SWM site plan preparation requirements, including §§ 170-15, 170-16, 170-17, 170-20 and 170-21 of this chapter, when justification is provided that there is no increase in stormwater volume, rate and no adverse impacts to water quality or change in flow characteristics.

C. Regulated activities that create new disconnected impervious areas (DIA's) greater than 1,000 square feet and less than or equal to 5,000 square feet that are proven to meet the seventy-five-foot minimum pervious flow path requirement outlined in Appendix A[1] are exempt from implementing stormwater controls and SWM site plan preparation requirements in §§ 170-15, 170-16, 170-17, 170-20 and 170-21.

[1] Editor's Note: Appendix A is included at the end of this chapter.

D. Regulated activities meeting the following parcel size and square footage requirements are exempt from the peak rate control requirements but not the requirements of §§ 170-15, 170-16, 170-17, 170-20 and 170-21 of this chapter. These criteria shall apply to the total proposed development even if development is to take place in phases. The date of the municipal ordinance adoption shall be the starting point from which to consider tracts as "parent tracts" in which future subdivisions and respective impervious area computations shall be cumulatively considered.

	New Impervious Area Exemption Criteria for Peak Rate Control
	Total Parcel Size (acres)	Total Parcel Size (square feet)	New Impervious Area Exemption (square feet)
	<0.25	<10,890	1,500
	0.25 to 0.5	10,890 to 21,780	2,500
	>0.5	>21,780	5,000

E. Agricultural activities are exempt from the SWM site plan requirements, including §§ 170-15 and 170-16 of this chapter, provided that the activities are performed according to the requirements of 25 Pa. Code Chapter 102.

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

G. The use of land for domestic (i.e., for a single individual dwelling unit) gardening and landscaping is exempt from specific approval and permitting under this chapter so long as such activities comply with all other applicable ordinances and statutes.

H. The Township has the ability to deny or revoke any exemption from any requirements of this chapter. Exemption from any requirements of this chapter does not convey exemption from any requirements of any other applicable local codes or ordinances or from the responsibility to protect public health, safety, property or the environment. Qualifications for revocation shall be on the basis of noncompliance with § 170-4. The Township shall provide a written citation of each provision of § 170-4 that is deemed to be noncompliant.

§ 170-15 Volume controls.

Water volume controls shall be implemented using the Design Storm Method in Subsection A or the Simplified Method in Subsection B below for all regulated activities not otherwise exempted by § 170-14. 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. Volume control by way of infiltration facilities may be required to incorporate a pretreatment facility capable of separating grit and other fine materials from the stormwater where accessibility and maintenance ability is restricted.

A. The Design Storm Method (see Section 8.7 of the most current version of the Pennsylvania Stormwater Best Management Practices Manual) is applicable to any size of regulated activity. This method requires detailed modeling based on site conditions.

(1) The post-development total runoff volume for all storms equal to or less than the two-year, twenty-four-hour duration precipitation shall not be increased.

(2) For modeling purposes:

(a) Existing (predevelopment) nonforested pervious areas must be considered meadow in good condition or its equivalent.

(b) Twenty percent of existing impervious area, when present, shall be considered meadow in the model for existing conditions.

(3) Field infiltration tests shall be required in accordance with § 170-19.

B. The Simplified Method (see Section 8.7 of the most current version of the Pennsylvania Stormwater Best Management Practices Manual) provided below is independent of site conditions and should be used if the Design Storm Method is not followed. This method is not applicable to regulated activities greater than one acre or for projects that require design of stormwater detention or rate-control facilities. For new impervious surfaces:

(1) Stormwater facilities shall be sized to capture at least the first two inches of runoff from all new impervious surfaces.

(2) 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 the commonwealth. Removal options include reuse, evaporation, transpiration, and infiltration.

(3) Infiltration facilities shall be designed to accommodate infiltration of the entire permanently removed runoff; however, in all cases at least the first 1/2 inch of the permanently removed runoff should be infiltrated.

(4) The second one inch of runoff from new impervious surfaces shall be detained using structural and nonstructural BMPs (as outlined in the most current version of the Pennsylvania Stormwater Best Management Practices Manual) and released at a controlled rate.

(5) Regulated activities eligible under this method are exempt from the requirements of § 170-16, Rate controls.

(6) Field infiltration tests shall be required in accordance with § 170-19.

§ 170-16 Rate controls.

A. For computation of predevelopment peak discharge rates, 20% of existing impervious areas, when present, shall be considered meadow.

B. Post-development discharge rates shall be in accordance with the Release Rate Map found in Appendix B.[1] This release rate is a percentage of the predevelopment discharge rate (i.e., for the Upper Yellow Breeches Creek watershed, RR = 95 or 95% of the predevelopment discharge rate.) For the one-, two-, five-, ten-, twenty-five-, fifty-, and one-hundred-year storms, the post-development peak discharge rates will follow the applicable approved release rate.

[1] Editor's Note: Appendix B is included at the end of this chapter.

§ 170-17 Protection of water quality.

A. All developments shall be required to capture and treat 90% of the average annual stormwater runoff volume. This volume of stormwater runoff, the water quality volume (WQ_v), is equal to the following:

	WQv = [(P90)(Rv)(A)]/1 2
	Where:
		WQv	=	Water quality volume (in acre-feet).
		P90	=	Ninety-percent storm (90% of the annual rainfall will occur in storms of equal or smaller magnitude) = 1.95 inches.
		Rv	=	0.05 + 0.009(I), where (I) is the percent of impervious cover of the development site.
		A	=	Development site (in acres).

B. The following assumptions may be made when designing stormwater management facilities for treatment of the WQ_v:

(1) When a development site has more than one drainage area, a WQ_v shall be calculated for each drainage area.

(2) If the development site is located in a drainage area that encompasses area outside of the development site, only the proposed impervious coverage on the development site shall be used to calculate the WQ_v.

C. The Board of Supervisors, upon recommendation of the Township Engineer, may impose the following additional restrictions on stormwater discharges:

(1) Peak discharge may be further restricted where there is a probable risk to downstream structures or unique natural areas or where existing, severe flooding problems could be aggravated.

(2) Measures shall be imposed to protect against groundwater or surface water pollution where the type of business activity may result in significant non-point-source pollution (so-called "hot spots") or the nature of the soils or bedrock underlying a stormwater management structure constitutes substantial risk of contamination, such as might be the case in limestone formations.

D. Significant loadings of non-point-source pollutants shall not be discharged into either surface water or groundwater. "Significant" is defined as resulting in an increase greater than 10% of existing background concentrations of all water quality parameters of consequence identified in federal and state criteria. In particular, nutrients (nitrate and total phosphorus), metals (cadmium and lead), total petroleum hydrocarbons (TPH), and synthetic organic compounds identified by the United States Environmental Protection Agency (USEPA) as toxic or hazardous substances must be controlled. If the volume and peak rate standards above are met, then water quality impacts are assumed to be adequately controlled. If the volume standard cannot be achieved, then a water quality impact analysis must be performed, at the direction of the Township Engineer, confirming prevention of any significant increase in non-point-source pollution, with particular focus on the pollutants discussed above. Both structural and nonstructural (preventive) measures must be considered for reduction and prevention of non-point-source pollution.

E. Provisions shall be made so the one-year storm takes a minimum of 24 hours to drain from the facility from a point where the maximum volume of water from the one-year storm is captured (i.e., the maximum water surface elevation is achieved in the facility). Release of water can begin at the start of the storm (i.e., the invert of the water quality orifice is at the invert of the facility). The design of the facility shall consider and minimize the chances of clogging and sedimentation potential.

F. For commercial and industrial sites where there is a potential for toxic or hazardous substances to come into contact with stormwater runoff, pretreatment of the first-flush (first 1/2 inch) runoff over areas where commercial or industrial operations take place shall be provided. Pretreatment shall include means for separating light and heavy nonaqueous phase liquids from the stormwater before the stormwater is conveyed to the general stormwater management facility.

§ 170-18 General provisions for areas underlain by limestone bedrock.

A. In areas that are underlain by limestone bedrock, stormwater management facilities shall be designed to minimize the concentration of stormwater runoff. A detailed geologic evaluation of the project site shall be performed to determine the suitability of recharge facilities. The evaluation shall be performed by a qualified geologist and/or soil scientist and, at a minimum, address soil permeability, depth to bedrock, susceptibility to sinkhole formation, and subgrade stability. Where pervious pavement is permitted for parking lots, recreational facilities, nondedicated streets, or other areas, pavement construction specifications shall be noted on the plan. Stormwater management facilities for the recharge of groundwater in limestone bedrock areas must provide for infiltration opportunities distributed over a very large area. Examples include filter strips, large bioretention areas, and pervious pavement. Stormwater management facilities that create concentrated sources of infiltration, such as infiltration trenches or dry wells, shall not be used in limestone bedrock areas. Implementation of these infiltration requirements in defined HQ and EV watersheds is subject to the Department of Environmental Protection's Chapter 93, Water Quality Regulations and Antidegradation Regulations.

B. Whenever a basin will be located in an area underlain by limestone, a geological evaluation of the proposed location shall be conducted to determine susceptibility to sinkhole formations. The design of all facilities over limestone formations shall include measures to prevent groundwater contamination and, where necessary, sinkhole formation. Soils used for the construction of basins shall have low-erosion characteristics of soil factors ("K" factors). The Township may require the installation of an impermeable liner in detention basins.

C. It shall be the developer's responsibility to verify if the site is underlain by limestone. The following note shall be attached to all SWM site plans and signed and sealed by the developer's engineer/surveyor/landscape architect/architect: "____________________ certify that the proposed stormwater management facility (circle one) is/is not underlain by limestone."

D. In areas that are underlain by limestone bedrock, no stormwater management facility will be placed in, over, or immediately adjacent to the following features:

(1) Closer than 100 feet from sinkholes.

(2) Closer than 100 feet from closed depressions.

(3) Closer than 100 feet from caverns, intermittent lakes, or ephemeral streams.

(4) Closer than 50 feet from lineaments in carbonate areas.

(5) Closer than 50 feet from fracture traces.

(6) Closer than 25 feet from bedrock pinnacles (surface or subsurface).

E. If the developer can prove through analysis that the project site is an area underlain by carbonate geology (i.e., limestone), and such geologic conditions may result in sinkhole formations, then the project site is exempt from recharge requirements as described in § 170-15, Volume controls. However, the project site shall still be required to meet all other standards found in this chapter.

§ 170-19 Stormwater management site plan infiltration requirements.

Infiltration testing shall be required at all infiltration BMP locations in accordance with the following:

A. A detailed soils evaluation of the project site shall be performed to determine the suitability of recharge facilities. The evaluation shall be performed by a qualified professional and, at a minimum, address soil permeability, depth to bedrock, susceptibility to sinkhole formation, and subgrade stability. The general process for designing the infiltration BMPs shall be:

(1) Site evaluation to determine general areas of suitability for infiltration practices.

(2) Provide field test throughout the area proposed for development to determine appropriate infiltration rate and/or hydraulic conductivity. For subdivision and land development creating between one and five proposed lots, one infiltration test must be conducted for each lot. For any subdivision and land development creating five or more proposed lots, one infiltration test must be completed for every five lots. For cases where existing lots are proposed for new land development, an infiltration test must be completed in each area where infiltration facilities are proposed. Additional infiltration testing may be required where directed by the Township Engineer due to existing site conditions. Infiltration tests must be taken at the location and depth of all proposed infiltration structures.

(3) Design infiltration structures for the required storm volume based on all available data.

(4) A double-ring infiltrometer test shall be used for all infiltration tests in accordance with the current edition of the BMP Manual.

B. Plans for infiltration must show the locations of existing and proposed septic tank infiltration areas and wells. A minimum twenty-five-foot separation from on-lot disposal system (OLDS) infiltration areas, including replacement areas, is desired and will be evaluated by the Township on a case-by-case basis. However, the separation shall not be less than the PADEP required 10 feet.

C. Alternative BMPs.

(1) An alternative BMP shall be utilized where the applicant can demonstrate that infiltration is not possible due to the following reasons:

(a) Seasonal high-water table.

(b) Permeability rate below 0.5 inch per hour based on county soil inventory or infiltration testing.

(c) Soil depth less than three feet.

(d) Separation distances as follows:

[1] One hundred feet from water supply wells;

[2] Ten feet downgradient or 50 feet upgradient from building foundations;

[3] Fifty feet from septic system drain fields; and

[4] Fifty feet from a geologic contact with limestone bedrock.

(2) Examples of alternative BMPs include rooftop disconnection and establishing sheet flow in conjunction with the use of a bioretention area, forested buffer area or other acceptable BMP measure in accordance with the BMP Manual or as determined by the Township Engineer.

§ 170-20 Stormwater management facility design standards.

A. General requirements.

(1) All SWM site plans shall be designed and certified by a registered professional engineer qualified to perform such duties.

(2) All stormwater management facility designs shall consider entire watersheds and include provisions for sound and stable construction and proper use of readily available materials, with the primary consideration for public safety and preventing property damage.

(3) Innovative designs and methods for stormwater retention may be used with the Township's approval. Design methods may be tailored to meet unique site topography, site conditions, and building construction, including the use of existing ponds or basins, rooftop storage, parking lot storage, seepage pits, and reductions in existing impervious areas.

(4) Stormwater discharge from a proposed site onto adjacent property shall not exceed the existing (predeveloped) site discharge. Discharge from a proposed site shall discharge in the same manner as existing flows into existing channels and/or culverts capable of handling the proposed discharge. Where an existing channel is not available to discharge flows from a proposed site, provisions shall be made to further detain proposed discharge flows, or convert the proposed flows into a pattern which matches existing flow types and capabilities, or provide other means of downstream flow accommodation which will not cause undue hardship of erosion or concentrations of water on downstream property owners.

(5) The designer shall submit a report which includes a map (one inch equals 2,000 feet minimum) of the downstream drainage path from the proposed site to the nearest stream or the Dickinson Township boundary (whichever occurs first). The report shall identify existing flood-prone properties and drainage problem areas and describe any planned or proposed improvements to the off-site problem areas.

(6) Approval of a stormwater management site plan by the Township does not guarantee that all requirements are met for the state and federal agencies. Where applicable, stormwater management site plans shall comply with Chapter 105 (Water Obstructions and Encroachments), Chapter 102 (Erosion and Sediment Pollution Control), and stormwater NPDES permits of the Pennsylvania Department of Environmental Protection; the United States Army Corps of Engineers' joint permit for Pennsylvania waterways obstructions; Pennsylvania Department of Transportation highway occupancy; and other applicable permits.

(7) Where this chapter does not specify materials or construction procedures, PennDOT Form 408 Specifications and PennDOT RC Series Construction Standards shall be used.

(8) Stormwater drainage systems shall permit unimpeded flow along natural watercourses, except as modified by stormwater management facilities or open channels consistent with this chapter.

(9) The existing points of concentrated drainage that discharge onto adjacent property shall not be altered and shall be subject to any applicable discharge criteria specified in this chapter.

(10) Areas of existing, diffused drainage discharge shall be subject to any applicable discharge criteria in the general direction of existing discharge, whether proposed to be concentrated or maintained as diffused drainage areas, except as otherwise provided by this chapter. If diffused flow is proposed to be concentrated and discharged onto adjacent property, the developer 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.

(11) Where a development site is traversed by watercourses, drainage easements shall be provided which conform to the line of such watercourses. The terms of the easement(s) shall prohibit excavation, the placing of fill or structures, or any alteration that may adversely affect the flow of stormwater within any portion of the easement. Also, maintenance, including mowing of vegetation within the easement, shall be required unless exempted by the Board of Supervisors.

(12) When it can be shown that, due to topographic conditions, natural drainageways and depressions on the site cannot adequately provide for drainage, open channels may be constructed in substantial conformity to the line and grade of such natural drainageways. Work within natural drainageways shall be subject to approval by PADEP through the joint permit application process or, where deemed appropriate by PADEP, through the general permit process.

(13) Any stormwater management facilities located in or adjacent to waters of the commonwealth or wetlands shall be subject to approval by PADEP through the joint permit application process or, where deemed appropriate by PADEP, the general permit process. When there is a question of whether wetlands may be involved, it is the responsibility of the owner, or his agent, to show that the land in question cannot be classified as wetlands; otherwise approval to work in the area must be obtained from PADEP.

(14) Any stormwater management facilities proposed to be located on state highway rights-of-way shall be subject to approval by PennDOT.

(15) Where soil conditions permit, minimization of impervious surfaces and infiltration of runoff through seepage beds, infiltration trenches, etc., are encouraged to reduce the size or eliminate the need for detention facilities.

(16) Connection of roof drains to streets, sanitary or storm sewers, or roadside ditches is prohibited in order to promote overland flow and infiltration/percolation of stormwater. Where the Board of Supervisors determines that it is more advantageous to connect directly to streets or storm sewers, then connections may be permitted on a case-by-case basis.

(17) Special requirements are adopted for areas falling within defined exceptional value (EV) and high quality (HQ) watersheds. The temperature and quality of water and streams that have been declared as EV or HQ are to be maintained as defined in Chapter 93, Water Quality Standards, Title 25 of the PADEP Rules and Regulations. Temperature-sensitive BMPs and stormwater conveyance systems are used and designed with storage pool areas and supply outflow channels and should be shaded with trees. This will require modification of berms for permanent ponds and the relaxation of restrictions on planting vegetation within the facilities, provided that capacity for volumes and rate control is maintained. At a minimum, the southern half of pond shorelines shall be planted with shade or canopy trees within 10 feet of the shoreline. In conjunction with this requirement, the maximum slope allowed on the berm area to be planted is 10:1. This will lessen the destabilization of berm soils due to root growth. A long-term maintenance schedule and management plan for the thermal-control BMPs are to be established and recorded for all development sites.

(18) Any hydraulic analysis shall use the following criteria for determining adequacy for accepting increased peak flow rates:

(a) Natural or man-made channels or swales located on and off of the development site must be able to convey the increased runoff associated with a twenty-five-year return period event (permanent channels) or the five-year return period event (temporary channels) within their banks at velocities consistent with protection of the channels from erosion. Acceptable velocities shall be based upon criteria included in the PADEP Erosion and Sediment Pollution Control Program Manual. The twenty-five-year return period runoff shall not create any hazard to persons or property.

(b) For certain areas within the study area, it may be more cost-effective to provide one control facility for more than one development site than to provide an individual control facility for each development site. The initiative and funding for any regional runoff control alternatives are the responsibility of prospective developers. The design of any regional control basins must incorporate reasonable development of the entire upstream watershed.

(19) Preservation of natural drainage features and depressions. For all land development plans and stormwater management site plans, the elimination of natural drainageways or depressions shall only be accepted if the developer cannot reasonably comply with this requirement because of a property's configuration and/or topography.

(20) Avoiding adverse impacts on downstream properties. Downstream effects shall be minimized under the following controls:

(a) The natural stormwater drainage features and man-made drainage facilities that convey stormwater runoff from a development site's discharge point(s) of concentrated stormwater runoff to a perennial stream must be located in a recorded drainage easement, or a public utility or road right-of-way with a minimum width of 20 feet.

(b) The quantity, velocity and direction of the stormwater runoff discharge must be evaluated in order to determine that there is no increase in the risk to the health and safety of the public and no damage to property for the two-year through one-hundred-year storm events following development.

(c) The following criteria shall be used for the stormwater runoff discharge evaluation:

[1] The capacity in the downstream natural or man-made conveyance system between a development site's discharge point(s) of concentrated stormwater runoff to a perennial stream may be used only in proportion to the development site's area relative to the total upstream area draining to the conveyance system (i.e., if a development site represents 10% of the upstream area draining to a conveyance system, the development site may use 10% of the capacity available in the conveyance system).

[2] Open channels, in combination with the drainage easement in which the channel is located, must have the capacity to convey the peak rate of runoff for the twenty-five-year storm event within the drainage easement at velocities that will not lead to erosion of the channel or in the drainage easement.

[3] Storm sewers, culverts, bridges, or any other facilities in combination with the drainage easements in which the facilities are located must have the capacity to convey the peak rate of runoff for the twenty-five-year storm event within the drainage easement at velocities that will not lead to erosion in the drainage easement.

(21) Prevention of stream bank erosion. In order to minimize the occurrence of critical erosive velocities in natural stream channels, stormwater runoff from new development sites must be gradually released. To attain this objective, stormwater runoff from new development sites, for the one-year, twenty-four-hour storm event, must be released over a minimum period of 24 hours.

(22) Design criteria for stormwater management facilities. All facilities must meet the following requirements:

(a) Any stormwater management facility designed to store runoff and requiring a berm or earthen embankment required or regulated by this chapter shall be designed to provide an emergency spillway to handle flow up to and including the one-hundred-year post-development conditions. The height of embankment must be set as to provide a minimum six inches of freeboard. To compute the required minimum freeboard, the one-hundred-year storm event shall be routed through the spillway assuming the starting pool elevation is equal to the emergency spillway elevation. Should any stormwater management facility require a dam safety permit under PADEP Chapter 105, the facility shall be designed in accordance with Chapter 105 and meet the regulations of Chapter 105 concerning dam safety which may be required to pass storms larger than one-hundred-year event.

(b) Any facilities that constitute water obstructions (e.g., culverts, bridges, outfalls, or stream enclosures), and any work involving wetlands, as set forth in PADEP Chapter 105 regulations (as amended or replaced from time to time by PADEP), shall be designed in accordance with Chapter 105 and will require a permit from PADEP. Any other drainage conveyance facility that doesn't fall under Chapter 105 regulations must be able to convey, without damage to the drainage structure or roadway, runoff from the twenty-five-year design storm with a minimum one foot of freeboard measured below the lowest point along the top of the roadway. Roadway crossings located within designated floodplain areas must be able to convey runoff from a one-hundred-year design storm with a minimum one foot of freeboard measured below the lowest point along the top of the roadway. Any facility that constitutes a dam, as defined in PADEP Chapter 105 regulations, may require a permit under dam safety regulations. Any facility located within a PennDOT right-of-way must meet PennDOT minimum design standards and permit submission requirements.

(c) Any permanent drainage conveyance facility and/or channel that is not encompassed by Chapter 105 regulations must be able to convey runoff from the ten-year design storm without damage to the drainage structure or roadway. Conveyance facilities to or exiting from stormwater management facilities (i.e., detention basins) shall be designed to convey the design flow to or from that structure. Roadway crossings located within designated floodplain areas must be able to convey runoff from a one-hundred-year design storm. Any facility located within a PennDOT right-of-way must meet PennDOT minimum design standards and permit submission requirements.

B. Stormwater runoff calculations. Stormwater runoff calculations shall be analyzed in accordance with § 170-16 and the following:

(1) The existing runoff calculations shall analyze the on-site use assuming summer or good land cover conditions. Calculations of off-site and proposed land use shall assume winter or poor land cover conditions.

(2) On-site stormwater transporting facilities (swales, pipe systems) shall be designed to carry a proposed twenty-five-year storm event for stormwater generated on site and designed to carry a twenty-five-year storm event for stormwater generated off site. The site shall include design provisions to safely pass the proposed one-hundred-year storm without damage to private or public property or limiting access to streets and/or rights-of-way. Lots adjacent to the one-hundred-year storm elevation in critical areas, such as backwater conditions or swales with significant drainage areas, shall have a minimum finished floor elevation of two feet above the one-hundred-year water elevation.

(3) Calculation methodology. Stormwater runoff from all development sites shall be calculated using either the Rational Method (See Appendix C[1]) or a Soil Cover Complex Methodology.

[1] Editor's Note: Appendix C is included at the end of this chapter.

(4) Any stormwater runoff calculations involving drainage areas greater than 200 acres, including on- and off-site areas, shall use a generally accepted calculation technique that is based on the NRCS Soil Cover Complex Method. Table I summarizes acceptable computation methods. It is assumed that all methods will be selected by the design professional based on the individual limitations and suitability of each method for a particular site. The Township may approve the use of the Rational Method to estimate peak discharges from drainage areas that contain less than 200 acres.

	Table I Acceptable Computation Methodologies For Stormwater Management Plans
	Method	Method Developed By	Applicability
	TR-20 or commercial package based on TR-20	USDA-NRCS	When use of a full model is desirable or necessary
	TR-55 or commercial package based on TR-55	USDA-NRCS	Applicable for plans within the model's limitations
	HEC-1	United States Army Corps of Engineers	When full model is desirable or necessary
	Rational Method or commercial package based on Rational Method	Emil Kuiching (1889)	For sites less than 100 acres (as approved by the Township Engineer)
	Other methods	Various	As approved by the Township Engineer

(5) All calculations consistent with this chapter using the NRCS Soil Cover Complex Method shall use the appropriate design rainfall depths for the various return period storms presented. If a hydrologic computer model such as PSRM or HEC-RAS is used for stormwater runoff calculations, then the duration of rainfall shall be 24 hours. The SCS Rainfall Type II Curve shall be used for the rainfall distribution.

(6) The design storm precipitation depth estimates to be used in the analysis of peak rates of discharge should be obtained from the Precipitation-Frequency Atlas of the United States, Atlas 14, Volume 2, United States Department of Commerce, National Oceanic and Atmospheric Administration (NOAA), National Weather Service, Hydrometeorological Design Studies Center, Silver Spring, Maryland, 20910. NOAA's Atlas 14 can be accessed at internet address: http://hdsc.nws.noaa.gov/hdsc/pfds/.

(7) All calculations using the Rational Method shall use rainfall intensities consistent with appropriate times of concentration for overland flow and return periods from the design storm curves from PennDOT Design Rainfall Curves (1986). Region 4 curves will apply to this watershed.

(8) Times of concentration for overland flow shall be calculated using the methodology presented in Chapter 3 of Urban Hydrology for Small Watersheds, NRCS, TR-55 (as amended or replaced from time to time by NRCS). Times of concentration for channel and pipe flow shall be computed using Manning's Equation.

(9) Runoff curve numbers (CN) for both existing and proposed conditions to be used in the Soil Cover Complex Method shall be obtained from Chapter 3 of Urban Hydrology for Small Watersheds, NRCS, TR-55 (as amended or replaced from time to time by NRCS).

(10) Runoff coefficients (c) for both existing and proposed conditions for use in the Rational Method shall be obtained from Appendix C of this chapter.[2]

[2] Editor's Note: Appendix C is included at the end of this chapter.

(11) Peak discharge computed using the Rational Method should follow the formula:

	Q = CIA
	Where:
		Q	=	Peak runoff rate in cubic feet per second (cfs).
		C	=	Coefficient of runoff (see Appendix C).
		I	=	Intensity of rainfall in inches per hour, a function of the time of concentration (Tc).
		A	=	Area of the watershed in acres.

(12) Where uniform flow is anticipated, the Manning Equation shall be used for hydraulic computations such as the capacity of open channels, pipes, and storm sewers. Values for Manning's roughness coefficient (n) shall be consistent with Appendix D.[3]

[3] Editor's Note: Appendix D is included at the end of this chapter.

(13) Outlet structures for stormwater management facilities shall be designed to meet the performance standards of this chapter using any generally accepted hydraulic analysis technique or method.

(14) The design of any stormwater detention facilities intended to meet the performance standards of this chapter shall be verified by routing the design storm hydrograph through these facilities using the Storage-Indication Method. For drainage areas greater than 20 acres in size, the design storm hydrograph shall be computed using a calculation method that produces a full hydrograph. The Township may approve the use of any generally accepted full hydrograph approximation technique that shall use a total runoff volume that is consistent with the volume from a method that produces a full hydrograph.

(15) The Township has the authority to require that computed existing runoff rates be reconciled with field observations and conditions. If the designer can substantiate through actual physical calibration that more-appropriate runoff and time-of-concentration values should be utilized at a particular site, then appropriate variations may be made upon review and recommendations of the Municipal Engineer. Calibration shall require detailed gauge and rainfall data for the particular site in question.

(16) Time of concentration shall be calculated based on three flow patterns:

(a) Sheet flow shall be a maximum of 150 feet using the equation:

	Tc = (0.667 L n/S0.5)0.467
	Where:
		Tc	=	Time of concentration in minutes.
		L	=	Length of flow path in feet.
		n	=	0.02 for smooth, paved surfaces; 0.1 for bare, packed soil or stone areas; 0.3 for poor grass cover or cultivated fields; and 0.4 for average grass or thick brush cover.
		S	=	Average slope of the path in feet/foot.

(b) Shallow concentrated flow shall be calculated using the equations:

	Paved Areas: Tc = L/1220 S0.5
	Unpaved Areas: Tc = L/968 S0.5
	Where:
		Tc	=	Time of concentration in minutes.
		L	=	Length of flow path in feet.
		S	=	Average slope of the path in feet/foot.

(c) Concentrated flow in depressions, swales, gutters, curbs and pipes shall be calculated using Manning's Equation and/or other acceptable engineering practices.

(d) Times of concentration based on computer models which use the Rational Method or TR-55 as a basis of design may be used with prior approval of the Township.

C. Detention basin design standards.

(1) Basins shall be designed in accordance with soil conservation practices to accommodate sediment during construction.

(2) Detention basins shall be kept in a maintainable condition with a minimum bottom slope of 1%.

(3) Infiltration rates may be used in Township-approved areas to offset basin discharge rates as groundwater recharge. Infiltration tests shall be performed at the site for each soil type.

(4) The maximum water depth in basins shall be six feet, measured from the bottom of the emergency spillway to the lowest point in the basin. Depths of greater than six feet will be permitted only with permission from the Township and may require provisions for fencing around the basin and a construction design of the basin embankment.

(5) The minimum top width of basin berms shall be five feet.

(6) Basin embankment slopes shall be a maximum of 3:1 for slopes outside the impoundment area. A maximum embankment slope of 4:1 shall be used for all slopes within the impoundment area.

(7) Basin berms shall be designed with a relatively impervious soil cutoff trench from a depth of one foot below the toe of the interior basin berm to the fifty-year stormwater elevation. This requirement is waived for berms using undisturbed existing soils. The discharge pipe shall also be backfilled with relatively impervious soil (no stone backfill).

(8) Concrete or an approved polyethylene anti-seep collar shall be placed around all basin discharge pipes to increase the seepage length along the pipe by 15% within the saturated zone of the pipe based on a 4:1 phreatic line. The connection of the anti-seep collar to the discharge pipe shall be completely watertight.

(9) Basin principal spillway outlet structures shall consist of multistaged orifices in a reinforced concrete riser box and concrete or smooth, lined corrugated polyethylene discharge pipe, and welded structural steel inlet grates (with a bituminous coating). For temporary spillways with riser pipes where the designed water depth is deeper than 0.4 times the diameter of the riser, the riser shall have an anti-vortex device to prevent reduced capacity of the riser. (Example, if the top of an eighteen-inch riser is 100, the riser shall have an anti-vortex device if the water is higher than 100 + [(18 inches* 0.4)/12] = 100.60). Materials used for design shall be specified on the plans.

(10) The use of subsurface retention/detention will be permitted only in areas where the Township finds that the soils have adequate infiltration rates and that the underlying bedrock is not susceptible to solutioning. The design calculations shall include field testing results verifying the soil's infiltration rate and identification of the underlying bedrock.

(11) Lots adjacent to basins shall have a finished floor elevation of two feet above the one-hundred-year storm event. The plans shall explicitly indicate the minimum allowable finished floor elevation for these lots.

(12) Stormwater management facilities shall have a fifteen-foot-wide access easement around them, which provides ingress to and egress from a public right-of-way.

D. Swales.

(1) Swales shall be designed using Manning's Equation, with the following n values and maximum allowable velocities:

	Surface Cover	Manning's n Value (n =)	Permissible Velocity (feet per second)
	Concrete	0.014	12
	Asphalt	0.016	8
	Riprap 3 inches	0.025	6
	Riprap 6 inches	0.025	9
	Riprap 9 inches	0.030	11
	Riprap 12 inches	0.030	13
	Riprap 15 inches	0.035	14
	Seeded grass swales	0.030	4
	Sod grass swales	0.040	5
	Existing grass swales	0.050	6

(2) Swales shall be designed with six inches of freeboard above the twenty-five-year storm.

(3) Swales with bare soils are not permissible. All swales shall have ground cover, or have a paved or concrete surface up to the twenty-five-year storm depth.

(4) Side slopes for all stormwater conveyance swales shall be a maximum of 3:1.

(5) Bends in swales shall have adequate protection to avoid erosion and channel meandering.

(6) All swales and watercourses, whether existing or proposed, shall have a minimum twenty-foot-wide easement. The swale or watercourse shall be centered in the easement.

E. Storm sewer pipes and culverts.

(1) Storm sewer pipes and culverts shall have a minimum diameter of 15 inches and be smooth, lined corrugated polyethylene pipe (SLCPP) or reinforced concrete, except single-family on-lot systems shall have a minimum diameter of four inches.

(2) Storm sewer pipes and culverts shall have sufficient slope to provide a minimum velocity of two feet per second (fps) for the twenty-five-year storm.

(3) Storm sewers and culverts shall be designed using Manning's Equation, with the following allowable pipe types and n values. The pipe types used for design shall be specified on the plans.

	Pipe Type	Manning's n Value
	Smooth, lined polyethylene	0.012
	Corrugated polyethylene	0.021
	Reinforced concrete	0.013

(4) All pipes shall be laid with a minimum cover on top of the crown of the pipe of one foot below the proposed finished grade in unpaved areas or below the stone subgrade for paved areas. Pipes shall be bedded and backfilled with fine-graded, highly compactible soil or stone.

(5) Storm sewers shall run in a straight line. Bends in the system shall be done with a manhole or inlet.

(6) All pipe ends shall have headwalls and endwalls and shall be PennDOT Type D or DW, or galvanized-metal flared end sections. Polyethylene flared end sections are prohibited.

(7) All pipe ends shall have erosion protection which meets the velocities of the end pipe discharge for the design storm.

(8) Headwalls and endwalls of pipes, 24 inches in diameter or larger and with a total enclosed pipe length exceeding 50 feet, shall be protected from child entry by placing removable stainless steel bars (and mounting hardware), spaced eight inches apart, across the openings.

(9) Roof gutters shall be used to divert water towards a surface drainage area. Flows off of a pitched roof shall be assumed to fall onto the ground below the edge of the roofline. Roof drains shall not discharge into road rights-of-way or storm sewers. Pipes from downspouts maybe constructed of flexible or rigid plastic pipe of any diameter and shall be installed to prevent erosion.

(10) Storm sewers and culverts shall have a twenty-foot-wide easement minimum, unless they are within a street or other public right-of-way. The pipe shall be centered in the easement.

F. Roadside gutters and inlets.

(1) Inlets shall be placed on both sides of the road at low points, spaced no more than 600 feet apart, at every abrupt change in horizontal or vertical direction of the storm sewer, at points where necessary to prevent gutter flow depths from exceeding three inches for the twenty-five-year storm, and at all curb and road tangents.

(2) The depths of flow across a street intersection shall not exceed one inch for the twenty-five-year storm.

(3) Inlets shall be depressed two inches below proposed finished grades.

(4) Manholes may be substituted for inlets at locations where inlets are not necessary.

(5) Inlet capacities shall be per the most-restrictive prevailing manufacturer design information.

(6) Inlet boxes subjected to highway loading shall be concrete. The use of polyethylene manholes will be allowed when supported on a concrete base. All inlets and grates shall meet design loads. Steel inlet grates shall be bituminous-coated.

G. Floodplain delineation.

(1) Sites with streams, intermittent channels, water impounding areas, or low- or slow-draining areas shall have the one-hundred-year floodplain established.

(2) The Federal Emergency Management Agency, National Flood Insurance Program, detailed flood areas may serve as the principal means of establishing the one-hundred-year floodplain for an existing site.

(3) Where detailed flood area information is not available or a more-detailed analysis is desired for an existing site, the United States Army Corps of Engineers, Hydrology Engineering Center Method 2 (HEC RAS) Flood Study shall be performed. Other standard step methods for natural channels to determine water surface profiles for flows through a natural river or stream may be used with Township approval.

(4) Proposed sites, where improvements or changes are made in the existing one-hundred-year floodplain, shall be analyzed for the one-hundred-year floodplain using the HEC RAS or other standard step methods with Township approval.

(5) All flood studies shall be signed and sealed by a Pennsylvania registered professional engineer.

H. Erosion and sedimentation requirements.

(1) Whenever the vegetation and topography are to be disturbed, such activity must be in conformance with Chapter 102, Title 25, Rules and Regulations, Part I, PADEP, Subpart C, Protection of Natural Resources, Article II, Water Resources, Chapter 102, Erosion Control, and in accordance with the Cumberland County Conservation District and the standards and specifications of the Township.

(2) Additional erosion and sedimentation control design standards and criteria that must be applied where infiltration BMPs are proposed include the following:

(a) Areas proposed for infiltration BMPs shall be protected from sedimentation and compaction during the construction phase, so as to maintain their maximum infiltration capacity.

(b) Infiltration BMPs shall not be constructed nor receive runoff until the entire contributory drainage area to the infiltration BMP has received final stabilization.

(3) Storm sewers must be able to convey post-development runoff from a twenty-five-year design storm without surcharging inlets, where appropriate.

(4) Adequate erosion protection shall be provided along all open channels and at all points of discharge.

(a) The design of all stormwater management facilities shall incorporate sound engineering principles and practices. The Township shall reserve the right to disapprove any design that would result in the occupancy or continuation of an adverse hydrologic or hydraulic condition within the watershed.

(b) A copy of the soil erosion and sedimentation control plans along with post-construction stormwater management plans shall be submitted to the Township for approval. An adequacy letter from the County Conservation District must be secured prior to approval of the plan.