Township of Worcester, PA Article III Stormwater Management

§ 129-12 General requirements.

A. All applicants proposing regulated activities in the Township that do not fall under the exemption criteria shown in § 129-5B and C of this chapter shall submit a stormwater management site plan, consistent with this chapter, to the Township for review. All applicants proposing regulated activities that fall under the exemption criteria identified in § 129-5C shall submit a simplified stormwater management site plan, consistent with this chapter, to the Township for review. These criteria shall apply to the total proposed development even if development is to take place in stages. Impervious surface shall include, but not be limited to, any roof, parking or driveway areas and any new streets and sidewalks. Any areas designed to be gravel or crushed stone shall be assumed to be impervious unless designed as a BMP (e.g., pervious paver blocks, reinforced turf, gravel filled grids, etc.). (Refer to definition of "impervious surface" within § 129-11 of this chapter.)

B. All regulated activities shall include such measures as necessary to:

(1) Protect health, safety, and property;

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

(a) Minimize disturbance to floodplains, wetlands, and wooded areas.

(b) Create, maintain, repair 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 (i.e., create disconnected impervious areas, DIAs) by directing runoff to pervious areas, wherever possible;

(3) To the maximum extent practicable, incorporate the techniques for low-impact development practices (e.g., protecting existing trees, reducing area of impervious surface, cluster development, and protecting open space) described in the Pennsylvania Stormwater Best Management Practices Manual, Pennsylvania Department of Environmental Protection (PADEP) No. 363-0300-002 (December 30, 2006).

C. The Township may, after consultation with the Department of Environmental Protection (PADEP), approve measures for meeting the state water quality requirements other than those in this chapter, provided that they meet the minimum requirements of, and do not conflict with, state law including, but not limited to, the Clean Streams Law.[1]

[1] Editor's Note: See 35 P.S. § 691.1 et seq.

D. For all regulated earth disturbance activities, erosion and sediment (E&S) control best management practices (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 and to meet all requirements under Title 25 of the Pennsylvania Code and the Clean Streams Law.[2] Various BMPs and their design standards are listed in the Erosion and Sediment Pollution Control Program Manual (March 2012), as amended and updated.

[2] Editor's Note: See 35 P.S. § 691.1 et seq.

E. No approval of any subdivision or land development plan, or issuance of any building, zoning, grading and excavations permit, occupancy permit, or the commencement of any regulated earth disturbance at a project site within the Township shall proceed until the requirements of this chapter are met, including approval of a stormwater management permit pursuant to Article IV and a permit under PADEP regulations, where applicable.

F. Erosion and sediment control during land disturbance shall be addressed as required by § 129-20.

G. Infiltration and water quality protection shall be addressed as required by §§ 129-15 and 129-16.

H. All best management practices (BMPs) shall conform to the design criteria of this chapter and Pennsylvania Stormwater Best Management Practices Manual, December 30, 2006.

I. Low-impact development techniques as described in Pennsylvania Stormwater Best Management Practices Manual (December 30, 2006, as amended) are encouraged to reduce the costs of complying with the requirements of this chapter and state water quality requirements. Use of nonstructural BMPs is encouraged and design and applicability of such BMPs is identified pursuant to Chapter 5 of the Pennsylvania BMP Manual. For all proposed nonstructural BMPs, the applicant shall utilize and submit applicable checklists included in Chapter 8, Section 8.8, of the Pennsylvania BMP Manual, to demonstrate that the BMPs are applicable to the project and to determine the amount of volume credit that may be applied to the development activity.

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

K. Stormwater drainage systems shall be provided in order to permit unimpeded flow along natural watercourses, except as modified by stormwater management facilities designed to encourage infiltration, groundwater recharge, and improved water quality.

L. Existing points of concentrated drainage that discharge onto adjacent property shall not be altered without written approval of the affected property owner(s) and shall be subject to discharge criteria specified in this chapter.

M. Areas of existing sheet flow discharge shall be maintained wherever possible. If sheet flow is proposed to be concentrated and discharged onto 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, as verified by the Township or Township Engineer, and submit written approval from the affected adjacent property owner(s) if required by the Township or Township Engineer.

N. Where a development site is traversed by watercourses, drainage easements shall be provided conforming to the line of such watercourses. The width of the easement shall be adequate to provide for the unimpeded flow of stormwater runoff from the 100-year storm event. Terms of the easement shall prohibit excavation, the placing of fill or structures, and any alterations that may adversely affect the flow of stormwater within any portion of the easement. Periodic maintenance of the easement shall be required by the landowner to ensure proper runoff conveyance, as defined by the Commonwealth of Pennsylvania.

O. When it can be shown that, due to topographic conditions, natural drainageways on the site cannot adequately provide for drainage, open channels may be constructed conforming substantially to the line and grade of such natural drainageways. Work within natural drainageways shall be subject to approval by PADEP through the Chapter 105[3] permit process, or, where deemed appropriate by PADEP, through the general permit process.

[3] Editor's Note: See 25 Pa. Code Chapter 105.

P. Any stormwater management facilities regulated by this chapter that will be located in or adjacent to waters of the commonwealth or wetlands shall be subject to approval by PADEP through the Chapter 105[4] permit process, or, where deemed appropriate by PADEP, the general permit process. When there is a question whether wetlands may be involved, it is the responsibility of the applicant to show that the land in question cannot be classified as wetlands, otherwise approval to work in the area must be obtained from PADEP.

[4] Editor's Note: See 25 Pa. Code Chapter 105.

Q. Any stormwater management facilities regulated by this chapter that would be located on state highway rights-of-way, or discharge stormwater to facilities located within a state highway right-of-way, shall be subject to approval by the Pennsylvania Department of Transportation (PADOT).

R. Site disturbance and impervious surface shall be minimized. Infiltrating stormwater runoff through seepage beds, infiltration basins, etc., shall be required, where soil conditions permit, to reduce the size or eliminate the need for retention/detention facilities.

S. Roof drains and sump pumps shall discharge to an infiltration structure, natural watercourse, storm sewer system, drainage swale, or stormwater easement. Roof drains and sump pumps shall not be connected to storm sewer unless the storm sewer is designed as part of a stormwater BMP facility. In no case shall roof drains or sump pumps be connected to sanitary sewer or be permitted to discharge directly across a sidewalk or walkway or onto a street. If curbing is present, no drainage pipes shall pass through the curb to discharge onto the street. Sump pump and roof drain discharge pipes shall not extend beyond the building envelope for the lot unless they are directly connected to an infiltration facility, detention basin, storm sewer pipe or as approved by the Township.

T. All storm sewer inlets must be identified with a storm drain marker. Storm drain markers shall be stainless steel affixed to the inlet hood with adhesive, rivets, or bolts. (Marker may be bolted to the grate in off-road locations). Marker shall have a minimum diameter of 3 1/2 inches and include "No Dumping - Drains to Waterway" and a fish symbol. Alternate designs/sizes may be used if approved by the Township.

U. Whenever a watercourse is located within a development site, it shall remain open in its natural state and location and shall not be piped, impeded, or altered (except for permitted crossings). It is the responsibility of the applicant to stabilize existing eroded stream/channel bed and banks. (Refer to § 129-17.)

V. The temperature and quality of water of streams that have been declared as exceptional value (EV) and high quality (HQ) are to be maintained as defined in Chapter 93, Water Quality Standards, Title 25, Pennsylvania Department of Environmental Protection Rules and Regulations. All regulated development activities within HQ or EV watersheds must provide volume controls and water quality controls pursuant to the requirements of §§ 129-15 and 129-16 of this chapter.

W. All stormwater runoff shall be pretreated for water quality prior to discharge to surface or groundwater as required by § 129-16 of this chapter.

X. Hot spots.

(1) Hot spots are sites where the land use or activity produces a higher concentration of trace metals, hydrocarbons, or priority pollutants than normally found in urban runoff. Use of infiltration BMPs is prohibited on hot spot land use areas. Examples of hot spots include but are not limited to the following:

(a) Vehicle salvage yards and recycling facilities.

(b) Vehicle fueling stations.

(c) Vehicle service and maintenance facilities.

(d) Vehicle and equipment cleaning facilities.

(e) Fleet storage areas (bus, truck, etc.).

(f) Industrial sites (based on Standard Industrial Codes defined by the United States Department of Labor).

(g) Marinas (service and maintenance).

(h) Outdoor liquid container storage.

(i) Outdoor loading/unloading facilities.

(j) Public works storage areas.

(k) Facilities that generate or store hazardous materials.

(l) Commercial container nursery.

(m) Other land uses and activities as designated by the Township.

(2) Stormwater runoff from hot spot land uses shall be pretreated. In no case may the same BMP be employed consecutively to meet this requirement. Guidance regarding acceptable methods of pretreatment is located in the Pennsylvania Stormwater Best Management Practices Manual.

Y. West Nile guidance requirements. All wet basin designs shall incorporate biologic controls consistent with the West Nile Guidance found in Appendix C[5] of this chapter.

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

§ 129-13 Stormwater management performance standards.

A. In the design of stormwater management facilities, postdevelopment rates of runoff from any regulated activity shall not exceed 75% of the peak rates of runoff prior to development for two- and ten-year-frequency storms and 100% of the peak rates of runoff prior to development for the twenty-five-, fifty, and 100-year frequency storms. In all other cases where unretained stormwater directly discharges from the site by bypassing the stormwater management facilities, the postdevelopment runoff rate shall not exceed predevelopment runoff rate. The preceding requirements shall apply to each location of concentrated or diffused drainage discharge from the development site.

B. Site areas. Where the area of a site being impacted by a proposed development activity, not associated with a subdivision or land development, differs significantly from the total site area as determined by the Township Engineer, the Township may, but is not required to, permit only the proposed impact area, which includes areas of the site that would be compacted due to construction activity, to be subject to the release rate criteria (performance standards).

C. Off-site areas. Off-site areas that drain through a proposed development site are not subject to release rate criteria when determining allowable peak runoff rates or volume reduction. However, on-site drainage facilities shall be designed to safely convey off-site flows through the development site.

D. Stormwater conveyance corridor protection (riparian corridor preservation and vegetation). Runoff from developed areas of the site, including but not limited to areas of impervious surface, shall be managed through a series of riparian corridor vegetation facilities whenever possible. This will be accomplished in a manner satisfactory to the Township, utilizing the "Pennsylvania Handbook of Best Management Practices for Developing Areas," 1998, Riparian Forested Buffer, and the priority goal of the riparian vegetation will be the reduction of thermal impacts on stormwater runoff associated with impervious areas, with a secondary goal being the protection of capacity of existing stormwater conveyance channels. These goals will be achieved through the use of design criteria in § 129-18I of this chapter, and shall be in addition to any other Township ordinance provisions.

E. For all subdivision and land development applications, the tributary area discharging drainage to any location along the site property boundary shall not increase by more than 25% over the predevelopment condition without written notification to the adjacent affected property owner(s) receiving runoff from the site, and review and approval by the Township Engineer.

§ 129-14 Project design (sequencing to minimize stormwater impacts).

A. The design of all regulated activities shall include the following steps in sequence to minimize stormwater impacts.

(1) The applicant is required to find practicable alternatives to the surface discharge of stormwater, the creation of impervious surfaces, and the degradation of waters of the commonwealth, and must maintain as much as possible the natural hydrologic regime of the site.

(2) An alternative is practicable if it is available and capable of being completed after considering cost, existing technology, and logistics in light of overall project purposes, and other Township requirements.

(3) All practicable alternatives to the discharge of stormwater are presumed to have less adverse impact on quantity and quality of waters of the commonwealth unless otherwise demonstrated.

B. The applicant shall demonstrate that regulated activities are designed in the following sequence to minimize the increases in stormwater runoff and impacts to water quality:

(1) Prepare an existing resources and site analysis map (ERSAM), showing environmentally sensitive areas including, but not limited to, steep slopes, ponds, lakes, streams, wetlands, hydric soils, vernal pools, floodplains, riparian corridors, hydrologic soil groups A, B, C, and D, woodlands, surface waters regulated by the state or federal government, any existing recharge areas, and any other requirements outlined in the Subdivision and Land Development and Zoning Ordinances.[1]

[1] Editor's Note: See Ch. 130, Subdivision and Land Development, and Ch. 150, Zoning.

(2) Prepare a draft project layout avoiding sensitive areas identified in § 129-14B(1) and minimizing total site earth disturbance as much as possible. The ratio of disturbed area to the entire site area and measures taken to minimize earth disturbance shall be included in the ERSAM.

(3) Identify site-specific existing conditions, drainage areas, discharge points (points of interest), recharge areas, and hydrologic soil groups A and B.

(4) Evaluate nonstructural stormwater management alternatives (refer to Pennsylvania BMP Manual).

(a) Minimize earth disturbance.

(b) Minimize impervious surfaces.

(c) Break up large impervious surface areas.

(d) Protect existing trees [not within protected areas as described in § 129-14B(1)].

(e) Direct rooftop runoff to pervious areas.

(f) Revegetate and reforest disturbed areas.

(g) Utilize natural flow pathways.

(5) Satisfy volume control standards (§ 129-15).

(6) Satisfy water quality objective (§ 129-16).

(7) Satisfy stream bank erosion protection objective (§ 129-17).

(8) Prepare final project design to maintain predevelopment drainage areas and discharge points, to minimize earth disturbance and impervious surfaces, to reduce runoff to the maximum extent possible, and to minimize the use of surface or point discharges.

(9) Conduct a proposed conditions runoff analysis based on the final design, to meet the release rate criteria (performance standards).

(10) Manage any remaining runoff through treatment prior to discharge, as part of detention, biorentention, direct discharge or other structural control.

§ 129-15 Volume control and infiltration BMPs.

A. For all regulated activities not exempt from requirements of this chapter pursuant to § 129-5B and C, water volume mitigation controls shall be implemented. The total volume of runoff that must be infiltrated may be calculated based on the Design Storm Method, in which case the postdevelopment total runoff volume shall not be increased from predevelopment total runoff volume for all design storms equal to or less than the two-year, twenty-four-hour duration precipitation. The Design Storm Method requires detailed stormwater runoff modeling based on site conditions. The required recharge volume may also be determined based on Equation 129-15.1, described in § 129-15D. The recharge volume (Re_v) must be reused, evapotranspired, or infiltrated through structural and/or nonstructural means. An alternative standard is allowed in this chapter where it can be demonstrated that due to existing natural site conditions [refer to § 129-15A(1)(b)], substantial infiltration and recharge are not occurring, predevelopment, resulting in greater than anticipated runoff volume.

(1) Alternate standard for runoff volume.

(a) Applicants may request from Worcester Township that an alternate standard be applied, where a portion of the runoff volume requirement of § 129-15A is not achieved but at least 50% of the total required volume of infiltrated runoff is achieved. Use of this alternate standard is permitted by the Township only after thorough scrutiny has been directed toward all possible stormwater management options at all possible locations at the site, consistent with the process set forth in § 129-15A(1).

(b) Required analysis for allowing use of alternate standard for runoff volume. The alternate standard shall be used only in those situations where it is demonstrated to the satisfaction of the Township that due to natural site conditions infiltration is not occurring in the predevelopment condition, resulting in greater runoff volumes (than would normally be anticipated) due to bedrock near or at the surface (less than two feet in depth); presence of seasonal high water table (SHWT) (less than two feet in depth); and soils with low permeability (e.g., 0.20 inch per hour or less). Alternate standard shall be permitted by the Township only in those cases where the applicant has demonstrated that one or all of the above-described conditions exist throughout the site, such that there is no reasonable means of infiltrating required stormwater volumes and that the property cannot be reasonably developed utilizing a stormwater management system which infiltrates the two-year-frequency storm event volume (difference between the pre- and postdevelopment storm). The applicant must demonstrate that there is no area of the site where the runoff volume requirement can feasibly be infiltrated. It is not grounds for approval of the alternate standard that infiltrating the runoff volume requirement will utilize areas that could otherwise be developed to obtain the most building area or lots.

(c) Applicants requesting to utilize the alternate standard must provide a feasibility study for infiltration utilizing BMPs as well as other runoff volume stormwater management systems and provide the following information:

[1] Site plan demonstrating the extent of site area with seasonal high-water table (SHWT) (less than two feet): The site will be evaluated both as to the extent of site with SHWT and the actual locations of SHWT areas. Use of the alternate standard shall be permitted by the Township only in those cases where it is demonstrated that site areas free of SHWT are not feasible for use as stormwater BMPs (i.e., they are located upgradient from reasonable site building areas).

[2] Site plan demonstrating extent of site area with less than two feet to bedrock: The site will be evaluated both as to the extent of site with shallow depth to bedrock and actual locations of shallow bedrock areas. Use of the alternate standard shall be permitted by the Township only in those cases where it is demonstrated that site areas free of shallow bedrock constraints are not feasible for use as stormwater BMPs (i.e., they are located upgradient from reasonable site building areas).

[3] The site plan shall demonstrate the extent of site area with less than 0.20 inch/hour of permeability in accordance with the soil testing protocol set forth in § 129-15E and Appendix B.[1]

[1] Editor's Note: Appendix B is included as an attachment to this chapter.

[4] Sum total of limited infiltration areas.

[a] In order to utilize the alternate standard, the applicant must demonstrate that the sum total of limited infiltration areas {the total of areas described in § 129-15A(1)(c)[1] through [3]} exceed the following percentages of the total site:

[i] Sites less than five acres: 75%.

[ii] Sites five to 10 acres: 80%.

[iii] Sites greater than 10 acres: 85%.

[b] In addition, the applicant must demonstrate that there is no feasible site area free of the above-described infiltration constraining features which exist in a location such that the runoff volume requirement can be achieved.

B. If it is determined to the satisfaction of the Township that the recharge volume standard set forth in § 129-15A cannot be achieved, then the peak rate standards for postdevelopment runoff are modified so that peak rate discharges from the site for all storms up to the ten-year-frequency design storm must be additionally reduced to be equal to or less than 75% of the design peak rates permitted pursuant to § 129-13.

C. Water volume controls will mitigate increased runoff impacts, protect stream channel morphology, maintain groundwater recharge, and contribute to water quality improvements. The applicant must demonstrate how the required recharge volume is controlled through stormwater best management practices (BMPs) which shall provide the means necessary to capture, reuse, evaporate, transpire or infiltrate the total runoff volume. The low-impact development practices provided in the Pennsylvania BMP Manual shall be utilized for all regulated activities to the maximum extent practicable. Volume controls provided through nonstructural BMPs may be subtracted from the required recharge volume to determine the volume of structural BMPs necessary for compliance with § 129-15A of this chapter. Design and applicability of nonstructural BMPs is identified pursuant to Chapter 5 of the Pennsylvania BMP Manual. For all proposed nonstructural BMPs, the applicant shall utilize and submit applicable checklists included in Chapter 8, Section 8.8, of the Pennsylvania BMP Manual, to demonstrate that the BMPs are applicable to the project and to determine the amount of volume credit that may be applied to the development activity.

D. Recharge volume formula.

(1) To determine the volume of runoff that must be infiltrated at a site, the recharge volume (Re_v), the following calculation formula may be used:

Equation 129-15.1

Re_v = [(S)(R_v)(A)]/12 (inches/foot),

Where:
	Rev	=	Recharge volume (acre-feet)
	S	=	Soil specific recharge factor (inches)
	A	=	Site area contributing to the recharge facility (acres)
	Rv	=	Volumetric runoff coefficient, Rv = 0.05 + 0.009(I), where: I = percent impervious area, and
	"S" shall be obtained based upon hydrologic soil group based upon the table below:

Hydrologic Soil Group	Soil Specific Recharge Factor(S)
A	0.38
B	0.26
C	0.14
D	0.07

(2) If more than one hydrologic soil group (HSG) is present at a site, a composite recharge volume shall be computed based upon the proportion of total site area within each HSG.

E. Infiltration BMPs intended to receive runoff from developed areas shall be selected based on the suitability of soils and site conditions. All applicants proposing regulated activities that are not exempt from preparation and submission of a stormwater management site plan (SMSP) are required to perform a detailed soils evaluation of the project site by a qualified geotechnical engineer, geologist and/or soil scientist, pursuant to Appendix B[2] of this chapter, which at minimum addresses soil permeability, depth to bedrock, susceptibility to sinkhole formation, and subgrade stability. Infiltration/permeability tests shall be completed (in conjunction with the soils evaluation) with an infiltrometer or other method approved by the Township Engineer, pursuant to Appendix B, to determine the saturated hydraulic conductivity of the soil [at the location and the level of the proposed infiltration surface(s)]. "Percolation" tests are not permitted for design of infiltration BMPs, unless approved by the Township Engineer.

[2] Editor's Note: Appendix B is included as an attachment to this chapter.

F. Infiltration BMPs must include safeguards against groundwater contamination for uses where it is anticipated that pollutants may enter the facility, by mishap or spill or where salt or chloride might be a nonpoint source contaminant since soils do little to filter this pollutant. If it is anticipated that pollutants may enter the infiltration facility (or other stormwater facility impounding water), resulting in potential groundwater contamination, Worcester Township may require the developer to submit a hydrogeologic justification study of the site and proposed infiltration BMPs, prepared by a qualified design professional, to determine the risk for such contamination. The Township may require the installation of a mitigative layer or an impermeable liner in the BMP and/or detention basins where the possibility of groundwater contamination exists.

G. Infiltration BMPs within high quality/exceptional value waters shall be subject to PADEP's Title 25, Chapter 93 antidegradation regulations.

H. The requirements for volume control and infiltration are applied to all disturbed areas, even if they are ultimately to be a pervious or permeable land use given the extent to which development-related disturbance leads to compaction of the soils and reduces their infiltrative capacity.

I. If on-lot infiltration structures are proposed, it must be demonstrated that the soils are conducive to infiltrate on the lots identified, or that the applicant's design includes the addition of suitable amounts of material to facilitate infiltration and support the calculations as submitted.

J. Infiltration BMPs shall be designed in accordance with the design criteria and specifications of the Pennsylvania Stormwater BMP Manual (2006) and as additionally identified pursuant to § 129-18I(1) of this chapter.

§ 129-16 Water quality requirements.

A. In addition to the performance standards and design criteria requirements of Article III of this chapter, adequate treatment and storage facilities must be provided to capture and treat stormwater runoff from developed or disturbed areas, unless otherwise exempted by provisions of this chapter. The recharge volume computed under § 129-15 may be a component of the water quality volume if the applicant chooses to manage both components in a single facility. Only if the recharge volume is less than the water quality volume may the remaining water quality volume be captured and treated by methods other than recharge/infiltration BMPs. The required water quality volume (WQ_v) is the storage capacity needed to capture and to treat a portion of stormwater runoff from the developed areas of the site produced from 90% of the average annual rainfall (P). The following calculation formula is to be used to determine the required water quality storage volume, (WQ_v), in acre-feet of storage:

Equation 129-16.1

WQ_v = [(P)(R_v)(A)]/12 (inches/foot),

Where:
	P	=	Rainfall amount equal to 90% of events producing this rainfall (in) - the volume of rainfall for 90% of the storm events which produce runoff in the watershed annually.
	A	=	Area of the project contributing to the water quality BMP (acres).
	Rv	=	Volume runoff coefficient 0.05 + 0.009(I) where I is the percent of the area that is impervious surface (impervious area ÷ total project study area) x 100%.

B. Provisions shall be made (such as adding a small orifice at the bottom of the BMP facility outflow control structure) so that the proposed condition, one-year-frequency design 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). The design of the facility shall minimize clogging and sedimentation. Orifices smaller than three inches in diameter are not recommended. However, if the design engineer can verify that the smaller orifice is protected from clogging by use of trash racks, etc., smaller orifices may be permitted. Trash racks are required for any primary orifice.

C. To accomplish the requirements in Subsections A and B above, the applicant may submit original and innovative designs to the Township Engineer for review and approval. Such designs may achieve the water quality objectives through a combination of BMPs. Infiltration BMPs shall be used wherever feasible. Wet ponds, artificial wetlands, or other permanent BMP acceptable to the Township shall be used to the extent that infiltration BMPs are deemed not feasible.

D. Design of BMPs used for water quality control shall be in accordance with design specifications outlined in the Pennsylvania Stormwater BMP Manual or other applicable manuals. The following factors must be considered when evaluating the suitability of BMPs used to control water quality at a given development site:

(1) Total contributing drainage area.

(2) Permeability and infiltration rate of the site soils.

(3) Topographic slope and depth to bedrock.

(4) Seasonal high-water table.

(5) Proximity to building foundations and wellheads.

(6) Erodibility of soils.

(7) Land availability and configuration of the topography.

(8) Peak discharge and required volume control.

(9) Stream bank erosion.

(10) Efficiency of the BMPs to mitigate potential water quality problems.

(11) Volume of runoff that will be effectively treated.

(12) Nature of the pollutant being removed.

(13) Maintenance requirements.

(14) Creation/protection of aquatic and wildlife habitat.

(15) Recreational value.

(16) Enhancement of aesthetic and property value.

§ 129-17 Stream bank erosion requirements.

A. In addition to the water quality volume, to mitigate the impact of stormwater runoff on downstream stream bank erosion, BMPs must be designed to detain the proposed conditions two-year, twenty-four-hour design storm to the existing conditions one-year flow using the SCS Type II distribution.

B. Whenever a watercourse is located within a development site, it shall remain open in its natural state and location and shall not be piped, impeded, or altered (except for permitted crossings). The applicant shall stabilize all eroded stream/channel beds and banks within a subdivision or land development site and obtain all permits necessary from PADEP to do so. The applicant must submit pictorial documentation of existing stream/channel banks to determine whether existing banks must be stabilized.

§ 129-18 Design and construction criteria for stormwater management facilities and best management practices.

A. Stormwater runoff which may result from regulated activities identified in § 129-4 shall be controlled by permanent stormwater runoff BMPs that will provide the required standards within Article III. The methods of stormwater control or best management practices (BMPs) which may be used to meet the required standards are described in this chapter and the Pennsylvania Stormwater Best Management Practices Manual, December 30, 2006, as amended, and are the preferred methods of controlling stormwater runoff. The choice of BMPs is not limited to the ones appearing in this chapter and the Manual; however, any selected BMP must meet or exceed the runoff peak rate requirements of this chapter.

B. Any stormwater facility located on state highway rights-of-way shall be subject to approval by the Pennsylvania Department of Transportation.

C. Collection system standards.

(1) Curb inlets. Curb inlets shall be located at curb tangents on the uphill side of street intersections, and at intervals along the curbline to control the maximum amount of encroachment of runoff on the roadway pavement so that same does not exceed a width of four feet during the design storm event. Design and location of curb inlets shall be approved by the Township.

(2) Pipe materials. All storm sewer piping shall be Class III reinforced concrete pipe, except when pipe class and strength is required to be increased in accordance with PennDOT specification. Piping shall be saw-cut at ends, as needed, and not hammered or broken. All pipe joints and lift holes must be mortared except where designed for infiltration.

(3) Minimum pipe size. Minimum pipe diameter shall be 15 inches (or an equivalent flow area of 1.23 square feet).

(4) Inlet and manhole construction. Inlet and manhole castings and concrete construction shall be equivalent to PennDOT design standards. Manhole castings and covers shall have the word "STORM" cast in two-inch-high letters on the top of the cover. All inlet grates shall be "bicycle safe" heavy-duty structural steel. All storm sewer inlets must be identified with a storm drain marker ("environmental" type). Storm drain markers shall be stainless steel affixed to the inlet hood with adhesive, rivets or bolts. (Marker may be bolted to the grate in off-road locations). Marker shall have a minimum diameter of 3 1/2 inches and include "No Dumping - Drains to Waterway" and a fish symbol. Alternate designs/sizes may be used if approved by the Township.

(5) Open end pipes must be fitted with concrete endwalls or wing walls in accordance with PennDOT standards.

(6) Flow velocity. Stormwater collection systems shall be designed to produce a minimum velocity of three feet per second when flowing full. The maximum permissible velocity shall be 15 feet per second. Pipe slopes shall not be less than 1/2 of 1% (0.005 ft/ft), with the exception that terminal sections of pipe shall have a minimum slope of 1% (0.01 ft/ft).

(7) Inlets and manholes shall be spaced at intervals not exceeding 300 feet, and shall be located wherever branches are connected or sizes are changed, and wherever there is a change in alignment or grade. For drainage lines of at least 36 inches diameter, inlets and manholes may be spaced at intervals of 400 feet. Manholes shall be equipped with open grate lids.

(8) Storm sewer bedding/backfill requirements shall conform to the Worcester Township construction requirements/specifications.

(9) Inlets shall be located to intercept concentrated runoff prior to discharge over public/private rights-of-way, sidewalks, streets, and driveways.

(10) The capacity of all Type C inlets shall be based on a maximum surface flow to the inlets of four cfs, calculated based on the 100-year frequency design storm event. The maximum flow to Type C inlets located in low points (such as sag vertical curves) shall include the overland flow directed to the inlet as well as all bypass runoff from upstream inlets. The bypass flow from upstream inlets shall be calculated using inlet efficiency curves included in PennDOT Design Manual Part 2, latest edition. If the surface flow to an inlet exceeds four cfs, additional inlets shall be provided upstream of the inlet to intercept the excessive surface flow. A Type C inlet at a low point of a paved area may be designed to accept a maximum of six cubic feet per second (cfs). Type M inlets shall be designed to accept a maximum surface flow of six cfs based on the 100-year frequency design storm event, unless otherwise approved by the Township. Double inlets will not be permitted where additional pipe and inlets can be placed upstream to intercept excessive surface flow. A maximum of 12 cfs shall be permitted to be collected by a Type M inlet located in an isolated pervious area provided the designer can verify that such an inlet would not cause stormwater to accumulate on any adjoining public or private property, outside of a storm sewer easement, and that the depth of the accumulated stormwater would not exceed 12 inches.

(11) A minimum drop of two inches shall be provided between the inlet and outlet pipe invert elevations within all inlets and manholes. When varying pipe sizes enter an inlet or manhole, the elevation of crown of all pipes shall be matched. Storm sewer pipes shall enter and exit the sides of inlet boxes and shall not encroach into the corner, wherever possible.

(12) Stormwater pipes shall have a minimum depth of cover of 18 inches (including over the bell) or as designated by the American Concrete Pipe Association (whichever is greater), and in no case shall any part of the pipe project into the road subbase or curb. Where cover is restricted, equivalent pipe arches may be specified in lieu of circular pipe, to achieve required cover. Stormwater pipes conveying swale flow under driveway crossings shall have a minimum cover of 12 inches, including over the bell, but in no case shall the cover be less than that required for the anticipated traffic loading. For driveway culverts, cover may be less than 12 inches if the design engineer verifies proposed pipe has sufficient strength to withstand loading from anticipated design vehicles. Where cover is restricted, concrete trench drain with bolt-down metal grates may be used.

(13) The capacity of all stormwater pipes shall be calculated utilizing the Manning Equation for open channel flow as applied to closed conduit flow. The Manning's roughness coefficient shall be 0.13 for all concrete pipe. In cases where pressure flow may occur, the hydraulic grade line shall be calculated throughout the storm sewer system to verify that at least one foot of freeboard will be provided in all inlets and manholes for the design storm event.

(14) Culverts shall be designed based on procedures contained in Hydraulic Design of Highway Culverts, HDS No. 5, United States Department of Transportation, Federal Highway Administration. Where pressure flow is anticipated in storm sewer pipes (non-open channel flow), the applicant's designer shall be required to calculate the elevation of the hydraulic grade line through the storm sewer system. Wherever the hydraulic grade line elevation exceeds the pipe crown elevation for the design flow, pipes with watertight joints must be specified.

(15) Storm sewer structures (e.g., endwalls, inlets, pipe sections, etc.) may not be located on top of, or within 10 feet of, electric, communication, water, sanitary sewer, or gas services and/or mains, and structures, unless approval is received from the Township and the Authority or Utility having jurisdiction over same.

(16) Stormwater pipes must be oriented at right angles to electric, water, sanitary sewer, and gas utilities when crossing above or beneath same. Crossing angles of less than 90° will only be permitted at the discretion of the Township. When skewed crossings are permitted, interior angles between alignment of the storm sewer pipe and utility may not be less than 45°. Vertical and horizontal design of storm sewer must be linear.

(17) Roadway underdrain is required along both sides of all proposed roadways, existing roadways proposed to be widened, and within existing or proposed roadside swales as directed by the Township.

(18) Where a public storm sewer system is not located within a right-of-way, or dedicated public property, a twenty-foot-wide easement shall be established to encompass the storm sewer system and any required access from the public road. For multiple pipes or utilities, the width of the easement shall be a minimum of 30 feet.

(19) A minimum of one-foot of freeboard, between the inlet grate and the design flow elevation, shall be provided in all storm sewer systems (inlets and manholes) for the 100-year-frequency design storm event.

(20) Stormwater roof drains and sump pumps shall not discharge water directly onto a sidewalk or a street and shall be constructed to discharge to a dry well/seepage pit or aboveground entirely on the subject property, except where such discharge could flow across sidewalk or onto a street. If approved by the Township Engineer, roof drains and sump pumps may be discharged directly to a storm sewer system if such system discharges to a stormwater BMP or water quality facility.

D. Open swales and gutters. Open swales shall be designed on the basis of Manning's Formula as indicated for collection systems with the following considerations:

(1) Roughness coefficient. The roughness coefficient shall be 0.040 for earth swales.

(2) Bank slopes. Slopes for swale banks shall not be steeper than one vertical to four horizontal.

(3) Flow velocity. The maximum velocity of flow as determined by Manning's equation shall not exceed the allowable velocities as shown in the following table for the specific type of material, unless otherwise approved by the Township and the Montgomery County Conservation District.

Note: Source of the following design criteria is the Pennsylvania Department of Environmental Protection, Bureau of Soil and Water Conservation Publication, Erosion and Sediment Pollution Control Program Manual (Document No. 363-2134-008).

Allowable Velocity
Material		Velocity in feet per second (fps)
Well established grass on good soil
Short pliant bladed grass		4.0 to 5.0
Bunch grass - soil exposed		2.0 to 3.0
Stiff stemmed grass		3.0 to 4.0
Earth without vegetation
	Fine sand or silt	1.0
	Ordinary firm loam	2.0 to 3.0
	Stiff clay	3.0 to 5.0
	Clay and gravel	4.0 to 5.0
	Coarse gravel	4.0 to 5.0
	Soft shale	5.0 to 6.0
Shoulders
	Earth	(as defined above)
	Stabilized	6.0
	Paved	10.0 to 15.0

(4) Swales shall be stabilized with biodegradable erosion control blanket to permit establishment of permanent vegetation. Swales shall be of such shape and size to effectively contain the 100-year, Rational Method design storm, or greater, and to conform to all other specifications of the Township.

(5) To minimize sheet flow of stormwater across lots located on the lower side of roads or streets, and to divert flow away from building areas, the cross section of the street as constructed shall provide for parallel ditches or swales or curb on the lower side which shall discharge only at drainage easements, unless otherwise approved by the Township.

(6) Gutters and swales adjacent to road paving shall be permitted to carry a maximum flow of four cubic feet per second prior to discharge away from the street surface, unless it is proven to the satisfaction of the Township by engineering calculations that the road slopes or other factors would allow higher gutter or swale capacity.

(7) Flows larger than those permitted in gutters and roadside swales may be conveyed in swales outside the required road right-of-way in separate drainage easements, or may be conveyed in pipes or culverts inside or outside the required road right-of-way.

(8) Existing and proposed swales shall be provided with underdrains as deemed necessary by the Township should overland seepage result in potential maintenance problems. Underdrains must discharge into a natural drainage channel or stormwater management system.

(9) Where drainage swales are used to divert surface waters away from buildings, they shall be sodded, landscaped, or otherwise protected as required and shall be of a slope, shape, and size conforming to the requirements of the Township. Concentration of surface water runoff shall be permitted only in swales, watercourses, retention or detention basins, bioretention areas, or other areas designed to meet the objectives of this chapter.

(10) Except for drainage at roadway stream crossings, artificial swale discharge shall be set back 75 feet from a receiving waterway, and shall be diffused or spread out to reduce and eliminate high-velocity discharges to the impacted ground surface.

E. Bridge and culvert design. Any proposed bridge or culvert to convey flow within a watercourse, perennial stream, intermittent stream or ephemeral stream shall be designed in accordance with the following principles:

(1) Culverts and bridges shall be designed with an open bottom to maintain natural sediment transport and bed roughness, avoiding acceleration of water velocity above the natural (preexisting) condition. Rock (rip rap) lining (native material if possible) shall be installed within the culvert as needed to prevent erosion within the structure. Approximate top of rock lining must be at the level of the existing stream bottom so as to maintain unimpeded movement of native animal species and a normal water depth of 12 inches unless a greater depth is required by PADEP.

(2) Bottom of opening shall be designed to match the bankfull channel condition in terms of width and depth. The cross-sectional area of the bankfull channel (measured at a reference location upstream of the structure) shall be matched with area in the crossing structure.

(3) Above the bankfull elevation, the width shall increase a minimum of 30% to disperse the energy of higher flow volumes and avoid undermining of the supporting structure by secondary currents.

(4) The total cross-sectional area of the structure opening must be equal to or greater than the flood-prone area (cross-sectional stream area at a depth of twice the maximum bankfull depth, measured at a reference location upstream of the structure). The flood-prone area is approximately equal to the area flooded by a 50-year return period flood.

(5) All bridges, culverts, and drainage channels shall be designed to convey a flow rate equal to a 100-year, 24-hour storm as defined by the United States Department of Agriculture, Soil Conservation Service, Technical Release No. 55. All bridges and culverts shall be designed to convey the 100-year design storm without increasing the extent and depth of the 100-year floodplain, upstream or downstream of the structure.

F. Storm sewer design.

(1) Design flow rate. The storm sewer system shall be designed to carry the 100-year-frequency design storm peak flow rate. The drainage area and runoff coefficient to each inlet shall be indicated on the stormwater management plan. The 100-year flow rate shall be determined by the Rational Method formula:

Q = CIA

Where:
	Q	=	Peak runoff rate measured in cubic feet per second (cfs).
	C	=	Runoff coefficient - The coefficient of stormwater runoff includes many variables, such as ground slope, ground cover, shape of drainage area, etc.
	I	=	Intensity - Average rainfall intensity in inches per hour for a time equal to the time of concentration.
	A	=	Area - Drainage area in acres.
Values for the rainfall intensity shall be based on NOAA Atlas 14, Volume 2, Version 3.0, rain data found in Table A-3 and Figure A-2 of Appendix A[1] of this chapter.

[1] Editor's Note: Appendix A is included as an attachment to this chapter.

(2) Consideration shall be given to future land use changes in the drainage area in selecting the Rational ("C") coefficient. For drainage areas containing several different types of ground cover, a weighted value of "C" shall be used.

(3) In determining the peak flow rate to individual storm sewer inlets (or other collection structures) the time of concentration method (as referenced in § 129-19) shall be used for inlet drainage areas in excess of one acre, unless otherwise approved by the Township. For inlet drainage areas less than one acre, a five-minute time of concentration shall be used unless otherwise approved by the Township.

(4) In determining the required design flow rate through a storm sewer piping system, if a five-minute time of concentration (storm duration) results in a pipe size exceeding a thirty-inch-diameter pipe (or equivalent flow area of 4.9 square feet), the time of concentration approach (as defined herein) shall be used in determining storm duration.

(5) In determining the required design flow rate through a storm sewer piping system, if a five-minute time of concentration results in a pipe size exceeding 30 inches, within any run of pipe, the time of concentration approach may be used for sizing of pipes from that point on, by adjusting the time of concentration.

(6) Overflow system. An overflow system shall be provided to carry all bypass flow and/or flow in excess of storm sewer pipe design capacity, to the detention basin (or other approved outlet point) when the capacity of the system is exceeded. Stormwater runoff will not be permitted to surcharge from storm sewer structures [refer § 129-18C(19)].

(7) Except for drainage at roadway stream crossings, pipe discharge shall be set back 75 feet from a receiving waterway, and the pipe discharge shall be diffused or spread out to reduce and eliminate high-velocity discharges to the impacted ground surface.

G. Grading and drainage.

(1) After completion of rough grading, a minimum of eight inches of topsoil shall be returned to remaining disturbed areas prior to final grading and seeding.

(2) Lots shall be graded to secure proper drainage away from buildings and to prevent the collection of stormwater in pools. Minimum 2% slopes shall be maintained away from and around all structures. Separation between the top of foundation wall (or slab) and final grade shall comply with Worcester Township Building Code requirements.

(3) Construction. The applicant shall construct and/or install such drainage structures and/or pipe as are necessary to prevent erosion damage and to satisfactorily disperse, infiltrate or carry off such surface waters to the nearest practical BMP, storm drain or natural watercourse.

(4) Excavation. No excavation shall be made with a cut face steeper in slope than four horizontal to one vertical (4:1 = 25%), except under one or more of the following conditions:

(a) The material in which the excavation is made is sufficiently stable to sustain a slope of steeper than 4:1 and a written statement (certification) from a professional civil engineer, licensed in the Commonwealth of Pennsylvania and experienced in erosion control, to this effect is submitted to the Township Engineer for review. This statement shall indicate the site has been inspected and that the deviation from the slope specified herein will not result in injury to persons or damage to property.

(b) A concrete, segmental block, or stone masonry wall, constructed in accordance with Township requirements, is provided to support the face of the excavation.

(5) Fill. No fill shall be made which creates any exposed surface steeper in slope than four horizontal to one vertical (4:1 = 25%) except under one or more of the following conditions:

(a) The fill is located so that settlement, sliding, or erosion will not result in property damage or be a hazard to adjoining property, streets, alleys, or buildings.

(b) A written statement from a professional civil engineer, licensed in the Commonwealth of Pennsylvania and experienced in erosion control, certifying the site has been inspected and that the proposed deviation from the slope specified above will not endanger any property or result in property damage, is submitted to and approved by the Township.

(c) A concrete, segmental block, or stone masonry wall, constructed in accordance with Township requirements, is provided to support the face of the excavation.

(6) Slopes and fences. The top or bottom edge of slopes shall be a minimum of five feet from property or right-of-way lines of streets or alleys in order to permit the normal rounding of the edge without encroaching on the abutting property. Where walls or slopes (steeper than two horizontal to one vertical) are approved under the criteria in this chapter, and are four feet or more in height, a protective fence, no less than four feet in height, shall be required at the top of the wall (or bank).

(7) Cleanup. All lots must be kept free of any debris or nuisances whatsoever during construction.

(8) Design of erosion and sedimentation control facilities (particularly stormwater/sediment basins) shall incorporate best management practices as defined herein.

(9) Cut and fill operations shall be kept to a minimum. Wherever feasible, natural vegetation shall be retained, protected, and supplemented. Cut and fills shall not endanger or otherwise adversely impact adjoining property.

(10) No grading equipment shall be permitted to be loaded and/or unloaded on a public street, and no grading equipment shall be permitted to travel on or across a public street unless licensed for operation on public thoroughfares.

(11) Grading equipment shall not be permitted to cross intermittent and perennial streams. Temporary crossing shall be permitted only where application is made, and approval is received, from the Pennsylvania Department of Environmental Protection (where applicable), the Montgomery County Conservation District, and Worcester Township.

(12) Design of energy dissipation for high volume and/or high velocity discharge from storm sewerpipes and channels shall be in accordance with Hydraulic Engineering Circular No. 14, Hydraulic Design of Energy Dissipaters for Culverts and Channels, as published by Department of Transportation, FHA, when deemed necessary by the Township, and as approved by the Montgomery County Conservation District.

(13) To control the dissemination of mud and dirt on to public roads and driveways, tire cleaning areas constructed of AASHTO No. 1 stone (underlain by geotextile structural fabric), at least 50 feet in length shall be installed at each point of access to the site and individual lots (upon construction of internal streets in a binder condition). When deemed necessary by the Township, washing stations shall also be set up at every construction entrance in order to wash mud and dirt from exiting vehicles. Appropriate measures must be taken to control runoff from such locations. The applicant shall be responsible for the placement of appropriate signage identifying construction entrances and washing stations. Construction entrances shall be maintained by the applicant during construction, as determined by the Township.

(14) In the event any mud and/or debris is transported from the site onto a public roadway, the debris shall be removed immediately and the roadway swept and/or washed as deemed necessary by the Township at the owner's expense.

(15) Adequate provision shall be made to prevent surface water from damaging the cut face of excavation and the sloping surfaces of fills.

H. Stormwater detention/retention basins.

(1) If permanent ponds (retention basin) are proposed, the applicant shall demonstrate that such ponds are designed to protect the public's health and safety. Should any stormwater management facility require a dam safety permit under the PADEP Chapter 105 regulations, 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 a 100-year event.

(2) During construction, duly authorized representatives of Worcester Township may enter at any reasonable time upon any property within the Township to investigate whether construction activity is in compliance with this chapter.

(3) When basins are provided, they shall be designed to utilize the natural contours of the land whenever possible. When such design is not practical, the construction of the basin shall utilize slopes as flat as possible to blend the structure into the terrain. To minimize the visual impact of detention basins, they shall be designed to avoid the need for safety fencing. To meet this requirement, detention basins shall be designed as follows:

(a) Maximum depth of detained runoff shall be 24 inches for a two-year or ten-year storm event.

(b) Maximum depth of detained runoff shall be 36 inches for a 100-year storm event.

(c) The basin inflow and outflow structures shall not be located directly across from each other and shall not be in close proximity to one another. A length-to-width ratio in all detention/retention basins and other such storage facilities of at least 2:1 shall be provided to maximize the flow path between the inflow point and the outlet structure. The distance between these two structures must be at least 50% of the maximum length of the basin as measured at the top of berm elevation. Alternatively, a means for extending the time of surface flow from basin inflow point to basin outlet structure, designed to the satisfaction of the Township Engineer, may be utilized.

(4) Except with the one-year design storm, basins shall be designed so that they return to normal conditions within approximately 12 hours after the termination of the storm, unless the Township determines that downstream conditions may warrant other design criteria for stormwater release.

(5) Landscaping and planting in and around the perimeter of basins shall be provided. Proposed planting shall also be in accordance with the provisions of this chapter, the Subdivision and Land Development Ordinance,[2] and as recommended by the Township Engineer. When a detention basin is not designed as a stormwater management constructed wetland, it shall be planted with low-maintenance grass or similar satisfactory to the Township.

[2] Editor's Note: See Ch. 130, Subdivision and Land Development.

(6) If a stormwater management basin will serve as a temporary sediment control device, the temporary sediment control measures shall be shown including perforated riser pipes or standboxes, filter berms, cleanout stakes and other measures as may be required by Pennsylvania Department of Environmental Protection, Chapter 102 regulations.[3] Plans for such facilities shall require Montgomery County Conservation District approval prior to implementation. Sedimentation basins shall be in place prior to any earthmoving activities within their tributary drainage areas. A note identifying the above criteria shall be on all plan sheets required to be recorded as well as the development agreement with the Township and the stormwater management facilities operation and maintenance agreement.

[3] Editor's Note: See 25 Pa. Code Chapter 102.

(7) Stormwater management basins shall be in place before the creation of any new impervious surfaces on the site. As-built drawings of the basins(s) shall be submitted to the Township for review. The basin shall not be considered functional until it is proved by the developer that the basin meets the volume requirements and the outflow characteristics of the original design of the basin(s).

(8) Runoff shall not be directed to any infiltration structure until all tributary drainage areas are permanently stabilized.

(9) Except where otherwise identified herein, all detention or retention basins shall have slopes of four horizontal to one vertical (4:1 = 25%), or flatter on the basin's outer berm and five horizontal to one vertical or less on the basin's inner berm. The top or toe of any slope shall be located a minimum of five feet from any property line. The maximum difference between the top of berm elevation and the invert elevation of the outlet structure shall be seven feet.

(10) All portions of a detention basin bottom shall have a minimum slope of 2%. For portions of basin bottoms with grades less than 2%, the applicant shall provide a landscape design, which minimizes maintenance provisions and encourages infiltration. These requirements may be altered when approved by the Township Engineer.

(11) Basin berm construction requirements.

(a) Site preparation. Areas under the embankment and any structural works shall be cleared, grubbed, and the topsoil stripped to remove the trees, vegetation, roots or other objectionable material. In order to facilitate cleanout and restoration, the pool area will be cleared of all brush and excess trees except where designed to retain such existing vegetation as stormwater BMPs.

(b) Cutoff trench. A cutoff trench will be excavated along the center line dam on earth fill embankments. The minimum depth shall be two feet. The cutoff trench shall extend up both abutments to the riser crest elevation. The minimum bottom width shall be eight feet but wide enough to permit operation of compaction equipment. The side slopes shall be no steeper than 1:1. Compaction requirements shall be the same as those for the embankment. The trench shall be kept free from standing water during the backfilling operations.

(c) Embankment:

[1] The fill material shall be taken from the selected borrow areas. It shall be free of roots, wood vegetation, oversized stones, rocks or other objectionable material. Areas on which fill is to be placed shall be scarified prior to placement of fill.

[2] The fill material should contain sufficient moisture so that it can be formed by hand into a ball without crumbling. If water can be squeezed out of the ball, it is too wet for proper compaction.

[3] Fill material will be placed in six-to-eight-inch layers and shall be continuous over the entire length of the fill. Fill material must be compacted to a minimum of 95% of Modified Proctor Density as established by ASTM D-1557. Compaction testing by a certified soils engineer/geologist must be completed as directed by the Township Engineer to verify adequate compaction has been achieved. Compaction tests shall be run on the leading and trailing edge of the berm along with the top of the berm. Verification of required compaction shall be submitted to the Township prior to utilization of any basin for stormwater management.

(12) Emergency overflow facilities/spillway shall be provided within basins in order to convey basin inflow in excess of design flows, out of the basin, or in the event the outlet structure becomes blocked and is unable to convey flow. Emergency spillways discharging over embankments shall be constructed of reinforced concrete checkerblocks to protect the berm against erosion. The checkerblocks shall be back-filled with topsoil and seeded. Checkerblock lining shall extend to the toe of the embankment on the outside of the berm, and shall extend to an elevation of three feet below the spillway crest on the inside of the berm. Vegetated spillways may be utilized for spillways constructed entirely on undisturbed ground (i.e., not discharging over fill material). A dense cover of vegetation shall be rapidly established in such spillways by sodding or seeding with a geotextile anchor. The vegetated spillway must be stabilized before runoff is directed to the basin. The minimum capacity of all emergency spillways shall be equivalent to the peak flow rate of the 100-year, postdevelopment design storm (entering to the basin).

(13) In all cases, the discharge end of the basin shall be provided with a properly designed outlet control structure (headwall, orifice structure or other approved flow control structure), culvert pipe, and endwall. Perforated riser pipes alone, without provision for permanent outlet control structure (as stated above), and culvert pipe are not permitted for permanent basins.

(14) The minimum top of basin berm width (at the design elevation) shall be 10 feet.

(15) The minimum freeboard through the emergency spillway shall be one foot. Freeboard is defined as the difference between the design flow elevation through the spillway and the elevation of the top of the settled basin berm.

(16) Antiseep collars shall be installed around the pipe barrel and shall be centered within the normal saturation zone of the berm. The antiseep collars and their connections to the pipe barrel shall be watertight. The antiseep collars shall be cast-in-place in the field and extend a minimum of two feet beyond the outside of the principal pipe barrel. Precast collars shall be permitted if approved by the Township Engineer. A minimum of two collars shall be installed on each basin outlet pipe. Collars shall have a minimum thickness of 12 inches and may not be installed within two feet of pipe joints.

(17) A perforated sediment control structure, sized in accordance with Montgomery County Conservation District requirements, shall be provided at each basin outlet structure (if more than one is to be utilized) for sediment control. Sediment control structures shall not be removed until the entire area tributary to the basin has been permanently stabilized and until approved by the Montgomery County Conservation District.

(18) Stormwater management facility outlet piping shall be Class II reinforced O-ring concrete pipe. All joints shall be mortared. Crushed stone bedding/backfill shall not be utilized through basin berms.

(19) The grate of the basin outlet structure shall be at least six inches lower than the elevation of the earthen emergency spillway. Six inches, minimum, is also required between the routed 100-year water surface elevation and top of grate of the outlet structure.

(20) Energy dissipating devices (rock lining/rip rap, or other approved materials) shall be provided at all basin outlets and shall be sized in accordance with Pennsylvania Department of Environmental Protection, Bureau of Soil and Water Conservation Publication, Erosion and Sediment Pollution Control Program Manual, latest revision.

(21) Stone gabion baskets or concrete or segmental block retaining walls shall not be permitted for use in construction of detention/retention basins within the berm or within the 100-year water surface elevation (as measured through the earthen emergency spillway).

(22) An access easement and stabilized access drive to stormwater detention facilities shall be provided for maintenance and operation. This access easement shall be cleared and, when possible, be at least 20 feet in width. Multiple accesses shall be encouraged for major facilities. The developer shall provide access easements and drives of interlocked, reinforced pervious paving systems (back-filled with topsoil and seeded) or other similar paver acceptable to the Township Engineer, over a six-inch bed of compacted PennDOT Type 3A coarse aggregate (or approved equivalent). Accessways to basins shall be a minimum of 10 feet wide and be no steeper in slope than 10 feet horizontal to one foot vertical (10:1). In addition, depressed curb and reinforced concrete apron (six-inch minimum thickness) shall be provided where the accessway enters a street/driveway and the stabilized driveway shall extend from the bottom of the interior basin berm embankment to the point of access to the basin from a public right-of-way or paved driveway within an access easement. The access easement shall be owned and maintained by the same entity owning the stormwater management facility and shall allow access by Worcester Township or its designee for emergency inspection and/or maintenance at any reasonable time.

(23) If the basin is not designed to meet the requirements of § 129-18H(3) a split-rail fence must be provided as follows:

(a) A level area (two-percent slope) eight feet in width shall be provided on both the inside and outside of the fence, along the entire length of the fence for proper access by maintenance equipment. The total width of this generally level area shall be at least 16 feet.

(b) Each basin fence installation shall include two points of access with ten-feet-wide self-closing, self-latch gates to allow for maintenance equipment/vehicle access.

(c) Fence shall be split-rail consisting of locust posts (two or three rail), four feet high, minimum, with assorted hardwood rails (eight feet to 10 feet long), and epoxy coated wire mesh (black or green in color) installed six inches above finished grade. The mesh shall be installed on the outside of the fence.

(d) Split-rail fence shall also be required around any detention or retention basin, where directed by Worcester Township.

(24) Landscaping:

(a) The perimeter berms and embankments of retention/detention basins including wet ponds, and artificial wetland stormwater management BMPs shall be designed to create a natural appearance and reduce future maintenance requirements. Landscaping shall include a mixture of native tall grasses and perennial plants, ground cover, shrubs, and trees to eliminate the necessity of periodic mowing.

(b) Artificial wetland basins shall be designed pursuant to requirements of the Pennsylvania Stormwater BMP Manual. Plant material and arrangement shall be subject to approval of the Township Engineer.

(c) Retention/detention basins.

[1] The perimeter of the retention/detention basin shall be landscaped with a mixture of deciduous trees, evergreens, and shrubs arranged in an informal manner. Retention basin (wet ponds) and artificial wetland basin landscaping shall be designed to create a "natural" appearance. Minimum plant material shall include the following per 100 linear feet of basin perimeter measured at the 100-year water surface elevation:

[a] Three evergreen trees (minimum height five feet).

[b] Two deciduous trees (minimum caliper 2 1/2 inches).

[c] Five shrubs (minimum height three feet).

[2] Retention/detention basin landscaping design is subject to approval by the Township.

(25) Special requirements for stormwater detention/retention BMPs within defined exceptional-value and high-quality watersheds as defined in Chapter 93, Water Quality Standards, Title 25, Pennsylvania Department of Environmental Protection Rules and Regulations:

(a) Temperature-sensitive BMPs and stormwater conveyance systems are to be used and designed with storage pool areas and supply outflow channels, and shaded with trees. At a minimum, the southern half of pond shorelines shall be planted with shade or canopy trees and understory shrubs within 10 feet of the pond shoreline. In conjunction with this requirement, the maximum slope allowed on the berm area to be planted is 10 to 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 must be identified on the stormwater management site plan and recorded at the Montgomery County Recorder of Deeds for all development sites.

(b) As an alternative to mitigating the temperature of stormwater runoff as described in § 129-18H(25)(a), alternative temperature-sensitive BMPs may be utilized, if approved by the Township Engineer, upon the applicant demonstrating such BMPs will effectively reduce the temperature of detained runoff before it is released from the development site. Such alternative BMPs may include, but are not limited to, facilities that cool runoff through underground storage and filtration and retention ponds/basins where outflow from the facility is drawn from a depth of five feet (or greater) below the permanent pool surface.

(26) At the conclusion of all construction and after all stormwater facilities have received final approval, the applicant shall offer the facilities for dedication to the Township, with the following requirements:

(a) The dedicated area shall include the entire ponded area for the 100-year storm event and the outside slope at the berm.

(b) The dedicated area shall not be considered part of the open space and recreation land required elsewhere in the Subdivision and Land Development Ordinance and Zoning Ordinance.[4]

[4] Editor's Note: See Ch. 130, Subdivision and Land Development, and Ch. 150, Zoning.

(c) The applicant shall provide for the special financial burden the Township will be accepting if the Township accepts the detention basin maintenance. To help mitigate this future financial burden, the applicant shall contribute to the Township a cash payment in an amount to be calculated by the Township Engineer, which amount shall include all estimated costs to inspect, maintain, and repair the facilities during a ten-year period.

(27) If the Township declines dedication of the basin, the applicant shall provide written assurance, satisfactory to the Township, that the retention/detention basin will be properly maintained. Such assurances shall be in a form of a covenant that will run with the land and shall provide for Township maintenance at the cost of the landowner in case of default, and further provide for assessment of costs and penalties in case of default.

I. All developments that create impervious surface shall provide capacity for and treatment of the calculated water quality volume and recharge volume unless exempt under § 129-5. In potential stormwater BMPs, the order of preference is as follows: 1) infiltration BMPs; 2) flow attenuation methods (e.g. vegetated open swales and natural depressions); 3) artificial wetlands, bioretention structures, and wet ponds; 4) minimum first flush detention or dual purpose detention (where appropriate). Infiltration BMPs shall be utilized unless the applicant can demonstrate use of infiltration techniques is not feasible due to site conditions, based upon site-specific soil testing. Vegetated swales, wetlands or artificial wetlands and bioretention structures shall be utilized wherever possible if infiltration BMPs are deemed unfeasible. BMP techniques can and should be used in conjunction with each other (e.g., vegetated swales with infiltration or retention facilities).

(1) Infiltration best management practices (BMPs). Infiltration BMPs shall be designed in accordance with the design criteria and specifications of the Pennsylvania Stormwater BMP Manual (2006) and shall conform to the following minimum requirements:

(a) A soils evaluation and infiltration/permeability testing of the project site shall be conducted in accordance with Appendix B[5] of this chapter.

[5] Editor's Note: Appendix B is included as an attachment to this chapter.

(b) A minimum soil depth of 18 inches shall be provided between the bottom of the infiltration BMPs and the top of bedrock or seasonally high-water table. The minimum required separation between the infiltration surface and these limiting zones shall be increased by the Township should project-specific conditions exist (such as anticipated increased contaminants) which dictate greater prevention of groundwater contamination.

(c) Infiltration BMPs must have an infiltration rate sufficient to accept the design stormwater load and dewater completely as determined by field permeability tests. The minimum field-tested infiltration rate permitted for construction of infiltration BMPs shall be 0.2 inch/hour (in/hr). A safety factor of 50% shall be applied to field-tested rates to determine the infiltration rate that must be utilized for design of infiltration BMPs (e.g., for soil which measured 0.4 in/hr, the BMP design rate shall be 0.2 in/hr to insure effective infiltration after construction).

(d) Infiltration BMPs intended to receive rooftop runoff shall include appropriate measures such as leaf traps and cleanouts to prevent clogging by vegetation. Surface inflows shall be designed to prevent direct discharge of sediment into the infiltration system.

(e) Adequate storage shall be provided to accommodate the volume of runoff calculated as the difference between the predevelopment runoff volume and postdevelopment runoff volume based on the 100-year design storm.

(f) The facility shall be designed to control the postdevelopment peak rate of runoff to the predevelopment peak rate of runoff for all design storms identified in § 129-13 of this chapter.

(g) An overflow or spillway shall be provided that safely permits the passing of runoff greater than that occurring during the 100-year design storm event.

(h) Underground infiltration basins and BMPs shall have positive overflow controls to prevent storage within one foot of the finished surface over the basin.

(i) When infiltration methods such as seepage pits, beds, or trenches are proposed, the locations of existing and proposed septic tanks, infiltration areas, and wells must be shown. A separation distance of no less than 50 feet shall be provided between any septic system and any facility used for stormwater management and infiltration.

(j) A minimum of 30 feet of undisturbed soil shall separate the foundation wall of any building and an infiltration BMP, unless a lesser distance is approved by the Township or Township engineer, based on site conditions or selected BMP.

(k) All infiltration facilities shall be designed to completely infiltrate runoff volume within two days (48 hours) from the peak of the design storm.

(l) Special attention shall be paid to proper installation of infiltration-oriented stormwater management systems during the construction and to careful avoidance of soil compaction during site development. Areas proposed for infiltration BMPs shall be protected from sedimentation and compaction during the construction phase, so as to maintain their maximum infiltration capacity.

(m) The Township may require the installation of a mitigative layer or an impermeable liner in an infiltration BMP and/or other stormwater structure that impounds runoff, where the possibility of groundwater contamination exists. A detailed hydrogeologic investigation may be required by the Township.

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

(o) Infiltration BMPs shall be designed based on field-tested infiltration/permeability rates at the level of the proposed infiltration surface(s) and based on a safety factor of 50%.

(2) Noninfiltration facilities used as best management practices (BMPs). All facilities shall be designed in accordance to the design criteria and specifications in the Pennsylvania Stormwater BMP Manual.

(3) Artificial wetlands, wet ponds, and bioretention structures.

(a) Wet pond BMPs shall meet the following requirements:

[1] Wet ponds shall be constructed on hydric or wet soils and/or soils which have an infiltration rate of less than 0.2 inch/hour.

[2] A minimum drainage area of five acres shall be directed to the pond unless a source of recharge is utilized such as a natural spring or well.

[3] The length of the pond between the inflow and outlet points shall be maximized. In addition, an irregular shoreline shall be provided. By maximizing the flow length through the pond and providing an irregular shoreline, the greatest water quality benefit will be achieved by minimizing "short circuiting" of runoff flowing through the pond.

[4] A shallow forebay shall be provided adjacent to all inflow areas. The forebay shall be planted as a marsh with emergent wetland vegetation. The forebay serves to enhance sediment trapping and pollutant removal, as well as concentrating accumulated sediment in an area where it can be readily removed.

[5] All wet ponds shall be designed with public safety as a primary concern. An aquatic safety bench shall be provided around the perimeter of the permanent pool. The depth of the bench shall be a maximum of 15 inches and a minimum of 12 inches for a width of at least 10 feet. A 3:1 slope shall lead from the edge of the safety bench toward the deep water portion of the pond. At least 15 feet of 3:1 slope shall be provided from the edge of the safety bench. Slopes in the remainder of the pond below the permanent pool elevation shall be a maximum of 2:1.

[6] The perimeter slope above the permanent pool shall have a maximum slope of 5:1.

[7] Wet ponds shall have a deep water zone of at least five feet to encourage gravity settling of suspended fines, and prevent stagnation and possible eutrophication.

[8] Wet ponds shall be capable of being substantially drained by gravity flow. Wet ponds shall be equipped with a manually operated drain that can be secured against unauthorized operation.

[9] A planting plan shall be developed for the wet pond, showing all proposed aquatic, emergent, and upland plantings required pursuant to this chapter and the Zoning and Subdivision and Land Development Ordinances[6] (where specifically identified).

[6] Editor's Note: See Ch. 130, Subdivision and Land Development, and Ch. 150, Zoning.

[10] Wet ponds shall be designed to discourage use by Canada geese. Techniques employed shall include the following:

[a] Elimination of straight shorelines, islands, and peninsulas;

[b] Placement of walking paths (where applicable) along the shoreline;

[c] Placement of grassed areas (i.e., playing fields) at least 450 feet from the water surface;

[d] Vegetative barriers;

[e] Rock barriers;

[f] Installation of tall trees within 10 feet of the water surface;

[g] Use of ground covers not palatable to Canada geese.

[h] Other techniques as approved by the Township Engineer.

(b) Artificial wetland BMPs shall meet the following requirements:

[1] Artificial wetlands shall be constructed on hydric or wet soils and/or soils which have an infiltration rate of less than 0.2 inch/hour.

[2] Runoff entering artificial wetlands shall be filtered through a sediment removal device before entering the wetland.

[3] A planting plan shall be developed for the artificial wetland showing all proposed aquatic, emergent, and upland plantings required pursuant to this chapter and the Zoning and Subdivision and Land Development Ordinances[7] (where specifically identified). The planting plan shall be developed to provide a diversity of species resulting in a dense stand of wetland vegetation.

[7] Editor's Note: See Ch. 130, Subdivision and Land Development, and Ch. 150, Zoning.

[4] At least 75% of the surface area of the wetland shall be developed as a shallow water emergent wetland, with a water depth of less than 12 inches. The remainder shall be constructed as open water with depths between two feet and four feet.

(4) Minimum first flush detention/dual purpose BMPs.

(a) Minimum first flush detention/dual purpose detention basin BMPs shall be designed to meet the following requirements:

[1] Postdevelopment runoff from a "water quality storm" (a one-year, twenty-four-hour event) shall be released over a minimum period of 24 hours.

[2] Two stage basins shall be utilized where first flush detention will be employed for water quality and conventional detention used for peak rate control of storms exceeding the one-year, twenty-four-hour event.

[3] Two stage basins shall be constructed so that the lower part of the basin is graded to detain stormwater from the "water quality storm," and the remainder of the basin graded as a flat overbank area to provide storage only for the larger, less frequent storm events. The overbank area is encouraged to be developed as an active or passive recreational area.

[4] The area inundated by the "water quality storm" is encouraged to be maintained as a wetland environment, which will increase the water quality benefits of the first flush/dual purpose detention basin, and will prevent the need for mowing of a frequently saturated area.

J. Riparian corridor restoration. Within all subdivisions and nonresidential land developments, from the top of watercourse bank, 75 feet on either side of the watercourse, which contains wetlands and/or floodplain, shall be planted to establish a Zone 1 and Zone 2 buffer as defined and in accordance with the Pennsylvania Handbook of Best Management Practices for Developing Areas, 1998, Riparian Forested Buffer. Where existing vegetation on the site essentially duplicates buffer requirements, this provision shall not apply. Additionally, this requirement may be modified or waived by the Board of Supervisors where existing man-made improvements or agricultural operations to be retained encroach within the buffer area.

K. General design requirements.

(1) Prior to finish grading of a development site and final overlay of streets, roads, and driveways, temporary measures, acceptable to the Township, shall be taken to ensure that all runoff intended to be intercepted and collected by an inlet or other facility will be collected. The plan shall include such details, notes, or specification including bituminous "eyebrows" at inlets, diversion berms, etc.

(2) Water originating from other than natural sources, such as air conditioning units, sump pumps, or other dry weather flow, wherever practical and possible, shall be connected first to an infiltration BMP, and if that is not possible, then to a storm sewer, street drainage structure, or other approved stormwater conveyance facility that is designed as part of a stormwater management BMP.

(3) All stormwater runoff and floodplain calculations and stormwater management facilities design shall be prepared by a professional engineer licensed in the Commonwealth of Pennsylvania.

(4) When subdivisions or land developments are submitted to the Township for approval in sections, a complete storm sewer design for the proposed subdivision and land development shall be submitted. The proposed design must include the entire tract and not a portion.

(5) 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.

L. All stormwater control facility designs shall conform to the applicable standards and specifications of the following governmental and institutional agencies:

(1) American Society of Testing and Materials (ASTM).

(2) Asphalt Institute (AI).

(3) Montgomery County Conservation District (MCCD).

(4) Federal Highway Administration (FHWA).

(5) National Crushed Stone Association (NCSA).

(6) National Sand and Gravel Association (NSGA).

(7) Pennsylvania Department of Environmental Protection (PADEP).

(8) Pennsylvania Department of Transportation (PADOT).

(9) United States Department of Agriculture, Natural Resources Conservation Service, Pennsylvania (USDA, NRCS, PA).

§ 129-19 Calculation methodology.

A. Stormwater runoff peak discharges from all development sites with a drainage area equal to or greater than 50 acres shall be calculated using a generally accepted calculation technique that is based on the NRCS Soil Cover Complex Method. The Rational Method may be used to estimate peak discharges from drainage areas that contain less than 100 acres as approved by the Township Engineer. The Rational Method is recommended for watershed areas under 50 acres. Table 129-19.1 summarizes acceptable computation methods. The method shall be selected by the applicant based on the individual limitations and suitability of each method for a particular site.

Table 129-19.1 Acceptable Computation Methodologies for Stormwater Management Designs
Method	Method Developed By	Applicability
TR-20 (or commercial computer package based on TR-20)	USDA NRCS	Applicable where use of full hydrology computer model is desirable or necessary
TR-55 (or commercial computer package based on TR-55)	USDA NRCS	Applicable for land development plans within limitations described in TR-55
HEC-1, HEC-HMS	U.S. Army Corps of Engineers	Applicable where use of full hydrologic computer model is desirable or necessary
PSRM	Penn State University	Applicable where use of a hydrologic computer model is desirable or necessary; simpler than TR-20 or HEC-1
Rational Method (or commercial computer package based on Rational Method)	Emil Kuichling (1889)	Applicable sites less than 50 acres, or as approved by the Township Engineer
Other methods	Varies	Other computation methodologies approved by the Township Engineer

B. All calculations consistent with this chapter using the Soil Cover Complex Method shall use the appropriate design rainfall depths for the various return period storms according to the National Oceanic and Atmospheric Administration (NOAA) Atlas 14, Volume 2, Version 3.0, rain data corresponding to the Graterford 1E rain gage (No. 36-3437), Schwenksville, Pennsylvania, as presented in Table A-1 of Appendix A of this chapter. The SCS Type II rainfall curve data from NOAA is listed in Figure A-1 in Appendix A[1] of this chapter. This data may also be directly retrieved from the NOAA Atlas 14, Volume 2, Version 3.0 website: hdsc.nws.noaa.gov/hdsc/pfds. If a hydrologic computer model such as PSRM or HEC-1/HEC-HMS is used for stormwater runoff calculations, then the duration of rainfall shall be 24 hours.

[1] Editor's Note: Appendix A is included as an attachment to this chapter.

C. Runoff curve numbers (CN) for both existing and proposed conditions to be used in the Soil Cover Complex Method shall be obtained from Table A-2 in Appendix A[2] of this chapter.

[2] Editor's Note: Appendix A is included as an attachment to this chapter.

D. Suggested runoff coefficients (C) for both existing and proposed conditions for use in the Rational Method are contained in Table A-4 in Appendix A[3] of this chapter.

[3] Editor's Note: Appendix A is included as an attachment to this chapter.

E. All calculations using the Rational Method shall use rainfall intensities consistent with appropriate time of concentration for overland flow and return periods from NOAA Atlas 14, Volume 2, Version 3.0, rain data corresponding to the Graterford 1E rain gage (No. 36-3437), Schwenksville, Pennsylvania, as presented in Table A-3 of Appendix A[4] of this chapter. The Rational Method rainfall curve data from NOAA is listed in Figure A-2 in Appendix A of this chapter. 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.

[4] Editor's Note: Appendix A is included as an attachment to this chapter.

F. For the purposes of existing conditions flow rate determination for all development activity, undeveloped land and existing impervious surfaces shall be considered as "meadow" in good condition, unless the natural ground cover generates a lower curve number (CN) or Rational "C" value (e.g., forest), as listed in Tables A-2 and A-4 in Appendix A[5] of this chapter. Wooded areas shall use a ground cover of "woods in good condition." An area shall be considered wooded if there is a contiguous canopy of trees existing over an area of 1/4 acre or more.

[5] Editor's Note: Appendix A is included as an attachment to this chapter.

G. Where uniform flow is anticipated, the Manning equation shall be used for hydraulic computations, and to determine the capacity of open channels, pipes, and storm sewers. Values for Manning's roughness coefficient (n) shall be consistent with Table A-5 in Appendix A[6] of this chapter.

[6] Editor's Note: Appendix A is included as an attachment to this chapter.

H. 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.

I. The design of any stormwater management 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 area, the design storm hydrograph shall be computed using a calculation method that produces a full hydrograph.

J. The time of concentration (Tc) is the time required for water to flow from the hydraulically most remote point of the drainage area to the point of interest (design point). Use of the Rational formula requires calculation of a Tc for each design point within the drainage basin. Travel time estimation for the Rational Method shall be based on NRCS Technical Release No. 55 (2nd Edition). For design purposes the time of concentration may not be less than five minutes. Travel time (Tt) is the time it takes runoff to travel from one location to another in a watershed (subreach) and is a component of time of concentration. Tc is computed by summing all the travel times for consecutive components of the drainage conveyance system.

K. Water moves through a watershed as sheet flow, shallow concentrated flow, open channel flow, or some combination of these. Sheet flow rates shall be calculated using the NRCS TR-55 (1986) variation of the kinematic wave equation. Sheet flow length may not exceed 50 feet over paved surfaces and 150 feet over unpaved surfaces. Maximum permitted sheet flow length shall be 150 feet unless site-specific conditions exist (that can be demonstrated) that warrant an increase of the sheet flow length. Under no circumstances shall sheet flow length exceed 300 feet. Shallow concentrated flow time and open channel flow time shall be calculated using standard engineering methodologies.

§ 129-20 Erosion and sedimentation control requirements.

A. Whenever vegetation and topography are to be disturbed, such activity must be in conformance with Chapter 102, Title 25, Rules and Regulations, Part 1, Commonwealth of Pennsylvania, Department of Environmental Protection, Subpart C, Protection of Natural Resources, Article II, Water Resources, Chapter 102, Erosion Control, and in accordance with the Montgomery County Conservation District and the standards and specifications of the Township. Various BMPs and their design standards are identified in the PADEP Erosion and Sediment Pollution Control Program Manual (March 2012), as amended and updated.

B. No regulated earth disturbance activities within the Township shall commence until approval by the Township of an erosion and sediment control plan for construction activities.

C. In addition, under 25 Pa.Code Chapter 92,[1] a PADEP NPDES Construction Activities Permit is required for regulated earth disturbance activities of one or more acres.

[1] Editor's Note: Chapter 92 of Title 25 of the Pennsylvania Code was reserved 10-8-2010, effective 10-9-2010. It was replaced by Chapter 92a, National Pollutant Discharge Elimination System (NPDES) Permitting, Monitoring and Compliance.

D. Evidence of any necessary permit(s) for regulated earth disturbance activities from the appropriate PADEP regional office or County Conservation District must be submitted to the Township.

E. A copy of the erosion and sediment control plan and any required permit, as required by PADEP or Montgomery County Conservation District regulations, shall be available at the project site at all times.

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

(1) Areas proposed for infiltration BMPs shall be protected from sedimentation and compaction during the construction phase, so as to maintain their maximum infiltration capacity. Thirty-three-inch super filter fabric fence (or other approved protection mechanism) must be installed around proposed infiltration areas to prevent encroachment and compaction by construction equipment.

(2) Infiltration BMPs shall not be constructed nor receive runoff until the entire contributory drainage area to the infiltration BMP has received final stabilization. If necessary, thirty-three-inch super filter fabric fence (or other approved protection mechanism) must be installed in the vicinity of infiltration area to prevent contamination by runoff containing suspended sediment.

(3) Areas of the site to remain undisturbed shall be protected from encroachment by construction equipment/vehicles to maintain the existing infiltration characteristics of the soil. Four-foot-high orange safety fence or other similar protection fence approved by the Township must be installed around the entire limit of disturbance/clearing prior to commencement of earthmoving activities, and maintained until completion of all construction activity.

G. Peak discharge rates from the site during land disturbance shall comply with the appropriate sections in this chapter related to allowable postdevelopment stormwater runoff rates, with the following additions:

(1) For purposes of calculating required detention storage during land disturbance, peak discharges shall be calculated based upon the runoff coefficients for bare soils during the period of maximum anticipated disturbance from clearing and grading, in combination with the entire quantity of proposed impervious surface installation, indicated on the development plan. Runoff controls shall insure that the peak rate of "during construction" runoff does not exceed predevelopment runoff rates for the one-year-frequency through 100-year-frequency design storm events. Detention storage during the period of land disturbance and prior to establishment of permanent cover may require additional detention facilities on a temporary basis. Such measures shall be located so as to preserve the natural soil infiltration capacities of the planned infiltration areas. Calculations based on the above parameters must be submitted to verify compliance with this requirement.

(2) Wherever soils, topography, cut and fill or grading requirements, or other conditions suggest substantial erosion potential during land disturbance, the Township may require that the entire volume of all storms up to a two-year storm from the disturbed areas be retained on site and that special sediment trapping facilities (such as check dams, etc.) be installed.

H. Areas of the site to remain undisturbed shall be protected from encroachment by construction equipment/vehicles to maintain the existing infiltration characteristics of the soil.