Design criteria for storm collection and conveyance facilities:
For the purpose of designing storm sewers, open swales, and other stormwater runoff collection and conveyance facilities, any of the calculation methods listed in § 99-20B shall be used. Stormwater conveyance pipes, swales, and other water-carrying structures shall be designed to handle the peak discharge from the twenty-five-year post-development storm event. However, the one-hundred-year post-development design storm shall be used wherein it is necessary for the storm sewers to convey the one-hundred-year design storm to the stormwater management facilities. Culverts intended to convey streams and natural drainageways shall be designed to handle the one-hundred-year post-development storm event. All pipes shall be provided with an end sections or end wall. Rainfall intensities for design should be obtained from the Pennsylvania Department of Transportation rainfall charts in Publication 584, Chapter 7, Appendix A entitled "Field Manual of Pennsylvania Design Rainfall Intensity Charts From NOAA Atlas 14 Version 3 Data," latest revision.
Stormwater inlets. The maximum spacing between stormwater inlets shall be designed according to the twenty-five-year storm flow and the capacity of the inlets, taking into account gradient of roadway, maximum allowable street flooding opening or combination inlets exists, use the capacity reduction factors shown in Table 1 applied to theoretical capacity of the inlet. The maximum amount of water that should be bypassed on to the next downstream inlet for inlets on continuous grades is 10%.
Pipes. The minimum allowable pipe diameter shall be 15 inches unless approved by the Township or the Township's representative in writing. Pipe shall be either reinforced concrete of smooth-lined corrugated plastic pipe, as approved by the Township Engineer. Horizontal and vertical curves with radii of 100 feet or greater are allowed for all pipe sizes. Other pipe design criteria include:
Spacing of structures. The maximum allowable spacing between structures to be used for inspecting and cleaning storm sewers shall be based on the pipe size and spacing shown in Table 2.
Open channels. Maximum allowable velocities of flow in swales, open channels and ditches as relating in slope and grass cover as are set forth in the PA DEP Stormwater Best Management Practices Manual.
Drain pipe, trenching, bedding, and backfilling requirements shall conform to the requirements of PennDOT Specifications, Form 408.
Appropriate grates shall be designed for all catch basins, stormwater inlets, and other entrance appurtenances.
Manholes shall be designed so that the top shall be at finished grade and sloped to conform to the slope of the finished grade. Top castings of structures located in roads or parking areas shall be machined or installed to preclude rattling.
Where a proposed sewer and/or detention or retention pond discharge connects with an existing storm sewer system and/or a natural drainage course, the applicant shall demonstrate that sufficient capacity exists in the downstream system to handle the additional flow.
Storm sewer outfalls shall be equipped with energy dissipation devices to prevent erosion and conform to applicable requirements of the Pennsylvania DEP for stream encroachments (Chapter 105 of Pennsylvania DEP Rules and Regulations). Exit velocities of the design storm for the stormwater conveyance systems must be considered in the design of the energy dissipation devices. A stable means of conveyance of the discharge from the outfall and energy dissipation device must be provided to the receiving channel.
All storm drain inlets shall have permanently affixed signage, castings or markings either on or directly adjacent to the inlet or as approved by the Township with the phrase "Do Not Dump, Drains to Stream" or similar message as approved by the Township.
Design criteria for stormwater detention/retention facilities:
No discharge of toxic or hazardous materials including but not limited to those identified in Title 25 Pa. Code § 109.503(a)(1) and 40 CFR 116.4 shall be permitted into any stormwater management system.
Spillway design. The outlets for the detention/retention ponds shall consist of a combination of principal and emergency spillways. The outlets must pass the peak runoff expected from the drainage area for the one-hundred-year storm without damage to the embankment of the pond.
Principal spillway. The principal spillway shall consist of a solid vertical reinforced concrete pipe or reinforced concrete box joined by a watertight connection to a horizontal reinforced concrete pipe (barrel) extending through the embankment and out-letting beyond the downstream toe of the fill. The principal spillway shall have a minimum capacity of 0.2 cfs per acre of drainage area when the water surface is at the crest of the emergency spillway. The construction materials must be approved by the Township.
Design elevations. The crest of the principal spillway shall be a minimum of 1.0 foot below the crest of the emergency spillway. The crest of the principal spillway shall be a minimum of three feet below the top of the embankment.
Anti-vortex and trash rack. An anti-vortex device and trash rack shall be attached to the top of the principal spillway to improve the flow of water into the spillway and prevent floating debris from being carried out of the basins.
Base. The base of the principal spillway must be firmly anchored to prevent its floating. If the riser of the spillway is greater than 10 feet in height, computations must be made to determine the anchoring requirements. As a minimum, a factor of safety of 1.25 shall be used (downward forces shall be no less than 1.25 times upward forces. For risers 10 feet or less in height, the anchoring may be done in one of the two following ways:
A concrete base 18 inches thick and twice the width of the riser diameter shall be used and the riser embedded six inches into the concrete.
A square steel plate, a minimum of 1/4 inch thick and having a width equal to twice the width or diameter of the riser shall be welded to the base of the riser. The plate shall then be covered with 2.5 feet of stone, gravel or compacted soil to prevent flotation.
Barrel. The barrel of the principal spillway, which extends through the embankment, shall be a circular reinforced concrete pipe set in a concrete cradle extending vertically to at least the midpoint of barrel along its length and extending horizontally at least six inches beyond the widest point of the barrel.
Anti-seep collars shall be used on the barrel of the principal spillway within the normal saturation zone of the embankment to increase the seepage length by at least 10% if either of the following conditions is met:
Anti-seep collars shall be installed within the saturation zone (phreatic line sloped at 4H:1V starting from interior face of dam at one-hundred-year maximum water surface elevation). Anti-seep collars shall be sized so that the flow length of water along the exterior of the outfall barrel is increased by a minimum of 15% for the length of the barrel within the saturation zone. The maximum spacing between collars shall be 14 times the projection of the collar above the barrel. Collars shall not be closer than two feet to a pipe joint. Connections between the collars and the barrel shall be watertight.
Emergency spillway. The emergency spillway shall consist of an open channel constructed adjacent to the embankment over undisturbed material. The emergency spillway shall be designed to carry the peak rate of runoff expected from the post-development one-hundred-year storm, with a minimum freeboard through the spillway of 1.0 foot.
Pond dam embankment. The pond dam embankment shall consist of all excavation on the project, except such material as may be determined unsuitable by the Township's representative, which includes, but is not limited to, frozen material, organic material, rock in excess of three inches in diameter, construction debris, sandy or other coarse grained soils, noncompactable soils and excessively wet or dry material. Acceptable material shall conform to PennDOT Publication 408, Section 206, latest revision, or shall be as approved by the Township's representative. The pond dam embankment shall be compacted to a minimum 95% standard proctor maximum dry density as per ASTM D698. The embankment shall be constructed and compacted in maximum eight-inch lifts and the principle spillway outfall barrel and anti-seep collar(s) shall be constructed integral with the dam embankment construction. Pond dam embankments shall not exceed a height of 15 feet as measured from the lowest downstream toe of the dam embankment to the dam crest. The minimum top width of the dam shall be six feet or equal to the dam's height, whichever is greater.
Side slopes for any embankments for detention/retention ponds shall be no steeper than a ratio of three to one (3:1) horizontal to vertical.
Inflow and outflow structures shall be designed and installed to prevent erosion, and bottoms of impoundment-type structures should be protected from soil erosion.
All inflow structures into detention/retention ponds shall discharge into a forebay designed in accordance with the Pennsylvania Stormwater Best Management Practices Manual.
Control and removal of debris both in the storage structure and in all inlet or outlet devices shall be a design consideration.
Inflow and outflow structures, pumping stations and other structures shall be designed and protected to minimize safety hazards.
The water depth at the perimeter of a storage pond should be limited to that which is safe for children. This is especially necessary if bank slopes are steep or if ponds are full and recirculating in dry periods. Restrictions of access (fence, walls, etc.) shall be necessary, depending on the location of the facility.
A safety bench with a minimum width of eight feet shall be provided around the entire interior perimeter of the pond at an elevation of six inches minimum above the two-year peak water surface elevation in the pond.
Landscaping shall be provided for the facility which harmonizes with the surrounding area. No trees or other woody vegetation shall be planted on the fill areas of the pond dam embankment that may result in loosening of the compacted soils of the embankment. Detention basin bottoms shall be vegetated with a diverse native planting mix and may include trees, woody shrubs and meadow/wetland herbaceous plants. Noxious and invasive plants that inhibit plant species diversity, as well as nonnative plant species, shall not be included in the plant mix. PennDOT seed mix Formulas B, C, D and L as per PennDOT Publication 408 latest revision, or approved equivalent mix shall be used for re-vegetation of embankment areas. Crownvetch and birdsfoot trefoil shall not be used due to their interference with performance of inspections of the dam embankment. For other suitable landscape plant species, refer to Appendix B of the Pennsylvania Stormwater Best Management Practices Manual.
Fence. Fences are not required for stormwater basins less than four feet in depth or for facilities having embankments sloped at 4H:1V or less, unless directed by the Township or its designee. For basins deeper than four feet or embankment slopes steeper than 4H:1V, a fence may be required to be installed around the pond at the discretion of the Township or its designee. Where fences are to be installed, a ten-foot wide gate with lock and keys must be provided to allow access for future maintenance. Fence details and specifications shall be submitted to the Township for approval. The fence requirements are waived where detention swales are utilized for storage. The maximum water depth in swales shall not exceed 2.0 feet with an average depth of 1.0 foot.
Access road. The developer shall provide a ten-foot wide access road constructed of 2B stone at a depth of four inches. Such access road shall run from the paved Township street to the stormwater management facility and extend to the bottom of the detention/retention facility.
Design criteria for stormwater infiltration facilities:
Stormwater infiltration facilities should be located as close to the runoff source as possible to replicate pre-development infiltration conditions to the maximum extent practicable.
Stormwater infiltration facilities shall be located a minimum horizontal distance of 20 feet from the nearest building or foundation wall.
Stormwater infiltration facilities shall not be located at the tops of fill slopes or on slopes greater than 15%.
Stormwater infiltration facilities shall be constructed to be as shallow as practicable with the bottoms of the facilities above the normal ground water elevation.
Where practicable, stormwater infiltration facilities shall be located in soils within the site having the highest in situ infiltration rates.
For infiltration systems located in moderately to poorly drained soils, underdrains shall be provided within the infiltration beds to allow for drainage of these systems between storm events. The design professional shall provide assurance that the infiltration systems will fully drain within no less than 24 hours but no greater than 72 hours between storm events.
A means for the appropriate handling of stormwater overflow shall be provided within the infiltration system.
The in situ soils within the vicinity of the infiltration systems shall be fenced off during site construction to minimize the compaction of these soils by heavy construction equipment.
Nonwoven geotextile fabric shall be used as a barrier between the infiltration media bed and the adjacent undisturbed earth.
Infiltration systems shall not receive any stormwater runoff until the drainage area to the infiltration system has been fully stabilized.
Easements shall be provided for all storm collection and conveyance facilities as well as all stormwater management facilities including easements for access of these facilities from a public right-of-way.
All outfalls from stormwater facilities shall discharge directly into an existing watercourse with defined bed and banks or tied directly into an existing storm sewer system. Where such existing watercourse or storm system is not available for direct discharge, a drainage easement must be established through all properties from the point of outfall to the existing receiving watercourse.
All outfalls from stormwater management facilities shall be equipped with energy dissipation devices to prevent erosion and conform to applicable requirements of the Pennsylvania DEP for stream encroachments (Chapter 105 of Pennsylvania DEP Rules and Regulations). Exit velocities of the design storms from the stormwater management facilities must be considered in the design of the energy dissipation devices. A stable means of conveyance of the discharge from the outfall and energy dissipation device must be provided to the receiving channel.