(A) Runoff. Stormwater runoff which may result from regulated activities shall be controlled by permanent stormwater BMPs that will meet the required standards within this subchapter. The methods of stormwater control or BMPs which may be used to meet the required standards are described in this chapter and the BMP Manual. While the choice of BMPs is not limited to the ones appearing in this chapter and the Manual, any selected BMP must meet or exceed the runoff peak rate requirements of this chapter for the applicable stormwater management district.
(B) Located near highways. Any stormwater facility located on state highway rights-of-way shall be subject to approval by PennDOT.
(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 curb line 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 to be dedicated to the township shall be Class III reinforced concrete pipe, except when pipe class and strength is required to be increased in accordance with PennDOT specifications. 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 18 inches (or an equivalent flow area of one and seventy-six hundredths (1.76) square feet).
(4) Inlet and manhole construction. Inlet and manhole castings and concrete construction shall be equivalent to PennDOT design standards. Manhole castings and grated 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.
(5) Storm drain notification marking. All storm sewer inlets must be identified with a storm drain marker. Storm drain markers shall be cast in the inlet hood, or a stainless steel marker affixed to the inlet hood with adhesive, rivets or bolts. (Marker may be bolted to the grate in off road locations). Markers shall have a minimum diameter of three and one-half inches and include “No Dumping - Drains to Waterway” and a fish symbol. Alternate designs/sizes may be used as approved by DEP.
(6) End pipes. Open end pipes must be fitted with concrete endwalls or wing walls in accordance with PennDOT standards.
(7) 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 0.5%.
(8) Inlets and manholes. Inlets and manholes shall be spaced at intervals not exceeding 300 feet, and shall be located wherever branches are connected or pipe 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.
(9) Bedding/backfill. Storm sewer bedding/backfill requirements shall conform to the construction details set forth in Appendix L of this chapter.
(10) Inlets. Inlets shall be located to intercept concentrated runoff prior to discharge over public/private rights-of-way, sidewalks, streets and driveways.
(11) Type C inlets. The capacity of all Type C inlets shall be based on a maximum surface flow to the inlets of four cubic feet per second (cfs), calculated based on the 100-year frequency design storm event. The maximum flow to Type C inlets located in low points (e.g. 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 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 design professional can verify that such an inlet would not cause stormwater to accumulate on any adjacent public or private property or outside of an associated storm sewer easement, and that the depth of the accumulated stormwater would not exceed 12 inches.
(12) Inlet and outlet elevation. 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.
(13) Depth cover. Stormwater pipes shall have a minimum depth of cover of 12 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.
(14) Capacity. 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 thirteen-hundredths (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.
(15) Culvert design. 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 elevation of the hydraulic grade line shall be calculated 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.
(16) Sewer structures location. Storm sewer structures (e.g. endwalls, inlets, sections and the like) may not be located on top of, or within ten feet of, electric, communication, water, sanitary sewer or gas services and/or mains, unless approval is received from the township and the authority or utility having jurisdiction over same.
(17) Pipe angles. 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 degrees 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 degrees. Vertical and horizontal design of storm sewer must be linear.
(18) Underdrain. 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.
(19) Right-of-way; public property. Where a public storm sewer system is not located within a right-of-way or dedicated public property, a 20-foot wide utility easement shall be established to encompass the storm sewer system. For multiple pipes or utilities, the width of the easement shall be a minimum of 30 feet.
(20) Freeboard. 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.
(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 fourty thousandths (0.040) for earth swales.
(2) Bank slopes. Slopes for swale banks shall not be steeper than one vertical to four horizontal (4:1).
(3) Flow velocity. The maximum velocity of flow as determined by Manning’s formula 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 Conservation District.
(4) Note. Source of the following design criteria is the DEP, Bureau of Soil and Water Conservation Publication, Erosion and Sediment Control Program Manual.
Allowable Velocity | |||
Material | Velocity in Feet per Second (fps) | ||
Allowable Velocity | |||
Material | Velocity in Feet per Second (fps) | ||
Earth without vegetation | |||
Clay and gravel | 4.0 to 5.0 | ||
Coarse gravel | 4.0 to 5.0 | ||
Fine sand or silt | 1.0 | ||
Ordinary firm loam | 2.0 to 3.0 | ||
Soft shale | 5.0 to 6.0 | ||
Stiff clay | 3.0 to 5.0 | ||
Shoulders | |||
Earth (as defined above) | |||
Paved | 10.0 to 15.0 | ||
Stabilized | 6.0 | ||
Well-established grass on good soil | |||
Bunch grass - soil exposed | 2.0 to 3.0 | ||
Short pliant bladed grass | 4.0 to 5.0 | ||
Stiff stemmed grass | 3.0 to 4.0 | ||
(5) Swales. Swales shall be stabilized with bio-degradable erosion control matting to permit establishment of permanent vegetation (or sodded). 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.
(6) Public ways. 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.
(7) Gutters and swales. 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.
(8) Excess flow. 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.
(9) Underdrains. 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.
(10) Swales diverting surface waters. 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 with 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.
(E) Bridge and culvert design. Any proposed bridge or culvert proposed to convey the flow of a perennial or intermittent stream shall be designed in accordance with the following principals.
(1) Culverts and bridges. 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 normal water level and unimpeded movement of native animal species.
(2) Culvert opening. Bottom of opening of a culvert shall, at a minimum, 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) Flow rate. 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, Natural Resources Conservation Service (NRCS), 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 flood plain.
(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 and the like.
I = Intensity: Average Rainfall Intensity in inches per hour for a time equal to the time of concentration (in./hr.). Use 6.20 inches for the 100-year event (Pennsylvania State Climatologist https://climate.met.psu.edu/links/).
A = Area: Drainage area in acres (ac).
(2) Rational C coefficient. 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) Peak flow rate. In determining the peak flow rate to individual storm sewer inlets (or other collection structures) the time of concentration method (as referenced in § 151.033) shall be used for inlet drainage areas greater than 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) Design flow rate for piping system. 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 30-inch diameter pipe (or equivalent flow area of four and nine-tenths (4.9) square feet), the time of concentration approach method (as referenced in § 151.033) shall be used in determining storm duration.
(5) Overflow system. An overflow system shall be provided to carry all bypass flow and/or flow in excess of storm sewer design capacity to the detention basin (or other approved outlet point) when the capacity of the storm sewer system is exceeded. Stormwater runoff will not be permitted to surcharge from storm sewer structures.
(G) Grading and drainage.
(1) Topsoil. After completion of rough grading, a minimum of eight inches of topsoil shall be returned to disturbed areas prior to final grading and seeding.
(2) Drainage. Lots shall be graded to secure proper drainage away from buildings and to prevent the collection of storm water in pools. Minimum 2% slopes shall be maintained away from and around all structures. Separation between the top of foundation wall (or slab) shall comply with Township Building Code requirements.
(3) Construction. The developer shall construct and/or install such drainage facilities which are necessary to prevent erosion damage and to satisfactorily carry off such surface waters to the nearest infiltration structure, street, 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 a ration of four to one (4:1) and a written statement (certification) from an engineer experienced in erosion control to this effect is submitted for review by the Township Engineer. 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; and
(b) A concrete, segmental block or stone masonry wall, constructed in accordance with requirements of the township zoning ordinance and Construction Codes, 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 (certification) from an engineer 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 for review by the Township Engineer; and
(c) A concrete, segmental block or stone masonry wall, constructed in accordance with requirements of the township zoning ordinance and Construction Codes, 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 lines 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 (2:1)) are approved under the criteria in this chapter, and are five feet or more in height, a protective fence conforming to Township Construction Codes shall be required at the top of the wall (or bank).
(7) Cut and fill operations. 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.
(8) Energy dissipation. Design of energy dissipation for high volume and/or high velocity discharge from storm sewer pipes 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 Conservation District.
(9) Surface water damage. Adequate provision shall be made to prevent surface water from damaging the cut face of excavation and the sloping surfaces of fills.
(H) Stormwater management basins.
(1) Basin design. 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.
(2) Prohibited in rights-of-way. Stormwater management basins shall not be permitted within the ultimate right-of-way of public streets, riparian buffers or conservation easements.
(3) Landscaping. Landscaping and planting in and around the perimeter of basins shall be provided. It shall be aesthetically pleasing and compatible with surrounding land uses and require minimum maintenance. Proposed planting shall also be in accordance with the provisions of this chapter, the township’s subdivision and land development ordinance and the township’s zoning ordinance.
(4) Temporary sediment control device. 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, clean-out stakes and other measures as may be required by State Department of Environmental Protection, Chapter 102 Regulations. Plans for such facilities shall require the 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 required on the record plan of subdivisions and land developments, as well as, in the development agreement with the township.
(5) Prior to construction. Stormwater management basins shall be in place before construction of any new impervious surfaces on the site.
(6) Runoff. Runoff shall not be directed to any infiltration structure until all tributary drainage areas are stabilized.
(7) Permanent retention facilities. Where permanent retention facilities (ponds) are proposed, there shall be a safety ledge, ten feet wide, no greater than 15 inches and no less than 12 inches below the permanent water surface level.
(8) Slope. All basins shall have slopes of four horizontal to one vertical (4:1 = 25%), or flatter on the basin’s outer berm and three horizontal to one vertical (3:1) or flatter 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 or ultimate right-of way line. The maximum difference between the top of berm elevation and the invert elevation of the outlet structure shall be seven feet.
(9) Dry detention basin slope. All portions of a dry detention basin bottom (non-wetland) shall have a minimum slope of 2%.
(10) Embankment. All basin embankments shall be placed in lifts not to exceed one foot in thickness and each lift shall be compacted to a minimum of 95% of modified proctor density as established by ASTM D-1557. Prior to proceeding to the next lift, the compaction shall be checked by a soils engineer employed by the applicant/developer. 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.
(11) Emergency overflow. Emergency overflow facilities/spillway shall be provided with 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 checker-blocks to protect the berm against erosion. The checker-blocks shall be backfilled with topsoil and seeded. Checker-block 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 (e.g., not discharging over fill material). A dense cover of vegetation shall be rapidly established in spillways by sodding or seeding with a geotextile anchor. The minimum capacity of all emergency spillways shall be equivalent to the peak flow rate of the 100-year, post-development design storm (entering to the basin).
(12) Basin discharge end. 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 and culvert pipe are not permitted for permanent basins.
(13) Basin berm. Minimum top of basin berm width (at the design elevation) shall be ten feet, unless otherwise approved by the township. A cut-off trench (keyway) of impervious material shall be provided under all embankments that require fill material. The cut-off trench shall be a minimum of eight feet wide, three feet deep and have side slopes of one horizontal to one vertical (1:1).
(14) Freeboard. 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.
(15) Anti-seep collars. Anti-seep collars shall be installed around the outfall pipe barrel and shall be centered within the normal saturation zone of the berm. The anti-seep collars and their connections to the pipe barrel shall be watertight. The anti-seep collars shall be cast-in-place 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. A minimum of two collars shall be installed on each basin outlet pipe.
(16) Basin outlet joints. All basin outlet pipes shall be watertight reinforced concrete having O-ring joints. All joints shall be mortared.
(17) Outlet structure elevation. A minimum of six inches is required between the top of outlet structure box and the emergency spillway elevation. Six inches, minimum, is also required between the 100-year water surface elevation and the top of outlet structure box.
(18) Energy dissipating devices required. 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 DEP, Bureau of Soil and Water Conservation Publication, Erosion and Sediment Control Program Manual, latest revision.
(19) Stone gabion baskets prohibited. Stone gabion baskets shall not be permitted for use in construction of detention/retention basins.
(20) An access easement and drive. An access easement and stabilized drive shall be provided to every stormwater detention facility 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 reinforced concrete checker-block (backfilled with topsoil and seeded) or other similar paver acceptable to the Township Engineer, over a six-inch bed of compacted PennDOT Tyle III. A coarse aggregate (or approved equivalent). Accessways to basins shall be a minimum of ten feet wide and be no steeper in slope than ten feet horizontal to one foot vertical (10:1). In addition, depressed curb and concrete apron shall be provided where the accessways 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. Access easement shall be owned and maintained by the individual lot owner(s) or homeowner’s association but shall be established to permit access by the township or its designee, for inspection, maintenance, repair, renovation, rehabilitation and/or replacement, at any reasonable time.
(21) Visual impact. To minimize the visual impact of detention basins, the basin shall be designed to avoid the need for safety fencing.
(I) General design requirements.
(1) Plan details. Prior to finish grading of a 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 specifications, such as bituminous “eyebrows” at inlets, diversion berms and the like.
(2) Discharge and drainage. Water originating from human-made sources, such as air conditioning units, sump pumps or other dry weather flow, wherever practical and possible, shall discharge to infiltration areas or vegetative BMPs, or at the approval of the township, shall be connected to a storm sewer, street drainage structure, or other approved drainage conveyance facility designed as part of a stormwater management BMP.
(3) Phases of completion. When subdivisions or land developments are submitted to the township for approval in phases, a complete storm sewer design for the entire proposed subdivision/land development shall be submitted. The proposed design must include the entire tract and not a phase or a portion.
(Ord. 239, passed 10-14-2020)