A. Stormwater detention facilities. Stormwater detention facilities include all structural measures which can reliably and predictably achieve the peak discharge requirements of this chapter. Stormwater detention facilities include, but are not necessarily limited to, detention basins, retention basins, bioretention areas, open (at-grade) sand filters, closed (below-grade) sand filters, water quality inlets, dry wells, below-grade detention chambers and rooftop detention.
B. Peak discharge design storms. The design storm criteria to be used in calculations for the watershed is to limit the post-development runoff for the one-, two-, ten-, fifty- and one-hundred-year storms to the corresponding predevelopment rates. Any stormwater detention facilities required by this chapter and subject to the water quality requirements and stormwater runoff peak rate requirements herein shall meet the applicable water quality and peak rate requirement for the one-, two-, ten-, fifty- and one-hundred-year return period runoff events (design storms) consistent with the standard and accepted calculation methodology and engineering standards and be satisfactory to the Township Engineer.
C. Runoff calculation methodology.
(1) Any stormwater runoff calculation involving drainage areas greater than 20 acres, including on- and off site areas, shall use a generally accepted calculation technique that is based on the NRCS soil cover complex method. It is assumed that all methods will be selected by the design professional based on the individual limitations and suitability of each method for a particular site.
(2) All calculations consistent with this chapter using the soil cover complex method shall use the appropriate design rainfall depths for the various return periods cited.
(3) For purposes of predevelopment flow rate determination, undeveloped land shall be considered as meadow, in good condition, unless the natural ground cover generates a lower curve number or Rational C value. Landowner is to use the lower runoff coefficient value.
(4) All calculations using the Rational Method shall use rainfall intensities consistent with appropriate times of concentration for overland flow and return periods from NCRS Methodology. Time of concentration for sheet flow (maximum 150 feet) and concentrated flow shall both be calculated using NRCS methodology. Times of concentration for channel and pipe flow shall be computed using Manning's Equation or NCRS Methodology.
(5) The design of any stormwater detention facilities intended to meet the performance standards of this chapter shall be verified by routing the design storm hydrograph through these facilities using accepted methods of practice. The Township Engineer may approve the use of any generally accepted reservoir routing technique which shall use a total runoff volume that is consistent with the volume from a method that produces a full hydrograph. The computer routing program used must take into account the tailwater effect of the discharge pipe on the outlet design as well as the submergence of the discharge pipe outlet.
(6) 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 approved by the Township Engineer.
D. Stormwater detention and retention facilities, Stormwater detention and retention facilities shall meet the following minimum design/construction standards:
(1) Detention basins shall be designed to allow regular maintenance, mowing and periodic silt removal and reseeding. Shallow broad basins are preferred to steep sided basins.
(2) The maximum slope of the basin embankment shall be 3 to 1. Whenever possible, the side slope and basin shape shall conform to the natural topography.
(3) Unless approved as a conditional use, detention basins shall not be located within riparian corridors, floodplains nor within areas of floodplain or alluvial soils.
(4) Detention basins shall be designed so they return to normal conditions within approximately 24 hours after termination of the storm, unless the Township Engineer finds that downstream conditions may warrant other design criteria for stormwater release.
(5) If retention basins are used, the applicant shall demonstrate that such ponds are designed to protect public health, safety and welfare.
(6) Fences may be required for any detention or retention basins where there is a permanent water surface or conditions warrant.
(7) The minimum top width of a basin berm shall be 10 feet. A cut-off trench (keyway) of relative impervious material shall be provided beneath all embankments requiring fill material. The keyway shall be a minimum 8 feet wide, minimum three feet deep and have 1 to 1 side slopes.
(8) Unless designed to be a naturalized basin, in order to ensure proper drainage on a basin bottom, a minimum grade of 2% shall be maintained for sheet flow. If deemed necessary by the Township Engineer, an underdrain shall be placed in the basin floor to help dry the basin.
(9) All basin embankments shall be placed in eight-inch maximum lifts to a minimum dry density of 95%. Prior to proceeding to the next lift, compaction shall be checked by the Township Engineer or an approved soils engineer who shall provide the Township Engineer with a written report. Compaction tests shall be performed using the modified proctor method in accordance with ASTM D-1557. Compaction tests shall be run on the leading and trailing edge as well as the top of the berm.
(10) Emergency overflow facilities shall be provided for facilities to accommodate runoff in excess of design flows. Emergency spillways for basins shall be constructed on undisturbed ground. If the emergency spillway is constructed on filled ground, the emergency spillways shall be constructed of concrete payers, gabions or other similar materials approved by the Township Engineer. All emergency spillways shall be constructed so that the detention basin berm is protected against downstream embankment erosion if the basin berm is overtopped. The minimum capacity of the emergency spillways shall be the peak flow rate of the one-hundred-year design storm after development. The construction material of the emergency spillway shall extend along the upstream and downstream berm embankment slopes. The upstream edge of the emergency spillway shall be a minimum of three feet below the spillway crest elevation. The downstream slope of the spillway shall, as a minimum, extend to the toe of the berm embankment. The emergency spillway shall not discharge over earthen fill or easily erodible material.
(11) The minimum freeboard shall be one foot as measured from the top of the post-development one-hundred-year flood elevation over the spillway and the top of the berm.
(12) Antiseep collars shall be installed around the pipe barrel within the normal saturation zone of the detention basin berms. The antiseep collars and their connections to the pipe bands shall be watertight. The antiseep collars shall extend a minimum of two feet beyond the outside of the principle pipe barrel. The maximum spacing between collars shall be 14 times the minimum projection of the collar measured perpendicular to the pipe. A minimum of two antiseep collars shall be installed on each outlet pipe.
(13) All outlet pipes through a basin berm shall be reinforced concrete pipe, designed to withstand the loading caused by a fully saturated berm and shall have watertight joints using 0-ring joint pipe. Outlet pipes shall be backfilled with material similar to the core material (semi-impervious).
(14) The invert of the inlet pipe(s) into a basin shall be above the basin floor or lining so that it can adequately drain after rainstorms. Inlet pipe(s) shall discharge to areas of the basin that slope toward the outlet structure, having the minimum slope as prescribed in Subsection D(8) above.
(15) Energy dissipaters and/or level spreaders shall be installed at points where pipes or drainageways drain to or from the basin. Energy dissipaters shall comply with criteria in Hydraulic Engineering Circular No. 15 - Design of Stable Channels with Flexible Linings, published by the Federal Highway Administration of the United States Department of Transportation or the Engineering Field Manual for Conservation Practices, NCRS energy dissipating device calculations shall be submitted for township review and approval.
(16) Inlet and outlet structures shall be located at a maximum distance from one another in order to promote water quality benefits. The Township Engineer may require a rock filter or rock-filled gabion for entrapping sediments carried in stormwater if sufficient separation of inlet and outlet structures cannot be achieved. The use of forebays as a sediment removal feature is encouraged for all basins.
(17) A perforated riser or similar sediment control device shall be provided at each outlet of all detention basins during construction for sediment control. The riser shall be constructed of metal or concrete. The riser shall extend to a maximum elevation of two feet below the crest elevation of the emergency spillway. The perforated riser shall be designed so that the rate of outflow is controlled by the pipe barrel through the basin berm when the depth of water within the basin exceeds the height of the riser. Circular perforations with a maximum diameter of one inch shall be spaced 12 inches vertically. The horizontal spacing shall be in accordance to DEP Soil Erosion and Sedimentation Control Manual Specifications. The perforations shall be cleanly cut and shall not be susceptible to enlargement. All metal risers shall be suitably coated to prevent corrosion. A trash rack or similar appurtenance shall be provided to prevent debris from entering the pipe. All risers shall have a concrete base attached with a watertight connection. The base shall be of sufficient weight to prevent flotation of the riser. An antivortex device consisting of a thin vertical plate normal to the base and berm shall be provided at the top of the riser. Unless this structure is part of the permanent outlet control, it shall be removed from the site when it has been adequately stabilized as determined by the Township Engineer.
(18) All drainage channels shall be designed to prevent erosion of the bed and banks. The maximum permissible flow velocity shall not exceed the design requirements outlined in the current Soil Erosion and Sedimentation Control Manual, published by the Pennsylvania Department of Environmental Protection. Suitable stabilization shall be provided where required to prevent erosion of the drainage channels. The design procedure used shall be that contained in the current Soil Erosion and Sedimentation Control Program Manual, published by the Pennsylvania Department of Environmental Protection.
(19) Any vegetated drainage channel requiring mowing of the vegetation shall have a maximum grade of three horizontal to one vertical on those areas to be mowed.
(20) Because of the critical nature of vegetated drainage channels, the design of all vegetated channels shall, as a minimum, conform to the design requirements outlined in the current Soil Erosion and Sedimentation Control Program Manual, published by the Pennsylvania Department of Environmental Protection.