(a) Stormwater runoff from all development sites shall be calculated using the Rational Method, Modified Rational Method, or a Soil Cover Complex methodology.
(1) Any stormwater runoff calculations involving drainage areas greater than 200 acres, including on- and off-site areas, shall use 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) The Municipality may allow the use of the Rational Method or Modified Rational Method to estimate peak discharges from drainage areas that contain less than 200 acres.
(3) All calculations consistent with this Ordinance using the Soil Cover Complex method shall us the appropriate design rainfall depths. If a hydrologic computer model such as PSRM or HEC-RAS is used for stormwater runoff calculations, then the duration of rainfall shall be 24 hours. The SCS Rainfall Type II curve shall be used for the rainfall distribution.
(4) For the purposes of pre-development flow rate determination, undeveloped land, including areas to be disturbed as part of the Regulated Activity, shall be considered as "meadow" in good condition, unless the natural ground cover generates a lower curve number or Rational "C" value (i.e., forest), as listed in Tables 1 and 2, respectively.
(5) All calculations using the Rational Method shall use rainfall intensities consistent with appropriate times-of-concentration for overland flow and return periods. 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). Time-of-concentration for channel and pipe flow shall be computed using Manning's equation.
(6) Runoff Curve Numbers (CN) for both existing and proposed conditions to be used in the Soil Cover Complex method shall be obtained from Table 1.
(7) Runoff coefficients (c) for both existing and proposed conditions for use in the Rational Method shall be obtained from Table 2.
(8) Where uniform flow is anticipated, the Manning equation shall be used for hydraulic computations such as the capacity of open channels, pipes, and storm sewers. Values for Manning's roughness coefficient (n) shall be consistent with Table 3.
(9) The design of any stormwater detention facilities intended to meet the performance standards of this Ordinance shall be verified by routing the design storm hydrograph through these facilities, using either manual methods or computerized routing. Routing shall be based upon the modified PULS method; other routing methodologies shall be subject to the approval of the Municipal Engineer.
(10) The stormwater collection system shall be designed using the peak discharge computed using the Rational Formula.
(b) Design Standards - Water Carrying Facilities.
(1) All storm sewer pipes, streets, and inlets (excluding detention and retention basin outfall structures) shall be designed for a 10-year storm event. Sole access structures (culverts and bridges) shall be designed to convey the 25-year flood without overtopping the roadway.
A. When a pipe or culvert is intended to convey the discharge from a stormwater management facility, its required capacity shall be computed by the rational method and compared to the peak outflow from the stormwater facility for the 100-Year storm. The greater flow shall govern the design of the pipe or culvert.
B. When a pipe is part of a storm sewer system and crosses the roadway, it shall be designed as a storm sewer with the same design storm as the remainder of the drainage system.
C. Greater design frequencies may be justified on individual projects.
D. A 100-year storm frequency may be required for design of the stormwater collection system to insure that the resultant stormwater runoff from the post-development storm is directed into the management facility.
(2) In general, inlets shall be spaced such that, based upon the Rational Method, tc = 5 min. and 10-year rainfall intensity, the area contributing to the inlet shall not produce a peak runoff of greater than 4 cfs. Also, inlets shall be spaced so that their efficiency, based upon efficiency curves published by the Pennsylvania Department of Transportation, is not less than 65%.
(3) Inlets shall be placed on both sides of the street at low spots and at the upper side of street intersections to prevent stormwater from crossing an intersection. Other devices such as high efficiency grates or perforated pipe may be required if conditions warrant. All inlets at low points along the roadway shall have a 10" curb reveal and shall be equipped with pavement base drain extending 50 feet in either direction, parallel to the centerline of the roadway.
(4) In all cases where drainage is picked up by means of a headwall, the pipe shall be designed as a culvert. Inlet and outlet conditions shall be analyzed. The minimum diameter of culvert shall be 18 inches. The procedure contained in Hydraulic Engineer Circulars No. 5 and No. 13, as prepared by the U. S. Department of Transportation, Federal Highway Administration, Washington, D.C., shall be used for the design of culverts. All culverts shall include concrete headwalls and endwalls.
(5) Guards shall be provided on all intake and outfall structures as well as outlet structures. The guard bars shall be one-half inch (½") diameter galvanized bars on six inch (6") centers attached to the structure with three- eighth inch (3/8") diameter stainless steel anchors. Guards shall also be provided for any pipe opening, 18" in diameter or larger.
(6) Manholes, inlets, headwalls, and endwalls shall conform to the requirements of the PennDOT Publication 408, as modified by the adopted Municipal Standards.
(7) Proposed channels or swales must be able to convey the increased runoff associated with a proposed 100-year return period event within their banks at velocities consistent with protection of the channels from erosion. Acceptable velocities shall be based upon criteria included in the PA DEP Erosion and Sediment Pollution Control Program Manual.
(8) Existing natural or man-made channels or swales must be able to convey proposed 100-year return period runoff without creating any hazard to persons or property.
(9) Stormwater runoff on roadways (i.e. gutter spread, lane encroachment, etc.) shall be controlled in accordance with PennDOT Publications 13M, "Design Manual, Part 2" and 584, "Drainage Manual".
(c) Design Standards - Detention and Retention Basins.
(1) Permanent Detention and Retention Basins shall be designed to meet the following standards:
A. Outlet Control Structures - Outlet control shall be accomplished utilizing (6" diameter or 6" maximum) perforations arranged vertically to provide for positive control of stormwater runoff. Outlet controls shall also provide for modification off the orifice to a smaller diameter through the use of removable plates.
B. Discharge Dispersion - Discharges from piping outlets of management facilities shall be provided with a concrete "level spreader" to convert point discharge back to simulated sheet flow. The length of the level spreader shall be equal to 10 times the outlet pipe diameter (e.g., an 18" discharge pipe would require a 15" wide level spreader).
C. Minimum Bottom Slope - All detention basins shall have a minimum bottom slope of two percent (2%) unless infiltration facilities are provided.
D. The maximum permitted depth for detention or retention basins shall be 6 feet, measured from the bottom of the emergency spillway to the lowest point in the basin.
E. The minimum top width of all basin embankments shall be 8 feet.
F. The maximum permitted side slopes for detention or retention basins shall be 4 horizontal to 1 vertical. In order to obtain a waiver for slopes steeper than 4:1, the plan must include a planting schedule to stabilize the embankments. The proposed vegetation shall be low maintenance varieties.
G. Any stormwater management facility (i.e., detention basin) designed to store runoff and requiring a berm or earthen embankment required or regulated by this Ordinance shall be designed to provide an emergency spillway to handle flow up to and including the 100-year, 24 hour design storm at post-development conditions, assuming the principal outlet structure to be clogged. The height of embankment must be set as to provide a minimum 1 foot of freeboard above the maximum elevation computed for the clogged orifice condition. Should any stormwater management facility require a dam safety permit under PA DEP 25 PA Code Chapter 105, the facility shall be designed in accordance with PA DEP 25 PA Code Chapter 105 and meet the regulations of PA DEP 25 PA Code Chapter 105 concerning dam safety which may be required to pass storms larger than 100-year event.
H. A cutoff trench of impervious material shall be provided within all basin embankments.
I. Where a basin embankment is constructed using fill on an existing 15% or greater slope, the basin must be keyed into the existing grade.
J. Fencing. Any above-ground stormwater management detention/retention facility, that is designed to store at least a two foot (2') depth of runoff, shall be subject to the following fencing requirements:
1. Stormwater facility must be completely surrounded by a chain link fence of not less than four (4) feet in height. Alternative fences and barriers may be permitted upon request to and approval by the Municipality.
2. All gates or doors opening through such enclosure shall be equipped with a self-closing and self-latching device for keeping the gate or door securely closed at all times, when not in actual use.
K. All outlet structures and emergency spillways shall include a satisfactory means of energy dissipation at its outlet to assure conveyance and flow without endangering the safety and integrity of the basin and the downstream drainage area.
L. A concentrated discharge of stormwater to an adjacent property shall be within a natural drainage way or watercourse, or an easement shall be required.
M. Easement - Plans showing outlet control structures shall contain an easement dedication as follows: "An easement is hereby granted to the City of York to access and modify the basin outlet control device at the expense of the Developer so as to function within design parameters."
N. Plans for infiltration must show the locations of existing and proposed septic tank infiltration areas and wells. A minimum 25 foot separation from On Lot Disposal Systems (OLDS) infiltration areas, including replacement areas, is desired and will be evaluated by the Municipality on a case by case basis. However, the separation shall not be less than the PA DEP required 10 feet. Infiltration rates shall be based upon perk and probe tests conducted at the site of the proposed facility.
(Ord. 32-2011. Passed 10-4-11.)