A. General.
(1) Storm sewers, culverts, bridges and related installations shall be provided to:
(a) Permit unimpeded flow of natural watercourses and in such a manner as to protect the natural character of the watercourses and to provide regulated discharge.
(b) Ensure adequate drainage of all low points along the line of streets.
(c) Intercept stormwater runoff along streets at intervals reasonably related to the extent and grade of the area drained and to prevent substantial flow of water across intersections.
(2) All storm sewer system components shall conform to current Upper Dublin Township Engineering and Construction Standards and PennDOT standards for materials, manufacture and construction.
(3) Drainage structures, which drain watershed areas in excess of one-half square mile (320 acres) or which have a span of eight feet or more, shall be designed for a maximum expected runoff as calculated using the Soil Conservation Service Technical Release 55, Urban Hydrology for Small Watersheds (Less Than 2000 Acres).
(4) The design storm of all cross roadway culverts shall be one-, two-, five-, ten-, twenty-five, fifty- and one-hundred-year storm. A water obstruction permit shall be obtained from DEP for the waterway opening before final design is undertaken.
(5) The cartway over a culvert or bridge shall be as wide as the ultimate width of the roadway approaches. Additional width may be required to provide sidewalks on one or both sides of the cartway.
B. Storm sewer design and construction requirements.
(1) Minimum pipe size is 18 inches.
(2) Minimum pipe slope shall be 0.005 foot per foot.
(3) Minimum drop across junctions shall be two inches. At changes in pipe diameter, pipe crowns shall be matched at junctions (manhole, inlet or junction box).
(4) Maximum distance between junctions shall be 300 feet.
(5) Runoff to proposed storm sewers and inlets shall be calculated using the Rational Method.
(6) The time of concentration shall be assumed five minutes for pipes under 30 inches. For pipes 30 inches or greater, the calculated time of concentration can be utilized.
(7) The time of concentration to inlets for grate capacity calculations shall be five minutes.
(8) All storm sewer pipes shall be designed at a minimum to accommodate a ten-year storm. Twenty-five-year storms shall be used if required by the Township Engineer.
(9) All storm sewer pipes downstream of inlets in sump condition shall be designed to accommodate the fifty-year storm.
(10) All storm sewer pipes and inlets intended to drain to detention facilities shall be designed to accommodate the one-hundred-year storm if any of the stormwater runoff bypass or overflow will not reach the basin by overland flow. In cases where the bypass or overflow will flow over land, a stable swale shall be constructed to accommodate the excess floods in a non-erosive manner
(11) All inlets in sump condition shall be six-foot inlets or dual four-foot inlets, as needed.
(12) All storm sewer systems shall be analyzed for both inlet and outlet control (including tailwater effects) by using the equations and nomographs as shown in the FHA's Hydraulic Design Services No. 5. In lieu of this, computer programs that calculate the actual hydraulic grade line for the storm sewer system can be used, provided that all losses (friction, bend, junction, etc.) are taken into account. Documentation for the program must be submitted for approval.
(13) Minimum cover over pipes is two feet from finished grade to outside of pipe bell.
(14) Inlet capacities shall be calculated using PennDOT or manufacturer's nomographs. Documentation for manufacturer's nomograph must be provided to the Township Engineer.
C. Shoulders in cut areas (without swales).
(1) Water flowing in the shoulder shall not encroach more than two-thirds the shoulder width during a twenty-five-year frequency storm of five-minute duration.
(2) The maximum velocity as determined by Manning's Equation shall not exceed the allowable velocities for the specific type of shoulder material.
(3) Inlets shall be provided to control the shoulder encroachment and water velocity.
D. Swales adjacent to shoulders.
(1) Swales in cut areas shall be designed to prevent the passage of water on the cartway during a twenty-five-year frequency storm of five-minute duration.
(2) The maximum velocity as determined by Manning's Equation shall not exceed the allowable velocities for the specific type of shoulder material.
E. Curbed sections.
(1) Inlets shall be provided to control the encroachment of water on the pavement.
F. Inlets, general.
(1) At street intersections, inlets shall be placed in the tangent portion, rather than the curved portion, of the curbing.
(2) If the capacity of the shoulder, swale, curb section or depressed median section exceeds the assumed inlet capacities, the inlet capacities shall govern the spacing of inlets.
(3) If the capacity of the shoulder, swale, curb section or depressed median section is less than the inlet capacities, then the shoulder, swale, curb section or depressed section capacity shall govern the spacing of inlets.