A. 
Method of computation. Computations for determining stormwater runoff and peak discharge for the design of stormwater management facilities shall be based upon the Soil Cover Complex Method described in TR-55, Urban Hydrology for Small Watersheds; the United States Department of Agriculture, Natural Resource Engineering Field Manual; or TR-20, where appropriate; excepting that the Rational Method may be used for computing roof and driveway water runoff figures for seepage trench, seepage tank, seepage pit use, etc., for individual residences or drainage areas less than 10 acres.
B. 
Rainfall frequency data. The frequency of rainfall shall be a two-year base frequency; a five-year base frequency; a ten-year base frequency; a twenty-five-year base frequency; a fifty-year base frequency; and a 100-year frequency.
C. 
Maintenance of natural drainageways. All natural streams, channels, swales, drainage systems and/or areas of surface water concentration shall be maintained in their existing condition unless a modification is approved by the municipality. All encroachment activities shall comply with Chapter 105, Water Obstructions and Encroachments of Title 25 of the Pennsylvania Code, Rules and Regulations of the Pennsylvania Department of Environmental Protection, latest revision.
D. 
Methods of stormwater runoff detention and control. The following is a list of detention and control methods suitable for stormwater management systems:
(1) 
Seepage pits, trenches, seepage tanks or other infiltrations structures.
(2) 
Detention and/or retention structures.
(3) 
Cisterns and underground reservoirs.
(4) 
Grass channels and vegetative strips.
(5) 
Routed flow over grass.
(6) 
Decreased impervious areas coverage.
(7) 
Porous pavement and concrete lattice block surface.
(8) 
Rooftop storage.
(9) 
Parking lot and street ponding.
(10) 
Other control methods which meet the criteria of this section, when approved by the Municipal Engineer.
A. 
Total system requirements. All predevelopment calculations shall be based upon existing land use features; however, agricultural lands shall be considered as using conservation treatment or in good condition irrespective of the current use. Predevelopment stormwater runoff shall be calculated for all storm frequencies.
(1) 
Release rates shall be based on the runoff from the two-year predevelopment storm event.
(2) 
Storage structures shall be designed that the post-development, ten-year peak discharge will not exceed the predevelopment, two-year peak discharge. Twenty-five-year, fifty-year and 100-year peak discharge rates shall not exceed the predevelopment peak discharge for that storm event. The height of the impoundment embankment shall be at least two feet above the discharge elevation of the emergency spillway.
(a) 
Detention or retention basins in carbonate areas shall be lined so that the permeability of all wetted areas shall not exceed a permeability factor of K = 1x10-6.
(b) 
Basins designated to be wet bottom ponds shall have sufficient constant flow to eliminate stagnation and health hazards. Alternate mechanical means will be deemed suitable if approved by the Municipal Engineer.
(3) 
All storage structures or facilities will be designed with emergency spillways sufficient to handle the 100-year, post-development storm event without causing major property damage even when the principal spillway is completely blocked. The emergency spillway shall be at least six inches above the principal spillway.
(4) 
Culverts, pipes and other water carrying structures shall be designed to handle the peak discharge from the twenty-five-year, post-development storm event. All pipes shall be provided with end section or end wall.
(5) 
All storage structures shall be designed that the post-development, twenty-five-year, fifty-year and 100-year peak discharges will not exceed the equivalent predevelopment peak discharge.
B. 
Stormwater runoff volumes.
(1) 
Stormwater runoff shall be based on the following 24-hour storm events or other valid data as deemed suitable by the Municipal Engineer:
Storm Frequency
Storm Volume in
(inches of rainfall)
2-year
3.1
5–year
4.1
10-year
4.9
25-year
5.5
50-year
6.1
100-year
7.0
(2) 
Rational Method. Storm volumes shall be in accordance with PennDOT Field Manual, Storm Intensity Duration, Frequency Charts, last revision for Region 5.
C. 
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 maximum allowable street flooding and drainageway capacity. When a possibility of clogging of grates, side opening or combination inlets exists, the capacity reduction factors shown in Table 425-49 must be applied to the theoretical capacity of the inlet. The maximum amount of water that shall be bypassed on to the next downstream inlet or inlets on continuous grades is 10%.
D. 
Pipes. The minimum allowable pipe diameter shall be 15 inches, unless reduced pipe size is approved by the Municipal Engineer. Horizontal and vertical curves with radii of 100 feet or greater are allowed for all pipe sizes 30 inches in diameter or greater. Friction losses in the pipe shall be calculated using the Manning Formula. Values for "n" are shown in Table 425-50. The minimum value for "v" in pipes shall be 3.0 feet per second. The maximum value for "v" in pipes shall be based on engineering judgment and experience. Pressure flow is permitted in storm sewers. The elevation of the hydraulic gradient shall be at least one foot below ground level. Pressure heads up to 25 feet can be used with concrete pipe with rubber gasketed joints.
E. 
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 425-51. Manholes, catch basins or junction boxes shall be installed at all changes in grade or horizontal alignment except as noted in § 425-48D.
F. 
Open channels. Maximum allowable velocities of flow in swales, open channels, and ditches as relating to slope and grass cover are shown in Pennsylvania Department of Environmental Protection Soil Erosion and Sedimentation Control Manual, latest revision. Higher velocities require invert stabilization. Velocity dissipators may be used, if approved by the Municipal Engineer.
G. 
Seepage pits. Where seepage pits, seepage tanks, seepage trenches and/or french drains are proposed, the applicant shall include an analysis of the potential for accelerated sinkhole development in the specific geology of the site due to the concentration of water introduction to the subsurface.
H. 
Seepage report. The applicant shall submit a seepage report containing a test pit soils analysis, prepared by a soil scientist, and percolation test results. The bottom of the test pits shall be the elevation at which the soil/seepage interface is designed, i.e., the bottom of the trench, pit, etc. Percolation tests shall be run at this interface. Percolation rates shall be 15 minutes per inch maximum and 90 minutes per inch minimum.
I. 
Equivalent discharge. Alternative means of stormwater discharge and retention, resulting in the equivalent discharge as required by this chapter, may be permitted when approved by the Municipal Engineer.
Table 425-49
Inlet Capacity Reduction Factors
Assuming Partial Clogging
Condition
Inlet Type
Reduction Factors*
(percentage of theoretical capacity)
Sump
Side opening
0.80
Sump
Grate
0.50
Sump
Combination
0.65
Continuous grade
Side opening
0.80
Continuous grade
Side opening with deflector
0.75
Continuous grade
Longitudinal bars
0.60
Continuous grade
Transverse bars
0.50
Continuous grade
Combination
0.60
Table 425-50
"n" Values for Manning Formula
Type of Pipe
"n" Value*
Concrete culvert pipe
0.013
Concrete sewer pipe
0.013
Cast/ductile/steel iron
0.013
Corrugated metal (plain)
0.024
Corrugated metal (coated)
0.021
Corrugated metal (lined)
0.013
Plastic
0.011
Vitrified clay
0.013
*
Adjustments for specific conditions shall be based on engineering experience and judgment and submitted to the Municipal Engineer for approval.