(A) Stormwater collection system.
(1) The stormwater collection system shall be designed and approved based upon the following criteria.
(2) Peak discharge shall be computed using the rational formula.
Q = CIA |
where: |
Q = Peak discharge in cubic feet per second |
C = Run-off factor expressed as a percent of the total water falling on an area |
I = The rate of rainfall for the time of concentration of the drainage area in inches per hour for a given storm frequency (rainfall Intensity) |
A = The drainage area expressed in acres |
(a) The run-off factor “C” is a percentage factor which represents the proportion of the total quantity of water falling on the area that remains as run-off.
(b) The run-off factors for various types of drainage areas, as presented in the following table, shall be used for design.
Run-off Factors for the Rational Equation | |
Type of Drainage Area or Surface Run-off Factor “C” |
Run-off Factors for the Rational Equation | |
Type of Drainage Area or Surface Run-off Factor “C” | |
Cultivated field | 0.40 |
Impervious surfaces such as, but not limited to, roof surface, pavement, concrete or bituminous concrete, gravel | 0.95 |
Lawn | 0.25 |
Meadow | 0.20 |
Wooded | 0.15 |
Notes: | |
1. Consideration should be given to future land use changes in the drainage area in selecting the “C” factor. | |
2. For drainage area containing several different types of ground cover, a weighted value of “C” factor must be used. | |
3. a. In special situations where sinkholes, stripped abandoned mines and the like exist, careful evaluation shall be given to the selection of a suitable run-off factor with consideration given to possible reclamation of the land in the future. | |
(1) Storm frequency: | |
(a) The following storm frequency shall be used for design: | |
i. Local streets -10-year, see storm duration; | |
ii. Culvert cross drains - 25-year rural, 50-year suburban, 100-year urban; and | |
iii. Swales - 100-year. | |
(b) 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 management facility for the 100-year storm. The greater flow shall govern the design of the pipe or culvert. | |
(c) A 100-year design storm frequency may be required for design of the stormwater collection system to ensure that the resultant stormwater run-off from the post-development design storm is directed into the stormwater management facility. | |
(d) In all cases where drainage is collected by means of a head wall or pipe end, the pipe shall be designed as a culvert. The minimum diameter of the culvert shall be 18 inches. The minimum diameter of storm sewer shall be 15 inches, when located in a public right-of-way or easement. | |
(e) Where the collection system may be under inlet or outlet control, the Municipal Engineer may request additional calculations, such as, but not limited to, hydraulic grade lines. | |
(2) Storm duration: | |
(a) A 5-minute storm duration shall be used if this duration does not result in a maximum expected discharge that exceeds the capacity of a 30-inch pipe; and | |
(b) If a 5-minute storm duration results in a pipe size exceeding 30 inches, the time of concentration approach shall be used in determining storm duration. | |
(3) Inlet placement: in general, catch basins shall be placed as required by hydraulic capacity. For design purposes, a capture ratio (intercepted flow / design flow) of 70% or greater is required. However, the width of flow in a street cannot exceed one-half of the travel lane. In any event, the maximum distance between conveyed inlets shall not exceed 400 feet. | |
(4) Pipe and swale capacity: Manning’s equation shall be used for the design of all storm sewer pipes and for open channel design: | |
V = 1.486 R2/3 S1/2 n | |
where: | |
V = Velocity of the water in feet per second | |
R = Hydraulic radius which is equal to the net effective areas (A) divided by the wetted perimeter (W.P.): | |
R = A W.P. | |
The wetted perimeter is the lineal feet of the drainage facility cross-section which is wetted by the water. | |
S = Slope of the hydraulic gradient (for approximation, use the water surface slope in a wetted stream and the stream bed slope in dry stream or the pipe slope). | |
n = The roughness coefficient. Roughness coefficients are as follows: | |
Value of Manning’s roughness coefficient - n | |
Bare earth channel | 0.020 |
Concrete | 0.012 |
Grass-lined channel | 0.035 |
Paved bituminous channel | 0.016 |
Rip-rap | 0.040 |
Turf reinforcement matting (TRM) | Per manufacturer’s recommendation |
The maximum permitted velocity in a lined or unlined swale shall be in accordance with the USDA Engineering Field Manual, PA DEP, applicable Pennsylvania codes and state law, whichever is less. | |
The maximum permitted velocity in storm sewer pipe is 20 FPS. If 20 FPS is exceeded, the pipe must be anchored in accordance with the following table: | |
Velocity of Flow (FPS) | Anchor Spacing (ft.) |
20—24.99 | 20 |
25—30 | 10 |
Under no circumstances shall flow velocity exceed 30 FPS.
| |
(B) Stormwater management facilities. The plan shall be designed and approved based upon the following criteria.
(1) General. For drainage areas 320 acres or larger the peak discharge and run-off shall be computed using the soil-cover complex method contained in Urban Hydrology for Small Watersheds, Technical Release No. 55 published by the Engineering Division, Soil Conservation Services, United States Department of Agriculture, dated June, 1986 or latest revision, except as modified herein. For drainage areas less than 320 acres the modified rational method may be utilized. Alternate methods of analysis may be considered if approved by the Municipal Engineer.
(2) Outflow determination.
(a) The maximum permitted stormwater discharge, in cubic feet per second, from any site shall not exceed the calculated peak discharge from the site at pre-development ground cover and soil conditions for all design storms specified in § 101-16.B. For the purpose of this chapter, pre-development ground cover conditions shall be assumed to be “meadow” for all non-forested pervious areas as defined in Urban Hydrology for Small Water Sheds, Technical Release No. 55 published by Engineering Division, Soil Conservation Service, United States Department of Agriculture, dated June, 1986 or latest revision or if using the modified rational method a “C” factor of 0.20 shall be used for meadow conditions for all non-forested pervious areas. For existing impervious surfaces see § 53.043(A). The maximum permitted stormwater discharge shall be calculated using the SCS method or alternative method approved by the Municipal Engineer for rainfalls having recurrence intervals of one, two, five, ten, 25, 50 and 100 years. Time of concentration (Tc) should be calculated using the SCS segmental approach in accordance with the current recommendations by SCS. For the purpose of this chapter, the rainfall depths shall be per § 53.040(L).
(b) Rainfall intensity shall be per § 53.040(L) for design if using the modified rational method.
(c) If alternate methods of analysis are utilized, the design storms recurrence interval in years shall be the same as used in the SCS TR-55 Method.
(3) Minimum required detention storage. The minimum required detention storage shall be determined by routing the approved post-development hydrographs through the stormwater management facility, 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.
(4) Emergency spillway. Emergency spillways or overflow structures shall be designed to pass the peak flow resulting from a 100-year recurrence interval design storm computed at post-development conditions, assuming that the principal outlet structure is non-functional. All retention basins and detention basins shall provide an emergency spillway. Emergency spillways shall be located in cut where feasible, if not, adequate permanent stabilization is required. All emergency spillways shall be permanently stabilized for the design peak flow rate and velocity.
(5) Minimum bottom slope. All detention basins shall have a minimum bottom slope of 2%, if not being utilized for infiltration purposes.
(6) Side slopes. The maximum side slopes for detention or retention basins shall be three horizontal to one vertical in cut and four horizontal to one vertical in fill.
(7) Freeboard. The stormwater management facility shall have a minimum one-half foot of freeboard determined after routing the 100-year recurrence interval design storm per division (B)(4) above or a minimum of one foot of freeboard above the normal 100-year storm routing elevation, whichever is greater.
(8) Seepage trench. All stormwater management detention basins shall provide as a minimum a two-foot wide by ten-foot long by six-foot deep seepage trench in the bottom of the basin near the outlet control structure, unless field conditions deem the seepage trench nonfunctional and concurred by the Municipal Engineer. This seepage trench is not required if the basin is being utilized for infiltration purposes.
(9) Required installations. Fencing, trash racks and installation of child-proof facilities may be required by the municipality.
(Ord. 2013-2, passed 10-7-2013)