[1]
Editor's Note: The former title of Art. V, Stony Creek and Saw Mill Run Watershed Areas, was specifically repealed 7-18-2006 by Ord. No. 295.
A. 
Unimpeded flow. Storm drainage systems shall be provided in order to permit unimpeded flow of natural watercourses except as modified by stormwater detention facilities or open channels consistent with this Part 1.
B. 
Concentrated discharge points. The existing points of concentrated drainage discharge onto adjacent property shall not be altered.
C. 
Diffused discharge areas. Areas of existing diffused drainage discharge onto adjacent property shall be managed such that, at minimum, the peak diffused flow does not increase in the general direction of discharge, except as otherwise provided in this Part 1. If diffused flow is proposed to be concentrated and discharged onto adjacent property, the developer must document that there are adequate downstream conveyance facilities to safely transport the concentrated discharge or otherwise prove that no harm will result from the concentrated discharge. Areas of existing diffuse drainage discharge shall be subject to any applicable release rate criteria in the general direction of existing discharge whether they are proposed to be concentrated or maintained as diffused drainage areas.
D. 
Drainage easements. Where a subdivision is traversed by watercourses other than permanent streams, there shall be provided a drainage easement conforming substantially with the line of such watercourse. The width of the easement and watercourse shall be adequate to provide unimpeded flow of storm runoff based on calculations made in conformance with the rest of § 125-24 for the one-hundred-year return period runoff and to provide a freeboard allowance of 5/10 foot above the design water surface level. The terms of the easement shall prohibit excavation, the placing of fill or structures and any alterations which may adversely affect the flow of stormwater within any portion of the easement. Also, periodic maintenance of the easement to ensure proper runoff conveyance shall be required.
[Amended 9-20-2016 by Ord. No. 350]
E. 
State highways. Any drainage facilities required by this Part 1 that are located on state highway rights-of-way shall be subject to approval by the Pennsylvania Department of Transportation.
F. 
Open channel alternatives. When it can be shown that, due to topographic conditions, natural drainage swales on the site cannot adequately provide for drainage, open channels may be constructed conforming substantially to the line and grade of such natural drainage swales. Capacities of such open channels shall be calculated using the Manning equation.
G. 
Minimize erosion. Storm drainage facilities and appurtenances shall be so designed and provided as to minimize erosion in watercourse channels and at all points of discharge.
H. 
Use of volume controls. Consideration should be given to the design and use of volume controls for stormwater management, where geology permits.
I. 
Special dam requirements. Should any stormwater management facility qualify as a dam under PaDEP Chapter 105, the facility shall be designed in accordance with Chapter 105 and meet the regulations of Chapter 105 concerning the dam's safety.
[Amended 9-20-2016 by Ord. No. 350]
J. 
Minimum roadway crossing requirements. Roadway crossing, including pipes, bridges, storm sewers or any other drainage conveyance facility must be able to convey, without damage to the drainage structure or roadway, runoff from the twenty-five-year design storm with a minimum of 1.0 foot of freeboard measured below the lowest point along the roadway. Roadway crossings located within designated floodplain areas must be able to convey runoff from a one-hundred-year design storm with a minimum of 1.0 foot of freeboard measured below the lowest point along the top of the roadway. Any facilities that constitute stream enclosures, as described in PaDEP Chapter 105 regulations (as amended or replaced from time to time by PaDEP), shall be designed in accordance with Chapter 105 and will require a permit from PaDEP. Other requirements can be added, such as methods of detention or control, which may be utilized in stormwater management systems. These would include detention basins, rooftop storage, parking lot and street ponding, seepage pits and cisterns, porous pavement, grassed channels, swales, vegetative strips and decrease of impervious coverage.
K. 
Additional design calculations. Stormwater management systems for individual projects must provide control of postdevelopment flows to predevelopment peaks for the greater-than-ten-year storm through the one-hundred-year storm.[1]
[1]
Editor's Note: Former Subsection L, Release rate implementation standards, which immediately followed this subsection, was repealed 7-18-2006 by Ord. No. 295.
A. 
Mapping of stormwater management districts.
(1) 
In order to implement the provisions of the Stony Creek/Saw Mill Run Stormwater Management Plan, Whitpain Township is hereby divided into stormwater management districts consistent with the Stony Creek/Saw Mill Run Release Rate Map presented in the plan. The boundaries of the stormwater management districts are shown on an official map which is available for inspection at the Township office.
(2) 
Three broad categories of stormwater management districts, namely the Release-Rate District, the Provisional No-Detention District and the One-Hundred-Percent Release-Rate District, may be applied in Whitpain Township, as described following.
B. 
Categories of districts. The stormwater management districts are hereinafter divided into three groups or categories as follows:
(1) 
Release-Rate Districts. Within a given district, the postdevelopment peak rate of storm runoff must be controlled to the stated percentage of the predevelopment peak rate of storm runoff in order to protect downstream watershed areas. There are several release rate districts which differ in the extent to which postdevelopment runoff must be controlled. The release rates range from 50% to 100%.
(2) 
Provisional No-Detention Districts. These watershed areas may discharge postdevelopment peak runoff without detention without adversely affecting the total watershed peak flow. In certain instances, however, the local runoff conveyance facilities, which transport runoff from the site to the main channel, may not have adequate capacity to safely transport increased peak flows associated with no detention for a proposed development. In those instances, the developer shall either use a one-hundred-percent release-rate control or provide increased capacity of downstream drainage elements to convey increased peak flows consistent with § 125-23J. In determining if adequate capacity exists in the local watershed drainage network, the developer must assume that the entire local watershed is developed per current zoning and that all new development would use the runoff controls specified by this Part 1. Similarly, any capacity improvements must be designed to convey runoff from development of all areas tributary to the improvement consistent with the capacity criteria specified in § 125-23C.
(3) 
One-Hundred-Percent Release-Rate Districts. These areas should use a one-hundred-percent release-rate control to protect the areas downstream of the development site. A one-hundred-percent release-rate control would result in the postdevelopment peak rate being controlled to the predevelopment level.
[1]
Editor's Note: The title of this § 125-22 was specifically amended 7-18-2006 by Ord. No. 295.
A. 
Compliance with release-rate criteria. Any stormwater management controls required by this Part 1 and subject to release rate criteria (50% to 100%) shall meet the applicable release rate criteria for each of the two-year, five-year, ten-year, twenty-five-year, fifty-year and one-hundred-year return period runoff events consistent with the calculation methodology specified in § 125-24.
B. 
Boundary criteria. The exact location of the stormwater management district boundaries as they apply to a given development shall be determined by mapping the boundaries using the two-foot topographic contours provided as part of the drainage plan. The district boundaries as originally drawn coincide with topographic divides or, in certain instances, are drawn from the intersection of the watercourse and a physical feature such as the confluence with another watercourse or a potential flow obstruction (road, culvert, bridge, etc.) to the topographic divide consistent with topography.
C. 
Downstream capacity analysis criteria. Any downstream capacity analysis conducted in accordance with this Part 1 shall use the following criteria for determining adequacy for accepting peak flow rates:
(1) 
Natural or man-made channels or swales must be able to convey the increased runoff associated with a two-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 DEP Soil Erosion and Sedimentation Control Manual (February 1988).
(2) 
Natural or man-made channels or swales must be able to convey the increased twenty-five-year return period runoff peak within their banks or to otherwise not create any hazards to persons or property.
(3) 
Culverts, bridges or storm sewers or any other facilities that must pass or convey flows from the tributary area must have sufficient capacity to pass or convey the increased flows associated with the twenty-five-year return period runoff event, except for facilities located within a designated floodplain area that must be capable of passing or conveying the one-hundred-year return period runoff. Any facilities that constitute stream enclosures per DEP's Chapter 105 regulations shall be designed to convey the one-hundred-year return period runoff.
D. 
Single release-rate criteria. For a proposed development site located within a single release-rate category area, the total runoff from the site shall meet the applicable release rate criteria. For development sites with multiple points of concentrated runoff discharge, individual drainage points may be designed for up to a one-hundred-percent release rate, so long as the total runoff from the site is controlled to the applicable release rate.
E. 
Two or more release-rate criteria. For a proposed development site located within two or more release-rate category areas, the total runoff that may be discharged at any point is limited to the predevelopment peak rate of runoff at that point multiplied by the applicable release rate. The control rates shall apply regardless of any grading modifications that may change the drainage area that discharges at a given point.
F. 
Locations of mixed categories. For proposed development sites located partially within a release-rate category area and partially within a provisional no-detention area, in no event shall a significant portion of the site area subject to the release rate control be drained to the discharge point(s) located in the no-detention area.
G. 
No-harm option. For any proposed development site not located in a Provisional No-Detention District, the developer has the option of using a less restrictive runoff control (including no detention) if the developer can prove that no harm would be caused by discharging at a higher runoff rate than that specified by the plan. Proof of no harm must be shown from the development site through the remainder of the downstream drainage network to the confluence of the Stony Creek or Saw Mill Run with the Schuylkill River. Proof of no harm must be shown using the capacity criteria specified in § 125-23C if downstream capacity analysis is a part of the no-harm justification. Any no-harm justifications shall be submitted by the developer as part of the drainage plan submission per Article VI. Attempts to prove no harm based upon downstream peak flow versus capacity analysis shall be governed by the following provisions:
(1) 
The peak flow values to be used for downstream areas for the design return period storms (two-year, five-year, ten-year, twenty-five-year, fifty-year and one-hundred-year) shall be the values from the calibrated Penn State Runoff Model (PSRM) for the Stony Creek/Saw Mill Run Watershed. These flow values would be supplied to the developer by the municipal engineer upon request.
(2) 
Any available capacity in the downstream conveyance system as documented by a developer may be used by the developer only in proportion to his development site acreage relative to the total upstream undeveloped acreage from the identified capacity (i.e., if his site is 10% of the upstream undeveloped acreage, he may use up to 10% of the documented downstream available capacity.).
(3) 
Developer-proposed runoff controls that would generate increased peak flow rates at documented storm drainage problem areas would, by definition, be precluded from successful attempts to prove no harm, except in conjunction with proposed capacity improvements for the problem areas consistent with § 125-23J.
H. 
Individual lots within a subdivision. When a site is proposed to be subdivided for the sale and development of lots on an individual basis, either by the subdividing party or other party, the subdividing party shall, as part of the subdivision proposal, show all provisions for stormwater control, in accordance with the requirements of this Part 1.
I. 
Regional or subregional detention alternatives. For certain regions within the watershed, it may be more cost effective to provide one control facility for an entire subarea, group of subareas or portion of a subarea incorporating more than one development site than to provide an individual control facility for each development site. The initiative and funding for any regional or subregional runoff control alternatives are the responsibility of prospective developers. The design of any regional control basins must incorporate reasonable development of the entire upstream watershed. The peak outflow of a regional basin would be determined on a case-by-case basis using the hydrologic model of the watershed consistent with protection of the downstream watershed areas. "Hydrologic model" refers to the calibrated Stony Creek/Saw Mill Run version of the Penn State Runoff Model as developed for the stormwater management plan.
J. 
Capacity improvements.
(1) 
In certain instances, primarily within the provisional no-detention and provisional 100% release-rate category areas, local drainage conditions may dictate more stringent levels of runoff control than those based upon protection of the entire watershed. In these instances, if the developer could prove that it would be feasible to provide capacity improvements to relieve the capacity deficiency in the local drainage network, then the capacity improvements could be provided by the developer in lieu of development controls on the development site. Any capacity improvements would be designed based upon development of all areas tributary to the proposed improvement and the capacity criteria specified in § 125-23C. In addition, all new development upstream of a proposed capacity improvement shall be assumed to implement the applicable runoff controls consistent with this Part 1 except that all new development within the entire subarea(s) within which the proposed development site is located shall be assumed to implement the developer's proposed discharge control, if any.
(2) 
Capacity improvements may also be provided as necessary to implement any regional or subregional detention alternatives or to implement a modified no-harm option which proposes specific capacity improvements to document the validity of a less stringent discharge control that would not create any harm downstream.[1]
[1]
Editor's Note: Former Subsection K, Waiver of runoff control based on minimum additional impervious cover, which immediately followed this subsection, was repealed 9-21-2004 by Ord. No. 280-1.
[Amended 7-18-2006 by Ord. No. 295; 9-20-2016 by Ord. No. 350; 6-17-2025 by Ord. No. 426]
A. 
Approved methodology. Stormwater runoff from all development sites shall be calculated using either the Rational Method or a soil cover complex methodology.
B. 
Basin designs. The design of any detention basin intended to meet the requirements of this Part 1 shall be verified by routing the design storm hydrograph through the proposed basin. For basins designed using the modified Rational Method technique, the detention volume shall, at minimum, equal the volume derived from the approximate routing process as contained in SCS Technical Release Number 55 (TR 55).
C. 
Freeboard requirements. All stormwater management facilities shall provide a minimum of 1.0 foot of freeboard above the maximum pool elevation associated with the two-year through twenty-five-year events to the emergency spillway. An emergency spillway shall be designed to pass the one-hundred-year runoff event with a minimum 0.5 foot freeboard between the crest elevation and top of berm.
D. 
Soil Conservation Service twenty-four-hour rainfall distribution.
(1) 
All calculations using the soil cover complex method shall use the Soil Conservation Service Type II twenty-four-hour rainfall distribution. The twenty-four-hour rainfall depths for various return periods to be used consistent with this Part 1 are the PENNDOT data as set forth in Frequency Field Manual (May 1986) for Region 5, as follows:
24-Hour Return Period
(years)
Rainfall Depth
(inches)
2
3.3
5
4.2
10
5.0
25
5.8
50
6.4
100
7.2
(2) 
Other rainfall frequency data available to the applicant would be the United States Department of Commerce, National Weather Service or PaDEP.
E. 
Rational Method calculation. All calculations using the Rational Method shall use rainfall intensities consistent with appropriate times of concentration and return periods and the intensity-duration-frequency curves, subject to review and approval by the Township Engineer.
F. 
Runoff curve numbers. Runoff curve numbers (CNs) to be used in the soil cover complex method shall be based upon the matrix subject to review and approval by the Township Engineer.
G. 
Runoff coefficients. Runoff coefficients for use in the Rational Method shall be based upon the table presented subject to review and approval by the Township Engineer.
H. 
Watercourse capacity calculations. The Manning equation shall be used to calculate the capacity of watercourses. Manning "n" values used in the calculations shall be consistent with the table, subject to review and approval by the Township Engineer. Pipe capacities shall be determined by standard engineering methods acceptable to the Township Engineer.
I. 
Compliance with Dam Safety and Encroachments Act.[1] Any detention basin intended to meet the requirements of this Part 1 which requires a dam safety permit from PaDEP shall be designed consistent with the provisions of the Dam Safety and Encroachments Act and the PaDEP Chapter 105 rules and regulations.
[1]
Editor's Note: See 32 P.S. § 693.1 et seq.
J. 
Underground storage volume. Underground storage volume, above the frost line or below the water table, shall not be included in the calculations for storage volume to satisfy stormwater runoff criteria.