A. 
Applicants proposing regulated activities in Schuylkill River Watershed which do not fall under the exemption criteria shown in § 410-60 shall submit a drainage plan consistent with the Schuylkill River Watershed Stormwater Management Plan to the municipality for review. These criteria shall apply to the total proposed development even if development is to take place in stages.
B. 
The applicant is required to evaluate practicable alternatives to the surface discharge of stormwater, the creation of impervious surfaces and the degradation of waters of the commonwealth, and must maintain as much as possible the natural hydrologic regime.
C. 
The drainage plan must be designed consistent with the sequencing provisions of to ensure maintenance of the natural hydrologic regime and to promote groundwater recharge and protect groundwater and surface water quality and quantity. The drainage plan designer must proceed sequentially in accordance with Article XI of this Part 2.
D. 
Existing points of concentrated drainage that discharge onto adjacent property shall not be altered in any manner which could cause property damage without permission of the affected property owner(s) and shall be subject to any applicable discharge criteria specified in this Part 2.
E. 
Areas of existing diffused drainage discharge shall be subject to any applicable discharge criteria in the general direction of existing discharge, whether proposed to be concentrated or maintained as diffused drainage areas, except as otherwise provided by this Part 2. If diffused drainage discharge is proposed to be concentrated and discharged onto adjacent property, the applicant must document that adequate downstream conveyance facilities exist to safely transport the concentrated discharge, or otherwise prove that no erosion, sedimentation, flooding or other impacts will result from the concentrated discharge.
F. 
Where a development site is traversed by existing watercourses, drainage easements shall be provided conforming to the line of such watercourses. The terms of the easement shall conform to the stream buffer requirements contained in § 410-54C of this chapter.
[Amended at time of adoption of Code (see Ch. 1, General Provisions, Art. I)]
G. 
Any stormwater management facilities regulated by this Part 2 that would be located in or adjacent to waters of the commonwealth or wetlands shall be subject to approval by PADEP through the joint permit application process or, where deemed appropriate by PADEP, the general permit process. When there is a question whether wetlands may be involved, it is the responsibility of the applicant or his agent to show that the land in question cannot be classified as wetlands, otherwise approval to work in the area must be obtained from PADEP.
H. 
Any alteration that affects stormwater flow directly or indirectly toward a PennDOT facility shall be subject to PennDOT regulations.
I. 
Minimization of impervious surfaces and infiltration of runoff through seepage beds, infiltration trenches, etc., are encouraged, where soil conditions permit, to reduce the size or eliminate the need for detention facilities or other structural BMPs.
J. 
Roof drains shall not be connected to impervious surfaces in order to promote overland flow and infiltration/percolation of stormwater where advantageous to do so. When site conditions preclude infiltration/percolation, then it shall be permitted on a case-by-case basis by the municipality.
K. 
All stormwater runoff shall be treated for water quality.
L. 
Transference of runoff to or from an EV/HQ watershed is prohibited unless otherwise authorized by PADEP, DRBC or SRBC.
M. 
Site plan for regulated activities.
(1) 
For all regulated activities, unless preparation of an SWM site plan is specifically exempted by this Part 2:
(a) 
Preparation and implementation of an approved SWM site plan is required.
(b) 
No regulated activities shall commence until the municipality issues written approval of an SWM site plan, which demonstrates compliance with the requirements of this Part 2.
(2) 
SWM site plans approved by the municipality in accordance with this Part 2 shall be on site throughout the duration of the regulated activity.
(3) 
The municipality may, after consultation with PADEP, approve measures for meeting the state water quality requirements other than those in this Part 2, provided that they meet the minimum requirements of, and do not conflict with, state law, including but not limited to the Clean Streams Act.[1]
[1]
Editor's Note: See 35 P.S. § 691.1 et seq.
(4) 
For all regulated earth disturbance activities, erosion and sediment control BMPs shall be designed, implemented, operated, and maintained during the regulated earth disturbance activities (e.g., during construction) to meet the purposes and requirements of this Part 2 and to meet all the requirements under Title 25 of the Pennsylvania Code and the Clean Streams Act.[2] Various BMPs and their design standards are listed in the Erosion and Sediment Pollution Control Program Manual (E&S Manual) 2, Commonwealth of Pennsylvania, Department of Environmental Protection, No. 363-2134-008, as amended and updated. For all regulated activities, implementation of the volume controls in accordance with this Part 2 is required.
[2]
Editor's Note: See 35 P.S. § 691.1 et seq.
A. 
No regulated earth disturbance activities within the municipality shall commence until the municipality receives an approval from the Conservation District of an erosion and sediment control plan for construction activities.
B. 
PADEP has regulations that require an erosion and sediment control plan for any earth disturbance activity of 5,000 square feet or more, under 25 Pa. Code § 102.4(b).
C. 
In addition, under 25 Pa. Code Chapter 92a, a PADEP NPDES construction activities permit is required for regulated earth disturbance activities.
[Amended at time of adoption of Code (see Ch. 1, General Provisions, Art. I)]
D. 
Evidence of any necessary permit(s) for regulated earth disturbance activities from the appropriate PADEP Regional Office or County Conservation District must be provided to the municipality. The issuance of an NPDES construction permit, or permit coverage under the statewide general permit (PAG-2), satisfies the requirements § 410-51A.
E. 
A copy of the erosion and sediment control plan and any required permit, as required by PADEP regulations, shall be available at the project site at all times.
F. 
Additional erosion and sediment control design standards and criteria are recommended to be applied where infiltration BMPs are proposed shall include the following:
(1) 
Areas proposed for infiltration BMPs shall be protected from sedimentation and compaction during the construction phase to maintain maximum infiltration capacity.
(2) 
Infiltration BMPs shall not be constructed nor receive runoff until the entire contributory drainage area to the infiltration BMP has achieved final stabilization
A. 
For projects disturbing one acre or more, the design of all regulated activities shall include evaluation of practicable alternatives to the surface discharge of stormwater, the creation of impervious surfaces, and the degradation of waters of the commonwealth, and must maintain as much as possible the natural hydrologic regime of the site.
(1) 
An alternative is practicable if it is available and capable of being done after taking into consideration cost, existing technology and logistics, in light of overall project purposes, and other municipal requirements.
(2) 
All practicable alternatives to the discharge of stormwater are presumed to have less adverse impact on quantity and quality of waters of the commonwealth unless otherwise demonstrated.
B. 
The applicant shall demonstrate that they designed the regulated activities that disturb one acre or more included consideration of the following issues:
(1) 
Prepare an existing resource and site analysis map (ERSAM), showing environmentally sensitive areas, including, but not limited to, steep slopes, ponds, lakes, streams, wetlands, hydric soils, vernal pools, floodplains, stream buffer zones, hydrologic soil groups A and B (areas conducive to infiltration), special geologic features, any existing recharge areas and any other requirements outlined in Chapter 425, Subdivision and Land Development, of the Code of Caernarvon Township.
(2) 
Establish appropriate buffers for each of the delineated environmentally sensitive areas per Chapter 500, Zoning. (See § 410-54C for stream buffers and § 410-58F for special geologic feature buffers.)
[Amended at time of adoption of Code (see Ch. 1, General Provisions, Art. I)]
(3) 
Prepare a draft project layout avoiding sensitive areas identified in § 410-52B(1).
(4) 
Identify site specific existing conditions drainage areas, discharge points, recharge areas and hydrologic soil groups A and B.
(5) 
Evaluate nonstructural stormwater management alternatives.
(a) 
Minimize earth disturbance.
(b) 
Minimize impervious surfaces.
(c) 
Break up large impervious surfaces.
(6) 
Satisfy infiltration objective (§ 410-53) and provide for stormwater pretreatment prior to infiltration. Pretreatment may not be necessary for rooftop runoff which enters the infiltration facility directly from a roof leader.
(7) 
Satisfy water quality (§ 410-54) and streambank erosion protection objective (§ 410-55).
(8) 
Determine what management district the site falls into (Appendix D[1]) and conduct an existing conditions runoff analysis.
[1]
Editor's Note: Appendix D, Stormwater Management District Watershed Map, is included as an attachment to this chapter.
(9) 
Prepare final project design to maintain existing conditions drainage areas and discharge points, to minimize earth disturbance and impervious surfaces, and to the maximum extent possible, to ensure the remaining site development has no surface or point discharge.
(10) 
Conduct a proposed conditions runoff analysis based on the final design and to meet the release rate and in turn the overbank flow and extreme event requirements (§ 410-56).
(11) 
Manage any remaining runoff through treatment prior to discharge, as part of detention, bioretention, direct discharge or other structural control.
Maximizing the groundwater recharge capacity of the area being developed is required. Design of the infiltration stormwater management facilities shall give consideration to providing groundwater recharge to compensate for the reduction in the percolation that occurs when the ground surface is disturbed or impervious surface is created. It is recommended that roof runoff be directed to infiltration BMPs which can be over-designed to compensate for the infiltration losses due to parking areas. It is recommended that roof runoff be directed to infiltration BMPs which may be designed to compensate for the runoff from parking areas. These measures are required to be consistent with § 410-41, and take advantage of utilizing any existing recharge areas. Infiltration may not be feasible on every site due to site-specific limitations such as soil type. If it cannot be physically accomplished, due to seasonal high-water table, soil permeability rate, soil depth or setback distances from special geologic features, then the design professional shall be responsible to show that this cannot be physically accomplished. If it can be physically accomplished, then the volume of runoff to be infiltrated shall be determined from § 410-53A(3) depending on demonstrated site conditions and shall be the greater of the two volumes.
A. 
Infiltration BMPs shall meet the following minimum requirements:
(1) 
Infiltration requirements.
(a) 
Regulated activities will be required to infiltrate, where site conditions permit, a portion of the runoff created by the development as part of an overall stormwater management plan designed for the site. The volume of runoff to be infiltrated shall be determined from § 410-53A(3)(a) or (b), depending upon demonstrated site conditions.
(2) 
Infiltration BMPs intended to receive runoff from developed areas shall be selected based on suitability of soils and site conditions and shall be constructed on soils that have the following characteristics:
(a) 
A minimum depth of 24 inches between the bottom of the BMP and the limiting zone.
(b) 
An infiltration and/or percolation rate sufficient to accept the additional stormwater load and drain completely as determined by field tests conducted by the applicant's design professional.
(c) 
The infiltration facility shall be capable of completely infiltrating the required retention (infiltration) volume within four days (96 hours).
(d) 
Pretreatment shall be provided prior to infiltration.
(3) 
The size of the infiltration facility shall be based upon the following volume criteria:
(a) 
NRCS curve number equation.
[1] 
The NRCS runoff equation shall be utilized to calculate infiltration requirements (I) in inches.
Equation 410-53A
I (infiltration requirement, in inches) = (200/CN) - 2
Where:
CN
=
NRCS (SCS) curve number of existing conditions contributing to the infiltration facility.
[2] 
This equation is displayed graphically in, and the infiltration requirement can be determined from, Figure 410-53.
[3] 
It has been determined that infiltrating 0.46 inches of runoff from the impervious areas will aid in maintaining the hydrologic regime of the watershed. However, the rounded number 0.5 inches will be used.
Figure 410-53
Infiltration Requirement Based Upon NRCS Curve Number
410 Required Infiltration Graph.tif
[4] 
The retention (infiltration) volume (Rev) required to meet the infiltration requirement would therefore be computed as:
Equation 410-53B
Rev = (0.5 or I, whichever is greater) * impervious area (square feet)/(12 inches / feet) = cubic feet
Where:
I
=
Infiltration requirements (in inches).
(b) 
Annual recharge: water budget approach.
[1] 
If the goals of § 410-53A(3)(a) cannot be achieved, then 0.5 inches of rainfall shall be infiltrated from all impervious areas, up to an existing site conditions curve number of 81. Above a curve number of 81, Equation 410-53A or the curve in Figure 41-53 should be used to determine the infiltration requirement.
[2] 
The retention (infiltration) volume (Rev) required again would therefore be computed as:
Equation 410-53B
Rev = (0.5 or I, whichever is greater) * impervious area (square feet)/(12 inches / feet) = cubic feet
B. 
Soils. A detailed soils evaluation of the project site shall be required where practicable to determine the suitability of infiltration facilities. The evaluation shall be performed by a qualified design professional and, at a minimum, address soil permeability, depth to bedrock and subgrade stability. The general process for designing the infiltration BMP shall be:
(1) 
Analyze hydrologic soil groups as well as natural and man-made features within the site to determine general areas of suitability for infiltration practices. In areas where development on fill material is under consideration, conduct geotechnical investigations of subgrade stability; infiltration is not permitted to be ruled out without conducting these tests.
(2) 
Provide field tests such as double ring infiltrometer or hydraulic conductivity tests (at the level of the proposed infiltration surface) to determine the appropriate hydraulic conductivity rate. Percolation tests are not recommended for design purposes.
(3) 
Design the infiltration structure for the required retention volume (Rev) based on field determined capacity at the level of the proposed infiltration surface.
(4) 
If on-lot infiltration structures are proposed by the applicant's design professional, it must be demonstrated to the municipality that the soils are conducive to infiltrate on the lots identified.
C. 
Carbonate areas.
(1) 
The applicant is required to investigate the ability of all areas on the site which are not underlain by carbonate rock to meet the infiltration requirements of § 410-53A. If this investigation proves infeasible, infiltration can occur on areas underlain by carbonate rock by following the recommended procedure below in conjunction with Figure B-1 in Appendix B.[1] However, the applicant is not required to use infiltration in carbonate areas even if the site falls into the "recommended" range on Figure B-1 in Appendix B. If infiltration is not proposed, the calculated infiltration volume (§ 410-53A) shall be treated by an acceptable BMP.
[1]
Editor's Note: Appendix B, Stormwater Management Design Criteria, is included as an attachment to this chapter.
(2) 
Infiltration BMP loading rate percentages in Figure B-1 in Appendix B[2] shall be calculated as follows:
(
Area tributary to the infiltration BMP
)
*100%
Base area of the infiltration BMP
[2]
Editor's Note: Appendix B, Stormwater Management Design Criteria, is included as an attachment to this chapter.
(3) 
The area tributary to the infiltration BMP shall be weighted as follows:
Area Description
Weighting
(%)
All disturbed area to be made impervious
100
All disturbed areas to be made pervious
50
All undisturbed impervious areas
100
All undisturbed pervious areas
0
(4) 
Soil thickness is to be measured from the bottom of any proposed infiltration BMP. The effective soil thickness in Figure B-1 in Appendix B[3] is the measured soil thickness multiplied by the thickness factor based on soil permeability, as follows:
Permeability Range
(inches per hour)
Thickness Factor
6.0 to 12.0
0.8
2.0 to 6.0
1.0
1.0 to 2.0
1.4
0.75 to 1.0
1.2
0.5 to 0.75
1.0
[3]
Editor's Note: Appendix B, Stormwater Management Design Criteria, is included as an attachment to this chapter.
(5) 
The design of all facilities over karst shall include an evaluation of measures to minimize adverse effects.
D. 
Stormwater hotspots. Following is a list of examples of designated hotspots. If a municipality designates a site or use as a hotspot, it has important implications for how stormwater is managed. First and foremost, untreated stormwater runoff from hotspots shall not be allowed to recharge into groundwater where it may contaminate water supplies. Therefore, the Rev requirement shall not applied to development sites that fit into the hotspot category (the entire WQv must still be treated). Second, a greater level of stormwater treatment shall be considered at hotspot sites to prevent pollutant washoff after construction. EPA's NPDES stormwater program requires some industrial sites to prepare and implement a stormwater pollution prevention plan.
(1) 
Examples of hotspots.
(a) 
Vehicle salvage yards and recycling facilities.
(b) 
Vehicle fueling stations.
(c) 
Vehicle service and maintenance facilities.
(d) 
Vehicle and equipment cleaning facilities.
(e) 
Fleet storage areas (bus, truck, etc.)
(f) 
Industrial sites (based on Standard Industrial Codes).
(g) 
Marinas (service and maintenance).
(h) 
Outdoor liquid container storage.
(i) 
Outdoor loading/unloading facilities.
(j) 
Public works storage areas.
(k) 
Facilities that generate or store hazardous materials.
(l) 
Commercial container nursery.
(m) 
Other land uses and activities as designated by an appropriate review authority.
(2) 
The following land uses and activities are not normally considered hotspots:
(a) 
Residential streets and rural highways.
(b) 
Residential development.
(c) 
Institutional development.
(d) 
Office developments.
(e) 
Nonindustrial rooftops.
(f) 
Pervious areas, except golf courses and nurseries [which may need an integrated pest management (IPM) plan].
(3) 
While large highways [average daily traffic volume (ADT) greater than 30,000] are not designated as a stormwater hotspot, it is important to ensure that highway stormwater management plans adequately protect groundwater.
E. 
Where infiltration is proposed in source water protection areas as defined by the local municipality or Water Authority the applicant shall work with the appropriate entity to ensure protection of the water supply.
F. 
Infiltration facilities shall be used in conjunction with other innovative or traditional stormwater control facilities that are found within the PADEP State BMP Manual.
G. 
Where salt or chloride (salt storage) would be a pollutant, since soils do little to filter this pollutant and may contaminate the groundwater, a qualified design professional shall evaluate the possibility of groundwater contamination from the proposed infiltration facility and perform a hydrogeologic justification study if necessary.
H. 
The infiltration requirement in high quality or exceptional value waters shall be subject to the Department's Chapter 93 antidegradation regulations.[4]
[4]
Editor's Notes: See 25 Pa. Code § 93.4a et seq.
I. 
Dependant upon certain land use or hotspots an impermeable liner will be required in detention basins where the possibility of groundwater contamination exists. A hydrogeologic study may be required by the municipality.
J. 
The municipality may, upon its sole discretion, require the applicant to provide safeguards against groundwater contamination for land uses that may cause groundwater contamination should there be a mishap or spill.
K. 
For projects that disturb one acre or more, unless otherwise specified in Chapter 500, Zoning, the following setbacks for infiltration facilities shall apply:
(1) 
One hundred feet from water supply wells.
(2) 
Ten feet downslope or 100 feet upslope from building foundations.
(3) 
Fifty feet from septic system drainfields.
(4) 
Fifty feet from a geologic contact with carbonate bedrock, unless a preliminary site investigation is done in the carbonate bedrock to show the absence of special geologic features within 50 feet of the proposed infiltration area.
(5) 
One hundred feet from the property line, unless documentation is provided to show all setbacks from wells, foundations and drainfields on the neighboring property will be met.
The applicant shall comply with the following water quality requirements of this section.
A. 
For areas within defined special protection subwatersheds, which include exceptional value (EV) and high quality (HQ) waters, coldwater fishery (CWF), the temperature and quality of water and streams shall be maintained.
B. 
To accomplish the above, the applicant shall use innovative or traditional stormwater control facilities that are found within the PADEP State BMP Manual.
C. 
If a perennial or intermittent stream passes through the site, the applicant shall create a stream buffer extending a minimum of 50 feet to either side of the top-of-bank of the channel. The buffer area shall be maintained with appropriate native vegetation (Reference to Appendix H of Pennsylvania Handbook of Best Management Practices for Developing Area for plant lists). If the applicable rear or side yard setback is less than 50 feet, the buffer width may be reduced to 25% of the setback to a minimum of 10 feet. If an existing buffer is legally prescribed (i.e., deed, covenant, easement, etc.) and it exceeds the requirements of this Part 2, the existing buffer shall be maintained. [The municipality may select a smaller or larger buffer width if desired, but the selected buffer may not be less than 10 feet].[1] This does not include lakes or wetlands.
[1]
Editor's Note: So in original.
D. 
Evidence of any necessary permit(s) for regulated earth disturbance activities from the appropriate PADEP Regional Office must be provided to the municipality. The issuance of an NPDES construction permit, or permit coverage under the statewide general permit (PAG-2), satisfies the requirements of Subsection A of § 410-54. [*][2]
[2]
Editor's Note: So in original.
A. 
In addition to control of the water quality volume, in order to minimize the impact of stormwater runoff on downstream streambank erosion, the primary requirement is to design a BMP to detain the proposed conditions two-year, twenty-four-hour design storm to the existing conditions one-year peak flow using the NRCS Type II distribution. Additionally, provisions shall be made (such as adding a small orifice at the bottom of the outlet structure) so that the proposed conditions one-year storm takes a minimum of 24 hours to drain from the facility from a point where the maximum volume of water from the one-year storm is captured (i.e., the maximum water surface elevation is achieved in the facility). Release of water can begin at the start of the storm (i.e., the invert of the water quality orifice is at the invert of the facility).
B. 
The minimum orifice size in the outlet structure to the BMP shall be a three-inch-diameter orifice and a trash rack shall be installed to prevent clogging. On sites with small contributing drainage areas to this BMP that do not provide enough runoff volume to allow a twenty-four-hour attenuation with the three-inch orifice, the calculations shall be submitted showing this condition. Orifice sizes less than three inches can be utilized, provided that the design will prevent clogging of the intake.
A. 
The Schuylkill River Watershed has been divided into stormwater management districts as shown on the Management District Map in Appendix D.[1]
(1) 
In addition to the requirements specified in Table 410-56 below, the groundwater recharge (§ 410-53), water quality (§ 410-54), and streambank erosion control (§ 410-55) requirements shall be implemented.
(2) 
Standards for managing runoff from each subarea in the Schuylkill River Watershed for the two-year through 100-year design storms are shown in Table 410-56. Development sites located in each of the districts must control proposed conditions runoff rates to existing conditions runoff rates for the design storms in accord with Table 410-56.
Table 410-56
Water Quantity Requirements
Management District
Proposed Condition Design Storm
Existing Condition Design Storm
A
2-year
Reduce to
1-year
5-year
5-year
10-year
10-year
25-year
25-year
50-year
50-year
100-year
100-year
B
2-year
Reduce to
1-year
5-year
2-year
10-year
5-year
25-year
10-year
50-year
25-year
100-year
50-year
C
Reduce to
Equivalent Release Rate
2-year
1-year
100%
5-year
5-year
100%
10-year
10-year
100%
25-year
25-year
100%
50-year
50-year
100%
100-year
100-year
100%
(3) 
All areas, regardless of the release rate, must still meet the requirements of the groundwater recharge criteria (§ 410-53), water quality criteria (§ 410-54), and streambank erosion criteria (§ 410-55).
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
B. 
General. Proposed condition rates of runoff from any regulated activity shall not exceed the peak release rates of runoff prior to development for the design storms specified on the Stormwater Management District Watershed Map (Appendix D[2]) and § 410-56 of this Part 2.
[2]
Editor's Note: Said appendix is included as an attachment to this chapter.
C. 
District boundaries. The boundaries of the Stormwater Management Districts are shown on an Official Map that is available for inspection at the municipal office. A copy of the Official Map at a reduced scale is included in the Appendix D.[3] The exact location of the Stormwater Management District boundaries as they apply to a given development site shall be determined by mapping the boundaries using the two-foot topographic contours (or most accurate data required) provided as part of the drainage plan.
[3]
Editor's Note: Said appendix is included as an attachment to this chapter.
D. 
Sites located in more than one district. For a proposed development site located within two or more stormwater management district category subareas, the peak discharge rate from any subarea shall meet the management district criteria for which the discharge is located, as indicated in § 410-56. The calculated peak discharges shall apply regardless of whether the grading plan changes the drainage area by subarea. An exception to the above may be granted if discharges from multiple subareas recombine in proximity to the discharge site. In this case, peak discharge in any direction shall follow Management District A criteria provided that the overall site discharge meets the management district criteria for which the discharge is located.
E. 
Off-site areas. Off-site areas that drain through a proposed development site are not subject to release rate criteria when determining allowable peak runoff rates. However, on-site drainage facilities shall be designed to safely convey off-site flows through the development site.
F. 
Site areas. Where the site area to be impacted by a proposed development activity differs significantly from the total site area, only the proposed impact area utilizing stormwater management measures shall be subject to the management district criteria. In other words, unimpacted areas bypassing the stormwater management facilities would not be subject to the management district criteria.
A. 
Stormwater runoff from all development sites with a drainage area of greater than 200 acres shall be calculated using a generally accepted calculation technique that is based on the NRCS Soil Cover Complex Method. Table 410-57 summarizes acceptable computation methods and the method selected by the design professional shall be based on the individual limitations and suitability of each method for a particular site. The municipality may allow the use of the Rational Method to estimate peak discharges from drainage areas that contain less than 200 acres. The Soil Complex Method shall be used for drainage areas greater than 200 acres.
Table 410-57
Acceptable Computation Methodologies for Stormwater Management Plans
Method
Method Developed By
Applicability
TR-20 (or commercial computer package based on TR-20)
USDA NRCS
Applicable where use of full hydrology computer model is desirable or necessary.
TR-55 (or commercial computer package based on TR-55)
USDA NRCS
Applicable for land development plans within limitations described in TR-55.
HEC-1/HEC-HMS
US Army Corps of Engineers
Applicable where use of full hydrologic computer model is desirable or necessary.
PSRM
Penn State University
Applicable where use of a hydrologic computer model is desirable or necessary; simpler than TR-20 or HEC-1.
Rational Method or commercial computer package based on Rational Method)
Emil Kuichling (1889)
For sites less than 200 acres and with time of concentration less than 60 minutes (TC< 60 minutes), or as approved by the municipality
Other Methods
Varies
Other computation methodologies approved by the municipality.
NOTES:
*
Successors to the above methods are also acceptable. These successors include WINNTR55 for TR-55 and WINTR20 for TR20 and SWMM.
B. 
All calculations consistent with this Part 2 using the Soil Cover Complex Method shall use the appropriate design rainfall depths for the various return period storms according to the region in which they are located as presented in Table B-1 in Appendix B of this Part 2.[1] If a hydrologic computer model such as PSRM or HEC-1/HEC-HMS is used for stormwater runoff calculations, then the duration of rainfall shall be 24 hours. The rainfall distribution should reference to NOAA Atlas 14.
[1]
Editor's Note: Appendix B, Stormwater Management Design Criteria, is included as an attachment to this chapter.
C. 
For the purposes of existing conditions flow rate determination, undeveloped land 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 Table B-1 or B-2 in Appendix B of this Part 2.[2]
[2]
Editor's Note: Appendix B, Stormwater Management Design Criteria, is included as an attachment to this chapter.
D. 
All calculations using the Rational Method shall use rainfall intensities consistent with appropriate times of concentration for overland flow and return periods from the NOAA Atlas 14 Precipitation-Frequency Atlas of the United States (2004, revised 2006). 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). Times of concentration for channel and pipe flow shall be computed using Manning's Equation. NRCS lag equation divided by 0.6 as acceptable method for TC in undeveloped areas.
E. 
Runoff curve numbers (CN) for both existing and proposed conditions to be used in the Soil Cover Complex Method shall be obtained from Table B-1 in Appendix B of this Part 2.[3]
[3]
Editor's Note: Appendix B, Stormwater Management Design Criteria, is included as an attachment to this chapter.
F. 
Runoff coefficients (c) for both existing and proposed conditions for use in the Rational Method shall be obtained from Table B-2 in Appendix B of this Part 2.[4]
[4]
Editor's Note: Appendix B, Stormwater Management Design Criteria, is included as an attachment to this chapter.
G. 
Where uniform flow is anticipated, the Manning Equation shall be used for hydraulic computations, and to determine the capacity of open channels, pipes, and storm sewers. Values for Manning's roughness coefficient (n) shall be consistent with Table B-3 in Appendix B of this chapter.[5] Full flow shall be assumed for closed conduits.
[5]
Editor's Note: Appendix B, Stormwater Management Design Criteria, is included as an attachment to this chapter.
H. 
Outlet structures for stormwater management facilities shall be designed to meet the performance standards of this Part 2 using any generally accepted hydraulic analysis technique or method.
I. 
The design of any stormwater detention facilities intended to meet the performance standards of this Part 2 shall be verified by routing the design storm hydrograph through these facilities using the Storage-Indication Method. For drainage areas greater than 200 acres in size, the design storm hydrograph shall be computed using a calculation method that produces a full hydrograph (i.e., TR-20, TR-55, HEC-1, PSRM). The municipality may approve the use of any generally accepted full hydrograph approximation technique that shall use a total runoff volume that is consistent with the volume from a method that produces a full hydrograph.
A. 
All wet basin designs shall incorporate biologic minimization controls consistent with the West Nile Guidance found in Appendix F.[1]
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
B. 
The design of any proposed roadway drainage facilities shall be consistent with the PennDOT Publication 13.M Manual Part II Title 40 Chapter.[2]
[2]
Editor's Note: So in original.
C. 
Storm sewers must be able to convey proposed conditions runoff from a fifty-year design storm without flooding inlets, where appropriate.
D. 
Adequate erosion protection shall be provided along all open channels, and at all points of discharge (PADEP Erosion, Sediment and Pollution Control Manual).
E. 
The municipality reserves the right to disapprove any design that would result in the construction in or continuation of a stormwater problem area.
F. 
No stormwater detention facility shall be placed within 50 feet of a special geologic feature. No stormwater conveyance facility shall be constructed within 50 feet of a special geologic feature, unless it is constructed of durable pipe utilizing watertight joints.