A. 
All regulated activities in Greene Township which do not fall under the exemption criteria shown in § 80-20 shall submit a drainage plan consistent with the provisions of this chapter to the municipality for review. This criteria shall apply to the total proposed development even if development is to take place in stages. Impervious cover shall include, but not be limited to, any roof, parking or driveway areas and any new streets and sidewalks. Any areas designed to initially be gravel or crushed stone shall be assumed to be impervious for the purposes of comparison to the exemption criteria.
B. 
All stormwater management facility designs shall consider entire watersheds, include provisions for sound and stable construction, proper use of modern and readily available materials, with the primary consideration for public safety and preventing property damage.
C. 
Stormwater drainage systems shall be provided in order to permit unimpeded flow along natural watercourses, except as modified by stormwater management facilities or open channels consistent with this chapter.
D. 
Innovative designs and methods for stormwater retention may be used with the Township's approval. Design methods may be tailored to meet unique site topography, site conditions, and building construction, including the use of existing ponds or basins, rooftop storage, parking lot storage, seepage pits, and reductions in existing impervious areas.
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 chapter. If diffused drainage discharge is proposed to be concentrated and discharged onto adjacent property, the developer must document that adequate downstream conveyance facilities exist to safely transport the concentrated discharge, or otherwise prove that no erosion, sedimentation, flooding or other harm will result from the concentrated discharge. Stormwater discharge from a proposed site onto adjacent property shall not exceed existing (predeveloped) site discharge. Discharge from a proposed site shall discharge in the same manner as existing flows into existing channels and/or culverts capable of handling the proposed discharge. Where an existing channel is not available to discharge flows from a proposed site, provisions shall be made to further detain proposed discharge flows, or convert proposed flows into a pattern which matches existing flow type and capacities, or provide other means of downstream flow accommodation which will not cause undue hardship of erosion or concentrations of water on downstream property owners.
F. 
Where a development site is traversed by watercourses, drainage easements shall be provided conforming to the line of such watercourses. The terms of the easement shall prohibit excavation, the placing of fill or structures, and any alterations that may adversely affect the flow of stormwater within any portion of the easement. Maintenance, including mowing of vegetation within the easement, shall be required, unless if otherwise approved by the Greene Township Board of Supervisors.
G. 
When it can be shown that, due to topographic conditions, natural drainageways on the site cannot adequately provide for drainage, open channels may be constructed conforming substantially to the line and grade of such natural drainageways. Work within natural drainageways shall be subject to approval by PaDEP through the joint permit application process, or, where deemed appropriate by PaDEP, through the general permit process.
H. 
Any stormwater management facilities regulated by this chapter 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 developer 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.
I. 
Any stormwater management facilities regulated by this chapter that would be located on state highway rights-of-way shall be subject to approval by the Pennsylvania Department of Transportation (PennDOT).
J. 
Minimization of impervious surfaces and infiltration of runoff through seepage beds, infiltration trenches, etc. are encouraged strongly, where soil conditions permit, to reduce the size or eliminate the need for detention facilities.
K. 
Roof drains must not be connected to streets, sanitary or storm sewers or roadside ditches to promote overland flow and infiltration/ percolation of stormwater where advantageous to do so. When it is more advantageous to connect directly to streets or storm sewers, then it shall be permitted on a case-by-case basis by the Greene Township Board of Supervisors.
L. 
Special requirements for watersheds draining to high quality (HQ) and exceptional value (EV) waters. The temperature and quality of water and streams that have been declared as exceptional value and high quality are to be maintained as defined in Chapter 93, Water Quality Standards, Title 25 of Pennsylvania Department of Environmental Protection Rules and Regulations. Temperature-sensitive BMPs and stormwater conveyance systems are to be used and designed with storage pool areas and supply outflow channels and should be shaded with trees. This will require modification of berms for permanent ponds and the relaxation of restrictions on planting vegetation within the facilities, provided that capacity for volumes and rate control is maintained. At a minimum, the southern half on pond shorelines shall be planted with shade or canopy trees within 10 feet of the pond shoreline. In conjunction with this requirement, the maximum slope allowed on the berm area to be planted is 10 to 1. A long-term maintenance schedule and management plan for the thermal control BMPs is to be established and recorded for all development sites.
M. 
Where this chapter does not specify materials or construction procedures, PennDOT Form 408 Specifications and PennDOT RC Series Roadway Construction Standards shall be used.
A. 
In addition to the requirements specified below, the groundwater recharge (§ 80-16), water quality (§ 80-17), and streambank erosion (§ 80-18), requirements shall be implemented.
B. 
Standards for managing runoff in the Township are shown below. Development sites located in the Township must control postdevelopment runoff rates to predevelopment runoff rates for the design storms as follows:
Design Storm Postdevelopment
Design Storm Predevelopment
2-year
1-year
5-year
5-year
10-year
10-year
25-year
25-year
100-year
100-year
A. 
General. Postdevelopment rates of runoff from any regulated activity shall meet the peak release rates of runoff prior to development for the design storms specified in § 80-11 of this chapter.
B. 
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.
C. 
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.
D. 
Downstream hydraulic capacity analysis. Any downstream capacity hydraulic analysis conducted in accordance with this chapter shall use the following criteria for determining adequacy for accepting increased 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 Erosion and Sediment Pollution Control Program Manual.
(2) 
Natural or man-made channels or swales must be able to convey increased twenty-five-year return period runoff without creating any hazard to persons or property.
(3) 
Culverts, bridges, storm sewers or any other facilities which must pass or convey flows from the tributary area must be designed in accordance with DEP Chapter 105 regulations (if applicable) and, at minimum, pass the increased twenty-five-year return period runoff.
E. 
Regional detention alternatives. For certain areas within the Township, it may be more cost-effective to provide one control facility for more than one development site than to provide an individual control facility for each development site. The initiative and funding for any regional 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 model as developed for the stormwater management plan.
A. 
Any stormwater facility located on state highway rights-of-way shall be subject to approval by the Pennsylvania Department of Transportation (PennDOT).
B. 
Any stormwater management facility (i.e., detention basin) designed to store runoff and requiring a berm or earthen embankment required or regulated by this chapter shall be designed to provide an emergency spillway to handle flow up to and including the one-hundred-year postdevelopment conditions. The height of embankment must be set as to provide a minimum 1.0 foot of freeboard above the maximum pool elevation computed when the facility functions for the one-hundred-year postdevelopment inflow. Should any stormwater management facility require a dam safety permit under PaDEP Chapter 105, the facility shall be designed in accordance with Chapter 105 and meet the regulations of Chapter 105 concerning dam safety which may be required to pass storms larger than one-hundred-year event.
C. 
Any facilities that constitute water obstructions (e.g., culverts, bridges, outfalls, or stream enclosures), and any work involving wetlands as directed 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. Any other drainage conveyance facility that does not fall under Chapter 105 regulations must be able to convey, without damage to the drainage structure or roadway, runoff from the twenty-five-year design storm with a minimum 1.0 foot of freeboard measured below the lowest point along the top of 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 1.0 foot of freeboard measured below the lowest point along the top of roadway. Any facility that constitutes a dam as defined in PaDEP Chapter 105 regulations may require a permit under dam safety regulations. Any facility located within a PennDOT right-of-way must meet PennDOT minimum design standards and permit submission requirements.
D. 
Any drainage conveyance facility and/or channel that does not fall under Chapter 105 regulations must be able to convey, without damage to the drainage structure or roadway, runoff from the ten-year design storm. Conveyance facilities to or exiting from stormwater management facilities (i.e., detention basins) shall be designed to convey the design flow to or from that structure. Roadway crossings located within designated floodplain areas must be able to convey runoff from a one-hundred-year design storm. Any facility located within a PennDOT right-of-way must meet PennDOT minimum design standards and permit submission requirements.
Stormwater runoff from all development sites shall be calculated using either the rational method or a soil-cover-complex methodology.
A. 
Any stormwater runoff calculations shall use a generally accepted calculation technique that is based on the NRCS soil cover complex method. Table 80-14-1 summarizes acceptable computation methods. It is assumed that all methods will be selected by the design professional based on the individual limitations and suitability of each method for a particular site. The Township 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 is recommended for drainage areas greater than 200 acres.
B. 
All calculations consistent with this chapter using the soil cover complex method shall use the appropriate design rainfall depths for the various return period storms according to the region for which they are located as presented in Table B-1 in Appendix B of this chapter.[1] If a hydrologic computer model such as HEC-HMS or HEC-1 is used for stormwater runoff calculations, then the duration of rainfall shall be 24 hours. The alternating block method to develop design hyetographs, described in Figure B-1, Appendix B of this chapter, shall be used for the rainfall distribution.
C. 
For the purposes of predevelopment 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-2 or B-3 in Appendix B of 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 Design Storm Curves from PA Department of Transportation Design Rainfall Curves (1986) (Figures B-3 to B-4). 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.
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-2 in Appendix B of this chapter.[2]
F. 
Runoff coefficients (c) for both existing and proposed conditions for use in the rational method shall be obtained from Table B-3 in Appendix B of 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-4 in Appendix B of this chapter. Outlet structures for stormwater management facilities shall be designed to meet the performance standards of this chapter using any generally accepted hydraulic analysis technique or method.
H. 
Time-of-concentration method. Time of concentration shall be calculated based on three flow patterns:
(1) 
Sheet flow shall be a maximum of 150 feet using the equation:
Tc
=
(0.667 L n/S0.5)0.467
Where:
Tc
=
Time of concentration in minutes
L
=
Length of flow path in feet
n
=
0.02 for smooth paved surfaces
0.1 for bare packed soil or stone areas
0.3 for poor grass cover or cultivated fields
0.4 for average grass or light brush cover
0.8 for dense grass or thick brush cover
S
=
Average slope of the path in feet/foot
(2) 
Shallow concentrated flow shall be calculated using the equations:
Paved areas
Tc = L/1220 S0.5
Unpaved areas
Tc = L/968 S0.5
Where:
Tc
=
Time of concentration in minutes
L
=
Length of flow path in feet
S
=
Average slope of the path in feet/foot
(3) 
Concentrated flow in depressions, swales, gutters, curbs and pipes shall be calculated using the Manning's Equation and/or other acceptable engineering practices.
(4) 
Times of concentration based on computer models which use the rational method as a basis of design may be used with prior approval of the Township.
I. 
The design of any stormwater detention facilities intended to meet the performance standards of this chapter 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. The Township 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.
TABLE 80-14-1
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, or as approved by the Township and the Township Engineer
Other methods and/or
Varies
Other computation methodologies approved by the Township and the Township Engineer
A. 
Whenever the vegetation and topography are to be disturbed, such activity must be in conformance with Chapter 102, Title 25, Rules and Regulations, Part I, Commonwealth of Pennsylvania, Department of Environmental Protection, Subpart C, Protection of Natural Resources, Article II, Water Resources, Chapter 102, "Erosion Control," and in accordance with the Franklin County Conservation District.
B. 
Additional erosion and sedimentation control design standards and criteria that must be or 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 so as to maintain their maximum infiltration capacity.
(2) 
Infiltration BMPs shall not be constructed nor receive runoff until the entire contributory drainage area to the infiltration BMP has received final stabilization.
A. 
The ability to retain and maximize the groundwater recharge capacity of the area being developed is encouraged strongly. Design of the infiltration/recharge 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 paved and roofed over. These measures are encouraged, particularly in hydrologic soil groups A and B and should be utilized wherever feasible. Soils used for the construction of basins shall have low-erodibility factors ("K" factors).
B. 
Infiltration BMPs shall meet the following minimum requirements:
(1) 
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 facility and the seasonal high-water table and/or bedrock (limiting zones).
(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 owner's professional designer or as otherwise approved by the Township Engineer.
(2) 
Infiltration BMPs receiving only roof runoff may be placed in soils having a minimum depth of 24 inches between the bottom of the facility and the limiting zone.
(3) 
The size of the recharge facility shall be based upon the following equation (DEP):
Rev
=
[(S) (Rv) (A)]/12
Where:
Rev
=
Recharge Volume (acre-feet)
S
=
Soil specific recharge factor (inches)
Rv
=
Volumetric runoff coefficient
A
=
Site area contributing to the recharge facility (acres)
And:
Rv
=
0.05 + 0.009 (I)
Where:
I
=
percent impervious area
And:
S may be obtained based upon hydrologic soil group based upon the table below, recommended as general values by DEP. More site-specific data may be determine recharge if justified by the design engineer and approved by the Township Engineer.
Hydrologic Soil Group
Soil Specific Recharge Factor (S)
(inches)
A
0.38
B
0.25
C
0.13
D
0.06
If more than one hydrologic soil group (HSG) is present at a site, a composite recharge volume shall be computed based upon the proportion of total site area within each HSG.
(4) 
The recharge volume provided at the site shall be directed to the most permeable HSG available, if feasible.
(5) 
The recharge facility shall be capable of completely infiltrating the recharge volume within 48 hours.
C. 
A detailed soils evaluation of the project site may be required to determine the suitability of recharge facilities. The evaluation shall be performed by a qualified design professional and, at a minimum, address soil permeability, depth to bedrock, susceptibility to sinkhole formation 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 watershed to determine general areas of suitability for infiltration practices.
(2) 
Provide field tests to determine appropriate percolation rate and/or hydraulic conductivity.
(3) 
Design infiltration structures for required storm volume based on field-determined capacity at the level of the proposed infiltration surface.
D. 
Extreme caution shall be exercised where infiltration is proposed in geologically susceptible areas such as strip mine or limestone areas. Extreme caution shall also be exercised where salt or chloride would be a pollutant since soils do little to filter this pollutant and it may contaminate the groundwater. It is also extremely important that the qualified design professional evaluate the possibility of groundwater contamination from the proposed infiltration/recharge facility and recommend a hydrogeologic justification study be performed if necessary. Whenever a basin will be located in an area underlain by limestone, a geological evaluation of the proposed location shall be conducted to determine susceptibility to sinkhole formations. The design of all facilities over limestone formations shall include measures to prevent groundwater contamination and, where necessary, sinkhole formation. The infiltration requirement in the High Quality/Exceptional Waters shall be subject to the Department's Chapter 93 and Antidegradation Regulations. The Township may require the installation of an impermeable liner in detention basins. A detailed hydrogeologic investigation may be required by the Township. It shall be the developer's responsibility to verify if the site is underlain by limestone. The following note shall be attached to all drainage plans and signed and sealed by the developer's engineer/surveyor/geologist:
"____________________________________, certify that the proposed detention basin (circle one) is/is not underlain by limestone."
E. 
The Township may require the developer to provide safeguards against groundwater contamination for uses which may cause groundwater contamination should there be a mishap or spill.
F. 
Where pervious pavement is permitted for parking lots, recreational facilities, nondedicated streets, or other areas, pavement construction specifications and maintenance schedules shall be noted on the plan.
G. 
Recharge/infiltration facilities may be used in conjunction with other innovative or traditional BMPs, stormwater management facilities, and nonstructural stormwater management alternatives.
A. 
In addition to the performance standards and design criteria requirements of Article III of this chapter, the land developer shall comply with the following water quality requirements of this article unless otherwise exempted by provisions of this chapter.
(1) 
Developed areas will provide adequate stormwater management and treatment facilities necessary to capture and treat stormwater runoff. The recharge volume computed under § 80-16 may be a component of the water quality volume. If the recharge volume is less than the water quality volume, the remaining water quality volume may be captured and treated by methods other than recharge/infiltration BMPs.
(2) 
The water quality volume (WQv) is the storage capacity needed to treat stormwater runoff produced by "P" inch of rainfall (90% Rule) from the developed areas of the site (for "P" values, see Appendix B, Table B-5). The following calculation formula is used to determine the storage volume, WQv, in acre-feet of storage:
WQv
=
[(P)(Rv)(A)]/12
WQv
=
Water Quality Volume
P
=
Rainfall amount (90% of events producing this rainfall (Appendix B, Table B-5)
A
=
Area in acres
Rv
=
0.05 + 0.009(I) where I is the impervious surface ratio
B. 
WQv shall be designed as part of a stormwater management facility which incorporates water quality BMPs as a primary benefit of using that facility, in accordance with design specifications contained in Pennsylvania Handbook of Best Management Practices for Developing Areas.
(1) 
The following factors shall be considered when evaluating the suitability of BMPs used to control water quality at a given development site:
(a) 
Total contributing drainage area.
(b) 
Permeability and infiltration rate of the site soils.
(c) 
Slope and depth to bedrock.
(d) 
Seasonal high-water table.
(e) 
Proximity to building foundations and wellheads.
(f) 
Erodibility of soils.
(g) 
Land availability and configuration of the topography.
(2) 
The following additional factors should be considered when evaluating the suitability of BMPs used to control water quality at a given development site:
(a) 
Peak discharge and required volume control.
(b) 
Streambank erosion.
(c) 
Efficiency of the BMPs to mitigate potential water quality problems.
(d) 
The volume of runoff that will be effectively treated.
(e) 
The nature of the pollutant being removed.
(f) 
Maintenance requirements.
(g) 
Creation/protection of aquatic and wildlife habitat.
(h) 
Recreational value.
(i) 
Enhancement of aesthetic and property value.
Applying the water quality criteria in § 80-17 above will also help the streambank erosion problem. Detain the two-year postdevelopment storm to the one-year predevelopment storm and detaining the one-year postdevelopment storm to a minimum of 24 hours.