1. 
General Design Guidelines.
A. 
Stormwater shall not be transferred from one watershed to another, unless: (1) the watersheds are sub-watersheds of a common watershed which join together within the perimeter of the property; (2) the effect of the transfer does not alter the peak rate discharge onto adjacent lands; or (3) easements from the affected landowner(s) are provided.
B. 
Consideration shall be given to the relationship of the subject property to the drainage pattern of the watershed. A concentrated discharge of stormwater to an adjacent property shall be within an existing watercourse or confined in an easement or returned to a predevelopment flow type condition.
C. 
Stormwater BMPs and recharge facilities are encouraged (e.g., rooftop storage, drywells, cisterns, recreation area ponding, diversion structures, porous pavements, holding tanks, infiltration systems, stream channel storage, inline storage in storm sewers, and grading patterns). They shall be located, designed and constructed in accordance with the latest technical guidance published by PADEP, provided they are accompanied by detailed engineering plans and performance capabilities and supporting site specific soils, geology, runoff and groundwater and infiltration rate data to verify proposed designs. Additional guidance from other sources may be accepted at the discretion of the Municipal Engineer (a preapplication meeting is suggested).
D. 
All existing and natural watercourses, channels, drainage systems and areas of surface water concentration shall be maintained in their existing condition unless an alteration is approved by the appropriate regulatory agency.
E. 
No outlet structure from a stormwater management facility, or swale, shall discharge directly onto a municipal or state-owned roadway.
F. 
The invert of all stormwater management facilities and underground infiltration/storage facilities shall be located a minimum of two feet above the seasonal high groundwater table or other soil limiting zone. The invert of stormwater facilities may be lowered if adequate subsurface drainage, which does not alter the existing water table level, is provided.
G. 
Any stormwater management facility may be required to be fenced with a minimum four-foot-high fence of material acceptable to Susquehanna Township. Gates with a minimum opening of 10 feet shall be provided for access.
(1) 
Nothing shall be placed within the fence area of the facility to avoid impact to its functionality.
H. 
Stormwater management facilities excavated to carbonate rock must either be fitted with an impervious clay liner or over-excavated four feet and refilled with a suitable material mix. Suitable backfill material is subject to the approval of the Municipal Engineer.
I. 
The type, location, and number of landscaping and planting specification shall be provided for all stormwater management facilities and be specified for each type of facility.
J. 
Erosion and sedimentation control measures must be applied surrounding infiltration structures during installation to prevent the infiltrative surfaces from becoming clogged. Additional erosion and sedimentation control design standards and criteria must 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. 
Soils Evaluation.
A. 
A detailed soils evaluation of the project site shall be performed to determine the suitability of recharge facilities. The evaluation shall be performed by a qualified professional, and at a minimum, address soil permeability, depth of bedrock, susceptibility to sinkhole formation, and subgrade stability, hydrologic soil groups and natural and man-made features.
B. 
Soil percolation or infiltration rates must be based on actual field tests and may not be assumed from data contained within the Dauphin County Soils Survey.
C. 
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 pollutant since soils do little to filter this pollutant and it may contaminate the groundwater. It is also extremely important that the 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.
D. 
The Township may require the installation of an impermeable liner in detention basins. A detailed hydrogeologic investigation may be required by the Township. 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.
E. 
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/landscape architect/geologist: "I, __________, certify that the proposed detention basin (circle one) is/is not underlain by limestone."
F. 
Where pervious pavement is permitted for parking lots, recreational facilities, non-dedicated streets, or other areas, pavement construction specifications shall be noted on the plan.
G. 
Recharge/infiltration facilities may be used in conjunction with other innovative or traditional BMPs, stormwater control facilities, and nonstructural stormwater management alternatives.
H. 
All recharge/infiltration facilities shall be designed to completely drain within 72 hours from the end of the storm.
I. 
Infiltration structures shall be designed for required storm volume based on field-determined capacity at the elevation of the proposed infiltration surface.
3. 
Stormwater management facilities (with a depth of water equal to or greater than three feet measured from the lowest point inside a facility to the crest of the emergency spillway).
A. 
Any stormwater management facility designed to store runoff and requiring a berm or earthen embankment shall be designed to provide an emergency spillway to handle peak rate or stormwater runoff up to and including the 100-year post-development flow, with a blocked primary outlet structure. The height of embankment must be set as to provide a minimum one-foot of freeboard through the spillway, above the maximum water surface elevation, computed when the spillway functions for the 100-year post-development inflow, with a blocked outlet structure. The primary outflow structure must be designed to pass all design storms (up to and including the 100-year event) without discharge through the emergency spillway. The maximum water depth within any stormwater management facility shall be no greater than eight feet when functioning through the primary outlet structure.
B. 
Emergency spillways shall be armored to prevent erosion during the 100-year post-development flow, with blocked primary outlet structure. Synthetic liners or riprap must be used, and calculations sufficient to support proposed armor must be provided. An earthen plug must be used to accurately control the spillway invert if riprap is the proposed armoring material. Emergency spillway armor must extend up the sides of the spillway and continue at full width to a minimum 10 feet past the toe of slope.
C. 
A stormwater management facility berm cross section must be at least five feet wide at the top and eight feet wide through the emergency spillway. For all embankments, the side slope shall be no steeper than 3:1 on the inside of the facility and 2:1 on the outside of the facility. For cut slopes, the side slopes shall be no steeper than 2:1.
D. 
A cutoff and key trench of impervious material shall be provided under all embankments four feet or greater in height.
E. 
Soils used for the construction of stormwater management facilities shall have low-erodibility factors ("K" factors) (refer to E&S Manual) and be identified on the SWM Site Plan.
F. 
Trash racks must be provided to prevent clogging of primary outflow structure stages for all orifices equivalent to 12 inches or smaller in diameter.
G. 
Anti-seep collars must be provided on all outflow culverts in accordance with the methodology contained in the latest edition of the E&S Manual. An increase in seepage length of 15% must be used in accordance with the requirements for permanent anti-seep collars.
H. 
Conventional, non-BMP stormwater management facilities (i.e., dry detention basins) must empty over a period of time not less than 24 hours and not more than 72 hours from the end of the facility's inflow hydrograph. Infiltration tests performed at the facility locations and proposed basin bottom depths, in accordance with the BMP Manual, must support time-to-empty calculations if infiltration is a factor is the sizing of the stormwater management facility.
I. 
Impervious low-flow channels are not permitted within stormwater management facilities to promote water quality and groundwater recharge for frequent storm events. Facilities designed as water quality/infiltration BMPs may have a bottom slope of zero. Minimal maintenance, saturation tolerant vegetation must be provided in basins designed as water quality/infiltration BMPs. Conventional, non-BMP stormwater management facilities must have a minimum slope of 1% extending radially out from the primary outlet structure. Water storage below the lowest outlet structure stage (i.e., dead storage) is permitted in stormwater management facilities designed as water quality/infiltration BMPs.
J. 
Stormwater management facilities bottom elevations must be greater than adjacent floodplain elevations (FEMA or HEC-RAS analysis). If no floodplain is defined, bottom elevations must be higher than existing ground elevations 50 feet from top of stream bank in the facilities' vicinity.
K. 
Basin outflow culverts discharging into floodplains must account for tailwater. Tailwater corresponding to the 100-year floodplain elevation may be used for all design storms, or the applicant is assumed to be 50 feet from top of stream bank in areas where a floodplain is not designated, or where no other evidence is provided.
L. 
Exceptions to these requirements may be made at the discretion of Susquehanna Township for BMPs that retain or detain water, but are of a much smaller scale than traditional stormwater management facilities.
M. 
Any stormwater facility located on or discharging to state highway rights-of-way or state-owned stormwater management facilities shall be subject to approval by PennDOT. The existing points of concentrated drainage that discharge onto adjacent property shall not be altered without permission of the adjacent property owner(s) and shall be subject to any applicable discharge criteria specified in this Chapter.
N. 
If diffused flow 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,
4. 
Storm Sewer Facilities.
A. 
Storm sewers must be able to convey post-development runoff from a 10-year design storm without surcharging inlets where appropriate. When connecting to an existing storm sewer system, the applicant must demonstrate that the proposed system will not exacerbate any existing stormwater problems and that adequate downstream capacity exists. Refer to § 19-306.
B. 
In proposed curbed roadway sections, the maximum encroachment of water on the roadway pavement shall not exceed half of a through travel lane or one inch less than the depth of curb during the 10-year design storm of five-minute duration. Gutter depth shall be verified by inlet capture/capacity calculations that account for road slope and opening area. The maximum distance between inlets in curbed roadway sections shall be no more than 600 feet, however access to underground pipes shall be provided every 300 feet.
C. 
At all roadway low points, swales and easements shall be provided behind the curb or swale and through adjacent properties to channelize and direct any overflow of stormwater runoff away from dwellings and structures.
D. 
Inlets, manholes, pipes, and culverts shall be constructed in accordance with the specifications set forth in PennDOT's Publication 408, latest edition, and as detailed in the PennDOT's Publication 72M, Standards for Roadway Construction (RC), latest edition, or as approved by the Municipal Engineer. All material and construction details (inlets, manholes, pipe trenches, etc.) must be show on the SWM Site Plan, and a note added that all construction must be in accordance with PennDOT's Publication 408 and PennDOT's Publication 72M, latest edition. A note shall be added to the plan stating that all frames, concrete top units, and grade adjustment rings shall be set in a bed of full mortar according to Publication 408.
E. 
At a minimum, pipe capacities shall be computed using both the Manning Equation for full flow capacity and the Inlet Control Nomographs in Hydraulic Design Series 5 by the Federal Highway Administration, dated September 1985. For inlets, the maximum headwater elevation must be less than the top of grate. Outlet control calculations shall be required for special circumstances such as high tailwater conditions, long runs or pipe, excessive bends, and other losses, etc. Alternate methods of headwater computation may be accepted at the discretion of the Municipal Engineer.
F. 
Pavement base drain shall be provided at all low points in cut areas, toe of slope areas, and other areas as dictated by proven engineering principles and design judgement. All base drain shall be in accordance with PennDOT Publication 408.
G. 
In certain instances, primarily within the provisional no detention districts, 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 runoff 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 § 19-306. In addition, all new development upstream of a proposed capacity improvement shall be assumed to implement the applicable runoff controls consistent with this Chapter, 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.
H. 
Storm drainage facilities and appurtenances shall be so designed and constructed as to minimize the erosion at discharge points and in watercourse channels. Adequate erosion protection shall be provided along all open channels and at all points of discharge.
I. 
Pipe Design.
(1) 
A minimum pipe size of 15 inches in diameter shall be used in all roadway systems (public or private) proposed for construction. Pipes shall be designed to provide a minimum velocity of two and one-half feet per second when flowing full, but in all cases, the slope shall be no less than 0.5%. Arch pipe of equivalent cross-sectional area may be substituted in lieu of circular pipe where cover or utility conflict conditions exist.
(2) 
All storm drainage piping (equal to or greater than 12 inches) discharging to the ground surface shall be provided with either reinforced concrete headwalls and end sections or plastic and metal pipe end sections compatible with the pipe size involved in accordance with PennDOT Publication 408 and Publication 72M.
(3) 
Accessible drainage structures shall be located on continuous storm sewer system at all vertical dislocations, at all locations where a transition in storm sewer pipe sizing is required, at all vertical and horizontal angle points exceeding 5°, and at all points of convergence of two or more storm sewer pipes.
(4) 
Pipe sizes, lengths, and slopes shall be shown on the approved drawings.
J. 
Inlets.
(1) 
Standard Type "C" inlets with eight-inch hoods shall be used along vertical concrete curbs roadway networks. Type "C" inlets with 10-inch hoods that provide a two-inch sump condition may be used with approval of the Municipal Engineer when roadway longitudinal slopes are 1.0% or less.
(2) 
For inlets containing a change in pipe size, the elevation for the crow of the pipes shall be the same or the smaller pipe's crown shall be at a higher elevation.
(3) 
All inlets shall provide a minimum two-inch drop between the lowest inlet pipe invert elevation and the outlet pipe invert elevation.
(4) 
On curbed sections, a double inlet shall be placed at the low point of sag vertical curves, or an inlet shall be placed at the low point and on each side of the low point at a distance not to exceed 100 feet, or at an elevation not to exceed 0.2 feet above the low point.
(5) 
Inlets shall be placed so drainage cannot cross intersections or street center lines.
(6) 
All inlets in paved areas shall have heavy-duty bicycle-safe grating consistent with PennDOT Publication 72M. A note to this effect shall be added to the SWM Site Plan or inlet details therein.
(7) 
Inlets must be sized to accept the specified pipe sizes without knocking out any of the inlet corners. All pipes entering or exiting inlets shall be cut flush with the inlet wall. A note to this effect shall be added to the SWM Site Plan or inlet details therein.
(8) 
Inlets shall have weep holes covered with geotextile fabric placed at appropriate elevations to completely drain the subgrade prior to placing the base and surface course on roadways.
(9) 
Inlets, junction boxes, or manholes greater than five feet in depth shall be equipped with ladder rungs and shall be detailed on the SWM Site Plan.
(10) 
Inlets shall not have a sump condition in the bottom (unless designed as a water quality BMP). Pipes shall be flush with the bottom of the box or concrete channels shall be poured.
(11) 
The maximum allowable spread of water on streets is 1/2 of a travel lane. Computations should be provided which demonstrate compliance with this spread requirement.
K. 
Manholes.
(1) 
Manholes shall be constructed in accordance with the specifications as set forth in the PennDOT, Publication 408, as amended and as detailed on the Roadway Construction Standard drawings (RC-39).
(2) 
Manholes shall be pre-cast concrete units.
L. 
Roof Leaders, Foundation Drains, Springs and Sump Pumps.
(1) 
In order to promote overland flow and infiltration/percolation of stormwater where it is advantageous to do so, roof drains, foundation drains, sump pumps, etc., shall not be connected to streets, sanitary sewers, or storm sewers. When it is more advantageous to connect directly to streets or storm sewers, the Township shall permit it on a case-by-case basis.
(2) 
Stormwater, groundwater, and/or spring runoff drains shall not discharge water directly over a sidewalk.
(3) 
Stabilized outlets shall be provided for footer drains, flood drains, and downspouts.
(4) 
All underground or surface springs encountered during or after construction of roadways or buildings shall be adequately conveyed over land to a natural drainageway, to the maximum extent feasible. The Township may permit alternatives on a case-by-case basis. The Township Public Works Director or Township Engineer shall be contacted when a spring is encountered. The Director or Engineer may make a site investigation and make written recommendations to the developer for correcting the problem. Such recommendations shall be binding on the developer unless the Board of Commissioners, upon the request of the developer, agree to permit an alternative solution.
(5) 
Subbase drains shall be provided at all low points in cut areas, toe of slope areas and other areas as dedicated by proven engineering principles and design judgement. Subbase drains shall be designed and constructed in accordance with PennDOT Design Criteria and Publication 408 Specifications.
(6) 
All land development projects shall be designed and constructed in order to provide proper drainage of stormwater runoff with minimal impact on adjoining properties, unless an area specifically designed for stormwater detention is provided.
M. 
Outlets.
(1) 
Outlet protection shall be provided at all surface drainage points with storm drainage piping (equal to or greater than 12 inches) in order to minimize erosion consistent with the E&S Manual.
(2) 
Outlet structures shall include appropriate and adequately sized riprap dissipators to ensure velocity control is managed at the discharge point.
(3) 
Outlet structures shall discharge a minimum of 20 feet away from the proposed stream or next conveyance system.
5. 
Swale Conveyance Facilities.
A. 
Swales must be able to convey post-development runoff from a 10-year design storm with six inches of freeboard to top of the swale.
B. 
Swales shall have side slopes no steeper than 3:1.
C. 
All swales shall be designed, labeled on the SWM Site Plan, and details provided to adequately construct and maintain the design dimension of the swales.
D. 
Swales shall be designed for stability using velocity or shear criteria. Velocity criteria may be used for channels with less than 10% slope. Shear criteria may be used for all swales. Documentation must be provided to support velocity and/or shear limitations used in calculations.
E. 
Where swale bends occur, the computed velocities or shear stresses shall be multiplied by the following factor for the purpose of designing swale erosion protection:
1.75
When swale bend is 30° to 60°
2.00
When swale bend is 60° to 90°
2.50
When swale bend is 90° or greater
F. 
Swales must be designed for both temporary and permanent conditions in accordance with the latest E&S Manual.
1. 
All calculations shall be consistent with the guidelines set forth in the BMP Manual. Stormwater runoff from all development sites shall be calculated using either the Rational Method or a Soil Cover Complex Methodology. Methods shall be selected by a qualified professional based on the individual limitations and suitability of each method for a particular site.
2. 
Rainfall Values.
A. 
Rational Method. The PennDOT, Intensity-Duration-Frequency Curves, Publication 584, Chapter 7A, latest edition, shall be used in conjunction with the appropriate time of concentration and return period.
B. 
Soil Cover Complex Method. The Soil Conservation Service Type II, 24-hour rainfall distribution shall be used in conjunction with rainfall depths from NOAA Atlas 14 or consistent with the following table:
RETURN INTERVAL
(YEAR)
24-HOUR RAINFALL TOTAL
(INCHES)
1
2.40
2
2.90
10
4.36
25
5.43
50
6.38
100
7.48
3. 
Peak Flow Rates.
A. 
Rational Method. May be used for drainage areas up to 20 acres. Extreme caution should be used by the qualified professional if the watershed has more than one main drainage channel, if the watershed is divided so that hydrologic properties are significantly different in one versus the other, if the time of concentration exceeds 60 minutes, or if the stormwater runoff volume is an important factor. The combination of Rational Method hydrographs based on timing shall be prohibited.
(1) 
The use of the Modified Rational Method to design stormwater management facilities must be approved by the Municipal Engineer.
B. 
Soil Cover Complex Method. May be used for drainage areas greater than 20 acres. This method is recommended for design of stormwater management facilities where stormwater runoff volume must be taken into consideration.
C. 
For comparison of peak flow rates, flows shall be rounded to a tenth of a cubic foot per second (cfs).
4. 
Runoff Coefficients.
A. 
Rational Method. Use Table C-1 (Appendix C).[1]
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
B. 
Soil Cover Complex Method. Use Table C-2 (Appendix C).
C. 
For the purposes of predevelopment peak flow rate and volume determination, existing nonforested pervious areas condition shall be considered as meadow (good condition).
D. 
For the purposes of predevelopment peak flow rate and volume determination, 20% of existing impervious area, when present on the project site, and contained within the new proposed limit of disturbance, shall be considered meadow (good condition) for predevelopment hydrologic calculations for redevelopment.
(1) 
This does not apply when dealing in the circumstance of cumulative impervious area added to the property since the adoption of the Stormwater Management Ordinance (10/10/2010). Any impervious area added to the property after the adoption date and which did not reach the stormwater management volume control criteria (1,000 square feet of IA) must include the management of stormwater into the project, for all impervious area added over such time.
5. 
Design Storm.
A. 
All drainage facilities (inlets, pipes, and swales) shall be designed to safely convey 10-year storm.
B. 
All stormwater management facilities shall be verified by routing the proposed one-, two-, 10-, 25-, 50-, and 100-year hydrographs through the facility using the storage indication (Modified Puls) method. The design storm hydrographs shall be computed using a calculation method that produces a full hydrograph.
C. 
The stormwater management and drainage system shall be designed to safely convey the post-development 100-year storm event to stormwater detention facilities, for the purpose of meeting peak rate control.
D. 
All structures (culvert or bridges) proposed to convey stormwater runoff under a municipal road shall be designed to pass the 50-year design storm with a minimum one-foot of freeboard measured below the lowest point along the top of the roadway.
E. 
All design within state or federal rights-of-way or that falls under the design criteria of any higher authority must meet the requirements of that agency in addition to meeting the minimum requirements of this Chapter.
6. 
Time of Concentration.
A. 
Time of concentration shall be computed using the NRCS Segmental Method as described in TR-55 (SCS 1986 or most current update). The length of sheet flow shall be limited to 100 feet. The Manning's "n" Roughness Coefficient for TR-55 sheet flow can be found in Table C-4 (Appendix C). Time of concentration for channel and pipe flow shall be computed using Manning' equation.
B. 
For sites with insignificant channelized flow and less than 20% imperviousness coverage, the time of concentration may be computed using the NRCS equation for lag time:
Time of Concentration = Tc = [(Tlag/0.6)*60] (minutes)
 
Where:
Tlag
=
Lag times (hours)
L
=
Hydraulic length of watershed (feet)
Y
=
Average overland slope of watershed (percent)
S
=
Maximum retention in watershed as defined by: S = [(1,000/CN) - 10]
CN
=
NRCS Curve Number for watershed as defined by NRCS Loss Method
C. 
Additionally, the following provisions shall apply to calculations for time of concentration:
(1) 
The post-development time of concentration shall never be greater that the predevelopment time of concentration for any watershed or subwatershed.
(2) 
The minimum time of concentration for any watershed shall be five minutes.
(3) 
The designer may choose to assume a five-minute time of concentration for post-development watershed or subwatershed without providing any computations.
(4) 
The designer must provide computations for all predevelopment time of concentration paths. A five-minute time of concentration cannot be assumed for predevelopment.
(5) 
Undetained fringe areas (areas that are not tributary to a stormwater facility but where a reasonable effort has been made to convey runoff from all new impervious coverage to best management practices) may be assumed to represent the predevelopment conditions for purpose of time of concentration calculations.
7. 
Drainage areas tributary to sinkholes or closed depressions in areas underlain by limestone or carbonate geologic features shall be excluded from the modeled point of analysis defining predevelopment flows. If left undisturbed during construction activities, areas draining to closed depressions may also be removed from peak runoff rates in post-development analysis. New, additional contributing runoff shall not be directed to existing sinkholes or closed depressions.
8. 
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. The Manning Equation should not be used for analysis of pipes under pressure flow or for analysis of culverts. Manning's "n" values shall be obtained from Table C-3 (Appendix C). Inlet control shall be checked at all inlet boxes to ensure the headwater depth during the 10-year design event is contained below the top of grate for each inlet box.
9. 
Susquehanna 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.
10. 
Susquehanna Township has the authority to require that computed existing runoff rates be reconciled with field observations, conditions, and site history. If the designer can substantiate, through actual physical calibration, that more appropriate runoff and time of concentration values should be utilized at a particular site, then appropriate variations may be made upon review and approval of Susquehanna Township.
All elements stated above in this Chapter are applicable to this Section, as well as the following.
1. 
Stormwater Management Districts.
A. 
The Township is hereby divided into stormwater management districts in accordance with the Paxton Creek Stormwater Management Plan, the Paxton Creek Release Rate Map, the Spring Creek Stormwater Management Plan and the Spring Creek Release Rate Map. The official release rate district maps are available for inspection at the Township office and by written request. All areas within the districts are subject to the specified release rate. The two-, 10-, and 25-year design storms are all subject to the specified release rates.
B. 
Specific Watershed Requirements.
(1) 
Paxton Creek Watershed.
(a) 
Specified Release Rate Districts. For all districts in this category, the future runoff must be controlled to the specified release rate. That is, the post-development runoff rate must be less than or equal to the predevelopment rate multiplied by the specified release rate for that district.
(b) 
Provisional No Detention District. These watershed areas may discharge post-development peak runoff without detention facilities. However, the developer must prove that the "local" runoff conveyance facilities which transport runoff from the site to the main channel have adequate capacity to safely transport unattenuated increased peak flows for a 25-year storm in accordance with § 19-306. If there is inadequate capacity, the developer shall either use 100% release rate control or provide increased capacity of downstream drainage facilities to convey increase peak flows consistent with § 19-306. When 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 will use the least restrictive runoff controls specified by this Chapter.
(2) 
Spring Creek Watershed.
(a) 
Specified Release Rate Districts. For all districts in this category, the future runoff must be controlled to the specified release rate. That is, the post-development runoff rate must be less than or equal to the predevelopment rate multiplied by the specified release rate for that district.
(3) 
Areas Not Within the Paxton Creek or Spring Creek Watershed. For all areas within the Township but outside the Paxton Creek or Spring Creek Watersheds, the future peak rate of runoff for the two-, 10-, and 25-year design storms must be controlled to the existing rate.
(4) 
The exact location of any given development site, or activity regulated by this Chapter relative to the release rate district boundaries shall be determined by mapping the release rate district boundaries using two-foot topographic contour mapping provided as part of the Drainage Plan.
(5) 
For the proposed development site located entirely within one release rate district, the total runoff from the site shall meet the required release rate criteria. For sites with multiple discharge points, any individual point may be designed to a 100% release rate, provided that the total runoff from the site is controlled to the applicable release rate for that district.
(6) 
For a proposed development site which is located within two or more release rate districts, the maximum runoff discharged at any point shall be equal to the release rate for the district in which the discharge point is located. In the event that a portion of the site is located in a provisional no detention area, no runoff from portions of the site located in areas subject to release rate controls may be drained to or through the provisional no detention area.
(7) 
Each development site shall be considered separately and shall conform to the criteria of the district in which it is located. In no case may the release rate for an area be exceeded as credit for a reduction below the criteria or any other restrictions on a separate site.
(8) 
Regional detention facilities will be permitted, provided that adequate conveyance is available or provided from the site to the facility. The acceptability and discharge characteristics of the facility will be determined on a case-by-case basis, by the Township Engineer, using the calibrated model developed for the Paxton Creek Act 167 Plan or Spring Creek Act 167 Plan, as applicable.
2. 
Stormwater Management District Implementation Provisions.
A. 
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.
B. 
Regional Detention Alternatives. For certain areas within the watershed, 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 is the responsibility of prospective developer(s). 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 Paxton Creek Stormwater Management Plan.
C. 
Capacity improvements may be provided as necessary to permit direct discharge in a provisional no detention district, to implement any regional or subregional detention alternatives.
3. 
Design Standards for Stormwater Management and Conveyance Facilities.
A. 
The existing points of natural drainage discharge onto adjacent property shall not be altered without the written approval of the affected landowners.
B. 
Stormwater runoff or natural drainage water shall not be diverted so as to overload existing drainage systems, or create flooding or the need for additional drainage structures on other private properties or public lands, without approved provisions being made by the applicant for properly handling such conditions.
C. 
Where a watercourse runs across or through a subdivision, a drainage easement, which conforms with the line of such watercourse shall be provided.
D. 
Storm drainage facilities, as required, shall be placed in front of the curb line when located in a street right-of-way. When located in undedicated land, they shall be placed within an easement, as approved by the Township, who may require additional width of easement as circumstances warrant.
E. 
Retention/Detention Basin Criteria.
(1) 
Where a stormwater detention basin is proposed, the plan shall include a proposal to prevent such basin from becoming a safety hazard or public nuisance and for regular perpetual maintenance of the basin to insure its continuous effectiveness.
(2) 
All controls designed to meet the requirements of this Chapter shall meet the applicable release rate criteria for the two-, 10-, and 25-year return period storms.
(3) 
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 water obstruction that does not fall under Chapter 105 regulations must be able to convey, without damage to the drainage structure or roadway, runoff from the 25-year design storm with a minimum one-foot of freeboard measured below the lowest point along the top of the roadway.
(4) 
The design of all stormwater management facilities shall incorporate sound engineering principles and practices. The Township shall reserve the right to disapprove any design that would result in the occupancy or continuation of an adverse hydrologic or hydraulic condition within the watershed.
(5) 
Basins which are not designed to release all stormwater shall be specifically identified as retention basins or permanent pond basins. All other basins shall have provisions for dewatering, particularly the bottom, and shall not create swampy and/or unmaintainable conditions. Sod-lined or stone low flow channels may be used to dewater the bottom of the basin. The minimum slope of any surface on the bottom of the pond shall be 1.5%.
(6) 
Retention basins and/or detention basins which are designed with earth fill embankments shall incorporate the following standards:
(a) 
The minimum top width of embankments up to eight feet in height shall be equal to 2/3 of the embankment height, but in no case shall the tope width be less than five feet.
(b) 
The side slopes of the earth fill shall not be less than 3H:1V.
(c) 
A cut off trench and key trench of impervious material shall be provided under all embankments exceeding four feet in height.
(d) 
All pipes and culverts through embankments shall be fitted with properly spaced and sized cut off collars.
(e) 
No outlet structure from a detention basin or swale shall discharge directly onto a Township or state road, but may discharge into a culvert under a Township or state road.
(f) 
All facilities shall have access roads and easements where necessary.
(7) 
General Requirements for Stormwater Collection. The design criteria for storm sewers and man-made channels (i.e., swales) shall be the 10-year frequency storm, unless more restrictive frequencies are required by Chapter 105 of PADEP regulations, with intensities based on the times of concentration for PennDOT Intensity Duration Frequency (IDF) curves, and runoff coefficients based on land use for each drainage area. A minimum time of concentration of five minutes shall be used. Computation methods shall be based on the Rational Method or Soil Cover Complex Method. The Township, pursuant of recommendations of the Township Engineer, may require a design storm of higher intensity be used when on-site or surrounding conditions, such as history of flooding downstream or the capacity of the receiving stream or stormwater system indicate such a variation to be necessary.