A. 
Landowners or other applicants proposing regulated activities in the Township that do not fall under specified exemption criteria shall submit a stormwater management site plan consistent with this chapter and the applicable watershed stormwater management plan to the Township for review. The stormwater management criteria of this chapter shall apply to the total proposed development even if development is to take place in stages.
B. 
No regulated activity within the Township shall commence until the Township issues approval of a SWM plan that demonstrates compliance with the requirements of this chapter.
C. 
For any site with proposed regulated earth disturbance equal to or greater than one acre where, after a close evaluation of alternative site designs, it proves to be impracticable to meet the mandatory minimum volume and infiltration control standards of this chapter onsite, the Township may approve measures other than those in this chapter after consultation with and evaluation by DEP that the alternate site design meets state water quality requirements and does not conflict with state law, including, but not limited to, the Clean Streams Law.[1]
[1]
Editor's Note: See 35 P.S. § 691.1 et seq.
D. 
For any site with proposed regulated earth disturbance that is less than one acre in an area where, after a close evaluation of alternative site designs, it proves to be impracticable to meet any one or more of the mandatory minimum standards of this chapter onsite, the Township may approve measures other than those in this chapter.
E. 
The design of any site with proposed earth disturbance must minimize surface discharge of stormwater and the creation of impervious surfaces in order to maintain, as much as possible, the natural hydrologic regime.
F. 
A SWM site plan must be designed consistent with the sequencing provisions specified by this chapter to ensure maintenance of the natural hydrologic regime, to promote infiltration, and to protect groundwater and surface water quality and quantity.
G. 
Stormwater drainage systems shall be designed to preserve natural flow conditions to the maximum extent practicable.
H. 
Existing drainage discharge onto adjacent property shall not be altered in any manner without written permission from, and a maintenance access agreement with, the affected property owner(s). Such discharge shall be subject to any applicable discharge criteria specified in this chapter and still must meet the requirements of Act 167.
I. 
Areas of existing diffused drainage discharge, whether proposed to be concentrated or maintained as diffused drainage areas, shall be subject to any applicable discharge criteria in the general direction of existing discharge, except as otherwise provided by this chapter. 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.
J. 
Where a development site is traversed by a stream, drainage easements of 25 feet shall be provided on either side of, and conform to the line of, such streams.
K. 
Minimization of impervious surfaces and infiltration of runoff through seepage beds, infiltration trenches, etc., is encouraged where soil conditions permit in order to reduce the size or eliminate the need for detention facilities or other structural BMPs.
L. 
All stormwater runoff from new development or redevelopment shall be pretreated for water quality prior to discharge to surface or groundwater. Rooftop runoff may go directly to an infiltration BMP or be evapotranspirated.
M. 
All regulated activities within Newtown Township shall be designed, implemented, operated, and maintained to meet the purposes of this chapter, through these two elements:
(1) 
Erosion and sediment control during earth disturbance activities (e.g., during construction); and
(2) 
Water quality protection measures after completion of earth disturbance activities (i.e., after construction), including operations and maintenance.
N. 
BMPs shall be designed, implemented, and maintained to meet state water quality requirements and any other more stringent requirements as determined by Newtown Township.
O. 
Post-construction water quality protection shall be addressed as required by this chapter.
P. 
Operations and maintenance of permanent stormwater BMPs shall be addressed as required.
Q. 
All BMPs used to meet the requirements of this chapter shall conform to the state water quality requirements and any more stringent requirements as set forth by Newtown Township.
R. 
Techniques described in Appendix E (Low-Impact Development)[2] of this chapter shall be considered because they reduce the costs of complying with the requirements of this chapter and the state water quality requirements.
[2]
Editor's Note: Said appendix is included as an attachment to this chapter.
S. 
In selecting the appropriate BMPs or combinations thereof, a landowner or other applicant shall consider the following:
(1) 
Total contributing drainage area.
(2) 
Permeability and infiltration rate of the site's soils.
(3) 
Slope and depth to bedrock.
(4) 
Seasonal high water table.
(5) 
Proximity to building foundations and wellheads.
(6) 
Erodibility of soils.
(7) 
Land availability and configuration of the topography.
(8) 
Peak discharge and required volume control.
(9) 
Stream bank erosion.
(10) 
Efficiency of the BMPs to mitigate potential water quality problems.
(11) 
The volume of runoff that will be effectively treated.
(12) 
The nature of the pollutant being removed.
(13) 
Maintenance requirements.
(14) 
Creation/protection of aquatic and wildlife habitat.
(15) 
Recreational value.
(16) 
Enhancement of aesthetic and property values.
T. 
The design of all stormwater management facilities shall incorporate sound engineering principles and practices in a manner that does not aggravate existing stormwater problems. The Township reserves the right to disapprove any design that would result in construction in or continuation of a stormwater problem area.
U. 
A landowner or other applicant may meet the stormwater management criteria through off-site stormwater management measures as long as the proposed measures are in the same subwatershed as shown by Appendix A.[3]
[3]
Editor's Note: Said appendix is included as an attachment to this chapter.
V. 
Stormwater hotspots. Stormwater runoff from hotspots shall be pretreated prior to surface or groundwater infiltration to prevent pollutant runoff. Industrial sites referenced in 40 CFR 125 are examples of hotspots.
(1) 
Below is a list of 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 Classification Codes.
(g) 
Marinas (service and maintenance areas).
(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) 
Contaminated sites/brownfields.
(n) 
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 streets and highways [average daily traffic volume (ADT) greater than 30,000] are not considered stormwater hotspots, it is important to ensure that highway stormwater management facilities are designed to adequately protect receiving streams and/or groundwater.
(4) 
The Environmental Protection Agency's (EPA) NPDES stormwater program requires some industrial sites to prepare and implement a stormwater pollution prevention plan.
W. 
The following standards for protection of downgradient properties from off-site conveyance must be accomplished:
(1) 
For any location where a new concentrated discharge of stormwater from any frequency rainfall event, up to and including the one-hundred-year, twenty-four-hour event, will flow onto a downgradient property, the following are required.
(a) 
A drainage easement (or other legal agreement/approval) must be obtained for conveyance of discharges onto or through adjacent properties.
(b) 
The conveyance must be designed to avoid erosion, flooding, or other damage to the properties through which it is being conveyed.
The following permit requirements may apply to certain regulated earth disturbance activities and must be met prior to commencement of regulated earth disturbance activities, as applicable:
A. 
All regulated earth disturbance activities subject to permit requirements by DEP under regulations at 25 Pa. Code Chapter 102.
B. 
Work within natural drainageways subject to permit by DEP under 25 Pa. Code Chapter 105.
C. 
Any stormwater management facility that would be located in or adjacent to surface waters of the commonwealth, including wetlands, subject to permit by DEP under 25 Pa. Code Chapter 105.
D. 
Any stormwater management facility that would be located on or discharging to a state highway right-of-way, or require access to or from a state highway shall be subject to approval by PennDOT.
E. 
Culverts, bridges, storm sewers, or any other facilities which must pass or convey flows from the tributary area and any facility which may constitute a dam subject to permit by DEP under 25 Pa. Code Chapter 105.
A. 
No regulated earth disturbance activities within the Township shall commence until the Township receives an approval from DEP in compliance with 25 Pa. Code, Chapter 102 of an erosion and sediment control plan for construction activities if applicable.
B. 
DEP regulations regarding erosion and sediment control under 25 Pa. Code Chapter 102 shall be followed.
C. 
In addition, under 25 Pa. Code Chapter 92, a DEP NPDES construction activities permit is required for regulated earth disturbance activities.
D. 
Evidence of any necessary permit(s) for regulated earth disturbance activities from DEP or the County Conservation District must be provided to the Township. The issuance of an NPDES construction permit [or permit coverage under the statewide general permit (PAG-2)] satisfies the requirements of Subsection A.
E. 
A copy of the erosion and sediment control plan and any required permit, as required by DEP, shall be available on 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. At a minimum, they 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 drainage area contributory to the infiltration BMP has achieved final stabilization.
The design of all regulated activities shall include the following to minimize stormwater impacts to reduce the surface discharge of stormwater, reduce the creation of unnecessary impervious surfaces, prevent the degradation of waters of the commonwealth, and maintain as much as possible the natural hydrologic regime of the site:
A. 
Low-impact development (LID) methods such as those listed in Appendix E[1] shall be applied provided that use of this method does not conflict with other local codes.
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
B. 
The design process shall follow the sequence noted below. The goal of the sequence is to minimize the increases in stormwater runoff and impacts to water quality resulting from the proposed regulated activity:
(1) 
The following items shall be addressed prior to development of other stormwater management site plan design elements:
(a) 
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, stream buffers, hydrologic soil groups, and other existing recharge areas. Land development shall also be included.
(b) 
Establish stream buffer boundaries.
(c) 
Prepare a draft project layout avoiding identified sensitive areas.
(d) 
Identify site-specific existing conditions drainage areas, discharge points, recharge areas, and hydrologic soil groups A and B (areas conducive to infiltration).
(e) 
Evaluate nonstructural stormwater management alternatives:
[1] 
Minimize earth disturbance.
[2] 
Minimize impervious surfaces.
[3] 
Break up large impervious surfaces.
(f) 
Determine into what management district the site falls (see Appendix A),[2] and conduct an existing conditions runoff analysis.
[2]
Editor's Note: Said appendix is included as an attachment to this chapter.
(2) 
The following items in this subsection may be addressed in any order provided that all items in Subsection B have been completed.
(a) 
Satisfy infiltration objectives and provide for stormwater pretreatment prior to infiltration.
(b) 
Provide for water quality protection in accordance with applicable water quality requirements.
(c) 
Provide stream bank erosion protection in accordance with application stream bank erosion requirements.
(d) 
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 that the remaining site development has no surface or point discharge.
(e) 
Conduct a proposed conditions runoff analysis based on the final design that meets the management district requirements.
(f) 
Manage any remaining runoff prior to discharge through detention, bioretention, direct discharge, or other structural control.
A. 
Providing for infiltration consistent with the natural hydrologic regime is required. Design of the infiltration facilities shall consider infiltration to compensate for the reduction in the recharge that occurs when the ground surface is disturbed or impervious surface is created.
B. 
If infiltration cannot be physically accomplished, then the design professional shall be responsible for demonstrating to the satisfaction of the Township Engineer that infiltration cannot be physically accomplished on the site (e.g., shallow depth to bedrock or limiting zone, open voids, steep slopes, etc., rather than a financial hardship). If infiltration can be physically accomplished, the volume of runoff to be infiltrated shall be determined depending on demonstrated site conditions, and shall be the greatest volume that can be physically infiltrated.
C. 
Infiltration BMPs shall meet the following minimum requirements:
(1) 
Infiltration BMPs intended to receive runoff from developed or redeveloped 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 top of the limiting zone.
(b) 
An infiltration rate sufficient to accept the additional stormwater volume and dewater completely as determined by field tests conducted by a design professional.
(c) 
The infiltration facility shall be capable of completely draining the retention (infiltration) volume (Rev) within three days (72 hours) from the end of the design storm.
(2) 
The size of the infiltration facility and Rev shall be based upon the following volume criteria:
(a) 
Modified Control Guideline One (MCG-1) of the Pennsylvania Stormwater Best Management Practices (PA BMP) Manual - the retention (infiltration) volume (Rev) to be captured and infiltrated shall be the net two-year twenty-four-hour volume. The net volume is the difference between the post-development runoff volume and the predevelopment runoff volume. The post-development total runoff volume for all storms equal to or less than the two-year twenty-four-hour duration precipitation shall not be increased. For modeling purposes, existing (predevelopment) nonforested pervious areas must be considered meadow in good condition or its equivalent, and 20% of existing impervious area, when present, shall be considered meadow in good condition.
(b) 
Infiltrating the entire Rev volume may not be feasible on every site due to site-specific limitations such as shallow depth to bedrock or the water table. If it cannot be physically accomplished, then the following criteria from Modified Control Guideline Two (MCG-2) of the PA BMP Manual must be satisfied:
[1] 
At least the first one inch of runoff from new or replacement impervious surfaces shall be infiltrated.
Rev = 1 (inch) * impervious area (square feet) ÷ 12 (inches) = cubic feet (cf)
An asterisk (*) in equations denotes multiplication.
(c) 
Only if infiltrating the entire Rev volume cannot be physically accomplished, then the following minimum criteria from Modified Control Guideline Two (MCG-2) of the PA BMP Manual must be satisfied:
[1] 
Wherever possible, infiltration facilities should be designed to accommodate infiltration of the entire water quality volume (WQv) (§ 143-17A); however, in all cases at least the first 1/2 inch of the WQv shall be infiltrated. The minimum infiltration volume (Rev) required would, therefore, be computed as:
Rev = I * impervious area (square feet) ÷ 12 (inches) = cubic feet (cf)
An asterisk (*) in equations denotes multiplication.
Where:
I
=
The maximum equivalent infiltration amount (inches) that the site can physically accept or 0.50 inch, whichever is greater.
[2] 
The retention volume values derived from the methods specified by this chapter is the minimum volume the applicant must control through an infiltration BMP facility. If site conditions preclude capture of runoff from portions of the impervious area, the infiltration volume for the remaining area should be increased an equivalent amount to offset the loss.
[3] 
Only if the minimum of 0.50 inch of infiltration requirement cannot be physically accomplished, a waiver from infiltration volume requirements is required from Newtown Township.
D. 
Soils. A detailed soils evaluation of the project site shall be required to determine the suitability of infiltration facilities. The evaluation shall be performed by a qualified design professional and at 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 may not 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 (Rev) volume based on field-determined capacity at the level of the proposed infiltration surface.
(4) 
If on-lot infiltration structures are proposed by a design professional, it must be demonstrated to the Township Engineer that the soils are conducive to infiltrate on the lots identified.
E. 
Infiltration facilities should, to the greatest extent practicable, be located to avoid introducing contaminants via groundwater, and be in conformance with an approved source water protection assessment or source water protection plan.
F. 
Roadway drainage systems should provide an opportunity to capture accidental spills. Road deicing material storage facilities shall be designed to avoid salt and chloride runoff from entering waterways and infiltration facilities. A qualified design professional shall evaluate the possibility of groundwater contamination from the proposed infiltration facility and perform a hydrogeologic justification study if necessary.
G. 
The antidegredation analysis found in 25 Pa. Code Chapter 93 shall be applied in HQ or EV streams.
H. 
An impermeable liner will be required in detention basins where the possibility of groundwater contamination exists. The Township may require a detailed hydrogeologic investigation.
I. 
The applicant should provide safeguards against groundwater contamination for land uses that may cause groundwater contamination should there be a mishap or spill.
A. 
A landowner or other applicant shall comply with the following water quality requirements of this chapter.
B. 
To control post-construction stormwater impacts from regulated activities and conform to state water quality requirements, BMPs which replicate predevelopment stormwater infiltration and runoff conditions must be provided in the site design such that post-construction stormwater discharges do not degrade the physical, chemical, or biological characteristics of the receiving waters. This may be achieved by the following:
(1) 
Infiltration: replication of preconstruction stormwater infiltration conditions; and
(2) 
Treatment: use of water quality treatment BMPs to provide filtering of chemical and physical pollutants from the stormwater runoff; and
(3) 
Stream bank and stream bed protection: management of volume and rate of post-construction stormwater discharges to prevent physical degradation of receiving waters (e.g., from scouring).
C. 
Developed areas shall provide adequate storage and treatment facilities necessary to capture and treat stormwater runoff. The infiltration volume computed under § 143-16 may be a component of the water quality volume if the applicant chooses to manage both components in a single facility. If the calculated water quality volume (WQv) is greater than the volume required to be infiltrated as described by this chapter, then the difference between the two volumes shall be treated for water quality by an acceptable stormwater management practice(s). The required water quality volume (WQv) is the storage capacity needed to capture and treat a portion of stormwater runoff from the developed areas of the site. To achieve this requirement, the following criterion is established:
(1) 
From Control Guideline (CG-1) in the PA BMP Manual, the water quality volume shall be the net two-year twenty-four-hour volume. The net volume is the difference between the post-development runoff volume and the predevelopment runoff volume. The post-development total runoff volume for all storms equal to or less than the two-year twenty-four-hour duration precipitation shall not be increased. For modeling purposes, existing (predevelopment) nonforested pervious areas must be considered meadow in good condition or its equivalent, and 20% of existing impervious area, when present, shall be considered meadow in good condition.
(2) 
This volume requirement can be managed by the permanent volume of a wet basin or the detained volume from other BMPs. Where appropriate, wet basins shall be utilized for water quality control and shall follow the guidelines of the PA BMP manual referenced in Appendix G.[1]
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
(3) 
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). The design of the facility shall provide for protection from clogging and unwanted sedimentation.
D. 
The temperature of receiving waters shall be protected through the use of BMPs that moderate temperature.
E. 
If a perennial or intermittent stream passes through, or a water body (i.e., lake, pond, wetland) is present on the site, the landowner shall create a riparian buffer extending a minimum of 50 feet to either side of the top-of-bank of the channel, lake, or wetland. The buffer area shall be planted with native vegetation and maintained in a vegetated state (refer to Appendix B, Pennsylvania Native Plant List, contained in the PA BMP Manual).
(1) 
The following provisions also apply to riparian buffers on lots in existence at the time of adoption of this chapter:
(a) 
If the applicable rear or side yard setback is less than 50 feet, the buffer width may be reduced to 25% of the setback or 25 feet, whichever is greater.
(b) 
If a stream traverses a site in a manner that significantly reduces the use of the site, the buffer may be either:
[1] 
Reduced to 25 feet on either side, with Township approval; or
[2] 
Reduced to 10 feet with Township waiver.
(2) 
Permitted uses within the buffer include the following, subject to Township approval and provided that they comply with all federal, state, and local regulations:
(a) 
Recreational trails. See Appendix J, Riparian Buffer Trail Guidelines.[2]
[2]
Editor's Note: Said appendix is included as an attachment to this chapter.
(b) 
Utility rights-of-way.
(c) 
Bridges.
(d) 
Other uses subject to Township approval.
F. 
If an existing buffer is legally prescribed (i.e., deed, covenant, easement, etc.) and it exceeds the requirements of this chapter, the existing buffer shall be maintained.
A. 
In addition to controlling water quality volume (in order to minimize the impact of stormwater runoff on downstream stream bank erosion), the primary requirement to control stream bank erosion is to design a BMP to detain the proposed conditions two-year, twenty-four-hour design storm to the existing conditions one-year flow using the SCS Type II distribution. Additionally, provisions shall be made (such as adding a small orifice at the bottom of the outlet structure) to release the proposed conditions one-year storm for a minimum of 24 hours from a point in time when the maximum volume of water from the one-year storm is stored in a proposed BMP (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 three inches in diameter where possible, and a trash rack shall be installed to prevent clogging. On sites with small drainage areas contributing to the 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. When the calculated orifice size is below three inches, gravel filters (or other methods) are recommended to discharge low-flow rates subject to the Township Engineer's satisfaction. When filters are utilized, maintenance provisions shall be provided to ensure filters meet the design function. All facilities shall make use of measures to extend the flow path and increase the travel time of flows in the facility.
A. 
Each watershed has been divided into either stormwater management districts or release rate districts as shown on the respective Management District or Release Rate Maps in Appendix A.[1]
(1) 
In addition to the watershed-specific requirements for each of the watersheds specified by the tables in this section, the erosion and sedimentation control, the nonstructural project design, the infiltration, the water quality, and the stream bank erosion requirements shall be implemented.
(2) 
Standards for managing runoff from each subarea in a watershed for the two-, five-, ten-, twenty-five-, fifty-, and one-hundred-year design storms are shown in tables in this section. Development sites located in each of the management/release rate districts must control proposed conditions runoff rates to existing conditions runoff rates for the design storms in accordance with the tables.
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
B. 
General: Proposed conditions rates of runoff from any regulated activity shall not exceed the peak release rates of runoff from existing conditions for the design storms specified on the Stormwater Management District Watershed Map (Appendix A)[2] and this section.
[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 Township and County Planning offices. A copy of the official map at a reduced scale is included in Appendix A.[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 SWM site plan.
[3]
Editor's Note: Said appendix is included as an attachment to this chapter.
D. 
Sites located in more than one district or watershed: For a proposed development site located within two or more stormwater management district subareas, the peak discharge rate from any subarea shall meet the management district criteria for which the discharge is located. The natural hydrology of each respective subarea shall be maintained, and drainage shall not be redirected from one subarea to another. Under circumstances where the landowner or other applicant shows this cannot be accomplished, a waiver is required from the Township.
Table 143-23.1
Peak Rate Control Standards in the Crum Creek Watershed
District
Proposed Condition
Existing Condition
Design Storm
Design Storm
A
2 year
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
1 year
5 year
2 year
10 year
5 year
25 year
10 year
50 year
25 year
100 year
100 year
Table 143-23.2
Peak Rate Control Standards by Stormwater Management District in the Darby-Cobbs Creek Watershed
District
Proposed Condition Design Storm
Existing Condition Design Storm
A
2 year
1 year
5 year
5 year
10 year
10 year
25 year
25 year
100 year
100 year
B-1
2 year
1 year
10 year
5 year
25 year
10 year
50 year
25 year
100 year
100 year
B-2
2 year
1 year
5 year
2 year
25 year
5 year
50 year
10 year
100 year
100 year
C
Conditional Direct Discharge District
(1) 
Conditional Direct Discharge District: In District C, development sites that can discharge directly to the Darby-Cobbs Creek main channel, major tributaries, or indirectly to the main channel through an existing stormwater drainage system (i.e., storm sewer or tributary) may do so without control of the proposed conditions' peak rate of runoff greater than the five-year storm. Sites in District C will still have to comply with the groundwater recharge criteria, the water quality criteria, and stream bank erosion criteria. If the proposed conditions' runoff is intended to be conveyed by an existing stormwater drainage system to the main channel, assurance must be provided that such system has adequate capacity to convey the flows greater than the two-year existing conditions' peak flow or will be provided with improvements to furnish the required capacity. When adequate capacity in the downstream system does not exist and will not be provided through improvements, the proposed conditions' peak rate of runoff must be controlled to the existing conditions' peak rate as required in District A provisions (i.e., ten-year proposed conditions' flows to ten-year existing conditions' flows) for the specified design storms.
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. On-site drainage facilities shall be designed to safely convey off-site flows through the development site.
F. 
Site areas: Where a 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 peak rate control standards noted above. Unimpacted areas for which the discharge point has not changed are not subject to the peak rate control standards.
G. 
Hardship option for regulated activities less than one acre: There may be certain instances where the peak rate criteria are too restrictive for a particular landowner or applicant. The existing drainage network in some areas may be capable of safely transporting slight increases in flows without causing a problem or increasing flows elsewhere. This must be demonstrated in order for the hardship option to be considered. If an applicant or homeowner cannot meet the stormwater standards due to lot conditions or if conformance would become a hardship to an owner, the hardship option may be applied. The applicant would have to plead his/her case to the Board of Supervisors with the final determination made by the Township. Any landowners pleading the hardship option will assume all liabilities that may arise due to exercising this option. Cost or financial burden cannot be considered as a hardship. A landowner or other applicant may consider off-site management controls or making a contribution to the Township Stormwater Control and BMP Operation and Maintenance Fund as long as the stormwater management controls are within the same subwatershed.
H. 
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 storm event within their banks at velocities consistent with protection of the channels from erosion. Velocities shall be based upon criteria and methodologies acceptable to the Township Engineer.
(2) 
Natural or man-made channels or swales must be able to convey increased twenty-five-year storm event runoff without creating any increased hazard to persons or property.
(3) 
Culverts, bridges, storm sewers or any other hydraulic facilities which must pass or convey flows from the tributary area must be designed in accordance with 25 Pa. Code Chapter 105 regulations (if applicable) and, at a minimum, pass the increased twenty-five-year storm event runoff.
(4) 
Water quality requirements defined herein must be met.
(5) 
Other than the water quality requirements in Subsection H(4) above, post-construction peak rates shall not exceed the existing peak rates for the respective subarea.
I. 
Alternate criteria for redevelopment sites: For redevelopment sites, one of the following minimum design parameters shall be accomplished, whichever is most appropriate for the given site conditions as determined by the Township Engineer:
(1) 
Meet the full requirements specified by this section; or
(2) 
Reduce the total impervious surface on the site by at least 20%; based upon a comparison of existing impervious surface to proposed impervious surface. In this case, calculations must be provided that show the peak rate has not increased.
A. 
Stormwater runoff from all development sites with a drainage area of greater than five acres shall be calculated using a generally accepted calculation technique that is based on the NRCS Soil Cover Complex Method. Table 143-24.1 summarizes acceptable computation methods. The method selected by the design professional shall be based on the individual limitations and suitability of each method for a particular site. The use of the Rational Method to estimate peak discharges for drainage areas greater than five acres shall be permitted only upon approval of Newtown Township.
Table 143-24.1
Acceptable Computation Methodologies for SWM Site Plan
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 where limitations described in TR-55
HEC-1/HEC-HMS
USACE
Applicable where use of a full hydrologic computer model is desirable or necessary
Rational Method (or commercial computer package based on Rational Method)
Emil Kuichling (1889)
For sites up to 5 acres if approved by Newtown Township and/or the Township Engineer
Other methods
Varies
Other computation methodologies approved by Newtown Township and/or Township Engineer
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. Rainfall depths shall be according to NOAA Atlas 14 values consistent with a partial duration series. When stormwater calculations are performed for routing procedures or water quality functions, the duration of rainfall shall be 24 hours.
C. 
The following criteria shall be used for peak rate runoff calculations:
(1) 
For development sites not considered redevelopment, the ground cover used in determining the existing conditions flow rates shall be as follows:
(a) 
Wooded sites shall use a ground cover of "woods in good condition." Portions of a site having more than one viable tree measuring a diameter at breast height (DBH) of six inches or greater per 1,500 square feet shall be considered wooded where such trees existed within three years of application.
(b) 
The undeveloped portion of the site including agriculture, bare earth, and fallow ground shall be considered as "meadow in good condition," unless the natural ground cover generates a lower curve (CN) number or Rational "c" value (i.e., woods) as listed in Tables F-1 or F-2 in Appendix F of this chapter.[1]
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
(2) 
For redevelopment sites, the ground cover used in determining the existing conditions flow rates for the developed portion of the site shall be based upon actual land cover conditions.
D. 
All calculations using the Rational Method shall use rainfall intensities consistent with appropriate times-of-concentration (duration) and storm events with rainfall intensities obtained from NOAA Atlas 14 partial duration series estimates, or the latest version of the PennDOT Drainage Manual (PDM Publication 584). Times-of-concentration shall be calculated based on the methodology recommended in the respective model used. 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 F-1 in Appendix F of this chapter.[2]
[2]
Editor's Note: Said appendix 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 F-2 in Appendix F of this chapter.[3]
[3]
Editor's Note: Said appendix is included as an attachment to this chapter.
G. 
Hydraulic computations to determine the capacity of pipes, culverts, and storm sewers shall be consistent with methods and computations contained in the Federal Highway Administration Hydraulic Design Series Number 5 (Publication No. FHWA-NHI-01-020 HDS No. 5). Hydraulic computations to determine the capacity of open channels shall be consistent with methods and computations contained in the Federal Highway Administration Hydraulic Engineering Circular Number 15 (Publication No. FHWA-NHI-05-114 HEC 15). Values for Manning's roughness coefficient (n) shall be consistent with Table F-3 in Appendix F of this chapter.[4]
[4]
Editor's Note: Said appendix 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 chapter 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 chapter shall be verified by routing the design storm hydrograph through these facilities using an acceptable method. The design storm hydrograph shall be computed using a calculation method that produces a full hydrograph. Newtown 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.
A. 
All wet basin designs shall incorporate biologic controls consistent with the West Nile Guidance found in Appendix H, DEP document 363-0300-001 "Design Criteria - Wetlands Replacement/Monitoring," or contact the Pennsylvania State Cooperative Wetland Center (www.wetlands.psu.edu/) or the Penn State Cooperative Extension Office (www.extension.psu.edu/extmap.html).
B. 
Any stormwater basin required or regulated by this chapter designed to store runoff and requiring a berm or earthen embankment shall be designed to provide an emergency spillway to handle flow up to and including the one-hundred-year proposed conditions. The height of embankment must provide a minimum 1.0 feet of freeboard above the maximum pool elevation computed for the one-hundred-year proposed conditions inflow through the emergency spillway. Freeboard is the difference between the design flow elevations in the emergency spillway and the top of the settled basin embankment. Should any stormwater management facility require a dam safety permit under 25 Pa. Code Chapter 105, the facility shall be designed in accordance with Chapter 105 and meet the regulations of Chapter 105 concerning dam safety. Chapter 105 may require the passing of storms larger than one-hundred-year event.
C. 
Any drainage conveyance facility and/or channel not governed by Chapter 105 regulations must be able to convey, without damage to the drainage structure or roadway, runoff from the twenty-five-year storm event. The larger events (fifty-year and one-hundred-year) must also be safely conveyed in the direction of natural flow without creating additional damage to any drainage structures, nearby structures, or roadways.
D. 
Conveyance facilities to or exiting from stormwater management facilities (i.e., detention basins) shall be designed to convey the design flow to or from the facility.
E. 
Roadway crossings or structures located within designated floodplain areas must be able to convey runoff from a one-hundred-year design storm consistent with Federal Emergency Management Agency National Flood Insurance Program floodplain management requirements.
F. 
Any facility located within a PennDOT right-of-way must meet PennDOT minimum design standards and permit submission requirements.
G. 
Adequate erosion protection and energy dissipation shall be provided along all open channels and at all points of discharge. Design methods shall be consistent with the Federal Highway Administration Hydraulic Engineering Circular Number 11 (Publication No. FHWA-IP-89-016) and the DEP Erosion and Sediment Pollution Control Program Manual (Publication No. 363-2134-008).