[Ord. 1-2012, 9/19/2012, § 301]
1. 
Applicants proposing regulated activities in the Township which do not fall under the exemption criteria shown in § 19-106 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.
2. 
No regulated activity within Thornbury Township shall commence until Thornbury Township issues approval of an SWM plan which demonstrates compliance with the requirements of this chapter.
[Amended by Ord. No. 3-2022, 9/21/2022]
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 on site, the Township may approve measures other than those in this chapter after consultation with and evaluation by PADEP 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, 35 P.S. § 691.1 et seq. The evaluation referenced above shall include, but not be limited to, in depth soil profile and infiltration testing at locations across the entire site and at different elevations as witnessed by and to the satisfaction of the Township Engineer.
For any site with proposed regulated earth disturbance that is less than one acre where, after a close evaluation of alternative site designs, it proves to be impracticable to meet any one or more of the mandatory minimum volume and infiltration standards of this chapter on site, the Township may approve measures other than those in this chapter. The evaluation referenced above shall include, but not be limited to, in depth soil profile and infiltration testing at locations across the entire site and at different elevations as witnessed by and to the satisfaction of the Township Engineer.
"Regulated earth disturbance" is defined to mean any activity involving earth disturbance subject to regulation under 25 Pa.Code, Chapter 92, 25 Pa.Code, Chapter 102, or the Clean Streams Law, 35 P.S. § 691.1 et seq.
3. 
The applicant is required to design the site to minimize surface discharge of stormwater and the creation of impervious surfaces in order to maintain, as much as possible, the natural hydrologic regime.
4. 
The SWM site plan must be designed consistent with the sequencing provisions of § 19-304 to ensure maintenance of the natural hydrologic regime, to promote infiltration, and to protect groundwater and surface water quality and quantity. The SWM site plan designer must proceed sequentially in accordance with this article of this chapter.
5. 
Stormwater drainage systems shall be designed in order to preserve natural flow conditions to the maximum extent practicable.
6. 
Alteration of existing drainage discharge onto adjacent property shall only be proposed in accordance with PADEP guidance document "Chapter 102 Off-Site Discharges of Stormwater to Non-Surface Waters - Frequently Asked Questions (FAQ)" dated January 2, 2019, or latest guidance document from PADEP. Such discharge shall be subject to any applicable discharge criteria specified in this chapter and still must meet the requirements of Act 167.
[Amended by Ord. No. 3-2022, 9/21/2022]
7. 
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.
8. 
Where a development site is traversed by a stream, drainage easements of 50 feet or meeting the one-hundred-year flood elevation as established by FEMA mapping, or the calculated one-hundred-year floodplain, but in no case less than 10 feet from the top of bank shall be provided on either side of, and conform to the line of, such streams.
9. 
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.
10. 
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.
11. 
All regulated activities within the Township shall be designed, implemented, operated, and maintained to meet the purposes of this chapter, through these two elements:
A. 
Erosion and sediment control during earth disturbance activities (e.g., during construction).
B. 
Water quality protection measures after completion of earth disturbance activities (i.e., after construction), including operations and maintenance.
12. 
The BMPs shall be designed, implemented, and maintained to meet state water quality requirements and any other more stringent requirements as determined by Thornbury Township. Applicants shall utilize the Pennsylvania Stormwater Best Management Practices Manual (PA BMP Manual), as amended, or other sources acceptable to the Municipal Engineer, for testing and design standards for BMPs, and where there is a conflict with the provisions of this chapter, the most restrictive applies.
[Amended by Ord. No. 3-2022, 9/21/2022]
13. 
Post-construction water quality protection shall be addressed as required by § 19-306.
14. 
Operations and maintenance of permanent stormwater BMPs shall be addressed as required by Article 7.
15. 
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 the Township.
16. 
Techniques described in Appendix 19-E (Low Impact Development) 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.
17. 
In selecting the appropriate BMPs or combinations thereof, the applicant shall consider the following:
A. 
Total contributing drainage area.
B. 
Permeability and infiltration rate of the site's 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.
H. 
Peak discharge and required volume control.
I. 
Stream bank erosion.
J. 
Efficiency of the BMPs to mitigate potential water quality problems.
K. 
The volume of runoff that will be effectively treated.
L. 
The nature of the pollutant being removed.
M. 
Maintenance requirements.
N. 
Creation/protection of aquatic and wildlife habitat.
O. 
Recreational value.
P. 
Enhancement of aesthetic and property values.
18. 
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.
19. 
The 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 in Appendix 19-A.
20. 
Stormwater Hotspots. Stormwater runoff from hot spots shall be pretreated prior to surface or groundwater infiltration to prevent pollutant runoff. Industrial sites referenced in 40 CFR 125 are examples of hot spots.
A. 
Below is a list of examples of hot spots:
(1) 
Vehicle salvage yards and recycling facilities.
(2) 
Vehicle fueling stations.
(3) 
Vehicle service and maintenance facilities.
(4) 
Vehicle and equipment cleaning facilities.
(5) 
Fleet storage areas (bus, truck, etc.).
(6) 
Industrial sites based on Standard Industrial Classification Codes.
(7) 
Marinas (service and maintenance areas).
(8) 
Outdoor liquid container storage.
(9) 
Outdoor loading/unloading facilities.
(10) 
Public works storage areas.
(11) 
Facilities that generate or store hazardous materials.
(12) 
Commercial container nursery.
(13) 
Contaminated sites/brownfields.
(14) 
Other land uses and activities as designated by an appropriate review authority.
B. 
The following land uses and activities are not normally considered hot spots:
(1) 
Residential streets and rural highways.
(2) 
Residential development.
(3) 
Institutional development.
(4) 
Office developments.
(5) 
Nonindustrial rooftops.
(6) 
Pervious areas, except golf courses and nurseries (which may need an integrated pest management (IPM) plan).
C. 
While streets and highways (average daily traffic volume (ADT) greater than 30,000 are not considered stormwater hot spots, it is important to ensure that highway stormwater management facilities are designed to adequately protect receiving streams and/or groundwater.
D. 
The Environmental Protection Agency's (EPA) NPDES stormwater program requires some industrial sites to prepare and implement a stormwater pollution prevention plan.
21. 
The following standards for protection of adjacent and downgradient properties from off-site conveyance must be accomplished:
[Amended by Ord. No. 3-2022, 9/21/2022]
A. 
For any location where a new concentrated discharge of stormwater from any frequency rainfall event, up to and including the 100-year storm, and the volume of runoff, up to and including the two-year storm, onto or through adjacent property(ies) or downgradient property(ies), the following are required:
(1) 
A drainage easement (or other legal agreement/approval) must be obtained for conveyance of discharges onto or through adjacent properties, per the PADEP guidance document, "Chapter 102 Off-Site Discharges of Stormwater to Non-Surface Waters - Frequently Asked Questions (FAQ)," dated January 2, 2019, or latest guidance document from PADEP.
(2) 
The conveyance must be designed to avoid erosion, flooding, or other damage to the properties through which it is being conveyed.
22. 
Additional Water Quality Requirements. The Township may require additional stormwater control measures for stormwater discharges to special management areas including, but not limited to:
A. 
Water bodies listed as "impaired" by PADEP.
B. 
Any water body or watershed with an approved total maximum daily load (TMDL). Areas of known existing flooding problems.
C. 
Critical areas with sensitive resources (e.g., state-designated special protection waters, cold water fisheries, carbonate geology or other groundwater recharge areas that may be highly vulnerable to contamination, drainage areas to water supply reservoirs, etc.).
[Ord. 1-2012, 9/19/2012, § 302]
1. 
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 PADEP under regulations at 25 Pa.Code, Chapter 102.
B. 
Work within natural drainageways subject to permit by PADEP 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 PADEP 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 PADEP under 25 Pa.Code, Chapter 105.
[Ord. 1-2012, 9/19/2012, § 303]
1. 
No regulated earth disturbance activities within the Township shall commence until the Township receives an approval from the PADEP in compliance with 25 Pa.Code, Chapter 102, of an erosion and sediment control plan for construction activities if applicable.
2. 
PADEP has regulations regarding an erosion and sediment control under 25 Pa.Code, Chapter 102.
3. 
In addition, under 25 Pa.Code, Chapter 92, a PADEP "NPDES Construction Activities" Permit is required for regulated earth disturbance activities.
4. 
Evidence of any necessary permit(s) for regulated earth disturbance activities from the appropriate PADEP regional office or County Conservation District must be provided to the Township. The issuance of an NPDES Construction Permit (or permit coverage under the Statewide General Permit (PAG-2)) satisfies the requirements of § 19-403, Subsection 1A.
5. 
A copy of the erosion and sediment control plan and any required permit, as required by PADEP regulations, shall be available on the project site at all times.
6. 
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:
A. 
Areas proposed for infiltration BMPs shall be protected from sedimentation and compaction during the construction phase to maintain maximum infiltration capacity.
B. 
Infiltration BMPs shall not be constructed nor receive runoff until the entire drainage area contributory to the infiltration BMP has achieved final stabilization.
[Ord. 1-2012, 9/19/2012, § 304]
1. 
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. 
The applicant shall apply low impact development (LID) methods such as those listed in Appendix 19-E, provided that use of this method does not conflict with other local codes.
B. 
The applicant shall demonstrate that the design process follows 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 in this subparagraph 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, and hydrologic soil groups. Land development, any existing recharge areas, and other requirements outlined in the Township SALDO [Chapter 22] shall also be included.
(b) 
Establish a stream buffer according to § 19-311.
[Amended by Ord. No. 3-2022, 9/21/2022]
(c) 
Prepare a draft project layout avoiding sensitive areas identified in Subsection 1B(1)(a).
(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 (Appendix 19-A), and conduct an existing conditions runoff analysis.
(2) 
The following items in this subsection may be addressed in any order provided that all items in Subsection 1B(1) have been completed.
(a) 
Satisfy the infiltration objective (§ 19-305) and provide for stormwater pretreatment prior to infiltration.
(b) 
Provide for water quality protection in accordance with § 19-306 water quality requirements.
(c) 
Provide stream bank erosion protection in accordance with § 19-307 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 (§ 19-308).
(f) 
Manage any remaining runoff prior to discharge through detention, bioretention, direct discharge, or other structural control.
[Ord. 1-2012, 9/19/2012, § 305]
1. 
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.
2. 
If it cannot be physically accomplished, then the design professional shall be responsible for demonstrating to the satisfaction of Thornbury Township that this cannot be physically accomplished on the site (e.g., shallow depth to bedrock or limiting zone, open voids, steep slopes, etc. A financial hardship, as defined in § 19-202, is not acceptable to avoid implementing infiltration facilities. If infiltration can be physically accomplished, the volume of runoff to be infiltrated shall be determined from Subsection 3B depending on demonstrated site conditions, and shall be the greatest volume that can be physically infiltrated or alternative methods consistent with the PA BMP Manual (as amended) or other PADEP guidance, such as the Managed Release Concept, may be used to manage this volume with approval from the Municipal Engineer. For example:
[Amended by Ord. No. 3-2022, 9/21/2022]
A. 
Any applicant (developer or redeveloper) shall first attempt to infiltrate the volume required in Subsection 3B(1).
B. 
If the Subsection 3B(1) requirement cannot be physically accomplished, then the applicant is required to attempt to infiltrate the volume required in Subsection 3B(2).
C. 
Finally, if the Subsection 3B(2) infiltration volume cannot be physically accomplished, the applicant must, at a minimum, infiltrate the volume required in Subsection 3B(3).
3. 
Infiltration BMPs shall meet the following minimum requirements:
A. 
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:
(1) 
A minimum depth of 24 inches between the bottom of the BMP and the top of the limiting zone.
(2) 
An infiltration rate sufficient to accept the additional stormwater volume and dewater completely as determined by field tests conducted by the applicant's design professional.
(3) 
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.
B. 
The size of the infiltration facility and Rev shall be based upon the following volume criteria:
(1) 
Modified Control Guideline One (MCG-1) of the 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.
[Amended by Ord. No. 3-2022, 9/21/2022]
(2) 
Infiltrating the entire Rev volume in subparagraph (1) (above) 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:
(a) 
At least the first one-inch of runoff from new or replacement impervious surfaces shall be infiltrated.
Rev = one (inch) * impervious area (square feet)
12 (inches) = cubic feet
An asterisk (*) in equations denotes multiplication.
(3) 
Only if infiltrating the entire Rev volume in subparagraph (2) (above) cannot be physically accomplished, then the following minimum criteria from Modified Control Guideline Two (MCG-2) of the PA BMP Manual must be satisfied:
(a) 
Wherever possible, infiltration facilities should be designed to accommodate infiltration of the entire water quality volume (WQv) (§ 19-306, Subsection 3); 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
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.
(b) 
The retention volume values derived from the methods in subparagraphs (1), (2) or (3) 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.
(c) 
Only if the minimum of 0.50 inch of infiltration requirement cannot be physically accomplished, a waiver from this section, infiltration volume requirements is required from the Township.
4. 
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:
A. 
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 sub-grade stability; infiltration may not be ruled out without conducting these tests.
B. 
Provide field tests as required in the PA BMP Manual.
[Amended by Ord. No. 3-2022, 9/21/2022]
C. 
Design the infiltration structure for the required retention (Rev) volume based on field determined capacity at the level of the proposed infiltration surface.
D. 
If on-lot infiltration structures are proposed by the applicant's design professional, it must be demonstrated to the Township that the soils are conducive to infiltrate on the lots identified.
5. 
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.
6. 
Roadway drainage systems should provide an opportunity to capture accidental spills. Road de-icing material storage facilities shall be designed to avoid salt and chloride runoff from entering waterways and infiltration facilities. The qualified design professional shall evaluate the possibility of groundwater contamination from the proposed infiltration facility and perform a hydrogeologic justification study if necessary.
7. 
The Township may require additional water quality and runoff control measures for stormwater discharging to special management areas such as those listed in § 19-301, Subsection 22. The antidegredation analysis found in [25 Pa.Code] Chapter 93 shall be applied in HQ or EV streams.
8. 
An impermeable liner will be required in detention basins where the possibility of groundwater contamination exists. The Township may require a detailed hydrogeologic investigation.
9. 
The applicant should provide safeguards against groundwater contamination for land uses that may cause groundwater contamination should there be a mishap or spill.
[Ord. 1-2012, 9/19/2012, § 306]
1. 
The applicant shall comply with the following water quality requirements of this article.
2. 
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. The green infrastructure and low-impact development (LID) practices provided in the PA BMP Manual, as well as the guidance on green infrastructure and LID provided in Appendix 19-E, shall be utilized for all regulated activities wherever possible. This may be achieved by the following:
[Amended by Ord. No. 3-2022, 9/21/2022]
A. 
Infiltration: replication of preconstruction stormwater infiltration conditions;
B. 
Treatment: use of water quality treatment BMPs to provide filtering of chemical and physical pollutants from the stormwater runoff; and
C. 
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).
3. 
Developed areas shall provide adequate storage and treatment facilities necessary to capture and treat stormwater runoff. The infiltration volume computed under § 19-305 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 in § 19-305, Subsection 3B, 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.
4. 
To achieve this requirement, the following criterion is established:
A. 
The post-construction total runoff volume shall not exceed the predevelopment total runoff volume for all storms equal to or less than the two-year, twenty-four-hour duration precipitation (design storm). If the Municipal Engineer concurs that this criterion cannot be met, a minimum of 0.5 inch of runoff from all regulated impervious surfaces shall be managed. 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.
[Amended by Ord. No. 3-2022, 9/21/2022]
B. 
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 19-G.
C. 
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.
5. 
The temperature of receiving waters shall be protected through the use of BMPs that moderate temperature.
6. 
Evapotranspiration may be quantified and credited towards meeting volume requirements according to the PADEP Post Construction Stormwater Management (PCSM) Spreadsheet and Instructions (December 2020) or the most recent guidance from PADEP.
[Amended by Ord. No. 3-2022, 9/21/2022]
7. 
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.
[Ord. 1-2012, 9/19/2012, § 307]
1. 
In addition to controlling the 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).
2. 
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 this BMP that do not provide enough runoff volume to allow a twenty-four-hour attenuation with the three-inch orifice, the calculations shall be submitted showing this condition. 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.
[Ord. 1-2012, 9/19/2012, § 308]
1. 
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 19-A.
A. 
In addition to the watershed-specific requirements specified in Tables 19-308-1 and 19-308-2 below, the erosion and sedimentation control (§ 19-303), the nonstructural project design (§ 19-304), the infiltration (§ 19-305), the water quality (§ 19-306), and the stream bank erosion (§ 19-307) requirements shall be implemented.
B. 
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 19-308-1 and 19-308-2. 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.
C. 
In all cases, and in all subareas, the rate of runoff for the proposed condition two-year storm shall not exceed the existing condition one-year storm.
2. 
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 Maps (Appendix 19-A) and this section of this chapter.
3. 
District Boundaries. The boundaries of the stormwater management districts are shown on official maps that are available for inspection at the Township and County planning offices. A copy of the official maps at a reduced scale are included in Appendix 19-A. 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.
4. 
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 applicant shows this cannot be accomplished, a waiver is required by the Township.
Table 19-308-1
Peak Rate Control Standards in the
Ridley Creek Watershed
Sub-area
Release Rate Percentage
Sub-area
Release Rate Percentage
3
85
26, 27
50
4-7
50
28
60
8
80
29, 30
50
9-14
50
31
60
16
95
32-39
50
17-22
50
31
60
23
70
32-39
50
24
80
40
85
25
95
49-65
50
Table 19-308-2
Control Criteria for Chester Creek Watershed
Stormwater Management Districts
District
Control Criteria
100%
Post-development peak discharge for all design storms must be no greater than predevelopment peak discharges.
75%
Post-development peak discharge for all design storms must be no greater than 75% of the predevelopment peak discharges.
50%
Post-development peak discharge for all design storms must be no greater than 50% of the predevelopment peak discharges.
5. 
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.
6. 
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 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.
7. 
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:
A. 
Meet the full requirements specified by Tables 19-308-1, 19-308-2 and Subsections 1 through 4.
B. 
Reduce the total predevelopment impervious surface on the site by at least 20%; based upon a comparison of existing impervious surface to regulated impervious surface. In this case, calculations must be provided that show the peak rate has not increased.
[Amended by Ord. No. 3-2022, 9/21/2022]
[Ord. 1-2012, 9/19/2012, § 309]
1. 
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 19-309-1 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 the Township.
Table 19-309-1
Acceptable Computation Methodologies for SWM Site Plan
Method
Developed by
Applicability
TR-20 (or commercial computer package based on TR-20)
USDA NCRS
Applicable where use of full hydrology computer model is desirable or necessary.
Applicable where use of full TR-55 (or commercial computer package based on TR-55)
USDA NCRS
Applicable for land development plans where limitations described in TR-55.
HEC-1/HEC-HMS
US Army Corps of Engineers
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 five acres, or as approved by the Township and/or Township Engineer.
Other methods
Varies
Other computation methodologies approved by the Township and/or Township Engineer.
2. 
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.
3. 
The following criteria shall be used for peak rate runoff calculations:
A. 
For all development sites including redevelopment, the ground cover used in determining the existing conditions flow rates shall be as follows:
(1) 
Wooded sites shall use a ground cover of "woods in good condition." (CN=55) 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.
(2) 
All portions of the site regardless of existing ground cover shall be considered as "meadow in good condition," (CN=58, c=0.12) unless the natural ground cover generates a lower curve (CN) number or Rational "c" value (i.e., woods) as listed in Tables 19-F-1 or 19-F-2 in Appendix 19-F of this chapter.
B. 
Through Township waiver, and only for redevelopment sites, the ground cover used in determining the existing conditions flow rates for the developed portion of the site could be based upon actual land cover conditions. Additional conditions and stormwater requirements may apply with this waiver.
4. 
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 a minimum of five minutes.
[Amended by Ord. No. 3-2022, 9/21/2022]
5. 
Runoff curve numbers (CN) for both existing and proposed conditions to be used in the Soil Cover Complex Method shall be obtained from Table 19-F-1 in Appendix 19-F of this chapter.
6. 
Runoff coefficients (c) for both existing and proposed conditions for use in the Rational Method shall be obtained from Table 19-F-2 in Appendix 19-F of this chapter.
7. 
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 No. 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 No. 15 (Publication No. FHWANHI-05-114 HEC 15). Values for Manning's roughness coefficient (n) shall be consistent with Table 19-F-3 in Appendix 19-F of this chapter.
8. 
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.
9. 
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. 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.
[Ord. 1-2012, 9/19/2012, § 310]
1. 
All wet basin designs shall incorporate biologic controls consistent with the West Nile Guidance found in Appendix H, PADEP 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).
2. 
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 of freeboard above the maximum pool elevation computed when the facility functions for the one-hundred-year proposed conditions inflow. Should any stormwater management facility require a dam safety permit under PADEP [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.
3. 
Any drainage conveyance facility and/or channel not governed by [25 Pa.Code] 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.
4. 
Conveyance facilities transporting flow to or exiting from stormwater management facilities (i.e., detention basins) shall be designed to convey the 100-year frequency storm.
[Amended by Ord. No. 3-2022, 9/21/2022]
5. 
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.
6. 
Any facility located within a PennDOT right-of-way must meet PennDOT minimum design standards and permit submission requirements.
7. 
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 No. 11 (Publication No. FHWA-IP-89-016) and the PADEP Erosion and Sediment Pollution Control Program Manual (Publication No. 363-2134-008).
[Added by Ord. No. 3-2022, 9/21/2022]
1. 
Except as required by Chapter 102,[1] if a perennial or intermittent stream passes through, or a waterbody (i.e., lake, pond, wetland) is present on the site, the applicant shall create a riparian buffer, extending a minimum of 75 feet, to either side of the top-of-bank of the channel, stream, lake, and/or from the edge of wetlands, whichever results in the greatest buffer distance.
[1]
Editor’s Note: Refers to 25 Pa. Code Chapter 102.
2. 
The riparian buffer shall be planted with native vegetation and maintained in a vegetated state (Refer to Appendix 19-B, Pennsylvania Native Plant List, contained in the PA BMP Manual or latest guidance document from PADEP).
A. 
The following provisions also apply to riparian buffers on lots in existence at the time of original adoption of this chapter:
(1) 
If the applicable rear or side yard setback is less than 75 feet, the buffer width may be reduced to 25% of the setback or 25 feet, whichever is greater, by Township waiver.
(2) 
If a stream traverses a site in a manner that significantly reduces the use of the site, the buffer may be reduced to 25 feet on either side, with Township waiver.
B. 
Permitted uses within the riparian buffer include the following, subject to municipal approval and provided that they comply with all federal, state, and local regulations:
(1) 
Recreational trails. See Appendix 19-J, Riparian Buffer Trail Guidelines.
(2) 
Utility rights-of-way.
(3) 
Bridges.
(4) 
Other uses subject to Township approval.
C. 
If an existing riparian buffer is legally prescribed (i.e., deed, covenant, easement, etc.) and it exceeds the requirements of this chapter, the existing buffer shall be observed and maintained.
3. 
In order to protect and improve water quality, a riparian buffer easement shall be created and recorded as part of any subdivision or land development that encompasses a riparian buffer.
4. 
Minimum Management Requirements for Riparian Buffers.
A. 
Existing native vegetation shall be protected and maintained within the riparian buffer easement.
B. 
Whenever practicable, invasive vegetation shall be actively removed and the riparian buffer easement shall be planted with native trees, shrubs and other vegetation to create a diverse native plant community appropriate to the intended ecological context of the site.
5. 
The riparian buffer easement shall be enforceable by the municipality and shall be recorded in the Delaware County Recorder of Deeds Office, so that it shall run with the land and shall limit the use of the property located therein.
6. 
Any permitted use within the riparian buffer easement shall be conducted in a manner that will maintain the extent of the existing 100-year floodplain, improve or maintain the stream stability, and preserve and protect the ecological function of the floodplain.
7. 
The following conditions shall apply when public and/or private recreation trails are permitted within riparian buffers:
A. 
Trails shall be for nonmotorized use only.
B. 
Trails shall be designed to have the least impact on native plant species and other sensitive environmental features.
8. 
Septic drainfields and sewage disposal systems shall not be permitted within the riparian buffer easement and shall comply with setback requirements established under 25 Pa. Code Chapter 73.