[Ord. No. 337, 5/15/2024]
(a) 
Applicants proposing regulated activities in Amity Township which do not fall under the exemption criteria shown in § 25-402 shall submit a drainage plan consistent with this chapter to the Township for review. These criteria shall apply to the total proposed development even if development is to take place in stages. No regulated activities shall commence until the Township issues written approval of a drainage plan, which demonstrates compliance with the requirements of this chapter.
(b) 
The applicant is required to evaluate practicable alternatives to the surface discharge of stormwater, the creation of impervious surfaces and the degradation of waters of the commonwealth, and must maintain as much as possible the natural hydrologic regime
(c) 
The drainage plan must be designed consistent with the sequencing provisions of § 25-304 to ensure maintenance of the natural hydrologic regime and to promote groundwater recharge and protect groundwater and surface water quality and quantity. The drainage plan designer must proceed sequentially in accordance with Part 3 of this chapter.
(d) 
Existing points of concentrated drainage that discharge onto adjacent property shall not be altered in any manner which could cause property damage without permission of the affected property owner(s) and shall be subject to any applicable discharge criteria specified in this chapter.
(e) 
Areas of existing diffused drainage discharge shall be subject to any applicable discharge criteria in the general direction of existing discharge, whether proposed to be concentrated or maintained as diffused drainage areas, except as otherwise provided by this chapter. If diffused drainage discharge is proposed to be concentrated and discharged onto adjacent property, the applicant must document that adequate downstream conveyance facilities exist to safely transport the concentrated discharge, or otherwise prove that no erosion, sedimentation, flooding or other impacts will result from the concentrated discharge.
(f) 
Where a development site is traversed by existing watercourses, drainage easements shall be provided conforming to the line of such watercourses. The terms of the easement shall conform to the stream buffer requirements contained in § 25-306(d) of this chapter.
(g) 
Any stormwater management facilities regulated by this chapter that would be located in or adjacent to waters of the commonwealth or wetlands shall be subject to approval by PaDEP through the joint permit application process, or, where deemed appropriate by PaDEP, the general permit process. When there is a question whether wetlands may be involved, it is the responsibility of the applicant or his agent to show that the land in question cannot be classified as wetlands; otherwise approval to work in the area must be obtained from PaDEP.
(h) 
Any alteration that affects stormwater flow directly or indirectly toward a PennDOT facility shall be subject to PennDOT regulations.
(i) 
Minimization of impervious surfaces and infiltration of runoff through seepage beds, infiltration trenches, etc., are encouraged, where soil conditions permit, to reduce the size or eliminate the need for detention facilities or other structural BMPs.
(j) 
Roof drains shall not be connected to impervious surfaces in order to promote overland flow and infiltration/percolation of stormwater where advantageous to do so. When site conditions preclude infiltration/percolation, then it shall be permitted on a case-by-case basis by the Township.
(k) 
All stormwater runoff shall be treated for water quality.
(l) 
Transference of runoff to or from an EV/HQ watershed is prohibited unless otherwise authorized by DEP, DRBC or SRBC.
(m) 
The Board may require that a landowner or developer provide reasonable corrective measures to alleviate an existing off-site drainage problem that may be affected by the proposed subdivision and/or land development. It shall be the responsibility of the landowner or developer to obtain all drainage easements on, over, or through other properties, and the Township, its agents, workmen, servants and employees shall be indemnified and held harmless from any liability.
(n) 
Drainage plans approved by the municipality, in accordance with § 25-406, shall be on site throughout the duration of the regulated activity.
(o) 
The municipality may, after consultation with DEP, approve measures for meeting the state water quality requirements other than those in this chapter, provided that they meet the minimum requirements of, and do 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.
(p) 
For all regulated earth disturbance activities, erosion and sediment control BMPs shall be designed, implemented, operated, and maintained during the regulated earth disturbance activities (e.g., during construction) to meet the purposes and requirements of this chapter and to meet all requirements under Title 25 of the Pennsylvania Code and the Clean Streams Law. Various BMPs and their design standards are listed in the Erosion and Sediment Pollution Control Program Manual (E&S Manual), No. 363-2134-008, as amended and updated.
(q) 
Impervious areas:
(1) 
The measurement of impervious areas shall include all of the impervious areas in the total proposed development even if development is to take place in stages.
(2) 
For development taking place in stages, the entire development plan must be used in determining conformance with this chapter.
(3) 
For projects that add impervious area to a parcel, the total impervious area on the parcel is subject to the requirements of this chapter; except that the volume controls in § 25-303 and the peak rate controls of § 25-304 do not need to be retrofitted to existing impervious areas that are not being altered by the proposed regulated activity.
(r) 
Stormwater flows onto adjacent property shall not be created, increased, decreased, relocated, or otherwise altered without written notification to the adjacent property owner(s). Such stormwater flows shall be subject to the requirements of this chapter.
(s) 
All regulated activities shall include such measures as necessary to:
(1) 
Protect health, safety, and property.
(2) 
Meet the water quality goals of this chapter by implementing measures to:
(i) 
Minimize disturbance to floodplains, wetlands, and wooded areas.
(ii) 
Maintain or extend riparian buffers.
(iii) 
Avoid erosive flow conditions in natural flow pathways.
(iv) 
Minimize thermal impacts to waters of this commonwealth.
(v) 
Disconnect impervious surfaces by directing runoff to pervious areas, wherever possible.
(3) 
Incorporate methods described in the Pennsylvania Stormwater Best Management Practices Manual (BMP Manual).
(t) 
The design of all facilities over karst shall include an evaluation of measures to minimize adverse effects.
(u) 
Infiltration BMPs should be spread out, made as shallow as practicable, and located to maximize use of natural on-site infiltration features while still meeting the other requirements of this chapter.
(v) 
Normally dry, open-top, storage facilities should completely drain both the volume control and rate control capacities over a period of time not less than 24 and not more than 72 hours from the end of the design storm.
(w) 
The design storm volumes to be used in the analysis of peak rates of discharge should be obtained from the latest version of the Precipitation-Frequency Atlas of the United States, National Oceanic and Atmospheric Administration (NOAA), National Weather Service, Hydrometeorological Design Studies Center, Silver Spring, Maryland. NOAA's Atlas 14 can be accessed at: http://hdsc.nws.noaa.gov/hdsc/pfds/.
(x) 
For all regulated activities, SWM BMPs shall be designed, implemented, operated, and maintained to meet the purposes and requirements of this chapter and to meet all requirements under Title 25 of the Pennsylvania Code, the Clean Streams Law,[2] and the Storm Water Management Act.[3]
[2]
Editor's Note: See 35 P.S. § 691.1 et seq.
[3]
Editor's Note: See 32 P.S. § 680.1 et seq.
(y) 
Various BMPs and their design standards are listed in the BMP Manual.
[Ord. No. 337, 5/15/2024]
(a) 
No regulated earth disturbance activities within the Township shall commence until the Township receives an approval from the Conservation District of an erosion and sediment control plan for construction activities.
(b) 
DEP has regulations that require an erosion and sediment control plan for any earth disturbance activity of 5,000 square feet or more, under 25 Pa. Code § 102.4(b).
(c) 
In addition, under 25 Pa. Code Chapter 92,[1] a DEP "NPDES Construction Activities" permit is required for regulated earth disturbance activities greater than 1.0 acre.
[1]
Editor's Note: See now 25 Pa. Code Ch. 92a.
(d) 
Evidence of any necessary permit(s) for regulated earth disturbance activities from the appropriate DEP 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 Subsection (a).
(e) 
A copy of the Erosion and Sediment Control plan and any required permit, as required by DEP regulations, shall be available at the project site at all times.
(f) 
Additional erosion and sediment control design standards and criteria are recommended to be applied where infiltration BMPs are proposed and shall include the following:
(1) 
Areas proposed for infiltration BMPs shall be protected from sedimentation and compaction during the construction phase to maintain maximum infiltration capacity.
(2) 
Infiltration BMPs shall not be constructed nor receive runoff until the entire contributory drainage area to the infiltration BMP has achieved final stabilization.
[Ord. No. 337, 5/15/2024]
(a) 
For projects disturbing one acre or more, the design of all regulated activities shall include evaluation of practicable alternatives to the surface discharge of stormwater, the creation of impervious surfaces, and the degradation of waters of the commonwealth, and must maintain as much as possible the natural hydrologic regime of the site.
(1) 
An alternative is practicable if it is available and capable of being done after taking into consideration cost, existing technology and logistics in light of overall project purposes, and other Township requirements.
(2) 
All practicable alternatives to the discharge of stormwater are presumed to have less adverse impact on quantity and quality of waters of the commonwealth unless otherwise demonstrated.
(b) 
The applicant shall demonstrate that they designed the regulated activities that disturb one acre or more to include consideration of the following issues:
(1) 
Prepare an Existing Resource and Site Analysis Map (ERSAM), showing environmentally sensitive areas, including, but not limited to, steep slopes, ponds, lakes, streams, wetlands, hydric soils, vernal pools, floodplains, stream buffer zones, hydrologic soil groups A and B (areas conducive to infiltration), special geologic features, any existing recharge areas and any other requirements outlined in the Township Subdivision and Land Development Ordinance.[1]
[1]
Editor's Note: See Ch. 27 of this Code.
(2) 
Establish appropriate buffers for each of the delineated environmentally sensitive areas per the Township Zoning Ordinance [See § 25-306(d) for stream buffers and § 25-310(h) for special geologic feature buffers].
(3) 
Prepare a draft project layout avoiding sensitive areas identified in Subsection (b)(1).
(4) 
Identify site-specific existing conditions drainage areas, discharge points, recharge areas and hydrologic soil groups A and B.
(5) 
Evaluate nonstructural stormwater management alternatives:
(i) 
Minimize earth disturbance.
(ii) 
Minimize impervious surfaces.
(iii) 
Break up large impervious surfaces.
(6) 
Satisfy infiltration objective (§ 25-305) and provide for stormwater pretreatment prior to infiltration. Pretreatment may not be necessary for rooftop runoff which enters the infiltration facility directly from a roof leader.
(7) 
Satisfy water quality (§ 25-306) and stream bank erosion protection objectives (§ 25-307).
(8) 
Determine what management district the site falls into (Appendix D[2]) and conduct an existing conditions runoff analysis.
[2]
Editor's Note: Said appendix is included as an attachment to this chapter.
(9) 
Prepare final project design to maintain existing conditions drainage areas and discharge points, to minimize earth disturbance and impervious surfaces, and to the maximum extent possible, to ensure the remaining site development has no surface or point discharge.
(10) 
Conduct a proposed conditions runoff analysis based on the final design and to meet the release rate and in turn the overbank flow and extreme event requirements (§ 25-308).
(11) 
Manage any remaining runoff through treatment prior to discharge, as part of detention, bioretention, direct discharge or other structural control.
[Ord. No. 337, 5/15/2024]
Maximizing the groundwater recharge capacity of the area being developed is required. Design of the infiltration stormwater management facilities shall give consideration to providing groundwater recharge to compensate for the reduction in the percolation that occurs when the ground surface is disturbed or impervious surface is created. It is recommended that roof runoff be directed to infiltration BMPs which can be over-designed to compensate for the infiltration losses due to parking areas. It is recommended that roof runoff be directed to infiltration BMPs which may be designed to compensate for the runoff from parking areas. These measures are required to be consistent with § 25-103, and take advantage of utilizing any existing recharge areas.
Infiltration may not be feasible on every site due to site-specific limitations such as soil type. If it cannot be physically accomplished, due to seasonal high-water table, soil permeability rate, soil depth or setback distances from special geologic features, then the design professional shall be responsible to show that this cannot be physically accomplished. If it can be physically accomplished, then the volume of runoff to be infiltrated shall be determined from Subsection (a)(3) depending on demonstrated site conditions and shall be the greater of the two volumes.
The green infrastructure and low impact development practices provided in the BMP Manual shall be utilized for all regulated activities wherever possible.
(a) 
Infiltration BMPs shall meet the following minimum requirements:
(1) 
Infiltration Requirements. Regulated activities will be required to infiltrate, where site conditions permit, a portion of the runoff created by the development as part of an overall stormwater management plan designed for the site. The volume of runoff to be infiltrated shall be determined from Subsection (a)(3)(i), (ii) or (iii), depending upon demonstrated site conditions.
(2) 
Infiltration BMPs intended to receive runoff from developed areas shall be selected based on suitability of soils and site conditions and shall be constructed on soils that have the following characteristics:
(i) 
A minimum depth of 24 inches between the bottom of the BMP and the limiting zone.
(ii) 
An infiltration and/or percolation rate sufficient to accept the additional stormwater load and drain completely as determined by field tests conducted by the applicant's design professional.
(iii) 
The infiltration facility shall be capable of completely infiltrating the required retention (infiltration) volume within four days (96 hours).
(iv) 
Pretreatment shall be provided prior to infiltration.
(3) 
The size of the infiltration facility shall be based upon the following volume criteria:
(i) 
NRCS Curve Number equation.
The NRCS runoff equation shall be utilized to calculate infiltration requirements (1) in inches.
I (Infiltration requirement, in inches) = (200/CN) - 2
Eqn: 305.1
Where:
CN = SCS (NRCS) curve number of existing conditions contributing to the infiltration facility.
This equation is displayed graphically in, and the infiltration requirement can be determined from Figure 305.1.
It has been determined that infiltrating 0.46 inches of runoff from the impervious areas will aid in maintaining the hydrologic regime of the watershed. However, the rounded number 0.5 inches will be used.
Figure 305.1. Infiltration requirement based upon NRCS Curve Number.
The retention (infiltration) volume (Rev) required to meet the infiltration requirement would therefore be computed as:
Rev = (0.5 or I, whichever is greater) * impervious area (square feet)/(12 in/ft) = Cubic Feet
Eqn: 305.2
Where:
I = infiltration requirements (in inches)
(ii) 
Annual Recharge - Water Budget Approach. If the goals of Subsection (a)(3)(i) or (ii) cannot be achieved, then 0.5 inches of rainfall shall be infiltrated from all impervious areas, up to an existing site conditions curve number of 81. Above a curve number of 81, Equation 305.1 or the curve in Figure 305.1 should be used to determine the infiltration requirement.
(iii) 
The retention (infiltration) volume (Rev) required again would therefore be computed as:
Rev = (0.5 or I, whichever is greater) * impervious area (sq. ft.)/(12in/ft) = Cubic Feet
(b) 
Soils Testing. If on-lot infiltration is proposed as part of a project, the applicant's design professional must demonstrate to the satisfaction of the Township that the soils are conducive to infiltration at the proposed location of infiltration facilities. Soils testing shall be completed as follows:
(1) 
Soils testing must be conducted by a qualified design professional and at a minimum shall address depth to limiting zone, soil permeability, and subgrade stability. Soils testing must be observed by a representative of the Township.
(2) 
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.
(3) 
Conduct field testing, including: test pits to determine soil horizons and depth to limiting zone and permeability tests, at the elevation of the proposed infiltration facility surface, to determine the appropriate hydraulic conductivity rate. Double ring infiltrometer or hydraulic conductivity tests should be used to determine soil permeability (percolation tests are not recommended for design purposes). Site evaluation and soils testing should be conducted in accordance with Appendix C of the Pennsylvania Stormwater Best Management Practices Manual.
(4) 
The proposed infiltration facilities shall be designed for the required recharge (Rev) volume based on the field determined capacity at the surface elevation of the proposed infiltration facility.
(c) 
Carbonate Areas. The applicant is required to investigate the ability of all areas on the site which are not underlain by carbonate rock to meet the infiltration requirements of Subsection (a). If this investigation proves infeasible, infiltration can occur on areas underlain by carbonate rock by following the recommended procedure below in conjunction with Figure B-1 in chapter Appendix B.[1] However, the applicant is not required to use infiltration in carbonate areas even if the site falls into the "Recommended" range on Figure B-1 in chapter Appendix B. If infiltration is not proposed, the calculated infiltration volume [Subsection (a)] shall be treated by an acceptable BMP.
Infiltration BMP loading rate percentages in Figure B-1 in chapter Appendix B shall be calculated as follows:
The area tributary to the infiltration BMP shall be weighted as follows:
Area Description
Weighting
All disturbed area to be made impervious
100%
All disturbed areas to be made pervious
50%
All undisturbed impervious areas
100%
All undisturbed pervious areas
0%
Soil thickness is to be measured from the bottom of any proposed infiltration BMP. The effective soil thickness in Figure B-1 in chapter Appendix B is the measured soil thickness multiplied by the thickness factor based on soil permeability, as follows:
Permeability Range
Thickness Factor
6.0 to 12.0 inches/hr
0.8
2.0 to 6.0 inches/hr
1.0
1.0 to 2.0 inches/hr
1.4
0.75 to 1.0 inches/hr
1.2
0.5 to 0.75 inches/hr
1.0
The design of all facilities over karst shall include an evaluation of measures to minimize adverse effects.
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
(d) 
Stormwater Hotspots. Following is a list of examples of designated hotspots. If a site is designated as a hotspot, it has important implications for how stormwater is managed. First and foremost, untreated stormwater runoff from hotspots shall not be allowed to recharge into groundwater where it may contaminate water supplies. Therefore, the Rev requirement shall not be applied to development sites that fit into the hotspot category (the entire WQv must still be treated). Second, a greater level of stormwater treatment shall be considered at hotspot sites to prevent pollutant washoff after construction. EPA's NPDES stormwater program requires some industrial sites to prepare and implement a stormwater pollution prevention plan.
(1) 
Examples of Hotspots:
(i) 
Vehicle salvage yards and recycling facilities.
(ii) 
Vehicle fueling stations.
(iii) 
Vehicle service and maintenance facilities.
(iv) 
Vehicle and equipment cleaning facilities.
(v) 
Fleet storage areas (bus, truck, etc.).
(vi) 
Industrial sites (based on Standard Industrial Codes).
(vii) 
Marinas (service and maintenance).
(viii) 
Outdoor liquid container storage.
(ix) 
Outdoor loading/unloading facilities.
(x) 
Public works storage areas.
(xi) 
Facilities that generate or store hazardous materials.
(xii) 
Commercial container nursery.
(xiii) 
Other land uses and activities as designated by an appropriate review authority.
(2) 
The following land uses and activities are not normally considered hotspots:
(i) 
Residential streets and rural highways.
(ii) 
Residential development.
(iii) 
Institutional development.
(iv) 
Office developments.
(v) 
Non-industrial rooftops.
(vi) 
Pervious areas, except golf courses and nurseries [which may need an integrated pest management (IPM) plan].
(3) 
While large highways [average daily traffic volume (ADT) greater than 30,000] are not designated as a stormwater hotspot, it is important to ensure that highway stormwater management plans adequately protect groundwater.
(e) 
Caution shall be exercised where infiltration is proposed in source water protection areas as defined by the Township or local Water Authority.
(f) 
Infiltration facilities shall be used in conjunction with other innovative or traditional stormwater control facilities that are found within the PADEP State BMP Manual.
(g) 
Caution shall be exercised where salt or chloride (municipal salt storage) would be a pollutant since soils do little to filter this pollutant and it may contaminate the groundwater. The qualified design professional shall evaluate the possibility of groundwater contamination from the proposed infiltration facility and perform a hydrogeologic justification study if necessary.
(h) 
The infiltration requirement in high quality or exceptional value waters shall be subject to the Department's Chapter 93 antidegradation regulations.
(i) 
Dependent upon certain land use or hotspots, an impermeable liner will be required in detention basins where the possibility of groundwater contamination exists. A detailed hydrogeologic investigation may be required by the Township.
(j) 
The Township shall require the applicant to provide safeguards against groundwater contamination for land uses that may cause groundwater contamination should there be a mishap or spill.
(k) 
Infiltration Design Criteria.
(1) 
All infiltration systems shall have appropriate positive overflow controls to prevent storage within one foot of the finished surface or grade.
(2) 
All infiltration systems shall have a minimum setback of 15 feet from principal structures, 10 feet from property lines, 100 feet from wells, 50 feet from septic system drain fields, and 50 feet from a geologic contact with carbonate bedrock. Care should be taken to prevent any seepage into subgrade structures.
(3) 
Surface inflows shall be treated to prevent the direct discharge of sediment and pollutants into the infiltration system; accumulated sediment reduces stormwater storage capacity and ultimately clogs the infiltration mechanism.
(4) 
No sand or other particulate matter may be applied to a porous paving surface for winter ice conditions.
(5) 
During site construction, all recharge system components shall be protected from compaction due to heavy equipment operation or storage of fill or construction material. Recharge areas shall be protected from sedimentation. All areas designated for recharge shall not receive runoff until the contributory drainage area has achieved final stabilization.
(6) 
The following procedures and materials shall be required during the construction of all subsurface facilities:
(i) 
Excavation for the infiltration facility shall be performed with equipment which will not compact the bottom of the seepage bed/trench, or like facility.
(ii) 
The bottom of the bed and/or trench shall be scarified prior to the placement of aggregate.
(iii) 
Only clean aggregate, free of fines, shall be allowed.
(iv) 
The top and sides of all seepage beds, trenches, or like facilities shall be covered with drainage filtration fabric. Fabric shall meet the specifications of PennDOT Publication 408, Section 735, Construction Class 1.
(v) 
Perforated distribution pipes connected to centralized catch basins and/or manholes with provision for the collection of debris shall be provided in all facilities. The perforated pipes shall distribute stormwater throughout the entire seepage bed/trench, or like facility.
(7) 
All infiltration facilities which service more than one lot and are considered a common facility shall have an easement provided to the Township for future access if necessary.
(8) 
No more than 50% of the required infiltration volume may be provided in detention basin bottoms. The remaining 50% of infiltration volumes shall be provided at or near the proposed impervious coverage.
[Ord. No. 337, 5/15/2024]
The applicant shall comply with the following water quality requirements of this Part.
(a) 
Developed areas shall provide adequate storage and treatment facilities necessary to capture and treat stormwater runoff. The infiltration volume computed under § 25-305 may be a component of the water quality volume if the applicant chooses to manage both components in a single facility. If the infiltration volume is less than the water quality volume, the remaining water quality volume may be captured and treated by methods other than infiltration BMPs. 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 goal, the following criterion is established:
The following calculation formula is to be used to determine the water quality storage volume, (WQv), in acre-feet of storage for Amity Township.
WQv = [(P)(Rv)(A)]/12
Eqn: 306.1
WQv
=
Water Quality Volume (acre-feet)
P
=
1 inch
A
=
Total contributing drainage area to the water quality BMP (acres)
Rv
=
0.05 + 0.009(I) where I is the percent of the area that is impervious surface ((impervious area/A) * 100)
This volume requirement can be accomplished by the permanent volume of a wet basin or the detained volume from other BMPs.
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.
(b) 
For areas within defined special protection subwatersheds which include exceptional value (EV) and high quality (HQ) waters, and cold water fishery (CWF), the temperature and quality of water and streams shall be maintained.
(c) 
To accomplish the above, the applicant shall use innovative or traditional stormwater control facilities that are found within the PADEP State BMP Manual.
(d) 
If a perennial or intermittent stream passes through the site, the applicant shall create a stream buffer extending a minimum of 50 feet to either side of the top-of-bank of the channel. The buffer area shall be maintained with appropriate native vegetation. (Refer to Appendix H of Pennsylvania Handbook of Best Management Practices for Developing Area for plant lists.) If the applicable rear or side yard setback is less than 50 feet, the buffer width may be reduced to 25% of the setback to a minimum of 10 feet. If an existing buffer is legally prescribed (i.e., deed, covenant, easement, etc.) and it exceeds the requirements of this chapter, the existing buffer shall be maintained. This does not include lakes or wetlands.
(e) 
Evidence of any necessary permit(s) for regulated earth disturbance activities from the appropriate DEP regional office must be provided to the Township.
[Ord. No. 337, 5/15/2024]
(a) 
In addition to control of the water quality volume, in order to minimize the impact of stormwater runoff on downstream stream bank erosion, the primary requirement is to design a BMP to detain the proposed conditions 2-year, 24-hour design storm to the existing conditions one-year peak flow using the SCS Type II distribution.
(b) 
Additionally, provisions shall be made (such as adding a small orifice at the bottom of the outlet structure) so that the proposed conditions one-year storm takes a minimum of 24 hours to drain from the facility from a point where the maximum volume of water from the one-year storm is captured (i.e., the maximum water surface elevation is achieved in the facility). Release of water can begin at the start of the storm (i.e., the invert of the water quality orifice is at the invert of the facility).
(c) 
The minimum orifice size in the outlet structure to the BMP shall be a 3-inch diameter orifice, and a trash rack shall be installed to prevent clogging. On sites with small contributing drainage areas to this BMP that do not provide enough runoff volume to allow a 24-hour attenuation with the 3-inch orifice, the calculations shall be submitted showing this condition. Orifice sizes less than three inches can be utilized provided that the design will prevent clogging of the intake.
[Ord. No. 337, 5/15/2024]
(a) 
Amity Township has been divided into stormwater management districts as shown on the Stormwater Management Districts Map in Appendix D.[1]
In addition to the requirements specified in Table 308.1 below, the groundwater recharge (§ 25-305), water quality (§ 25-306), and stream bank erosion control (§ 25-307), requirements shall be implemented.
Standards for managing runoff from each subarea for the 2-year through 100-year design storms are shown in Table 308.1. Development sites located in each of the districts must control proposed conditions runoff rates to existing conditions runoff rates for the design storms in accord with Table 308.1.
Table 308.1 - Water Quantity Requirements
Management District
Proposed Condition Design Storm
Existing Condition Design Storm
B
2-year
Reduce To
1-year
5-year
2-year
10-year
5-year
25-year
10-year
50-year
25-year
100-year
50-year
C
2-year
Reduce To
1-year
5-year
5-year
10-year
10-year
25-year
25-year
50-year
50-year
100-year
100-year
All areas, regardless of the release rate, must still meet the requirements of the groundwater recharge criteria (§ 25-305), water quality criteria (§ 25-306), and stream bank erosion criteria (§ 25-307).
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
(b) 
General. Proposed condition rates of runoff from any regulated activity shall not exceed the peak release rates of runoff prior to development for the design storms specified on the Stormwater Management Districts Map (Chapter Appendix D[2]) and § 25-308, of this chapter.
[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 in Chapter Appendix D. 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 2-foot topographic contours (or most accurate data required) provided as part of the drainage plan.
(d) 
Sites Located in More Than One District. For a proposed development site located within two or more stormwater management district category subareas, the peak discharge rate from any subarea shall meet the management district criteria for which the discharge is located, as indicated in § 25-308. The calculated peak discharges shall apply regardless of whether the grading plan changes the drainage area by subarea.
(e) 
Off-Site Areas. Off-site areas that drain through a proposed development site are not subject to release rate criteria when determining allowable peak runoff rates. However, on-site drainage facilities shall be designed to safely convey off-site flows through the development site.
(f) 
Site Areas. Where the site area to be impacted by a proposed development activity differs significantly from the total site area, only the proposed impact area utilizing stormwater management measures shall be subject to the management district criteria. In other words, unimpacted areas bypassing the stormwater management facilities would not be subject to the management district criteria.
(g) 
For areas not covered by a release rate map from an approved Act 167 Stormwater Management Plan: Post-development discharge rates shall not exceed the pre-development discharge rates for the 1-, 2-, 5-, 10-, 25-, 50-, and 100-year, 24-hour storm events. If it is shown that the peak rates of discharge indicated by the post-development analysis are less than or equal to the peak rates of discharge indicated by the pre- development analysis for 1-, 2-, 5-, 10-, 25-, 50-, and 100-year, 24-hour storms, then the requirements of this section have been met. Otherwise, the applicant shall provide additional controls as necessary to satisfy the peak rate of discharge requirement.
[Ord. No. 337, 5/15/2024]
(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 309-1 summarizes acceptable computation methods, and the method selected by the design professional shall be based on the individual limitations and suitability of each method for a particular site. The Township may allow the use of the Rational Method to estimate peak discharges from drainage areas that contain less than five acres. The Soil Cover Complex Method shall be used for drainage areas greater than five acres.
Table 309-1
Acceptable Computation Methodologies For Stormwater Management Plans
Method
Method Developed By
Applicability
TR-20 (or commercial computer package based on TR-20)
USDA NRCS
Applicable where use of full hydrology computer model is desirable or necessary.
TR-55 (or commercial computer package based on TR-55)
USDA NRCS
Applicable for land development plans within limitations described in TR-55.
HEC-1/HEC-HMS
US Army Corps of Engineers
Applicable where use of full hydrologic computer model is desirable or necessary.
PSRM
Penn State University
Applicable where use of a hydrologic computer model is desirable or necessary; simpler than TR-20 or HEC-1.
Rational Method (or commercial computer package based on Rational Method)
Emil Kuichling (1889)
For sites less than 5 acres and with time of concentration less than 60 minutes (Tc < 60 min), or as approved by the Township.
Other Methods
Varies
Other computation methodologies approved by the Township.
*Note: Successors to the above methods are also acceptable. These successors include WinTR55 for TR-55 and WinTR20 for TR-20 and SWMM.
(b) 
All calculations consistent with this chapter using the Soil Cover Complex Method shall use the appropriate design rainfall depths for the various return period storms according to the region in which they are located as presented in Table B-1 in Appendix B of this chapter.[1] If a hydrologic computer model such as PSRM or HEC-1/HEC-HMS is used for stormwater runoff calculations, then the duration of rainfall shall be 24 hours. The rainfall distribution should reference to NOAA Atlas 14.
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
(c) 
For the purposes of existing conditions flow rate determination, undeveloped land shall be considered as "meadow" in good condition, unless the natural ground cover generates a lower curve number or Rational "C" value (i.e., forest), as listed in Table B-2 or B-3 in Appendix B of this chapter. No less than 20% of existing impervious area, when present, shall be considered meadow in good condition.
(d) 
All calculations using the Rational Method shall use rainfall intensities consistent with appropriate times of concentration for overland flow and return periods from the NOAA Atlas 14 Precipitation-Frequency Atlas of the United States (2004, revised 2006). Times of concentration for overland flow shall be calculated using the methodology presented in chapter 3 of Urban Hydrology for Small Watersheds, NRCS, TR-55 (as amended or replaced from time to time by NRCS). Times of concentration for channel and pipe flow shall be computed using Manning's Equation. The NRCS lag equation divided by 0.6 is an acceptable method for Tc in undeveloped areas.
(e) 
Runoff curve numbers (CN) for both existing and proposed conditions to be used in the Soil Cover Complex Method shall be obtained from Table B-2 in Appendix B of this chapter.
(f) 
Runoff coefficients (c) for both existing and proposed conditions for use in the Rational Method shall be obtained from Table B-3 in Appendix B of this chapter.
(g) 
Where uniform flow is anticipated, the Manning Equation shall be used for hydraulic computations, and to determine the capacity of open channels, pipes, and storm sewers. Values for Manning's roughness coefficient (n) shall be consistent with Table B-4 in Appendix B of the chapter. Full flow shall be assumed for closed conduits.
(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 the Storage-Indication Method. For drainage areas greater than five acres in size, the design storm hydrograph shall be computed using a calculation method that produces a full hydrograph (i.e., TR-20, TR-55, HEC-1, PSRM). The 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. No. 337, 5/15/2024]
(a) 
All wet basin designs shall incorporate biologic minimization controls consistent with the West Nile Guidance found in Appendix F.[1]
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
(b) 
Any stormwater management facility (i.e., detention basin) designed to store runoff and requiring a berm or earthen embankment required or regulated by this chapter shall be designed to provide an emergency spillway to handle flow up to and including the 100-year proposed conditions and may be subject to PaDEP Chapter 105 regulations.
(c) 
Any facilities that constitute water obstructions (e.g., culverts, bridges, outfalls, or stream enclosures), water encroachments, and any work involving wetlands governed by PaDEP Chapter 105 regulations (as amended or replaced from time to time by PaDEP), are subject to PaDEP Chapter 105 regulations.
(d) 
Any proposed roadway drainage facilities shall be designed according to PennDOT Design Manual Part II.
(e) 
Storm sewers must be able to convey proposed conditions runoff from a 50-year design storm without flooding inlets, where appropriate. Roadway crossings must be able to convey runoff from a 100-year design storm.
(f) 
Adequate erosion protection shall be provided along all open channels, and at all points of discharge (DEP erosion and Erosion, Sediment and Pollution Control Manual).
(g) 
The Township reserves the right to disapprove any design that would result in the construction in or continuation of a stormwater problem area.
(h) 
No stormwater detention facility shall be placed within 50 feet of a special geologic feature. No stormwater conveyance facility shall be constructed within 50 feet of a special geologic feature, unless it is constructed of durable pipe utilizing watertight joints.
[Ord. No. 337, 5/15/2024]
This section does not apply to projects that qualify for preparation of a minor drainage plan or rain garden design. Rain garden design shall comply with the PADEP Best Management Practices (BMP) Manual (latest edition).
(a) 
The following criteria shall be used for the design of detention and wet basins. Any reference to detention basin shall also include wet basins.
(1) 
When detention basins are provided, they shall be designed to utilize the natural contours of the land wherever possible. When such design is impracticable, the construction of the basin shall utilize slopes as flat as possible to blend the structure into the terrain. All basins shall have maximum side slope of three horizontal to one vertical (3:1).
(2) 
A routed hydrograph and calculations shall be furnished for each storm through the detention basin.
(3) 
Emergency overflow facilities (i.e., emergency spillway) shall be provided for all detention facilities which shall pass a 100-year storm under orifice block conditions.
(b) 
In the design of detention basins, the following items listed below shall be submitted to the Township for review. Any reference to detention basin shall also include retention basin.
(1) 
Design computations for the sizing of the outlet device.
(2) 
A stage-storage curve for said detention/retention basin.
(3) 
Flood routing and/or storage requirement calculations.
(4) 
A plan showing the berm embankment and outlet structure. The plan shall indicate top of berm elevations, width of the top of the berm, side slopes, emergency spillway elevation, and elevations of the outlet structure, including riser, dimensions and spacing of anti-seep collars.
(5) 
A cross section through the outlet structure, emergency spillway and berm embankment.
(6) 
A detailed plan of the trash rack and anti-vortex device (if required).
(7) 
The maximum side slope of earthen detention embankments shall be three horizontal to one vertical (3:1). The top and/or toe (whichever is closer to a property line) of any slope shall be located a minimum of 25 feet from any property line. Whenever possible, the side slopes and basin shape shall conform to the natural topography.
(8) 
The minimum top width of the detention basin berm shall be 10 feet. A cutoff trench (key-way) of impervious material shall be provided under all embankments that require fill material. The cutoff trench shall be a minimum of eight feet wide, two feet deep and minimum of two feet over the pipe, and have maximum side slopes of one horizontal to one vertical (1:1).
(9) 
In order to ensure proper drainage on the floor of the basin, a minimum grade of 2% shall be maintained for areas of sheet flow. For channel flow, a minimum grade of 1% shall be maintained. Under certain circumstances, such as continuous seasonal flow, the Township may require a low flow channel to be constructed. Wet basins may be designed with a flat bottom upon approval by the Township Engineer.
(10) 
All detention basin embankments shall be placed in a maximum of 8-inch lifts to a minimum of 95% of maximum dry density as established by ASTM D-1557. Prior to proceeding to the next lift, the compaction shall be checked by the Township Engineer. The developer's contractor shall obtain the services of a qualified laboratory technician to conduct compaction tests on the leading and the trailing edge of the berm along with the top of the berm. All tests shall be furnished to the Township for review.
(11) 
Whenever possible, the emergency spillway for detention basins shall be constructed on undisturbed ground. Emergency spillways shall be constructed of reinforced concrete checker blocks or other permanent material approved by the Township Engineer. All emergency spillways shall be constructed so that the detention basin berm is protected against erosion. The minimum capacity of all emergency spillways shall be the peak flow rate from the 100-year design storm after development. The construction material of the emergency spillway shall extend along the upstream and downstream berm embankment slopes. Construction material on the upstream slope of the emergency spillway shall extend to a minimum of two feet below the spillway crest elevation. The downstream slope of the spillway shall as a minimum extend to the toe of the berm embankment. The emergency spillway shall not discharge over earthen fill and/or easily erodible material.
(12) 
A minimum freeboard of one foot shall be provided between the 100-year design stormwater elevation and the invert of the emergency spillway, and between the design flow through the emergency spillway and the top of the berm.
(13) 
Anti-seep collars shall be installed around the pipe barrel within the normal saturation zone of the detention basin berms. The anti-seep collars and their connections to the pipe barrel shall be watertight. The anti-seep collars shall extend a minimum of two feet beyond the outside of the principal pipe barrel. The maximum spacing between collars shall be 14 times the minimum projection of the collar measured perpendicular to the pipe. A minimum of two anti-seep collars shall be installed on each pipe outlet.
(14) 
All outlet pipes through the basin berm shall be reinforced concrete pipe having O-ring joints. All outlet structures shall be concrete.
(15) 
Energy dissipating devices shall be placed at all basin outlets.
(16) 
Easements of all basins shall be provided.
(17) 
Permanent detention basins outlet structures shall be designed to incorporate multiple stage outlet release devices.
(18) 
A cross section through the basin from the proposed pipe termination into the basin to the termination of the outlet pipe shall be provided.
(19) 
At the discretion of the Township, the perimeter of the detention/retention pond shall be enclosed with a standard chain-link security fence or approved equal with a minimum height of four feet with locking gates. Each basin shall be provided with two gates, one wide enough for maintenance vehicles and a second gate for pedestrian access. The security fence and gate shall be subject to the approval of the Township.
(20) 
Permanent plantings for wet ponds shall be designed by a wetland biologist to have a mixture of plants that thrive in wet areas.
[Ord. No. 337, 5/15/2024]
This section does not apply to projects that qualify for preparation of a minor drainage plan.
(a) 
Drainage easements shall be provided to accommodate all storm drainage requirements and shall be a minimum of 20 feet in width. Easements shall be provided for all watercourses and storm drainage piping that are not located within street rights-of-way.
(b) 
Storm sewers, as required, shall be placed in the cartway of curbed streets and parallel to the roadway shoulders of streets without curbs. When located in undedicated land with approval by the Board, storm sewers shall be placed within an easement not less than 20 feet wide.
(c) 
Storm Drainage Pipes. The minimum diameter of all storm drainage pipes shall be 15 inches or an equivalent thereto. All storm drainage piping shall be laid in a straight line. Storm drainage piping shall not be permitted under buildings or structures. The minimum grade of piping shall be 0.5%. All pipe shall be reinforced concrete with O-rings joints, or smooth bore high density polyethylene pipe. All structures exposed to the surface shall be reinforced concrete.
(d) 
When proposed, manholes and inlets (catch basins) shall not be spaced more than 400 feet apart for pipes of less than or equal to 24-inch diameter and 500 feet apart for pipes of greater than 24-inch diameter. Additionally, manholes or inlets shall be placed at all changes in alignment, grade or pipe size, and at all points of convergence of two or more influent storm sewer lines. Inlets may be substituted for manholes where they will serve a useful purpose. In addition, the following standards shall apply:
(1) 
Manholes and inlets must conform to the standards established by PennDOT and must be supplied by a PennDOT Bulletin 15 approved supplier. Such requirement shall be listed on the plan.
(2) 
At street intersections, inlets shall be placed to prevent the flow of water across intersections.
(3) 
Inlets shall be spaced to limit the gutter spread to within the parking lane, not more than eight feet during the 10-year storm. Capacity of the inlets shall not exceed four cubic feet per second (cfs) for 4-foot inlets and five cfs for 6-foot inlets in non-ponding areas.
(4) 
Inlets with a depth greater than five feet must be provided with ladder rungs and noted on the plan as such.
(5) 
When there is a change in pipe size in the inlet, the elevation of the top of pipes shall be the same or the smaller pipe higher. A minimum drop of two inches shall be provided at the inlet pipe invert elevation and the outlet pipe elevation.
(6) 
Inlets shall contain a marker which discourages the discharge of anything other than stormwater into the inlet.
(7) 
Inlets in paved areas shall be equipped with bicycle-safe grates. Inlets in non-paved areas shall be equipped with standard grates.
(8) 
Manhole covers shall have the word "STORM" cast on the top of the cover.
(e) 
Properly designed, graded and lined drainage swales may be permitted in lieu of storm sewers in commercial, industrial and residential areas where approved by the Township. Swale lining must meet the Conservation District design standards. All drainage channels shall have a maximum side slope grade of three horizontal to one vertical (3:1). All drainage swales shall be provided with a minimum 6-inch freeboard, measured from the top of the design storm flow to the top of the swale.
(f) 
Curb requirements shall vary according to street hierarchy and intensity of the proposed development. Curbing may also be required for any one or more of the following reasons:
(1) 
For stormwater management.
(2) 
To stabilize the pavement/cartway edge.
(3) 
To delineate parking areas.
(4) 
To delineate vehicular access lanes.
(5) 
At intersections, corners and tight radii.
(g) 
The maximum encroachment of water on the roadway pavement shall not exceed four inches in depth at the curbline during a 10-year frequency storm. Inlets shall be provided to control the encroachment of water on the pavement.
(h) 
Pipe underdrains and/or pavement base drains shall be provided in areas delineated as having a "seasonal high-water table" and in areas deemed necessary by the Township Engineer during the construction phase of the project. The installation of the underdrain system shall be approved by the Township Engineer and paid for by the developer. Pipe underdrains and pavement base drains shall be constructed in accordance with PennDOT Pub. 408, Section 610, as amended.
(i) 
Pipe end sections and/or head walls shall be utilized at all terminated pipe segments.
(j) 
All drainage structures located within a state highway right-of-way shall be reviewed and approved by the PennDOT. A letter from PennDOT indicating such approval shall be submitted to the Township prior to Township approval.
(k) 
Energy dissipaters shall be provided at all pipe end treatments.
(l) 
At the discretion of the Township, in situations in which the design standards and requirements do not apply to the site conditions, the Township Engineer shall suggest or provide additional and/or alternative design methods to meet the objectives of this chapter.