A. 
Applicants proposing regulated activities in the Borough which do not fall under the exemption criteria shown in § 513-6 shall submit a drainage plan consistent with this chapter and the respective stormwater management plan to the Borough for review. The stormwater management criteria of this chapter shall apply to the total proposed development even if development is to take place in stages.
B. 
The applicant is required to find 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 § 513-19 to ensure maintenance of the natural hydrologic regime, to promote groundwater recharge and to protect groundwater and surface water quality and quantity. The drainage plan designer must proceed sequentially in accordance with Article IV of this chapter.
D. 
Stormwater drainage systems shall be designed in order to permit unimpeded flow along natural watercourses, except as modified by stormwater management facilities or open channels consistent with this chapter.
E. 
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.
F. 
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.
G. 
Where a development site is traversed by existing streams, drainage easements shall be provided conforming to the line of such streams. The terms of the easement shall conform to the stream buffer requirements contained in § 513-21G of this article.
H. 
Any stormwater management facilities regulated by this chapter that would be located in or adjacent to waters of the commonwealth or delineated wetlands shall be subject to approval by DEP through the joint permit application or the environmental assessment approval process or, where deemed appropriate, by the DEP general permit process. When there is a question as to 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 DEP.
I. 
Any proposed stormwater management facilities regulated by this chapter that would be located on state highway rights-of-way shall be subject to approval by PennDOT.
J. 
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.
K. 
All stormwater runoff shall be pretreated for water quality prior to discharge to surface water or groundwater.
L. 
All regulated activities within the Borough shall be designed, implemented, operated and maintained to meet the purposes of this chapter through these two elements:
(1) 
Erosion and sediment control during earth disturbance activities (e.g., during construction); and
(2) 
Water quality protection measures after completion of earth disturbance activities (i.e., after construction), including operations and maintenance.
M. 
No regulated earth disturbance activities within the Borough shall commence until the requirements of this chapter are met.
N. 
Post-construction water quality protection shall be addressed as required by § 513-21.
O. 
Operations and maintenance of permanent stormwater BMPs shall be addressed as required by Article VII.
P. 
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 Borough.
Q. 
Techniques described in Appendix E[1] (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.
[1]
Editor's Note: Appendix E and other appendixes referenced in this chapter are on file in the Borough's offices.
R. 
In selecting the appropriate BMPs or combinations thereof, the applicant shall consider the following:
(1) 
Total contributing area.
(2) 
Permeability and infiltration rate of the site's soils.
(3) 
Slope and depth to bedrock.
(4) 
Seasonal high-water table.
(5) 
Proximity to building foundations and wellheads.
(6) 
Erodibility of soils.
(7) 
Land availability and configuration of the topography.
(8) 
Peak discharge and required volume control.
(9) 
Stream bank erosion.
(10) 
Efficiency of the BMPs to mitigate potential water quality problems.
(11) 
The volume of runoff that will be effectively treated.
(12) 
The nature of the pollutant being removed.
(13) 
Maintenance requirements.
(14) 
Creation/protection of aquatic and wildlife habitat.
(15) 
Recreational value.
S. 
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 A.[2]
[2]
Editor's Note: Appendix A and other appendixes referenced in this chapter are on file in the Borough's offices.
The following permit requirements may apply to certain regulated earth disturbance activities and must be met prior to commencement of regulated earth disturbance activities, as applicable:
A. 
All regulated earth disturbance activities subject to permit requirements by DEP under regulations at 25 Pa. Code Chapter 102.
B. 
Work within natural drainageways subject to permit by DEP under 25 Pa. Code Chapter 105.
C. 
Any stormwater management facility that would be located in or adjacent to surface waters of the commonwealth, including wetlands, subject to permit by DEP under 25 Pa. Code Chapter 105.
D. 
Any stormwater management facility that would be located on a state highway right-of-way or require access from a state highway shall be subject to approval by PennDOT.
E. 
Culverts, bridges, storm sewers or any other facilities which must pass or convey flows from the tributary area and any facility which may constitute a dam subject to permit by DEP under 25 Pa. Code Chapter 105.
A. 
No regulated earth disturbance activities within the Borough shall commence until the Borough 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, a DEP NPDES construction activities permit is required for regulated earth disturbance activities.[1]
[1]
Editor’s Note: Former 25 Pa. Code Ch. 92 was repealed effective 10-9-2010. 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 municipality. The issuance of an NPDES construction permit or permit coverage under the statewide general permit (PAG-2) satisfies the requirements of Subsection A.
E. 
A copy of the erosion and sediment control plan and any required permit, as required by DEP regulations, shall be available on the project site at all times.
F. 
Additional erosion and sediment control design standards and criteria are recommended to be applied where infiltration BMPs are proposed. They shall include the following:
(1) 
Areas proposed for infiltration BMPs shall be protected from sedimentation and compaction during the construction phase to maintain maximum infiltration capacity.
(2) 
Infiltration BMPs shall not be constructed nor receive runoff until the entire drainage area contributory to the infiltration BMP has achieved final stabilization.
A. 
The design of all regulated activities shall include the following to minimize stormwater impacts:
(1) 
The applicant shall find practicable alternatives to the surface discharge of stormwater, such as those listed in Appendix F, Table F-5,[1] 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]
Editor's Note: Appendix F and other appendixes referenced in this chapter are on file in the Borough's offices.
(2) 
An alternative is practicable if it is available and capable of implementation after taking into consideration existing technology and logistics in light of overall project purposes and other municipal requirements.
(3) 
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 the regulated activities were designed in the following sequence. 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) 
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 Chapter 530, Subdivision and Land Development, shall also be included.
(2) 
Establish a stream buffer according to § 513-21G.
(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 (area conducive to infiltration).
(5) 
Evaluate nonstructural stormwater management alternatives:
(a) 
Minimize earth disturbance.
(b) 
Minimize impervious surfaces.
(c) 
Break up large impervious surfaces.
(6) 
Satisfy the groundwater recharge (infiltration) objective § 513-20 and provide for stormwater pretreatment prior to infiltration.
(7) 
Provide for water quality protection in accordance with § 513-21, Water quality requirements.
(8) 
Provide stream bank erosion protection in accordance with § 513-22, Stream bank erosion requirements.
(9) 
Determine into what management district the site falls (Appendix A[2]) and conduct an existing conditions runoff analysis.
[2]
Editor's Note: Appendix A and other appendixes referenced in this chapter are on file in the Borough's offices.
(10) 
Prepare final project design to maintain existing conditions drainage areas and discharge points, to minimize earth disturbance and impervious surfaces and, to the maximum extend possible, to ensure that the remaining site development has no surface or point discharge.
(11) 
Conduct a proposed conditions runoff analysis based on the final design that meets the management district requirements, § 513-23.
(12) 
Manage any remaining runoff prior to discharge through detention, bioretention, direct discharge or other structural control.
Maximizing the groundwater recharge capacity of the area being developed is required. Design of the infiltration shall consider groundwater recharge to compensate for the reduction in the recharge that occurs when the ground surface is disturbed or impervious surface is created. It is recommended that roof runoff be directed to infiltration BMPs that may be designed to compensate for the runoff from parking areas. These measures are required to be consistent with § 513-3 and to 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, 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(2), depending on demonstrated site conditions, and shall be the greater of the volumes.
A. 
Infiltration BMPs shall meet the following minimum requirements:
(1) 
Infiltration BMPs intended to receive runoff from developed areas shall be selected based on suitability of soil and site conditions and shall be constructed on soils that have the following characteristics:
(a) 
A minimum depth of 24 inches between the bottom of the BMP and the top of the limiting zone.
(b) 
An infiltration rate sufficient to accept the additional stormwater load and dewater completely, as determined by field tests conducted by the applicant's design professional.
(c) 
The infiltration facility shall be capable of completely infiltrating the retention (infiltration) volume (Rev) within four days (96 hours).
(2) 
The size of the infiltration facility shall be based upon the following volume criteria:
(a) 
Net two-year volume approach. In HQ/EV watersheds, the retention (infiltration) volume (Rev) to be captured and infiltrated shall be the net two-year volume. The net two-year volume shall be determined by plotting the two-year project site post-development hydrograph, drawing a straight line from the point-of-inflection of the rising limb of the hydrograph to the predevelopment two-year storm, and measuring the volume under the curve as shown in Figure 513-20A.
Figure 513-20A
Infiltration Hydrograph
513 Infiltration Hydrograph.tif
(b) 
One inch from impervious surface. In other portions of the watershed that are not classified as HQ/EV, the retention (infiltration) volume (Rev) will be equal to capturing one inch of rainfall over all proposed impervious surfaces.
Rev = I * impervious area (square feet) ÷ 12 (inches) = cubic feet (cf)
An asterisk (*) in equations denotes multiplication.
(c) 
Obtaining the Rev volume in Subsection A(2)(a) (above) may not be feasible on every site due to site-specific limitations such as soil type If it cannot be physically accomplished, then the design professional shall be responsible for showing that this cannot be physically accomplished. If it cannot be physically accomplished, then the retention (infiltration) volume Rev required shall be as much as can be physically accomplished with a minimum of 0.50 inch depending on demonstrated site conditions. It has been determined that capturing and infiltrating 0.50 inch of runoff from the impervious areas will aid in maintaining the hydrologic regime (base flow) of the watershed. If the goals of Subsection A(2)(a) or (b) cannot be achieved, then 0.50 inch of rainfall shall be retained and infiltrated from all impervious areas. The minimum recharge volume (Rev) required would, therefore, be computed as:
Rev = I * impervious area (square feet) ÷ 12 (inches) = cubic feet (cf)
An asterisk (*) in equations denotes multiplication.
Where:
I
=
The maximum equivalent infiltration amount (inches) that the site can physically accept or 0.50 inch, whichever is greater.
(3) 
The retention volume value derived from the methods in Subsection A(2)(a), (b) and/or (c) is the minimum volume the applicant must control through an infiltration BMP facility. However, if a site has areas of soils where additional volume of retention can be achieved, the applicant is encouraged to infiltrate as much of the stormwater runoff from the site as possible. If the minimum of 0.50 inch of infiltration requirement cannot be achieved, a waiver from this section, Groundwater recharge, would be required from the Borough.
B. 
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 a minimum, address soil permeability, depth to bedrock and subgrade stability. The general process for designing the infiltration BMP shall be:
(1) 
Analyze hydrologic soil groups as well as natural and man-made features within the site to determine general areas of suitability for infiltration practices. In areas where development on fill material is under consideration, conduct geotechnical investigations of subgrade stability; infiltration may not be ruled out without conducting these tests.
(2) 
Provide field tests such as double-ring infiltrometer or hydraulic conductivity tests (at the level of the proposed infiltration surface) to determine the appropriate hydraulic conductivity rate. Percolation tests are not recommended for design purposes.
(3) 
Design the infiltration structure for the required retention (Rev) volume based on field-determined capacity at the level of the proposed infiltration surface.
(4) 
If on-lot infiltration structures are proposed by the applicant's design professional, it must be demonstrated to the Borough that the soils are conducive to infiltrate on the lots identified.
C. 
Stormwater hot spots.
(1) 
Below is a list of examples of designated hot spots. If a site is designated as a hot spot, it has important implications for how stormwater is managed. First and foremost, untreated stormwater runoff from hot spots 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 hot spot category (the entire WQv must still be treated). Second, a greater level of stormwater treatment shall be considered at hot spot sites to prevent pollutant wash off after construction. The Environmental Protection Agency's (EPA) NPDES stormwater program requires some industrial sites to prepare and implement a stormwater pollution prevention plan.
(a) 
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 Codes.
[7] 
Marinas (service and maintenance).
[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] 
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].
(2) 
While large highways [average daily traffic volume (ADT) greater than 30,000] are not designated as stormwater hot spots, it is important to ensure that highway stormwater management plans adequately protect groundwater.
D. 
Extreme caution shall be exercises where infiltration is proposed in SWPAs as defined by the local municipality or water authority.
E. 
Infiltration facilities shall be used in conjunction with other innovative or traditional BMPs, stormwater control facilities, and nonstructural stormwater management alternatives.
F. 
Extreme 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.
G. 
The infiltration requirement in HQ or EV waters shall be subject to the Department's Chapter 93 Antidegradation Regulations.
H. 
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 Borough.
I. 
The Borough 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.
The applicant shall comply with the following water quality requirements of this article:
A. 
No regulated earth disturbance activities within the Borough shall commence until approval by the Borough of a plan which demonstrates compliance with post-construction state water quality requirements.
B. 
The BMPs shall be designed, implemented and maintained to meet state water quality requirements and any other more stringent requirements as determined by the Borough.
C. 
To control post-construction stormwater impacts from regulated earth disturbance activities, state water quality requirements can be met by BMPs, including site design, which provide for replication of preconstruction stormwater infiltration and runoff conditions so that post-construction stormwater discharges do not degrade the physical, chemical or biological characteristics of the receiving waters. As described in the DEP Comprehensive Stormwater Management Policy (No. 392-0300-002, September 28, 2002), this may be achieved by the following:
(1) 
Infiltration: replication of preconstruction stormwater infiltration conditions.
(2) 
Treatment. Use of water quality treatment BMPs to ensure filtering out of the chemical and physical pollutants from the stormwater runoff.
(3) 
Stream bank and streambed protection. Management of volume and rate of post-construction stormwater discharges to prevent physical degradation of receiving waters (e.g., from scouring).
D. 
Developed areas shall provide adequate storage and treatment facilities necessary to capture and treat stormwater runoff. The retention volume computed under § 513-20 may be a component of the water quality volume if the applicant chooses to manage both components in a single facility. If the retention 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:
(1) 
The following calculation formula is to be used to determine the water quality storage volume (WQv) in acre-feet of storage required by this chapter:
WQv = [(P)(Rv)(A)] ÷ 12
Where:
WQv = Water quality volume (acre-feet)
P
=
1 inch
A
=
Area of the project contributing 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]
(2) 
This volume requirement can be accomplished by the permanent volume of a wet basin or the detained volume from other BMPs. Where appropriate, wet basin shall be utilized for water quality control and shall follow the guidelines of the BMP manuals referenced in Appendix G.[1]
[1]
Editor's Note: Appendix G and other appendixes referenced in this chapter are on file in the Borough's offices.
(3) 
Release of water can begin at the start of the storm (i.e., the invert of the water quality orifice is at the invert of the facility). The design of the facility shall provide for protection from clogging and unwanted sedimentation.
E. 
For areas within defined special protection subwatersheds that include EV and HQ waters, the temperature and quality of water and streams shall be maintained through the use of temperature-sensitive BMPs and stormwater conveyance systems.
F. 
To accomplish the above, the applicant shall submit original and innovative designs to the Borough Engineer for review and approval. Such designs may achieve the water quality objectives through a combination of different BMPs.
G. 
If a perennial or intermittent stream passes through the site, the applicant shall create a stream buffer extending a minimum of 10 feet to either side of the top-of-bank of the channel. The buffer area shall be maintained with and encouraged to use appropriate native vegetation (refer to Appendix H of the Pennsylvania Handbook of Best Management Practices for Developing Areas for plant lists). 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.
H. 
Evidence of any necessary permit(s) for regulated earth disturbance activities from the appropriate DEP regional office must be provided to the Borough. The issuance of an NPDES construction permit [or permit coverage under the statewide general permit (PAG-2)] satisfies the requirements of Subsection A.
A. 
In addition to the control of 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 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) 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).
B. 
The minimum orifice size in the outlet structure to the BMP shall be three inches in diameter where possible, and a trash rack shall be installed to prevent clogging. On sites with small drainage areas contributing to 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. Orifice sizes less than three inches can be utilized, provided that the design will prevent clogging of the intake.
C. 
In "conditional direct discharge districts" (District C) only (see § 513-23), the objective is not to attenuate the storms greater than the two-year recurrence interval. This can be accomplished by configuring the outlet structure not to control the larger storms or by a bypass channel that diverts only the two-year stormwater runoff into the basin or, conversely, diverts flows in excess of the two-year storm away from the basin.
A. 
The Darby and Cobbs Creeks Watershed has been divided into stormwater management districts as shown on the Stormwater Management District Map in Appendix A.[1] In addition to the requirements specified in Table 513-23A below, the erosion and sedimentation control (§ 513-18), the nonstructural project design (§ 513-19), the groundwater recharge (§ 513-20), the water quality (§ 513-21), and the stream bank erosion (§ 513-22) requirements shall be implemented.
(1) 
Standards for managing runoff from each subarea in the Darby and Cobbs Creeks watershed for the two-, five-, ten-, twenty-five-, fifty- and one-hundred-year design storms are shown in Table 513-23A. Development sites located in each of the management districts must control proposed conditions runoff rates to existing conditions runoff rates for the design storms in accordance with Table 513-23A.
Table 513-23A
Peak Rate Control Standards by Stormwater Management District in the Darby-Cobbs Creek Watershed
District
Proposed Condition
Design Storm
Existing Condition
Design Storm
A
2-year
1-year
5-year
5-year
10-year
10-year
25-year
25-year
100-year
100-year
B-1
2-year
1-year
10-year
5-year
25-year
10-year
50-year
25-year
100-year
100-year
B-2
2-year
1-year
5-year
2-year
25-year
5-year
50-year
10-year
100-year
100-year
C
Conditional Direct Discharge District
(2) 
In District C, development sites that can discharge directly to the Darby-Cobbs Creek main channel, major tributaries, or indirectly to the main channel through an existing stormwater drainage system (i.e., storm sewer or tributary) may do so without control of the proposed conditions peak rate of runoff greater than the five-year storm. Sites in District C will still have to comply with the groundwater recharge criteria, the water quality criteria and stream bank erosion criteria. If the proposed conditions runoff is intended to be conveyed by an existing stormwater drainage system to the main channel, assurance must be provided that such system has adequate capacity to convey the flows greater than the two-year existing conditions peak flow or will be provided with improvements to furnish the required capacity. When adequate capacity in the downstream system does not exist and will not be provided through improvements, the proposed conditions peak rate of runoff must be controlled to the existing conditions peak rate as required in District A provisions (i.e., ten-year proposed conditions flows to ten-year existing conditions flows) for the specified design storms.
[1]
Editor's Note: Appendix A and other appendixes referenced in this chapter are on file in the Borough's offices.
B. 
General. Proposed conditions rates of runoff from any regulated activity shall not exceed the peak release rates of runoff from existing conditions for the design storms specified on the Stormwater Management District Watershed Map (Appendix A[2]) and this section of the chapter.
[2]
Editor's Note: Appendix A and other appendixes referenced in this chapter are on file in the Borough's offices.
C. 
District boundaries. The boundaries of the stormwater management districts are shown on the official map that is available for inspection at the municipal and county planning offices. A copy of the official map at a reduced scale is included in Appendix A.[3] The exact location of the stormwater management district boundaries as they apply to a given development site shall be determined by mapping the boundaries using the two-foot topographic contours (or most accurate data required) provided as part of the drainage plan.
[3]
Editor's Note: Appendix A and other appendixes referenced in this chapter are on file in the Borough's offices.
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 The calculated peak discharges shall apply regardless of whether the grading plan changes the drainage area by subarea. An exception to the above may be granted if discharges from multiple subareas recombine in proximity to the discharge site. In this case, peak discharge in any direction may follow Management District A criteria, provided that the overall site discharge meets the management district criteria for which the discharge is located.
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. 
"No harm" option. For any proposed development site not located in a conditional direct discharge district, the applicant has the option of using a less-restrictive runoff control (including no detention) if the applicant can prove that no harm would be caused by discharging at a higher runoff rate than that specified by the stormwater management plan. The "no harm" option is used when an applicant can prove that the proposed conditions hydrographs can match existing conditions hydrographs and if it can be proved that the proposed conditions will not cause increases in peaks at all points downstream. Proof of no harm must be shown based upon the following downstream impact evaluation, which shall include a downstream hydraulic capacity analysis consistent with Subsection H to determine if adequate hydraulic capacity exists. The applicant shall submit to the municipality this evaluation of the impacts due to increased downstream stormwater flows in the watershed.
(1) 
The hydrologic regime of the site must be maintained.
(2) 
The downstream impact evaluation shall include hydrologic and hydraulic calculations necessary to determine the impact of hydrograph timing modifications due to the proposed development upon a dam, highway, structure, natural point of restricted stream flow, or any stream channel section established with the concurrence of the Borough.
(3) 
The evaluation shall continue downstream until the increase in flow diminishes due to additional flow from tributaries and/or stream attenuation.
(4) 
The peak flow values to be used for downstream areas for the design return period storms (two-, five-, ten-, twenty-five-, fifty- and one-hundred-year) shall be the values from the calibrated model for the respective watershed. These flow values can be obtained from the original Act 167 watershed stormwater management plans.
(5) 
Applicant-proposed runoff controls which would generate increased peak flow rates at storm drainage problem areas would, by definition, be precluded from successful attempts to prove no harm, except in conjunction with proposed capacity improvements for the problem areas consistent with Subsection H.
(6) 
Financial distress shall not constitute grounds for the Borough to approve the use of the "no harm" option.
(7) 
Capacity improvements to conveyance facilities or obstructions may be provided as necessary to implement the "no harm" option as long as it can be demonstrated through the downstream hydraulic capacity analysis that the improvements would not create any harm downstream.
(8) 
Any "no harm" justifications shall be submitted by the applicant as part of the drainage plan submission per Article III.
H. 
Downstream hydraulic capacity analysis. Any downstream hydraulic capacity analysis conducted in accordance with this chapter shall use the following criteria for determining adequacy for accepting increased peak flow rates:
(1) 
Natural or man-made channels or swales must be able to convey the increased runoff associated with a two-year return period event within their banks at velocities consistent with protection of the channels from erosion. Acceptable velocities shall be based upon criteria included in the DEP Erosion and Sediment Pollution Control Program Manual.
(2) 
Natural or man-made channels or swales must be able to convey increased twenty-five-year return period runoff without creating any hazard to persons or property.
(3) 
Culverts, bridges, storm sewers or any other facilities which need to pass or convey flows from the tributary area must be designed in accordance with DEP Chapter 105 regulations (if applicable) and, at minimum, pass the increased twenty-five-year return period runoff.
I. 
Hardship option. The standards and criteria outlined in § 513-23 of this chapter are designed to maintain existing peak flows and volumes throughout the Borough's watersheds as they become developed. There may be certain instances, however, where the standards and criteria established are too restrictive for a particular landowner or applicant. The existing drainage network in some areas may be capable of safely transporting slight increases in flows without causing a problem or increasing flows elsewhere. This might be demonstrated as per Subsection H above in order for the hardship option to be considered. If an applicant or landowner cannot meet the stormwater standards due to lot conditions or if conformance would become a hardship to an owner, the hardship option may be applied. The applicant would have to plead his/her case to the Borough Council with the final determination made by the Borough. Any landowners pleading the hardship option will assume all liabilities that may arise due to exercising this option. Cost or financial burden cannot be considered a hardship. The applicant may consider off-site management controls or contributing to the municipal stormwater control and BMP operation and maintenance fund (§ 513-37) as long as the stormwater management controls are within the same subwatershed as shown in Appendix A.[4]
[4]
Editor's Note: Appendix A and other appendixes referenced in this chapter are on file in the Borough's offices.
J. 
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 Collingdale Borough:
(1) 
Meet the full requirements specified by Table 513-23A and Subsections A through J; or
(2) 
Reduce the total impervious surface on the site by at least 20%, based upon a comparison of existing impervious surface to proposed impervious surface.
A. 
Stormwater runoff from all development sites with a drainage area of greater than 200 acres shall be calculated using a generally accepted calculation technique that is based on the NRCS Soil Cover Complex Method. Table 513-24A 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 Borough may allow the use of the Rational Method to estimate peak discharges from drainage areas that contain less than 200 acres. The Soil Cover Complex Method shall be used for drainage areas greater than 200 acres.
Table 513-24A
Acceptable Computation Methodologies for Stormwater Management Plans
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 with limitations described in TR-55.
HEC-1/HEC-HMS
U.S. Army Corps of Engineers
Applicable where use of a hydrologic computer is desirable or necessary.
PSRM
Penn State University
Applicable where use of a hydrologic model is desirable or necessary; simpler than TR-20 or HEC-1.
Rational Method (or commercial computer package based on Rational Method)
Emil Kuichling (1889)
For sites less than 200 acres or as approved by the Borough and/or Borough Engineer.
B. 
All calculations consistent with this chapter using the Soil Cover Complex Method shall use the appropriate design rainfall depths for the various return period storms according to the region in which they are located as presented in Table F-1 in Appendix F of this chapter. 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 Alternating Block Method shown in Figure F-1 or the SCS Type II S Curve, Figure F-3 in Appendix F, shall be used for the rainfall distribution.[1]
[1]
Editor's Note: Appendix F and other appendixes referenced in this chapter are on file in the Borough's offices.
C. 
The following criteria shall be used for runoff calculations:
(1) 
For development sites not considered redevelopment, the ground cover used in determining the existing conditions flow rates shall be as follows:
(a) 
Wooded sites shall use a ground cover of "woods in good condition." Portions of a site having more than one viable tree of a DBH of six inches or greater per 1,500 square feet shall be considered wooded where such trees existed within three years of application.
(b) 
The undeveloped portion of the site, including agriculture, bare earth and fallow ground, shall be considered as "meadow in good condition," unless the natural ground cover generates a lower curve (CN) number or Rational "c" value (i.e., woods) as listed in Table F-2 or F-3 in Appendix F of this chapter.[2]
[2]
Editor's Note: Appendix F and other appendixes referenced in this chapter are on file in the Borough's offices.
(2) 
For development and redevelopment sites, the ground cover used in determining the existing conditions flow rates for the developed portion of the site shall be based upon actual land cover conditions.
D. 
All calculations using the Rational Method shall use rainfall intensities consistent with appropriate times-of-concentration for overland flow and return periods presented in the Region 5 Curves from the PennDOT Storm-Duration-Frequency Chart (Figure F-4). Times-of-concentration for overland flow shall be calculated using the methodology presented in the PennDOT Design Manual Part 2 - Chapter 10. Times-of-concentation for channel and pipe flow shall be computed using Manning's equation.
E. 
Runoff curve number (CN) for both existing and proposed conditions to be used in the Soil Cover Complex Method shall be obtained from Table F-2 in Appendix F of this chapter.[3]
[3]
Editor's Note: Appendix F and other appendixes referenced in this chapter are on file in the Borough's offices.
F. 
Runoff coefficients (c) for both existing and proposed conditions for use in the Rational Method shall be obtained from Table F-3 in Appendix F 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 F-4 in Appendix F of this chapter.
H. 
Outlet structures for stormwater management facilities shall be designed to meet the performance standards of this chapter using any generally accepted hydraulic analysis technique or method.
I. 
The design of any stormwater detention facilities intended to meet the performance standards of this chapter shall be verified by routing the design storm hydrograph through these facilities using the Storage-Indication Method. The design storm hydrograph shall be computed using a calculation method that produces a full hydrograph. The Borough may approve the use of any generally accepted full hydrograph approximation technique that shall use a total runoff volume that is consistent with the volume from a method that produces a full hydrograph.
A. 
Any stormwater facility located on state highway rights-of-way shall be subject to approval by PennDOT.
B. 
All wet basin designs shall incorporate biologic controls consistent with the West Nile guidance found in Appendix H.[1]
[1]
Editor's Note: Appendix H and other appendixes referenced in this chapter are on file in the Borough's offices.
C. 
Any stormwater management facility (i.e., detention 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 one foot 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 DEP 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 be required to pass storms larger than the one-hundred-year event.
D. 
Any facilities that constitute water obstructions (e.g., culverts, bridges, outfalls or stream enclosures) and any work involving wetlands governed by DEP Chapter 105 regulations (as amended or replaced from time to time by DEP) shall be designed in accordance with Chapter 105 and will require a permit from DEP.
E. 
Any other drainage conveyance facility that does not fall under Chapter 105 regulations must be able to convey, without damage to the drainage structure or roadway, runoff from the twenty-five-year design storm with a minimum one foot of freeboard measured below the lowest point along the top of the roadway. Any facility that constitutes a dam as defined in DEP Chapter 105 regulations may require a permit under dam safety regulations. Any facility located within a PennDOT right-of-way must meet PennDOT minimum design standards and permit submission requirements.
F. 
Any drainage conveyance facility and/or channel not governed by Chapter 105 regulations must be able to convey, without damage to the drainage structure or roadway, runoff from the twenty-five-year design storm. Conveyance facilities to or exiting from stormwater management facilities (i.e., detention basins) shall be designed to convey the design flow to or from that structure. Roadway crossings located within designated floodplain areas must be able to convey runoff from a one-hundred-year design storm. Any facility located within a PennDOT right-of-way must meet PennDOT minimum design standards and permit submission requirements.
G. 
Storm sewers must be able to convey proposed conditions runoff from a twenty-five-year design storm without surcharging inlets, where appropriate.
H. 
Adequate erosion protection shall be provided along all open channels and at all points of discharge.
I. 
The design of all stormwater management facilities shall incorporate sound engineering principles and practices. The Borough reserves the right to disapprove any design that would result in construction in or continuation of a stormwater problem area.