Township of Upper Chichester, PA
Delaware County
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Table of Contents
Table of Contents
A. 
Applicants proposing regulated activities in the municipality which do not fall under the exemption criteria shown in § 490-6 shall submit a stormwater management site plan consistent with this chapter and the applicable watershed stormwater management plan to the municipality for review. The stormwater management criteria of this chapter shall apply to the total proposed development even if development is to take place in stages.
B. 
No regulated activity within the municipality shall commence until the municipality issues approval of a SWM plan, which demonstrates compliance with the requirements of this chapter.
C. 
The applicant is required to design the site to minimize surface discharge of stormwater and the creation of impervious surfaces in order to maintain, as much as possible, the natural hydrologic regime.
D. 
The SWM site plan must be designed consistent with the sequencing provisions of § 490-14 to ensure maintenance of the natural hydrologic regime, to promote infiltration, and to protect groundwater and surface water quality and quantity. The SWM site plan designer must proceed sequentially in accordance with Article III of this chapter.
E. 
Stormwater drainage systems shall be designed in order to preserve natural flow conditions to the maximum extent practicable.
F. 
Existing drainage discharge onto adjacent property shall not be altered in any manner without written permission, and a maintenance access agreement with, the affected property owner(s). Such discharge shall be subject to any applicable discharge criteria specified in this chapter and still must meet the requirements of Act 167.
G. 
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.
H. 
Where a development site is traversed by a stream, drainage easements of 25 feet shall be provided on either side of, and conform to the line of such streams.
I. 
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.
J. 
All stormwater runoff from new development or redevelopment shall be pretreated for water quality prior to discharge to surface or groundwater. Rooftop runoff may go directly to an infiltration BMP or be evapotranspirated.
K. 
All regulated activities within the municipality 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.
L. 
The BMPs shall be designed, implemented, and maintained to meet state water quality requirements and any other more stringent requirements as determined by the municipality.
M. 
Post-construction water quality protection shall be addressed as required by § 490-16.
N. 
Operations and maintenance of permanent stormwater BMPs shall be addressed as required by Article VII.
O. 
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 municipality.
P. 
Techniques described in Appendix C (low-impact development)[1] 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: Said appendix is included as an attachment to this chapter.
Q. 
In selecting the appropriate BMPs or combinations thereof, the applicant shall consider the following:
(1) 
Total contributing drainage area.
(2) 
Permeability and infiltration rate of the site's soils.
(3) 
Slope and depth to bedrock.
(4) 
Seasonal high-water table.
(5) 
Proximity to building foundations and wellheads.
(6) 
Erodibility of soils.
(7) 
Land availability and configuration of the topography.
(8) 
Peak discharge and required volume control.
(9) 
Stream bank erosion.
(10) 
Efficiency of the BMPs to mitigate potential water quality problems.
(11) 
The volume of runoff that will be effectively treated.
(12) 
The nature of the pollutant being removed.
(13) 
Maintenance requirements.
(14) 
Creation/protection of aquatic and wildlife habitat.
(15) 
Recreational value.
(16) 
Enhancement of aesthetic and property values.
R. 
The design of all stormwater management facilities shall incorporate sound engineering principles and practices in a manner that does not aggravate existing stormwater problems. The municipality reserves the right to disapprove any design that would result in construction in or continuation of a stormwater problem area.
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 Ordinance Appendix A.[2]
[2]
Editor's Note: Said appendix is included as an attachment to this chapter.
T. 
Stormwater hotspots. Stormwater runoff from hotspots shall be pretreated prior to surface or groundwater infiltration to prevent pollutant runoff. Industrial sites referenced in 40 CFR 125 are examples of hotspots.
(1) 
Below is a list of examples of hotspots:
(a) 
Vehicle salvage yards and recycling facilities.
(b) 
Vehicle fueling stations.
(c) 
Vehicle service and maintenance facilities.
(d) 
Vehicle and equipment cleaning facilities.
(e) 
Fleet storage areas (bus, truck, etc.).
(f) 
Industrial sites based on Standard Industrial Classification Codes.
(g) 
Marinas (service and maintenance areas).
(h) 
Outdoor liquid container storage.
(i) 
Outdoor loading/unloading facilities.
(j) 
Public works storage areas.
(k) 
Facilities that generate or store hazardous materials.
(l) 
Commercial container nursery.
(m) 
Contaminated sites/brownfields.
(n) 
Other land uses and activities as designated by an appropriate review authority.
(2) 
The following land uses and activities are not normally considered hotspots:
(a) 
Residential streets and rural highways.
(b) 
Residential development.
(c) 
Institutional development.
(d) 
Office developments.
(e) 
Nonindustrial rooftops.
(f) 
Pervious areas, except golf courses and nurseries [which may need an integrated pest management (IPM) plan].
(3) 
While streets and highways [average daily traffic volume (ADT) greater than 30,000] are not considered stormwater hotspots, it is important to ensure that highway stormwater management facilities are designed to adequately protect receiving streams and/or groundwater.
(4) 
The Environmental Protection Agency's (EPA) NPDES stormwater program requires some industrial sites to prepare and implement a stormwater pollution prevention plan.
U. 
The following standards for protection of downgradient properties from off-site conveyance must be accomplished:
(1) 
For any location where a new concentrated discharge of stormwater from any frequency rainfall event, up to and including the one-hundred-year, twenty-four-hour event, will flow onto a downgradient property, the following are required:
(a) 
A drainage easement (or other legal agreement/approval) must be obtained for conveyance of discharges onto or through adjacent properties.
(b) 
The conveyance must be designed to avoid erosion, flooding, or other damage to the properties through which it is being conveyed.
The following permit requirements may apply to certain regulated earth disturbance activities and must be met prior to commencement of regulated earth disturbance activities, as applicable:
A. 
All regulated earth disturbance activities subject to permit requirements by PADEP under regulations at Title 25 Pennsylvania Code Chapter 102.
B. 
Work within natural drainageways subject to permit by PADEP under Title 25 Pennsylvania Code Chapter 105.
C. 
Any stormwater management facility that would be located in or adjacent to surface waters of the commonwealth, including wetlands, subject to permit by PADEP under Title 25 Pennsylvania Code Chapter 105.
D. 
Any stormwater management facility that would be located on or discharging to a state highway right-of-way, or require access to or from a state highway, shall be subject to approval by PennDOT.
E. 
Culverts, bridges, storm sewers, or any other facilities which must pass or convey flows from the tributary area and any facility which may constitute a dam subject to permit by PADEP under Title 25 Pennsylvania Code Chapter 105.
A. 
No regulated earth disturbance activities within the municipality shall commence until the municipality receives an approval from the PADEP in compliance with Title 25 Pennsylvania Code Chapter 102 of an erosion and sediment control plan for construction activities if applicable.
B. 
PADEP has regulations regarding an erosion and sediment control under Title 25 Pennsylvania Code Chapter 102.
C. 
In addition, under Title 25 Pennsylvania Code Chapter 92,[1] a PADEP "NPDES Construction Activities" permit is required for regulated earth disturbance activities.
[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 PADEP 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 § 490-23A.
E. 
A copy of the erosion and sediment control plan and any required permit, as required by PADEP regulations, shall be available on the project site at all times.
F. 
Additional erosion and sediment control design standards and criteria are recommended to be applied where infiltration BMPs are proposed. At a minimum, they shall include the following:
(1) 
Areas proposed for infiltration BMPs shall be protected from sedimentation and compaction during the construction phase to maintain maximum infiltration capacity.
(2) 
Infiltration BMPs shall not be constructed nor receive runoff until the entire drainage area contributory to the infiltration BMP has achieved final stabilization.
The design of all regulated activities shall include the following to minimize stormwater impacts to reduce the surface discharge of stormwater, reduce the creation of unnecessary impervious surfaces, prevent the degradation of waters of the commonwealth, and maintain as much as possible the natural hydrologic regime of the site.
A. 
The applicant shall apply low-impact development (LID) methods such as those listed in Appendix C,[1] provided that use of this method does not conflict with other local codes.
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
B. 
The applicant shall demonstrate that the design process follows the sequence noted below. The goal of the sequence is to minimize the increases in stormwater runoff and impacts to water quality resulting from the proposed regulated activity:
(1) 
The following items in this subsection shall be addressed prior to development of other stormwater management site plan design elements:
(a) 
Prepare an Existing Resource and Site Analysis Map (ERSAM) showing environmentally sensitive areas including, but not limited to, steep slopes, ponds, lakes, streams, wetlands, hydric soils, vernal pools, stream buffers, and hydrologic soil groups. Land development, any existing recharge areas, and other requirements outlined in the municipal SALDO shall also be included.
(b) 
Establish a stream buffer according to § 490-16D.
(c) 
Prepare a draft project layout avoiding sensitive areas identified in § 490-15B(1)(a).
(d) 
Identify site-specific existing conditions drainage areas, discharge points, recharge areas, and hydrologic soil groups A and B (areas conducive to infiltration).
(e) 
Evaluate nonstructural stormwater management alternatives:
[1] 
Minimize earth disturbance.
[2] 
Minimize impervious surfaces.
[3] 
Break up large impervious surfaces.
(f) 
Determine into what management district the site falls (Ordinance Appendix A)[2], and conduct an existing conditions runoff analysis.
[2]
Editor's Note: Said appendix is included as an attachment to this chapter.
(2) 
The following items in this subsection may be addressed in any order provided that all items in § 490-14B(1) have been completed.
(a) 
Satisfy the infiltration objective (§ 490-15) and provide for stormwater pretreatment prior to infiltration.
(b) 
Provide for water quality protection in accordance with § 490-16, Water quality requirements.
(c) 
Provide stream bank erosion protection in accordance with § 490-17, Stream bank erosion requirements.
(d) 
Prepare final project design to maintain existing conditions drainage areas and discharge points, to minimize earth disturbance and impervious surfaces, and, to the maximum extent possible, to ensure that the remaining site development has no surface or point discharge.
(e) 
Conduct a proposed conditions runoff analysis based on the final design that meets the management district requirements (§ 490-18).
(f) 
Manage any remaining runoff prior to discharge through detention, bioretention, direct discharge, or other structural control.
Providing for infiltration consistent with the natural hydrologic regime is required. Design of the infiltration facilities shall consider infiltration to compensate for the reduction in the recharge that occurs when the ground surface is disturbed or impervious surface is created.
A. 
If it cannot be physically accomplished, then the design professional shall be responsible for demonstrating to the satisfaction of the municipality that this cannot be physically accomplished on the site (e.g., shallow depth to bedrock or limiting zone, open voids, steep slopes, etc., vs. a "financial hardship" as defined in § 490-10). If it can be physically accomplished, the volume of runoff to be infiltrated shall be determined from § 490-15B(2) depending on demonstrated site conditions, and shall be the greatest volume that can be physically infiltrated. For example:
(1) 
Any applicant (developer or redeveloper) shall first attempt to infiltrate the volume required in § 490-15B(2)(a).
(2) 
If the § 490-15B(2)(a) requirement cannot be physically accomplished, then the applicant is required to attempt to infiltrate the volume required in § 490-15B(2)(b).
(3) 
Finally, if the § 490-15B(2)(b) infiltration volume cannot be physically accomplished, the applicant must, at a minimum, infiltrate the volume required in § 490-15B(2)(c).
B. 
Infiltration BMPs shall meet the following minimum requirements:
(1) 
Infiltration BMPs intended to receive runoff from developed or redeveloped areas shall be selected based on suitability of soils and site conditions and shall be constructed on soils that have the following characteristics:
(a) 
A minimum depth of 24 inches between the bottom of the BMP and the top of the limiting zone.
(b) 
An infiltration rate sufficient to accept the additional stormwater volume and dewater completely as determined by field tests conducted by the applicant's design professional.
(c) 
The infiltration facility shall be capable of completely draining the retention (infiltration) volume (Rev) within three days (72 hours) from the end of the design storm.
(2) 
The size of the infiltration facility and Rev shall be based upon the following volume criteria:
(a) 
Modified Control Guideline One (MCG-1) of the Pennsylvania Stormwater Best Management Practices (PA BMP) Manual. The retention (infiltration) volume (Rev) to be captured and infiltrated shall be the net two-year twenty-four-hour volume. The net volume is the difference between the post-development runoff volume and the predevelopment runoff volume. The post-development total runoff volume for all storms equal to or less than the two-year twenty-four-hour duration precipitation shall not be increased. For modeling purposes, existing (predevelopment) nonforested pervious areas must be considered meadow in good condition or its equivalent, and 20% of existing impervious area, when present, shall be considered meadow in good condition.
(b) 
Infiltrating the entire Rev volume in § 490-15B(2)(a) (above) may not be feasible on every site due to site-specific limitations such as shallow depth to bedrock or the water table. If it cannot be physically accomplished, then the following criteria from Modified Control Guideline Two (MCG-2) of the PA BMP Manual must be satisfied:
[1] 
At least the first 1.0 inch of runoff from new or replacement impervious surfaces shall be infiltrated.
Rev = 1 (inch) * impervious area (square feet) ÷ 12 (inches) = cubic feet (cf)
An asterisk (*) in equations denotes multiplication.
(c) 
Only if infiltrating the entire Rev volume in § 490-15B(2)(b) (above) cannot be physically accomplished, then the following minimum criteria from Modified Control Guideline Two (MCG-2) of the PA BMP Manual must be satisfied:
[1] 
Wherever possible, infiltration facilities should be designed to accommodate infiltration of the entire water quality volume (WQv) (§ 490-16B); however, in all cases at least the first 0.5 inch of the WQv shall be infiltrated. The minimum infiltration volume (Rev) required would, therefore, be computed as:
Rev = I * impervious area (square feet) ÷ 12 (inches) = cubic feet (cf)
An asterisk (*) in equations denotes multiplication.
Where:
I
=
The maximum equivalent infiltration amount (inches) that the site can physically accept or 0.50 inch, whichever is greater.
[2] 
The retention volume values derived from the methods in § 490-15B(2)(a), 490-15B(2)(b) or 490-15B(2)(c) is the minimum volume the applicant must control through an infiltration BMP facility. If site conditions preclude capture of runoff from portions of the impervious area, the infiltration volume for the remaining area should be increased an equivalent amount to offset the loss.
[3] 
Only if the minimum of 0.50 inch of infiltration requirement cannot be physically accomplished, a waiver from § 490-15, Infiltration volume requirements, is required from the municipality.
C. 
Soils. A detailed soils evaluation of the project site shall be required to determine the suitability of infiltration facilities. The evaluation shall be performed by a qualified design professional and at minimum address soil permeability, depth to bedrock, and subgrade stability. The general process for designing the infiltration BMP shall be:
(1) 
Analyze hydrologic soil groups as well as natural and man-made features within the site to determine general areas of suitability for infiltration practices. In areas where development on fill material is under consideration, conduct geotechnical investigations of subgrade stability; infiltration may not be ruled out without conducting these tests.
(2) 
Provide field tests such as double ring infiltrometer or hydraulic conductivity tests (at the level of the proposed infiltration surface) to determine the appropriate hydraulic conductivity rate. Percolation tests are not recommended for design purposes.
(3) 
Design the infiltration structure for the required retention (Rev) volume based on field-determined capacity at the level of the proposed infiltration surface.
(4) 
If on-lot infiltration structures are proposed by the applicant's design professional, it must be demonstrated to the municipality that the soils are conducive to infiltrate on the lots identified.
D. 
Infiltration facilities should, to the greatest extent practicable, be located to avoid introducing contaminants via groundwater, and be in conformance with an approved source water protection assessment or source water protection plan.
E. 
Roadway drainage systems should provide an opportunity to capture accidental spills. Road de-icing material storage facilities shall be designed to avoid salt and chloride runoff from entering waterways and infiltration facilities. The qualified design professional shall evaluate the possibility of groundwater contamination from the proposed infiltration facility and perform a hydrogeologic justification study if necessary.
F. 
The antidegradation analysis found in Chapter 93[1] shall be applied in HQ or EV streams.
[1]
Editor's Note: See 25 Pa. Code Ch. 93.
G. 
An impermeable liner will be required in detention basins where the possibility of groundwater contamination exists. The municipality may require a detailed hydrogeologic investigation.
H. 
The applicant should 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. 
To control post-construction stormwater impacts from regulated activities and conform to state water quality requirements, BMPs which replicate predevelopment stormwater infiltration and runoff conditions must be provided in the site design such that post-construction stormwater discharges do not degrade the physical, chemical, or biological characteristics of the receiving waters. This may be achieved by the following:
(1) 
Infiltration. Replication of preconstruction stormwater infiltration conditions;
(2) 
Treatment. Use of water quality treatment BMPs to provide filtering of chemical and physical pollutants from the stormwater runoff; and
(3) 
Stream bank and streambed protection. Management of volume and rate of post-construction stormwater discharges to prevent physical degradation of receiving waters (e.g., from scouring).
B. 
Developed areas shall provide adequate storage and treatment facilities necessary to capture and treat stormwater runoff. The infiltration volume computed under § 490-15 may be a component of the water quality volume if the applicant chooses to manage both components in a single facility. If the calculated water quality volume (WQv) is greater than the volume required to be infiltrated as described in § 490-15B(2), then the difference between the two volumes shall be treated for water quality by an acceptable stormwater management practice(s). The required water quality volume (WQv) is the storage capacity needed to capture and treat a portion of stormwater runoff from the developed areas of the site. To achieve this requirement, the following criterion is established:
(1) 
From control guideline (CG-1) in the PA BMP Manual, the water quality volume shall be the net two-year twenty-four-hour volume. The net volume is the difference between the post-development runoff volume and the predevelopment runoff volume. The post-development total runoff volume for all storms equal to or less than the two-year twenty-four-hour duration precipitation shall not be increased. For modeling purposes, existing (predevelopment) nonforested pervious areas must be considered meadow in good condition or its equivalent, and 20% of existing impervious area, when present, shall be considered meadow in good condition.
(2) 
This volume requirement can be managed by the permanent volume of a wet basin or the detained volume from other BMPs. Where appropriate, wet basins shall be utilized for water quality control and shall follow the guidelines of the PA BMP manual referenced in Ordinance Appendix H.[1]
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
(3) 
Release of water can begin at the start of the storm (i.e., the invert of the water quality orifice is at the invert of the facility). The design of the facility shall provide for protection from clogging and unwanted sedimentation.
C. 
The temperature of receiving waters shall be protected through the use of BMPs that moderate temperature.
D. 
If a perennial or intermittent stream passes through, or a water body (i.e., lake, pond, wetland) is present on the site, the applicant shall create a riparian buffer extending a minimum of 50 feet, subject to federal and state buffer policies and regulation, to either side of the top-of-bank of the channel, lake, or wetland. The buffer area shall be planted with native vegetation and maintained in a vegetated state. (Refer to Appendix B, Pennsylvania Native Plant List, contained in the PA BMP Manual.)
(1) 
The following provisions also apply to riparian buffers on lots in existence at the time of adoption of this chapter:
(a) 
If the applicable rear or side yard setback is less than 50 feet, the buffer width may be reduced to 25% of the setback or 10 feet, whichever is greater.
(b) 
If a stream traverses a site in a manner that significantly reduces the use of the site, the buffer may be either:
[1] 
Reduced to 25 feet on either side, with municipal approval; or
[2] 
Reduced to 10 feet with municipal waiver.
(2) 
Permitted uses within the buffer include the following, subject to municipal approval and provided that they comply with all federal, state, and local regulations:
(a) 
Recreational trails. See Ordinance Appendix G, Riparian Buffer Trail Guidelines.[2]
[2]
Editor's Note: Said appendix is included as an attachment to this chapter.
(b) 
Utility rights-of-way.
(c) 
Bridges.
(d) 
Other uses subject to municipal approval.
(3) 
If an existing buffer is legally prescribed (i.e., deed, covenant, easement, etc.) and it exceeds the requirements of this chapter, the existing buffer shall be maintained.
A. 
In addition to controlling the water quality volume (in order to minimize the impact of stormwater runoff on downstream stream bank erosion), the primary requirement to control stream bank erosion is to design a BMP to detain the proposed conditions two-year, twenty-four-hour design storm to the existing conditions one-year flow using the NRCS Type II distribution. Additionally, provisions shall be made (such as adding a small orifice at the bottom of the outlet structure) to release the proposed conditions one-year storm for a minimum of 24 hours from a point in time when the maximum volume of water from the one-year storm is stored in a proposed BMP (i.e., the maximum water surface elevation is achieved in the facility). Release of water can begin at the start of the storm (i.e., the invert of the water quality orifice is at the invert of the facility).
B. 
The minimum orifice size in the outlet structure to the BMP shall be three inches in diameter where possible, and a trash rack shall be installed to prevent clogging. On sites with small drainage areas contributing to this BMP that do not provide enough runoff volume to allow a twenty-four-hour attenuation with the three-inch orifice, the calculations shall be submitted showing this condition. When the calculated orifice size is below three inches, gravel filters (or other methods) are recommended to discharge low-flow rates subject to the Municipal Engineer's satisfaction. When filters are utilized, maintenance provisions shall be provided to ensure filters meet the design function. All facilities shall make use of measures to extend the flow path and increase the travel time of flows in the facility.
A. 
Each watershed has been divided into either stormwater management districts or release rate districts as shown on the respective Management District or Release Rate Maps in Appendix A.[1]
(1) 
In addition to the watershed-specific requirements specified in Tables 490-18.1 and 490-18.2 for each of the watersheds below, the erosion and sedimentation control (§ 490-13), the nonstructural project design (§ 490-14), the infiltration (§ 490-15), the water quality (§ 490-16), and the stream bank erosion (§ 490-17) requirements shall be implemented.
(2) 
Standards for managing runoff from each subarea in a watershed for the two-, five-, ten-, twenty-five-, fifty-, and one-hundred-year design storms are shown in Tables 490-18.1 and 490-18.2. Development sites located in each of the management/release rate districts must control proposed conditions runoff rates to existing conditions runoff rates for the design storms in accordance with the tables.
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
B. 
General. Proposed conditions rates of runoff from any regulated activity shall not exceed the peak release rates of runoff from existing conditions for the design storms specified on the Stormwater Management District Watershed Map (Ordinance Appendix A[2]) and this section 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 on an 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 Ordinance Appendix A.[3] The exact location of the stormwater management district boundaries as they apply to a given development site shall be determined by mapping the boundaries using the two-foot topographic contours (or most accurate data required) provided as part of the SWM site plan.
[3]
Editor's Note: Said appendix is included as an attachment to this chapter.
D. 
Sites located in more than one district or watershed. For a proposed development site located within two or more stormwater management district subareas, the peak discharge rate from any subarea shall meet the management district criteria for which the discharge is located. The natural hydrology of each respective subarea shall be maintained, and drainage shall not be redirected from one subarea to another. Under circumstances where the applicant shows this cannot be accomplished, a waiver is required by the municipality.
Table 490-18.1
Control Criteria for Chester Creek Watershed Stormwater Management Districts
District
Control Criteria
100%
Post-development peak discharge for all design storms must be no greater than predevelopment peak discharges.
75%
Post-development peak discharge for all design storms must be no greater than 75% of the predevelopment peak discharges.
50%
Post-development peak discharge for all design storms must be no greater than 50% of the predevelopment peak discharges.
NOTE:
For Chester Creek Watershed: watershed subareas with a 100% should confirm that the peak rate control requirement is at least as stringent as the Naamans Creek/Marcus Hook Creek requirement (below).
Table 490-18.2
Peak Rate Control Standards in the Naamans Creek/Marcus Hook Creek Watersheds
Proposed Condition Design Storm
Reduce to
Existing Condition Design Storm
2-year
1-year
5-year
5-year
10-year
10-year
25-year
25-year
50-year
50-year
100-year
100-year
E. 
Off-site areas. Off-site areas that drain through a proposed development site are not subject to release rate criteria when determining allowable peak runoff rates. On-site drainage facilities shall be designed to safely convey off-site flows through the development site.
F. 
Site areas. Where the site area to be impacted by a proposed development activity differs significantly from the total site area, only the proposed impact area utilizing stormwater management measures shall be subject to the peak rate control standards noted above. Unimpacted areas for which the discharge point has not changed are not subject to the peak rate control standards.
G. 
Downstream hydraulic capacity analysis. Any downstream capacity hydraulic analysis conducted in accordance with this chapter shall use the following criteria for determining adequacy for accepting increased peak flow rates:
(1) 
Natural or man-made channels or swales must be able to convey the increased runoff associated with a two-year storm event within their banks at velocities consistent with protection of the channels from erosion. Velocities shall be based upon criteria and methodologies acceptable to the municipality.
(2) 
Natural or man-made channels or swales must be able to convey increased twenty-five-year storm event runoff without creating any increased hazard to persons or property.
(3) 
Culverts, bridges, storm sewers or any other hydraulic facilities which must pass or convey flows from the tributary area must be designed in accordance with PADEP Chapter 105 regulations (if applicable) and, at a minimum, pass the increased twenty-five-year storm event runoff.
(4) 
Water quality requirements defined in § 490-16 must be met.
(5) 
Post-construction peak rates shall not exceed the existing peak rates for the respective subarea.
H. 
Alternate criteria for redevelopment sites. For redevelopment sites, one of the following minimum design parameters shall be accomplished, whichever is most appropriate for the given site conditions as determined by the municipality;
(1) 
Meet the full requirements specified by Tables 490-18.1 and 490-18.2 and § 490-18A through H; or
(2) 
Reduce the total impervious surface on the site by at least 20%; based upon a comparison of existing impervious surface to proposed impervious surface. In this case, calculations must be provided that show the peak rate has not increased.
A. 
Stormwater runoff from all development sites with a drainage area of greater than five acres shall be calculated using a generally accepted calculation technique that is based on the NRCS Soil Cover Complex Method. Table 490-19 summarizes acceptable computation methods. The method selected by the design professional shall be based on the individual limitations and suitability of each method for a particular site. The use of the Rational Method to estimate peak discharges for drainage areas greater than five acres shall be permitted only upon approval of the municipality.
Table 490-19
Acceptable Computation Methodologies for SWM Site Plan
Method
Developed By
Applicability
TR-20 (or commercial computer package based on TR-20)
USDA NRCS
Applicable where use of full hydrology computer model is desirable or necessary
TR-55 (or commercial computer package based on TR-55)
USDA NRCS
Applicable for land development plans where limitations described in TR-55
HEC-1/HEC-HMS
United States Army Corps of Engineers
Applicable where use of a full hydrologic computer model is desirable or necessary
Rational method (or commercial computer package based on Rational Method)
Emil Kuichling (1889)
For sites up to 5 acres, or as approved by the municipality and/or Municipal Engineer
Other methods
Varies
Other computation methodologies approved by the municipality and/or Municipal Engineer
B. 
All calculations consistent with this chapter using the Soil Cover Complex Method shall use the appropriate design rainfall depths for the various return period storms. Rainfall depths shall be according to NOAA Atlas 14 values consistent with a partial duration series, or the latest version of the PennDOT Drainage Manual (PDM Publication 584), whichever is greater. When stormwater calculations are performed for routing procedures or water quality functions, the duration of rainfall shall be 24 hours.
C. 
The following criteria shall be used for peak rate runoff calculations:
(1) 
For development sites not considered redevelopment, the ground cover used in determining the existing conditions flow rates shall be as follows:
(a) 
Wooded sites shall use a ground cover of "woods in good condition." Portions of a site having more than one viable tree measuring a diameter at breast height (DBH) of six inches or greater per 1,500 square feet shall be considered wooded where such trees existed within three years of application.
(b) 
The undeveloped portion of the site including agriculture, bare earth, and fallow ground shall be considered as "meadow in good condition," unless the natural ground cover generates a lower curve (CN) number or Rational "c" value (i.e., woods) as listed in Tables D-1 or D-2 in Appendix D[1] of this chapter.
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
(2) 
For redevelopment sites, the ground cover used in determining the existing conditions flow rates for the developed portion of the site shall be based upon actual land cover conditions.
D. 
All calculations using the Rational Method shall use rainfall intensities consistent with appropriate times-of-concentration (duration) and storm events with rainfall intensities obtained from NOAA Atlas 14 partial duration series estimates, or the latest version of the PennDOT Drainage Manual (PDM Publication 584), whichever is greater. Times-of-concentration shall be calculated based on the methodology recommended in the respective model used. Times-of-concentration for channel and pipe flow shall be computed using Manning's equation.
E. 
Runoff curve numbers (CN) for both existing and proposed conditions to be used in the Soil Cover Complex Method shall be obtained from Table D-1 in Appendix D[2] of this chapter.
[2]
Editor's Note: Said appendix is included as an attachment to this chapter.
F. 
Runoff coefficients (c) for both existing and proposed conditions for use in the Rational Method shall be obtained from Table D-2 in Appendix D[3] of this chapter.
[3]
Editor's Note: Said appendix is included as an attachment to this chapter.
G. 
Hydraulic computations to determine the capacity of pipes, culverts, and storm sewers shall be consistent with methods and computations contained in the Federal Highway Administration Hydraulic Design Series Number 5 (Publication No. FHWA-NHI-01-020 HDS No. 5). Hydraulic computations to determine the capacity of open channels shall be consistent with methods and computations contained in the Federal Highway Administration Hydraulic Engineering Circular Number 15 (Publication No. FHWA-NHI-05-114 HEC 15). Values for Manning's roughness coefficient (n) shall be consistent with Table D-3 in Appendix D[4] of this chapter.
[4]
Editor's Note: Said appendix is included as an attachment to this chapter.
H. 
Outlet structures for stormwater management facilities shall be designed to meet the performance standards of this chapter using any generally accepted hydraulic analysis technique or method.
I. 
The design of any stormwater detention facilities intended to meet the performance standards of this chapter shall be verified by routing the design storm hydrograph through these facilities using an acceptable method. The design storm hydrograph shall be computed using a calculation method that produces a full hydrograph. The municipality may approve the use of any generally accepted full hydrograph approximation technique that shall use a total runoff volume that is consistent with the volume from a method that produces a full hydrograph.
A. 
All wet basin designs shall incorporate biologic controls consistent with the West Nile Guidance found in Appendix E,[1] PADEP document 363-0300-001 "Design Criteria - Wetlands Replacement/Monitoring," or contact the Pennsylvania State Cooperative Wetland Center (www.wetlands.psu.edu/) or the Penn State Cooperative Extension Office (www.extension.psu.edu/extmap.html).
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
B. 
Any stormwater basin required or regulated by this chapter designed to store runoff and requiring a berm or earthen embankment shall be designed to provide an emergency spillway to handle flow up to and including the one-hundred-year proposed conditions. The height of embankment must provide a minimum recommended 1.0 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 PADEP Chapter 105, the facility shall be designed in accordance with Chapter 105 and meet the regulations of Chapter 105 concerning dam safety. Chapter 105 may require the passing of storms larger than one-hundred-year event.
C. 
Any drainage conveyance facility and/or channel not governed by Chapter 105 regulations must be able to convey, without damage to the drainage structure or roadway, runoff from the twenty-five-year storm event. The larger the events (fifty-year and one-hundred-year) must also be safely conveyed in the direction of natural flow without creating additional damage to any drainage structures, nearby structures, or roadways.
D. 
Conveyance facilities to or exiting from stormwater management facilities (i.e., detention basins) shall be designed to convey the design flow to or from the facility.
E. 
Roadway crossings or structures located within designated floodplain areas must be able to convey runoff from a one-hundred-year design storm consistent with Federal Emergency Management Agency National Flood Insurance Program - Floodplain Management Requirements.
F. 
Any facility located within a PennDOT right-of-way must meet PennDOT minimum design standards and permit submission requirements.
G. 
Adequate erosion protection and energy dissipation shall be provided along all open channels and at all points of discharge. Design methods shall be consistent with the Federal Highway Administration Hydraulic Engineering Circular Number 11 (Publication No. FHWA-IP-89-016) and the PADEP Erosion and Sediment Pollution Control Program Manual (Publication No. 363-2134-008).