ARTICLE IV - STORMWATER MANAGEMENT

SECTION 401.  GENERAL PROCEDURES FOR WATER QUALITY AND QUANTITY CONTROL

SECTION 402.  STORMWATER MANAGEMENT DISTRICTS 

SECTION 403.  STORMWATER MANAGEMENT DISTRICT IMPLEMENTATION PROVISIONS

SECTION 404.  WATER QUALITY REQUIREMENTS    

SECTION 405.  CALCULATION METHODOLOGY

SECTION 406.  DESIGN CRITERIA FOR STORMWATER MANAGEMENT FACILITIES  

SECTION 407.  EROSION AND SEDIMENTATION REQUIREMENTS 

SECTION 401.  GENERAL PROCEDURES FOR WATER QUALITY AND QUANTITY CONTROL 

A.                 All regulated activities in the Township which do not fall under the exemption criterion shown in Ordinance Section 302 shall submit a drainage plan consistent with the Chester Creek Stormwater Management Plan to the Township for review.  This criterion shall apply to the total proposed development even if development is to take place in stages.  Impervious cover shall include, but not be limited to, any roof, parking, or driveway areas and any new streets and sidewalks.  Any areas designed to initially be gravel or crushed stone shall be assumed to be impervious. 

B.                 Drainage plans shall be prepared in accordance with the provisions contained in this article.  The process for implementing these provisions is illustrated in Figure 4-1, Water Quality and Quantity Control Drainage Plan Preparation Procedures. 

C.                The Chester Creek Stormwater Management Plan requires water quality and water quantity controls as illustrated on the flow chart shown in Figure 4-1 and detailed in Section 404.  The flow chart illustrates a three-step hierarchical process:  Step 1 - Infiltration, Step 2 - Extended detention, and Step 3 - Implementation of additional design controls.  Applicants must evaluate the outcome of each step before proceeding to the next.   In addition, riparian buffers are required where applicable, in accordance with Section 404.A.2. 

D.                Applicants are highly encouraged to meet the post-development peak discharge control criteria indicated in Sections 402 and 403 through use of BMPs and innovative site designs that minimize the amount of new impervious surface.   

SECTION 402.  STORMWATER MANAGEMENT DISTRICTS 

A.       The Chester Creek watershed is divided into districts that represent three (3) levels of stormwater management.  The boundaries of the stormwater management districts are shown on an official release rate map, included as part of the Chester Creek Stormwater Management Plan (see Plate 6, Release Rate Map).  A copy of the official release rate map at a reduced scale is included as Plate 1 in Appendix A of this Ordinance.  This map is for reference only.  The exact location of the stormwater management district boundaries as they apply to a given development site must be determined by mapping the boundaries using the two-foot topographic contours (or the most accurate data required) provided as part of the drainage plan.

B.    All other areas of the municipality not in the Chester Creek watershed shall be required to utilize a 100% release rate when developing stormwater controls in  conformance with Section 403 of this ordinance.

WATER QUALITY   and QUANTITY CONTROL DRAINAGE PLAN PREPARATION PROCEDURE FLOWCHART

SECTION 403.  STORMWATER MANAGEMENT DISTRICT IMPLEMENTATION PROVISIONS  (Performance Standards) 

A.     General - Post-development 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 official stormwater management release rate map, Ordinance Appendix A, and Section 402 of the Ordinance [,whichever applies]. 

B.     Standards for managing runoff for new development from each subarea in the Chester Creek watershed for the 2-, 5-, 10-, 25-, 50-, and 100-year design storms are shown in Table 403-1.  Development sites located in each of the districts must control post-development peak runoff rates to the specified percentage of pre-development peak runoff rates for the design storms as shown in the table. 

                                    SOURCE: Gannett Fleming, 2001   

All other areas of the Township shall utilize a 100% release rate for the purpose of implementation of this Ordinance. 

C.    Redevelopment projects shall meet peak discharge requirements based on the adjusted runoff control number (RCN) or “C” value illustrated by Figure B-3 in Appendix B.   

D.  Sites Located in More than One District - for a proposed development site located within two or more release category subareas, the peak discharge rate from any subarea shall be the pre-development peak discharge for each subarea multiplied by the applicable release rate.  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, as determined by the municipal Engineer, only the proposed development area and areas contributory to the proposed stormwater management facilities shall be subject to the release rate criteria.  

SECTION 404.  WATER QUALITY REQUIREMENTS

A.    In addition to the performance standards and design criteria requirements of Sections 402 and 403 and Sections 405 through 407 of this Ordinance, the applicant shall comply with the following water quality requirements unless otherwise exempted by provisions of this Ordinance. 

1. The applicant shall first provide infiltration facilities in areas where soils are suitable for infiltration and shall direct the runoff from impervious surfaces into those infiltration facilities.  The volume of storage to be provided shall be no less than the net increase in runoff from the 2-year storm event, or one (1) inch of runoff from the total area draining to the infiltration facility, whichever is greater.    

2. If a perennial or intermittent stream passes through the site, the applicant shall create a riparian buffer extending a minimum of fifty (50) feet to either side of the top of the bank of the channel.  The buffer area shall be maintained with appropriate native vegetation (see list of technical references in Appendix C of this Ordinance).  If the applicable rear or side yard setback is less than fifty (50) feet, the buffer width may be reduced to twenty-five (25) percent of the setback to a minimum of ten (10) feet.  If an existing buffer is legally prescribed (e.g., deed covenant, easement, etc.) and it exceeds the requirement of this Ordinance, the existing buffer shall be maintained.   

3. Detain the 2-year, 24-hour design storm runoff based on using the SCS Type II distribution.  Provisions shall be made so that the detained runoff takes a minimum of twenty-four (24) hours to drain from the facility from a point where the maximum volume of water 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).  The design of the facility shall consider and minimize the chances of clogging and sedimentation potential.  The applicant may also utilize infiltration facilities in lieu of extended detention.  The volume of infiltration provided for the contributing area may be deducted from the volume requirement for extended detention. 

B.    The applicant shall submit designs for water quality facilities to the Township Engineer for review and approval.  Such designs may achieve the water quality objectives through a combination of BMPs. 

C.   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 soils

3.                  Slope and depth to bedrock

4.                  Seasonal high water table

5.                  Proximity to building foundations and well heads

6.                  Erodibility of soils

7.                  Land availability and configuration of the topography

8.                  Consistency with approved watershed and stormwater management plans or regulations.  

D.   The following additional factors should be considered when evaluating the suitability of BMPs used to control water quality at a given development site: 

1.         Peak discharge and required volume control

2.         Streambank erosion

3.         Efficiency of the BMPs to mitigate potential water quality problems

4.         The volume of runoff that will be effectively treated

5.         The nature of the pollutant being removed

6.         Maintenance requirements

7.         Creation/protection of aquatic and wildlife habitat

8.         Recreational value

9.         Enhancement of aesthetic and property value 

SECTION 405.  CALCULATION METHODOLOGY

A.     Any stormwater runoff calculations involving drainage areas greater than two hundred (200) acres, including on-and off-site areas, shall use a generally accepted calculation technique that is based on the NRCS soil-cover complex method.  Table 405-1 summarizes acceptable computation methods.  It is assumed that all methods will be selected by the design professional based on the individual limitations and suitability of each method for a particular site. 

  *     Use of the Rational Method to estimate peak discharges from drainage areas that contain more than one hundred (100) acres must be approved by the Township Engineer. 

SOURCE: Gannett Fleming, 2001 

B.     All calculations consistent with this Ordinance using the soil-cover complex method shall use the appropriate design rainfall depths for the various return period storms presented in Table B-1 in Appendix B of this Ordinance.  If a hydrologic computer model such as PSRM or HEC-1 is used for stormwater runoff calculations, then the duration of rainfall shall be twenty-four (24) hours.  The NRCS “S” curve shown in Figure B-1, Appendix B of this Ordinance shall be used for the rainfall distribution. 

C.    For the purposes of pre-development flow rate determination, undeveloped land shall be considered as “meadow” good condition, type “B” soils, (RCN = 58, Rational “C” = 0.12) unless the natural ground cover generates a lower curve number or Rational “C” value (i.e., forest).  If a proposed development meets the definition of redevelopment as defined in Article II of this Ordinance, the applicant may adjust the pre-development RCN or “C” value based on the curves presented in Figure B-3. 

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 design storm curves from PA Department of Transportation Design Rainfall Curves (1986) (Figure B-2).  Times of concentration for overland flow shall be calculated using the methodology presented in Chapter 10 of Pa Department of Transportation Design Manual Part 2.  Times of concentration for channel and pipe flow shall be computed using Manning’s Equation. 

E.     RCNs 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 Ordinance. 

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 Ordinance. 

G.    Runoff characteristics of off-site areas that drain through a proposed development shall be based on actual existing conditions, not RCN=58 or C=0.12, and shall be assumed to not have any controls implemented on future development (i.e., no release rate restrictions). 

H.     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 Ordinance. 

I.         Outlet structures for stormwater management facilities shall be designed to meet the performance standards of this Ordinance using any generally accepted hydraulic analysis technique or method.  Acceptable methods are presented in Handbook of Hydraulics, by King and Brater (McGraw Hill).  In addition, application of computer programs such as HY-8 (Federal Highway Administration) or FlowMaster (Haestad Methods) will also be accepted. 

J.      The design of any stormwater detention facilities intended to meet the performance standards of this Ordinance shall be verified by routing the design storm hydrograph through these facilities using the Storage-Indication Method.  For drainage areas greater than twenty (20) acres in size, 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 uses a total runoff volume that is consistent with the volume from a method that produces a full hydrograph.   

K.     The Township has the authority to require that computed existing runoff rates be  reconciled with field observations and conditions.  If the designer can substantiate through actual physical calibration that more appropriate runoff and time-of-concentration values should be utilized at a particular site, then appropriate variations may be made upon review and recommendation of the Township Engineer.  Calibration shall require detailed gauge and rainfall data for the particular site in question.   

SECTION 406.  DESIGN CRITERIA FOR STORMWATER MANAGEMENT FACILITIES  

A.     Any stormwater management facility (i.e., detention basin) designed to store runoff and requiring a berm or earthen embankment required or regulated by this Ordinance shall be designed to provide an emergency spillway to handle flow up to and including the 100-year post-development conditions.  The height of the embankment must be set so as to provide a minimum two (2.0) feet of freeboard above the maximum pool elevation computed when the facility functions for the 100-year post-development 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 which may be required to pass storms larger than the 100-year event. 

B.     Any facilities that constitute water obstructions (e.g., culverts, bridges, outfalls, or stream enclosures) and any work involving wetlands as directed in 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.  Any other drainage conveyance facility that does not fall under Chapter 105 regulations shall be designed to convey, without damage to the drainage structure or roadway, runoff from a minimum 25-year design storm. The Township may require design based on a larger storm event.   Open channels shall be designed with a minimum of one (1.0) foot of freeboard.  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.  If the primary drainage facilities do not have capacity for future flows, then a safe drainage path must be provided to convey up to the 100-year design storm (without impacting structures). 

C.    Storm sewers must be able to convey post-development runoff from a minimum 25- year design storm without surcharging inlets. 

D.    Adequate erosion protection shall be provided along all open channels and at all points of discharge.

E.     The design of all stormwater management facilities shall incorporate sound engineering principles and practices.  The Township shall reserve the right to disapprove any design that would result in the occurrence or continuation of an adverse hydrologic or hydraulic condition within the watershed. 

F.     Stormwater drainage systems shall be provided in order to permit unimpeded flow along natural watercourses, except as modified by stormwater management facilities or open channels consistent with this Ordinance. 

G.    The existing points of concentrated drainage that discharge onto adjacent property shall not be altered without permission of the adjacent property owner(s) and shall be subject to any applicable discharge criteria specified in this Ordinance. 

H.     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 Ordinance.  If diffused flow is proposed to be concentrated and discharged onto adjacent property, the applicant must document to the Township in accordance with Section 405 that adequate downstream conveyance exists to safely transport the concentrated discharge, or the applicant must obtain drainage easements from affected downstream property owners and provide the facilities to safely convey the flow. 

I.         Downstream Hydraulic Capacity Analysis - Any downstream capacity hydraulic analysis conducted in accordance with this Ordinance 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 2-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 the increased 25-year return period runoff without creating any hazard to persons or property. 

3.                  Culverts, bridges, storm sewers, or any other facilities which must pass or convey flows from the tributary area must be designed in accordance with DEP, Chapter 105 regulations (if applicable) and, at a minimum, pass the increased 25-year return period runoff. 

J.    Where a development site is traversed by watercourses, riparian buffers shall be provided conforming to the line of such watercourses.  The width of the buffers shall be determined as set forth in Section 404.A.2.  Excavating, placing of fill, building structures, or making any alterations that may adversely affect the flow of stormwater within any portion of the riparian buffer shall be prohibited unless the proposed work is associated with a regulated wetlands mitigation program.  The buffer must be defined through a deed covenant. 

K.    When it can be shown that, due to topographic conditions, natural drainageways on the site cannot adequately provide for drainage, open channels may be constructed conforming substantially to the line and grade of such natural drainageways.  Work within natural drainageways shall be subject to approval by DEP through the Joint Permit Application process, or, where deemed appropriate by DEP, through the General Permit process. 

L.    Any stormwater management facilities regulated by this Ordinance that would be located in or adjacent to waters of the Commonwealth or wetlands shall be subject to approval by DEP through the Joint Permit Application process, or, where deemed appropriate by DEP, the 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. 

M.   Any stormwater management facilities regulated by this Ordinance that would be located on state highway rights-of-way shall be subject to approval by PennDOT. 

N.    Minimization of impervious surfaces and infiltration of runoff through seepage beds, infiltration trenches, etc. are required, where soil conditions permit, to reduce the size or eliminate the need for detention facilities. 

O.   In order to promote overland flow and infiltration/percolation of stormwater, roof drains must discharge into an accepted BMP providing infiltration and filtering of the stormwater.

SECTION 407.  EROSION AND SEDIMENTATION REQUIREMENTS

A.                 Whenever the vegetation and topography are to be disturbed, such activity must be in conformance with Chapter 102, Title 25, Rules and Regulations, Part I, Commonwealth of Pennsylvania, DEP, Subpart C, protection of Natural Resources, Article II, Water Resources, Chapter 102, “Erosion Control,” and in accordance with the Delaware County Conservation District and the standards and specifications of the Township. 

B.                 Additional erosion and sedimentation control design standards and criteria that must be applied where infiltration BMPs are proposed include the following: 

1.                  Areas proposed for infiltration BMPs shall be protected from sedimentation and compaction during the construction phase to maintain their maximum infiltration capacity. 

2.                  In order to ensure compliance with Chapter 102, the timing of the installation and operation of the infiltration BMP shall be at the discretion of the Township Engineer.