A. Stormwater runoff from all development sites shall be calculated using either the rational method or a soil cover complex methodology.
B. Any stormwater runoff calculations shall use generally accepted calculation technique that is based on the NRCS soil cover complex method. Table 309-1 summarizes acceptable computation methods. Method must be selected by the applicant based on the individual limitations and suitability of each method for a particular site.
The Rational Method may be used to estimate peak discharges from drainage areas that contain less than two hundred (200) acres. The Rational Method is recommended for drainage areas under one hundred (100) acres.
C. 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 according to the region for which they are, located as presented in Table A-1 in Appendix A of this Ordinance. If a hydrologic computer model such as HEC-1 or HEC-HMS is used for stormwater runoff calculations, the duration of rainfall shall be twenty-four (24) hours. The NACS `S' curve shown in Figure A-1, Appendix A of this Ordinance shall be used for the rainfall distribution.
D. Runoff Curve Numbers (CN) for both existing and proposed conditions to be used in the soil cover complex method shall be obtained from Table A-2 in Appendix A of Ordinance.
E.Suggested runoff coefficients (C) for both existing and proposed conditions for use in the Rational Method are noted in Table A-3 in Appendix A of this Ordinance.
F. 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 A-2). Times of concentration for overland flow shall be calculated using the methodology presented in Chapter 3 of Urban Hydrology for Small Watersheds, NRCS, TR-55 (as amended or replaced from time to time by NRCS). Times of concentration for channel and pipe flow shall be computed using Manning's equation.
G. For the purposes of existing conditions flow rate determination for all subdivision and land development applications, undeveloped land and existing impervious surfaces shall be considered as "meadow" in good condition, unless the natural ground cover generates a lower curve number (ON) or Rational 'C' value (i.e. forest), as listed in Table A-2 or A-3 in Appendix A of this Ordinance.
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 A-4 in Appendix A of this 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.
J. The design of any stormwater management 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 area, the design storm hydrograph shall be computed using a calculation 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 design professional engineer 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 recommendations of the Township Engineer. Calibration shall require detailed gauge and rainfall data for the particular site in question.
L. Infrequent Storm - The rational runoff coefficients given in this Ordinance are applicable for the two (2) year through ten (10) year frequency design storm event. Less frequent, higher intensity storms require modification of the runoff coefficient because infiltration and other losses have a proportionally smaller effect on runoff (Wright-McLaughlin 1969). The adjustment of the rational method for use with major storms shall be made by multiplying the rational formula by a frequency factor Cf. Cf values are as follows: twenty-five (25) year frequency design storm 1.1; fifty (50) year frequency design storm 1.2; one hundred (100) year frequency design storm 1.25. The product of Cf and Rational Coefficient C shall not exceed 1.0.
M. The time of concentration (Tc) is the time required for water to flow from the hydraulically most remote point of the drainage area to the point of interest (design point). Use of the rational formula requires calculation of a Tc for each design point within the drainage basin. Travel Time Estimation for the rational method shall be based on NRCS Technical Release No. 55 (2nd Edition). For design purposes the time of concentration may not be less than five (5) minutes. Travel time (Tt) is the time it takes runoff to travel from one location to another in a watershed (subreach) and is a component of time of concentration. Tc is computed by summing all the travel times for consecutive components of the drainage conveyance system.
Water moves through a watershed as sheet flow, shallow concentrated flow, open channel flow, or some combination of these. Sheet flow rates shall be calculated using the NRCS TR-55 (1986) variation of the kinematic wave equation. Sheet flow length may not exceed fifty (50) feet over paved surfaces and one hundred and fifty (150) feet over unpaved surfaces. Maximum permitted sheet flow length shall be one hundred and fifty (150) feet unless site specific conditions exist (that can be demonstrated) that warrant an increase of the sheet flow length. Under no circumstances shall sheet flow length exceed three hundred (300) feet. Shallow concentrated flow time and open channel flow time shall be calculated using standard engineering methodologies.
A. Whenever vegetation and topography are to be disturbed, such activity must be in conformance with Chapter 102, Title 25, Rules and Regulations, Part 1, Commonwealth of Pennsylvania, Department of Environmental Protection, Subpart C, protection of Natural Resources, Article II, Water Resources, Chapter 102, "Erosion Control," and in accordance with the Bucks Conservation District and the standards and specifications of the Township.
B. No Regulated Earth Disturbance activities within the Township shall commence until approval by the Township of an Erosion and Sediment Control and Grading Plan for construction activities.
C. Pursuant to 25 PA Code Chapter 92, a PADEP 'NPDES Construction Activities" permit is required for Regulated Earth Disturbance activities.
D. Evidence of any necessary permit(s) for Regulated Earth Disturbance activities from the appropriate PADEP regional office or Bucks County Conservation District must be submitted to the Township.
E. A copy of the Erosion and Sediment Control Plan and any required permit, as required by PADEP regulations shall be available at the project site at all times.
F. 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, so as to maintain their maximum infiltration capacity. Thirty-three (33) inch super filter fabric fence (or other approved protection mechanism) must be installed around proposed infiltration areas to prevent encroachment and compaction by construction equipment.
2. Infiltration BMPs shall not be constructed nor receive runoff until the entire contributory drainage area to the infiltration BMP has received final stabilization. If necessary, thirty-three (33) inch super filter fabric fence (or other approved protection mechanism) must be installed in the vicinity of infiltration area to prevent contamination by runoff containing suspended sediment.
G. Peak discharges and discharge volumes from the site shall comply with the appropriate sections above, with the following additions:
1. For purposes of calculating required detention storage during land disturbance, peak discharges and discharge volumes shall be calculated based upon the runoff coefficients for bare soils during the maximum period and extent of disturbance from clearing, and impervious surface installation, indicated on the development plan. Controls shall insure that the difference in volume and rate of peak discharges before disturbance and during shall not exceed those peak discharges required in Section 303 of this Ordinance. Detention storage during the period of land disturbance and prior to establishment of permanent cover may require additional facilities on a temporary basis. Such measures shall be located so as to preserve the natural soil infiltration capacities of the planned infiltration areas. Calculations based on the above parameters must be submitted to verify "during construction" runoff does not exceed predevelopment runoff for the one (1) year frequency through one hundred (100) year frequency design storm events.
2. Wherever soils, topography, cut and fill or grading requirements, or other conditions suggest substantial erosion potential during land disturbance, the Township may require that the entire volume of all storms up to a two (2) year storm from the disturbed areas be retained on site and that special sediment trapping facilities (such as check dams, etc.) be installed.
H. Areas of the site to remain undisturbed shall be protected from encroachment by construction equipment/vehicles to maintain the existing infiltration characteristics of the soil. Four (4) feet high orange safety fence or other similar protection fence approved by the Township must be installed around the entire limit of disturbance/clearing prior to commencement of earthmoving activities, and maintained until completion of all construction activity.
A. No Regulated Earth Disturbance activities within the Township shall commence until approval by the Township of a plan which demonstrates compliance with State Water Quality Requirements after construction is complete.
B. The BMPs must be designed, implemented, and maintained to meet State Water Quality Requirements and any other more stringent requirements as determined by the Township.
C. To control postconstruction stormwater impacts from Regulated Earth Disturbance activities, State Water Quality Requirements may be met by BMPs, including site design, which provide for replication of preconstruction stormwater infiltration and runoff conditions, so that postconstruction stormwater discharges do not degrade the physical, chemical or biological characteristics of receiving waters. As described in the PADEP Comprehensive Stormwater Management Policy (#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 filter out the chemical and physical pollutants from the stormwater runoff, and
3. Streambank and Streambed Protection: management of volume and rate of postconstruction stormwater discharges to prevent physical degradation of receiving waters (e.g. from scouring).
D. PADEP has regulations that require Townships to ensure design, implementation, and maintenance of Best Management Practices (BMPs) that control runoff from new development and redevelopment after Regulated Earth Disturbance activities are complete.These requirements include the need to implement postconstruction stormwater BMPs with assurance of long-term operations and maintenance of those BMPs.
E. Evidence of any necessary permit(s) for Regulated Earth Disturbance activities from the appropriate PADEP regional office must be submitted to the Township.
F. BMP operations and maintenance requirements are described in Article IV of this Ordinance.
Development on lands adjacent to the Delaware Canal which propose to discharge stormwater into the canal must obtain special right-of-way approval from Pennsylvania Department of Conservation and Natural Resources (PADCNR). The PADCNR has established a separate policy for granting right-of-way for stormwater drainage into the Delaware Canal, which shall be used in conjunction with this Ordinance. Feeder streams that flow or feed into the Delaware Canal shall be protected against soil erosion, water quality degradation, and sedimentation.
