Newtown Township D Stormwater Management.

§ 26-231 General Requirements.

[Ord. 2011-O-2, 5/25/2011, § 218]

1. Applicants proposing regulated activities in Newtown Township which do not fall under the exemption criteria shown in § 206 shall submit a stormwater management site plan consistent with this Part, to Newtown Township for review. The stormwater management criteria of this Part shall apply to the total proposed development even if development is to take place in stages. Preparation and implementation of an approved stormwater management site plan is required. No regulated activities shall commence until Newtown Township issues written approval of a stormwater management site plan, which demonstrates compliance with the requirements of this Part.

2. The applicant is required to find practicable alternatives to the surface discharge of stormwater including alternatives to impervious surfaces, in order to prevent the degradation of waters of the Commonwealth, and to maintain, as much as possible, the natural hydrologic regime.

3. The stormwater management site plan must be designed consistent with the sequencing provisions of § 234 to ensure maintenance of the natural hydrologic regime and to promote groundwater recharge and protect groundwater and surface water quality and quantity.

4. Stormwater drainage systems shall be designed and/or installed as necessary to:

A. Permit unimpeded flow along natural watercourses, except as modified by stormwater management facilities or open channels consistent with this Part.

B. To prevent erosion and to satisfactorily carry off or detain and control the release of surface waters.

C. Encourage all run-off control measures to percolate the stormwater into the ground to aid in the recharge of ground waters.

D. Carry surface water to the nearest adequate street, storm drain, detention basin, natural watercourse or drainage facility.

E. Maintain the adequacy of the natural stream channels. Accelerated bank erosion shall be prevented by controlling the rate and velocity of run-off discharge to these water courses, so as to avoid increasing occurrence of stream-bank overflow.

F. If in the course of reviewing the stormwater management site plan, the Township determines that off-site improvements are necessary to satisfactorily handle the stormwater from the proposed development the developer shall be responsible for design and installation of the off-site improvements.

5. 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 Part.

6. 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 Part. 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.

7. Where a development site is traversed by existing streams, drainage easements shall be provided conforming to the line of such streams. If a perennial or intermittent stream passes through the site, the applicant shall create stream buffer extending a minimum of 50 feet to either side of the top-of-bank of the channel. The buffer area shall be maintained with and encouraged to use appropriate native vegetation. If the applicable rear or side yard setback is less than 50 feet, the buffer width may be reduced to 25% of the setback to a minimum of 10 feet. If an existing buffer is legally prescribed (i.e., deed, covenant, easement, etc.) and it exceeds the requirements of this Part, the existing buffer shall be maintained.

8. Any stormwater management facilities regulated by this Part 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.

9. Any proposed stormwater management facilities regulated by this Part that would be located on State highway rights-of-way shall be subject to approval by the Pennsylvania Department of Transportation (PennDOT).

10. All regulated activities within Newtown Township shall be designed, implemented, operated and maintained to meet the purposes and requirements of this Part and to meet the requirements under 25 Pa. Code, the Clean Stream Law and the Stormwater Management Act. No regulated activities within Newtown Township shall commence until the requirements of this Part are met. Various BMPs and their design standards are listed in the Erosion and Sediment Pollution Control Program Manual, as amended and updated.

11. Operations and maintenance of permanent stormwater BMPs shall be addressed as required by this Part.

12. Stormwater management site plans approved by Newtown Township shall be on-site throughout the duration of the regulated activity.

13. Newtown Township may, after consultation with PADEP approve measures for meeting the State water quality requirements other than those in this Part, provided that they meet the minimum requirements of, and do not conflict with, State law including but not limited to the Clean Streams Law.

14. For all regulated activities, implementation of the volume controls of this Part is required.

15. Impervious Areas.

A. The measurement of impervious areas shall include all of the impervious areas in the total proposed development even if development is to take place in stages.

B. For development taking place in stages, the entire development plan must be used in determining conformance with this Part.

C. For projects that add impervious area to a parcel, the total impervious area on the parcel is subject to the requirements of this Part.

16. All regulated activities shall include such measures as necessary to:

A. Protect health, safety, and property.

B. Meet the water quality goals of this Part by implementing measures to:

(1) Minimize disturbance to floodplains, wetlands, and wooded areas.

(2) Create, maintain, repair or extend riparian buffers.

(3) Avoid erosive flow conditions in natural flow pathways.

(4) Minimize thermal impacts to waters of this Commonwealth.

(5) Disconnect impervious surfaces (i.e., disconnect impervious areas, DIAs) by directing runoff to pervious areas, wherever possible.

17. To the maximum extent practicable, incorporate the techniques for low impact development practices (e.g., protecting existing trees, reducing area of impervious surface, cluster development, and protecting open space) described in the Pennsylvania Stormwater Best Management Practices Manual.

18. Infiltration BMPs should be spread out, made as shallow as practicable, and located to maximize the use of natural on-site infiltration features while still meeting the other requirements of this Part.

19. The design of all facilities over karst shall include an evaluation of measures to minimize the risk of adverse effects.

20. Storage facilities should completely drain both the volume control and rate control capacities over a period of time not less than 24 and not more than 72 hours from the end of the design storm.

21. The design storm volumes to be used in the analysis of peak rates of discharge should be obtained from the Precipitation-Frequency Atlas of the United States, Atlas 14, Volume 2, Version 3.0, United States Department of Commerce, National Oceanic and Atmospheric Administration (NOAA), National Weather Service, Hydrometeorological Design Studies Center, Silver Spring, Maryland. NOAA's Atlas 14 can be accessed at http://hdsc.nws.noaa.gov/hdsc/pfds/.

22. In selecting the appropriate BMPs or combinations thereof, the applicant shall consider the following:

A. Total contributing area.

B. Permeability and infiltration rate of the site soils.

C. Slope and depth to bedrock.

D. Seasonal high water table.

E. Proximity to building foundations and well heads.

F. Erodibility of soils.

G. Land availability and configuration of the topography.

H. Peak discharge and required volume control.

I. Stream bank erosion.

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

K. The volume of runoff that will be effectively treated.

L. The nature of the pollutant being removed.

M. Maintenance requirements.

N. Creation/protection of aquatic and wildlife habitat.

O. Recreational value.

23. 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 D to this Part.

24. Retention of Existing Watercourses and Natural Drainage Features.

A. Whenever a watercourse, stream, or intermittent stream is located within a development site, it shall remain open in its natural state and location and shall not be piped.

B. No stormwater run-off or natural drainage shall be so diverted as to overload existing drainage systems, or create flooding or the need for additional drainage structures on other private properties or public lands.

25. Requirements for Drainage Structures and/or Pipes.

A. Drainage Easements. Drainage easements shall be provided to accommodate all storm drainage requirements and shall be a minimum of 20 feet in width.

B. Storm Drainage Pipe. The minimum diameter of all storm drainage pipes shall be 15 inches or an equivalent thereto, with the exception of detention basins where smaller pipe may be utilized where necessary to provide the required detention. The minimum grade of piping shall be 0.5%. Structures shall conform to Pennsylvania Department of Transportation specifications.

C. Stormwater Detention Areas.

(1) When detention basins are provided, they shall be designed to utilize the natural contours of the land whenever possible. When such design is impracticable, the construction of the basin shall utilize slopes as flat as possible to blend the structure into the terrain. All basins shall have slopes less than four horizontal to one vertical.

(2) Detention ponds shall be designed so that they return to normal conditions within approximately 12 hours after the termination of the storm, unless the Township Engineer finds that downstream conditions may warrant other design criteria for stormwater release.

(3) Emergency overflow facilities shall be provided for detention facilities to handle run-off in excess of design flows.

D. Storm sewers, culverts, and related installations shall be provided:

(1) To permit unimpeded flow of natural watercourses and in such a manner as to protect the natural character of the watercourses and to provided regulated discharge.

(2) To insure adequate drainage of all low points along the line of streets.

(3) To intercept stormwater runoff along streets at intervals reasonably related to the extent and grade of the area drained and to prevent substantial flow of water across intersections.

E. Storm sewers, as required, shall be placed in the right-of-way, parallel to the roadway and shall be designed as a combination storm sewer and underdrain. When located in undedicated land, they shall be placed within an easement, as approved by the Township Engineer.

F. Manholes shall be not more than 300 feet apart on sizes up to 24 inches and not more than 450 feet apart on greater sizes. Inlets may be substitute for manholes, on approval by the Township Engineer as the same spacing required for manholes. In no case shall inlets be spaced more than 450 feet apart.

G. Inlets and manhole covers and frames shall conform to Pennsylvania Department of Transportation Specifications. At street intersections, inlets shall be placed in the tangent and not in the curved potion of the curbing, unless approved by the Township Engineer.

H. Drainage structures, which drain watershed areas in excess of 1/2 square mile (320 acres), or which have a span of eight feet or more, shall be designed for a maximum expected run-off as calculated using the Soil Conservation Service Technical Release 55, "Urban Hydrology for Small Watersheds." The design storm shall be a minimum fifty-year storm. A one-hundred-year storm shall be used if the structure will significantly affect the existing one-hundred-year floodplain. All necessary permits from the Pennsylvania Department of Environmental Protection shall be obtained for the waterway opening before final design is undertaken.

The cartway area over the bridge shall be as wide as the widest road connecting with the bridge, or if the character of the road is expected to change for future planning, the cartway of the bridge shall be made to anticipate this condition. All drainage piping and culverts shall extend the full-width of the road right-of-way.

I. Where headroom is restricted, equivalent pipe arches may be used in lieu of circular pipe.

J. The minimum diameter of storm pipe shall be 15 inches.

K. Storm pipes will be provided in three inch increments of diameter.

L. Abrupt changes in direction or slope of pipe shall be avoided. Where such abrupt changes are required, an inlet or manhole shall be placed at the point of change.

M. The minimum slope in a pipe shall provide a minimum velocity of 2.5 fps but shall not have a minimum slope of less than 0.5%.

N. The top of storm pipes shall be at least six inches below grade elevation, except cast iron pipe, which may be within three inches.

26. All drainage channels shall be designed to prevent erosion of the bed and banks. The maximum permissible flow velocity shall not exceed those outlined in the following table. Suitable stabilization shall be provided where required to prevent erosion of the drainage channels.

	Allowable Water Velocities
	Material			Allowable Velocity Feet Per Second
	1.	Well established grass on good soil
		A.	Short pliant bladed grass	5-6
		B.	Bunch grass - soil exposed	2-4
		C.	Stiff stemmed grass	2-3
	2.	Earth without vegetation
		A.	Fine sand or silt	1-1
		B.	Ordinary firm loam	2-3
		C.	Stiff clay	3-5
		D.	Clay and gravel	4-5
		E.	Coarse gravel	4-5
		F.	Soft shale	5-6
	3.	Shoulders
		A.	Earth	See 2. above
		B.	Stabilized	6

27. Any vegetated drainage channel requiring mowing of the vegetation shall have a maximum grade of four horizontal to one vertical on those areas to be mowed.

28. Because of the critical nature of vegetated drainage channels, the design of all vegetated channels shall, as a minimum, conform to the design procedures outlined in the PADEP Erosion and Sediment Pollution Control Program Manual.

29. Maximum Expected Discharge.

A. The maximum expected discharge (M.E.D.) shall be defined as the maximum expected quantity of water, created by the design storm, arriving at a particular location (inlet, ditch, etc.).

B. The design storm is a selected intensity of rainfall, which tends to occur once during a specified period of years.

C. The Soil Conservation Service Technical Release # 55, "Urban Hydrology for Small Watersheds (less than 2,000 acres)," shall be used for the calculation of the maximum expected discharge.

D. The maximum expected discharge from drainage areas contributing to the storm drain pipe system may be determined by the use of the rational equation, when the total drainage area does not exceed 100 acres.

	The rational equation is as follows: Q = cIA
	Where:
	Q	=	Maximum expected discharge in cubic feet per second.
	C	=	Runoff factor expressed as a percent of the total water falling on an area.
	I	=	The rate of rainfall for the time of concentration of the drainage area in inches per hour for a given storm frequency.
	A	=	The drainage area expressed in acres.

E. Consideration should be given to future land use changes in the drainage area in selecting the "c" factor. For drainage areas containing several different types of ground cover, a weighted value of "c" must be used.

F. The rational "c" values are listed in Table B-4 and B-7 in Appendix B.

G. Drainage Area. The extent of an individual drainage area shall be outlined on a map of highest order available and submitted to the Township Engineer with the design calculations for storm sewers. The plan shall also indicate the "Q" anticipated at each proposed inlet and the slope of the street or swale used to compute the inlet capacity. Care should be taken to assure that all areas delivering run-off to the point under consideration shall be included.

30. Additional design criteria for specific drainage facilities are required as follows:

A. Inlets.

(1) When there is a change in pipe size in the inlet, the elevation for the top of pipes should be the same or the smaller pipe higher. A minimum drop of two inches should be provided in the inlet between the lowest inlet pipe invert elevation and the outlet pipe invert elevation.

(2) If the capacity of the shoulder, swale, curb section or depressed median section exceeds the assumed inlet capacities, the inlet capacities shall govern the spacing of inlets.

(3) If the capacity of the shoulder, swale, curb section, or depressed median section is less than the inlet capacities, then the shoulder, swale, curb section or depressed section capacity shall govern the spacing of inlets.

(4) Type C Inlets.

(a) This type of inlet is designated for installation in non-mountable curbs.

(b) The capacity of an inlet at a low point of a street vertical curve may be designed to accept 2.0 cfs from each direction or a maximum of 4.0 cfs.

(5) Type M and S Inlets.

(a) Type S Inlets are designated for installation in shoulder swale areas with back slopes 6:1 and steeper. Type M Inlets are designated to be used in swale areas where the back slope is flatter than 6:1.

(b) The capacity of an inlet at a low point in the swale (sump condition) is at eight cfs maximum.

§ 26-232 Permit Requirements by Other Government Entities.

[Ord. 2011-O-2, 5/25/2011, § 219]

1. 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 the Pennsylvania Department of Transportation (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.

§ 26-233 Erosion and Sediment Control During Regulated Activities.

[Ord. 2011-O-2, 5/25/2011, § 220]

1. No regulated activities within Newtown Township shall commence until Newtown Township approves an erosion and sediment control plan for construction activities.

2. 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. The Bucks County Conservation District requires their approval of an erosion and sedimentation control plan for any earth disturbance activity of 1,000 square feet or more.

3. In addition, under 25 Pa. Code, Chapter 102.5, a DEP "NPDES Permit for Stormwater Discharges Associated with Construction Activities" permit is required for any earth disturbance activity that involves equal to or greater than one acre of earth disturbance prior to commencing the earth disturbance activity, this includes earth disturbance on any portion, part or during any stage of a larger common plan of development or sale.

4. Detailed construction schedules shall be included in all erosion and sedimentation plans.

5. Evidence of any necessary permit(s) for regulated earth disturbance activities from the appropriate DEP regional office or the Bucks County Conservation District must be provided to Newtown Township. The issuance of an NPDES stormwater permit for construction activities or permit under the Statewide general permit (PAG-2) satisfies the requirements of this subsection.

6. 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.

7. Additional erosion and sediment control design standards and criteria are recommended to be applied where infiltration BMPs are proposed. They shall include the following:

A. Areas proposed for infiltration BMPs shall be protected from sedimentation and compaction during the construction phase to maintain maximum infiltration capacity. Barriers such as construction fencing should be used to protect and keep construction traffic out of the area where infiltration BMPs are proposed. Inlet or pipes that are installed to these systems should be completely sealed or protected and monitored until the drainage areas are completely stabilized.

B. Infiltration BMPs shall not be constructed nor receive runoff until the entire contributory drainage area to the infiltration BMP has achieved final stabilization, or as approved by the Engineer.

8. General Performance Standards.

A. Measures used to control erosion and reduce sedimentation shall, as a minimum, meet the standards and specifications of the Bucks County Conservation District. In cases where the Bucks County Conservation District does not have standards and specifications for erosion and sedimentation control, other known and commonly accepted standards and specifications may be used as approved by the Township Engineer.

B. The following measures to minimize erosion and sedimentation shall be included where applicable in the control plan:

(1) Stripping of vegetation, re-grading, or other development shall be done in a way that will minimize erosion.

(2) Development plans shall preserve salient natural features, keep fill operations to a minimum, and ensure conformity with topography so as to create the least erosion potential and adequately handle the volume and velocity of surface water run-off.

(3) Whenever feasible, natural vegetation shall be retained, protected, and supplemented.

(4) The disturbed area and the duration of exposure shall be kept to a practical minimum, but shall not exceed 60 days.

(5) Disturbed soils shall be stabilized as quickly as practical, but within a time not to exceed 90 days.

(6) Temporary vegetation and/or mulching shall be used to protect exposed critical areas during development.

(7) The permanent (final) vegetation and mechanical erosion control and drainage shall be installed as soon as practical in the development.

(8) Provisions shall be made to accommodate effectively the increased run-off caused by changed soil and surface conditions during and after development within the site. Where necessary, the rate of surface water run-off will be mechanically retarded.

(9) Sediment in the run-off water shall be trapped until the disturbed area is stabilized by the use of debris basins, sediment basins, silt traps, or similar measures.

C. Whenever sedimentation is caused by stripping of vegetation, regrading or other development, it shall be the responsibility of the person, corporation of other entity causing such sedimentation to remove it from all affected surfaces, drainage systems and watercourses on and off-site, and to repair any damage at his expense as quickly as possible.

D. It is the responsibility of any person, corporation, or entity doing any work on or across a stream, watercourse, or swale, or upon the floodplain or right-of-way during the period of the work, to return it to its original or equal condition after such activity is completed.

E. No person, corporation, or other entity shall block, impede the flow of, alter, construct any structure or deposit any material or thing or perform any work which will affect normal or flood flow in any stream or watercourse without having obtained prior approval from the Township and/or the Department of Environmental Protection, whichever is applicable.

F. Each person, corporation, or other entity which makes any surface changes shall be required to:

(1) Collect on-site surface run-off and control it to a point of discharge into the natural watercourse of the drainage area.

(2) Handle existing off-site run-off through the development by designing it to adequately handle all upstream run-off.

(3) Provide and install at their expense, in accordance with Township requirements, all drainage and erosion control improvements (temporary and permanent) as required by the approved erosion and sedimentation control plan.

9. Site Grading for Erosion Control. In order to provide more suitable sites for building and other uses, improve surface drainage, and control erosion, the following requirements shall be met:

A. All lots, tracts or parcels shall be graded to provide proper drainage away from buildings and dispose of it without ponding, and all land within a development shall be graded to drain and dispose of surface water without ponding, except where ponding (detention basins, etc.) is part of the stormwater management plan for the site.

B. Concentration of surface water run-off shall be permitted only in swales, watercourses, or detention basins. Subject to the approval of the Township Engineer, swales shall be sodded, utilize jute matting, or other similar measures to insure proper growth of the ground cover.

C. Grading shall in no case be done in such a way as to divert water onto the property of another landowner.

D. During grading operations, necessary measures for dust control must be exercised.

E. Grading equipment will not be allowed to cross live streams. Provisions shall be made for the installation of temporary culverts or bridges.

F. Tire cleaning areas shall be provided at each point of egress from the development site.

10. Excavations and Fills.

A. No excavation or fill shall be made with a face steeper than four horizontal to one vertical, except under the following condition:

A concrete, timber, masonry or other approved retaining wall constructed in accordance with approved standards is provided to support the face of the excavation.

B. Edges of slope shall be a minimum of five feet from property lines or right-of-way lines of streets in order to permit the normal rounding of the edge without encroaching on the abutting property.

C. Adequate provisions shall be made to prevent surface water from damaging the cut face of excavations and the sloping surfaces of fills.

D. Cut and fill shall not endanger adjoining property.

E. Fill shall be placed and compacted so as to minimize sliding or erosion of the soil.

F. Fills shall not encroach on natural watercourses or constructed channels.

G. Fills placed adjacent to natural watercourses or constructed channels shall have suitable protection against erosion during periods of flooding.

§ 26-234 Nonstructural Project Design (Sequencing to Minimize Stormwater Impacts).

[Ord. 2011-O-2, 5/25/2011, § 221]

1. The design of all regulated activities shall include the following to minimize stormwater impacts:

A. The applicant shall 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 of the site.

B. 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 Township requirements.

C. 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.

2. 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:

A. The stormwater calculations shall show on plans or list in the narrative all environmentally sensitive areas including, but not limited to, steep slopes, ponds, lakes, streams, wetlands, hydric soils, vernal pools, stream buffers, hydrologic soil groups, any existing recharge areas, potential infiltration areas or permeable soil area and other requirements outlined in the Newtown Township Subdivision and Land Development Ordinance [Chapter 22].

B. Prepare a project layout avoiding sensitive areas identified in subsection (2)(A).

C. Identify site specific existing conditions drainage areas, discharge points, recharge areas and hydrologic soil groups A and B (areas conducive to infiltration).

D. Evaluate Nonstructural Stormwater Management Alternatives.

(1) Minimize earth disturbance.

(2) Minimize impervious surfaces.

(3) Break up large impervious surfaces.

E. Determine what management district the site falls into (Appendix D) and conduct an existing conditions runoff analysis.

F. 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, ensure the remaining site development has no surface or point discharge.

G. Conduct a proposed conditions runoff analysis based on the final design that meets the management district requirements.

H. Manage any remaining runoff prior to discharge, through detention, bioretention, direct discharge or other structural control.

§ 26-235 Volume Control.

[Ord. 2011-O-2, 5/25/2011, § 222]

1. Volume controls will mitigate increased runoff impacts, protect stream channel morphology, maintain groundwater recharge, and contribute to water quality improvements. Stormwater runoff volume control methods are based on the net change in runoff volume for the two-year storm event.

Volume controls shall be implemented using the design storm method in subsection (A) or the simplified method in subsection (B) below. For regulated activities equal to or less than one acre, this Part establishes no preference for either methodology; therefore, the applicant may select either methodology on the basis of economic considerations, the intrinsic limitations of the procedures associated with each methodology, and other factors. All regulated activities greater than one acre must use the design-storm method.

A. Design-Storm Method (Any Regulated Activity). This method requires detailed modeling based on site conditions. For modeling assumptions refer to § 237.

(1) Post-development total runoff should not be increased from pre-development total runoff for all storms equal to or less than the two year twenty-four hour duration precipitation.

(2) The following must be used in order to estimate the increased volume of runoff for the two year twenty-four hour duration precipitation event:

To calculate the runoff volume (cubic feet) for existing site conditions (pre-development) and for the proposed developed site conditions (post-development), it is recommended to use the soil cover complex method as shown on the following page. Table B-3 in Appendix B is available to guide a qualified professional and/or an applicant to calculate the stormwater runoff volume. The calculated volume shall be either reused, evapotranspired, or infiltrated through structural or nonstructural means.

	Soil Cover Complex Method:
	Step 1:	Runoff (in) = Q =	(P-0.2S)2 (P + 0.8S) where
	P = 2-year Rainfall (in)
	S =	(1000) (CN)	- 10, the potential maximum retention (including initial abstraction, 1a)
	Step 2: Runoff Volume (Cubic Feet) = Q x Area x 1/12
	Q = Runoff (in)
	Area = SWM Area (sq ft)

B. Simplified Method (Regulated activities less than or equal to one acre):

(1) Stormwater facilities shall capture the runoff volume from at least the first two inches of runoff from all new impervious surfaces.

Volume (cubic feet) = (2" runoff/12 inches) X impervious surface (sq. ft.)

(2) At least the first inch of runoff volume from the new impervious surfaces shall be permanently removed from the runoff flow, i.e., it shall not be released into the surface waters of the Commonwealth. The calculated volume shall be either reused, evapotranspired or infiltrated through structural or nonstructural means.

Volume (cubic feet) = (1" runoff/12 inches) X impervious surface (sq. ft.)

(3) Infiltration facilities should be designed to accommodate the 0.5 inch of the permanently removed runoff.

(4) No more than one inch of runoff volume from impervious surfaces shall be released from the site. The release time must be over 24 to 72 hours.

C. Stormwater Control Measures. The applicant must demonstrate how the required volume is controlled through stormwater best management practices (BMPs) which shall provide the means necessary to capture, reuse, evaporate, transpire or infiltrate the total runoff volume.

(1) If natural resources exist on the site, the applicant is required to determine the total acreage of protected area where no disturbance is proposed. The acreage of the protected area should be subtracted from the total site area and not be included in the stormwater management site area acreage used in determining the volume controls.

Stormwater Management Site Area = {Total Site Area (for both pre and post development conditions) - Protected Area}

Natural resource areas should be calculated based upon the Newtown Township Zoning Ordinance. See Table B-2 in Appendix B for guidance to assess the total protected area. For additional reference see Chapter 5, § 5.4.1 of the PA BMP manual.

(2) Calculate the volume controls provided through non-structural BMPs. Table B-5 in Appendix B is recommended as guidance.

(3) Volume controls provided through nonstructural BMPs should be subtracted from the required volume to determine the necessary structural BMPs.

Required Volume Control (ft3)

–

Nonstructural Volume Control (ft3)

–

Structural Volume Requirement (ft3)

(4) Calculate the volume controls provided through structural BMPs. Table B-6 in Appendix B is recommended as guidance. See PA BMP Manual Chapter 6 for description of the BMPs.

(5) Infiltration BMPs intended to receive runoff from developed areas shall be selected based on the suitability of soils and site conditions (see Table B-6 in Appendix B for a list of Infiltration BMPs), Infiltration BMPs shall be constructed on soils that have the following characteristics:

(a) A minimum soil depth of 24 inches between the bottom of the infiltration BMPs and the top of bedrock or seasonally high water table.

(b) An infiltration rate sufficient to accept the additional stormwater load and dewater completely as determined by field tests. A minimum of 0.2 inches/hour (in/hr) should be utilized and for acceptable rates a safety factor of 50% should be applied for design purposes (e.g., for soil which measured four-tenths in/hr, the BMP design should use two-tenths in/hr to insure safe infiltration rates after construction).

(c) All open-air infiltration facilities shall be designed to completely infiltrate runoff volume within three days (72 hours) from the start of the design storm.

(6) Soils. A soils evaluation of the project site shall be required to determine the suitability of infiltration facilities. All regulated activities are required to perform a detailed soils evaluation by a qualified design professional which at minimum address' soil permeability, depth to bedrock, and subgrade stability. The general process for designing the infiltration BMP shall be:

(a) 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 sub-grade stability; infiltration may not be ruled out without conducting these tests.

(b) 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.

(c) Design the infiltration structure based on field determined capacity at the level of the proposed infiltration surface and based on the safety factor of 50%.

(d) If on-lot infiltration structures are proposed, it must be demonstrated to the Township that the soils are conducive to infiltrate on the lots identified.

(e) An impermeable liner will be required in detention basins where the possibility of groundwater contamination exists. A detailed hydrogeologic investigation may be required by the Township.

§ 26-236 Stormwater Peak Rate Control and Management Districts.

[Ord. 2011-O-2, 5/25/2011, § 223]

1. The Neshaminy Creek Watershed has been divided into two stormwater management districts in Newtown Township (District A and District B) as shown on the Peak Rate Control Management District Map in Appendix D. The Delaware River South Watershed contains only one stormwater management district. The release rate requirements for the Neshaminy Creek Watershed District 'A' and the Delaware River South Watershed District 'A' are the same.

Standards for managing runoff from each subarea in the Neshaminy Creek and Delaware River South Watersheds for the two-, five-, ten-, twenty-five-, fifty- and one-hundred-year design storms are shown in Table 236.1. Development sites located in each of the Management Districts must control proposed development conditions runoff rates to existing conditions runoff rates for the design storms in accordance with Table 236.1.

	Table 236.1 Peak Rate Runoff Control Standards by Stormwater Management Districts In the Neshaminy Creek Watershed and the Delaware River South Watershed (includes Little Neshaminy Creek)
	District	Design Storm Postdevelopment (Proposed Conditions)	Design Storm Predevelopment (Existing Conditions)
	A	2-year	1-year
		5-year	5-year
		10-year	10-year
		25-year	25-year
		50-year	50-year
		100-year	100-year
	B	2-year	1-year
		5-year	2-year
		10-year	5-year
		25-year	10-year
		50-year	25-year
		100-year	50-year

2. 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 Peak Rate Control Management District Map (Appendix D) and in this Section of this Part.

3. District Boundaries. The boundaries of the stormwater management districts are shown on official maps and are available for inspection at the Township office and County planning offices. A copy of the map at a reduced scale with zoomed-in extents is included in Appendix D. The exact location of the stormwater management district boundaries as they apply to a given development site shall be determined by mapping the boundaries using the two-foot topographic contours (or most accurate data required) provided as part of the SWM site plan.

4. 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 the district in which the discharge is located.

5. Off-Site Areas. When calculating the allowable peak runoff rates, developers do not have to account for runoff draining into the subject development site from an off-site area. On-site drainage facilities shall be designed to safely convey off-site flows through the development site.

6. Site Areas. The stormwater management site area is the only area subject to the management district criteria. Non-impacted areas or non-regulated activities bypassing the stormwater management facilities would not be subject to the management district criteria.

7. 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 Newtown Township:

A. Meet the full requirements specified by Table 236.1 and subsections (1) through (6); or,

B. Reduce the total impervious surface on the site by at least 20% based upon a comparison of existing impervious surface to proposed impervious surface.

§ 26-237 Calculation Methodology.

[Ord. 2011-O-2, 5/25/2011, § 225]

1. The following criteria shall be used for runoff calculations:

A. For development sites not considered redevelopment, the ground cover used to determine the existing conditions runoff volume and flow rate shall be as follows:

(1) Wooded sites shall use a ground cover of "woods in good condition." A site is classified as wooded if a continuous canopy of trees exists over a 1/4 acre.

(2) 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 number (CN) or rational "c" value (i.e., woods) as listed in Tables B-4 or B-7 in Appendix B of this Part.

B. For development and redevelopment sites, the ground cover used to determine the existing conditions runoff volume and flow rate for the developed portion of the site shall be based upon actual land cover conditions. If the developed site contains impervious surfaces, 20% of the impervious surface area shall be considered meadow in the model for existing conditions.

2. Stormwater runoff peak discharges 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 237-1 summarizes acceptable computation methods and the method selected by the design professional shall be based on the individual limitations and suitability of each method for a particular site. Newtown Township may allow the use of the rational method (Q=CIA) to estimate peak discharges from drainage areas that contain less than 200 acres.

	Q	=	Peak flow rate, cubic feet per second (cfs)
	C	=	Runoff coefficient, dependent on land use/cover
	I	=	Design rainfall intensity, inches per hour
	A	=	Drainage Area, acres.

	Table 237-1 Acceptable Computation Methodologies for Stormwater Management Plans
	Method	Method Developed By	Applicability
	TR-20 (or commercial computer package based on TR-20)	USDA NRCS	Applicable where use of full hydrology computer model is desirable or necessary.
	TR-55 (or commercial computer package based on TR-55)	USDA NRCS	Applicable for land development plans within limitations described in TR-55.
	HEC-1/HEC-HMS	U.S. Army Corps of Engineers	Applicable where use of full hydrologic computer model is desirable or necessary.
	PSRM	Penn State University	Applicable where use of a hydrologic computer model is desirable or necessary; simpler than TR-20 or HEC-1.
	Rational method (or commercial computer package based on rational method)	Emil Kuichling (1889)	For sites less than 200 acres, or as approved by Newtown Township and/or the Township Engineer.
	Other methods	Varies	Other computation methodologies approved by Newtown Township and/or the Township Engineer.

3. All calculations consistent with this Part using the soil cover complex method shall use the appropriate design rainfall depths for the various return period storms according to NOAA Atlas 14, rain data corresponding to the Doylestown rain gauge, seen in Table B-1 in Appendix B. The SCS Type II rainfall curve from NOAA is found on Figure B-1 in Appendix B. This data may also be directly retrieved from the NOAA Atlas 14 website. 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.

4. All calculations using the rational method shall use rainfall intensities consistent with appropriate times-of-concentration for overland flow and return periods from NOAA Atlas 14, Volume 2, Version 2.1. 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.

5. Runoff curve numbers (CN) for both existing and proposed conditions to be used in the soil cover complex method shall be obtained from Table B-4 in Appendix B.

Runoff coefficients (c) for both existing and proposed conditions for use in the rational method shall be obtained from Table B-7 in Appendix B.

6. The designer shall consider that the runoff from proposed sites graded to the subsoil will not have the same runoff conditions as the site under existing conditions because of the soil compaction, even after topsoiling or seeding. The designer may increase his proposed condition "CN" or "c" by 5% to better reflect proposed soil conditions.

7. The Manning equation is preferred for one-dimensional, gradually-varied open channel flow. In other cases, appropriate, applicable methods should be applied; however, early coordination with Newtown Township is necessary.

8. Outlet structures for stormwater management facilities shall be designed to meet the performance standards of this Part using any generally accepted hydraulic analysis technique or method.

9. The design of any stormwater detention facilities intended to meet the performance standards of this Part 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. Newtown Township 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.

§ 26-238 Other Requirements.

[Ord. 2011-O-2, 5/25/2011, § 225]

1. Any stormwater facility located on State highway rights-of-way shall be subject to approval by the Pennsylvania Department of Transportation (PennDOT).

2. All wet basin designs shall incorporate biologic controls consistent with accepted West Nile Virus Guidance materials.

3. Any stormwater management facility (i.e., detention basin) required or regulated by this Part 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.

4. 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.

5. Any other drainage conveyance facility that does not fall under Chapter 105 regulations, other than storm sewers, 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 or embankment. 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. Roadway crossings located within designated floodplain areas must be able to convey runoff from a one-hundred-year design storm.

6. Storm sewers must be able to convey proposed conditions runoff from a twenty-five-year design storm without surcharging inlets, where appropriate.

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

8. The design of all stormwater management facilities shall incorporate sound engineering principles and practices. Newtown Township reserves the right to disapprove any design that would result in the construction in or continuation of a stormwater problem area.

A. It is the responsibility of any person, corporation, or entity doing any work on or across a stream, watercourse, or swale, or upon the floodplain or right-of-way during the period of the work, to return it to its original or equal condition after such activity is completed.

B. No person, corporation, or other entity shall block, impede the flow of, alter, construct any structure or deposit any material or thing or perform any work which will affect normal or flood flow in any stream or watercourse without having obtained prior approval from the Township and/or Department of Environmental Protection, whichever is applicable.

C. Each person, corporation or other entity which makes any surface changes shall be required to:

(1) Collect on-site surface run-off and control it to a point of discharge into the natural watercourse of the drainage area.

(2) Handle existing off-site run-off through his development by designing it to adequately handle all upstream run-off.

(3) Provide and install at his expense, in accordance with Township requirements, all drainage and erosion control improvements (temporary and permanent) as required by the approved erosion and sediment control plan.