A. 
Applicants proposing regulated activities in the Neshaminy Creek Watershed that do not fall under the exemption criteria shown in § 148-6 shall submit a stormwater management (SWM) site plan consistent with the Neshaminy Creek Watershed SWM plan to the municipality for review. The SWM criteria of this chapter shall apply to the total proposed development even if development is to take place in stages. Preparation and implementation of an approved SWM site plan is required. No regulated activities shall commence until the municipality issues written approval of a SWM site plan, which demonstrates compliance with the requirements of this chapter.
B. 
SWM site plans approved by the municipality, in accordance with Article IV, shall be on-site throughout the duration of the regulated activity.
C. 
The municipality may, after consultation with the Department of Environmental Protection (PADEP), approve measures for meeting the state water quality requirements other than those in this chapter, provided that they meet the minimum requirements of, and do not conflict with, state law, including but not limited to the Clean Streams Law.[1]
[1]
Editor's Note: See 35 P.S. § 691.1 et seq.
D. 
For all regulated earth-disturbance activities, erosion and sediment (E&S) control best management practices (BMPs) shall be designed, implemented, operated, and maintained during the regulated earth-disturbance activities (e.g., during construction) to meet the purposes and requirements of this chapter and to meet all requirements under Title 25 of the Pennsylvania Code and the Clean Streams Law.[2] Various BMPs and their design standards are listed in the Erosion and Sediment Pollution Control Program Manual, No. 363-2134-008 (April 15, 2000), as amended and updated.
[2]
Editor's Note: See 35 P.S. § 691.1 et seq.
E. 
For all regulated activities, implementation of the volume controls in § 148-16 of this chapter is required.
F. 
Impervious areas:
(1) 
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.
(2) 
For development taking place in stages, the entire development plan must be used in determining conformance with this chapter.
(3) 
For projects that add impervious area to a parcel, the total impervious area on the parcel is subject to the requirements of this chapter, except that the volume controls in § 148-16 and the peak rate controls of § 148-17 do not need to be retrofitted to existing impervious areas that are not being altered by the proposed regulated activity.
G. 
Stormwater flows onto adjacent property shall not be created, increased, decreased, relocated, or otherwise altered without written notification of the adjacent property owner(s) from the developer. Such stormwater flows shall be subject to the requirements of this chapter.
H. 
All regulated activities shall include such measures as necessary to:
(1) 
Protect health, safety, and property;
(2) 
Meet the water quality goals of this chapter by implementing measures to:
(a) 
Minimize disturbance to floodplains, wetlands, and wooded areas.
(b) 
Create, maintain, repair or extend riparian buffers.
(c) 
Avoid erosive flow conditions in natural flow pathways.
(d) 
Minimize thermal impacts to waters of this commonwealth.
(e) 
Disconnect impervious surfaces [i.e., disconnected impervious areas (DIAs)] by directing runoff to pervious areas, wherever possible. See Appendix F[3] for detail on DIAs.
[3]
Editor's Note: Appendix F is included as an attachment to this chapter.
(3) 
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, Pennsylvania Department of Environmental Protection (PADEP) No. 363-0300-002 (2006) if methods other than green infrastructure and LID methods are proposed to achieve the volume and rate controls required under this chapter, the SWM site plan must include a detailed justification demonstrating that the use of LID and green infrastructure is not practicable. See Ordinance Appendix E[4] for a summary description.
[4]
Editor's Note: Appendix E is included as an attachment to this chapter.
I. 
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 chapter.
J. 
The design of all facilities over karst shall include an evaluation of measures to minimize the risk of adverse effects.
K. 
Normally dry, open-top, storage facilities should completely drain both the volume control and rate control capacities over a period of time not less than 24 hours and not more than 72 hours from the end of the design storm.
L. 
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, U.S. 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/
M. 
For all regulated activities, SWM BMPs shall be designed, implemented, operated, and maintained to meet the purposes and requirements of this chapter and to meet all requirements under Title 25 of the Pennsylvania Code, the Clean Streams Law,[5] and the Stormwater Management Act.[6]
[5]
Editor's Note: See 35 P.S. § 691.1 et seq.
[6]
Editor's Note: See 32 P.S. § 680.1 et seq.
N. 
Various BMPs and their design standards are listed in the Pennsylvania Stormwater Best Management Practices Manual (Pennsylvania BMP Manual).
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 chapter 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 § 148-19A.
(1) 
Post-development total runoff should not be increased from predevelopment total runoff for all storms equal to or less than the two-year, twenty-four-hour duration precipitation.
(2) 
The following applies in order to estimate the increased volume of runoff for the two-year, twenty-four-hour duration precipitation event:
(a) 
To calculate the runoff volume (cubic feet) for existing site conditions (predevelopment) 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 Ordinance Appendix B[1] 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 (inches) = Q = (P - 0.2S)2/(P + 0.8S)
Where:
P
=
2-year rainfall (inches)
S
=
(1,000/CN) - 10, the potential maximum retention (including initial abstraction, Ia)
Step 2: Runoff volume (cubic feet) = Q x area x 1/12
Where:
Q
=
Runoff (inches)
Area
=
SWM area (square feet)
[1]
Editor's Note: Appendix B is included as an attachment to this chapter.
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 inches runoff/12 inches) * impervious surface (square feet)
(2) 
At least the first one 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. Removal options include reuse, evaporation, transportation, and infiltration.
Volume (cubic feet) = (1 inch runoff/12 inches) * impervious surface (square feet)
(3) 
Infiltration facilities should be designed to accommodate the first 1/2 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 hours 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 and a SWM site plan submission is required for the regulated activity, the applicant shall determine and display the total acreage of protected area where no disturbance is proposed on the plan. The acreage of the protected area should be subtracted from the total site area and not 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 shall be calculated in accordance with the environmental protection standards specified in the Doylestown Township Zoning Ordinance.[2]
[2]
Editor's Note: See Ch. 175, Zoning.
(2) 
Calculate the volume controls provided through nonstructural BMPs. Table B-5 in Ordinance Appendix B[3] is recommended as guidance.
[3]
Editor's Note: Appendix B is included as an attachment to this chapter.
(3) 
Volume controls provided through nonstructural BMPs should be subtracted from the required volume to determine the necessary structural BMPs.
Required volume control (feet3)
-
Nonstructural volume control (feet3)
=
Structural volume requirement (feet3)
(4) 
Calculate the volume controls provided through structural BMPs. Table B-6 in Ordinance Appendix B[4] is recommended as guidance. See Pennsylvania BMP Manual Chapter 6 for description of the BMPs.
[4]
Editor's Note: Appendix B is included as an attachment to this chapter.
(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 Ordinance Appendix B[5] 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 inch/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 0.4 in/hr, the BMP design should use 0.2 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.
[5]
Editor's Note: Appendix B is included as an attachment to this chapter.
(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 addresses 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 subgrade 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. Percolation tests are not recommended for design purposes.
(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 two.
(d) 
If on-lot infiltration structures are proposed, it must be demonstrated to the municipality 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 municipality.
Peak rate controls for large storms, up to the 100-year event, is essential in order to protect against immediate downstream erosion and flooding. The following peak rate controls have been determined through hydrologic modeling of the Neshaminy Creek Watershed.
A. 
Standards for managing runoff from each subarea in the Neshaminy Creek Watershed for the two-, five-, ten-, twenty-five-, fifty-, and 100-year design storms are shown in the table below. 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 the following table:
Peak Rate Runoff Control Standards by Stormwater Management Districts in the Neshaminy Creek Watershed
(includes Little Neshaminy Creek)
District
Design Storm Post-development
(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
C
2-year
2-year
5-year
5-year
10-year
10-year
25-year
25-year
50-year
50-year
100-year
100-year
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 D[1]) and in this section of the chapter.
[1]
Editor's Note: Appendix D is included as an attachment to this chapter.
C. 
District boundaries. The boundaries of the stormwater management districts are shown on official maps and are available for inspection at the municipal office and county planning offices. A copy of the Doylestown Township area of the map with zoomed-in extents is included in Ordinance Appendix D.[2] 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.
[2]
Editor's Note: Appendix D is included as an attachment to this chapter.
D. 
Sites located in more than one district. For a proposed development site located within two or more stormwater management district category subareas, the peak discharge rate from any subarea shall meet the management district criteria for the district in which the discharge is located.
E. 
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.
F. 
Site areas. The stormwater management site area is the only area subject to the management district criteria. Nonimpacted areas or nonregulated activities bypassing the stormwater management facilities would not be subject to the management district criteria.
G. 
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 Doylestown Township:
(1) 
Meet the full requirements specified by the table in § 148-17A and by § 148-17A through F; or
(2) 
Reduce the total impervious surface on the site by at least 20% based upon a comparison of existing impervious surface to proposed impervious surface.
A. 
In order to protect and improve water quality, a riparian buffer easement shall be created and recorded as part of any subdivision or land development that encompasses a riparian buffer.
B. 
Except as required by Chapter 102, the riparian buffer easement shall be measured to be the greater of the limit of the 100-year floodplain or a minimum of 35 feet from the top of the streambank (on each side).
(1) 
Minimum management requirements for riparian buffers.
(a) 
Existing native vegetation shall be protected and maintained within the riparian buffer easement.
(b) 
Whenever practicable invasive vegetation shall be actively removed and the riparian buffer easement shall be planted with native trees, shrubs and other vegetation to create a diverse native plant community appropriate to the intended ecological context of the site.
C. 
The riparian buffer easement shall be enforceable by the municipality and shall be recorded in the appropriate county Recorder of Deeds office, so that it shall run with the land and shall limit the use of the property located therein. The easement shall allow for the continued private ownership and shall count toward the minimum lot area a required by zoning, unless otherwise specified in the municipal Zoning Ordinance.[1]
[1]
Editor's Note: See Ch. 175, Zoning.
D. 
Any permitted use within the riparian buffer easement shall be conducted in a manner that will maintain the extent of the existing 100-year floodplain, improve or maintain the stream stability, and preserve and protect the ecological function of the floodplain.
E. 
The following conditions shall apply when public and/or private recreation trails are permitted within riparian buffers:
(1) 
Trails shall be for nonmotorized use only, except for the use of e-bikes.
(2) 
Trails shall be designed to have the least impact on native plant species and other sensitive environmental features.
F. 
Septic drainfields and sewage disposal systems shall not be permitted within the riparian buffer easement and shall comply with setback requirements established under 25 Pa. Code Chapter 73.
A. 
The following criteria shall be used for runoff calculations:
(1) 
For development sites not considered redevelopment, the ground cover used to determine the existing conditions' runoff volume and flow rate shall be as follows:
(a) 
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 1/4 acre.
(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 number (CN) or Rational "C" value (i.c., woods) as listed in Tables B-4 or B-7 in Appendix B[1] of this chapter.
[1]
Editor's Note: Appendix B is included as an attachment to this chapter.
(2) 
For sites considered redevelopment, 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.
B. 
Stormwater runoff peak discharges from all development sites with a drainage area equal to or greater than 200 acres shall be calculated using a generally accepted calculation technique that is based on the NRCS Soil Cover Complex Method. The table in Subsection D 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 municipality 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
C. 
All calculations consistent with this chapter using the Soil Cover Complex Method shall use the appropriate design rainfall depths for the various return period storms according to the National Oceanic and Atmospheric Administration (NOAA) Atlas 14 rain data corresponding to the Doylestown rain gauge, seen in Table B-1 in Ordinance Appendix B.[2] The SCS Type II rainfall curve from NOAA is found on Figure B-1 in Ordinance Appendix B. This data may also be directly retrieved from the NOAA Atlas 14 website: hdsc.nws.noaa.gov/hdsc/pfds/orb/pa_pfds.html. 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.
[2]
Editor's Note: Appendix B is included as an attachment to this chapter.
D. 
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 the municipality and/or Municipal Engineer
Other methods
Varies
Other computation methodologies approved by the municipality and/or Municipal Engineer
E. 
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.
F. 
Runoff curve numbers (CN) for both existing and proposed conditions to be used in the soil cover complex method shall be based on Table B-4 in Ordinance Appendix B.[3]
[3]
Editor's Note: Appendix B is included as an attachment to this chapter.
G. 
Runoff coefficients (C) for both existing and proposed conditions for use in the Rational Method shall be consistent with Table B-7 in Ordinance Appendix B.
H. 
Runoff from proposed sites graded to the subsoil will not have the same runoff conditions as the site under existing conditions because of soil compaction, even after top-soiling or seeding. The proposed condition "CN" or "C" shall increase by 5% to better reflect proposed soil conditions.
I. 
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 the municipality is necessary.
J. 
Outlet structures for stormwater management facilities shall be designed to meet the performance standards of this chapter using the generally accepted hydraulic analysis technique or method of the municipality.
K. 
The design of any stormwater detention facilities intended to meet the performance standards of this chapter shall be verified by routing the design storm hydrograph through these facilities using the Storage-Indication Method. For drainage areas greater than 200 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 shall use a total runoff volume that is consistent with the volume from a method that produces a full hydrograph.
A. 
Hot spots.
(1) 
The use of infiltration BMPs is prohibited on hot spot land use areas. Examples of hot spots are listed in Ordinance Appendix G.[1]
[1]
Editor's Note: Appendix G is included as an attachment to this chapter.
(2) 
Stormwater runoff from hot spot land uses shall be pretreated. In no case may the same BMP be employed consecutively to meet this requirement. Guidance regarding acceptable methods of pretreatment is located in Appendix G.
B. 
West Nile guidance requirements. All wet basin designs shall incorporate biologic controls consistent with the West Nile Guidance found in Appendix H.[2]
[2]
Editor's Note: Appendix H is included as an attachment to this chapter.