A.
Applicants proposing regulated activities in the Neshaminy Creek, Pennypack Creek, and Poquessing Creek watersheds that do not fall under the exemption criteria shown in § 153-6 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 an 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. 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.
E.
For all regulated activities, implementation of the volume controls in § 153-16 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 § 153-16 and the peak rate controls of § 153-17 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:
(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 Appendix E[3] for a summary description.
[3]
Editor's Note: See 35 P.S. § 1701 et seq.
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 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, and the Stormwater Management Act.[4]
[4]
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).
Approvals issued and actions taken under this chapter do not
relieve the applicant of the responsibility to secure required permits
or approvals for activities regulated by any other code, law, regulation
or ordinance.
A.
An erosion and sediment control plan shall be submitted to the Bucks
County Conservation District if earth disturbance is 1,000 square
feet or greater.
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, Control Guideline 1 (any regulated activity). This method requires detailed modeling based on site conditions. For modeling assumptions refer to § 153-16A.
(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 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
|
=
|
(1000/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: Said appendix is included as an attachment to this chapter.
B.
Simplified Method, Control Guideline 2 (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 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 an 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 Upper Southampton
Township Zoning Ordinance.[2]
|
(2)
Calculate the volume controls provided through nonstructural BMPs.
Table B-6 in Appendix B[3] is recommended as guidance.
[3]
Editor's Note: Said appendix 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
|
Nonstructural
|
Structural volume
| ||
|
-
|
=
| |||
|
Volume control (feet)
|
volume control (feet)
|
requirement (feet)
|
(4)
Calculate the volume controls provided through structural BMPs. Table
B-6 in Appendix B[4] is recommended as guidance. See Pennsylvania BMP Manual
Chapter 6 for description of the BMPs.
[4]
Editor's Note: Said appendix 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 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: Said appendix 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 soil scientist or other 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 2.
(d)
If on-lot infiltration structures are proposed, it must be demonstrated
to the municipality through soil testing 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,
are 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, Pennypack Creek,
and Poquessing Creek watersheds.
A.
Standards for managing runoff from each subarea in the Neshaminy
Creek, Pennypack Creek, and Poquessing Creek watersheds for the two-,
five-, ten-, twenty-five-, fifty-, and 100-year design storms are
shown in Table 153-17A. 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:
|
Table 153-17A Peak Rate Runoff standards by Stormwater Management
District Neshaminy Creek, Pennypack Creek and Poquessing Creek Watershed
| ||
|---|---|---|
|
District
|
Design Storm Post Developmen
(Proposed Conditions)
|
Design Storm Predevelopment
(Existing Conditions)
|
|
Neshaminy Creek
|
2-year
|
1-year
|
|
District B and Southampton Creek
|
5-year
|
2-year
|
|
Pennypack District B
|
10-year
|
5-year
|
|
25-year
|
10-year
| |
|
50-year
|
25-year
| |
|
100-year
|
50-year
| |
|
Southampton Creek
|
2-year
|
1-year
|
|
Pennypack District A and Poquessing Creek
|
5-year
|
5-year
|
|
District A
|
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 (Appendix D[1]) and in this section of the chapter.
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
C.
District boundaries. The boundaries of the stormwater management
districts are shown on official maps and are available for inspection
at the municipal office and county planning offices. A copy of the
Upper Southampton Township area of the map with zoomed-in extents
is included in 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: Said appendix 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 Upper Southampton Township:
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 PADEP 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).
C.
Minimum management requirements for riparian buffers.
(1)
Existing native vegetation shall be protected and maintained within
the riparian buffer easement.
(2)
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.
D.
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 as required by zoning, unless otherwise specified in the municipal
Zoning Ordinance.
E.
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.
F.
The following conditions shall apply when public and/or private recreation
trails are permitted within riparian buffers:
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 over
1/4 acre exists.
(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.e., woods) as listed in Tables
B-2, B-3 or B-4 in Appendix B[1] of this chapter.
[1]
Editor's Note: Said appendix 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 discharge 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 Upper Southampton rain gauge, seen in Table B-1 in Appendix
B.[2] 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: 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: Said appendix 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-2 in Appendix B.[3]
[3]
Editor's Note: Said appendix 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-3
in Appendix B.[4]
[4]
Editor's Note: Said appendix is included as an attachment to this chapter.
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 Appendix G.[1]
[1]
Editor's Note: Said appendix 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.[2]
[2]
Editor's Note: Said appendix is included as an attachment to this chapter.
B.
West Nile guidance requirements. All wet basin designs shall incorporate
biologic controls consistent with the West Nile guidance found in
Appendix H.[3]
[3]
Editor's Note: Said appendix is included as an attachment to this chapter.
