A.
Applicants proposing regulated activities in the Neshaminy Creek Watershed that do not fall under the exemption criteria shown in § 431-106 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 (PA DEP), 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 § 431-303 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
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). 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
(PA DEP) No. 363-0300-002 (2006). See Appendix E for a summary description.[3]
[3]
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.
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, 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/.
M.
N.
Various BMPs and their design standards are listed in the Pennsylvania
Stormwater Best Management Practices Manual (PA 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.
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.
B.
Volume controls shall be implemented using the Design Storm Method in Subsection B(1) or the Simplified Method in Subsection B(2) 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.
(1)
Design Storm Method (any regulated activity). This method requires detailed modeling based on site conditions. For modeling assumptions refer to § 431-305A.
(a)
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.
(b)
The following applies in order to estimate the increased volume
of runoff for the two-year twenty-four-hour duration precipitation
event:
[1]
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.
[1]
Editor's Note: Appendix B is included as an attachment to
this chapter.
[2]
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)
| ||
(2)
Simplified Method (regulated activities less than or equal to
one acre):
(a)
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-inch runoff/12 inches) * impervious
surface (square feet)
|
(b)
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-inch runoff/12 inches) * impervious
surface (square feet)
|
(c)
Infiltration facilities should be designed to accommodate the
0.5 inch of the permanently removed runoff.
(d)
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.
(3)
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.
(a)
If natural resources exist on the site, the applicant is required
to submit a SWM site plan and shall 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
included in the stormwater management site area acreage used in determining
the volume controls.[2]
|
Stormwater Management Site Area =
Total Site Area (for both predevelopment and post-development
conditions) - Protected Area
|
[1]
Natural resource areas should be calculated based upon the municipality's
own natural resource protection ordinance. If no ordinance exists,
see Table B-2 in Appendix B for guidance to assess the total protected
area.[3] For additional reference see Chapter 5, Section 5.4.1
of the PA BMP Manual.
[3]
Editor's Note: Appendix B is included as an attachment to
this chapter.
(c)
Volume controls provided through nonstructural BMPs should be
subtracted from the required volume to determine the necessary structural
BMPs.
|
Required Volume Control (cubic feet)
|
—
|
Nonstructural Volume Control (cubic feet)
|
=
|
Structural Volume Requirement (cubic feet)
|
(e)
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).[6] Infiltration BMPs shall be constructed on soils that have
the following characteristics:
[1]
A minimum soil depth of 24 inches between the bottom of the
infiltration BMPs and the top of bedrock or seasonally high-water
table.
[2]
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 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 inch/hour, the BMP design should use 0.2 inch/hour
to ensure safe infiltration rates after construction).
[3]
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]
Editor's Note: Appendix B is included as an attachment to
this chapter.
(f)
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:
[1]
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.
[2]
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.
[3]
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%.
[4]
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.
[5]
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 one-hundred-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 Watershed.
A.
Standards for managing runoff from each subarea in the Neshaminy
Creek Watershed for the two-, five-, ten-, twenty-five-, fifty- and
one-hundred-year design storms are shown in Table 304.1. 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 304.1 following.
|
Table 304.1
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
5-year
10-year
25-year
50-year
100-year
|
1-year
5-year
10-year
25-year
50-year
100-year
| |
|
B
|
2-year
5-year
10-year
25-year
50-year
100-year
|
1-year
2-year
5-year
10-year
25-year
50-year
| |
|
C
|
2-year
5-year
10-year
25-year
50-year
100-year
|
2-year
5-year
10-year
25-year
50-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: 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
map at a reduced scale, and four other maps with zoomed-in extents,
are 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: 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 the Township:
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.e., woods) as listed in Table
B-4 or B-7 in Appendix B of this chapter.[1]
[1]
Editor's Note: Appendix B is included as an attachment to
this chapter.
(2)
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.
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. Table 305.1 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 gage, 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.
|
Table 305.1
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 the municipality
and/or Municipal Engineer.
| |
|
Other methods
|
Varies
|
Other computation methodologies approved by the municipality
and/or Municipal Engineer.
| |
[2]
Editor's Note: Appendix B is included as an attachment to
this chapter.
D.
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 coefficients ("C") for both existing and proposed conditions
for use in the Rational Method shall be consistent with Table B-7
in Appendix B.
G.
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 topsoiling or seeding. The proposed
condition "CN" or "C" shall increase by 5% to better reflect proposed
soil conditions.
H.
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.
I.
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.
J.
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.
