[Ord. No. 1358, 9/12/2022]
1.
Applicants proposing regulated activities in the Neshaminy Creek watershed that do not fall under the exemption criteria shown in § 26-215 shall submit a stormwater management (SWM) site plan consistent with the Neshaminy Creek Watershed SWM plan to the Borough for review. The SWM 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 SWM site plan is required. No regulated activities shall commence until the Borough issues written approval of a SWM site plan, which demonstrates compliance with the requirements of this Part.
2.
SWM site plans approved by the Borough, in accordance with Part 2D,
shall be on-site throughout the duration of the regulated activity.
3.
The Borough may, after consultation with the DEP, 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.[1]
[1]
Editor's Note: See 35 P.S. § 691.1 et seq.
4.
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 Part 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, as amended and updated.
5.
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.
6.
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 Part.
7.
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., disconnected impervious
areas/DIAs) by directing runoff to pervious areas, wherever possible.
See Appendix F[2] for detail on DIAs.
[2]
Editor's Note: Said appendix is included as an attachment to this chapter.
C.
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 (PA BMP Manual) No. 363-0300-002(2006),
as amended and updated. See Appendix E for a summary description.
8.
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.
9.
The design of all facilities over karst shall include an evaluation
of measures to minimize the risk of adverse effects.
10.
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.
11.
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, using data from the Doylestown station (36-2221),
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. NOAA's Atlas 14 can
be accessed at http://hdsc.nws.noaa.gov/hdsc/pfds/orb/pa_pfds.html.
12.
13.
Various BMPs and their design standards are listed in the Pennsylvania
Stormwater Best Management Practices Manual (PA BMP Manual).
[Ord. No. 1358, 9/12/2022]
Approvals issued and actions taken under this Part 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.
[Ord. No. 1358, 9/12/2022]
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.
2.
The green infrastructure and low-impact development practices provided in the PA BMP Manual shall be utilized for all regulated activities wherever possible. Runoff volume controls shall be implemented using the design storm method in Subsection 2A or the simplified method in Subsection 2B below. For regulated activities equal to or less than one acre that do not require hydrologic routing to design the stormwater facilities, 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 and those that require hydrologic routing to design the stormwater facilities must use the design storm method.
A.
The design storm method (CG-1 in the PA BMP Manual) is applicable to any size of regulated activity. This method requires detailed modeling based on site conditions. For modeling assumptions, refer to § 26-218, Subsection 1.
(1)
Post-development total runoff cannot 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 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 below. The calculated volume shall be either reused,
evapotranspired, or infiltrated through structural or nonstructural
means. Runoff volume must be calculated for each land use type and
soil. The use of a weighted CN value for volume calculations is not
acceptable. Table B-3 in Appendix B[1] is available to guide a qualified professional and/or
an applicant to calculate the stormwater runoff volume.
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Soil Cover Complex Method:
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Step 1: Runoff (in) = Q = (P-0.2S)2/(P + 0.8S)
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|
Where:
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P
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=
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Two-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
|
=
|
Stormwater management area (square feet)
|
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
B.
The simplified method (CG-2 in the PA BMP Manual) is independent
of site conditions and should be used if the design storm method is
not followed. This method is not applicable to regulated activities
greater than one acre or for projects that require design of stormwater
storage facilities. For new impervious surfaces:
(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) = (two inches runoff/12 inches) * impervious
surface (square feet)
(2)
At least the first one inch of runoff volume from new impervious
surfaces shall be permanently removed from the runoff flow, i.e.,
it shall not be released into the surface waters of this commonwealth.
The calculated volume shall be removed through reuse, evapotranspiration,
or infiltration through structural or nonstructural means.
Volume (cubic feet) = (one inch runoff/12 inches) * impervious
surface (square feet)
(3)
Wherever possible, infiltration facilities should be designed to accommodate infiltration of the entire permanently removed runoff; however, in all cases where soils are suitable for infiltration based on the criteria of § 26-216, Subsection 2C(5) and (6), at least the first 1/2 inch (0.5) of the permanently removed runoff should be infiltrated.
(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 who is
required to submit a SWM site plan 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.
|
Stormwater Management Site Area =
Total Site Area (for both pre and post development conditions)
minus Protected Area
|
|
Natural resource areas should be calculated based upon the Borough's own natural resource protection ordinance requirements in the Zoning Ordinance. See Table B-2 in Appendix B for guidance to assess the total protected area. For additional reference, see Chapter 5 Section 5.4.1 of the PA BMP manual.
|
(2)
Calculate the volume controls provided through nonstructural
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
(feet3)
|
minus
|
Nonstructural Volume Control
(feet3)
|
=
|
Structural Volume Requirement
(feet3)
|
(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, seasonally high-water table,
or other limiting zone.
(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 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).
(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 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 50%.
(d)
If on-lot infiltration structures are proposed, it must be demonstrated
to the Borough 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 Borough.
[Ord. No. 1358, 9/12/2022]
1.
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 Table 26-217.1. 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 26-217.1 below:
|
Table 26-217.1
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
Pre-development
(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 (Appendix D)[1] and in this section.
[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
map at a reduced scale, and four other maps with zoomed-in extents,
are 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.
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. On-site
drainage facilities shall be designed to safely convey flows from
an undisturbed area of the property through the developed portion
of the site.
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 Borough.
[Ord. No. 1358, 9/12/2022]
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.[1]
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
B.
For 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, except
that 20% of the existing impervious surface area shall be considered
meadow in good condition in the model for existing conditions.
2.
Stormwater runoff peak discharges from all development sites with
a drainage area equal to or greater than one acre shall be calculated
using a generally accepted calculation technique that is based on
the NRCS soil cover complex method. Table 26-218.1 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 Borough may allow the use of the
rational method (Q=CIA) to estimate peak discharges from drainage
areas that contain less than one acre.
|
Where:
| ||
|
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
|
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 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. 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 26-218.1
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 1 acre, or as approved by the Borough and/or
municipal engineer
|
|
Other methods
|
Varies
|
Other computation methodologies approved by the Borough and/or
municipal engineer
|
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 3 corresponding
to the Doylestown rain gage, seen in Table 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. 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 based on Table
B-4 in Appendix B.
6.
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.
7.
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.
8.
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 Borough
is necessary.
9.
Outlet structures for stormwater management facilities shall be designed
to meet the performance standards of this Part using the generally
accepted hydraulic analysis technique or method of the Borough.
10.
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. 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 Borough 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.
[Ord. No. 1358, 9/12/2022]
1.
Hot spots.
A.
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.
B.
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.
West Nile Guidance Requirements. All wet basin designs shall incorporate
biologic controls consistent with the West Nile Guidance found in
Appendix H.
