A.
Applicants proposing regulated activities in the Neshaminy Creek Watershed that do not fall under the exemption criteria shown in § 172-6 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 Part 1 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 Part 1.
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 Part 1, 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 Part 1 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.
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 Part 1.
(3)
For projects that add impervious area to a parcel, the total impervious
area on the parcel is subject to the requirements of this Part 1.
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 Part 1.
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 article 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 for detail on DIAs.[3]
[3]
Editor's Note: Said appendix is on file in the Township offices.
The updated stormwater ordinances and appendixes can also be found
online at www.lowermoreland.org.
(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 Ordinance Appendix E for a summary
description.[4]
[4]
Editor's Note: Said appendix is on file in the Township offices.
The updated stormwater ordinances and appendixes can also be found
online at www.lowermoreland.org.
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 Part 1.
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 Part 1 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.
Volume controls will mitigate increased runoff impacts, protect stream channel morphology, maintain groundwater recharge, and contribute to water quality improvements. Stormwater runoff volume control methods are based on the net change in runoff volume for the two-year storm event. Volume controls shall be implemented using the Design Storm Method in Subsection A or the Simplified Method in Subsection B below. For regulated activities equal to or less than one acre, this Part 1 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 § 172-14A.
(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 below. 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
| ||
|
Q = Runoff (inches)
| ||
|
Area = SWM Area (square feet)
| ||
[1]
Editor's Note: Said appendix is on file in the Township offices.
The updated stormwater ordinances and appendixes can also be found
online at www.lowermoreland.org.
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-inch runoff/12 inches) * impervious
surface (square feet)
|
(2)
At least the first inch of runoff volume from the new impervious
surfaces shall be permanently removed from the runoff flow, i.e.,
it shall not be released into the surface waters of the commonwealth.
The calculated volume shall be either reused, evapotranspired or infiltrated
through structural or nonstructural means.
|
Volume (cubic feet) = (1-inch runoff/12 inches) * impervious
surface (square feet)
|
(3)
Infiltration facilities should be designed to accommodate the first
half 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)
Stormwater management site area.
(a)
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}
|
(b)
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 Ordinance Appendix B[2] for guidance to assess the total protected area. For additional
reference, see Chapter 5, Section 5.4.1, of the PA BMP Manual.
[2]
Editor's Note: Said appendix is on file in the Township offices.
The updated stormwater ordinances and appendixes can also be found
online at www.lowermoreland.org.
(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: Said appendix is on file in the Township offices.
The updated stormwater ordinances and appendixes can also be found
online at www.lowermoreland.org.
(3)
Volume controls provided through nonstructural BMPs should be subtracted
from the required volume to determine the necessary structural BMPs.
|
Required
Volume Control (ft3)
|
-
|
Nonstructural
Volume Control (ft3)
|
=
|
Structural Volume
Requirement (ft3)
|
(4)
Calculate the volume controls provided through structural BMPs. Table
B-6 in Ordinance Appendix B[4] is recommended as guidance. See PA BMP Manual Chapter
6 for a description of the BMPs.
[4]
Editor's Note: Said appendix is on file in the Township offices.
The updated stormwater ordinances and appendixes can also be found
online at www.lowermoreland.org.
(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 per 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 per hour, the BMP design should use
0.2 inch per hour 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 on file in the Township offices.
The updated stormwater ordinances and appendixes can also be found
online at www.lowermoreland.org.
(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 50%.
(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 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 172-13. 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 172-13 below.
|
Table 172-13
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 (Ordinance Appendix D)[1] and in this section of this Part 1.
[1]
Editor's Note: Said appendix is on file in the Township offices.
The updated stormwater ordinances and appendixes can also be found
online at www.lowermoreland.org.
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 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: Said appendix is on file in the Township offices.
The updated stormwater ordinances and appendixes can also be found
online at www.lowermoreland.org.
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 Lower Moreland Township:
H.
Areas covered by a release rate map from an alternative approved
Act 167 Stormwater Management Plan.
I.
For the one-, two-, five-, ten-, twenty-five-, fifty-, and one-hundred-year
storms, the post-development peak discharge rates will follow the
applicable approved release rate maps. For any areas not shown on
the release rate maps, the post-development discharge rates shall
not exceed the predevelopment discharge rates.
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 a 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 Tables B-4 or B-7
in Appendix B[1] of this Part 1.
[1]
Editor's Note: Said appendix is on file in the Township offices.
The updated stormwater ordinances and appendixes can also be found
online at www.lowermoreland.org.
(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 172-14[2] 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 Part 1 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 Ordinance Appendix B.[3] 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.
|
Table 172-14
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
| |
[3]
Editor's Note: Said appendix is on file in the Township offices.
The updated stormwater ordinances and appendixes can also be found
online at www.lowermoreland.org.
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.
E.
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.[4]
[4]
Editor's Note: Said appendix is on file in the Township offices.
The updated stormwater ordinances and appendixes can also be found
online at www.lowermoreland.org.
F.
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.[5]
[5]
Editor's Note: Said appendix is on file in the Township offices.
The updated stormwater ordinances and appendixes can also be found
online at www.lowermoreland.org.
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 Part 1 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 Part 1 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: Said appendix is on file in the Township offices.
The updated stormwater ordinances and appendixes can also be found
online at www.lowermoreland.org.
(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 on file in the Township offices.
The updated stormwater ordinances and appendixes can also be found
online at www.lowermoreland.org.
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 on file in the Township offices.
The updated stormwater ordinances and appendixes can also be found
online at www.lowermoreland.org.
