A.
All applicants proposing regulated activities in the Township which are not exempt based on criteria contained in § 134-5C of this chapter shall submit a stormwater management plan consistent with this chapter to the Township for review. These criteria shall apply to the total proposed development even if development is to take place in stages.
B.
All regulated activities shall include such measures as necessary
to:
(1)
Protect health, safety, and property;
(2)
Meet the water quality goals of this chapter by implementing measures
to:
(a)
Minimize disturbance to floodplains, wetlands, and wooded areas.
(b)
Create, maintain, repair or extend riparian buffers.
(c)
Avoid erosive flow conditions in natural flow pathways.
(d)
Minimize thermal impacts to waters of this commonwealth.
(e)
Disconnect impervious surfaces (i.e., disconnected impervious
areas, DIAs) by directing runoff to pervious areas, wherever possible.
(3)
Incorporate methods 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.
C.
The Township may, after consultation with the Department of Environmental
Protection (PADEP), approve measures for meeting the state water quality
requirements other than those in this chapter, provided that they
meet the minimum requirements of, and do not conflict with, state
law, including, but not limited to, the Clean Streams Law.[1]
[1]
Editor's Note: See 35 P.S. § 691.1 et seq.
D.
For all regulated earth-disturbance activities, erosion and sediment
(E&S) control best management practices (BMPs) shall be designed,
implemented, operated, and maintained during the regulated earth disturbance
activities (e.g., during construction) to meet the purposes and requirements
of this chapter and to meet all requirements under Title 25 of the
Pennsylvania Code and the Clean Streams Law.[2] Various BMPs and their design standards are listed in
the Erosion and Sediment Pollution Control Program Manual, No. 363-2134-008
(April 15, 2000), as amended and updated.
[2]
Editor's Note: See 35 P.S. § 691.1 et seq.
E.
No approval of any subdivision or land development plan, or issuance of any building, zoning, erosion and sedimentation control and grading permit, occupancy permit, or the commencement of any earth disturbance at a project site within the Township, shall proceed until the requirements of this chapter are met, including approval of a stormwater management plan under § 134-26 and a permit under PADEP regulations, where applicable.
I.
All best management practices (BMPs) shall conform to the design
criteria of this chapter and Pennsylvania Stormwater Management Practices
Manual, December, 2006, as amended.
J.
Techniques described in Appendix D[3] (Low-Impact Development Techniques) of this chapter are
encouraged because they reduce the costs of complying with the requirements
of this chapter and the state water quality requirements.
[3]
Editor's Note: Appendix D is included as an attachment to this chapter.
K.
Infiltration BMPs should be spread out, made as shallow as practicable,
and located to minimize the use of natural on-site infiltration features
while still meeting the other requirements of this chapter.
L.
Stormwater drainage systems shall be provided in order to permit
unimpeded flow along natural watercourses, except as modified by stormwater
management facilities designed to encourage infiltration, groundwater
recharge, and improved water quality.
M.
Existing points of concentrated drainage that discharge onto adjacent
property shall not be altered without written approval of the affected
property owner(s) and shall be subject to any applicable discharge
criteria specified in this chapter.
N.
Areas of existing sheet flow discharge shall be maintained wherever
possible. If sheet flow is proposed to be concentrated and discharged
onto adjacent property, the developer must document that adequate
downstream conveyance facilities exist to safely transport the concentrated
discharge, or otherwise prove that no erosion, sedimentation, flooding
or other harm will result from the concentrated discharge; and submit
written approval from the affected adjacent property owner(s).
O.
For all subdivision and land development applications, the tributary
area discharging drainage to any location along the site property
boundary shall not increase by more than 25% over the predevelopment
condition without written approval from the adjacent affected property
owner(s).
P.
Where a development site is traversed by watercourses, drainage easements
shall be provided conforming to the line of such watercourses. The
width of the easement shall be adequate to provide for the unimpeded
flow of stormwater runoff from the 100-year storm event. However,
in no case shall the easement be less than 30 feet in width. Terms
of the easement shall prohibit excavation, the placing of fill or
structures, and any alterations that may adversely affect the flow
of stormwater within any portion of the easement. Periodic maintenance
of the easement shall be required by the landowner to ensure proper
runoff conveyance. The developer shall retain the easement until such
time as one of the following is accomplished:
(1)
For subdivisions or land developments, the individual lot owner assumes
responsibility for the maintenance of the portion of their property
through which the easement passes. The record plan shall contain a
description of such easement(s) and notation indicating maintenance
responsibilities.
(2)
A homeowners' association or other legal entity approved by
the Township assumes responsibility for the maintenance of the development,
including the watercourse easement. The record plan shall contain
a description of such easement(s) and notation indicating the maintenance
responsibilities.
Q.
When it can be shown that, due to topographic conditions, natural
drainageways on the site cannot adequately provide for drainage, open
channels may be constructed conforming substantially to the line and
grade of such natural drainageways. Work within natural drainageways
shall be subject to approval by PA DEP through the joint permit application
process, or, where deemed appropriate by PA DEP, through the general
permit process.
R.
Any stormwater management facilities regulated by this chapter that
will be located in or adjacent to waters of the commonwealth or wetlands
shall be subject to approval by PA DEP through the joint permit application
process, or, where deemed appropriate by PA DEP, the general permit
process. When there is a question whether wetlands may be involved,
it is the responsibility of the developer or his agent to show that
the land in question cannot be classified as wetlands, otherwise approval
to work in the area must be obtained from PA DEP.
S.
Any stormwater management facilities regulated by this chapter that
would be located on state highway rights-of-way, or discharge stormwater
to facilities located within a state highway right-of-way, shall be
subject to approval by the Pennsylvania Department of Transportation
(PADOT).
T.
Site disturbance and impervious surface shall be minimized. Infiltrating
stormwater runoff through seepage beds, infiltration trenches, etc.,
shall be required, where soil conditions permit, to reduce the size
or eliminate the need for retention/detention facilities.
U.
Roof drains and sump pumps shall discharge to an infiltration bed,
natural watercourse, storm sewer system, or drainage swale (within
a stormwater easement). Roof drains and sump pumps shall not be connected
to a storm sewer or street drainage structure unless designed as part
of a stormwater management facility. In no case shall roof drains
or sump pumps be connected to a sanitary sewer or permitted to discharge
across a sidewalk, walkway, or to a street through the curb.
V.
All storm sewer inlets must be identified with a storm drain marker.
Storm drain markers shall be stainless steel affixed to the inlet
hood with adhesive, rivets, or bolts. (Marker may be bolted to the
grate in off-road locations). Marker shall have a minimum diameter
of 3 1/2 inches and include "No Dumping - Drains to Waterway"
and a fish symbol. Alternate designs/sizes may be used if approved
by the Township.
W.
Whenever a watercourse is located within a development site, it shall
remain open in the natural state and location and shall not be piped,
impeded, or altered (except for road crossings). It is the responsibility
of the developer to stabilize existing eroded stream/channel banks.
X.
Special requirements for watersheds draining to high quality (HQ)
and exceptional value (EV) waters. The temperature and quality of
water and streams that have been declared as exceptional value and
high quality are to be maintained as defined in Chapter 93, Water
Quality Standards, Title 25 Pennsylvania Department of Environmental
Protection Rules and Regulations. Temperature-sensitive BMPs and stormwater
conveyance systems are to be used and designed with storage pool areas
and supply outflow channels, and shaded with trees. This will require
modification of berms for permanent ponds and the relaxation of restrictions
on planting vegetation within the facilities, provided that capacity
for volumes and rate control is maintained. At a minimum, the southern
half on pond shorelines shall be planted with shade or canopy trees
within 10 feet of the pond shoreline. In conjunction with this requirement,
the maximum slope allowed on the berm area to be planted is 10 to
1. This will lessen the destabilization of berm soils due to root
growth. A long-term maintenance schedule and management plan for the
thermal-control BMPs is to be established and recorded for all development
sites.
Y.
All stormwater runoff shall be pretreated for water quality prior
to discharge to surface water or groundwater.
A.
Mapping of stormwater runoff peak rate districts. In order to implement
the provisions of this chapter, the Neshaminy Creek Watershed Stormwater
Management Plan, Tohickon Creek Watershed Stormwater Management Plan,
and East Branch Perkiomen Creek Watershed Stormwater Management Plan,
Hilltown Township is hereby divided into stormwater runoff peak rate
districts consistent with the plans. The boundaries of the districts
are indicated on the runoff peak rate district map that is available
for inspection at the municipal building. A large-scale boundary map
is included as Appendix F[1] for reference.
[1]
Editor's Note: Appendix F is included as an attachment to this chapter.
B.
The exact location of the stormwater runoff peak rate district boundary
as it applies to a given development site shall be determined by mapping
the boundaries using the two-foot or five-foot topographic contours
provided as part of the stormwater management plan developed for the
site in accordance with the Subdivision and Land Development Ordinance.[2] The district boundaries as originally drawn coincide with
topographic divides or, in certain instances, are drawn from the intersection
of the watercourse or a potential flow obstruction to the topographic
divide consistent with topography. The locations determined on the
stormwater management plan shall be reviewed and verified by the Township
Engineer.
C.
For the purposes of implementing the provisions of the Tohickon Creek
Watershed Stormwater Management Plan, East Branch Perkiomen Creek
District A, and Neshaminy Creek District A, design storm proposed
conditions shall be controlled to design storm existing conditions
as follows:
|
Design Storm Proposed Conditions
|
to
|
Design Storm Existing Conditions
|
|---|---|---|
|
2-year
|
1-year
| |
|
5-year
|
5-year
| |
|
10-year
|
10-year
| |
|
25-year
|
25-year
| |
|
50-year
|
50-year
| |
|
100-year
|
100-year
|
D.
For the purpose of implementing the provisions of the Neshaminy Creek
Watershed Stormwater Management Plan, District B, and East Branch
Perkiomen Creek-Watershed Stormwater Management Plan, District B,
design storm proposed conditions shall be controlled to design storm
existing conditions as follows:
|
Design Storm Proposed Conditions
|
to
|
Design Storm Existing Conditions
|
|---|---|---|
|
2-year
|
1-year
| |
|
5-year
|
2-year
| |
|
10-year
|
5-year
| |
|
25-year
|
10-year
| |
|
50-year
|
25-year
| |
|
100-year
|
50-year
|
E.
Within the Skippack Creek Watershed, design storm proposed conditions
shall be controlled to design storm existing conditions as follows:
|
Design Storm Proposed Conditions
|
to
|
Design Storm Existing Conditions
|
|---|---|---|
|
2-year
|
1-year
| |
|
5-year
|
5-year
| |
|
10-year
|
10-year
| |
|
25-year
|
25-year
| |
|
50-year
|
50-year
| |
|
100-year
|
100-year
|
A.
General. Proposed conditions peak rates of runoff from any regulated
activity shall meet the peak release rates of runoff prior to development
for the design storms specified on the Stormwater Management District
Watershed Map (Appendix F)[1] and § 134-12, of this chapter.
[1]
Editor's Note: Appendix F is included as an attachment to this chapter.
B.
District boundaries. The boundaries of the stormwater management
districts are shown on an official map, which is available for inspection
at the municipal office. A copy of the official map at a reduced scale
is included in the Appendix F[2] of this chapter. The exact location of stormwater management
district boundaries as they apply to a given development site shall
be determined by mapping the boundaries using topographic contours
at an appropriate level of detail, but in no case less than two-foot
intervals (or five-foot intervals as applicable). This information
shall be provided as part of the stormwater management plan.
[2]
Editor's Note: Appendix F is included as an attachment to this chapter.
C.
Sites located in more than one district. For a proposed development
site located within two or more release category subareas, the peak
discharge rate from any subarea shall be the predevelopment peak discharge
for that subarea multiplied by the applicable release rate. The calculated
peak discharges shall apply regardless of whether the grading plan
changes the drainage area by subarea.
D.
Off-site areas. Off-site areas that drain through a proposed development
site are not subject to release rate criteria when determining allowable
peak runoff rates or volume reduction. However, on-site drainage facilities
shall be designed to safely convey off-site flows through the development
site.
E.
Site areas. Where the site area to be impacted by a proposed development
activity differs significantly from the total site area as determined
by the municipality, the municipality may, but is not required to,
permit only the proposed impact area to be subject to the release
rate criteria. Nonimpacted or undisturbed areas that do not flow into,
or are bypassing, the stormwater management facilities shall not be
subject to the stormwater management district criteria.
F.
Stormwater conveyance corridor protection (riparian corridor preservation and vegetation). Runoff from developed areas of the site, including, but not limited to, areas of impervious surface, shall be managed through a series of riparian corridor vegetation facilities whenever possible. This will be accomplished in a manner satisfactory to the Township, utilizing the Pennsylvania Stormwater Best Management Practices Manual, December 2006, as amended. Riparian forested buffer: the priority goal of the riparian vegetation will be the reduction of thermal impacts on stormwater runoff associated with impervious areas, with a secondary goal being the protection of capacity of existing stormwater conveyance channels. These goals will be achieved through the use of design criteria in § 134-24 of this chapter and the Pennsylvania Stormwater Best Management Practices Manual, December 2006, as amended, and shall be in addition to any other Township ordinance provisions.
G.
Regional detention alternatives. For certain areas within the study
area, it may be more cost-effective to provide one control facility
for more than one development site than to provide an individual control
facility for each development site. The initiative and funding for
any regional runoff control alternatives are the responsibility of
prospective developers. The design of any regional control basins
must incorporate reasonable development of the entire upstream watershed.
The peak outflow of a regional basin would be determined on a case-by-case
basis using the hydrologic model of the watershed consistent with
protection of the downstream watershed areas. "Hydrologic model" refers
to the calibrated model as developed for the stormwater management
plan.
H.
A downstream hydraulic capacity analysis shall be performed to ascertain
whether runoff from a regulated activity could increase the potential
and/or frequency of flooding of existing downstream drainage facilities.
In the event that a downstream hydraulic capacity analysis determines
that the regulated activity will increase the potential and/or frequency
of flooding of existing downstream drainage facilities, runoff from
the regulated activity must be controlled to mitigate the potential
and/or frequency of flooding of existing downstream drainage facilities.
For regulated activities in conditional no-detention (direct discharge)
districts that seek to discharge stormwater runoff without detention
facilities, the following criteria shall be used for determining adequacy
of downstream drainage facilities for accepting increased peak flow
rates from the regulated activity:
(1)
Natural or man-made channels or swales must be able to convey the
increased runoff associated with a two-year return period event within
their banks at velocities consistent with protection of the channels
from erosion. Acceptable velocities shall be based upon criteria included
in the DEP Erosion and Sediment Pollution Control Program Manual.
(2)
Natural or man-made channels or swales must be able to convey the
increased twenty-five-year return period runoff without creating any
hazard to persons or property.
(3)
Culverts, bridges, storm sewers or any other facilities which must
pass or convey flows from the tributary area must be designed in accordance
with DEP, Chapter 105 regulations (if applicable) and, at a minimum,
pass the increased twenty-five-year return period runoff.
A.
For design and applicability of nonstructural BMPs refer to Chapter 5 of the Pennsylvania Stormwater Best Management Practices Manual, December 2006, as amended. For the nonstructural BMPs proposed, the applicant shall utilize and submit appropriate checklists included in Chapter 8, Section 8.8, of the Pennsylvania Stormwater Best Management Practices Manual, December 2006, as amended (refer to Appendix E)[1] to demonstrate that the BMPs are applicable to the project
and to determine the amount of volume or peak rate credit is applicable.
[1]
Editor's Note: Appendix E is included as an attachment to this chapter.
B.
The applicant shall demonstrate that regulated activities are designed
in the following sequence to minimize the increases in stormwater
runoff and impacts to water quality:
(1)
Prepare an Existing Resource and Site Analysis Map (ERSAM), showing
environmentally sensitive areas, including, but not limited to, steep
slopes, ponds, lakes, streams, wetlands, hydric soils, vernal pools,
floodplains, stream buffer zones, hydrologic soil Groups A, B, C,
and D, any existing recharge areas and any other requirements outlined
in the Subdivision and Land Development Ordinance.[2]
(2)
Prepare a draft project layout avoiding sensitive areas identified in § 134-14B(1) and minimizing total site earth disturbance as much as possible. The ratio of disturbed area to the entire site area and measures taken to minimize earth disturbance shall be included in the ERSAM.
(3)
Identify site specific existing conditions drainage areas, discharge
points, recharge areas, and hydrologic soil Groups A and B.
(8)
Determine the watershed management district within which the site
is located (Appendix F)[4] and conduct a predevelopment runoff analysis.
[4]
Editor's Note: Appendix F is included as an attachment to this chapter.
(9)
Prepare final project design to maintain predevelopment drainage
areas and discharge points, to minimize earth disturbance and impervious
surfaces, to reduce runoff to the maximum extent possible, and to
minimize the use of surface or point discharges.
(10)
Conduct a proposed conditions runoff analysis based on the final
design and to meet the release rate and in turn the overbank flow
and extreme event requirements.
(11)
Manage any remaining runoff through treatment prior to discharge,
as part of detention, biorentention, direct discharge or other structural
control.
A.
In addition to the performance standards and design criteria requirements of this chapter, adequate storage and treatment facilities must be provided to capture and treat stormwater runoff from developed or disturbed areas. The recharge volume computed under § 134-16 may be a component of the water quality volume if the applicant chooses to manage both components in a single facility. If the recharge volume is less than the water quality volume, the remaining water quality volume may be captured and treated by methods other than recharge/infiltration BMPs. The required water quality volume (WQv) is the storage capacity needed to capture and to treat a portion of stormwater runoff from the developed areas of the site produced from 90% of the average annual rainfall (P). The following calculation formula is to be used to determine the water quality storage volume, (WQv), in acre-feet of storage:
|
Equation: 134-15.1
|
|
WQv = [(P)(Rv)(A)]/12 (inches/foot)
|
|
WQv = water quality volume (acre-feet)
|
|
P = Rainfall amount equal to 90% of events producing
this rainfall (in.) - the volume of rainfall for 90% of the storm
events which produce runoff in the watershed annually.
|
|
P = 2.0 inches
|
|
A = Area of the project contributing to the water
quality BMP (acres)
|
|
Rv = Volume runoff coefficient 0.05 + 0.009(I) where
I is the percent of the area that is impervious surface (impervious
area ÷ total project area) x 100%
|
B.
Design of BMPs used for water quality control shall be in accordance
with design specifications outlined in the Pennsylvania Stormwater
Best Management Practices Manual, December 2006, as amended. The following
factors must be considered when evaluating the suitability of BMPs
used to control water quality at a given development site:
(1)
Total contributing drainage area.
(2)
Permeability and infiltration rate of the site soils.
(3)
Topographic slope and depth to bedrock.
(4)
Seasonal high-water table.
(5)
Proximity to building foundations and wellheads.
(6)
Erodibility of soils.
(7)
Land availability and configuration of the topography.
(8)
Peak discharge and required volume control.
(9)
Streambank erosion.
(10)
Efficiency of the BMPs to mitigate potential water quality problems.
(11)
Volume of runoff that will be effectively treated.
(12)
Nature of the pollutant being removed.
(13)
Maintenance requirements.
(14)
Creation/protection of aquatic and wildlife habitat.
(15)
Recreational value.
(16)
Enhancement of aesthetic and property value.
C.
To accomplish the above, the applicant shall submit original and
innovative designs for review. Such designs may achieve the water
quality objectives through a combination of BMPs (best management
practices).
A.
In calculating the volume of runoff that can be infiltrated at a
site, the following methodology shall be used:
|
Methodology:
|
|
Rev = [(S)(Rv)(A)]/12 (inches/foot)
|
|
Where:
| ||
|
Rev
|
=
|
Recharge volume (acre-feet)
|
|
S
|
=
|
Soil specific recharge factor (inches)
|
|
A
|
=
|
Site area contributing to the recharge facility (acres)
|
|
Rv
|
=
|
Volumetric runoff coefficient
|
Rv = 0.05 + 0.009 (I), where:
I = percent impervious area, and S = shall be obtained based upon
hydrologic soil group based upon the table below:
|
Hydrologic Soil Group
|
Soil Specific Recharge Factor (S)
|
|---|---|
|
A
|
0.38
|
|
B
|
0.25
|
|
C
|
0.13
|
|
D
|
0.06
|
If more than one hydrologic soil group (HSG) is present
at a site, a composite recharge volume shall be computed based upon
the proportion of total site area within each HSG.
B.
In selecting the appropriate infiltration BMPs, the applicant shall
consider the following:
(1)
Permeability and infiltration rate of the site soils.
(2)
Topographic slope and depth to bedrock.
(3)
Seasonal high-water table.
(4)
Proximity and elevation relative to building foundations, basements,
and wellheads. (Infiltration BMP should be located downgrade of these
structures.)
(5)
Erodibility of soils.
(6)
Land availability, configuration, and topography.
(7)
Peak discharge and required volume control.
(8)
Streambank erosion.
(9)
Efficiency of the BMPs to mitigate potential water quality problems.
(10)
Volume of runoff that will be effectively treated.
(11)
Nature of the pollutant being removed.
(12)
Maintenance requirements.
(13)
Creation/protection of aquatic and wildlife habitat.
(14)
Recreational value.
(15)
Enhancement of aesthetic and property value.
C.
A detailed soils evaluation of the project site shall be performed
to determine the suitability of infiltration BMPs. The evaluation
shall be performed by a qualified professional and, at a minimum,
address soil permeability, depth to bedrock, susceptibility to sinkhole
formation, and subgrade stability. The site testing shall include
adequate sampling of all portions of the site not limited by 100%
protected natural resources to determine areas of the property which
are suitable for infiltration BMPs. 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 watershed to determine general areas of suitability for
infiltration BMPs.
(2)
Provide field testing data at the elevation of the proposed infiltration
zone (bottom surface of infiltration facilities) to determine appropriate
percolation rate and/or hydraulic conductivity. Field testing guidelines
are identified in Appendix C.[1]
[1]
Editor's Note: Appendix C is included as an attachment to this chapter.
(3)
Design infiltration BMPs for required stormwater volume based on
field-determined capacity at the level of the proposed infiltration
surface.
D.
Soil characteristics:
(1)
Infiltration BMPs are particularly appropriate in hydrologic soil
Groups A and B, as described in the Natural Resources Conservation
Service Manual TR-55.
(2)
Low-erodibility factors ("K" factors) are preferred for the construction
of basins.
(3)
There must be a minimum depth of 12 inches between the bottom of
any facility and the seasonal high-water table and/or bedrock (limiting
zones). The minimum required separation between the limiting zone
may be increased, if required by the Township, should project specific
conditions exist (such as anticipated increased contaminants) which
dictate greater prevention of groundwater contamination.
(4)
There must be an infiltration and/or percolation rate sufficient
to accept the additional stormwater load, and to drain completely
as determined by field tests.
(5)
A minimum of 30 feet of undisturbed fill shall separate the foundation
wall of any building and an infiltration BMP.
(6)
The infiltration system shall have positive overflow controls to
prevent storage within one foot of the finished surface of grade.
(7)
Infiltration rates shall not be used in computing the storage volume
of the infiltration system.
(8)
Surface inflows shall be designed to prevent direct discharge of
sediment into the infiltration system.
E.
The recharge volume provided at the site shall be directed to the
most permeable HSG (hydrologic soil group) available, except where
other considerations apply such as in limestone geology.
F.
Any infiltration BMP shall be capable of completely infiltrating
the impounded water within 48 hours from the peak of the storm.
G.
Special attention shall be paid to proper installation of infiltration-oriented
stormwater management systems during construction, and to careful
avoidance of soil compaction during site development. During the period
of land disturbance, runoff shall be controlled prior to entering
any proposed infiltration area. Areas proposed for infiltration BMPs
shall be protected from sedimentation and compaction during the construction
phase, so as to maintain their maximum capacity.
H.
Extreme caution shall be exercised where salt or chloride would be
a pollutant since soils do little to filter this pollutant and it
may contaminate the groundwater. Extreme caution shall be exercised
where infiltration is proposed in source water protection areas. The
qualified design professional shall evaluate the possibility of groundwater
contamination from the proposed infiltration/recharge facility and
perform a hydrogeologic justification study if necessary. The infiltration
requirement in high quality/exceptional value waters shall be subject
to DEP's Title 25, Chapter 93, Antidegradation Regulations. The
Township may require the installation of an impermeable liner in BMP
and/or detention basins where the possibility of groundwater contamination
exists. A detailed hydrogeologic investigation may be required by
the Township.
I.
The plan must include safeguards against groundwater contamination
for uses which may cause groundwater contamination, should there be
a mishap or spill.
J.
Recharge/infiltration facilities shall be used in conjunction with
other innovative or traditional BMPs, stormwater control facilities,
and nonstructural stormwater management alternatives.
K.
Infiltration BMPs shall not be constructed nor receive runoff until
the entire contributory drainage area to the infiltration BMP has
achieved final stabilization.
L.
The requirements for volume control and infiltration are applied
to all disturbed areas, even if they are ultimately to be a pervious
or permeable land use such as lawn or other landscaped area, given
the extent to which development-related disturbance leads to compaction
of the soils and reduces their infiltrative capacity.
A.
Volume control. 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(1) or the Simplified Method in Subsection A(3) below. For regulated activities which propose one acre or less of impervious surface, 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 § 134-21.
(a)
Post-development total runoff shall not be increased from predevelopment
total runoff for all storms equal to or less than the two-year, twenty-four-hour
duration precipitation.
(b)
To estimate the increased volume of runoff (cubic feet) for
the two-year, twenty-four-hour duration precipitation event 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 in this section. Appendix A[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 (in.) = Q = (P - 0.2S)2/(P + 0.8S)
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|
Where:
|
|
P = 2-year rainfall (in.) P = 3.26 inches
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|
S = (1,000/CN) - 10, the potential maximum retention
(including initial abstraction, Ia)
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Step 2: Runoff volume (cubic feet) = Q x Area x 1/12
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Q = Runoff (in.)
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Area = SWM area (sq. ft.), as calculated per § 134-21
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[1]
Editor's Note: Appendix A is included as an attachment to this chapter.
(2)
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 determine the total acreage of protected
area where no disturbance is proposed. The acreage of the protected
area is subtracted from the total site area and not included in the
stormwater management site area (SWM area) acreage used in determining
the volume controls.
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Stormwater management site area = total site area (for both
predevelopment and post-development conditions) - protected area
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Natural resource areas must be calculated based upon
the natural resource protection requirements in the Hilltown Township
Zoning Ordinance.[2] Appendix A[3] provides for guidance to assess the total protected area. For additional reference refer to Chapter 5, Section 5.4.1, of the PA BMP manual.
[3]
Editor’s Note: Appendix A is included as an attachment to this chapter.
(b)
Calculate the volume controls provided through nonstructural
BMPs. Table A-5 in Appendix A[4] is recommended as guidance.
[4]
Editor's Note: Appendix A is included as an attachment to this chapter.
(c)
Volume controls provided through nonstructural BMPs are subtracted
from the required volume to determine the necessary structural BMPs.
|
Required Volume Control (ft.3)
|
-
|
Nonstructural Volume Control (ft.3)
|
=
|
Structural Volume Requirement (ft.3)
|
(d)
Calculate the volume controls provided through structural BMPs.
Table A-6 in Appendix A[5] is recommended as guidance. Refer PA BMP Manual, Chapter
6, for description of the BMPs.
[5]
Editor's Note: Appendix A is included as an attachment to this chapter.
(e)
Infiltration BMPs intended to receive runoff from developed
areas shall be selected based on the suitability of soils and site
conditions. Infiltration BMPs shall be constructed on soils that have
the following characteristics:
[1]
A minimum soil depth of 12 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 (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).
[3]
All infiltration facilities shall be designed to completely
infiltrate runoff volume within two days (48 hours) from the peak
of the design storm.
(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 addresses 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
that the soils are conducive to infiltrate on the lots identified.
[5]
An impermeable liner is required in detention/retention basins
where the possibility of groundwater contamination exists. A detailed
hydrogeologic investigation may be required by the Township.
(3)
Simplified Method (regulated activities less than one acre only).
(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 inches runoff/12 inches/ft.) x impervious
surface area (sq. ft.)
|
(b)
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.
|
Volume (cubic feet) = (1 inch runoff/12 inches/ft.) x impervious
surface area (sq. ft.)
|
(c)
Infiltration facilities shall be designed to accommodate the
first 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 between
24 hours and 72 hours.
The requirements of this section apply to all watersheds. In the event that individual watershed volume requirements listed in §§ 134-15, 134-16, and 134-17 of this chapter result in a required volume that differs from the volume-control requirements of this section, BMP design must comply with the greater volume control requirement, as applicable. The green infrastructure and low-impact development practices provided in the BMP Manual shall be utilized for all regulated activities wherever possible. Water volume controls shall be implemented using the Design Storm Method in Subsection A or the Simplified Method in Subsection B below. For regulated activity areas of one acre or less 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 on applicability of the analytical procedures associated with each methodology and other factors.
A.
The Design Storm Method (CG-1 in the BMP Manual) is applicable to
any size of regulated activity. This method requires detailed modeling
based on site conditions.
(1)
Do not increase the post-development total runoff volume for all
storms equal to or less than the two-year, twenty-four-hour duration
precipitation.
B.
The Simplified Method (CG-2 in the BMP Manual) provided below is
independent of site conditions and shall be used if the Design Storm
Method is not followed. This method is not applicable to regulated
activities of one acre or greater or for projects that require design
of stormwater storage facilities. For new impervious surfaces:
(1)
Stormwater facilities shall capture at least the first two inches
of runoff from all new impervious surfaces.
(2)
At least the first one inch of runoff 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. Removal
options include reuse, evaporation, transpiration, and infiltration.
(3)
Wherever possible, infiltration facilities shall be designed to accommodate
infiltration of the entire permanently removed runoff; however, in
all cases, at least the first 0.5 inch of the permanently removed
runoff must be infiltrated.
A.
To minimize the impact of stormwater runoff on downstream stream
bank erosion, a BMP must be designed to detain the proposed conditions
two-year, twenty-four-hour design storm to the existing conditions
one-year flow using the SCS Type II distribution. Additionally, provisions
shall be made (such as adding a small orifice at the bottom of the
outlet structure) so that the proposed conditions one-year storm takes
a minimum 24 hours to drain from the facility from a point where the
maximum volume of water from the one-year storm is captured (i.e.,
the maximum water surface elevation is achieved in the facility).
B.
Release of water may begin at the start of the storm (i.e., the invert
of the water quality orifice is at the invert of the facility). The
design of the facility shall minimize clogging and sedimentation.
Orifices smaller than three inches in diameter are not recommended.
However, if the design engineer can verify that the smaller orifice
is protected from clogging by use of trash racks, etc., smaller orifices
may be permitted. Trash racks are required for any primary orifice.
C.
Whenever a watercourse is located within a development site, it shall
remain open in the natural state and location and shall not be piped,
impeded, or altered (except for road crossings). It is the responsibility
of the developer to restore existing eroded stream/channel banks within
a subdivision or land development site and obtain all permits necessary
from PADEP, to do so. The developer must submit pictorial documentation
of existing stream/channel banks to determine whether existing banks
must be stabilized.
A.
Stormwater runoff which may result from regulated activities listed in § 134-4 shall be controlled by permanent stormwater runoff control measures that will provide the required standards of this chapter. The methods of stormwater control or best management practices (BMPs) which may be used to meet the required standards are described in this chapter and Pennsylvania Stormwater Best Management Practices Manual, December 2006, as amended. The choice of BMPs is not limited to the ones appearing in this chapter and the Pennsylvania Stormwater Best Management Practices Manual, however, any selected BMP must meet or exceed the runoff peak rate requirements of this chapter for the applicable hydrologic district.
B.
Any stormwater facility located on state highway rights-of-way shall
be subject to approval by the Pennsylvania Department of Transportation.
C.
Any stormwater management facility designed to store runoff and requiring
a berm or earthen embankment required or regulated by this chapter
shall be designed to provide an emergency spillway to handle flow
up to and including the 100-year post-development conditions. The
height of embankment must be set as to provide a minimum 1.0 foot
of freeboard above the maximum pool elevation computed when the facility
functions for the 100-year post-development inflow.
D.
Emergency spillways discharging over embankment fill shall be constructed
of reinforced concrete checker blocks to protect the berm against
erosion. The checker block lining shall extend to the toe of the fill
slope on the outside of the berm and shall extend to an elevation
three feet below the spillway crest on the inside of the berm.
E.
Vegetated spillways may be utilized for spillways constructed entirely
on undisturbed ground (i.e., not discharging over fill) if the designer
can demonstrate that flow velocities through the spillways will not
cause erosion of the spillway. A dense cover of vegetation shall be
rapidly established in such spillways by sodding or seeding with a
geotextile anchor. Such a vegetated spillway must be stabilized before
runoff is directed to the basin.
F.
Should any stormwater management facility require a dam safety permit
under PA DEP Chapter 105, the facility shall be designed in accordance
with Chapter 105 and meet the regulations of Chapter 105 concerning
dam safety which may be required to pass storms larger than 100-year
event.
G.
Stormwater management facility outlet piping shall be Class III reinforced
O-ring concrete pipe. A minimum of one concrete antiseep collar shall
be required. Precast collars shall have a minimum thickness of eight
inches; field-poured collars shall have a minimum thickness of 12
inches. Collars may not be installed within two feet of pipe joints.
Collars must be designed to project a minimum of two feet around the
perimeter of the pipe. Maximum collar spacing is 14 times the design
projection around the perimeter.
H.
No stone gabion baskets may be used in the construction of stormwater
management facilities.
I.
Retention/detention basins:
(1)
Pipe outlet shall permit complete drainage of all detained water,
unless the stormwater management facility is designed as a retention
basin/pond or provides for stormwater renovation with constructed
wetlands.
(2)
When a detention basin is not designed as a stormwater management
constructed wetland, the stormwater management facility shall be planted
with low-maintenance grass or substitute satisfactory to the Township.
(3)
All detention basin bottoms intended to be maintained as lawn (e.g.,
recreational fields) shall be designed with a minimum grade of 2%.
As an alternative, the detention basin may be designed with a minimum
grade of 1% with underdrains to ensure complete drainage.
(4)
Visual impact.
(a)
To minimize the visual impact of detention basins, the detention
basin shall be designed to avoid the need for safety fencing. To meet
this requirement, basins shall be designed to the following specifications:
[1]
Maximum depth of detained runoff shall be 24 inches for a two-year
or ten-year storm event.
[2]
Maximum depth of detained runoff shall be 36 inches for a 100-year
storm event.
[3]
Interior slopes shall not be steeper than a ratio of 4:1 horizontal
to vertical.
[4]
Ponded water shall never exceed a depth of 24 inches for more
than four hours.
(b)
Depths and slopes may be exceeded by permission of the Township
on a case-by-case basis if lot runoff, topography and/or existing
downstream systems make the required pond area unreasonably large.
In such case, fence and landscape screens will be required.
(5)
An access ramp of 10:1, 10 feet wide, shall be provided to allow
maintenance equipment to reach the basin floor. The ramp shall coincide
with the required gate if fencing is needed.
(6)
When required by the Township, fencing shall provide a suitable barrier
at least four feet in height of material approved by the Township,
such as split-rail fencing with wire backing. Access to the basin
shall be provided by a gate or gates having a total opening of at
least 10 feet at such location(s) as to permit ready access to the
detention basin with maintenance equipment.
(7)
Landscaping:
(a)
The perimeter berms and embankments of retention/detention basins,
including wet ponds, and artificial wetland stormwater management
facilities, shall be designed to create a natural appearance and reduce
future maintenance requirements. Landscaping shall include a mixture
of native tall grasses and perennial plants, ground cover, shrubs,
and trees to eliminate the necessity of periodic mowing.
(b)
Artificial wetland basins shall be designed pursuant to requirements
of the Pennsylvania Stormwater Best Management Practices Manual, December
2006, as amended. Plant material and arrangement shall be subject
to approval of the Township.
(c)
The perimeter of the retention/detention basin shall be landscaped
with a mixture of deciduous trees, evergreens, and shrubs arranged
in an informal manner. Retention basin (wet ponds) and artificial
wetland basin landscaping shall be designed to create a natural appearance.
Minimum plant material shall include the following per 100 linear
feet of basin perimeter measured at the 100-year reoccurrence stormwater
elevation:
(d)
Retention/detention basin landscaping design is subject to approval
by the Township.
(8)
Retaining walls shall not be specified for use within the 100-year
water surface elevation area of any detention/retention facility or
as part of any embankment or cut slope that is appurtenant to the
construction of a detention/retention facility.
(9)
The developer shall provide written assurance, satisfactory to the
Township, that the retention/detention basin will be properly maintained.
Such assurances shall be in a form to act as a covenant that will
run with the land, and shall provide Township maintenance at the cost
of the landowner in case of default, and further provide for assessment
of costs and penalties in case of default.
(10)
As an alternate to the above subsection, the Township may, at
its own option, assume responsibility of the basin and may accept
dedication of the basin by the developer. If the retention/detention
basin is dedicated or offered to the Township for long-term maintenance,
the following regulations shall apply:
(a)
The dedicated area shall include the entire ponded area for
the 100-year storm event and the outside slope at the berm.
(c)
If fencing is necessary, the basin design shall provide a level
area (2% slope) eight feet in width on both the inside and outside
of the fence, along the entire length of the fence for proper access
by Township maintenance equipment. The total width of this generally
level area shall be at least 16 feet.
(d)
The developer shall provide for the special financial burden
the Township will be accepting if the Township accepts the detention
basin maintenance. To help mitigate this future financial burden,
the developer shall contribute to the Township a cash payment in the
amount established by resolution of the Board of Supervisors for any
detention/retention basin site or area dedicated to the Township and
being accepted by the Township.
(11)
Basin berm construction requirements:
(a)
Site preparation. Areas under the embankment and any structural
works shall be cleared, grubbed, and the topsoil stripped to remove
the trees, vegetation, roots or other objectionable material. In order
to facilitate clean-out and restoration, the pool area will be cleared
of all brush and excess trees.
(b)
Cut off trench. A cut-off trench will be excavated along the
centerline dam on earth fill embankments. The minimum depth shall
be two feet. The cut-off trench shall extend up both abutments to
the riser crest elevation. The minimum bottom width shall be eight
feet but wide enough to permit operation of compaction equipment.
The side slopes shall be no steeper than 1:1. Compaction requirements
shall be the same as those for the embankment. The trench shall be
kept free from standing water during the backfilling operations.
(c)
Embankment.
[1]
The fill material shall be taken from the selected borrow areas.
It shall be free of roots, wood vegetation, oversized stones, rocks
or other objectionable material. Areas on which fill is to be placed
shall be scarified prior to placement of fill.
[2]
The fill material should contain sufficient moisture so that
it can be formed by hand into a ball without crumbling. If water can
be squeezed out of the ball, it is too wet for proper compaction.
[3]
Fill material will be placed in six- to eight-inch layers and
shall be continuous over the entire length of the fill. Fill material
must be compacted to a minimum of 95% of Modified Proctor Density
as established by ASTM D-1557. Compaction testing by a certified soils
engineer/geologist must be completed as directed by the Township Engineer
to verify adequate compaction has been achieved.
(12)
Bridge and culvert design. Any proposed bridge or culvert to
convey flow within a perennial or intermittent stream shall be designed
in accordance with the following principals:
(a)
Culverts and bridges shall be designed with an open bottom to
maintain natural sediment transport and bed roughness, avoiding acceleration
of water velocity above the natural (preexisting) condition. Rock
(riprap) lining (native material if possible) shall be installed within
the culvert as needed to prevent erosion within the structure. Approximate
top-of-rock lining must be at the level of the existing stream bottom
so as to maintain normal water level and unimpeded movement of native
animal species.
(b)
Bottom of opening shall be designed to match the bankfull channel
condition in terms of width and depth. The cross-sectional area of
the bankfull channel (measured at a reference location upstream of
the structure) shall be matched with area in the crossing structure.
(c)
Above the bankfull elevation, the width shall increase a minimum
of 30% to disperse the energy of higher flow volumes and avoid undermining
of the supporting structure by secondary currents.
(d)
The total cross-sectional area of the structure opening must
be equal to or greater than the flood-prone area (cross-sectional
stream area at a depth of twice the maximum bankfull depth, measured
at a reference location upstream of the structure). The flood-prone
area is approximately equal to the area flooded by a fifty-year return
flood.
(e)
All bridges, culverts, and drainage channels shall be designed
to convey a flow rate equal to a 100-year, twenty-four-hour storm
as defined by the U.S. Department of Agriculture, Natural Resource
Conservation Service (previously SCS), Technical Release No. 55. All
bridges and culverts shall be designed to convey the 100-year design
storm without increasing the extent and depth of the 100-year floodplain.
J.
Any facilities that constitute water obstructions (e.g., culverts,
bridges, outfalls, or stream enclosures), and any work involving wetlands,
as directed in PA DEP Chapter 105 regulations (as amended or replaced
from time to time by PA DEP), shall be designed in accordance with
Chapter 105 and will require a permit from PA DEP. Any other drainage
conveyance facility that doesn't fall under Chapter 105 regulations
must be able to convey, without damage to the drainage structure or
roadway, runoff from the twenty-five-year design storm with a minimum
1.0 foot of freeboard measured below the lowest point along the top
of the roadway. Roadway crossings located within designated floodplain
areas must be able to convey runoff from a 100-year design storm with
a minimum 1.0 foot of freeboard measured below the lowest point along
surface of the roadway. Any facility that constitutes a dam as defined
in PA DEP Chapter 105 regulations may require a permit under dam safety
regulations. Any facility located within a PA DOT right-of-way must
meet PA DOT minimum design standards and permit submission requirements.
K.
Any drainage conveyance facility and/or channel that doesn't
fall under Chapter 105 regulations must be able to convey, without
damage to the drainage structure or roadway, runoff from the twenty-five-year
design storm. Conveyance facilities to or exiting from stormwater
management facilities shall be designed to convey the design flow
to or from that structure. Roadway crossings located within designated
floodplain areas must be able to convey runoff from a 100-year design
storm. Any facility located within a PA DOT right-of-way must meet
PA DOT minimum design standards and permit submission requirements.
L.
Adequate erosion protection shall be provided along all open channels,
and at all points of discharge.
M.
Except for drainage at roadway stream crossings, pipe or artificial
swale discharge shall be set back 75 feet from a receiving waterway,
and the pipe discharge shall be diffused or spread out to reduce and
eliminate high-velocity discharges to the impacted ground surface.
The conveyance mechanism shall minimize disturbance and velocity of
discharge.
N.
Developers shall utilize BMPs to provide for additional water quality
improvement and groundwater recharge.
(1)
In evaluating potential stormwater BMPs, the order of preference
is as follows:
(2)
Infiltration BMPs shall be utilized unless the applicant can demonstrate
use of infiltration techniques is not feasible due to site conditions
based upon site specific soil testing. Vegetated swales, wetlands
or artificial wetlands and bioretention structures shall be utilized
wherever possible if infiltration BMPs are deemed unfeasible. BMP
techniques can and should be used in conjunction with each other (e.g.,
vegetated swales with infiltration or retention facilities).
(3)
Infiltration best management practices (BMPs). Infiltration devices
shall be selected based upon suitability of soils and site conditions.
Soil infiltration tests shall be performed on all sites to determine
suitability of the site for infiltration BMPs. Testing shall include
evaluation of selected soil horizons by soil probes, deep pits and/or
percolation measurements. The soil infiltration rate of discharge
from the infiltration area being used in the proposed design shall
be based on these measurements. Infiltration BMPs shall be designed
in accordance with the design criteria and specifications in Section
5 of the Pennsylvania Stormwater Best Management Practices Manual,
December 2006, as amended, and shall meet the following minimum requirements:
(a)
Infiltration BMPs shall be constructed on soils with a minimum
depth of 24 inches between the intended bottom of the facility and
the seasonal high-water table and/or bedrock (limiting zones).
(b)
Infiltration BMPs intended to receive rooftop runoff shall include
appropriate measures such as leaf traps and cleanouts to prevent clogging
by vegetation.
(c)
Where direct discharge is permitted under the requirements of § 134-12, infiltration BMPs shall be designed to provide adequate storage to accommodate the post-development first flush design storm (one-year twenty-four-hour storm) volume with outlet and overflow controls to convey runoff larger than the first flush design storm volume safely to a natural outfall.
(d)
In areas where runoff release rates are specified under the requirements of § 134-12, regardless of the specified release rate percentage, if infiltration BMPs are intended, they shall be designed to, as a minimum:
[1]
Provide adequate storage to accommodate the volume of runoff
calculated as the difference between the predevelopment runoff volume
and post-development runoff volume based on the 100-year design storm.
[2]
Control the post-development peak rate of runoff to the predevelopment peak rate of runoff for all design storms identified in § 134-12 of this chapter.
[3]
Provide an overflow or spillway that safely permits the passing
of runoff greater than that occurring during the 100-year design storm.
(4)
Non-infiltration facilities used as best management practices (BMPs).
All facilities shall be designed in accordance with the design criteria
and specifications in the Pennsylvania Stormwater Best Management
Practices Manual, December 2006, as amended. This design shall be
in particular coordination with Section 8, Descriptions of Selected
Best Management Practices.
(5)
Artificial wetlands, wet ponds, and bioretention structures.
(a)
Wet pond BMPs shall meet the following requirements:
[1]
Wet ponds shall be constructed on hydric or wet soils and/or
soils which have an infiltration rate of less than 0.2 inch/hour.
[2]
A minimum drainage area of five acres shall be directed to the
pond unless a source of recharge is utilized such as a natural spring
or well.
[3]
The length of the pond between the inflow and outlet points
shall be maximized. In addition, an irregular shoreline shall be provided.
By maximizing the flow length through the pond and providing an irregular
shoreline, the greatest water quality benefit will be achieved by
minimizing short circuiting of runoff flowing through the pond.
[4]
A shallow forebay shall be provided adjacent to all inflow areas.
The forebay shall be planted as a marsh with emergent wetland vegetation.
The forebay serves to enhance sediment trapping and pollutant removal,
as well as concentrating accumulated sediment in an area where it
can be readily removed.
[5]
All wet ponds shall be designed with public safety as a primary
concern. An aquatic safety bench shall be provided around the perimeter
of the permanent pool. The depth of the bench shall be a maximum of
one foot for a width of at least three feet. A 3:1 slope shall lead
from the edge of the safety bench toward the deep-water portion of
the pond. At least 15 feet of 3:1 slope shall be provided from the
edge of the safety bench. Slopes in the remainder of the pond below
the permanent pool elevation shall be a maximum of 2:1.
[6]
The perimeter slope above the permanent pool shall have a maximum
slope of 4:1 for a distance of at least 20 feet. The remaining areas
above the permanent pool shall have a maximum slope of 3:1.
[7]
Wet ponds shall have a deep water zone to encourage gravity
settling of suspended fines and prevent stagnation and possible eutrophication.
[8]
Wet ponds shall be capable of being substantially drained by
gravity flow. Where possible, wet ponds shall be equipped with a manually
operated drain that can be secured against unauthorized operation.
[9]
A planting plan shall be developed for the wet pond, showing
all proposed aquatic, emergent, and upland plantings.
[10]
Wet ponds shall be designed to discourage use
by Canada geese. Techniques employed shall include the following:
[a]
Elimination of straight shorelines, islands, and
peninsulas;
[b]
Placement of walking paths (where applicable) along
the shoreline;
[c]
Placement of grassed areas (i.e., playing fields)
at least 450 feet from the water surface;
[d]
Vegetative barriers;
[e]
Rock barriers;
[f]
Installation of tall trees within 10 feet of the
water surface;
[g]
Use of ground covers not palatable to Canada geese.
(b)
Artificial wetland BMPs shall meet the following requirements:
[1]
Artificial wetlands shall be constructed on hydric or wet soils
and/or soils which have an infiltration rate of less than 0.2 inch/hour.
[2]
Runoff entering artificial wetlands shall be filtered through
a sediment removal device before entering the wetland.
[3]
A planting plan shall be developed for the artificial wetland
showing all proposed aquatic, emergent, and upland plantings. The
planting plan shall be developed to provide a diversity of species
resulting in a dense stand of wetland vegetation.
[4]
At least 75% of the surface area of the wetland shall be developed
as a shallow water emergent wetland, with a water depth of less than
12 inches. The reminder shall be constructed as open water with depths
between two feet and four feet.
(6)
Minimum first flush detention/dual purpose BMPs.
(a)
Minimum first flush detention/dual purpose detention basin BMPs
shall be designed to meet the following requirements:
[1]
Post-development runoff from a water quality storm (a one-year,
twenty-four-hour event) shall be released over a minimum period of
24 hours.
[2]
Two stage basins shall be utilized where first flush detention
will be employed for water quality and conventional detention used
for peak rate control of storms exceeding the one-year, twenty-four-hour
event.
[3]
Two-stage basins shall be constructed so that the lower part
of the basin is graded to detain stormwater from the water quality
storm, and the remainder of the basin graded as a flat overbank area
to provide storage only for the larger, less frequent storm events.
The overbank area is encouraged to be developed as an active or passive
recreational area.
[4]
The area inundated by the water quality storm is encouraged
to be maintained as a wetland environment, which will increase the
water quality benefits of the first flush/dual purpose detention basin,
and will prevent the need for mowing of a frequently saturated area.
O.
All stormwater control facility designs shall conform to the applicable
standards and specifications of the following governmental and institutional
agencies:
(1)
American Society of Testing and Materials (ASTM).
(2)
Asphalt Institute (AI).
(3)
Bucks Conservation District (BCD).
(4)
Federal Highway Administration (FHWA).
(5)
National Crushed Stone Association (NCSA).
(6)
National Sand and Gravel Association (NSGA).
(7)
Pennsylvania Department of Environmental Protection (PADEP).
(8)
Pennsylvania Department of Transportation (PADOT).
(9)
U.S. Department of Agriculture, Natural Resources Conservation Service,
Pennsylvania (USDA, NRCS, PA).
P.
If special geological hazards or soil conditions, such as carbonate
derived soils, are identified on the site, the developer's professional
engineer shall consider the effect of proposed stormwater management
measures on these conditions. In such cases, the municipality shall
require an in-depth report by a registered professional geologist.
Q.
The design of all stormwater management facilities shall incorporate
sound engineering principles and practices. Guidelines established
by the Pennsylvania Stormwater Best Management Practices Manual, December
2006, as amended, shall be utilized in determining stormwater management
facility design except where specifically modified by this chapter
or other municipal ordinance. The municipality shall reserve the right
to disapprove any design that would result in the occupancy or continuation
of an adverse hydrologic or hydraulic condition within the watershed.
Stormwater runoff from all development sites shall be calculated
using either the Rational Method or a Soil Cover Complex Methodology.
A.
Any stormwater runoff calculations shall use generally accepted calculation
technique that is based on the NRCS Soil Cover Complex Method. Table
134-21 summarizes acceptable computation methods. Method must be selected
by the applicant based on the individual limitations and suitability
of each method for a particular site. The Rational Method may be used
to estimate peak discharges from drainage areas that contain less
than 200 acres. The Rational Method is recommended for drainage areas
under 100 acres.
|
Table 134-21. 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 Kuiching (1889)
|
For sites less than 200 acres or as approved by the Municipal
Engineer
|
|
Other methods
|
Various
|
Other computation methodologies approved by the Municipal Engineer
|
B.
All calculations consistent with this chapter using the Soil Cover
Complex Method shall use the appropriate design rainfall depths for
the various return period storms according to the National Oceanic
and Atmospheric Administration (NOAA) Atlas 14 rain data corresponding
to the Doylestown rain gauge. The SCS Type 11 rainfall curve from
NOAA is found on Figure A-1 in Appendix A[1] of this chapter. This data may also be directly retrieved
from the NOAA Atlas 14 website: https://hdsc.nws.noaa.gov/hdsc/pfds/pfds_map_cont.html?bkmrk=pa.
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.
[1]
Editor's Note: Appendix A is included as an attachment to this chapter.
C.
Runoff curve numbers (CN) for both existing and proposed conditions
to be used in the Soil Cover Complex Method shall be obtained from
Table A-4 in Appendix A[2] of this chapter.
[2]
Editor's Note: Appendix A 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 (Figure
A-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 coefficients (C) for both existing and proposed conditions
for use in the Rational method shall be obtained from Table A-7 in
Appendix A[3] of this chapter.
[3]
Editor's Note: Appendix A is included as an attachment to this chapter.
F.
For the purposes of existing conditions flow rate determination,
undeveloped land and existing impervious surfaces shall be considered
as meadow in good condition, unless the natural ground cover generates
a lower curve number (CN) or rational C value (e.g., forest), as listed
in Table A-4 or A-7 in Appendix A[4] of this chapter. Wooded areas shall use a ground cover
of woods in good condition. An area shall be considered wooded if
there is a contiguous canopy of trees existing over an area of 1/4
acre or more.
[4]
Editor's Note: Appendix A is included as an attachment to this chapter.
G.
Where uniform flow is anticipated, the Manning equation shall be
used for hydraulic computations, and to determine the capacity of
open channels, pipes, and storm sewers. Values for Manning's
roughness coefficient (n) shall be consistent with Table A-8 in Appendix
A[5] of this chapter.
[5]
Editor's Note: Appendix A is included as an attachment to this chapter.
H.
Outlet structures for stormwater management facilities shall be designed
to meet the performance standards of this chapter using any generally
accepted hydraulic analysis technique or method accepted by the Township.
I.
The design of any stormwater management 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. The design storm hydrograph shall be computed using
a calculation method that produces a full hydrograph.
J.
The Township has the authority to require that computed existing
runoff rates be reconciled with field observations and conditions.
If the design professional engineer can substantiate through actual
physical calibration that more appropriate runoff and time-of-concentration
values should be utilized at a particular site, then appropriate variations
may be made upon review and recommendations of the Municipal Engineer.
Calibration shall require detailed gauge and rainfall data for the
particular site in question.
K.
Township reserves the right to require stormwater management calculations
and analyses related to existing downstream drainage facilities, in
order to ascertain whether the stormwater management design for the
regulated activity adversely effects existing downstream facilities
by increasing flooding potential and/or frequency from the predevelopment
condition.
A.
Whenever vegetation and topography are to be disturbed, such activity must be in conformance with Chapter 102, Title 25, Rules and Regulations, Part 1, Commonwealth of Pennsylvania, Department of Environmental Protection, Subpart C, Protection of Natural Resources, Article II, Water Resources, Chapter 102, Erosion Control, and in accordance with the Bucks County Conservation District and the standards and specifications of the Township.
B.
All activities or projects involving earth disturbance of 5,000 square
feet or greater require erosion and sediment control plan adequacy
approval from Bucks County Conservation District. It is the applicant's
responsibility to make submission to Bucks County Conservation District
for review and approval of the erosion and sediment control plan.
A copy of erosion and sediment control plan adequacy approval correspondence
from Bucks County Conservation District must be received by the Township
prior to commencing earth-disturbance activities and issuance of building/zoning
permits or approvals by the Township.
C.
No regulated earth-disturbance activities within the Township shall
commence until approval by the Township of an erosion and sediment
control plan for construction activities.
D.
Pursuant to 25 Pa. Code Chapter 92, a PADEP NPDES construction activities permit is required for regulated earth-disturbance activities.
E.
Evidence of any necessary permit(s) for regulated earth-disturbance
activities from the appropriate PADEP regional office or County Conservation
District must be submitted to the Township.
F.
A copy of the erosion and sediment control plan and any required
permit, as required by PADEP regulations shall be available at the
project site at all times.
G.
Additional erosion and sedimentation control design standards and
criteria that must be applied where infiltration BMPs are proposed
include the following:
(1)
Areas proposed for infiltration BMPs shall be protected from sedimentation
and compaction during the construction phase, so as to maintain their
maximum infiltration capacity.
(2)
Infiltration BMPs shall not be constructed nor receive runoff until
the entire contributory drainage area to the infiltration BMP has
received final stabilization.
H.
Peak discharges and discharge volumes from the site shall comply
with the appropriate sections above, with the following additions:
(1)
For purposes of calculating required detention storage during land disturbance, peak discharges rates and volumes shall be calculated based upon the runoff coefficients for bare soils during the maximum period and extent of disturbance which shall be clearing, grading, and impervious surface installation indicated on the development plan. Controls shall insure that the difference in volume and rate of peak discharges and volumes before disturbance and during shall not exceed those peak discharges and discharge volumes required in §§ 134-12, 134-17 and 134-18 of this chapter. Detention storage during the period of land disturbance and prior to establishment of permanent cover may require additional facilities on a temporary basis. Such measures shall be located so as to preserve the natural soil infiltration capacities of the planned infiltration bed areas. Calculations based on the above parameters must be submitted to verify during construction runoff rate does not exceed predevelopment runoff rate for the one-year frequency through 100-year frequency design storm events.
(2)
Wherever soils, topography, cut and fill or grading requirements,
or other conditions suggest substantial erosion potential during land
disturbance, the Township may require that the entire volume of all
storms up to a two-year storm from the disturbed areas be retained
on-site and that special sediment trapping facilities (such as check
dams, etc.) be installed.
I.
Areas of the site to remain undisturbed shall be protected from encroachment
by construction equipment/vehicles to maintain the existing infiltration
characteristics of the soil. Four-foot-high orange safety fence or
other similar protection fence approved by the Township must be installed
around the entire limit of disturbance/clearing prior to commencement
of earthmoving activities and maintained until completion of all construction
activity.
A.
No regulated earth-disturbance activities within the Township shall
commence until approval by the Township of a plan which demonstrates
compliance with state water quality requirements after construction
is complete.
B.
The BMPs must be designed, implemented, and maintained to meet state
water quality requirements and any other more stringent requirements
as determined by the Township.
C.
To control post-construction stormwater impacts from regulated earth-disturbance
activities, state water quality requirements may be met by BMPs, including
site design, which provide for replication of preconstruction stormwater
infiltration and runoff conditions, so that post-construction stormwater
discharges do not degrade the physical, chemical or biological characteristics
of receiving waters. As described in the PADEP Comprehensive Stormwater
Management Policy (#392-0300-002, September 28, 2002), this may be
achieved by the following:
(1)
Infiltration: replication of preconstruction stormwater infiltration
conditions.
(2)
Treatment: use of water quality treatment BMPs to filter out the
chemical and physical pollutants from the stormwater runoff.
(3)
Streambank and streambed protection: management of volume and rate
of post- construction stormwater discharges to prevent physical degradation
of receiving waters (e.g., from scouring).
D.
PADEP has regulations that require municipalities to ensure design,
implementation, and maintenance of best management practices (BMPs)
that control runoff from new development from new development and
redevelopment after regulated earth-disturbance activities area complete.
These requirements include the need to implement post-construction
stormwater BMPs with assurance of long-term operations and maintenance
of those BMPs.
E.
Evidence of any necessary permit(s) for regulated earth-disturbance
activities from the appropriate PADEP regional office must be submitted
to the Township.
A.
Riparian corridor restoration. Within all major subdivisions and
land developments, from the top of watercourse bank, 75 feet on either
side of the watercourse, which contains wetlands and/or floodplain,
shall be planted to establish a Zone 1 and Zone 2 buffer in accordance
with the Pennsylvania Stormwater Best Management Practices Manual,
December 2006, as amended, Riparian Forest Buffers. Where existing
vegetation on the site essentially duplicates buffer requirements,
this provision shall not apply. Additionally, this requirement may
be modified or waived by the Board of Supervisors where existing man-made
improvements or agricultural operations to be retained encroach within
the buffer area.
B.
In order to protect and improve water quality, a riparian corridor
easement shall be created and recorded as part of any major subdivision
or land development that encompasses a riparian corridor.
C.
Except as required by PADEP Chapter 102 regulations, the riparian
corridor 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
bank of the watercourse (on each side).
D.
Minimum management requirements for riparian corridor.
(1)
Existing native vegetation shall be protected and maintained within
the riparian corridor easement.
(2)
Whenever practicable, invasive vegetation shall be actively removed
and the riparian corridor 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.
E.
The riparian corridor easement shall be enforceable by the Township
and shall be recorded at Bucks County Courthouse, 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
of the property.
F.
Any permitted use within the riparian corridor 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.
H.
Septic drainfields and sewage disposal systems shall not be permitted
within the riparian corridor easement and shall comply with setback
requirements established under 25 Pa. Code Chapter 73.
A.
Hot spots.
(1)
Use of infiltration BMPs is prohibited on hot spot land use areas.
Examples of hot spots are listed in Ordinance Appendix G.[1]
[1]
Editor's Note: Appendix G is included as an attachment to this chapter.
(2)
Stormwater runoff from hot spot land uses shall be pretreated. In
no case may the same BMP be employed consecutively to meet this requirement.
Guidance regarding acceptable methods of pretreatment is located in
Appendix G.[2]
[2]
Editor's Note: Appendix G 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: Appendix H is included as an attachment to this chapter.
