[Ord. 2011-6, 5/19/2011, § 301]
1.
All applicants proposing regulated activities within the Township which do not fall under the exemption criteria contained within § 23-105 of this chapter shall submit a stormwater management site 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. (Refer definition of "impervious surface" within § 23-201 of this chapter).
[Amended Ord. No. 2022-005, 9/6/2022]
2.
All applicants proposing regulated activities shall implement such
measures as necessary to:
A.
Protect health, safety and property.
B.
Meet the water quality goals of this chapter by implementing measures
to:
(1)
Minimize disturbance to floodplains, wetlands and wooded areas.
(2)
Create, maintain, repair or extend riparian buffers.
(3)
Avoid erosive flow conditions in natural flow pathways.
(4)
Minimize thermal impacts to waters of the commonwealth.
(5)
Disconnect impervious surfaces (i.e., disconnected impervious
areas, DIAs) by directing runoff to pervious areas, wherever possible.
C.
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) and 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.
[Amended Ord. No. 2022-005, 9/6/2022]
3.
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, 35 P.S. § 691.1
et seq.
4.
For all regulated earth disturbance activities, erosion and sediment
(E&S) control best management practices (BMPs) shall be designed,
implemented, operated and maintained during the regulated earth disturbance
activities (e.g., during construction) to meet the purposes and requirements
of this chapter and to meet all requirements under Title 25 of the
Pennsylvania Code and the Clean Streams Law, 35 P.S. § 691.1
et seq. 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.
5.
No approval of any subdivision or land development plan, or issuance of any building, zoning, erosion and sedimentation control and grading, or occupancy permit; or the commencement of any earth disturbance at a project site within the Township, shall proceed until the requirements of this part are met, including approval of a stormwater management site plan under § 23-401 and a permit under PADEP regulations, where applicable.
[Amended Ord. No. 2022-005, 9/6/2022]
6.
Erosion and sediment control during regulated activities shall be addressed as required by § 23-311.
[Amended Ord. No. 2022-005, 9/6/2022]
8.
Volume controls (e.g. water quality volume, recharge/infiltration volume) for all watersheds shall be addressed pursuant to § 23-303, unless more stringent requirements are specified for individual watersheds (as identified in §§ 23-305, 23-306 and/or 23-307), in which case the more stringent requirement(s) shall apply.
[Amended Ord. No. 2022-005, 9/6/2022]
9.
All best management practices (BMPs) shall conform to the design
criteria of this chapter and Pennsylvania Stormwater Management Practices
Manual, December 30, 2006.
10.
Techniques described in Appendix 23-C, "Low Impact Development Practices,"
of this chapter are encouraged because they reduce the costs of complying
with the requirements of this chapter and the state water quality
requirements.
11.
Infiltration BMPs should be spread out, made as shallow as practicable,
and located to maximize the use of natural on-site infiltration features
while still meeting the other requirements of this chapter.
12.
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.
13.
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.
14.
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).
15.
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).
16.
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 one-hundred-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 will retain the easement until such
time as one of the following is accomplished:
A.
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 the maintenance
responsibilities.
B.
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.
17.
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 the Township and PADEP through the
joint permit application process, or, where deemed appropriate by
PADEP, through the general permit process.
18.
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 PADEP and/or U.S. Army Corps of Engineers
through the joint permit application process, or, where deemed appropriate
by PADEP and/or U.S. Army Corps of Engineers, 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 PADEP and/or U.S. Army Corps
of Engineers.
19.
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).
20.
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.
21.
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 be connected
to a storm sewer, drainage structure, or other approved stormwater
conveyance facility that is designed as part of a stormwater management
BMP. 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.
22.
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).
23.
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 determined to be exceptional value and high quality are to be maintained as defined in 25 Pa. Code, Chapter 93, "Water Quality Standards," Pennsylvania Department of Environmental Protection Rules and Regulations. Maintaining the multiple values of these special protection waters occurs through maintaining the predevelopment or natural water cycle; not decreasing infiltration and recharge; and not increasing runoff. In so doing, critical temperature considerations are provided (optimal temperature control is achieved through infiltration of precipitation to groundwater which maximizes temperature-moderated stream base flow). In those cases where runoff volume control can not be achieved as per § 23-303, Subsection 1A, temperature sensitive BMPs and stormwater conveyance systems are to be used and designed with storage pool areas (drawing outflow from the bottom of the pool) and supply outflow channels shall be 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 of 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.
24.
All stormwater runoff shall be pretreated for water quality prior
to discharge to surface or groundwater as required by this chapter.
[Amended Ord. No. 2022-005, 9/6/2022]
25.
All regulated activities which result in earth disturbance shall comply with the requirements of the Solebury Township Erosion and Sedimentation Control and Grading Ordinance [Chapter 9].
26.
Completed stormwater management facilities, including detention/retention
basins, shall be surveyed by a professional land surveyor or engineer
licensed in the Commonwealth of Pennsylvania, to verify compliance
with the character of stormwater management facilities as depicted
on the approved final plan (or subsequently approved revision, thereof).
As-constructed plans shall be submitted to the Township for review
and approval, upon completion of construction of all facilities and
prior to offer of dedication of any public facilities and/or submission
of financial security for the required maintenance period. Public
facilities will not be accepted by the Township until such time the
as-constructed plans have been reviewed and approved by the Township
Engineer.
27.
The record plan and development agreement for the approved subdivision
or land development shall define the ownership and maintenance responsibilities
as well as access rights for all drainage related easements. Specifically,
the record plan shall contain a provision permitting access to such
easement(s), at any reasonable time, for inspection and/or emergency
repair/maintenance, by the Township or its designee, of all facilities
deemed critical to public welfare. In the event the lot owner or homeowners'
association fails to honor their maintenance responsibilities set
forth herein, in any manner, the Township shall have the right of
entry upon and within the area of the easement to undertake any required
corrective or maintenance effort. The total cost of such, including
administrative, engineering and legal costs for enforcement, may be
imposed upon the responsible party as determined by the Township.
Failure to remedy all associated costs described above, may be subject
of the imposition of a lien by the Township against the owner(s) in
question, in the same manner as the Township might otherwise be empowered
by law to assess or impose a lien against a property for municipal
improvements.
[Ord. 2011-6, 5/19/2011, § 302]
1.
Mapping of Stormwater Runoff Peak Rate Districts. In order to implement
the provisions of this chapter, the Delaware River (South) Watershed
Stormwater Management Plan and Neshaminy Creek Watershed Stormwater
Management Plan, the 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 Township building. A large-scale
boundary map is included as Appendix 23-D for reference.
2.
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 [Chapter 22]. 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.
3.
For the purposes of implementing the provisions of the Delaware River
(South) Watershed Stormwater Management Plan, the peak release rates
of runoff, predevelopment, for the design storms as specified in the
table below; this table of release rates is keyed to Stormwater Management
District Watershed Map (refer Appendix 23-D).
|
Development site located in each of the A, B, C, or D Districts
must control postdevelopment runoff rates to predevelopment runoff
rates for the design storms as follows:
|
|
District
|
Design Storm Postdevelopment
|
Design Storm Predevelopment
|
|---|---|---|
|
A
|
2-year
|
1-year
|
|
5-year
|
5-year
| |
|
10-year
|
10-year
| |
|
25-year
|
25-year
| |
|
50-year
|
50-year
| |
|
100-year
|
100-year
| |
|
B
|
2-year
|
1-year
|
|
5-year
|
2-year
| |
|
10-year
|
5-year
| |
|
25-year
|
10-year
| |
|
50-year
|
50-year
| |
|
100-year
|
100-year
| |
|
C*
|
2-year
|
1-year
|
|
5-year
|
2-year
|
|
*
|
In District C, development sites which can discharge directly
to the Delaware River South main channel or major tributaries or indirectly
to the main channel through an existing stormwater drainage system
(i.e., storm sewer or tributary) may do so without control of postdevelopment
peak rate of runoff greater than the five-year storm. If the postdevelopment
runoff is intended to be conveyed by an existing stormwater drainage
system to the main channel, assurance must be provided that such system
has adequate capacity to convey the flows greater than the two year
predevelopment peak flow or will be provided with improvements to
furnish the required capacity. When adequate capacity of downstream
system does not exist and will not be provided through improvements,
the postdevelopment peak rate of runoff must be controlled to the
predevelopment peak rate as required in District A provisions (e.g.,
ten-year postdevelopment flows to 10 predevelopment flows) for the
specified design storms.
|
4.
For the purpose of implementing the provisions of the Neshaminy 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
|
[Ord. 2011-6, 5/19/2011, § 303; as amended by Ord. No. 2022-005, 9/6/2022]
1.
General Standards for Volume Controls. The requirements of Subsection 1A and B below apply to all watersheds. In the event that individual watershed volume requirements listed in this section or §§ 23-305 through 23-307 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 1A or the simplified method in Subsection 1B 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 should be used if the design storm
method is not followed. This method is not applicable to regulated
activities greater than one acre or for projects that require design
of stormwater storage facilities. For new impervious surfaces:
(1)
Stormwater facilities shall capture 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 should 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 should be infiltrated.
2.
General Standards.
A.
Runoff Volume Standard. Post-development stormwater runoff volume being discharged from any regulated activity shall not exceed pre -development stormwater runoff volume being discharged for up to the two-year frequency rainfall (for each watershed on-site). An alternative standard is allowed in this chapter where it can be demonstrated that due to existing natural site conditions [refer to Subsection 2A(2)], substantial infiltration and recharge are not occurring, predevelopment, resulting in greater than anticipated runoff volume.
(1)
Alternate Standard for Runoff Volume. Applicants may request
from the Township that an alternate standard be applied, where a portion
of the Runoff Volume Standard is not achieved but at least 50% of
the total required volume of infiltrated runoff is achieved. Use of
this alternate standard is permitted by the Township only after thorough
scrutiny has been directed toward all possible stormwater management
options at all possible locations at the site, consistent with the
process set forth in the following subsections.
(2)
Required Analysis for Allowing Use of Alternate Standard for
Runoff Volume. The alternate standard shall be used only in those
situations where it is demonstrated to the satisfaction of the Township
that due to natural site conditions infiltration is not occurring
in the pre-development condition, resulting in greater runoff volumes
(than would normally be anticipated) due to bedrock near or at the
surface (less than two feet in depth); presence of seasonal high water
table (SHWT) (less than two feet in depth); and soils with low permeability
(e.g., 0.20 inches per hour). Alternate standard shall be permitted
by the Township only in those cases where the applicant has demonstrated
that one or all of the above described conditions exist throughout
the site, such that there is no reasonable means of infiltrating required
stormwater volumes and that the property cannot be reasonably developed
utilizing a stormwater management system which infiltrates the two
year frequency storm event volume (pre to postdevelopment). The applicant
must demonstrate that there is no area of the site where the runoff
volume standard can feasibly be infiltrated. It is not grounds for
approval of the alternate standard that infiltrating the standard
runoff volume will utilize areas that could otherwise be developed
to obtain the most building area or lots.
(3)
Applicants requesting to utilize the alternate standard must provide a Feasibility Study (also refer to § 23-402) for infiltration utilizing BMPs as well as other runoff volume stormwater management systems and provide the following information:
(a)
Site plan demonstrating the extent of site area with seasonal
high water table (SHWT) (less than two feet). The site will be evaluated
both as to the extent of site with SHWT and the actual locations of
SHWT areas. Use of the alternate standard shall be permitted by the
Township only in those cases where it is demonstrated that site areas
free of SHWT are not feasible for use as stormwater BMPs (i.e., they
are located upgradient from reasonable site building areas).
(b)
Site plan demonstrating extent of site area with less than two
feet to bedrock. The site will be evaluated both as to the extent
of site with shallow depth to bedrock and actual locations of shallow
bedrock areas. Use of the alternate standard shall be permitted by
the Township only in those cases where it is demonstrated that site
areas free of shallow bedrock constraints are not feasible for use
as stormwater BMPs (i.e., they are located upgradient from reasonable
site building areas).
(c)
The site plan shall demonstrate the extent of site area with
less than 0.20 inches/hour of permeability in accordance with the
soil testing protocol set forth in Appendix 23-B.
(d)
In order to utilize the alternate standard, the applicant must
demonstrate that the total of infiltration "challenged" areas (the
total of areas described in (a) through (c) hereof) exceed the following
percentages of the total site:
1)
Seventy-five percent (sites less than five acres).
2)
Eighty percent (sites five to 10 acres).
3)
Eighty-five percent (sites greater than 10 acres).
|
In addition, the applicant must demonstrate that there is no
feasible site area free of the above described infiltration constraining
features which exist in a location such that the runoff volume standard
can be achieved.
|
(4)
If the applicants professional engineer can demonstrate based upon site specific soil testing to the satisfaction of the Township that due to existing soil, bedrock, water table or other conditions on the parcel, that such a standard is not achievable on the site (all or in part), the standard contained in § 23-303, Subsection 2C, shall apply.
B.
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 23-D) and § 23-302 of this chapter.
C.
If it is determined to the satisfaction of the Township that the volume standard set forth in § 23-303, Subsection 2A, cannot be achieved, then the peak rate standards are modified so that post-development peak rate discharges from the site for all storms up to the ten-year storm must be equal to or less than 75% of the design peak rates permitted within § 23-302.
3.
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 pre-development 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.
4.
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 or around
the development site to existing points of discharge from the property.
The future use of undeveloped areas upstream shall be taken into account
in the calculation of pipe sizes for storm sewer system designs. The
capacity and maximum anticipated present flow of the body or system
receiving the proposed system(s) discharge shall be calculated to
verify its capability of receiving any additional flow caused by the
development or subdivision. The runoff from any proposed development
shall be subject to an evaluation that includes the anticipated runoff
from other existing or proposed developments with the same watershed.
5.
Where the area of a site being impacted by a proposed development
activity differs significantly from the total site area, only the
proposed disturbed area utilizing stormwater management measures shall
be subject to the management district criteria. Unimpacted 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.
6.
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 30, 2006," as amended. Riparian forested buffer, and 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 within this chapter and the Pennsylvania
Stormwater Best Management Practices Manual, and shall be in addition
to any other Township ordinance provisions.
7.
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.
8.
Downstream Hydraulic Capacity Analysis. Any downstream capacity hydraulic
analysis conducted in accordance with this chapter shall use the following
criteria for determining adequacy for accepting increased peak flow
rates:
A.
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.
B.
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, or wildlife and aquatic habitat. Habitat
impact should be minimized or avoided.
C.
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, 25 Pa. Code, Chapter 105 regulations (if applicable) and,
at a minimum, pass the increased twenty-five-year return period runoff.
[Ord. 2011-6, 5/19/2011, § 304]
1.
The design of all regulated activities shall include the following
steps in sequence to minimize stormwater impacts.
A.
The applicant is required to find practicable alternatives to the
surface discharge of stormwater, the creation of impervious surfaces
and the degradation of waters of the commonwealth, and must maintain
as much as possible the natural hydrologic regime of the site.
B.
An alternative is practicable if it is available and capable of being
completed after considering cost, existing technology and logistics
in light of overall project purposes, and other Township requirements.
C.
All practicable alternatives to the discharge of stormwater are presumed
to have less adverse impact on quantity and quality of waters of the
commonwealth unless otherwise demonstrated.
2.
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:
A.
Prepare an existing resource and site analysis plan (ERSAP), as required by the Subdivision and Land Development Ordinance [Chapter 22], showing environmentally sensitive areas including, but not limited to, steep slopes, ponds, lakes, streams, wetlands, hydric soils, vernal pools, floodplains, riparian corridors, hydrologic soil groups A, B, C and D, "prime agricultural soils" (prime farmland and farmland of statewide importance as designated in the soil survey of Bucks County), woodlands, surface waters regulated by the state or federal government, any existing recharge areas, and any other requirements outlined in the Subdivision and Land Development Ordinance [Chapter 22].
B.
Prepare a draft project layout avoiding sensitive areas identified in Subsection 2A (and as further defined by Part 22, "Natural Resources," of the Zoning Ordinance [Chapter 27]) 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 ERSAP.
C.
Identify site specific existing conditions, drainage areas, discharge
points (points of interest), recharge areas, and hydrologic soil groups
A and B.
E.
Volume controls (e.g. water quality volume, recharge/infiltration volume) for all watersheds shall be addressed pursuant to § 23-303, unless more stringent requirements are specified for individual watersheds (as identified in §§ 23-305, 23-306 and/or 23-307), in which case the more stringent requirement(s) shall apply.
[Amended by Ord. No. 2022-005, 9/6/2022]
G.
Determine the management district within which the site is located
(refer Appendix 23-D) and conduct a predevelopment runoff analysis.
H.
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.
I.
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.
J.
Manage any remaining runoff through treatment prior to discharge,
as part of detention, biorentention, direct discharge or other structural
control.
[Ord. 2011-6, 5/19/2011, § 305]
1.
The project plan shall specify permanent stormwater BMPs to be implemented,
operated, and maintained to meet water quality requirements. Because
water quality requirements vary depending on the "uses" of the water
bodies in the watershed, a framework methodology is provided here.
2.
In order to protect and maintain water quality, additional stormwater
runoff created by the development project must be captured, stored
and treated. In addition, post construction stormwater infiltration
of runoff must replicate preconstruction infiltration of runoff to
the maximum extent possible, in high quality and exceptional value
watersheds, special requirements may apply.
3.
The volume of additional stormwater runoff to be captured, stored
and treated is called the water quality volume ("WQv") in acre-feet
of storage.
A.
The formula for determining WQv is:
(1)
WQv = [(P)(Rv)(A)]/12, where:
(a)
P = Rainfall depth in inches, using the "90% storm" -the volume
of rainfall for 90% of the storm events which produce runoff in the
watershed annually. For PennDOT Region 5, the current P value is 2.04
inches.
(b)
A = Project Area in acres.
(c)
Rv = Volume Runoff Coefficient [0.05 + 0.009(I)], where I is
the impervious surface percentage (impervious area ÷ total
project area) X 100%.
B.
In special protection watersheds, as described in 25 Pa. Code, Chapter
93, this volume is required to remain on-site through infiltration
and other methods, to protect water quality. Guidance can be obtained
from PADEP.
4.
The following factors must be considered when evaluating the suitability
of BMPs used to control water quality at a given development site:
A.
Total contributing drainage area.
B.
Permeability and infiltration rate of the site soils.
C.
Slope and depth to bedrock.
D.
Seasonal high water table.
E.
Proximity to building foundations and wellheads.
F.
Erodibility of soils.
G.
Land availability and configuration of the topography.
H.
Peak discharge and required volume control.
I.
Streambank erosion.
J.
Efficiency of the BMPs to mitigate potential water quality problems.
K.
Volume of runoff that will be effectively treated.
L.
Nature of the pollutant being removed.
M.
Maintenance requirements.
N.
Creation/protection of aquatic and wildlife habitat.
O.
Recreational value.
P.
Enhancement of aesthetic and property value.
|
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).
|
5.
The applicant may, subject to approval of the Township, use any of
the following non-structural stormwater credits, generally described
in the following table, in computing the required water quality volume.
|
Stormwater Credit
|
Description
| |
|---|---|---|
|
Natural Area Conservation
|
Conservation of natural areas such as forest, wetlands or other
sensitive areas in a protected easement thereby retaining their predevelopment
hydrologic and water quality characteristics. Using this credit, a
designer may subtract conservation areas from total site area when
computing the required water quality volume.
| |
|
Vegetated Roof
|
Credit may be given for water quality and volume benefits for
vegetated roof covers where vegetation is grown on, and completely
covers, an otherwise flat or pitched roof (less than or equal to 30°
slope).
| |
|
Disconnection of Rooftop Runoff
|
Credit may be given when rooftop runoff is disconnected and
then directed over a previous area where it may either infiltrate
into the soil or filter over it. Credit is typically obtained by grading
the site to promote overland flow or by providing bioretention on
single-family residential lots. If a rooftop area is adequately disconnected,
the impervious area may be deducted from the total impervious cover.
| |
|
Disconnection of Non-Rooftop Runoff
|
Credit may be given for practices that disconnect surface impervious
cover by directing it to pervious areas where it is either infiltrated
or filtered through the soil. As with rooftop runoff, the impervious
area may be deducted from the total impervious cover thereby reducing
the required water quality volume.
| |
|
Stream Buffer Credit
|
Credit may be given when a stream buffer effectively treats
stormwater runoff. Effective treatment constitutes capturing runoff
from pervious and impervious areas adjacent to the buffer and treating
the runoff through overland flow across a grass or forested area.
Areas treated in this manner may be deducted from total site area.
| |
|
Grass Channel (Open Section Roads)
|
Credit may be given when open grass channels are used to reduce
the volume of runoff and pollutants during smaller storms. If designed
according to appropriate criteria, these channels may meet water quality
criteria for certain types of residential development.
| |
|
Environmentally Sensitive Rural Development
|
Credit may be given when a group of environmental site design
techniques are applied to low density or rural residential development.
This credit eliminates the need for structural practices to address
water quality volume.
|
|
For design and applicability of non-structural BMPs refer to Chapter 5 of the Pennsylvania Stormwater Management Practices Manual, December 2006, as amended. For the nonstructural BMPs proposed, the applicant shall utilize and submit appropriate checklists included in Chapter 8, § 8.8 of the Pennsylvania Stormwater Best Management Practices Manual, December 2006, as amended (refer Appendix 23-E) to demonstrate that the BMPs are applicable to the project and to determine the amount of volume or peak rate credit is applicable.
|
6.
The volume and rate of any stormwater discharges allowed under this
chapter must be managed to prevent the physical degradation of receiving
waters, such as by streambank scour and erosion. If a detention facility
is proposed which is part of the BMPs approved for the project, the
facility(ies) must be designed to provide for a twenty-four-hour extended
detention of the one year, twenty-four-hour storm event (i.e., the
stormwater runoff will be released over a minimum 24 hours for the
one-year, twenty-four-hour storm event from the time of peak inflow
to zero outflow).
[Ord. 2011-6, 5/19/2011, § 306]
1.
Prevention of stormwater runoff is key objective of 25 Pa. Code,
Chapter 93, of the DEP regulations, because runoff can change the
physical, chemical and biological integrity of waterbodies thereby
impacting water quality.
2.
The project plan shall describe how water quality protection requirements will be met. Infiltration BMPs shall be evaluated and utilized to the maximum extent possible to manage the net change in stormwater runoff generated so that post construction discharges do not degrade the physical, chemical or biological characteristics of the receiving waters. These BMPs may be used to satisfy all or part of the requirements in § 23-305.
3.
Post construction stormwater infiltration of runoff shall replicate
preconstruction infiltration of runoff to the maximum extent possible.
In high quality and exceptional value watersheds, special requirements
apply (refer applicable sections in this chapter).
4.
In calculating the volume of runoff that can be infiltrated at a
site, the following methodology shall be used:
A.
Methodology:
(1)
Rev = [(S)(Rv)(A)]/12, where:
(a)
Rev = Recharge Volume (acre-feet).
(b)
S = Soil specific recharge factor (inches).
(c)
A = Site area contributing to the recharge facility (acres).
(d)
Rv = Volumetric runoff coefficient, Rv
= 0.05 + 0.009 (I), where:
1)
I = percent impervious area.
2)
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)
Slope and depth to bedrock.
(3)
Seasonal high water table.
(4)
Proximity and elevation relative to building foundations, basements
and well heads. (Infiltration BMPs 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 (refer to § 23-402, Subsection 2A). 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 23-B.
(3)
Design infiltration BMPs for required stormwater volume based
on field-determined capacity at the level of the proposed infiltration
surface.
D.
Soil characteristics. Subject to the specific considerations in Subsection 4D(8) below.
(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 18 inches between the bottom
of any facility and the seasonal high water table and/or bedrock (limiting
zones), except for infiltration BMPs receiving only roof runoff which
shall be placed in soils having a minimum depth of 12 inches between
the bottom of the facility and the limiting zone. 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 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 the construction and
to careful avoidance of soil compaction during site development.
H.
Infiltration is not permitted in geologically susceptible limestone
areas including the Carbonate Geology Overlay Zoning District. All
stormwater facilities in this overlay district must be constructed
with an impermeable liner approved by the Township to prevent seepage
of water into the ground. All proposed channels or swales receiving
or conveying concentrated stormwater runoff must be protected with
an impermeable liner approved by the Township.
I.
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 25
Pa. Code, Chapter 93, antidegradation regulations.
J.
The plan must include safeguards against groundwater contamination
for uses which may cause groundwater contamination, should there be
a mishap or spill. The Township may require the installation of a
mitigative layer or 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.
K.
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 infiltration
capacity.
L.
Infiltration BMPs shall not be constructed nor receive runoff until
the entire contributory drainage area to the infiltration BMP has
achieved final stabilization.
M.
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 given the extent to which development-related
disturbance leads to compaction of the soils and reduces their infiltrative
capacity.
[Ord. 2011-6, 5/19/2011, § 307]
1.
Volume Control.
A.
Volume controls will mitigate increased runoff impacts, protect stream
channel morphology, maintain groundwater recharge and contribute to
water quality improvements. Stormwater runoff volume control methods
are based on the net change in runoff volume for the two-year storm
event.
B.
Volume controls shall be implemented using the design storm method in Subsection 1B(1) or the simplified method in Subsection 1B(3) below. For regulated activities which propose 2,500 square feet 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 2,500 square feet 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 § 23-310.
(a)
Post-development total runoff shall not be increased from pre-development
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 (pre-development) 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 23-A 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.
(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.
1)
Stormwater Management Site Area = Total Site Area (for both
pre- and post-development conditions) - Protected Area.
(b)
Calculate the volume controls provided through nonstructural
BMPs. Table 23-A-5 in Appendix 23-A is recommended as guidance.
(c)
Volume controls provided through nonstructural BMPs are subtracted
from the required volume to determine the necessary structural BMPs.
|
Required Volume Control (ft3) - Nonstructural
Volume Control (ft3) - Structural Volume
Requirement (ft3)
|
(d)
Calculate the volume controls provided through structural BMPs.
Table 23-A-6 in Appendix 23-A is recommended as guidance. Refer PA
BMP Manual Chapter 6 for a description of these BMPs.
(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 applicants
proposing 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 will be required in detention basins where
the possibility of groundwater contamination exists. A detailed hydrogeologic
investigation may be required by the Township.
(3)
Simplified Method (Regulated Activities involving less than 2,500 square feet of impervious surface). Individual home construction projects on single-family lots which result in less than 2,500 square feet of new impervious area (including the building footprint, driveway, sidewalks and parking areas) and less than 5,000 square feet of earth disturbance may utilize the simplified procedure contained in Appendix 23-J to meet requirements of this chapter and are not required to submit detailed stormwater management plans prepared pursuant to § 23-402 of this chapter. This procedure may not be utilized for proposed subdivision or land developments.
[Ord. 2011-6, 5/19/2011, § 308]
1.
To minimize erosion, all BMPs shall be designed to comply with the
following:
A.
In addition to the water quality volume, 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 of 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 four 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.
2.
Whenever a watercourse is located within a site proposed for major
subdivision or land development, 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.
[Ord. 2011-6, 5/19/2011, § 309]
1.
Stormwater runoff which may result from regulated activities listed in § 23-104 shall be controlled by permanent stormwater runoff BMPs that will provide the required standards within Part 3. 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 Practice Manual, December 30, 2006, as amended. The choice of BMPs is not limited to the ones appearing in this chapter and the Manual, however, any selected BMP must meet or exceed the runoff peak rate requirements of this chapter for the applicable hydrologic district.
2.
Any stormwater facility located on state highway rights-of-way shall
be subject to approval by the Pennsylvania Department of Transportation.
3.
Collection System Standards.
A.
Curb Inlets. Curb inlets shall be located at curb tangents on the
uphill side of street intersections and at intervals along the curbline
to control the maximum amount of encroachment of runoff on the roadway
pavement so that same does not exceed a width of four feet during
the design storm event. Design and location of curb inlets shall be
approved by the Township.
B.
Pipe Materials. All storm sewer piping shall be Class III reinforced
concrete pipe, except when pipe class and strength is required to
be increased in accordance with PennDOT specifications. Piping shall
be saw-cut at ends, as needed, and not hammered or broken. All pipe
joints and lift holes must be mortared except where designed for infiltration.
C.
Minimum Pipe Size. Minimum pipe diameter shall be 18 inches (or an
equivalent flow area of 1.76 square feet).
D.
Inlet and Manhole Construction. Inlet and manhole castings and concrete
construction shall be equivalent to PennDOT design standards. Manhole
castings and covers shall have the word "STORM" cast in two-inch high
letters on the top of the cover. All inlet grates shall be "bicycle
safe" heavy duty structural steel. 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.
E.
Open end pipes must be fitted with concrete endwalls or wing walls
in accordance with PennDOT standards.
F.
Flow Velocity. Stormwater collection systems shall be designed to
produce a minimum velocity of three feet per second when flowing full.
The maximum permissible velocity shall be 15 feet per second. Pipe
slopes shall not be less than 0.05%.
G.
Inlets and manholes shall be spaced at intervals not exceeding 300
feet, and shall be located wherever branches are connected or sizes
are changed, and wherever there is a change in alignment or grade.
For drainage lines of at least 36 inches diameter, inlets and manholes
may be spaced at intervals of 400 feet. Manholes shall be equipped
with open grate lids.
H.
Storm sewer bedding/backfill requirements shall conform to the construction
details in Appendix 23-H.
I.
Inlets shall be located to intercept concentrated runoff prior to
discharge over public/private rights-of-way, sidewalks, streets and
driveways.
J.
The capacity of all Type "C" inlets shall be based on a maximum surface
flow to the inlets of four cfs, calculated based on the one-hundred-year
frequency design storm event. The maximum flow to Type "C" inlets
located in low points (such as sag vertical curves) shall include
the overland flow directed to the inlet as well as all bypass runoff
from upstream inlets. The bypass flow from upstream inlets shall be
calculated using inlet efficiency curves included in PennDOT Design
Manual, Part 2, latest edition. If the surface flow to an inlet exceeds
four cfs, additional inlets shall be provided upstream of the inlet
to intercept the excessive surface flow. A Type "C" inlet at a low
point of a paved area may be designed to accept a maximum of six cubic
feet per second (CFS). Type "M" inlets shall be designed to accept
a maximum surface flow of six CFS based on the one-hundred-year frequency
design storm event, unless otherwise approved by the Township. Double
inlets will not be permitted where additional pipe and inlets can
be placed upstream to intercept excessive surface flow. A maximum
of 12 cfs shall be permitted to be collected by a Type "M" inlet located
in an isolated pervious area provided the designer can verify that
such an inlet would not cause stormwater to accumulate on any adjacent
public or private property, outside of an associated storm sewer easement,
and that the depth of the accumulated stormwater would not exceed
12 inches.
K.
A minimum drop of two inches shall be provided between the inlet
and outlet pipe invert elevations within all inlets and manholes.
When varying pipe sizes enter an inlet or manhole, the elevation of
crown of all pipes shall be matched.
L.
Stormwater pipes shall have a minimum depth of cover of 12 inches
(including over the bell) or as designated by the American Concrete
Pipe Association (whichever is greater), and in no case shall any
part of the pipe project into the road subbase or curb. Where cover
is restricted, equivalent pipe arches may be specified in lieu of
circular pipe.
M.
The capacity of all stormwater pipes shall be calculated utilizing
the Manning equation for open channel flow as applied to closed conduit
flow. Manning's roughness coefficient shall be 0.13 for all concrete
pipe. In cases where pressure flow may occur, the hydraulic grade
line shall be calculated throughout the storm sewer system to verify
that at least one foot of freeboard will be provided in all inlets
and manholes for the design storm event.
N.
Culverts shall be designed based on procedures contained in "Hydraulic
Design of Highway Culverts," HDS #5, U.S. Department of Transportation,
Federal Highway Administration. Where pressure flow is anticipated
in storm sewer pipes (non-open channel flow), the applicant's designer
shall be required to calculate the elevation of the hydraulic grade
line through the storm sewer system. Wherever the hydraulic grade
line elevation exceeds the pipe crown elevation for the design flow,
pipes with watertight joints must be specified.
O.
Storm sewer structures (e.g., endwalls, inlets and sections, etc.)
may not be located on top of, or within 10 feet of electric, communication,
water, sanitary sewer or gas services and/or mains, unless approval
is received from the Township and the Authority or utility having
jurisdiction over same.
P.
Stormwater pipes must be oriented at right angles to electric, water,
sanitary sewer and gas utilities when crossing above or beneath same.
Crossing angles of less than 90° will only be permitted at the
discretion of the Township. When skewed crossings are permitted, interior
angles between alignment of the storm sewer pipe and utility may not
be less than 45°. Vertical and horizontal design of storm sewer
must be linear.
Q.
Roadway underdrain is required along both sides of all proposed roadways,
existing roadways proposed to be widened and within existing or proposed
roadside swales as directed by the Township.
R.
Where a public storm sewer system is not located within a right-of-way,
or dedicated public property, a 20 feet wide easement shall be established
to encompass the storm sewer system. For multiple pipes or utilities,
the width of the easement shall be a minimum of 30 feet.
S.
A minimum of one foot of freeboard, between the inlet grate and the
design flow elevation, shall be provided in all storm sewer systems
(inlets and manholes) for the one-hundred-year frequency design storm
event.
4.
Open Swales and Gutters. Open swales shall be designed on the basis
of Manning's Formula as indicated for collection systems with the
following considerations:
A.
Roughness Coefficient. The roughness coefficient shall be 0.040 for
earth swales.
B.
Bank Slopes. Slopes for swale banks shall not be steeper than one
vertical to four horizontal.
C.
Flow Velocity. The maximum velocity of flow as determined by Manning's
equation shall not exceed the allowable velocities as shown in the
following table for the specific type of material, unless otherwise
approved by the Township and the Bucks County Conservation District.
|
Note: Source of the following design criteria is the Pennsylvania
Department of Environmental Protection, Bureau of Soil and Water Conservation
Publication, Erosion and Sediment Control Program Manual.
|
|
Allowable Velocity
| ||||
|---|---|---|---|---|
|
Material
|
Velocity
(fps)
| |||
|
Well established grass on good soil
| ||||
|
Short Pliant bladed grass
|
4.0 to 5.0
| |||
|
Bunch grass-soil exposed
|
2.0 to 3.0
| |||
|
Stiff stemmed grass
|
3.0 to 4.0
| |||
|
Earth without vegetation
| ||||
|
Fine sand or silt
|
1.0
| |||
|
Ordinary firm loam
|
2.0 to 3.0
| |||
|
Stiff clay
|
3.0 to 5.0
| |||
|
Clay and gravel
|
4.0 to 5.0
| |||
|
Coarse gravel
|
4.0 to 5.0
| |||
|
Soft shale
|
5.0 to 6.0
| |||
|
Shoulders
| ||||
|
Earth
|
(as defined above)
| |||
|
Stabilized
|
6.0
| |||
|
Paved
|
10.0 to 15.0
| |||
D.
Swales shall be stabilized with biodegradable erosion control matting
to permit establishment of permanent vegetation. Swales shall be of
such shape and size to effectively contain the one-hundred-year, Rational
Method design storm, or greater, and to conform to all other specifications
of the Township.
E.
To minimize sheet flow of stormwater across lots located on the lower
side of roads or streets, and to divert flow away from building areas,
the cross-section of the street as constructed shall provide for parallel
ditches or swales or curb on the lower side which shall discharge
only at drainage easements, unless otherwise approved by the Township.
F.
Gutters and swales adjacent to road paving shall be permitted to
carry a maximum flow of four cubic feet per second prior to discharge
away from the street surface, unless it is proven to the satisfaction
of the Township by engineering calculations that the road slopes or
other factors would allow higher gutter or swale capacity.
G.
Flows larger than those permitted in gutters and roadside swales
may be conveyed in swales outside the required road right-of-way in
separate drainage easements, or may be conveyed in pipes or culverts
inside or outside the required road right-of-way.
H.
Existing and proposed swales shall be provided with underdrains as
deemed necessary by the Township should overland seepage result in
potential maintenance problems. Underdrains must discharge into a
natural drainage channel or stormwater management system.
I.
Where drainage swales are used to divert surface waters away from
buildings, they shall be sodded, landscaped or otherwise protected
as required and shall be of a slope, shape and size conforming with
the requirements of the Township. Concentration of surface water runoff
shall be permitted only in swales, watercourses, retention or detention
basins, bioretention areas or other areas designed to meet the objectives
of this chapter.
5.
Bridge and Culvert Design. Any proposed bridge or culvert to convey
flow within a watercourse, perennial stream, intermittent stream or
ephemeral 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
(rip rap) 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 one-hundred-year, twenty-four-hour storm
as defined by the U.S. Department of Agriculture, Soil Conservation
Service, Technical Release No. 55. All bridges and culverts shall
be designed to convey the one-hundred-year design storm without increasing
the extent and depth of the one-hundred-year floodplain.
6.
Storm Sewer Design.
A.
Design flow rate. The storm sewer system shall be designed to carry
the one-hundred-year frequency design storm peak flow rate. The drainage
area and runoff coefficient to each inlet shall be indicated on the
stormwater management plan. The one-hundred-year flow rate shall be
determined by the "Rational" method formula: Q = CIA where:
(1)
Q = Peak runoff rate measured in cubic feet per second (cfs).
(2)
C = Runoff coefficient. The coefficient of stormwater runoff
includes many variables, such as ground slope, ground cover, shape
of drainage area, etc.
(3)
I = Intensity. Average rainfall intensity in inches per hour
for a time equal to the time of concentration.
B.
Consideration shall be given to future land use changes in the drainage
area in selecting the Rational ("C") coefficient. For drainage areas
containing several different types of ground cover, a weighted value
of "C" shall be used.
C.
In determining the peak flow rate to individual storm sewer inlets (or other collection structures) the time of concentration method (as referenced in § 23-309) shall be used for inlet drainage areas in excess of one acre, unless otherwise approved by the Township. For inlet drainage areas less than one acre, a five-minute time of concentration shall be used unless otherwise approved by the Township.
D.
In determining the required design flow rate through a storm sewer
piping system, if a five-minute time of concentration (storm duration)
results in a pipe size exceeding a thirty-inch diameter pipe (or equivalent
flow area of 4.9 square feet), the time of concentration approach
(as defined herein) shall be used in determining storm duration.
E.
In determining the required design flow rate through a storm sewer
piping system, if a five-minute time of concentration results in a
pipe size exceeding 30 inches, within any run of pipe, the time of
concentration approach may be used for sizing of pipes from that point
on, by adjusting the time of concentration.
F.
Overflow System. An overflow system shall be provided to carry all
bypass flow and/or flow in excess of storm sewer design capacity,
to the detention basin (or other approved outlet point) when the capacity
of the storm sewer system is exceeded. Stormwater runoff will not
be permitted to surcharge from storm sewer structures.
7.
Grading and Drainage.
A.
After completion of rough grading, a minimum of eight inches of topsoil
shall be returned to remaining disturbed areas prior to final grading
and seeding.
B.
Lots shall be graded to secure proper drainage away from buildings and to prevent the collection of stormwater in pools. Minimum 2% slopes shall be maintained away from and around all structures. Separation between the top of foundation wall (or slab) shall comply with Township Building Code [Chapter 5] requirements.
C.
Construction. The developer shall construct and/or install such drainage
structures and/or pipe which are necessary to prevent erosion damage
and to satisfactorily carry off such surface waters to the nearest
practical street, storm drain or natural watercourse.
D.
Excavation. No excavation shall be made with a cut face steeper in
slope than four horizontal to one vertical (4:1 = 25%), except under
one or more of the following conditions:
(1)
The material in which the excavation is made is sufficiently
stable to sustain a slope of steeper than 4:1 and a written statement
(certification) from a professional civil engineer, licensed in the
Commonwealth of Pennsylvania and experienced in erosion control, to
this effect is submitted to the Township Engineer for review. This
statement shall indicate the site has been inspected and that the
deviation from the slope specified herein will not result in injury
to persons or damage to property.
(2)
A concrete, segmental block or stone masonry wall, constructed
in accordance with Township requirements, is provided to support the
face of the excavation.
E.
Fill. No fill shall be made which creates any exposed surface steeper
in slope than four horizontal to one vertical (4:1 = 25%) except under
one or more of the following conditions:
(1)
The fill is located so that settlement, sliding, or erosion
will not result in property damage or be a hazard to adjoining property,
streets, alleys or buildings.
(2)
A written statement from a professional civil engineer, licensed
in the Commonwealth of Pennsylvania and experienced in erosion control,
certifying the site has been inspected and that the proposed deviation
from the slope specified above will not endanger any property or result
in property damage, is submitted to and approved by the Township.
(3)
A concrete, segmental block or stone masonry wall, constructed
in accordance with Township requirements, is provided to support the
face of the excavation.
F.
Slopes and Fences. The top or bottom edge of slopes shall be a minimum
of five feet from property or right-of-way lines of streets or alleys
in order to permit the normal rounding of the edge without encroaching
on the abutting property. Where walls or slopes (steeper than two
horizontal to one vertical) are approved under the criteria in this
chapter, and are five feet or more in height, a protective fence shall
be required at the top of the wall (or bank).
G.
Clean up. All lots must be kept free of any debris or nuisances whatsoever
during construction.
H.
Design of erosion and sedimentation control facilities (particularly
stormwater/sediment basins) shall incorporate best management practices
as defined herein.
I.
Cut and fill operations shall be kept to a minimum. Wherever feasible,
natural vegetation shall be retained, protected and supplemented.
Cut and fills shall not endanger or otherwise adversely impact adjoining
property.
J.
No grading equipment shall be permitted to be loaded and/or unloaded
on a public street, and no grading equipment shall be permitted to
travel on or across a public street unless licensed for operation
on public thoroughfares.
K.
Grading equipment shall not be permitted to cross intermittent and
perennial streams. Temporary crossing shall only be permitted where
application is made, and approval is received, from the Pennsylvania
Department of Environmental Protection (where applicable), the Bucks
County Conservation District, and the Township.
L.
Design of energy dissipation for high volume and/or high velocity
discharge from storm sewer pipes and channels shall be in accordance
with Hydraulic Engineering Circular No. 14, "Hydraulic Design of Energy
Dissipaters for Culverts and Channels," as published by Department
of Transportation, FHA, when deemed necessary by the Township, and
as approved by the Bucks County Conservation District.
M.
To control the dissemination of mud and dirt on to public roads and
driveways, tire cleaning areas constructed of AASHTO #1 stone (underlain
by geotextile structural fabric), at least 50 feet in length shall
be installed at each point of access to the site and individual lots
(upon construction of internal streets in a binder condition). When
deemed necessary by the Township, washing stations shall also be set-up
at every construction entrance in order to wash mud and dirt from
exiting vehicles. Appropriate measures must be taken to control runoff
from such locations. The developer shall be responsible for the placement
of appropriate signage identifying construction entrances and washing
stations. Construction entrances shall be maintained by the developer
during construction, as determined by the Township.
N.
In the event any mud and/or debris is transported from the site onto
a public roadway, the debris shall be removed immediately and the
roadway swept and/or washed as deemed necessary by the Township at
the owner's expense.
O.
Adequate provision shall be made to prevent surface water from damaging
the cut face of excavation and the sloping surfaces of fills.
8.
Stormwater Detention/Retention Basins.
A.
If permanent ponds (retention basin) are proposed, the developer
shall demonstrate that such ponds are designed to protect the public's
health and safety.
B.
During construction, duly authorized representatives of the Township
may enter at any reasonable time upon any property within the Township
to investigate whether construction activity is in compliance with
this chapter.
C.
When basins are provided, they shall be designed to utilize the natural
contours of the land whenever possible. When such design is not practical,
the construction of the basin shall utilize slopes as flat as possible
to blend the structure into the terrain.
D.
Except with the one-year design storm, basins shall be designed so
that they return to normal conditions within approximately 12 hours
after the termination of the storm, unless the Township determines
that downstream conditions may warrant other design criteria for stormwater
release.
E.
Landscaping and planting in and around the perimeter of basins shall be provided. Proposed planting shall also be in accordance with the provisions of this chapter, the Subdivision and Land Development Ordinance [Chapter 22], and as recommended by the Township landscape architect/planner.
F.
If a stormwater management basin will serve as a temporary sediment
control device, the temporary sediment control measures shall be shown
including perforated riser pipes or standboxes, filter berms, clean-out
stakes and other measures as may be required by Pennsylvania Department
of Environmental Protection, 25 Pa. Code, Chapter 102 regulations.
Plans for such facilities shall require Bucks County Conservation
District approval prior to implementation. Sedimentation basins shall
be in place prior to any earthmoving activities within their tributary
drainage areas. A note identifying the above criteria shall be required
on the Record Plan of subdivisions and land developments as well as
in the development agreement with the Township.
G.
Stormwater management basins shall be in place before the creation
of any new impervious surfaces on the site.
H.
Runoff shall not be directed to any infiltration structure until
all tributary drainage areas are stabilized.
I.
Where permanent retention (pond) facilities are proposed, there shall
be a safety ledge, 10 feet wide, no greater than 15 inches and no
less than 12 inches below the permanent water surface level.
J.
All basins shall have slopes of four horizontal to one vertical (4:1
= 25%), or flatter on the basin's outer berm and five horizontal to
one vertical or less on the basin's inner berm. The top or toe of
any slope shall be located a minimum of five feet from any property
line. The maximum difference between the top of berm elevation and
the invert elevation of the outlet structure shall be seven feet.
K.
All portions of a dry detention basin bottom shall have a minimum
slope of 2%.
L.
All basin embankments shall be placed in lifts not to exceed one
foot in thickness and each lift shall be compacted to a minimum of
95% of Modified Proctor Density as established by A.S.T.M. D-1557.
Prior to proceeding to the next lift, the compaction shall be checked
by a soils engineer employed by the applicant/developer. Compaction
tests shall be run on the leading and trailing edge of the berm along
with the top of the berm. Verification of required compaction shall
be submitted to the Township prior to utilization of any basin for
stormwater management.
M.
Emergency overflow facilities/spillway shall be provided within basins
in order to convey basin inflow in excess of design flows, out of
the basin, or in the event the outlet structure becomes blocked and
is unable to convey flow. Emergency spillways discharging over embankments
shall be constructed of reinforced concrete checker-blocks to protect
the berm against erosion. The checkerblocks shall be back-filled with
topsoil and seeded. Checkerblock lining shall extend to the toe of
the embankment on the outside of the berm, and shall extend to an
elevation of three feet below the spillway crest on the inside of
the berm. Vegetated spillways may be utilized for spillways constructed
entirely on undisturbed ground (i.e., not discharging over fill material).
A dense cover of vegetation shall be rapidly established in such spillways
by sodding or seeding with a geotextile anchor. The vegetated spillway
must be stabilized before runoff is directed to the basin. The minimum
capacity of all emergency spillways shall be equivalent to the peak
flow rate of the one-hundred-year, post-development design storm (entering
to the basin).
N.
In all cases, the discharge end of the basin shall be provided with
a properly designed outlet control structure (headwall, orifice structure
or other approved flow control structure), culvert pipe and endwall.
Perforated riser pipes alone, without provision for permanent outlet
control structure (as stated above), and culvert pipe are not permitted
for permanent basins.
O.
The minimum top of basin berm width (at the design elevation) shall
be 10 feet. A cut-off trench (keyway) of impervious material shall
be provided under all embankments that require fill material. The
cut-off trench shall be a minimum of eight feet wide, three feet deep
and have side slopes of one horizontal to one vertical.
P.
The minimum freeboard through the emergency spillway shall be one
foot. Freeboard is defined as the difference between the design flow
elevation through the spillway and the elevation of the top of the
settled basin berm.
Q.
Anti-seep collars shall be installed around the pipe barrel and shall
be centered within the normal saturation zone of the berm. The anti-seep
collars and their connections to the pipe barrel shall be watertight.
The anti-seep collars shall be cast-in-place and extend a minimum
of two feet beyond the outside of the principal pipe barrel. Precast
collars shall be permitted if approved by the Township. A minimum
of two collars shall be installed on each basin outlet pipe.
R.
A perforated sediment control structure, sized in accordance with
Bucks County Conservation District requirements, shall be provided
at each basin outlet structure (if more than one is to be utilized)
for sediment control. Sediment control structures shall not be removed
until the entire area tributary to the basin has been permanently
stabilized and until approved by the Bucks County Conservation District.
S.
All basin outlet pipes shall be watertight reinforced concrete having
"O-Ring" joints. All joints shall be mortared. Crushed stone bedding/backfill
shall not be utilized through basin berms.
T.
A minimum of six inches is required between the top of outlet structure
box and the emergency spillway elevation. Six inches, minimum, is
also required between the one-hundred-year water surface elevation
and the top of outlet structure box.
U.
Energy dissipating devices (rock lining/rip rap, or other approved
materials) shall be provided at all basin outlets and shall be sized
in accordance with Pennsylvania Department of Environmental Protection,
Bureau of Soil and Water Conservation Publication, Erosion and Sediment
Control Program Manual, latest revision.
V.
Stone gabion baskets shall not be permitted for use in construction
of detention/retention basins.
W.
Access easement and stabilized drive to stormwater detention facilities
shall be provided for maintenance and operation. This access easement
shall be cleared and, when possible, be at least 20 feet in width.
Multiple accesses shall be encouraged for major facilities. The developer
shall provide access easements and drives of reinforced concrete checker-block
(back-filled with topsoil and seeded) or other similar paver acceptable
to the Township Engineer, over a six-inch bed of compacted PennDOT
type 3A coarse aggregate (or approved equivalent). Accessways to basins
shall be a minimum of 10 feet wide and be no steeper in slope than
10 feet horizontal to one feet vertical (10:1). In addition, depressed
curb and concrete apron shall be provided where the accessway enters
a street/driveway and the stabilized driveway shall extend from the
bottom of the interior basin berm embankment to the point of access
to the basin. Access easement shall be owned and maintained by the
individual lot owner(s) or homeowners' association but shall be established
to permit access by the Township or its designee, for emergency inspection
and/or maintenance, at any reasonable time.
X.
Split rail fence with wire mesh backing may be required around all
detention or retention basin where directed by the Township. Fence
shall consist of locust posts (two or three rail), four feet high,
with assorted hardwood rails (eight feet to 10 feet long), and epoxy
coated wire mesh (black or green in color) installed six inches above
finished grade. Each basin fence installation shall include two points
of access with 10 feet wide self-closing, self-latch gates.
9.
General Design Requirements.
A.
Prior to finish grading of a development site and final overlay of
streets, roads and driveways, temporary measures, acceptable to the
Township, shall be taken to ensure that all runoff intended to be
intercepted and collected by an inlet or other facility, will be collected.
The plan shall include such details, notes or specification including
bituminous "eyebrows" at inlets, diversion berms, etc.
B.
Water originating from other than natural sources, such as air conditioning
units, sump pumps or other dry weather flow, wherever practical and
possible, shall be connected to a storm sewer, or street drainage
structure, or other approved stormwater conveyance facility that is
designed as part of a stormwater management BMP.
C.
All stormwater runoff and floodplain calculations and stormwater
management facilities design shall be prepared by a professional engineer
licensed in the Commonwealth of Pennsylvania.
D.
When subdivisions or land developments are submitted to the Township
for approval in sections, a complete storm sewer design for the proposed
subdivision and land development shall be submitted. The proposed
design must include the entire tract and not a portion.
[Ord. 2011-6, 5/19/2011, § 310]
1.
Stormwater runoff from all development sites shall be calculated
using either the rational method or a soil cover complex methodology.
2.
Any stormwater runoff calculations shall use generally accepted calculation
technique that is based on the NRCS soil cover complex method. Table
23-310-1 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 23-310-1 Acceptable Computation Methodologies For
Stormwater Management Plans
| |||
|---|---|---|---|
|
METHOD
|
METHOD DEVELOPED BY
|
APPLICABILITY
| |
|
TR-20 (or commercial computer package based on TR-20)
|
USDA NRCS
|
Applicable where use of full hydrology computer model is desirable
or necessary
| |
|
TR-55 (or commercial computer package based on TR-55)
|
USDA NRCS
|
Applicable for land development plans within limitations described
in TR-55
| |
|
HEC-1, HEC-HMS
|
U.S. Army Corps of Engineers
|
Applicable where use of full hydrologic computer model is desirable
or necessary
| |
|
PSRM
|
Penn State University
|
Applicable where use of a hydrologic computer model is desirable
of necessary; simpler than TR-20 or HEC-1
| |
|
Rational Method (or commercial computer package base on Rational
Method)
|
Emil Kuichling (1889)
|
Applicable sites less than 200 acres, or as approved by the
Township Engineer
| |
|
Other methods
|
Varies
|
Other computation methodologies approved by the Township Engineer
| |
3.
All calculations consistent with this chapter using the soil cover
complex method shall use the appropriate design rainfall depths for
the various return period storms according to the National Oceanic
and Atmospheric Administration (NOAA) Atlas 14 rain data corresponding
to the Doylestown rain gage, as presented in Table 23-A-1 of Appendix
23-A of this chapter. The SCS Type 11 rainfall curve data from NOAA
is listed in Figure 23-A-1 in Appendix 23-A 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-1/1-IEC-HMS is used
for stormwater runoff calculations, then the duration of rainfall
shall be 24 hours.
[Amended by Ord. No. 2022-005, 9/6/2022]
4.
Runoff curve numbers (CN) for both existing and proposed conditions
to be used in the soil cover complex method shall be obtained from
Table 23-A-2 in Appendix 23-A of this chapter.
5.
Suggested runoff coefficients (C) for both existing and proposed
conditions for use in the rational method are contained in Table 23-A-3
in Appendix 23-A of this chapter.
6.
All calculations using the rational method shall use rainfall intensities consistent with appropriate time-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.
7.
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 23-A-24 or 23-A-37 in Appendix 23-A of this part.
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.
[Amended by Ord. No. 2022-005, 9/6/2022]
8.
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 23-A-4 in Appendix
23-A of this chapter.
9.
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.
10.
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. For drainage areas greater than 20 acres in area,
the design storm hydrograph shall be computed using a calculation
method that produces a full hydrograph.
11.
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 Township Engineer.
Calibration shall require detailed gauge and rainfall data for the
particular site in question.
12.
Infrequent Storm. The rational runoff coefficients given in this
chapter are applicable for the two-year through ten-year frequency
design storm event. Less frequent, higher intensity storms require
modification of the runoff coefficient because infiltration and other
losses have a proportionally smaller effect on runoff (Wright-McLaughlin
1969). The adjustment of the rational method for use with major storms
shall be made by multiplying the rational formula by a frequency factor
Cf. Cf values are as follows: twenty-five-year frequency design storm-1.1;
fifty-year frequency design storm-1.2; one-hundred-year frequency
design storm-1.25. The product of Cf and rational coefficient C shall
not exceed 1.0.
13.
The time of concentration (Tc) is the time required for water to
flow from the hydraulically most remote point of the drainage area
to the point of interest (design point). Use of the rational formula
requires calculation of a Tc for each design point within the drainage
basin. Travel time estimation for the rational method shall be based
on NRCS Technical Release No. 55 (2nd Edition). For design purposes
the time of concentration may not be less than five minutes. Travel
time (Tt) is the time it takes runoff to travel from one location
to another in a watershed (subreach) and is a component of time of
concentration. Tc is computed by summing all the travel times for
consecutive components of the drainage conveyance system.
14.
Water moves through a watershed as sheet flow, shallow concentrated
flow, open channel flow, or some combination of these. Sheet flow
rates shall be calculated using the NRCS TR-55 (1986) variation of
the kinematic wave equation. Sheet flow length may not exceed 50 feet
over paved surfaces and 150 feet over unpaved surfaces. Maximum permitted
sheet flow length shall be 150 feet unless site specific conditions
exist (that can be demonstrated) that warrant an increase of the sheet
flow length. Under no circumstances shall sheet flow length exceed
300 feet. Shallow concentrated flow time and open channel flow time
shall be calculated using standard engineering methodologies.
[Ord. 2011-6, 5/19/2011, § 311]
1.
Whenever vegetation and topography are to be disturbed, such activity
must be in conformance with 25 Pa. Code, Chapter 102, "Erosion and
Sedimentation Control," and in accordance with the regulations of
the Bucks Conservation District and the standards and specifications
of the Township.
2.
No regulated earth disturbance activities within the Township shall
commence until approval by the Township of an erosion and sediment
control and grading plan for construction activities.
3.
Pursuant to 25 Pa. Code, Chapter 92, a PADEP "NPDES construction
activities" permit is required for regulated earth disturbance activities.
4.
Copies of any necessary permit(s) for regulated earth disturbance
activities from the appropriate PADEP regional office or Bucks County
Conservation District must be submitted to the Township.
5.
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.
6.
Whenever infiltration BMPs are proposed, the following additional
erosion and sedimentation control design standards and criteria must
be applied:
A.
Areas proposed for infiltration BMPs shall be protected from sedimentation
and compaction during the construction phase, so as to maintain their
maximum infiltration capacity. Thirty-three-inch super filter fabric
fence (or other approved protection mechanism) must be installed around
proposed infiltration areas to prevent encroachment and compaction
by construction equipment.
B.
Infiltration BMPs shall not be constructed nor receive runoff until
the entire contributory drainage area to the infiltration BMP has
received final stabilization. If necessary, thirty-three-inch super
filter fabric fence (or other approved protection mechanism) must
be installed in the vicinity of infiltration area to prevent contamination
by runoff containing suspended sediment.
7.
Peak discharges and discharge volumes from the site shall comply
with the appropriate sections above, with the following additions:
A.
For purposes of calculating required detention storage during land disturbance, peak discharges and discharge volumes shall be calculated based upon the runoff coefficients for bare soils during the maximum period and extent of disturbance from clearing, and impervious surface installation, indicated on the development plan. Controls shall insure that the difference in volume and rate of peak discharges before disturbance and during shall not exceed those peak discharges required in § 23-303 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 areas. Calculations based on the above parameters must be submitted to verify "during construction" runoff does not exceed predevelopment runoff for the one year frequency through one-hundred-year frequency design storm events.
B.
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.
8.
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 feet 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.
[Ord. 2011-6, 5/19/2011, § 312]
1.
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.
2.
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.
3.
To control postconstruction 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 postconstruction
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:
A.
Infiltration. Replication of preconstruction stormwater infiltration
conditions.
B.
Treatment. Use of water quality treatment BMPs to filter out the
chemical and physical pollutants from the stormwater runoff.
C.
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).
4.
PADEP has regulations that require Townships to ensure design, implementation,
and maintenance of best management practices (BMPs) that control runoff
from new development and redevelopment after regulated earth disturbance
activities are complete. These requirements include the need to implement
postconstruction stormwater BMPs with assurance of long-term operations
and maintenance of those BMPs.
5.
Copies of any necessary permit(s) for regulated earth disturbance
activities from the appropriate PADEP regional office must be submitted
to the Township.
[Ord. 2011-6, 5/19/2011, § 313]
Development on lands adjacent to the Delaware Canal which propose
to discharge stormwater into the canal must obtain special right-of-way
approval from Pennsylvania Department of Conservation and Natural
Resources (PADCNR). The PADCNR has established a separate policy for
granting right-of-way for stormwater drainage into the Delaware Canal,
which shall be used in conjunction with this chapter. Feeder streams
that flow or feed into the Delaware Canal shall be protected against
soil erosion, water quality degradation and sedimentation.
[Ord. 2011-6, 5/19/2011, § 314]
1.
Hot Spots.
A.
Use of infiltration BMPs is prohibited on hot spot land use areas.
Examples of hot spots are listed in Appendix 23-G.
B.
Stormwater runoff from hot spot land uses shall be pretreated. In
no case may the same BMP be employed consecutively to meet this requirement.
Guidance regarding acceptable methods of pre-treatment is located
in Appendix 23-G.
2.
West Nile Guidance Requirements. All wet basin designs shall incorporate
biologic controls consistent with the West Nile Guidance found in
Appendix 23-H.
