A.
All regulated activities in the Delaware River South watershed that do not fall under the exemption criteria shown in § 228-19 shall require submission of a drainage plan consistent with the Delaware River South Watershed Stormwater Management Plan to the municipality for review. The criteria of this chapter shall apply to the total proposed development even if development is to take place in stages. Impervious cover shall include, but not be limited to, any roof, parking or driveway areas and any new streets and sidewalks. Any areas designed to initially be gravel or crushed stone shall be assumed to be impervious for the exemption criteria.
B.
Stormwater drainage systems shall be provided in order
to permit unimpeded flow along natural watercourses, except as modified
by stormwater management facilities or open channels consistent with
this chapter.
C.
The drainage plan must be designed consistent with the sequencing provisions of § 228-8 of this chapter to ensure maintenance of the natural hydrologic regime and to promote groundwater recharge and protect groundwater and surface water quality and quantity. The drainage plan designer must proceed sequentially in accordance with Article III of this chapter.
D.
The existing points of concentrated drainage that
discharge onto adjacent property shall not be altered without permission
of the affected property owner(s) and shall be subject to any applicable
discharge criteria specified in this chapter.
E.
Areas of existing diffused drainage discharge shall
be subject to any applicable discharge criteria in the general direction
of existing discharge, whether proposed to be concentrated or maintained
as diffused drainage areas, except as otherwise provided by this chapter.
If diffused flow is proposed to be concentrated and discharged onto
adjacent property, the applicant 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.
F.
Whenever a watercourse is located within a development
site, it shall remain open in its natural state and location and shall
not be piped, impeded, or altered (except for road crossings). It
is the responsibility of the developer to stabilize existing eroded
stream/channel banks.
G.
Where a development site is traversed by watercourses,
drainage easements shall be provided conforming to the line of such
watercourses. The 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.
H.
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 municipality and the DEP through
the Joint Permit Application process, or, where deemed appropriate
by DEP, through the General Permit process.
I.
Any stormwater management facilities regulated by
this chapter that would be located in or adjacent to waters of the
commonwealth or wetlands shall be subject to approval by DEP through
the Joint Permit Application process, or, where deemed appropriate
by DEP, the General Permit process. When there is a question of whether
wetlands may be involved, it is the responsibility of the applicant
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 DEP.
J.
Any stormwater management facilities regulated by
this chapter that would be located on or discharge into state highway
rights-of-way shall be subject to approval by the Pennsylvania Department
of Transportation (PennDOT).
K.
Minimization of impervious surfaces and infiltration
of runoff through seepage beds, infiltration trenches, etc., are required
to reduce the size or eliminate the need for detention facilities.
L.
Roof drains must not be discharged to streets or roadside
ditches or connected to sanitary or storm sewers. Overland flow and
infiltration/percolation of stormwater shall be promoted where advantageous
to do so. Only when it is more advantageous to connect directly to
streets or storm sewers, shall it be permitted on a case-by-case basis
by the municipality.
A.
The design of all regulated activities shall include
the following steps in sequence to minimize stormwater impacts.
(1)
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.
(2)
An alternative is practicable if it is available and
capable of being done after taking into consideration cost, existing
technology, and logistics in light of overall project purposes and
other municipal requirements.
(3)
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.
B.
The applicant shall demonstrate that they designed
the regulated activities in the following sequence to minimize the
increases in stormwater runoff and impacts to water quality:
(1)
Prepare an Existing Resource and Site Analysis Map
(ERSAM), showing environmentally sensitive areas, including, but not
limited to, steep slopes, ponds, lakes, streams, wetlands, hydric
soils, vernal pools, floodplains, stream buffer zones, hydrologic
soil groups A, B, C, and D, any existing recharge areas and any other
requirements outlined in the New Hope Borough Subdivision and Land
Development Ordinance.[1] Establish stream buffer according to recommended criteria
or applicable ordinances.
(2)
Prepare a draft project layout avoiding sensitive areas identified in Subsection B(1) herein and minimizing total site earth disturbance as much as possible. The ratio of disturbed area to the entire site area and measures taken to minimize earth disturbance shall be included in the ERSAM.
(3)
Identify site specific existing conditions drainage
areas, discharge points, recharge areas, and hydrologic soil groups
A and B.
(6)
Satisfy groundwater recharge (infiltration) objective (§ 228-10 herein) and provide for stormwater treatment prior to infiltration.
(8)
Determine what Management District the site falls
into (Appendix D herein[3]) and conduct a predevelopment runoff analysis.
[3]
Editor's Note: Appendix D is included at the end of this chapter.
(9)
Prepare final project design to maintain predevelopment
drainage areas and discharge points, to minimize earth disturbance
and impervious surfaces, to reduce runoff to the maximum extent possible,
and the use of surface or point discharges.
(10)
Conduct a proposed conditions runoff analysis based on the final design and to meet the release rate and, in turn, the overbank flow and extreme event requirements (§ 228-12 herein).
(11)
Manage any remaining runoff through treatment
prior to discharge, as part of detention, bioretention, direct discharge
or other structural control.
In addition to the performance standards and
design criteria requirements of this article, the applicant shall
comply with the following water quality requirements of this article.
A.
Adequate storage and treatment facilities will be provided to capture and treat stormwater runoff from developed or disturbed areas. The recharge volume computed under § 228-10 herein may be a component of the water quality volume if the applicant chooses to manage both components in a single facility. If the recharge volume is less than the water quality volume, the remaining water quality volume may be captured and treated by methods other than recharge/infiltration BMPs. The required water quality volume (WQv) is the storage capacity needed to capture and to treat a portion of stormwater runoff from the developed areas of the site produced from 90% of the average annual rainfall (P). To achieve this goal, the following criterion is established: The following calculation formula is to be used to determine the water quality storage volume (WQv), in acre-feet of storage for the Delaware River South Watershed:
|
Equation § 228-9:1
| |||
|---|---|---|---|
|
WQv
|
=
|
[(P)(Rv)(A)]/12
| |
|
Where:
| |||
|
WQv
|
=
|
Water quality volume (acre-feet)
| |
|
P
|
=
|
Rainfall amount equal to 90% of events producing
this rainfall (inches)
| |
|
A
|
=
|
Area of the project contributing to the water
quality BMP (acres)
| |
|
Rv
|
=
|
0.05 + 0.009(I) where I is the percent of the
area that is impervious surface (impervious area/A* 100)
| |
|
The P value for the five PennDOT rainfall regions
is shown in Figure B-2 in Appendix B of this chapter and as shown
in Appendix Table B-5.[1] Since the Delaware River South is in PennDOT Region 5,
the P value to be utilized to meet this requirement is 2.04 inches.
| |||
[1]
Editor's Note: Appendix B is included at the end of this chapter.
B.
Design of BMPs used for water quality control shall
be in accordance with design specifications outlined in the Pennsylvania
Stormwater Best Management Practices Manual or other applicable manuals.
The following factors shall be considered when evaluating the suitability
of BMPs used to control water quality at a given development site:
[Amended 9-20-2022 by Ord. No. 2022-04]
(1)
Total contributing drainage area.
(2)
Permeability and infiltration rate of the site soils.
(3)
Slope and depth to bedrock.
(4)
Seasonal high water table.
(5)
Proximity to building foundations and well heads.
(6)
Erodibility of soils.
(7)
Land availability and configuration of the topography.
(8)
Peak discharge and required volume control.
(9)
Stream bank erosion.
(10)
Efficiency of the BMPs to mitigate potential
water quality problems.
(11)
The volume of runoff that will be effectively
treated.
(12)
The nature of the pollutant being removed.
(13)
Maintenance requirements.
(14)
Creation/protection of aquatic and wildlife
habitat.
(15)
Recreational value.
(16)
Enhancement of aesthetic and property value.
C.
To accomplish the above, the applicant shall submit
original and innovative designs to the municipality for review and
approval. Such designs may achieve the water quality objectives through
a combination of BMPs (best management practices).
A.
Infiltration BMPs shall meet the following minimum
requirements.
(1)
Regulated activities will be required to recharge (infiltrate) a portion of the runoff created by the development as part of an overall stormwater management plan designed for the site. The volume of runoff to be recharged shall be determined from Subsection A(1)(b)[1] or A(1)(b)[2], depending upon demonstrated site conditions.
(a)
Infiltration BMPs intended to receive runoff
from developed areas shall be selected based on suitability of soils
and site conditions and shall be constructed on soils that have the
following characteristics.
[1]
A minimum depth of 24 inches between the bottom
of the BMP and the limiting zone.
[2]
An infiltration rate sufficient to accept the
additional stormwater load and drain completely as determined by field
tests conducted by the applicant's professional soil scientist. The
minimum infiltration rate shall be 0.2 inches per hour regardless
of the results of applicant's field tests. If the minimum rate can
not be achieved due to site characteristics or unsuitable soil conditions,
the applicant shall consider other innovative or traditional BMPs.
[3]
The recharge facility shall be capable of completely
infiltrating the recharge volume within four days (96 hours).
[4]
Pretreatment shall be provided prior to infiltration.
[5]
The requirements for recharge are applied to
all disturbed areas, even if they are ultimately to be an undeveloped
land use such as grass, since studies have found that compaction of
the soils during disturbance reduces their infiltrative capacity.
(b)
The recharge volume (Re) shall be computed by first obtaining the infiltration requirement using methods in either Subsection A(1)(b)[1] or [2], then multiplying by the total proposed impervious area. The overall required recharge volume for a site is computed by multiplying total impervious area by the infiltration requirement.
[1]
NRCS curve number equation. The following criteria
shall apply.
[a]
The NRCS runoff shall be utilized
to calculate infiltration requirements (P) in inches. For zero runoff:
|
Equation § 228-10:1
| ||||
|
P = I (Infiltration) = (200/CN) - 2
| ||||
|
Where:
| ||||
|
P=I
|
=
|
Infiltration requirement (inches)
| ||
|
CN
|
=
|
SCS (NRCS) curve number of the existing conditions
contributing to the recharge facility
| ||
[2]
Annual recharge water budget approach. It has been determined that infiltrating 0.5 inch of runoff from the impervious areas will aid in maintaining the hydrologic regime of the watershed. If the goals of Subsection A(1)(b)[1] cannot be achieved, then 0.5 inch of rainfall shall be infiltrated from all impervious areas, up to an existing site conditions curve number of 81. Above a curve number of 81, Equation § 228-10:1 or the curve in Figure § 228-10:1 herein should be used to determine the infiltration requirement. Where I = 0.5 inch, the recharge volume (Rev) required
would therefore be computed as:
|
Rev = I * percent impervious
area (SF) / 12 = (CF)
| ||||
(2)
The recharge values derived from these methods are
the minimum volumes the applicant must control through an infiltration/recharge
BMP facility. However, if a site has areas of soils where additional
volume of infiltration can be achieved, the applicant is encouraged
to recharge as much of the stormwater runoff from the site as possible.
|
Figure 228-10:1. Infiltration Requirement
Based Upon NRCS Curve Number.
|
|---|
 |
B.
The general process for designing the infiltration
BMP shall be: A detailed soils evaluation of the project site shall
be required to determine the suitability of recharge facilities. The
evaluation shall be performed by a qualified applicant and, at a minimum,
address soil permeability, depth to bedrock, and subgrade stability.
(1)
Analyze hydrologic soil groups as well as natural
and man-made features within the watershed to determine general areas
of suitability for infiltration practices.
(2)
Provide field tests, such as double ring infiltration
tests at the level of the proposed infiltration surface to determine
the appropriate hydraulic conductivity rate.
(3)
Design the infiltration structure for the required
storm volume based on field determined capacity at the level of the
proposed infiltration surface.
(4)
Where the recharge volume requirement cannot be physically
accomplished due to the results of the field soils testing, supporting
documentation and justification shall be supplied to the municipality
with the drainage plan. If on-lot infiltration structures are proposed
by the applicant's design professional, it must be demonstrated to
the municipality that the soils are conducive to infiltration on the
lots identified.
C.
Extreme caution shall be exercised where infiltration
is proposed in geologically susceptible areas such as strip mine or
limestone areas. Extreme caution shall also 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 the DEP's Title 25: Chapter 93 Antidegradation
Regulations. The municipality may require the installation of an impermeable
liner in BMPs and/or detention basins where the possibility of groundwater
contamination exists. A detailed hydrogeologic investigation may be
required by the municipality.
D.
New Hope Borough shall require the applicant to provide
safeguards against groundwater contamination for uses which may cause
groundwater contamination, should there be a mishap or spill.
E.
Recharge/infiltration facilities shall be used in
conjunction with other innovative or traditional BMPs, stormwater
control facilities, and nonstructural stormwater management alternatives.
A.
In addition to the water quality volume, to minimize
the impact of stormwater runoff on downstream streambank erosion,
the requirement is to design a BMP to detain the post-development
two-year, twenty-four-hour design storm to the predevelopment 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 post-development 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 can 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 consider and minimize
the chances of clogging and sedimentation. Orifices smaller than three
inches in diameter are not recommended. However, if the design engineer
can provide proof that the smaller orifices are protected from clogging
by use of trash racks, etc., smaller orifices may be permitted.
A.
Delaware River South watershed has been divided into
stormwater management districts as shown on the Watershed Map in Appendix
D.[1] In addition to the requirements specified below, the water quality (§ 228-9), groundwater recharge (§ 228-10), and streambank erosion (§ 228-11) requirements shall be implemented.
[1]
Editor's Note: Appendix D is included at the end of this chapter.
B.
Standards for managing runoff from each subarea in
the Delaware River South watershed is shown below. Development sites
located in each of the A, B, or C Districts must control post-development
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 may do so without
control of postdevelopment peak rate of runoff greater than the five-year
storm, upon the recommendation of the Municipal Engineer and approval
by the New Hope Borough Council. Sites in District C will still have
to comply with the groundwater recharge criteria, the water quality
criteria, and streambank erosion criteria. 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 one-hundred-year predevelopment
peak flow, or will be provided with improvements to furnish the required
capacity. The downstream hydraulic capacity analysis must include
the assumption of complete development build-out of the tributary
areas to the channel being evaluated, based on future land use per
the latest zoning after the plan adoption. When adequate capacity
in the downstream system does not exist and will not be provided through
improvements, the post-development peak rate of runoff must be controlled
to the predevelopment peak rate as required in District A provisions
(i.e., one-hundred-year postdevelopment flows to one-hundred-year
predevelopment flows) for the specified design storms.
|
A.
General. Post-development peak rates of runoff from any regulated activity shall meet the peak release rates of runoff prior to development for the design storms specified in § 228-12 of this chapter.
B.
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. However, on-site drainage
facilities shall be designed to safely convey off-site flows through
the development site.
C.
Site areas. Where the area of a site being impacted
by a proposed development activity differs significantly from the
total site area, only the proposed impact area utilizing stormwater
management measures shall be subject to the Management District Criteria.
Unimpacted or undisturbed areas that do flow into or are bypassing
the stormwater management facilities would not be subject to the Management
District Criteria.
D.
"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.
(1)
Natural or man-made channels or swales must be able
to convey the increased runoff associated with a 1.5-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 Department of Environmental Protection's
Erosion and Sediment Pollution Control Program Manual.
(2)
Natural or man-made channels or swales must be able
to convey increased one-hundred-year return period runoff without
creating any hazards to persons or property.
(3)
Culverts, bridges, storm sewers or any other facilities, which must pass or convey flows from the tributary area, must be designed in accordance with the Department of Environmental Protection's Chapter 105 regulations (if applicable) and, at minimum, pass the increased one-hundred-year return period runoff.
E.
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 applicants. 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. It is a requirement that, even if regional basins are proposed
for the water quantity control, that the water quality, streambank
erosion and recharge criteria be accomplished on-site, or as close
to the source of the runoff as possible.
A.
Any stormwater facility located on state highway rights-of-way
shall be subject to approval by the Pennsylvania Department of Transportation
(PennDOT) and New Hope Borough.
B.
Any stormwater management facility (i.e., detention basin) designed to store runoff and requiring a berm or earthen embankment required or regulated by this chapter shall be designed to provide an emergency spillway to handle flow up to and including the one-hundred-year post-development conditions. The height of embankment must be set as to provide a minimum one foot of freeboard above the maximum pool elevation computed when the facility functions for the one-hundred-year post-development inflow. Should any stormwater management facility require a dam safety permit under Title 25, Environmental Protection, Chapter 105, Dam Safety and Waterway Management, the facility shall be designed in accordance with Chapter 105 and meet the regulations of Chapter 105 concerning dam safety which may be required to pass storms larger than one-hundred-year event.
C.
Any facilities that constitute water obstructions (e.g., culverts, bridges, outfalls, or stream enclosures) and any work involving wetlands, as directed in DEP Chapter 105 regulations (as amended or replaced from time to time by DEP), shall be designed in accordance with Chapter 105 and will require a permit from DEP. Any other drainage conveyance facility that does not fall under Chapter 105 regulations must be able to convey, without damage to the drainage structure or roadway, runoff from the one-hundred-year design storm with a minimum one foot of freeboard measured below the lowest point along the top of the roadway. Roadway crossings located within designated floodplain areas must be able to convey runoff from a one-hundred-year design storm with a minimum one foot of freeboard measured below the lowest point along the top of the roadway. Any facility that constitutes a dam, as defined in DEP Chapter 105 regulations, may require a permit under dam safety regulations. Any facility located within a PennDOT right-of-way must meet PennDOT minimum design standards and permit submission requirements.
D.
Any drainage conveyance facility and/or channel that does not fall under Chapter 105 Regulations must be able to convey, without damage to the drainage structure or roadway, runoff from the one-hundred-year design storm. Conveyance facilities to or exiting from stormwater management facilities (i.e., detention basins) shall be designed to convey the design flow to or from that structure. Roadway crossings located within designated floodplain areas must be able to convey runoff from a one-hundred-year design storm. Any facility located within a PennDOT right-of-way must meet PennDOT and New Hope Borough minimum design standards and permit submission requirements.
E.
Storm sewers must be able to convey post-development
runoff from a one-hundred-year design storm without surcharging inlets.
F.
Adequate erosion protection shall be provided along
all open channels and at all points of discharge.
G.
The design of all stormwater management facilities
shall incorporate sound engineering principles and practices. The
municipality shall reserve the right to disapprove any design that
would result in the creation, exacerbation, or continuation of an
adverse hydrologic or hydraulic condition within the watershed.
H.
General design requirements.
(1)
The design of all stormwater management facilities
shall incorporate sound engineering principles and practices. If the
Subdivision and Land Development Ordinance (SALDO)[1] has more stringent design criteria, then the more stringent
criteria shall apply.
(2)
Where stormwater runoff will be collected within a
development and discharged over lands within or beyond the boundaries
of the property/lot boundary, the applicant shall reserve or obtain
easements over all lands affected. The stormwater easement width shall
be of adequate width to collect the stormwater runoff, and for access
to the stormwater facility for maintenance and repairs.
(3)
All as-constructed stormwater management facilities
shall be surveyed by a professional land surveyor licensed in the
commonwealth to verify compliance with the stormwater management facilities
as depicted on the plan. As-constructed plans shall be submitted to
the Borough for review and approval upon completion of all facilities.
(4)
All sump pump and roof drain leaders shall be connected
to an existing or proposed storm sewer system, or discharged to an
approved drainage swale. Sump pumps and roof drain leaders shall not
be discharged over or through a curb onto a public street, or connected
to a roadway underdrain system.
(5)
All new public streets shall be constructed with a
minimum six-inch underdrain system on both sides of the cartway. A
minimum six-inch underdrain system shall also be installed along existing
roadways where widening is proposed with curb.
I.
Detention/retention basin facilities.
(1)
Detention basins 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. Detention
basins shall be designed to facilitate regular maintenance, mowing
and periodic de-silting.
(2)
Within proposed subdivisions, detention basins shall
be located in open space, on an individual lot, or common area, but
shall not cross individual lot boundaries, unless a homeowners' or
condominium association will own and maintain the detention basin.
(3)
All basin berms, including emergency spillways, shall
have a maximum interior and exterior side slope of three horizontal
to one vertical. The toe of any slope shall be located a minimum of
five feet from any property line or street right-of-way line. Also,
all portions of the detention basin bottom shall be sloped towards
the outlet structure at a minimum slope of 2%.
(4)
The minimum basin berm width at the design elevation
shall be 10 feet. A cutoff trench (keyway) of impervious material
shall be provided under all embankments that require fill material.
The cutoff trench shall be a minimum of eight feet wide, three feet
deep and have side slopes of one horizontal to one vertical.
(5)
Outlet pipes from detention facilities shall be designed
to control the peak rate for the one-hundred-year design storm. All
basin outlet pipes shall be watertight reinforced concrete having
"O-Ring" joints, with a minimum size of 18 inches. Also, two anti-seep
collars shall be installed around the basin outlet pipe 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.
(6)
An emergency spillway shall be provided for a detention
basin in order to convey basin inflow in excess of design flows, or
in the event the outlet structure becomes blocked and is unable to
convey the design flow. The minimum capacity of the emergency spillway
shall be equivalent to the one-hundred-year peak inflow rate for the
post-development design storm. Emergency spillways shall be constructed
of interlocking grass paver system, backfilled with topsoil and seeded,
to protect the berm against erosion. The emergency spillway lining
shall extend to the toe of the embankment on the exterior of the berm,
and shall extend to an elevation of three feet below the spillway
crest on the interior of the berm. Also, the minimum freeboard through
the emergency spillway shall be one foot, and the maximum spillway
length shall not exceed 75 feet.
(7)
For all detention basins, the permanent outlet control
structure shall be a Type "M" inlet grate and box. The inlet grate
shall only be used as an emergency outflow and the grate elevation
shall be equal to or higher than the one-hundred-year water surface
elevation, but at least six inches below the emergency spillway elevation.
(8)
Detention/retention basin screening shall consist
of one deciduous tree (three-inch caliper) or evergreen (six feet
high) planted every 20 feet, plus one deciduous/evergreen shrub (30
inches high) planted every 10 feet along the basin perimeter. The
screening may be planted in an informal arrangement as approved by
the Municipal Engineer.
(9)
Where a retention basin (pond) is proposed, the applicant
shall demonstrate the retention basin is designed to protect and provide
for the public health, safety and welfare. At a minimum, the retention
basin design shall include a ten-foot-wide safety ledge, clay liner,
cutoff trench, fountain aerator, and specifications for embankment
construction and soil testing.
(10)
Underground detention basins shall meet the
following criteria:
(a)
All pipe shall be constructed of high density
corrugated polyethylene pipe, or approved equal. The required volume
shall be provided utilizing only underground pipe capacity. Each pipe
shall have a minimum slope of 0.5%.
(b)
Inlets or storm manhole structures with steps
shall be located at each corner of the underground detention basin
to provide access for maintenance.
(c)
A minimum of one foot of freeboard shall be
provided between the one-hundred-year water surface elevation and
the top of all inlet grates or manhole rims.
(d)
An emergency overflow weir shall be provided
for the outlet structure in order to convey basin inflow in excess
of design flows, or in the event the outlet structure becomes blocked
and is unable to convey the design flow. The minimum capacity of the
emergency overflow weir shall be equivalent to the one-hundred-year
peak inflow rate for the post-development design storm.
(e)
Proposed site improvements and grading downstream
of the underground detention basin shall be designed to convey the
one-hundred-year peak inflow rate to a natural outfall, storm sewer
system, or other drainage facility, without adversely affecting downstream
properties.
J.
Storm sewer and drainage facilities.
(1)
All storm sewer systems shall provide the required
capacity for the one-hundred-year design storm based on the Rational
Method, as determined by the PennDOT Design Manual, Part 2, as amended,
and the U.S. Department of Commerce, Hydraulic Design Series No. 5,
Hydraulic Design of Highway Culverts, as amended. Rainfall intensity
values found in the PennDOT Design Manual Part 2, as amended, shall
be used for calculating peak flow rates.
(2)
Storm sewer shall be reinforced concrete pipe with
a minimum diameter of 15 inches or equivalent. Storm sewer located
outside a public ultimate right-of-way or easement may be either high
density corrugated polyethylene or approved aluminum corrugated pipe.
(3)
Storm sewer shall be installed with a slope such that
a minimum full-flow velocity of two feet per second will be attained;
however, the slope shall not be less than 0.50%. No pipe shall exceed
a slope that will permit full-flow velocity in excess of 15 feet per
second.
(4)
The top of all storm sewer shall be at least six inches
below the grade elevation of the pavement stone subbase. For storm
sewer located in pervious areas, the top of pipe elevation shall be
a minimum of one foot below the finished grade elevation.
(5)
A minimum of one foot of freeboard shall be provided
between the one-hundred-year water surface elevation and the top of
inlet grate or manhole rim.
(6)
When there is a change in storm sewer size at an inlet
or manhole structure, the top of pipe elevations shall be the same.
For equal storm sewer sizes, a minimum of two inches shall be provided
between the inlet pipe and the outlet pipe invert elevations.
(7)
Inlets to be utilized within a public right-of-way
shall be Type "C" or Type "M," with standard concrete top unit and
"bicycle safe" structural steel grate. Storm sewer manhole covers
shall have the word "STORM" cast in two-inch-high letters.
(8)
Storm sewer inlet and manhole spacing shall not exceed
300 feet. Inlet spacing in paved areas shall be arranged so that a
minimum of 80% of the gutter flow tributary to the inlet will be captured.
The designer shall be required to verify that bypass surface runoff
from the one-hundred-year design storm will enter the storm sewer
system at some point, prior to discharge into a detention basin or
other approved outlet point.
(9)
At street intersections, inlets shall be placed at
the curb tangent and not in the curved portion of the curb. At least
one inlet shall be required at the curb tangent on the upstream side
of all access drive and driveway intersections with public roadways.
(10)
A Type "DW" Endwall shall be provided at the
termination of all storm sewer systems, unless conditions warranted
an alternate type as approved by the Municipal Engineer.
(11)
All drainage swales shall be designed to convey
the one-hundred-year peak flow rate, as determined by the Rational
Method. All swales shall have maximum side slopes of four horizontal
to one vertical and a minimum center-line slope of 2%.
(12)
Bridges and culverts shall be designed and constructed
to meet current PennDOT construction and loading standards (HS-25
loading standards). They shall be constructed to the full width of
the right-of-way or to an adequate dimension to accommodate special
grade conditions. Bethlehem Precast Cable Concrete CC-20, or approved
equal, shall be required at the upstream and downstream ends of all
bridges and culverts.
Stormwater runoff from all development sites
shall be calculated using the following methodology.
A.
Stormwater runoff calculations for the design of stormwater
detention/retention facilities shall use a generally accepted calculation
technique that is based on the NRCS soil cover complex method. Table
228-15:1 summarizes acceptable computation methods. It is assumed
that all methods will be selected by the applicant based on the individual
limitations and suitability of each method for a particular site.
B.
All calculations consistent with this chapter using
the soil cover complex method shall use the appropriate design rainfall
depths for the various return period storms according to the region
for which they are located as presented in Table B-1 in Appendix B
of this chapter.[1] If a hydrologic computer model such as HEC-1 or HEC-HMS
is used for stormwater runoff calculations, then the duration of rainfall
shall be 24 hours. The SCS 'S' curve shown in Figure B-1, Appendix
B of this chapter shall be used for the rainfall distribution.
[1]
Editor's Note: Appendix B is included at the end of this chapter.
C.
For the purposes of predevelopment flow rate determination,
undeveloped land shall be considered as "meadow" in good condition,
unless the natural ground cover generates a lower curve number (i.e.,
forest), as listed in Table B-2 in Appendix B of this chapter. For
areas of prior mining disturbance (i.e., strip mining, mine spoil
areas, etc.), the designer must first identify in which mining area
the site is located, using the Management District Map in Appendix
D herein.[2] The appropriate curve number from Table B-2 or Table B-3
herein shall then be used.
[2]
Editor's Note: Appendix D is included at the end of this chapter.
D.
Times-of-concentration for overland flow shall be
calculated using the methodology presented in Chapter 3 of Urban Hydrology
for Small Watersheds, NRCS, TR-55 (as amended or replaced from time
to time by NRCS). Times-of-concentration for channel and pipe flow
shall be computed using Manning's equation.
E.
For stormwater detention/retention facilities, runoff
curve numbers (CN) for both existing and proposed conditions to be
used in the soil cover complex method shall be obtained from Table
B-2 in Appendix B of this chapter.
F.
For storm sewer facilities, runoff coefficients (c)
for both existing and proposed conditions for use in the Rational
Method shall be obtained from Table B-3 in Appendix B of this chapter.
G.
Where uniform flow is anticipated, the Manning equation
shall be used for hydraulic computations and to determine the capacity
of open channels, pipes, and storm sewers. Values for Manning's roughness
coefficient (n) shall be consistent with Table B-4 in Appendix B of
the chapter.
H.
Outlet structures for stormwater management facilities
shall be designed to meet the performance standards of this chapter
using any generally accepted hydraulic analysis technique or method.
I.
The design of any stormwater detention facilities
intended to meet the performance standards of this chapter shall be
verified by routing the design storm hydrograph through these facilities
using the Storage-Indication Method. For drainage areas greater than
200 acres in size, the design storm hydrograph shall be computed using
a calculation method that produces a full hydrograph. The municipality
may approve the use of any generally accepted full hydrograph approximation
technique that shall use a total runoff volume that is consistent
with the volume from a method that produces a full hydrograph.
|
Table 228-15:1. Acceptable Computation
Methodologies For Stormwater Detention/Retention Facilities
| |||
|---|---|---|---|
|
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
|
US Army Corps of Engineers
|
Applicable where use of full hydrologic computer
model is desirable or necessary
| |
|
PSRM
|
Penn State University
|
Applicable where use of a hydrologic computer
model is desirable or necessary; simpler than TR-20 or HEC-1
| |
A.
Whenever the vegetation and topography are to be disturbed, such activity must be in conformance with Chapter 102, Title 25, Rules and Regulations, Part I, Commonwealth of Pennsylvania, Department of Environmental Protection, Subpart C, Protection of Natural Resources, Article II, Water Resources, Chapter 102, "Erosion Control," and in accordance with the Bucks County Conservation District.
B.
Additional erosion and sedimentation control design
standards and criteria that must be applied where infiltration BMPs
are proposed shall include the following.
(1)
Areas proposed for infiltration BMPs shall be protected
from sedimentation and compaction during the construction phase, so
as to maintain their maximum infiltration capacity.
(2)
Infiltration BMPs shall not be constructed nor receive
runoff until the entire contributory drainage area to the infiltration
BMP has received final stabilization.
Developments on lands adjacent to the Delaware
Canal which propose to discharge stormwater into the canal must obtain
special right-of-way approval from the Department of Conservation
and Natural Resources (DCNR). The DCNR 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.
