A.
All regulated activities in the Township which do not fall under the exemption criteria shown in § 132-5 shall submit a drainage 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. The starting point from which to consider tracts as parent tracts is February 25, 2004. All impervious surface area constructed on or after February 25, 2004, shall be considered cumulatively. Impervious surface area existing on the parent tract prior to this date shall not be considered in cumulative impervious surface area calculations for determining exemption applicability. No regulated activities shall commence until the Township issues written approval of a drainage plan, demonstrating compliance with the requirements of this chapter, or issues a building or zoning permit where a proposed activity is exempt from the requirements of this chapter related to preparation of a drainage plan.
B.
For all regulated earth disturbance activities, erosion and sediment
control 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.[1] Various BMPs and their design standards are listed in
the Erosion and Sediment Pollution Control Program Manual (E&S
Manual) dated March 2012, as amended and updated.
[1]
Editor's Note: See 35 P.S. § 691.1 et seq.
C.
All regulated activities shall be designed in a manner to protect
health, safety, and property and meet water quality goals of this
chapter by minimizing disturbance to floodplains, wetlands, and wooded
areas, maintaining or extending riparian buffers, preventing erosive
flow conditions in natural flow pathways, minimizing thermal impacts
to surface waters of the commonwealth, and by disconnecting impervious
surfaces by directing stormwater runoff over pervious areas wherever
possible. Stormwater management facilities/BMPs shall be designed,
implemented, operated, and maintained with all regulated activities
to meet the purposes and requirements of this chapter and to meet
all requirements under Title 25 of the Pennsylvania Code, the Clean
Streams Law, and the Storm Water Management Act.[2]
[2]
Editor's Note: See 32 P.S. § 680.1 et seq.
D.
To the maximum extent practicable, all regulated activities shall
be designed in a manner to incorporate techniques for low impact development
practices described in the Pennsylvania Stormwater Best Management
Practices Manual.
E.
Stormwater drainage systems shall be provided in order to permit
unimpeded flow along natural watercourses, except as modified by stormwater
management facilities, stormwater BMPs or open channels consistent
with this chapter.
F.
The existing points of concentrated drainage flow that discharge
onto an adjacent downstream property shall not be altered without
written consent of the affected property owner(s) in the form of a
drainage easement. The Township Engineer shall determine the length
and width of the drainage easement as well as the extent of the properties
from which written consent must be obtained by the developer or applicant.
An increase in runoff shall be subject to review for the effect of
increased flow rate and velocity on the downstream receiving stormwater
facilities to evaluate the potential for flooding and erosion. The
applicant must provide supporting calculations showing a "no harm"
condition relative to the same prior to approval by the Township Engineer.
G.
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 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. The developer shall provide
documentation indicating that permission has been obtained from the
affected downstream property owner(s) for the purpose of discharging
formerly diffused flow in a concentrated manner onto the property
owner's property and/or improving the downstream off-site stormwater
facilities as directed by the Township Engineer.
H.
Stormwater runoff peak rate control.
(1)
In the design of stormwater management facilities, postdevelopment
rates of runoff from any regulated activity shall not exceed 75% of
the peak rates of runoff prior to development for two- and ten-year-frequency
storms and 100% of the peak rates of runoff prior to development for
the twenty-five-, fifty-, and one-hundred-year frequency storms. In
all other cases where unretained stormwater directly discharges from
the site by bypassing the stormwater management facilities, the postdevelopment
runoff rate shall not exceed the predevelopment runoff rate.
(2)
For all regulated activities within the Township, 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).
The preceding requirements shall apply to each location of concentrated
or diffused drainage discharge from the development site.
(3)
For redevelopment sites, one of the following minimum design parameters
shall be accomplished, whichever is most appropriate for the given
site conditions as determined by the Township Engineer:
(4)
For all regulated activities within the Neshaminy Creek Watershed,
postdevelopment rates of runoff shall not exceed those peak rates
identified in Appendix 132F of this chapter.
I.
Where the area of a site being impacted by a proposed development activity not associated with a subdivision or land development differs significantly from the total site area, only the proposed disturbed area and other areas compacted due to construction activity shall be subject to the peak release rate criteria of § 132-11H. The postdevelopment runoff rate from unimpacted or undisturbed areas that do not flow into or are bypassing the stormwater management facilities shall not exceed the predevelopment runoff rate.
J.
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), and 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, the removal of trees/vegetation, and any alterations
that may adversely affect the flow of stormwater within any portion
of the easement. The width of the easement shall be defined as the
Riparian Corridor Zone 1 limits, a minimum of 25 feet wide, measured
horizontally on both sides from the top of the bank of the identified
watercourse. Also, the property owner shall be responsible for maintenance
of vegetation within the easement as governed by the requirements
referenced under the definition of the riparian corridor. For all
subdivisions and land developments, the developer shall stabilize
all eroded stream/channel beds and banks 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.
K.
For all regulated activities proposed in conjunction with a subdivision
and land development, whether or not those disturbance activities
are adjacent to a watercourse, sediment-reducing best management practices
(BMPs) must be provided to the watercourse in a manner approved by
the Township Engineer. Calculations must be included with the subdivision
and land development application which document the amount of sediment
expected to be removed from the watercourse on a yearly basis (as
defined pursuant to the Pennsylvania BMP Manual) as a result of the
proposed BMPs.
L.
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 of the Township Engineer and by PADEP
through the Joint U.S. Army Corps of Engineers/PADEP Stream Encroachment
application or, where deemed appropriate by PADEP, through the general
permit process.
M.
Any stormwater management facilities regulated by this chapter that
would be located in or adjacent to surface waters of the commonwealth
or wetlands shall be subject to approval by PADEP through the joint
permit application process or, where deemed appropriate by PADEP,
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.
N.
Any stormwater management facilities regulated by this chapter that
would be located on state highway rights-of-way shall be subject to
approval by the Pennsylvania Department of Transportation (PennDOT).
O.
Minimization of impervious surfaces and infiltration of runoff through seepage beds, infiltration trenches, etc., are required, where soil conditions permit, to reduce the size or eliminate the need for detention facilities. Soil conditions that are not considered suitable are identified in § 132-12. Sinkhole-prone areas where karst bedrock exists and steep slope areas as defined by Towamencin Township Code, Chapter 153, Zoning, are not considered suitable for infiltration BMPs.
P.
Infiltration BMPs should be spread out, made as shallow as practicable,
and be located to maximize use of natural on-site infiltration features
while still meeting the other requirements of this chapter. As a selection
guide and design references for developing these methods, the applicants'
designers should use the Pennsylvania Stormwater Best Management Practices
Manual.
Q.
Roof drains must not be connected to streets or sanitary sewers.
They should be connected to storm sewers or roadside ditches only
when necessary and only when they discharge to infiltration or vegetative
stormwater BMPs. Infiltration/percolation of stormwater should be
provided by the means of dry wells, seepage or infiltration pits where
conditions permit.
R.
Prohibition against nonstormwater discharges.
(1)
Prohibited discharges.
(a)
No person shall allow, or cause to allow, discharges into the Township's regulated small MS4, or discharges into surface waters of the commonwealth, which are not composed entirely of stormwater, except as provided in § 132-11R(1)(b) below, or discharges allowed under a state or federal permit.
(b)
Discharges which may be allowed based on a finding by the Township
that the discharges do not significantly contribute to pollution to
the regulated small MS4 or to surface waters of the Commonwealth are:
[1]
Discharges from firefighting activities.
[2]
Potable water sources, including waterline and fire hydrant
flushing.
[3]
Uncontaminated water from foundation or from footing drains.
[4]
Flows from riparian habitats and wetlands.
[5]
Lawn watering.
[6]
Irrigation drainage.
[7]
Pavement wash waters where spills or leaks of toxic or hazardous
materials have not occurred (unless all spill material has been removed)
and where detergents are not used.
[8]
Routine external building wash down (which does not use detergents
or other compounds).
[9]
Air-conditioning condensate.
[10]
Water from individual residential car washing.
[11]
Dechlorinated swimming pool discharges.
[12]
Springs.
[13]
Uncontaminated groundwater.
[14]
Water from crawl space pumps or sump pumps.
[15]
Diverted stream flows.
(c)
In the event that the Township or PADEP determines that any of the discharges identified in § 132-11R(1)(b) significantly contribute pollutants to the regulated small MS4 or surface waters of the commonwealth, the Township or PADEP will notify the responsible person(s) to cease the discharge.
(d)
Upon notice provided by the Township or PADEP under § 132-11R(1)(c), the discharger will have a reasonable time, as determined by the Township, to cease the discharge consistent with the degree of pollution caused by the discharge.
(e)
Nothing in this section shall affect a discharger's responsibilities
under state law.
(2)
Prohibited connections. The following connections are prohibited, except as provided in § 132-11R(1) above:
(a)
Any drain or conveyance, whether on the surface or subsurface,
which allows any nonstormwater discharge, including sewage, process
wastewater, and wash water, to enter the regulated small MS4 or the
surface waters of the commonwealth, and any connections to the storm
drain system from indoor waste water drains and sinks; and
(b)
Any drain or conveyance connected from a commercial or industrial
land use to the regulated small MS4 which has not been documented
in plans, maps, or equivalent records, and approved by the Township.
S.
Hot spots.
(1)
Hot spots are sites where the land use or activity produces a higher
concentration of trace metals, hydrocarbons, or priority pollutants
than normally found in urban runoff. Use of infiltration BMPs is prohibited
on hot spot land use areas. Examples of hot spots include but are
not limited to the following:
(a)
Vehicle salvage yards and recycling facilities.
(b)
Vehicle fueling stations.
(c)
Vehicle service and maintenance facilities.
(d)
Vehicle and equipment cleaning facilities.
(e)
Fleet storage areas (bus, truck, etc.).
(f)
Industrial sites (based on Standard Industrial Codes defined
by the U.S. Department of Labor).
(g)
Marinas (service and maintenance).
(h)
Outdoor liquid container storage.
(i)
Outdoor loading/unloading facilities.
(j)
Public works storage areas.
(k)
Facilities that generate or store hazardous materials.
(l)
Commercial container nursery.
(m)
Other land uses and activities as designated by the Township.
(2)
Stormwater runoff from hot spot land uses shall be pretreated. In
no case may the same BMP be employed consecutively to meet this requirement.
Guidance regarding acceptable methods of pretreatment is located in
The Pennsylvania Stormwater Best Management Practices Manual.
T.
Additional stormwater management criteria applicable to properties
within the Neshaminy Creek Watershed. For regulated activities within
the Neshaminy Creek Watershed, the additional stormwater management
provisions identified in Appendix 132F of this chapter shall be applicable.
A.
For all regulated activities not exempt from preparation and submission of a drainage plan pursuant to § 132-5 of this chapter, water volume controls shall be implemented. Postdevelopment total runoff volume shall not be increased from predevelopment total runoff volume for all storms equal to or less than the two-year, twenty-four-hour duration precipitation. The Design Storm Method, which requires detailed stormwater runoff modeling based on site conditions, shall be utilized to calculate the volume of runoff that must either be reused, evapotranspired, or infiltrated through structural or nonstructural means. Water volume controls will mitigate increased runoff impacts, protect stream channel morphology, maintain groundwater recharge, and contribute to water quality improvements. The low impact development practices provided in the Pennsylvania BMP Manual shall be utilized for all regulated activities to the maximum extent practicable. For regulated activities within the Neshaminy Creek Watershed, the volume control criteria identified in Appendix 132F[1] of this chapter shall be applicable.
[1]
Editor's Note: Appendix 132F is included as an attachment to this chapter.
B.
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.
C.
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 18 inches between the bottom of the infiltration
BMPs and the top of bedrock or seasonally high water table (i.e.,
"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.
(2)
An infiltration rate sufficient to accept the additional stormwater
load and dewater completely as determined by field tests. The minimum
acceptable infiltration rate for field-tested soils shall be 0.2 inches/hour
(in/hr). A safety factor of 50% shall be applied to field-tested infiltration
rates to determine the rate to be utilized for design purposes (e.g.,
for soil which measured 0.4 in/hr in the field, the BMP design shall
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.
(4)
When infiltration methods such as seepage pits, beds, or trenches
are proposed, the locations of existing and proposed septic tanks,
infiltration areas, and wells must be shown. A separation distance
of no less than 20 feet shall be provided between any septic system
and any facility used for stormwater management and infiltration.
(5)
A minimum of 30 feet of undisturbed soil 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)
Surface inflows shall be designed to prevent direct discharge of
sediment into the infiltration system.
(8)
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. Areas proposed
for infiltration BMPs shall be protected from sedimentation and compaction
during the construction phase, so as to maintain their maximum infiltration
capacity.
(9)
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. The qualified design professional shall evaluate
the possibility of groundwater contamination from the proposed infiltration/recharge
facility if it is anticipated that such pollutants or similar pollutants
may enter the facility.
(10)
The drainage 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 the BMP and/or detention
basins where the possibility of groundwater contamination exists.
A detailed hydrogeologic investigation may be required by the Township.
If impermeable liner or other containment measures are required to
prevent contaminated runoff from infiltrating into the ground, additional
measures conforming to the requirements of this chapter must be implemented
to improve the quality of stormwater runoff from the development site.
(11)
Infiltration BMPs shall not be constructed nor receive runoff
until the entire contributory drainage area to the infiltration BMP
has achieved final stabilization.
(12)
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.
D.
A soils evaluation of the project site shall be required to determine the suitability of infiltration facilities. All applicants proposing regulated activities that are not exempt from preparation and submission of a drainage plan pursuant to § 132-5 of this chapter are required to perform a detailed soils evaluation by a qualified geotechnical engineer, geologist and/or soil scientist, in accordance with Appendix 132C[2] of this chapter, which at a minimum addresses soil permeability,
depth to bedrock, susceptibility to sinkhole formation, 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 BMP 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.
[2]
Editor's Note: Appendix 132C is included as an attachment to this chapter.
A.
Stormwater runoff which may result from regulated activities identified in § 132-4 shall be controlled by permanent stormwater runoff BMPs that will provide the required standards within Article III. 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 the "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.
B.
Any stormwater facility located on state highway rights-of-way shall
be subject to approval by the Pennsylvania Department of Transportation.
C.
Collection system standards.
(1)
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.
(2)
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.
(3)
Minimum pipe size. Minimum pipe diameter shall be 15 inches (or an
equivalent flow area of 1.23 square feet).
(4)
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 ("environmental" type). Storm
drain markers shall be stainless steel affixed to the inlet hood with
adhesive, rivets or bolts. (Markers may be bolted to the grate in
off-road locations). Markers 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.
(5)
Open-end pipes must be fitted with concrete endwalls or wing walls
in accordance with PennDOT standards.
(6)
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 1/2 of 1% (0.005 ft/ft), with the exception
that terminal sections of pipe shall have a minimum slope of 1% (0.01
ft/ft).
(7)
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.
(8)
Stormwater collection facilities shall be designed and constructed
to intercept concentrated runoff prior to discharge over public/private
rights-of-way, sidewalks, streets, and driveways.
(9)
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. 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. 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. 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 adjoining public or
private property, outside of a storm sewer easement, and that the
depth of the accumulated stormwater would not exceed 12 inches.
(10)
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. Storm sewer pipes shall enter
and exit the sides of inlet boxes and shall not encroach into the
corner, wherever possible.
(11)
Stormwater pipes shall have a minimum depth of cover of 18 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, to achieve required cover. Stormwater pipes conveying
swale flow under driveway crossings shall have a minimum cover of
12 inches, including over the bell, but in no case shall the cover
be less than that required for the anticipated traffic loading. For
driveway culverts, cover may be less than 12 inches if the design
engineer verifies such pipe has sufficient strength for the anticipated
vehicle loading. Where cover is restricted, concrete trench drain
with bolt-down metal grate may be used.
(12)
The capacity of all stormwater pipes shall be calculated utilizing
the Manning Equation for open channel flow as applied to closed conduit
flow. The 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.
(13)
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.
(14)
Storm sewer structures (e.g., endwalls, inlets, pipe 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
and structures, unless approval is received from the Township and
the authority or utility having jurisdiction over same.
(15)
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
sewers must be linear.
(16)
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.
(17)
Where a public storm sewer system is not located within a right-of-way
or dedicated public property, a twenty-foot-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.
(18)
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.
(19)
Stormwater roof drains and sump pumps shall not discharge water
directly onto a sidewalk or a street and shall be constructed to discharge
to a dry well/seepage pit or above ground entirely on the subject
property, except where such discharge could flow across a sidewalk
or onto a street. If approved by the Township Engineer, roof drains
and sump pumps may be discharged directly to a storm sewer system
if such system discharges to a stormwater BMP or water quality facility.
D.
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:
(1)
Roughness coefficient. The roughness coefficient shall be 0.040 for
earth swales.
(2)
Bank slopes. Slopes for swale banks shall not be steeper than one
vertical to four horizontal.
(3)
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 Montgomery County Conservation District.
(NOTE: The 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.
|
Table 132-13-1 Allowable Velocity
| |||
|---|---|---|---|
|
Allowable Velocity
| |||
|
Material
|
Velocity in Feet per Second
(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
| ||
(4)
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.
(5)
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.
(6)
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.
(7)
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.
(8)
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.
(9)
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.
E.
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:
(1)
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.
(2)
The bottom of the 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.
(3)
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.
(4)
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.
(5)
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.
F.
Storm sewer design.
(1)
Design flow rate.
(a)
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:
|
Q
|
=
|
Peak runoff rate measured in cubic feet per second (cfs).
| |
|
C
|
=
|
Runoff coefficient. The coefficient of stormwater runoff includes
many variables, such as ground slope, ground cover, shape of drainage
area, etc.
| |
|
I
|
=
|
Intensity: average rainfall intensity in inches per hour for
a time equal to the time of concentration.
| |
|
A
|
=
|
Area: drainage area in acres.
|
(b)
Values for the rainfall intensity shall be based on NOAA Atlas
14, Volume 2, Version 3.0, rain data found in Table B-2 and Figure
B-2 of Appendix 132B of this chapter.[1]
[1]
Editor's Note: Appendix 132B is included as an attachment to this chapter.
(2)
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.
(3)
In determining the peak flow rate to individual storm sewer inlets (or other collection structures) the time of concentration method (as referenced in § 132-14) 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.
(4)
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 (or equivalent flow area of 4.9 square
feet), 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.
(5)
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.
G.
Grading and drainage.
(1)
After completion of rough grading, a minimum of six inches of topsoil
shall be returned to remaining disturbed areas prior to final grading
and seeding.
(2)
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) and final grade shall
comply with Township Building Code requirements.
(3)
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.
(4)
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:
(a)
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.
(b)
A concrete, segmental block, or stone masonry wall, constructed
in accordance with Township requirements, is provided to support the
face of the excavation.
(5)
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:
(a)
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.
(b)
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.
(c)
A concrete, segmental block, or stone masonry wall, constructed
in accordance with Township requirements, is provided to support the
face of the fill.
(6)
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. A protective fence shall be required at
the top of the wall (or embankment), pursuant to the requirements
of the Pennsylvania Uniform Construction Code, where walls or slopes
(steeper than two horizontal to one vertical) are approved under the
criteria in this chapter.
(7)
Cleanup. All lots must be kept free of any debris or nuisances whatsoever
during construction.
(8)
Design of erosion and sedimentation control facilities (particularly
stormwater/sediment basins) shall incorporate best management practices
as defined herein.
(9)
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.
(10)
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.
(11)
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 Montgomery
County Conservation District, and Towamencin Township.
(12)
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 the Department
of Transportation, FHA, when deemed necessary by the Township, and
as approved by the Montgomery County Conservation District.
(13)
To control the dissemination of mud and dirt onto public roads
and driveways, tire cleaning areas constructed of AASHTO #1 stone
(underlain by geotextile structural fabric) and conforming to PADEP
standards 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.
(14)
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.
(15)
Adequate provision shall be made to prevent surface water from
damaging the cut face of excavation and the sloping surfaces of fills.
H.
Stormwater detention/retention basins.
(1)
If permanent ponds (retention basins) are proposed, the developer
shall demonstrate that such ponds are designed to protect the public's
health and safety. Should any stormwater management facility require
a dam safety permit under the PADEP Chapter 105 regulations, 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 a one-hundred-year event.
(2)
During construction, duly authorized representatives of Towamencin
Township may enter at any reasonable time upon any property within
the Township to investigate whether construction activity is in compliance
with this chapter.
(3)
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. To minimize the visual impact
of detention basins, they shall be designed to avoid the need for
safety fencing. To meet this requirement, detention basins shall be
designed as follows:
(a)
Maximum depth of detained runoff shall be 24 inches for a two-year
or ten-year storm event.
(b)
Maximum depth of detained runoff shall be 36 inches for a one-hundred-year
storm event.
(c)
The basin inflow and outflow structures shall not be located
directly across from each other and shall not be in close proximity
to one another. A length-to-width ratio in all detention/retention
basins and other such storage facilities of at least 2:1 shall be
provided to maximize the flow path between the inflow point and the
outlet structure. The distance between these two structures must be
at least 50% of the maximum length of the basin as measured at the
top of berm elevation. Alternatively, a means for extending the time
of surface flow from basin inflow point to basin outlet structure,
designed to the satisfaction of the Township Engineer, may be utilized.
(4)
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.
(5)
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,[2] the Towamencin Township Engineering Standards, and as
recommended by the Township Engineer.
(6)
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, cleanout
stakes and other measures as may be required by Pennsylvania Department
of Environmental Protection Chapter 102 Regulations. Plans for such
facilities shall require Montgomery 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 on the erosion and
sedimentation control plan.
(7)
Stormwater management basins shall be in place before the creation
of any new impervious surfaces on the site. As-built drawings of the
basins shall be submitted to the Township for review. The basin shall
not be considered functional until it is proved by the developer that
the basin meets the volume requirements and the outflow characteristics
of the original design of the basin(s).
(8)
Runoff shall not be directed to any infiltration structure until
all tributary drainage areas are stabilized.
(9)
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.
(10)
All basins shall have slopes of four horizontal to one vertical
(4:1 = 25%), or flatter. 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.
(11)
All portions of a detention basin bottom shall have a minimum
slope of 2%. For portions of basin bottoms with grades less than 2%,
the applicant shall provide a landscape design which minimizes maintenance
provisions and encourages infiltration. These requirements may be
altered when approved by the Township Engineer.
(12)
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 ASTMD-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.
(13)
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 checkerblocks
to protect the berm against erosion. Alternative spillway erosion
control lining may be utilized if approved by the Township Engineer.
The checkerblocks shall be backfilled 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, postdevelopment design storm (entering to the basin).
(14)
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.
(15)
The minimum top of 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 and three
feet deep and have side slopes of one horizontal to one vertical.
(16)
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.
(17)
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.
(18)
A perforated sediment control structure, sized in accordance
with Montgomery 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 Montgomery County
Conservation District.
(19)
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.
(20)
The top of outlet structure box shall be at least six inches
lower than the elevation of the emergency spillway. Six inches, minimum,
is also required between the one-hundred-year water surface elevation
and the top of outlet structure box.
(21)
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.
(22)
Stone gabion baskets or concrete or segmental block retaining
walls shall not be permitted for use in construction of detention/retention
basins.
(23)
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 interlocked,
reinforced pervious paving system (backfilled 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 foot 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 from a public right-of-way or
paved driveway within an access easement. Access easements shall be
owned and maintained by the individual lot owner(s) or homeowner's
association but shall be established to permit access by Towamencin
Township or its designee, for emergency inspection and/or maintenance,
at any reasonable time.
(24)
If the basin is not designed to meet the requirements of § 132-13H3,
a split-rail fence must be provided as follows:
(a)
A level area (two-percent slope) eight feet in width shall be
provided on both the inside and outside of the fence, along the entire
length of the fence, for proper access by maintenance equipment. The
total width of this generally level area shall be at least 16 feet.
(b)
Each basin fence installation shall include two points of access
with ten-foot-wide self-closing, self-latch gates to allow for maintenance
equipment/vehicle access.
(c)
Fences shall be split-rail consisting of locust posts (three
rail), four feet high, minimum, with assorted hardwood rails (eight
feet to 10 feet long), and epoxy-coated wire mesh (black or green
in color) installed on the outside of the fence.
(d)
Split-rail fences shall also be required around any detention
or retention basin, where directed by Towamencin Township.
(25)
All stormwater management facilities designed to permanently
retain water shall be constructed in a manner to allow such facilities
to be drained for periodic inspection or maintenance, as approved
by the Township Engineer.
I.
General design requirements.
(1)
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 design shall include such details, notes, or specifications, including
bituminous "eyebrows" at inlets, diversion berms, etc.
(2)
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.
(3)
All stormwater runoff and floodplain calculations and stormwater
management facilities design shall be prepared by a professional engineer
licensed in the Commonwealth of Pennsylvania.
(4)
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.
(5)
The design of all stormwater management facilities shall incorporate
sound engineering principles and practices. The Township shall reserve
the right to disapprove any design that would result in the occupancy
or continuation of an adverse hydrologic or hydraulic condition within
the watershed.
A.
Any stormwater runoff calculations shall use generally accepted calculation
techniques that are based on the NRCS Soil Cover Complex Method. Table
132-14-1 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.
|
Table 132-14-1
| |||
|---|---|---|---|
|
Acceptable Computation Methodologies for Stormwater Management
Designs
| |||
|
Method
|
Method Developed By
|
Applicability
| |
|
TR-20 (or commercial computer package based on TR-20)
|
USDA NRCS
|
Applicable where use of full hydrology computer model is desirable
or necessary
| |
|
TR-55 (or commercial computer package based on TR-55)
|
USDA NRCS
|
Applicable for land development plans within limitations described
in TR-55
| |
|
HEC-1, HEC-HMS
|
U.S. Army Corps of Engineers
|
Applicable where use of full hydrologic computer model is desirable
or necessary
| |
|
PSRM
|
Penn State University
|
Applicable where use of a hydrologic computer model is desirable
or necessary; simpler than TR-20 or HEC-1
| |
|
Rational Method (or commercial computer package based on Rational
Method)
|
Emil Kuichling (1889)
|
Applicable sites less than 200 acres, or as approved by the
Township Engineer
| |
|
Other methods
|
Varies
|
Other computation methodologies approved by the Township Engineer
| |
B.
All calculations consistent with this chapter using the Soil Cover
Complex Method shall use the appropriate design rainfall depths for
the various return period storms according to the National Oceanic
and Atmospheric Administration (NOAA) Atlas 14, Volume 2, Version
3.0, rain data corresponding to the Graterford 1E rain gage (No. 36-3437),
Schwenksville, Pennsylvania, as presented in Table B-1 of Appendix
132B of this chapter.[1] For regulated activities within the Neshaminy Creek Watershed,
rain data corresponding to the Doylestown rain gage (36-2221) shall
be utilized. The SCS Type II rainfall curve data from NOAA is listed
in Figure B-1 in Appendix 132B of this chapter. This data may also
be directly retrieved from the NOAA Atlas 14, Volume 2, Version 3.0
website: hdsc.nws.noaa.gov/hdsc/pfds. If a hydrologic computer model
such as PSRM or HEC-1/HEC-HMS is used for stormwater runoff calculations,
then the duration of rainfall shall be 24 hours.
[1]
Editor's Note: Appendix 132B is included as an attachment to this chapter.
C.
Runoff curve numbers (CN) for both existing and proposed conditions
to be used in the Soil Cover Complex Method shall be obtained from
Table B-4 in Appendix 132B of this chapter.
D.
Suggested runoff coefficients (C) for both existing and proposed
conditions for use in the Rational Method shall be obtained from Table
B-4 in Appendix 132B of this chapter.
E.
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 3.0, rain
data corresponding to the Graterford 1E rain gage (No. 36-3437), Schwenksville,
Pennsylvania, as presented in Table B-3 of Appendix 132B of this chapter.
For regulated activities within the Neshaminy Creek Watershed, rain
data corresponding to the Doylestown rain gage (36-2221) shall be
utilized. The Rational Method rainfall curve data from NOAA is listed
in Figure B-2 in Appendix 132B of this chapter. 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.
F.
For the purposes of existing conditions flow rate determination for
all development activity, undeveloped land 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 Tables B-2 and B-4 in Appendix 132B of this chapter. Wooded
areas shall use a ground cover of "woods in good condition." An area
shall be considered wooded if there is a contiguous canopy of trees
existing over an area of 1/4 acre or more.
G.
For development and redevelopment sites, the ground cover used to
determine the existing conditions runoff volume and flow rate for
the developed portion of the site shall be based upon actual land
cover conditions. If the developed site contains impervious surfaces,
20% of the impervious surface area shall be considered as "meadow"
in good condition in the model for existing conditions runoff volume
and flow rate.
H.
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-5 in Appendix
132B of this chapter.
I.
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.
J.
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.
K.
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.
L.
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.
M.
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.
A.
For all regulated earth disturbance activities, erosion and sediment
control 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, the Montgomery
County Conservation District, and Towamencin Township, and to meet
all requirements under Pennsylvania Code Title 25, Chapter 102, Erosion
and Sediment Control, and the Clean Streams Law.[1] Various BMPs and their design standards are identified
in the Erosion and Sediment Pollution Control Program Manual, No.
363-2134-008 (March 2012), as amended and updated.
[1]
Editor's Note: See 35 P.S. § 691.1 et seq.
B.
Additional erosion and sedimentation control design standards and
criteria that must be or are recommended to be applied where infiltration
BMPs are proposed include the following:
(1)
Areas proposed for infiltration BMPs shall be protected from sedimentation
and compaction during the construction phase, so as to maintain their
maximum infiltration capacity. 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.
(2)
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 the infiltration area to prevent contamination
by runoff containing suspended sediment.
(3)
Areas of the site to remain undisturbed shall be protected from encroachment
by construction equipment/vehicles to maintain the existing infiltration
characteristics of the soil. Four-foot-high orange safety fence or
other similar protection fence approved by the Township must be installed
around the entire limit of disturbance/clearing prior to commencement
of earthmoving activities, and maintained until completion of all
construction activity.
C.
Peak discharge rates from the site during land disturbance shall
comply with the appropriate sections in this chapter related to allowable
postdevelopment stormwater runoff rates, with the following additions:
(1)
For purposes of calculating required detention storage during land
disturbance, peak discharges shall be calculated based upon the runoff
coefficients for bare soils during the period of maximum anticipated
disturbance from clearing and grading, in combination with the entire
quantity of proposed impervious surface installation, indicated on
the development plan. Runoff controls shall insure that the peak rate
of "during construction" runoff does not exceed predevelopment runoff
rates for the two-year frequency through one-hundred-year frequency
design storm events. Detention storage during the period of land disturbance
and prior to establishment of permanent cover may require additional
detention 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 compliance with this requirement.
(2)
Wherever soils, topography, cut and fill or grading requirements,
or other conditions, suggest substantial erosion potential during
land disturbance, the Township may require that the entire volume
of all storms up to a two-year storm from the disturbed areas be retained
on site and that special sediment trapping facilities (such as check
dams, etc.) be installed.
D.
No regulated earth disturbance activities within the Township shall
commence until approval by the Township of an erosion and sediment
control plan for construction activities.
E.
The Pennsylvania Department of Environmental Protection (PADEP) has
regulations that require an erosion and sediment control plan for
any earth disturbance activity of 5,000 square feet or more, under
25 Pa. Code § 102.4(b).
F.
In addition, under 25 Pa. Code Chapter 92,[2] a PADEP NPDES construction activities permit is required
for any earth disturbance of one acre or more with a point source
discharge to surface waters or the regulated small municipal separate
storm sewer system, or five acres or more regardless of the planned
runoff (hereinafter collectively referred to as "regulated earth disturbance
activities"). This includes earth disturbance on any portion of, part
of, or during any stage of, a larger common plan of development.
[2]
Editor's Note: See now 25 Pa. Code Chapter 92a.
G.
Evidence of any necessary permit(s) for regulated earth disturbance activities from the appropriate PADEP regional office or Montgomery County Conservation District must be provided to the Township. The issuance of an NPDES construction permit or permit coverage under the statewide general permit (PAG-2) may satisfy the requirements of § 132-15D.
H.
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.
A.
In addition to the performance standards and design criteria of Article III of this chapter, the developer shall comply with the following water quality requirements unless otherwise exempted by provisions of this chapter.
B.
Detain the one-year, twenty-four-hour design storm using the SCS
Type II distribution. Provisions shall be made so that the 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). 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 the outlet orifice and prevent sediment buildup.
C.
In order to protect and maintain water quality, additional stormwater
runoff created by the development project must be captured, stored,
and treated and the volume that must be treated, called the Water
Quality Volume (WQv), shall be as indicated
below. The formula for determining WQv, which
shall be measured in acre-feet, is:
|
WQv = [(P)(Rv)(A)]/12,
where:
|
|
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.
| |
|
A
|
=
|
Project area in acres.
| |
|
Rv
|
=
|
Volume runoff coefficient [0.05 + 0.009(I)], where I is the
impervious surface percentage (impervious area ÷ total project
area) x 100%.
|
D.
To accomplish Subsections A and B above, the developer may submit original and innovative designs to the Township Engineer for review and approval. Such designs may achieve the water quality objectives through a combination of BMPs (best management practices). Infiltration BMPs shall be used wherever feasible. Wet ponds, artificial wetlands, or other permanent BMPs acceptable to the Township, shall be used if infiltration BMPs are deemed not feasible.
E.
In selecting the appropriate BMPs or combinations thereof, the land
developer shall consider the following:
F.
The following additional factors must be considered and addressed
when evaluating the suitability of BMPs used to control water quality
at a given development site:
(1)
Peak discharge and required volume control.
(2)
Streambank erosion.
(3)
Efficiency of the BMPs to mitigate potential water quality problems.
(4)
The volume of runoff that will be effectively treated.
(5)
The nature of the pollutant being removed.
(6)
Maintenance requirements.
(7)
Creation/protection of aquatic and wildlife habitat.
(8)
Recreational value.
(9)
Enhancement of aesthetic and property value.
G.
The applicant is required to schedule a meeting with the Township
Engineer to discuss the design of the BMPs chosen prior to plan submission.
H.
Postconstruction runoff control requirements.
(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 to protect and maintain existing uses (e.g., drinking water use, cold water fishery use) and maintain the level of water quality necessary to protect those uses in all streams, and to protect and maintain water quality in "special protection" streams, as required by statewide regulations at 25 Pa. Code Chapter 93 (collectively referred to herein as "state water quality requirements").
(3)
To control postconstruction stormwater impacts from regulated earth
disturbance activities, state water quality requirements can 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 the receiving waters. As described in the PADEP
Comprehensive Stormwater Management Policy (No. 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 ensure filtering
out of chemical and physical pollutants from the stormwater runoff;
and
(c)
Streambank and streambed protection: management of volume and
rate of postconstruction stormwater discharges to prevent physical
degradation of receiving waters (e.g., from scouring and erosion).
(4)
PADEP has regulations that require municipalities to ensure design,
implementation and maintenance of BMPs that control runoff from new
development and redevelopment (hereinafter "development") 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)
Evidence of any necessary permit(s) for regulated earth disturbance activities from the appropriate PADEP regional office or Montgomery County Conservation District must be provided to the Township. The issuance of an individual NPDES construction permit [or permit coverage under the statewide general permit (PAG-2)] may satisfy the requirements of § 132-16H(1).
