A.
All regulated activities, unless exempted by § 166-5 of this chapter, within the Township shall be designed, implemented, operated and maintained to meet the purpose of this chapter, through these elements:
(1)
Erosion and sediment control during the earth disturbance activities
(e.g., during construction).
(2)
Water quality protection measures after completion of earth disturbance
activities (e.g., after construction), including operations and maintenance.
(3)
Providing facilities and best management practices to reduce the runoff volume, in accordance with § 166-22 of this chapter.
B.
For all regulated activities, unless preparation of a stormwater management plan is exempted by § 166-5 of this chapter:
C.
Erosion and sediment control during regulated earth disturbance activities shall be addressed as required by § 166-19 of this chapter.
D.
A copy of the stormwater management plan approved by the Township
shall be on site throughout the duration of the regulated activity.
E.
For all regulated earth disturbance activities, erosion and sediment
(E&S) 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. Various BMPs and their design standards
are listed in the Erosion and Sediment Control Program Manual (E&S
Manual), No. 363-2134-008 (April 15, 2000), as amended and updated.[1]
G.
Stormwater flows onto adjacent property shall not be created, increased,
decreased, relocated, or otherwise altered without written notification
of the adjacent property owner(s) by the developer. Such stormwater
flows shall be subject to the requirements of this chapter.
H.
Infiltration BMPs should be spread out, made as shallow as practicable,
and located to maximize use of natural on-site infiltration features
while still meeting the other requirements of this chapter.
I.
Storage facilities should completely drain both the volume control
and rate control capacities over a period of time not less than 24
hours and not more than 72 hours from the end of the design storm.
J.
For all regulated activities, stormwater management BMPs shall be
implemented, operated, and maintained 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.
K.
All stormwater detention/retention facilities and sedimentation control
measures shall be in place and functioning prior to the creation of
any impervious surface.
L.
Any term not specifically defined in this chapter or in the East
Norriton Township Subdivision and Land Development Ordinance (SALDO)[3] shall have the meaning set forth in the latest edition
of Urban Hydrology for Small Watersheds, Technical Release 55, and
the National Engineering Handbook, Section 4, Hydrology.
N.
Hardship waiver procedure.
(1)
The Township may hear requests for waivers where it is alleged that
the stormwater management provisions of this chapter inflict unreasonable
or undue hardship upon the applicant. The waiver request shall be
in writing to the Township. The applicant shall fully document the
nature of the alleged hardship.
(2)
The Township Board of Supervisors may grant a waiver from the stormwater
management provisions, provided that all of the following findings
are made in a given case:
(a)
That there are unique physical circumstances or conditions,
including irregularity of lot size or shape, or exceptional topographic
or other physical conditions peculiar to the particular property,
and that the unreasonable or undue hardship is due to such conditions
and not the circumstances or conditions generally created by the provisions
applicable to the stormwater management district in which the property
is located.
(b)
That, because of such physical circumstances or conditions,
there is no possibility that the property can be developed in strict
conformance with the stormwater management provisions of this chapter
and that the authorization of a waiver is therefore necessary to enable
the reasonable use of the property.
(c)
That such unreasonable or undue hardship has not been created
by the applicant.
(d)
That the waiver, if authorized, will represent the minimum waiver
that will afford relief and will represent the least modification
possible of the regulations as set forth.
(3)
In granting any waiver, the municipality may attach such reasonable
conditions and safeguards as it may deem necessary to implement the
purposes of Pennsylvania Act 167.
O.
Stormwater hotspots.
(1)
If a site is designated as a hotspot, it has important implications
for how stormwater is managed. First and foremost, untreated stormwater
runoff from hotspots shall not be allowed to recharge into groundwater
where it may contaminate water supplies. Therefore, infiltration requirements
shall not apply to development sites that fit into the hotspot category
(the runoff volume must be managed using appropriate methods). Second,
a greater level of stormwater management shall be considered at hotspot
sites to prevent pollutant washoff after construction. The Environmental
Protection Agency's (EPA) NPDES stormwater program requires some
industrial sites to prepare and implement a stormwater pollution prevention
plan. Below is a list of examples of designated hotspots:
(a)
Vehicle salvage yard 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.
(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, as defined
by the Resource Conservation and Recovery Act of 1976.
(l)
Commercial container nursery.
(m)
Other land uses and activities as designated by an appropriate
review authority.
(2)
The following land uses and activities are not normally considered
hotspots:
(3)
While large highways [average daily traffic volume (ADT) greater
than 30,000] are not designated as stormwater hotspots, it is important
to ensure that highway stormwater management plans adequately protect
groundwater.
P.
Extreme caution shall be exercised where infiltration is proposed
in SWPAs as defined by the local municipal or water authority.
Q.
Infiltration facilities shall be used in conjunction with other innovative
or traditional BMPs. stormwater control facilities, and nonstructural
stormwater management alternatives.
R.
Extreme caution shall be exercised where salt or chloride (municipal
salt storage) 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 facility and perform
a hydrogeologic justification study if necessary.
S.
The infiltration requirement in HQ or EV waters shall be subject
to the Department's Chapter 93 Anti-Degradation Regulations.
T.
An impermeable liner will be required in detention basins where the
possibility of groundwater contamination exists. A detailed hydrogeologic
investigation may be required by the Township.
U.
The Township shall require the applicant to provide safeguards against
groundwater contamination for land uses that may cause groundwater
contamination should there be a mishap or spill.
The following permit requirements may apply to certain regulated
earth disturbance activities and must be met prior to commencement
of regulated earth disturbance activities, as applicable:
A.
All regulated earth disturbance activities subject to permit requirements
by the DEP under regulations at 25 Pa. Code Chapter 102.
C.
Any stormwater management facility that would be located in or adjacent to surface waters of the commonwealth, including wetlands, subject to permit by the DEP under 25 Pa. Code Chapter 105.
D.
Any stormwater management facility that would be located on a state
highway right-of-way or would require access from a state highway
shall be subject to approval by the Pennsylvania Department of Transportation
(PennDOT).
A.
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.
B.
The DEP 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).
C.
In addition, under 25 Pa. Code Chapter 92, a DEP NPDES construction activities permit is required for regulated earth disturbance activities.
D.
Evidence of any necessary permit(s) for regulated earth disturbance activities from the appropriate DEP regional office or 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)] satisfies the requirements of Subsection A.
E.
A copy of the erosion and sediment control plan and any required
permit, as required by DEP regulation, shall be available at the project
site at all times.
A.
No regulated earth disturbance activities within the Township shall
commence until approval by the Township of a plan which demonstrates
compliance with state water quality requirements after construction
is complete.
B.
The BMPs must be designed, implemented and maintained to meet state
water quality requirements and any other more-stringent requirements
as determined by the Township.
C.
To control post-construction stormwater impacts from regulated earth
disturbance activities, state water quality requirements can be met
by BMPs, including site design, which provide for replication of preconstruction
stormwater infiltration and runoff conditions, so that post-construction
stormwater discharges do not degrade the physical, chemical or biological
characteristics of the receiving waters. As described in the DEP Comprehensive
Stormwater Management Policy (No. 392-0300-002, September 28, 2002),
this may be achieved by the following:
(1)
Infiltration: replication of preconstruction stormwater infiltration
conditions;
(2)
Treatment: use of water quality treatment BMPs to ensure filtering
out of the chemical and physical pollutants from the stormwater runoff;
and
(3)
Stream bank and stream bed protection: management of volume and rate
of post-construction stormwater discharges to prevent physical degradation
of receiving waters (e.g., from scouring).
D.
The DEP has regulations that require municipalities to ensure design,
implementation and maintenance of best management practices (BMPs)
that control runoff from new development and redevelopment after regulated
earth disturbance activities are complete. These requirements include
the need to implement post-construction stormwater BMPs with assurance
of long-term operations and maintenance of those BMPs.
E.
Evidence of any necessary permit(s) for regulated earth disturbance activities from the appropriate DEP regional office must be provided to the Township. The issuance of an NPDES construction permit [or permit coverage under the statewide general permit (PAG-2)] satisfies the requirements of Subsection A.
The low-impact development practices provided in the BMP Manual shall be utilized for all regulated activities to the maximum extent practicable. Unless exempted from meeting volume control requirements by § 166-5 of this chapter, water volume controls shall be implemented using the Design Storm Method in Subsection A or the Simplified Method in Subsection B below. For regulated activities equal or less than one acre that do not require hydrologic routing to design the stormwater facilities, this chapter establishes no preference for either methodology. Therefore, the applicant may select either methodology on the basis of economic considerations, the intrinsic limitations on applicability of the analytical procedures associated with each methodology, and other factors.
A.
The Design Storm Method (CG-1 in the BMP Manual) is applicable to
any size of regulated activity. This method requires detailed modeling
based on site conditions.
(1)
Do not increase the post-development total runoff volume for all
storms equal to or less than the two-year frequency, twenty-four-hour
duration precipitation.
B.
The Simplified Method (CG-2 in the BMP Manual) provided below is
independent of site conditions and should be used if the Design Storm
Method is not followed. This method is not applicable to regulated
activities greater than one acre or for projects that require design
of stormwater storage facilities. For new impervious surfaces:
(1)
Stormwater facilities shall capture at least the first two inches
of runoff from all new impervious surfaces.
(2)
At least the first one inch of runoff from new impervious surfaces
shall be permanently removed from the runoff flow (i.e., the first
one inch of runoff shall not be released into the surface waters of
this commonwealth). Removal options include reuse, evaporation, transpiration
and infiltration.
(3)
Wherever possible, infiltration facilities should be designed to
accommodate infiltration of the entire permanently removed runoff.
However, in all cases at least the first 0.5 inch of the permanently
removed runoff shall be infiltrated.
(a)
Analyze hydrologic soil groups as well as natural and man-made
features within the site to determine general areas of suitability
for infiltration practices. In areas where development on fill material
is under consideration, conduct a geotechnical investigation of subgrade
stability; infiltration may not be ruled out without conducting these
tests.
(b)
Provide field tests such as double-ring infiltrometer or hydraulic
conductivity tests (at the level of the proposed infiltration surface)
to determine the appropriate hydraulic conductivity rate. Percolation
tests are not recommended for design purposes.
(c)
Design the infiltration structure for the required retention
(Rev) volume based on field-determined capacity
at the level of the proposed infiltration surface.
(d)
If on-lot infiltration structures are proposed by the developer,
it must be demonstrated to the Township that the soils are conducive
to infiltrate on the lots identified.
A.
Management of stormwater management districts.
(1)
In order to implement the provisions of the Stony Creek/Saw Mill
Run Act 167 Stormwater Management Plan, developed by the Montgomery
County Planning Commission with input from the Montgomery County municipalities
and adopted by the Montgomery County Commissioners, dated June 1,
1991 (hereinafter the "Stony Creek Plan"), the Township is hereby
divided into stormwater management districts, consistent with the
designated watersheds for the Stony Creek/Saw Mill Run Watershed Release
Rate Map developed and adopted by the Montgomery County Planning Commission,
dated October 1991 (hereinafter the "Release Rate Map"). The boundaries
of the stormwater management districts are shown on an official map
which, along with the Stony Creek Plan and the Release Rate Map, is
incorporated herein by reference and all of which are available for
inspection at the Township administrative offices.
(2)
Three broad categories of stormwater management districts, namely
the Release Rate District, the Provisional No-Detention District and
the One-Hundred-Percent Release Rate District, shall be applied in
the Township.
B.
Categories of districts. The stormwater management districts are
hereinafter divided into three groups or categories as follows:
(1)
Release Rate District. Within a given district, the post-development
peak rate of stormwater runoff must be controlled to the stated percentage
of the predevelopment peak rate of stormwater runoff in order to protect
downstream watershed areas. There are numerous Release Rate Districts
which differ in the extent to which post-development stormwater runoff
must be controlled. The release rates range from 50% to 100%, and
a specific release rate for each district is shown on the Release
Rate Map and the Stony Creek Plan.
(2)
Provisional No-Detention District. These watershed areas may discharge post-development peak stormwater runoff without detention without adversely affecting the total watershed peak flow. In certain instances, however, the local conveyance facilities, which transport stormwater runoff from the site to the main channel, may not have adequate capacity to safely transport increased peak flows associated with no detention for a proposed development. In those instances, the developer shall either use a one-hundred-percent release rate control or provide increased capacity of downstream drainage elements to convey increased peak flows. In determining if adequate capacity exists in the local watershed drainage network, the developer must assume that the entire local watershed is developed per the existing current Chapter 205, Zoning, and that all new development would use the runoff controls specified within this chapter.
(3)
One-Hundred-Percent Release Rate District. This is an area that should
use a one-hundred-percent release rate control to protect the areas
downstream of the development site. A one-hundred-percent release
rate control would result in the post-development peak rate stormwater
runoff being controlled to the predevelopment level, as further defined
in this chapter.
A.
Compliance with release rate criteria. Any stormwater management
controls required by this chapter and subject to release rate criteria
(50% to 100%) shall meet the applicable release rate criteria for
each of the two-, five-, ten-, twenty-five-, fifty- and one-hundred-year
return period runoff events consistent with the calculation methodology
set forth in this chapter.
B.
Boundary criteria. The exact location of the stormwater management
district boundaries as they apply to a given development within the
Township shall be determined by mapping the boundaries using a two-foot
topographic contours provided as part of the stormwater management
plan. The district boundaries as originally drawn on the Stony Creek
Plan coincide with topographic divides or, in certain instances, are
drawn from the intersection of the watercourse and a physical feature
such as the confluence with another watercourse or a potential flow
obstruction (road, culvert, bridge, etc.) to the topographic divide
consistent with topography.
C.
Downstream capacity analysis criteria. Any downstream capacity analysis
conducted in accordance with this chapter shall use the following
criteria for determining adequacy for accepting peak flow rates:
(1)
Natural or man-made channels or swales must be able to convey the
increased runoff associated with a two-year return period event within
their banks at velocities consistent with protection of the channels
from erosion. Acceptable velocities shall be based upon criteria included
in the DEP Erosion and Sediment Pollution Control Program Manual,
latest edition.
(2)
Natural or man-made channels or swales must be able to convey the
increased twenty-five-year return period runoff peak within their
banks or to otherwise not create any hazards to persons or property.
(3)
Culverts, bridges or storm sewers or any other facilities that must pass or convey flows from the tributary area must have sufficient capacity to pass or convey the increased flows associated with the twenty-five-year return period event, except for facilities located within a designated floodplain area that must be capable of passing or conveying the one-hundred-year return period runoff. Any facilities that constitute stream enclosures per DEP's Chapter 105 regulations shall be designed to convey the one-hundred-year return period runoff.
D.
Single release rate criteria. For a proposed development site located
within a single release rate category area, the total runoff from
the site shall meet the applicable release rate criteria. For development
sites with multiple points of concentrated runoff discharge, individual
drainage points may be designed for up to a one-hundred-percent release
rate, so long as the total runoff from the site is controlled to the
applicable release rate.
E.
Two or more release rate criteria. For a proposed development site
located within two or more release rate category areas, the total
runoff that may be discharged at any point is limited to the predevelopment
peak rate of runoff at that point multiplied by the applicable release
rate. The control rates shall apply regardless of any grading modifications
that may change the drainage area that discharges to a given point.
F.
Locations of mixed categories. For proposed development sites located
partially within a release rate category area and partially within
a provisional no-detention area, in no event shall a significant portion
of the site area subject to the release rate be drained to the discharge
point(s) located in the no-detention area.
G.
No-harm option. For any proposed development site not located in a provisional no-detention district, the developer has the option of using a less-restrictive runoff control (including no detention) if the developer can prove that no harm would be caused by discharging at a higher runoff rate than that specified by the plan. Proof of no harm must be shown from the development site through the remainder of the downstream drainage network to the confluence of the Stony Creek or Saw Mill Run with the Schuylkill River. Proof of no harm must be shown using the capacity criteria specified in § 166-24J of this chapter if the downstream capacity analysis is a part of the no-harm justification. Any no-harm justifications shall be submitted by the developer as part of the stormwater management plan submission. Attempts to prove no harm based upon downstream peak flow versus capacity analysis shall be governed by the following provisions:
(1)
The peak flow values to be used for the downstream areas for the
design return period storms (two-, five-, ten-, twenty-five-, fifty-
and one-hundred-year) shall be the values from the calibrated Penn
State Runoff Model (PSRM) for the Stony Creek/Saw Mill Run watershed.
These flow values would be supplied to the developer by the Township
Engineer upon request.
(2)
Any available capacity in the downstream conveyance system as documented
by a developer may be used by the developer only in proportion to
his development site acreage relative to the total upstream undeveloped
acreage from the identified capacity (i.e., if the developer's
site is 10% of the upstream undeveloped acreage, the developer may
use up to 10% of the documented downstream available capacity).
(3)
Developer-proposed runoff controls that would generate increased peak flow rates at documented storm drainage problem areas would, by definition, be precluded from successful attempts to prove no harm, except in conjunction with proposed capacity improvements for the problem areas consistent with § 166-24J of this chapter.
H.
Individual lots within a subdivision. When a site is proposed to
be subdivided for the sale and development of lots on an individual
basis, either by the subdividing party or other party, the subdividing
party shall, as part of the subdivision proposal, show all provisions
for stormwater control, in accordance with the requirements of this
chapter.
I.
Regional or subregional detention alternatives. For certain regions
within the watershed, it may be more cost effective to provide one
control facility for an entire subarea, group of subareas or portion
of a subarea incorporating more than one development site plan to
provide an individual control facility for each development site.
The initiative and funding for any regional or subregional runoff
control alternatives are the responsibility of prospective developers.
The design of any regional control basins must incorporate reasonable
development of the entire upstream watershed. The peak outflow of
a regional basin would be determined on a case-by-case basis using
the hydrologic model of the watershed consistent with protection of
the downstream watershed areas. "Hydrologic model" refers to the calibrated
Stony Creek/Sawmill Run version of the Penn State Runoff Model as
developed for the requirements of this chapter.
J.
Capacity improvements.
(1)
In certain instances, primarily within the provisional no-detention and provisional one-hundred-percent release rate category areas, local drainage conditions may dictate more-stringent levels of runoff control than those based upon protection of the entire watershed. In these instances, if the developer could prove that it would be feasible to provide capacity improvements to relieve the capacity deficiency in the local drainage network, then the capacity improvements could be provided by the developer in lieu of development controls on the development site. Any capacity improvements would be designed based upon the development of all areas tributary to the proposed improvement and the capacity criteria specified in § 166-24C of this chapter. In addition, all new development upstream of a proposed capacity improvement shall be assumed to implement the applicable runoff controls consistent with this chapter, except that all new development within the entire subarea(s) within which the proposed development site is located shall be assumed to implement the developer's proposed discharge control, if any.
(2)
Capacity improvements may also be provided as necessary to implement
any regional or subregional detention alternatives or to implement
a modified no-harm option which proposes specific capacity improvements
to document the validity of a less-stringent discharge control that
would not create any harm downstream.
A.
Approved methodology. Stormwater runoff from all development sites
with a drainage area of greater than 200 acres shall be calculated
using a generally accepted calculation technique that is based on
the NRCS Soil Cover Complex Method. The Township may allow the use
of the Rational Method to estimate peak discharges from drainage areas
that contain less than 200 acres. The Soil Cover Complex Method shall
be used for drainage areas greater than 200 acres. The following computation
methods may be selected by the design professional, based on the individual
limitations and suitability of each method for a particular use:
(1)
TR-20 (or commercial computer package based on TR-20), developed
by the USDA and NRCS; applicable where use of full hydrology computer
model is desirable or necessary.
(2)
TR-55 (or commercial computer package based on TR-55), developed
by the USDA and NRCS; applicable for land development plans within
the limitations described in TR-55.
(3)
HEC-1/HEC-HMS, developed by the United States Army Corps of Engineers;
applicable where use of a full hydrologic computer model is desirable
or necessary.
(4)
PSRM, developed by Penn State University; applicable where use of
a hydrologic model is desirable or necessary; simpler than TR-20 or
HEC-1.
(5)
Rational Method (or commercial computer package based on Rational
Method), developed by Emil Kuichling (1889); applicable for sites
less than 200 acres, or as approved by the Township.
(6)
Other methods: other computation methodologies approved by the Township.
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 in which
they are located as presented in the PennDOT Drainage Manual (Publication
584). 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. The Alternating Block Method shown in Figure F-1
or the SCS Type II S Curve, Figure F-3 in Chapter Appendix F, shall
be used for the rainfall distribution.
C.
The following criteria shall be used for runoff calculations:
(1)
For development and redevelopment sites, the ground cover used in
determining the existing conditions' flow rates for the developed
portion of the site shall be considered as "meadow in good condition,"
unless the natural ground cover generates a lower curve number (CN)
or Rational "c" value (i.e., woods) as presented in the PennDOT Drainage
Manual (Publication 584).
D.
All calculations using the Rational Method shall use the rainfall
intensities consistent with appropriate times of concentration for
overland flow and return periods presented in the Region five Curves
from the PennDOT Storm-Duration-Frequency Chart (Figure F-4). Times
of concentration for overland flow shall be calculated using the methodology
presented in Chapter 3 of Urban Hydrology for Small Watersheds, NRCS,
TR-55 (as amended or replaced from time to time by NRCS). Times of
concentration for channel and pipe flow shall be computed using Manning's
Equation.
E.
Runoff curve numbers (CN) for both existing and proposed conditions
to be used in the Soil Cover Complex Method shall be obtained from
the PennDOT Drainage Manual (Publication 584).
F.
Runoff coefficients (c) for both existing and proposed conditions
for use in the Rational Method shall be obtained from the PennDOT
Drainage Manual (Publication 584).
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 the PennDOT Drainage Manual
(Publication 584).
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.
Basin designs. The design of any stormwater detention facilities
intended to meet the requirements of this chapter shall be verified
by routing the design storm hydrograph through the facilities using
the Storage-Indication Method. The storm hydrograph shall be computed
using a calculation method that produces a full hydrograph. The Township
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. For
basins designed using the modified Rational Method technique, the
detention volume shall, at a minimum, equal the volume derived from
the approximate routing process as contained in SCS Technical Release
Number 55 (TR-55).
J.
Freeboard requirements. All stormwater detention facilities shall
provide a minimum of one foot of freeboard above the maximum pool
elevation associated with the two-year through twenty-five-year events.
An emergency spillway shall be designed to pass the one-hundred-year
runoff event with a minimum six-inch freeboard.
A.
General.
(1)
The developer shall construct and/or install such drainage structures
as necessary to:
(a)
Prevent erosion damage and to satisfactorily carry off or detain
and control the rate of release of surface waters.
(b)
Encourage all runoff control measures to percolate the stormwater
into the ground to aid in the recharge of groundwater.
(c)
Carry surface water to the nearest adequate street, storm drain,
detention basin, natural watercourse or drainage facility.
(d)
Take surface water from the bottom of grades, to lead water
away from springs and to avoid excessive use of cross gutters at street
intersections and elsewhere.
(e)
Control the anticipated peak discharge from the property being
subdivided or developed and the existing runoff contributed from all
land at a higher elevation in the same watershed.
(f)
Maintain the adequacy of the natural stream channels. Accelerated
bank erosion shall be prevented by controlling the rate and velocity
of runoff discharge to these watercourses so as to avoid increasing
the occurrence of stream bank overflow.
(g)
Preserve the adequacy of existing culverts and bridges by suppressing
the new flood peaks created by new land development.
(2)
If, in the course of preparing or reviewing the stormwater management
plan, the Township determines that off-site improvements are necessary
to satisfactorily control the stormwater from the proposed development,
the developer shall be responsible for the off-site improvements.
(3)
All stormwater detention/retention facilities and erosion and sedimentation
control measures shall be in place and functioning prior to the creation
of any impervious surface.
(4)
Any term not specifically defined in § 166-10 or in the Subdivision and Land Development Ordinance of East Norriton Township[1] shall have the meaning set forth in the latest edition
of Urban Hydrology for Small Watersheds, Technical Release 55, and
the National Engineering Handbook, Section 4, Hydrology.
B.
Retention of existing watercourses and natural drainage features.
(1)
Whenever a watercourse, stream or intermittent stream is located
within a development site, it shall remain open in its natural state
and location and shall not be piped. However, if the stream banks
are of excessive slope (steeper than 3:1), it shall be the responsibility
of the developer to improve the stream banks along the watercourse.
(2)
The existing points of natural drainage discharge onto adjacent property
shall not be altered without the written approval of the affected
landowners in a form satisfactory to the Township Solicitor.
(3)
No stormwater runoff or natural drainage shall be so diverted as
to overload existing drainage systems or create flooding or the need
for additional drainage structures on other private properties or
public lands.
C.
Requirements for drainage structures and/or pipes.
(1)
Drainage easements. Drainage easements to accommodate all storm drainage
requirements shall be a minimum of 20 feet in width and shall be offered
for dedication to the Township. Easements shall be provided for all
improved watercourses and storm drainage piping that are not located
within street rights-of-way.
(2)
Storm drainage pipe. The minimum diameter of all storm drainage pipe
shall be 18 inches or an equivalent with equal area. No storm drainage
piping shall be permitted under buildings. The minimum grade of piping
shall be reinforced concrete and conform to Pennsylvania Department
of Transportation specifications. All joints shall be mortared.
(3)
Stormwater detention/retention. The following criteria shall be used
for design of retention/detention areas. Any reference to detention
basin shall also include retention basin.
(a)
The design of stormwater detention areas shall be based upon
criteria set forth in the latest edition of Urban Hydrology for Small
Watersheds, Technical Release 55, and the National Engineering Handbook,
Section 4, Hydrology, as published by the United States Department
of Agriculture, Soil Conservation Service, as revised.
(b)
Detention areas shall be designed so that the time and rate
of runoff from the site, when developed at its maximum potential use
or development, will not exceed the rate of runoff for the stormwater
management district in which the site is located. Further definitions
and limitations affecting the rate of runoff are set forth within
the Stony Creek Plan and Release Rate Map for the particular stormwater
management districts.
(c)
Discharge from detention basins and from detention areas shall be designed to control the time and rate of runoff as referred to in § 166-25C(3)(b) above and also as set forth within the stormwater management district and within the Stony Creek Plan.
(d)
If permanent ponds are used, the developer shall demonstrate,
by a report to be submitted by a Commonwealth of Pennsylvania registered
certified professional engineer, that such ponds are designed to protect
the public health and safety.
(e)
Prior to the granting of final approval of any subdivision or
land development plan, the Township must be satisfied through contractual
arrangements that all stormwater facilities will be properly maintained.
(f)
When detention basins are provided, they shall be designed to
utilize the natural contours of the land wherever possible. When such
design is impracticable, the construction of the basin shall utilize
slopes as flat as possible to blend the structure into the terrain.
All basins shall have slopes of four horizontal to one vertical or
less.
(g)
Detention basins shall be designed so that they return to normal
conditions within 12 hours after the termination of the storm, unless
the Township Engineer finds that downstream conditions may warrant
other design criteria for stormwater release.
(h)
Emergency overflow facilities (emergency spillways) shall be
provided for all detention facilities.
(i)
If the land of the proposed subdivision or land development
will be conveyed to two or more separate owners, the developer shall
provide written assurance and deed restrictions to the Township that
the detention basins will be properly maintained by the owners.
(j)
The detention basin area will be enclosed by a four-foot-high
chain-link fence (green) or an equal approved by the Township Engineer
constructed along the berm of the basin. A double gate shall be provided
for access of maintenance equipment.
(k)
In all cases, the bottom of the detention basin shall be provided
with a pipe underdrain and outletted from the basin.
D.
Design criteria.
(1)
Stormwater release rate.
(a)
For a proposed development site located within a single Release
Rate District, the total stormwater runoff from the site shall meet
the applicable release rate criteria. For a development site with
multiple points of concentrated stormwater runoff discharge, individual
drainage points may be designed for up to a one-hundred-percent release
rate, so long as the total stormwater runoff from the site is controlled
to the applicable release rate.
(b)
For a proposed development site located within two or more Release
Rate Districts, the maximum peak rate of stormwater runoff that may
be discharged at any point is limited to the predevelopment peak rate
of runoff at that point, multiplied by the applicable release rate.
The control rates shall apply regardless of any grading modifications
that may change the drainage area that discharges to a given point.
(c)
For a proposed development site located partially within a Release
Rate District and partially within a Provisional No-Detention District,
in no event shall a significant portion of the site area subject to
the release rate control be drained to the discharge points located
in the no-detention area.
(2)
The following rainfall intensities shall be used for the twenty-four-hour
Type II distribution storm with average antecedent moisture conditions
for the frequencies shown:
(3)
All developments shall limit the rate of stormwater runoff as previously
set forth within this chapter to be in strict conformance with the
Stony Creek Plan and this section.
(4)
The increased stormwater runoff which may result from subdivision
or land development shall be controlled by permanent stormwater runoff
control measures and facilities. The design for such facilities is
as follows:
(a)
For the two-, five-, and ten-year storm frequencies, the control
facility must achieve the required release rate as set forth in this
section and within the Stony Creek Plan.
(b)
For storms greater than a ten-year storm frequency, but up to
and including the one-hundred-year storm frequency, the control facility
must achieve a release rate of 100%.
(5)
An overflow system shall be provided to carry runoff to the detention
basin when the capacity of the storm drainage pipe system is exceeded.
The overflow system shall have sufficient capacity to carry the runoff
difference between the one-hundred-year-storm peak flow rate and the
capacity of the storm drain pipe. The one-hundred-year-storm peak
shall be calculated by the Soil Cover Complex Method or by an equal
method as may be approved by the Township Engineer.
(6)
Detention basins shall be designed to facilitate regular maintenance,
mowing and periodic desilting and reseeding. Basins shall not be located
within floodplains or floodplain soils. In residential subdivisions
and residential developments, shallow broad basins shall be used.
(7)
If the side slopes of the basin area exceed 3:1 over at least a five-foot
area, a four-foot-high chain-link fence (green) shall be required.
The fence should be located on the outside of the berm or slope. Plantings
as required in this section shall be located next to the fence on
the outside to soften the appearance. If the side slopes are less
than 3:1, no fencing is required.
(8)
The minimum top width of the detention basin berm shall be 10 feet.
A cutoff trench (keyway) comprised 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 two feet deep and
have side slopes of 1:1.
(9)
In order to ensure proper drainage on the floor of the basin, a minimum
grade of 2% shall be maintained for grassed areas. For concrete channel
flow, a minimum grade of 1% shall be maintained.
(10)
All detention/retention basin embankments shall be placed in
a maximum of eight-inch concrete lifts to a minimum of 95% of maximum
dry density as established by the American Society for Testing and
Materials D-1557. Prior to proceeding to the next lift, the compaction
shall be checked by the Township Engineer or soils engineer. Compaction
tests will be run on the leading and the trailing edge of the berm
as well as the top of the berm.
(11)
Whenever possible, the emergency spillway for detention basins
shall be constructed on undisturbed ground. In situations where the
spillway is constructed within a fill area, the emergency spillways
shall be constructed of reinforced concrete checker blocks. All emergency
spillways shall be constructed so that the detention basin berm is
protected against erosion. The minimum capacity of all emergency spillways
shall be the peak flow rate from the one-hundred-year design storm
after development. The construction material of the emergency spillway
shall extend along the upstream and downstream berm embankment slopes.
The upstream edge of the emergency spillway shall be a minimum of
three feet below the spillway crease elevation. The downstream slope
of the spillway shall, as a minimum, extend to the toe of the berm
embankment. The emergency spillway shall not discharge over earth
fill and/or easily erodible material.
(12)
All stormwater management detention facilities shall provide
a minimum one foot of freeboard above the maximum pool elevation associated
with the storm event. Any emergency spillway shall be designed to
pass the one-hundred-year after-construction stormwater runoff event
with a minimum one foot of freeboard from the design flow elevation
in the emergency spillway to the top of a settled detention basin
embankment.
(13)
Antiseep collars shall be installed around the pipe barrel within
the normal saturation zone of the detention basin berms. The antiseep
collars and their connections to the pipe barrel shall be watertight.
The antiseep collars shall extend a minimum of two feet beyond the
outside of the principal pipe barrel. The maximum spacing between
collars shall be 14 times the minimum projection of the collar measured
perpendicular to the pipe. A minimum of two antiseep collars shall
be installed on each pipe outlet.
(14)
All outlet pipes through the basin berm shall be reinforced
concrete pipe having O-ring joints. All joints shall be mortared.
(15)
Energy-dissipating devices, including, but not limited to, riprap
and endwalls, shall be placed at all basin outlets.
(16)
During construction, a perforated riser pipe shall be provided
at each outlet of all detention basins for sediment control. The riser
shall be constructed of metal or concrete. The riser shall extend
to a maximum elevation of two feet below the crest elevation of the
emergency spillway. The perforated riser shall be designed so that
the rate of outflow is controlled by the pipe barrel through the basin
berm when the depth of water within the basin exceeds the height of
the riser. Circular perforations with a maximum diameter of 1 1/2
inches shall be spaced eight inches vertically and 12 inches horizontally
on the upper 2/3 of the pipe. The perforations shall be cleanly cut
and shall not be susceptible to enlargement. All metal risers shall
be coated to prevent rust. A trash rack or similar appurtenances shall
be provided to prevent debris from entering the riser. All risers
shall have a concrete base attached with a watertight connection.
The base shall be of sufficient weight to prevent flotation of the
riser. An antivortex device, consisting of a thin vertical plate normal
to the basin berm, shall be provided on the top of the riser.
(17)
All drainage channels shall be designed to prevent erosion of
the bed and banks. The maximum permissible flow velocity shall not
exceed those outlined in the following table. Suitable stabilization
shall be provided where required to prevent erosion of the drainage
channels. Allowable water velocities shall be as follows:
(a)
Well-established grass on good soil, short pliant bladed: five
feet to six feet per second.
(b)
Well-established grass on good soil, bunch grass, soil exposed:
two feet to four feet per second.
(c)
Well-established grass on good soil, stiff-stemmed grass: two
feet to three feet per second.
(d)
Earth without vegetation, fine sand and silt: one feet to two
feet per second.
(e)
Earth without vegetation, ordinary firm loam: two feet to three
feet per second.
(f)
Earth without vegetation, stiff clay: three feet to five feet
per second.
(g)
Earth without vegetation, clay and gravel: four feet to five
feet per second.
(h)
Earth without vegetation, coarse gravel: four feet to five feet
per second.
(i)
Earth without vegetation, soft shale: five feet to six feet
per second.
(j)
Shoulders, earth: (see "earth without vegetation" entry above).
(k)
Shoulders, stabilized: six feet per second.
(l)
Shoulders, paved: 10 feet to 15 feet per second.
(18)
All vegetated drainage channels shall have a maximum side slope
grade of three horizontal to one vertical.
(19)
Because of the critical nature of vegetated drainage channels,
the design of all vegetated drainage channels shall, as a minimum,
conform to the design procedures outlined in the Pennsylvania Department
of Environmental Resources Erosion and Sediment Control Handbook (April
1990, or as further revised).
(20)
Access ramps (for maintenance equipment) shall be 12 feet in
width and have a maximum slope of 12 1/2% for all detention/retention
basins. These ramps shall be constructed of concrete checker blocks
on a six-inch compacted layer of PennDOT No. 2A coarse aggregate or
approved equal.
(21)
The following standards shall apply to underground detention
basins:
(a)
All underground storage shall be constructed of approved culvert
pipes or concrete vaults. No storage shall be permitted within the
voids of stone bedding.
(b)
All underground storage pipes shall meet the requirements of storm drainage pipe as outlined in § 166-25C(2) and § 166-25L. All underground vaults shall have a minimum wall thickness of eight inches and be designed to withstand the anticipated vehicular loads.
(c)
If an underground detention basin is to be privately owned,
operated, and maintained, underground storage pipes may be constructed
of a material other than reinforced concrete. The pipe and material
shall meet all applicable PennDOT standards and shall be subject to
the approval of the Township Engineer. The applicant shall provide
satisfactory documentation on the strength, durability and installation
requirements of the proposed material.
(d)
All pipes, regardless of material, shall be installed on a six-inch
stone bedding. The remaining backfill shall be of crushed stone if
required by the manufacturer's specification or the Township
Engineer.
(e)
All parts of an underground detention basin shall lie completely
outside and be adequately separated from proposed or existing public
rights-of-way, utility easements, foundations, wetlands and floodplains,
woods and landscaping, or other sensitive areas.
(f)
The design of underground detention basins shall provide for
complete access for regular cleaning and inspection. The design shall
incorporate an emergency spillway or other fail-safe means of addressing
overflow conditions.
E.
General storm and surface drainage.
(1)
All subdivision and land development plans shall include methods
to withhold and release stormwater at a controlled rate onto adjacent
property.
(2)
Lots shall be laid out and graded to provide positive drainage away
from buildings.
(3)
Storm sewers, culverts and related installations shall be provided
to:
(a)
Permit the unimpeded flow of natural watercourses in such a
manner as to protect the natural character of the watercourses and
to provide regulated discharge.
(b)
Ensure adequate drainage of all low points along the line of
streets.
(c)
Intercept stormwater runoff streets at intervals reasonably
related to the extent and grade of the area drained and to prevent
substantial flow of water across intersections.
(4)
Storm sewers are required and shall be placed in the right-of-way
parallel to the roadway. When located in undedicated land, they shall
be placed within an easement not less than 20 feet wide.
(5)
Manholes shall be spaced not more than 300 feet apart on pipe sizes
up to and including 24 inches in diameter and not more than 450 feet
apart for pipe sizes greater than 24 inches in diameter. Inlets may
be substituted for manholes, on approval by the Township Engineer,
at the same spacing as required for manholes. In no case shall inlets
be spaced more than 450 feet apart. Manholes or inlets shall be provided
at all changes of direction of storm sewer piping.
(6)
Inlets and manhole cover frames shall conform to Pennsylvania Department
of Transportation specifications. At street intersections, inlets
shall be placed in the tangent and not in the curved portion of the
curbing. Manhole covers shall have the word "STORM" cast in two-inch-high
letters on the top of the cover.
(7)
Stormwater roof drains and sump pumps shall not discharge water directly
onto a sidewalk or a street and shall be constructed to discharge
wholly to a storm sewer system.
(8)
Drainage structures which drain watershed areas in excess of 1/2
square mile (320 acres) or which have a span of eight feet or more
shall be designed for a maximum expected runoff as calculated using
the Soil Conservation Service Technical Release 55, Urban Hydrology
for Small Watersheds (Less Than 2,000 Acres). The design storm shall
be a minimum one-hundred-year storm. A water obstruction and encroachment
permit shall be obtained from the Pennsylvania Department of Environmental
Resources or a waiver from the Pennsylvania Department of Environmental
Resources for all waterway openings prior to final plan approval.
The cartway width over a structure shall be as wide as the widest
road connecting with the structure, or, if the character of the road
is expected to change for the future planning, the cartway over the
structure shall be made to anticipate this condition.
F.
Storm sewer design.
(1)
Storm frequency.
(a)
Design flow rate. The storm drain system shall be designed to
carry the ten-year peak flow rate. The design ten-year peak flow rate
into each inlet shall be indicated on the stormwater drainage plan.
The ten-year flow rate shall be determined by the Rational Formula.
|
Q = C*I*A
| ||||
|
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)
Overflow system. An overflow system shall be provided to carry
flow to the detention basin when the capacity of the storm drain pipe
system is exceeded. The overflow system shall be of sufficient capacity
to carry the difference between the one-hundred-year and ten-year
peak flow rates.
G.
Storm duration.
(1)
A five-minute storm duration shall be used if the duration does not
result in a maximum expected discharge that exceeds the capacity of
a thirty-inch pipe.
(2)
If a five-minute storm duration results in a pipe size exceeding
30 inches, the time-of-concentration approach shall be used in determining
storm duration.
(3)
If a five-minute storm duration results in a pipe size exceeding
30 inches, within any run of pipe, the time-of-concentration approach
may be used for the sizing of pipes from that point on by adjusting
the time of concentration.
H.
Additional design criteria. Additional design criteria for specific
drainage facilities are required as follows:
(1)
Shoulders in cut areas (without swales).
(a)
Water flowing in the shoulder shall not encroach more than 2/3
of the shoulder width during a ten-year-frequency storm of five-minute
duration.
(b)
The maximum velocity as determined by Manning's Equation shall not exceed the allowable velocities as shown in § 166-25D(17) for the specific type of shoulder material.
(c)
Inlets shall be provided to control the shoulder encroachment
and water velocity.
I.
Swales adjacent to shoulders.
(1)
When swales are provided in cut areas, the water shall not encroach
upon the roadway area during a ten-year-frequency storm of five-minute
duration.
(2)
The maximum velocity as determined by Manning's Equation shall not exceed the allowable velocities as shown in § 166-25D(17) for the specific type of swale material.
J.
Curbed sections.
K.
Inlets.
(1)
When there is a change in pipe size in the inlet, the elevation of
the top of pipes shall be the same or the smaller pipe higher. A minimum
drop of two inches shall be provided at the inlet pipe invert elevation
and the outlet pipe invert elevation.
(2)
If the capacity of the shoulder, swale, curb section or depressed
median section is less than the inlet capacity, the shoulder, swale,
curb section or depressed medial section capacity shall govern inlet
spacing.
(3)
Inlet capacities shall be based on a maximum flow of 5.0 cubic feet
per second.
L.
Storm pipes.
(1)
To facilitate the solution of Manning's Equation as applied
to storm pipes, charts are presented in Design Charts for Open Channel
Flow, prepared by the United States Department of Commerce, as last
revised, which permit a direct determination of the capacity of circular
pipes.
(2)
Where headroom is restricted, equivalent pipe arches may be used
in lieu of circular pipe.
(3)
The minimum diameter of storm pipe shall be 18 inches.
(4)
Inlets or manholes shall be placed at changes in vertical or horizontal
direction of pipe.
(5)
Pipe shall be designed with a slope such that the minimum velocity
of 2.5 feet per second will be attained; however, the slope shall
not be less than 0.5%.
(6)
All inlets, manholes and piping shall be designed to provide a minimum
of 1.0 foot of freeboard at all inlets and manholes. The stormwater
piping system shall be designed to meet inlet and outlet control.
(7)
All pipes shall be designed to have a minimum of 18 inches of cover
over the bell of the pipe.
(8)
Subbase and U-drains in combination with six-inch pipe U-drains and/or
combination storm sewers and U-drains shall be installed on roadways
where the finished roadway grade is within one foot of the seasonal
high-water table as designated for the soils by the Montgomery County
Soil Survey.
(9)
All plans showing the proposed storm sewer construction must be accompanied
by a complete design submitted by the registered engineer.
(10)
When subdivisions or land development 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.
(11)
If only a section of a subdivision or land development is contemplated
for construction, the engineer shall show how he proposes to handle
stormwater from this section in order to prevent damage to adjacent
properties. If temporary construction is required, the engineer shall
include such structures in the plans submitted.
(12)
In the event that such temporary measures cannot ensure protection
to adjacent properties, then the main outfall line of the storm sewer
shall be included as part of the construction for the proposed section.
M.
Stormwater basin landscaping. It is the intent of this section to
conserve the value of land and buildings on surrounding properties
and neighborhoods by mitigating the visual impact of stormwater management
facilities, and to minimize soil erosion. Landscaping shall be required
in and around all stormwater management facilities with a minimum
surface area of 1,000 square feet. Basins may be natural or maintained
as described below. A planting plan shall be submitted in accordance
with the one of the following two types of landscape treatments for
stormwater basins:
(1)
Landscaping for naturalized basins. The following shall apply to
basins left in a natural state, especially those in parks or large
open spaces:
(a)
Basin floors. Basin floors shall be planted with wildflowers
and grasses, the intent being to create a mixed meadow of such plantings,
where appropriate. Selection of plantings should be based on whether
the area in question is usually well-drained or permanently wet.
(b)
Wet edges. Wet edges which remain wet all or most of the year
shall be planted with wildflowers, grasses and shrubs tolerant of
wet conditions. Plants to be located on rims or banks which remain
dry most of the year should be planted with species tolerant to dry
soil conditions.
(d)
Slopes.
[1]
Where slopes are 30% or less, a mixture of wildflowers and/or
grasses shall be planted.
[2]
No woody plant materials or trees shall be located on a berm
acting as the impoundment structure of a detention/retention basin.
Trees shall be located around the perimeter of the downstream side
of a berm a sufficient distance from the toe of the slope to assure
that the toe of the slope is outside the dripline of the mature species
planted.
(2)
Landscaping for maintained basins. The following shall be applied
for basins in developed areas where a manicured lawn is possible.
Lawn mowing shall take place at intervals sufficient to maintain the
grass ground cover at no higher than six inches.
(a)
Basin floors and slopes. Basin floors and slopes shall be planted
with lawn grass and maintained as a lawn with a height of not more
than six inches.
(b)
Wet areas. Low areas subject to prolonged wet conditions, such
as low-flow channels or areas with poor drainage, shall be stabilized
to allow mowing and prevent erosion. Such stabilization may include
geotextile fabrics or other porous materials such as grass pavers
or PVC stabilizers.
(c)
Drainage structures in residential districts. Embankments surrounding
drainage structures, such as head- or endwalls, are to be planted
with shrubs or ornamental trees along the top of the structure. Shrubs
shall be planted three feet apart.
(d)
No trees shall be located on a berm acting as the impoundment
structure of a detention/retention basin. Trees shall be located around
the perimeter of the downstream side of a berm a sufficient distance
from the toe of the slope to assure that the toe of the slope is outside
the dripline of the mature species planted.
