A.
All applicants proposing regulated activities in the Township that do not fall under the exemption criteria shown in § 129-6B and C of this chapter shall submit a stormwater management site plan, consistent with this chapter, to the Township for review. All applicants proposing regulated activities that fall under the exemption criteria identified in § 129-6C shall submit a simplified stormwater management site plan, consistent with this chapter, to the Township for review. These criteria shall apply to the total proposed development even if development is to take place in stages. Impervious surface shall include, but not be limited to, any roof, parking or driveway areas and any new streets and sidewalks. Any areas designed to be gravel or crushed stone shall be assumed to be impervious unless designed as a BMP (e.g., pervious paver blocks, reinforced turf, gravel-filled grids, etc.). (Refer to definition of "impervious surface" within § 129-12 of this chapter).
B.
All regulated activities shall include such measures as necessary
to:
(1)
Protect health, safety, and property;
(2)
Meet the water quality goals of this chapter by implementing measures
to:
(a)
Minimize disturbance to floodplains, wetlands, and wooded areas.
(b)
Create, maintain, repair or extend riparian buffers.
(c)
Avoid erosive flow conditions in natural flow pathways.
(d)
Minimize thermal impacts to waters of this commonwealth.
(e)
Disconnect impervious surfaces (i.e. create disconnected impervious
areas, DIAs) by directing runoff to pervious areas, wherever possible.
(3)
To the maximum extent practicable, incorporate the techniques for
low-impact development practices (e.g., protecting existing trees,
reducing area of impervious surface, cluster development, and protecting
open space) described in the Pennsylvania Stormwater Best Management
Practices Manual, Pennsylvania Department of Environmental Protection
(PADEP) No. 363-0300-002 (December 30, 2006).
C.
The Township may, after consultation with the Department of Environmental
Protection (PADEP), approve measures for meeting the state water quality
requirements other than those in this chapter, provided that they
meet the minimum requirements of, and do not conflict with, state
law including, but not limited to, the Clean Streams Law.[1]
[1]
Editor's Note: See 35 P.S. § 691.1 et seq.
D.
For all regulated earth disturbance activities, erosion and sediment
(E&S) control best management practices (BMPs) shall be designed,
implemented, operated, and maintained during the regulated earth disturbance
activities (e.g., during construction) to meet the purposes and requirements
of this chapter and to meet all requirements under Title 25 of the
Pennsylvania Code and the Clean Streams Law. Various BMPs and their
design standards are listed in the Erosion and Sediment Pollution
Control Program Manual (March 2012), as amended and updated.
E.
No approval of any subdivision or land development plan, or issuance of any building, zoning, grading and excavations permit, occupancy permit, or the commencement of any regulated earth disturbance at a project site within the Township shall proceed until the requirements of this chapter are met, including approval of a stormwater management permit pursuant to Article IV and a permit under PADEP regulations, where applicable.
H.
All best management practices (BMPs) shall conform to the design
criteria of this chapter and Pennsylvania Stormwater Best Management
Practices Manual, December 30, 2006.
I.
Low-impact development techniques as described in Pennsylvania Stormwater Best Management Practices Manual (December 30, 2006, as amended) are encouraged to reduce the costs of complying with the requirements of this chapter and state water quality requirements. Use of nonstructural BMPs is encouraged, and design and applicability of such BMPs is identified pursuant to Chapter 5 of the Pennsylvania BMP Manual. For all proposed nonstructural BMPs, the applicant shall utilize and submit applicable checklists included in Chapter 8, Section 8.8 of the Pennsylvania BMP Manual, to demonstrate that the BMPs are applicable to the project and to determine the amount of volume credit that may be applied to the development activity.
J.
Where a subdivision or land development proposes the creation of
new residential lots of less than 20,000 square feet, no infiltration
BMPs, aboveground basins or similar stormwater management BMPs shall
be placed on individual lots where a single-family dwelling is proposed
to be constructed. All such developments shall consolidate their required
stormwater management facilities on a lot or lots not containing a
single-family dwelling unless approved otherwise by the Board of Supervisors.
This prohibition applies to aboveground and surface BMPs and not to
stormwater pipes, drains, inlets, or similar underground structural
elements of a stormwater management system.
[Amended 2-20-2020 by Ord. No. 664]
K.
Stormwater drainage systems shall be provided in order to permit
unimpeded flow along natural watercourses, except as modified by stormwater
management facilities designed to encourage infiltration, groundwater
recharge, and improved water quality.
L.
Existing points of concentrated drainage that discharge onto adjacent
property shall not be altered without written approval of the affected
property owner(s) and shall be subject to discharge criteria specified
in this chapter.
M.
Areas of existing sheet flow discharge shall be maintained wherever
possible. If sheet flow is proposed to be concentrated and discharged
onto adjacent property, the applicant must document that adequate
downstream conveyance facilities exist to safely transport the concentrated
discharge, or otherwise prove that no erosion, sedimentation, flooding
or other harm will result from the concentrated discharge, as verified
by the Township or Township Engineer; and submit written approval
from the affected adjacent property owner(s) if required by the Township
or Township Engineer.
N.
Where a development site is traversed by watercourses, drainage easements
shall be provided conforming to the line of such watercourses. The
width of the easement shall be adequate to provide for the unimpeded
flow of stormwater runoff from the one-hundred-year storm event. Terms
of the easement shall prohibit excavation, the placing of fill or
structures, and any alterations that may adversely affect the flow
of stormwater within any portion of the easement. Periodic maintenance
of the easement shall be required by the landowner to ensure proper
runoff conveyance, as defined by the Commonwealth of Pennsylvania.
O.
When it can be shown that, due to topographic conditions, natural drainageways on the site cannot adequately provide for drainage, open channels may be constructed conforming substantially to the line and grade of such natural drainageways. Work within natural drainageways shall be subject to approval by PADEP through the Chapter 105 permit process, or, where deemed appropriate by PADEP, through the general permit process.
P.
Any stormwater management facilities regulated by this chapter that will be located in or adjacent to waters of the commonwealth or wetlands shall be subject to approval by PADEP through the Chapter 105 permit process, or, where deemed appropriate by PADEP, the general permit process. When there is a question as to whether wetlands may be involved, it is the responsibility of the applicant to show that the land in question cannot be classified as wetlands; otherwise, approval to work in the area must be obtained from PADEP.
Q.
Any stormwater management facilities regulated by this chapter that
would be located on state highway rights-of-way, or discharge stormwater
to facilities located within a state highway right-of-way, shall be
subject to approval by the Pennsylvania Department of Transportation
(PADOT).
R.
Site disturbance and impervious surface shall be minimized. Infiltrating
stormwater runoff through seepage beds, infiltration basins, etc.,
shall be required, where soil conditions permit, to reduce the size
or eliminate the need for retention/detention facilities.
S.
Roof drains and sump pumps shall discharge to an infiltration structure,
natural watercourse, storm sewer system, drainage swale, or stormwater
easement. Roof drains and sump pumps shall not be connected to a storm
sewer unless the storm sewer is designed as part of a stormwater BMP
facility. In no case shall roof drains or sump pumps be connected
to sanitary sewer or be permitted to discharge directly across a sidewalk
or walkway or onto a street. If curbing is present, no drainage pipes
shall pass through the curb to discharge onto the street. Sump pump
and roof drain discharge pipes shall not extend beyond the building
envelope for the lot unless they are directly connected to an infiltration
facility, detention basin, storm sewer pipe or as approved by the
Township.
T.
All storm sewer inlets must be identified with a storm drain marker.
Storm drain markers shall be stainless steel, affixed to the inlet
hood with adhesive, rivets, or bolts. (Marker may be bolted to the
grate in off-road locations). Marker shall have a minimum diameter
of 3 1/2 inches and include "No Dumping - Drains to Waterway"
and a fish symbol. Alternate designs/sizes may be used if approved
by the Township.
U.
Whenever a watercourse is located within a development site, it shall remain open in its natural state and location and shall not be piped, impeded, or altered (except for permitted crossings). It is the responsibility of the applicant to stabilize existing eroded stream/channel beds and banks (Refer to § 129-18).
V.
The temperature and quality of water of streams that have been declared as exceptional value (EV) and high quality (HQ) are to be maintained as defined in Chapter 93, Water Quality Standards, Title 25 Pennsylvania Department of Environmental Protection Rules and Regulations. All regulated development activities within HQ or EV watersheds must provide volume controls and water quality controls pursuant to the requirements of §§ 129-16 and 129-17 of this chapter.
W.
All stormwater runoff shall be pretreated for water quality prior to discharge to surface or groundwater as required by § 129-17 of this chapter.
X.
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 United States 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.
A.
In the design of stormwater management facilities, post-development
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 post-development
runoff rate shall not exceed the predevelopment runoff rate. The preceding
requirements shall apply to each location of concentrated or diffused
drainage discharge from the development site.
B.
Site areas. 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 as determined by the
Township Engineer, the Township may, but is not required to, permit
only the proposed impact area, which includes areas of the site that
would be compacted due to construction activity, to be subject to
the release rate criteria (performance standards).
C.
Off-site areas. Off-site areas that drain through a proposed development
site are not subject to release rate criteria when determining allowable
peak runoff rates or volume reduction. However, on-site drainage facilities
shall be designed to safely convey off-site flows through the development
site.
D.
Stormwater conveyance corridor protection (riparian corridor preservation and vegetation). Runoff from developed areas of the site, including but not limited to areas of impervious surface, shall be managed through a series of riparian corridor vegetation facilities whenever possible. This will be accomplished in a manner satisfactory to the Township, utilizing the "Pennsylvania Handbook of Best Management Practices for Developing Areas," 1998, Riparian Forested Buffer, and the priority goal of the riparian vegetation will be the reduction of thermal impacts on stormwater runoff associated with impervious areas, with a secondary goal being the protection of capacity of existing stormwater conveyance channels. These goals will be achieved through the use of design criteria in § 129-19I of this chapter, and shall be in addition to any other Township ordinance provisions.
E.
For all subdivision and land development applications, the tributary
area discharging drainage to any location along the site property
boundary shall not increase by more than 25% over the predevelopment
condition without written approval from the adjacent affected property
owner(s) receiving runoff from the site.
A.
The design of all regulated activities shall include the following
steps in sequence to minimize stormwater impacts:
(1)
The applicant is required to find practicable alternatives to the
surface discharge of stormwater, the creation of impervious surfaces,
and the degradation of waters of the commonwealth, and must maintain
as much as possible the natural hydrologic regime of the site.
(2)
An alternative is practicable if it is available and capable of being
completed after considering cost, existing technology, and logistics
in light of overall project purposes and other Township requirements.
(3)
All practicable alternatives to the discharge of stormwater are presumed
to have less adverse impact on quantity and quality of waters of the
commonwealth unless otherwise demonstrated.
B.
The applicant shall demonstrate that regulated activities are designed
in the following sequence to minimize the increases in stormwater
runoff and impacts to water quality:
(1)
Prepare an existing resources and site analysis map (ERSAM), showing
environmentally sensitive areas including, but not limited to, steep
slopes, ponds, lakes, streams, wetlands, hydric soils, vernal pools,
floodplains, riparian corridors, hydrologic soil groups A, B, C, and
D, woodlands, surface waters regulated by the state or federal government,
any existing recharge areas, and any other requirements outlined in
the Subdivision and Land Development and Zoning ordinances.[1]
(2)
Prepare a draft project layout avoiding sensitive areas identified in § 129-15B(1) and minimizing total site earth disturbance as much as possible. The ratio of disturbed area to the entire site area and measures taken to minimize earth disturbance shall be included in the ERSAM.
(3)
Identify site-specific existing conditions, drainage areas, discharge
points (points of interest), recharge areas, and hydrologic soil groups
A and B.
(4)
Evaluate nonstructural stormwater management alternatives (refer
to the Pennsylvania BMP Manual).
(a)
Minimize earth disturbance.
(b)
Minimize impervious surfaces.
(c)
Break up large impervious surface areas.
(d)
Protect existing trees [not within protected areas as described in § 129-15B(1)].
(e)
Direct rooftop runoff to pervious areas.
(f)
Revegetate and reforest disturbed areas.
(g)
Utilize natural flow pathways.
(8)
Prepare final project design to maintain predevelopment drainage
areas and discharge points, to minimize earth disturbance and impervious
surfaces, to reduce runoff to the maximum extent possible, and to
minimize the use of surface or point discharges.
(9)
Conduct a proposed conditions runoff analysis, based on the final
design, to meet the release rate criteria (performance standards).
(10)
Manage any remaining runoff through treatment prior to discharge,
as part of detention, biorentention, direct discharge or other structural
control.
A.
For all regulated activities not exempt from requirements of this chapter pursuant to § 129-6B and C, water volume mitigation controls shall be implemented. The total volume of runoff that must be infiltrated may be calculated based on the Design Storm Method, in which case the post-development total runoff volume shall not be increased from pre-development total runoff volume for all design storms equal to or less than the two-year, twenty-four-hour duration precipitation. The Design Storm Method requires detailed stormwater runoff modeling based on site conditions. The required recharge volume may also be determined based on Equation 129-16.1, described in § 129-16D. The recharge volume (Rev) must be reused, evapotranspired, or infiltrated through structural and/or nonstructural means. An alternative standard is allowed in this chapter where it can be demonstrated that due to existing natural site conditions [Refer to § 129-16A(1)(b)], substantial infiltration and recharge are not occurring, pre-development, resulting in greater than anticipated runoff volume.
(1)
Alternate standard for runoff volume.
(a)
Applicants may request from Lower Providence Township that an alternate standard be applied, where a portion of the runoff volume requirement of § 129-16A is not achieved but at least 50% of the total required volume of infiltrated runoff is achieved. Use of this alternate standard is permitted by the Township only after thorough scrutiny has been directed toward all possible stormwater management options at all possible locations at the site, consistent with the process set forth in § 129-16A(1).
(b)
Required analysis for allowing use of alternate standard for
runoff volume. The alternate standard shall be used only in those
situations where it is demonstrated to the satisfaction of the Township
that, due to natural site conditions, infiltration is not occurring
in the predevelopment condition, resulting in greater runoff volumes
(than would normally be anticipated) due to bedrock near or at the
surface (less than two feet in depth); presence of seasonal high water
table (SHWT) (less than two feet in depth); and soils with low permeability
(e.g., 0.20 inch per hour or less). Alternate standard shall be permitted
by the Township only in those cases where the applicant has demonstrated
that one or all of the above described conditions exist throughout
the site, such that there is no reasonable means of infiltrating required
stormwater volumes and that the property cannot be reasonably developed
utilizing a stormwater management system which infiltrates the two-year
frequency storm event volume (difference between the pre and post-development
storm). The applicant must demonstrate that there is no area of the
site where the runoff volume requirement can feasibly be infiltrated.
It is not grounds for approval of the alternate standard that infiltrating
the runoff volume requirement will utilize areas that could otherwise
be developed to obtain the most building area or lots.
(c)
Applicants requesting to utilize the alternate standard must
provide a feasibility study for infiltration utilizing BMPs as well
as other runoff volume stormwater management systems and provide the
following information:
[1]
Site plan demonstrating the extent of site area with SHWT (less
than two feet): The site will be evaluated both as to the extent of
site with SHWT and the actual locations of SHWT areas. Use of the
alternate standard shall be permitted by the Township only in those
cases where it is demonstrated that site areas free of SHWT are not
feasible for use as stormwater BMPs (i.e., they are located upgradient
from reasonable site building areas).
[2]
Site plan demonstrating extent of site area with less than two
feet to bedrock: The site will be evaluated both as to the extent
of site with shallow depth to bedrock and actual locations of shallow
bedrock areas. Use of the alternate standard shall be permitted by
the Township only in those cases where it is demonstrated that site
areas free of shallow bedrock constraints are not feasible for use
as stormwater BMPs (i.e., they are located upgradient from reasonable
site building areas).
[4]
Sum of infiltration area.
[a]
In order to utilize the alternate standard, the applicant must demonstrate that the sum total of limited infiltration areas (the total of areas described in § 129-16A(1)(c)[1] through [3] exceed the following percentages of the total site:
[b]
In addition, the applicant must demonstrate that
there is no feasible site area free of the above-described infiltration
constraining features which exist in a location such that the runoff
volume requirement can be achieved.
B.
If it is determined to the satisfaction of the Township that the recharge volume standard set forth in § 129-16A cannot be achieved, then the peak rate standards for post-development runoff are modified so that peak rate discharges from the site for all storms up to the ten-year frequency design storm must be additionally reduced to be equal to or less than 75% of the design peak rates permitted pursuant to § 129-14.
C.
Water volume controls will mitigate increased runoff impacts, protect stream channel morphology, maintain groundwater recharge, and contribute to water quality improvements. The applicant must demonstrate how the required recharge 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. The low-impact development practices provided in the Pennsylvania BMP Manual shall be utilized for all regulated activities to the maximum extent practicable. Volume controls provided through nonstructural BMPs may be subtracted from the required recharge volume to determine the volume of structural BMPs necessary for compliance with § 129-16A of this chapter. Design and applicability of nonstructural BMPs is identified pursuant to Chapter 5 of the Pennsylvania BMP Manual. For all proposed nonstructural BMPs, the applicant shall utilize and submit applicable checklists included in Chapter 8, Section 8.8 of the Pennsylvania BMP Manual, to demonstrate that the BMPs are applicable to the project and to determine the amount of volume credit that may be applied to the development activity.
D.
To determine the volume of runoff that must be infiltrated at a site,
the Recharge Volume (Rev), the following calculation
formula may be used:
|
Equation 129-16.1
|
|
Rev = [(S)(Rv)(A)]/12 (inches/foot)
|
|
Where:
| |||
|
Rev
|
=
|
Recharge volume (acre-feet)
| |
|
S
|
=
|
Soil specific recharge factor (inches)
| |
|
A
|
=
|
Site area contributing to the recharge facility (acres)
| |
|
Rv
|
=
|
Volumetric runoff coefficient, Rv = 0.05
+ 0.009(I),
| |
|
Where:
| |||
|
I
|
=
|
Percent impervious area, "S" shall be obtained based upon hydrologic
soil group based upon the table below:
| |
|
Hydrologic Soil Group
|
Soil Specific Recharge Factor (S)
|
|---|---|
|
A
|
0.38
|
|
B
|
0.26
|
|
C
|
0.14
|
|
D
|
0.07
|
|
If more than one hydrologic soil group (HSG) is present at a
site, a composite recharge volume shall be computed based upon the
proportion of total site area within each HSG.
|
E.
Infiltration BMPs intended to receive runoff from developed areas
shall be selected based on the suitability of soils and site conditions.
All applicants proposing regulated activities that are not exempt
from preparation and submission of a stormwater management site plan
(SMSP) are required to perform a detailed soils evaluation of the
project site by a qualified geotechnical engineer, geologist and/or
soil scientist, pursuant to Appendix B of this chapter,[2] which at minimum addresses soil permeability, depth to
bedrock, susceptibility to sinkhole formation, and subgrade stability.
Infiltration/permeability tests shall be completed (in conjunction
with the soils evaluation) with an infiltrometer or other method approved
by the Township Engineer, pursuant to Appendix B, to determine the
saturated hydraulic conductivity of the soil (at the location and
the level of the proposed infiltration surface(s)). Percolation tests
are not permitted for design of infiltration BMPs, unless approved
by the Township Engineer.
[2]
Editor’s Note: Said appendix is on file in the Township’s
offices.
F.
Infiltration BMPs must include safeguards against groundwater contamination
for uses where it is anticipated that pollutants may enter the facility,
by mishap or spill or where salt or chloride might be a nonpoint source
contaminant since soils do little to filter this pollutant. If it
is anticipated that pollutants may enter the infiltration facility
(or other stormwater facility impounding water), resulting in potential
groundwater contamination, Lower Providence Township may require the
developer to submit a hydrogeologic justification study of the site
and proposed infiltration BMPs, prepared by a qualified design professional,
to determine the risk for such contamination. 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.
G.
Infiltration BMPs within high quality/exceptional value waters shall be subject to PADEP's Title 25, Chapter 93, Antidegradation Regulations.
H.
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.
I.
If on-lot infiltration structures are proposed, it must be demonstrated
that the soils are conducive to infiltrate on the lots identified,
or that the applicant's design includes the addition of suitable
amounts of material to facilitate infiltration and support the calculations
as submitted.
J.
Infiltration BMPs shall be designed in accordance with the design criteria and specifications of the Pennsylvania Stormwater BMP Manual (2006) and as additionally identified pursuant to § 129-19I(1) of this chapter.
A.
In addition to the performance standards and design criteria requirements of Article III of this chapter, adequate treatment and storage facilities must be provided to capture and treat stormwater runoff from developed or disturbed areas, unless otherwise exempted by provisions of this chapter. The recharge volume computed under § 129-15 may be a component of the water quality volume if the applicant chooses to manage both components in a single facility. Only if the recharge volume is less than the water quality volume may the remaining water quality volume be captured and treated by methods other than recharge/infiltration BMPs. The required water quality volume (WQv) is the storage capacity needed to capture and to treat a portion
of stormwater runoff from the developed areas of the site produced
from 90% of the average annual rainfall (P). The following calculation
formula is to be used to determine the required water quality storage
volume, (WQv), in acre-feet of storage:
|
Equation 129-17.1
|
|
WQv = [(P)(Rv)(A)]/12 (inches/foot)
|
|
Where:
| |||
|
P
|
=
|
Rainfall amount equal to 90% of events producing this rainfall
(inches): the volume of rainfall for 90% of the storm events which
produce runoff in the watershed annually.
| |
|
A
|
=
|
Area of the project contributing to the water quality BMP (acres).
| |
|
Rv
|
=
|
Volume runoff coefficient, 0.05 + 0.009(I), where I is the percent
of the area that is impervious surface (impervious area ÷ total
project study area) x 100%.
| |
B.
Provisions shall be made (such as adding a small orifice at the bottom
of the BMP facility outflow control structure) so that the proposed
condition, one-year frequency design 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). The design of the
facility shall minimize clogging and sedimentation. Orifices smaller
than three inches in diameter are not recommended. However, if the
design engineer can verify that the smaller orifice is protected from
clogging by use of trash racks, etc., smaller orifices may be permitted.
Trash racks are required for any primary orifice.
C.
To accomplish the requirements in Subsections A and B above, the applicant 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. Infiltration BMPs shall be used wherever feasible. Wet ponds, artificial wetlands, or other permanent BMPs acceptable to the Township shall be used to the extent that infiltration BMPs are deemed not feasible.
D.
Design of BMPs used for water quality control shall be in accordance
with design specifications outlined in the Pennsylvania Stormwater
BMP Manual or other applicable manuals. The following factors must
be considered when evaluating the suitability of BMPs used to control
water quality at a given development site:
(1)
Total contributing drainage area.
(2)
Permeability and infiltration rate of the site soils.
(3)
Topographic slope and depth to bedrock.
(4)
Seasonal high water table.
(5)
Proximity to building foundations and wellheads.
(6)
Erodibility of soils.
(7)
Land availability and configuration of the topography.
(8)
Peak discharge and required volume control.
(9)
Streambank erosion.
(10)
Efficiency of the BMPs, to mitigate potential water quality
problems.
(11)
Volume of runoff that will be effectively treated.
(12)
Nature of the pollutant being removed.
(13)
Maintenance requirements.
(14)
Creation/protection of aquatic and wildlife habitat.
(15)
Recreational value.
(16)
Enhancement of aesthetic and property value.
A.
In addition to the water quality volume, to mitigate the impact of
stormwater runoff on downstream stream bank erosion, BMPs must be
designed to detain the proposed conditions two-year, twenty-four-hour
design storm to the existing conditions one-year flow using the SCS
Type II distribution.
B.
Whenever a watercourse is located within a development site, it shall
remain open in its natural state and location and shall not be piped,
impeded, or altered (except for permitted crossings). The applicant
shall stabilize all eroded stream/channel beds and banks within a
subdivision or land development site and obtain all permits necessary
from PADEP to do so. The applicant must submit pictorial documentation
of existing stream/channel banks to determine whether existing banks
must be stabilized.
A.
Stormwater runoff which may result from regulated activities identified in § 129-5 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, and are the preferred methods of controlling stormwater runoff. 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 curb
line 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 specification. 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. (Marker may be bolted to the grate in off-road locations).
Marker shall have a minimum diameter of 3 1/2 inches and include
"No Dumping - Drains to Waterway" and a fish symbol. Alternate designs/sizes
may be used if approved by the Township.
(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 one-half of one percent (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)
Storm sewer bedding/backfill requirements shall conform to the Lower
Providence Township construction requirements/specifications.
(9)
Inlets shall be located to intercept concentrated runoff prior to
discharge over public/private rights-of-way, sidewalks, streets, and
driveways.
(10)
The capacity of all Type 'C' inlets shall be based
on a maximum surface flow to the inlets of four cfs, calculated based
on the one-hundred-year frequency design storm event. The maximum
flow to Type 'C' inlets located in low points (such as sag
vertical curves) shall include the overland flow directed to the inlet
as well as all bypass runoff from upstream inlets. The bypass flow
from upstream inlets shall be calculated using inlet efficiency curves
included in PennDOT Design Manual Part 2, latest edition. If the surface
flow to an inlet exceeds four cfs, additional inlets shall be provided
upstream of the inlet to intercept the excessive surface flow. A Type 'C'
inlet at a low point of a paved area may be designed to accept a maximum
of six cubic feet per second (CFS). Type 'M' inlets shall
be designed to accept a maximum surface flow of six CFS based on the
one-hundred-year frequency design storm event, unless otherwise approved
by the Township. Double inlets will not be permitted where additional
pipe and inlets can be placed upstream to intercept excessive surface
flow. A maximum of 12 cfs shall be permitted to be collected by a
Type 'M' inlet located in an isolated pervious area, provided
the designer can verify that such an inlet would not cause stormwater
to accumulate on any adjoining public or private property, outside
of a storm sewer easement, and that the depth of the accumulated stormwater
would not exceed 12 inches.
(11)
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.
(12)
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 proposed pipe has sufficient strength to withstand
loading from anticipated design vehicles. Where cover is restricted,
concrete trench drain with bolt-down metal grates may be used.
(13)
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.
(14)
Culverts shall be designed based on procedures contained in
Hydraulic Design of Highway Culverts, HDS #5, United States 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.
(15)
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.
(16)
Stormwater pipes must be oriented at right angles to electric,
water, sanitary sewer, and gas utilities when crossing above or beneath
same. Crossing angles of less than 90° will only be permitted
at the discretion of the Township. When skewed crossings are permitted,
interior angles between alignment of the storm sewer pipe and utility
may not be less than 45°. Vertical and horizontal design of storm
sewer must be linear.
(17)
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.
(18)
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 and any required access
from the public road. For multiple pipes or utilities, the width of
the easement shall be a minimum of 30 feet.
(19)
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.
(20)
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 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: Source of the following design criteria
is the Pennsylvania Department of Environmental Protection, Bureau
of Soil and Water Conservation Publication, Erosion and Sediment Pollution
Control Program Manual (Document No. 363-2134-008).]
|
|
Allowable Velocity
| ||
|---|---|---|
|
Material
|
Velocity
(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 blanket
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 to
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.
(10)
Except for drainage at roadway stream crossings, artificial
swale discharge shall be set back 75 feet from a receiving waterway,
and shall be diffused or spread out to reduce and eliminate high-velocity
discharges to the impacted ground surface.
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
principles:
(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
(riprap) lining (native material if possible) shall be installed within
the culvert as needed to prevent erosion within the structure. Approximate
top of rock lining must be at the level of the existing stream bottom
so as to maintain unimpeded movement of native animal species and
a normal water depth of 12 inches unless a greater depth is required
by PADEP.
(2)
Bottom of opening shall be designed to match the bankfull channel
condition in terms of width and depth. The cross-sectional area of
the bankfull channel (measured at a reference location upstream of
the structure) shall be matched with area in the crossing structure.
(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 period 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 United States 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, upstream
or downstream of the structure.
F.
Storm sewer design.
(1)
Design flow rate. The storm sewer system shall be designed to carry
the one-hundred-year frequency design storm peak flow rate. The drainage
area and runoff coefficient to each inlet shall be indicated on the
stormwater management plan. The one-hundred-year flow rate shall be
determined by the Rational Method formula:
|
Q = CIA
|
|
Where:
| |||
|
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.
| |
|
Values for the rainfall intensity shall be based on NOAA Atlas
14, Volume 2, Version 3.0, rain data found in Table A-3 and Figure
A-2 of Appendix A of this chapter.[1]
|
[1]
Editor's Note: Said appendix is on file in the Township's
offices.
(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 § 129-20) 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 (storm duration)
results in a pipe size exceeding a thirty-inch diameter pipe (or equivalent
flow area of 4.9 square feet), the time of concentration approach
(as defined herein) shall be used in determining storm duration.
(5)
In determining the required design flow rate through a storm sewer
piping system, if a five-minute time of concentration results in a
pipe size exceeding 30 inches, within any run of pipe, the time of
concentration approach may be used for sizing of pipes from that point
on, by adjusting the time of concentration.
(6)
Overflow system. An overflow system shall be provided to carry all bypass flow and/or flow in excess of storm sewer pipe design capacity to the detention basin (or other approved outlet point) when the capacity of the system is exceeded. Stormwater runoff will not be permitted to surcharge from storm sewer structures [Refer § 129-19C(19)].
(7)
Except for drainage at roadway stream crossings, pipe discharge shall
be set back 75 feet from a receiving waterway, and the pipe discharge
shall be diffused or spread out to reduce and eliminate high-velocity
discharges to the impacted ground surface.
G.
Grading and drainage.
(1)
After completion of rough grading, a minimum of eight inches of topsoil
shall be returned to remaining disturbed areas prior to final grading
and seeding.
(2)
Lots shall be graded to secure proper drainage away from buildings
and to prevent the collection of stormwater in pools. Minimum two-percent
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 Lower Providence Township Building Code requirements.
(3)
Construction. The applicant shall construct and/or install such drainage
structures and/or pipe as are necessary to prevent erosion damage
and to satisfactorily disperse, infiltrate or carry off such surface
waters to the nearest practical BMP, 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 excavation.
(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. Where walls or slopes (steeper than two
horizontal to one vertical) are approved under the criteria in this
chapter, and are four feet or more in height, a protective fence,
no less than four feet in height, shall be required at the top of
the wall (or bank).
(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 be permitted only
where application is made, and approval is received, from the PADEP
(where applicable), the Montgomery County Conservation District, and
Lower Providence 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 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 on to public roads
and driveways, tire cleaning areas constructed of AASHTO #1 stone
(underlain by geotextile structural fabric), at least 50 feet in length
shall be installed at each point of access to the site and individual
lots (upon construction of internal streets in a binder condition).
When deemed necessary by the Township, washing stations shall also
be set up at every construction entrance in order to wash mud and
dirt from exiting vehicles. Appropriate measures must be taken to
control runoff from such locations. The applicant shall be responsible
for the placement of appropriate signage identifying construction
entrances and washing stations. Construction entrances shall be maintained
by the applicant 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 basin) are proposed, the applicant 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 Lower Providence
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] and as recommended by the Township Engineer. When a detention
basin is not designed as a stormwater management constructed wetland,
it shall be planted with low-maintenance grass or similar satisfactory
to the Township.
(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, clean-out stakes and other measures as may be required by PADEP, 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 all plan sheets required to be recorded as well as the development agreement with the Township and the stormwater management facilities operation and maintenance agreement.
(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(s) 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 permanently stabilized.
(9)
Except where otherwise identified herein, all detention or retention
basins shall have slopes of four horizontal to one vertical (4:1 =
25%) or flatter on the basin's outer berm and five horizontal
to one vertical or less on the basin's inner berm. The top or
toe of any slope shall be located a minimum of five feet from any
property line. The maximum difference between the top of berm elevation
and the invert elevation of the outlet structure shall be seven feet.
(10)
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.
(11)
Basin berm construction requirements.
(a)
Site preparation. Areas under the embankment and any structural
works shall be cleared, grubbed, and the topsoil stripped to remove
the trees, vegetation, roots or other objectionable material. In order
to facilitate clean-out and restoration, the pool area will be cleared
of all brush and excess trees except where designed to retain such
existing vegetation as stormwater BMPs.
(b)
Cut-off trench. A cut-off trench will be excavated along the
centerline dam on earth fill embankments. The minimum depth shall
be two feet. The cut-off trench shall extend up both abutments to
the riser crest elevation. The minimum bottom width shall be eight
feet but wide enough to permit operation of compaction equipment.
The side slopes shall be no steeper than 1:1. Compaction requirements
shall be the same as those for the embankment. The trench shall be
kept free from standing water during the backfilling operations.
(c)
Embankment:
[1]
The fill material shall be taken from the selected borrow areas.
It shall be free of roots, wood vegetation, oversized stones, rocks
or other objectionable material. Areas on which fill is to be placed
shall be scarified prior to placement of fill.
[2]
The fill material should contain sufficient moisture so that
it can be formed by hand into a ball without crumbling. If water can
be squeezed out of the ball, it is too wet for proper compaction.
[3]
Fill material will be placed in six to eight-inch layers and
shall be continuous over the entire length of the fill. Fill material
must be compacted to a minimum of 95% of modified proctor density
as established by ASTM D-1557. Compaction testing by a certified soils
engineer/geologist must be completed as directed by the Township Engineer
to verify adequate compaction has been achieved. 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.
(12)
Emergency overflow facilities/spillway shall be provided within
basins in order to convey basin inflow in excess of design flows,
out of the basin, or in the event the outlet structure becomes blocked
and is unable to convey flow. Emergency spillways discharging over
embankments shall be constructed of reinforced concrete checker-blocks
to protect the berm against erosion. The checkerblocks shall be back-filled
with topsoil and seeded. Checkerblock lining shall extend to the toe
of the embankment on the outside of the berm, and shall extend to
an elevation of three feet below the spillway crest on the inside
of the berm. Vegetated spillways may be utilized for spillways constructed
entirely on undisturbed ground (i.e., not discharging over fill material).
A dense cover of vegetation shall be rapidly established in such spillways
by sodding or seeding with a geotextile anchor. The vegetated spillway
must be stabilized before runoff is directed to the basin. The minimum
capacity of all emergency spillways shall be equivalent to the peak
flow rate of the one-hundred-year, post-development design storm (entering
to the basin).
(13)
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.
(14)
The minimum top of basin berm width (at the design elevation)
shall be 10 feet.
(15)
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.
(16)
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 in the
field 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 Engineer. A minimum of two collars shall be installed on
each basin outlet pipe. Collars shall have a minimum thickness of
12 inches and may not be installed within two feet of pipe joints.
(17)
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.
(18)
Stormwater management facility outlet piping shall be Class
III reinforced O-ring concrete pipe. All joints shall be mortared.
Crushed stone bedding/backfill shall not be utilized through basin
berms.
(19)
The grate of the basin outlet structure shall be at least six
inches lower than the elevation of the earthen emergency spillway.
Six inches, minimum, is also required between the routed one-hundred-year
water surface elevation and top of grate of the outlet structure.
(20)
Energy-dissipating devices (rock lining/riprap, or other approved
materials) shall be provided at all basin outlets and shall be sized
in accordance with PADEP, Bureau of Soil and Water Conservation Publication,
Erosion and Sediment Pollution Control Program Manual, latest revision.
(21)
Stone gabion baskets or concrete or segmental block retaining
walls shall not be permitted for use in construction of detention/retention
basins within the berm or within the one-hundred-year water surface
elevation (as measured through the earthen emergency spillway).
(22)
An access easement and stabilized access 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 systems (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 reinforced concrete apron (six-inch minimum thickness) 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. The access easement shall
be owned and maintained by the same entity owning the stormwater management
facility and shall allow access by Lower Providence Township or its
designee for emergency inspection and/or maintenance at any reasonable
time.
(23)
If the basin is not designed to meet the requirements of § 129-19H(3), 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)
Fence shall be split-rail consisting of locust posts (two- or
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 six inches above finished grade. The mesh
shall be installed on the outside of the fence.
(d)
Split-rail fence shall also be required around any detention
or retention basin, where directed by Lower Providence Township.
(24)
Landscaping:
(a)
The perimeter berms and embankments of retention/detention basins,
including wet ponds and artificial wetland stormwater management BMPs,
shall be designed to create a natural appearance and reduce future
maintenance requirements. Landscaping shall include a mixture of native
tall grasses and perennial plants, ground cover, shrubs, and trees
to eliminate the necessity of periodic mowing.
(b)
Artificial wetland basins shall be designed pursuant to requirements
of the Pennsylvania Stormwater BMP Manual. Plant material and arrangement
shall be subject to approval of the Township Engineer.
(c)
Required planting.
[1]
The perimeter of the retention/detention basin shall be landscaped
with a mixture of deciduous trees, evergreens, and shrubs arranged
in an informal manner. Retention basin (wet ponds) and artificial
wetland basin landscaping shall be designed to create a natural appearance.
Minimum plant material shall include the following per 100 linear
feet of basin perimeter measured at the one-hundred-year water surface
elevation:
[2]
Retention/detention basin landscaping design is subject to approval
by the Township.
(25)
Special requirements for stormwater detention/retention BMPs within defined exceptional-value and high-quality watersheds as defined in Chapter 93, Water Quality Standards, Title 25, Pennsylvania Department of Environmental Protection Rules and Regulations:
(a)
Temperature-sensitive BMPs and stormwater conveyance systems
are to be used and designed with storage pool areas and supply outflow
channels and shaded with trees. At a minimum, the southern half of
pond shorelines shall be planted with shade or canopy trees and understory
shrubs within 10 feet of the pond shoreline. In conjunction with this
requirement, the maximum slope allowed on the berm area to be planted
is 10 to 1. This will lessen the destabilization of berm soils due
to root growth. A long-term maintenance schedule and management plan
for the thermal control BMPs must be identified on the stormwater
management site plan and recorded at the Montgomery County Recorder
of Deeds for all development sites.
(b)
As an alternative to mitigating the temperature of stormwater runoff as described in § 129-19H(25)(a), alternative temperature-sensitive BMPs may be utilized, if approved by the Township Engineer, upon the applicant demonstrating such BMPs will effectively reduce the temperature of detained runoff before it is released from the development site. Such alternative BMPs may include, but are not limited to, facilities that cool runoff through underground storage and filtration and retention ponds/basins where outflow from the facility is drawn from a depth of five feet (or greater) below the permanent pool surface.
(26)
At the conclusion of all construction and after all stormwater
facilities have received final approval, the applicant shall offer
the facilities for dedication to the Township, with the following
requirements:
(a)
The dedicated area shall include the entire ponded area for
the one-hundred-year storm event and the outside slope at the berm.
(c)
The Applicant shall provide for the special financial burden
the Township will be incurring if the Township accepts the detention
basin maintenance. To help mitigate this future financial burden,
the developer shall contribute to the Township a cash payment in an
amount to be calculated by the Township Engineer, which amount shall
include all estimated costs to inspect, maintain, and repair the facilities
during a ten-year period.
(27)
If the Township declines dedication of the basin, the applicant
shall provide written assurance, satisfactory to the Township, that
the retention/detention basin will be properly maintained. Such assurances
shall be in a form of a covenant that will run with the land and shall
provide for Township maintenance at the cost of the landowner in case
of default, and further provide for assessment of costs and penalties
in case of default.
I.
All developments that create impervious surface shall provide capacity for and treatment of the calculated water quality volume and recharge volume unless exempt under § 129-6. In potential stormwater BMPs, the order of preference is as follows: 1) infiltration BMPs; 2) flow attenuation methods (e.g., vegetated open swales and natural depressions); 3) artificial wetlands, bioretention structures, and wet ponds; 4) minimum first-flush detention or dual-purpose detention (where appropriate). Infiltration BMPs shall be utilized unless the applicant can demonstrate use of infiltration techniques is not feasible due to site conditions, based upon site-specific soil testing. Vegetated swales, wetlands or artificial wetlands and bioretention structures shall be utilized wherever possible if infiltration BMPs are deemed unfeasible. BMP techniques can and should be used in conjunction with each other (e.g., vegetated swales with infiltration or retention facilities).
(1)
Infiltration best management practices (BMPs). Infiltration BMPs
shall be designed in accordance with the design criteria and specifications
of the Pennsylvania Stormwater BMP Manual (2006) and shall conform
to the following minimum requirements:
(b)
A minimum soil depth of 18 inches shall be provided between
the bottom of the infiltration BMPs and the top of bedrock or seasonal
high water table. The minimum required separation between the infiltration
surface and these limiting zones shall be increased by the Township
should project-specific conditions exist (such as anticipated increased
contaminants) which dictate greater prevention of groundwater contamination.
(c)
Infiltration BMPs must have an infiltration rate sufficient
to accept the design stormwater load and dewater completely as determined
by field permeability tests. The minimum field-tested infiltration
rate permitted for construction of infiltration BMPs shall be 0.2
inches/hour (in/hr). A safety factor of 50% shall be applied to field-tested
rates to determine the infiltration rate that must be utilized for
design of infiltration BMPs (e.g., for soil which measured 0.4 in/hr,
the BMP design rate shall be 0.2 in/hr to insure effective infiltration
after construction).
(d)
Infiltration BMPs intended to receive rooftop runoff shall include
appropriate measures such as leaf traps and cleanouts to prevent clogging
by vegetation. Surface inflows shall be designed to prevent direct
discharge of sediment into the infiltration system.
(e)
Adequate storage shall be provided to accommodate the volume
of runoff calculated as the difference between the predevelopment
runoff volume and post-development runoff volume based on the one-hundred-year
design storm.
(f)
The facility shall be designed to control the post-development peak rate of runoff to the predevelopment peak rate of runoff for all design storms identified in § 129-14 of this chapter.
(g)
An overflow or spillway shall be provided that safely permits
the passing of runoff greater than that occurring during the one-hundred-year
design storm event.
(h)
Underground infiltration basins and BMPs shall have positive
overflow controls to prevent storage within one-foot of the finished
surface over the basin.
(i)
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 50 feet shall be provided between any septic system
and any facility used for stormwater management and infiltration.
(j)
A minimum of 30 feet of undisturbed soil shall separate the
foundation wall of any building and any infiltration BMP, unless a
lesser distance is approved by the Township or Township engineer,
based on site conditions or selected BMP.
(k)
All infiltration facilities shall be designed to completely
infiltrate runoff volume within two days (48 hours) from the peak
of the design storm.
(l)
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.
(m)
The Township may require the installation of a mitigative layer
or an impermeable liner in an infiltration BMP and/or other stormwater
structure that impounds runoff, where the possibility of groundwater
contamination exists. A detailed hydrogeologic investigation may be
required by the Township.
(n)
Infiltration BMPs shall not be constructed nor receive runoff
until the entire contributory drainage area to the infiltration BMP
has achieved final stabilization.
(o)
Infiltration BMPs shall be designed based on field-tested infiltration/permeability
rates at the level of the proposed infiltration surface(s) and based
on a safety factor of 50%.
(2)
Noninfiltration facilities used as best management practices (BMPs).
All facilities shall be designed in accordance to the design criteria
and specifications in the Pennsylvania Stormwater BMP Manual.
(3)
Artificial wetlands, wet ponds, and bioretention structures.
(a)
Wet pond BMPs shall meet the following requirements.
[1]
Wet ponds shall be constructed on hydric or wet soils and/or
soils which have an infiltration rate of less than 0.2 inch/hour.
[2]
A minimum drainage area of five acres shall be directed to the
pond unless a source of recharge is utilized, such as a natural spring
or well.
[3]
The length of the pond between the inflow and outlet points
shall be maximized. In addition, an irregular shoreline shall be provided.
By maximizing the flow length through the pond and providing an irregular
shoreline, the greatest water quality benefit will be achieved by
minimizing short circuiting of runoff flowing through the pond.
[4]
A shallow forebay shall be provided adjacent to all inflow areas.
The forebay shall be planted as a marsh with emergent wetland vegetation.
The forebay serves to enhance sediment trapping and pollutant removal,
as well as concentrating accumulated sediment in an area where it
can be readily removed.
[5]
All wet ponds shall be designed with public safety as a primary
concern. An aquatic safety bench shall be provided around the perimeter
of the permanent pool. The depth of the bench shall be a maximum of
15 inches and a minimum of 12 inches for a width of at least 10 feet.
A 3:1 slope shall lead from the edge of the safety bench toward the
deep water portion of the pond. At least 15 feet of 3:1 slope shall
be provided from the edge of the safety bench. Slopes in the remainder
of the pond below the permanent pool elevation shall be a maximum
of 2:1.
[6]
The perimeter slope above the permanent pool shall have a maximum
slope of 5:1.
[7]
Wet ponds shall have a deep water zone of at least five feet
to encourage gravity settling of suspended fines, and prevent stagnation
and possible eutrophication.
[8]
Wet ponds shall be capable of being substantially drained by
gravity flow. Wet ponds shall be equipped with a manually operated
drain that can be secured against unauthorized operation. A planting
plan shall be developed for the wet pond, showing all proposed aquatic,
emergent, and upland plantings required pursuant to this chapter and
the Zoning and Subdivision and Land Development ordinances (where
specifically identified).[5] Wet ponds shall be designed to discourage use by Canada
geese. Techniques employed shall include the following:
[a]
Elimination of straight shorelines, islands, and
peninsulas;
[b]
Placement of walking paths (where applicable) along
the shoreline;
[c]
Placement of grassed areas (i.e., playing fields)
at least 450 feet from the water surface;
[d]
Vegetative barriers;
[e]
Rock barriers;
[f]
Installation of tall trees within 10 feet of the
water surface;
[g]
Use of ground covers not palatable to Canada geese.
[h]
Other techniques as approved by the Township Engineer.
(b)
Artificial wetland BMPs shall meet the following requirements:
[1]
Artificial wetlands shall be constructed on hydric or wet soils
and/or soils which have an infiltration rate of less than 0.2 inch/hour.
[2]
Runoff entering artificial wetlands shall be filtered through
a sediment removal device before entering the wetland.
[3]
A planting plan shall be developed for the artificial wetland
showing all proposed aquatic, emergent, and upland plantings required
pursuant to this chapter and the Zoning and Subdivision and Land Development
ordinances (where specifically identified).[6] The planting plan shall be developed to provide a diversity
of species resulting in a dense stand of wetland vegetation.
[4]
At least 75% of the surface area of the wetland shall be developed
as a shallow water emergent wetland, with a water depth of less than
12 inches. The remainder shall be constructed as open water with depths
between two and four feet.
(4)
Minimum first flush detention/dual purpose BMPs.
(a)
Minimum first flush detention/dual purpose detention basin BMPs
shall be designed to meet the following requirements:
[1]
Post-development runoff from a water quality storm (a one-year,
twenty-four-hour event) shall be released over a minimum period of
24 hours.
[2]
Two stage basins shall be utilized where first flush detention
will be employed for water quality and conventional detention used
for peak rate control of storms exceeding the one-year, twenty-four-hour
event.
[3]
Two stage basins shall be constructed so that the lower part
of the basin is graded to detain stormwater from the water quality
storm, and the remainder of the basin graded as a flat overbank area
to provide storage only for the larger, less frequent storm events.
The overbank area is encouraged to be developed as an active or passive
recreational area.
[4]
The area inundated by the water quality storm is encouraged
to be maintained as a wetland environment, which will increase the
water quality benefits of the first flush/dual purpose detention basin,
and will prevent the need for mowing of a frequently saturated area.
J.
Riparian corridor restoration. Within all subdivisions and nonresidential
land developments, from the top of watercourse bank, 75 feet on either
side of the watercourse, which contains wetlands and/or floodplain,
shall be planted to establish a Zone 1 and Zone 2 buffer as defined
and in accordance with the Pennsylvania Handbook of Best Management
Practices for Developing Areas, 1998, Riparian Forested Buffer. Where
existing vegetation on the site essentially duplicates buffer requirements,
this provision shall not apply. Additionally, this requirement may
be modified or waived by the Board of Supervisors where existing man-made
improvements or agricultural operations to be retained encroach within
the buffer area.
K.
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 plan shall include such details, notes, or specification 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 first to an infiltration BMP, and if
that is not possible, then to a storm sewer, 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.
L.
All stormwater control facility designs shall conform to the applicable
standards and specifications of the following governmental and institutional
agencies:
(1)
American Society of Testing and Materials (ASTM).
(2)
Asphalt Institute (AI).
(3)
Montgomery County Conservation District (MCCD).
(4)
Federal Highway Administration (FHWA).
(5)
National Crushed Stone Association (NCSA).
(6)
National Sand and Gravel Association (NSGA).
(7)
Pennsylvania Department of Environmental Protection (PADEP).
(8)
Pennsylvania Department of Transportation (PADOT).
(9)
United States Department of Agriculture, Natural Resources Conservation
Service, Pennsylvania (USDA, NRCS, PA).
A.
Stormwater runoff peak discharges from all development sites with
a drainage area equal to or greater than 50 acres shall be calculated
using a generally accepted calculation technique that is based on
the NRCS Soil Cover Complex Method. The Rational Method may be used
to estimate peak discharges from drainage areas that contain less
than 100 acres as approved by the Township Engineer. The Rational
Method is recommended for watershed areas under 50 acres. Table 129-20.1
summarizes acceptable computation methods. The method shall be selected
by the applicant based on the individual limitations and suitability
of each method for a particular site.
|
Table 129-20.1 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
|
US Army Corps of Engineers
|
Applicable where use of full hydrologic computer model is desirable
or necessary
|
|
PSRM
|
Penn State University
|
Applicable where use of a hydrologic computer model is desirable
or necessary; simpler than TR-20 or HEC-1
|
|
Rational Method (or commercial computer package based on Rational
Method)
|
Emil Kuichiling (1889)
|
Applicable sites less than 50 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 A-1 of Appendix
A of this chapter.[1] The SCS Type II rainfall curve data from NOAA is listed
in Figure A-1 in Appendix A 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: Said appendix is on file in the Township's
offices.
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 IE rain gage (No. 36-3437), Schwenksville,
Pennsylvania as presented in Table A-3 of Appendix A of this chapter.[4] The Rational Method rainfall curve data from NOAA is listed
in Figure A-2 in Appendix A 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.
[4]
Editor's Note: Said appendix is on file in the Township's
offices.
F.
For the purposes of existing conditions flow rate determination for
all development activity, undeveloped land and existing impervious
surfaces shall be considered as meadow in good condition, unless the
natural ground cover generates a lower curve number (CN) or Rational 'C'
value (e.g., forest), as listed in Tables A-2 and A-4 in Appendix
A of this chapter.[5] 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.
[5]
Editor's Note: Said appendix is on file in the Township's
offices.
G.
Where uniform flow is anticipated, the Manning equation shall be
used for hydraulic computations, and to determine the capacity of
open channels, pipes, and storm sewers. Values for Manning's
roughness coefficient (n) shall be consistent with Table A-5 in Appendix
A of this chapter.[6]
[6]
Editor's Note: Said appendix is on file in the Township's
offices.
H.
Outlet structures for stormwater management facilities shall be designed
to meet the performance standards of this chapter using any generally
accepted hydraulic analysis technique or method.
I.
The design of any stormwater 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.
J.
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. Time of concentration is computed by summing all the
travel times for consecutive components of the drainage conveyance
system.
K.
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.
Whenever vegetation and topography are to be disturbed, such activity must be in conformance with Chapter 102, Title 25, Rules and Regulations, Part 1, Commonwealth of Pennsylvania, Department of Environmental Protection, Sub-Part C, Protection of Natural Resources, Article II, Water Resources, Chapter 102, Erosion Control, and in accordance with the Montgomery County Conservation District and the standards and specifications of the Township. Various BMPs and their design standards are identified in the PADEP Erosion and Sediment Pollution Control Program Manual (March 2012), as amended and updated.
B.
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.
C.
In addition, under 25 Pa. Code, Chapter 92, a PADEP NPDES Construction
Activities permit is required for regulated earth disturbance activities
of one or more acres.
D.
Evidence of any necessary permit(s) for regulated earth disturbance
activities from the appropriate PADEP regional office or county conservation
district must be submitted to the Township.
E.
A copy of the erosion and sediment control plan and any required
permit, as required by PADEP or Montgomery County Conservation District
regulations, shall be available at the project site at all times.
F.
Additional erosion and sedimentation control design standards and
criteria that must be applied where infiltration BMPs are proposed
include the following:
(1)
Areas proposed for infiltration BMPs shall be protected from sedimentation
and compaction during the construction phase, so as to maintain their
maximum infiltration capacity. 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 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.
G.
Peak discharge rates from the site during land disturbance shall
comply with the appropriate sections in this chapter related to allowable
post-development 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 one-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.
H.
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.
