[Amended 6-15-1988 by Ord. No. 3098; 5-17-1989 by Ord. No.
3155; 11-17-1999 by Ord. No. 3546]
A.
General requirements.
(1)
For all properties within the Township, the developer
is required to control stormwater runoff both during and after construction
in accordance with the provisions of this chapter.
(a)
Non-watershed plan districts. The developer
shall ensure that the peak discharge of runoff from all storms of
up to and including five-year frequency shall be controlled to that
peak discharge which occurred prior to development in a two-year frequency
storm. The peak discharge rates of storms of ten-year, twenty-five-year
and one-hundred-year frequencies shall be controlled to the peak discharges
which occurred prior to development in the respective storm frequencies.
(b)
Release rate districts.
[Amended 7-18-2007 by Ord. No. 3821]
[1]
Lower Merion Act 167 drainage areas. The two-year
postdevelopment peak rate of runoff shall be controlled to the one-year
predevelopment peak rate of runoff. Storms up to the five-year frequency
must be controlled to the lesser of the two-year predevelopment rate
or the percentage of the predevelopment rate as listed in Appendix
B for the particular subwatershed area. For the ten- and twenty-five-year
storm, the postdevelopment rate shall be controlled to the percentage
of the respective predevelopment rate as listed in Appendix B. The
fifty- and one-hundred-year storms shall be controlled to the peak
discharges which occurred prior to development in the respective storm
frequencies.
[2]
Darby-Cobbs Creek Act 167 drainage areas.
[a]
The peak rate of runoff must be controlled according
to the following table:
|
District
|
Proposed Condition Design Storm
|
Existing Condition Design Storm
|
|---|---|---|
|
A
|
2-year
|
1-year
|
|
5-year
|
2-year
| |
|
10-year
|
10-year
| |
|
25-year
|
25-year
| |
|
50-year
|
50-year
| |
|
100-year
|
100-year
| |
|
B-1
|
2-year
|
1-year
|
|
5-year
|
2-year
| |
|
10-year
|
5-year
| |
|
25-year
|
10-year
| |
|
50-year
|
25-year
| |
|
100-year
|
100-year
| |
|
B-2
|
2-year
|
1-year
|
|
5-year
|
2-year
| |
|
25-year
|
5-year
| |
|
50-year
|
10-year
| |
|
100-year
|
100-year
|
[b]
In addition to the general requirements for volume recharge applicable to all release rate districts, developments within the Darby-Cobbs drainage areas must document recharge and water quality controls in accordance with § 121-4B(2)(d).
(c)
Provisional infiltration district. The developer shall control stormwater as described in § 121-4B(2)(b) of this chapter.
(2)
In addition, seepage beds shall be provided to recharge
the full roof drainage from a two-year storm, with direct connection
to the seepage beds from roof drains established wherever feasible.
[Amended 3-15-2023 by Ord. No. 4262]
(3)
The design of all stormwater management facilities
shall be in accordance with this chapter and submitted to the Township
Engineer for review and, when required, to the Pennsylvania Department
of Environmental Protection, the United States Department of Agriculture
Soil Conservation Service or the United States Army Corps of Engineers.
[Amended 3-15-2023 by Ord. No. 4262]
(4)
Where retention basins and/or other facilities are
required, the responsibility for the continued maintenance and operation
of the retention basins and/or other facilities shall be the obligation
of the property owner or the condominium unit owners' association
or the council or the homeowners' association, as applicable.
(5)
Any deed given for the transfer of a lot where stormwater
piping and/or a retention facility is provided on a recorded lot shall
refer to such structures and facilities and provide that the maintenance
and operation shall be the continued responsibility of the property
owner or the condominium unit owners' association or the council or
the homeowners' association, as applicable.
(6)
The natural drainage system shall be preserved in
an undisturbed state to the extent possible. Storm drainage systems
shall be provided in order to permit unimpeded flow of natural watercourses
except as modified by stormwater detention facilities or open channels
consistent with this chapter.
(7)
The existing points of concentrated stormwater discharge
onto adjacent property shall not be altered without written approval
of the affected property owner(s). Alteration of existing drainage
discharge onto adjacent property shall only be proposed in accordance
with the PADEP guidance document "Chapter 102 Off-Site Discharges
of Stormwater to Non-Surface Waters - Frequently Asked Questions (FAQ),"
dated January 2, 2019, or latest guidance document from PADEP. For
any location where a new concentrated discharge of stormwater from
any frequency rainfall event, up to and including the 100-year storm
and the volume of runoff up to and including the two-year storm, or
through adjacent properties or a downgradient property(ies), the following
are required:
[Amended 3-15-2023 by Ord. No. 4262]
(a)
A drainage easement (or other legal agreement/approval) must be obtained
for conveyance of discharges onto or through adjacent properties per
the PADEP guidance document "Chapter 102 Off-Site Discharges of Stormwater
to Non-Surface Waters - Frequently Asked Questions (FAQ)," dated January
2, 2019, or latest guidance document from PADEP.
(b)
The conveyance must be designed to avoid erosion, flooding, or other
damage to the properties through which it is being conveyed.
(8)
Areas of existing diffused drainage onto adjacent
property shall be managed such that, at a minimum, the peak diffused
flow does not increase in the general direction of discharge, except
as otherwise provided for in this chapter. Written permission from
the adjacent property owner must be obtained first if diffused flow
is to be concentrated and discharged onto adjacent property. The developer
must also demonstrate that there are adequate downstream conveyance
facilities to transport the concentrated discharge safely and prove
that no harm will result from the concentrated discharge (including
damage to downstream property, persons or increased scouring and bank
erosion). Areas of existing diffused drainage discharge shall be subject
to any applicable release rate criteria in the general direction of
existing discharge, whether they are proposed to be concentrated or
maintained as diffused drainage areas.
(9)
In developments of more than one lot, stormwater control
is to be achieved on an individual lot basis wherever feasible. Where
control on an individual lot basis is determined to be infeasible,
the stormwater basins shall be located on a separate, non-building
lot or open space easement area. Maintenance of the basins shall be
achieved in accordance with this chapter.
(10)
Where a subdivision is traversed by watercourses other than those for which a one-hundred-year floodplain is defined by the municipality, there shall be provided drainage easements conforming substantially with the line of such watercourses. The width of any easement shall be adequate to provide for unimpeded flow of storm runoff based on calculations made in conformance with § 121-4D for the one-hundred-year return period runoff and to provide a freeboard allowance of one foot above the design water surface level and required maintenance. The terms of the easement shall prohibit excavation, the placing of fill or structures and any alterations which may adversely affect the flow of stormwater within any portion of the easement. Also, periodic maintenance of the easement by the fee property owner(s) to ensure proper runoff conveyance shall be required. Watercourses for which the one-hundred-year floodplain is formally defined are subject to the applicable municipal floodplain regulations.
(11)
When it can be shown that natural drainage swales on the site cannot adequately provide for drainage because of topographic conditions, open channels may be constructed conforming substantially to the line and grade of such natural drainage swales. Capacities of open channels shall be calculated using the Manning equation as described in § 121-4F(6).
(12)
Volume controls for stormwater management, such as infiltration basins, shall be required unless waived by the Township Engineer. Providing for infiltration consistent with the natural hydrologic regime is required. Design of the infiltration facilities shall consider infiltration to compensate for the reduction in the recharge that occurs when the ground surface is disturbed, or impervious surface is created. If it cannot be physically accomplished, then the design professional shall be responsible for demonstrating to the satisfaction of the Township Engineer that this cannot be physically accomplished on the site (e.g., shallow depth to bedrock or limiting zone, open voids, steep slopes, etc., per the PA BMP Manual). A financial hardship is not acceptable to avoid implementing infiltration facilities. If infiltration can be physically accomplished, the volume of runoff to be infiltrated shall be determined from § 121-4, depending on demonstrated site conditions, and shall be the greatest volume that can be physically infiltrated, or alternative methods consistent with the PA BMP Manual (as amended) or other PADEP guidance, such as the Managed Release Concept, may be used to manage this volume with approval from the Township Engineer.
[Amended 3-15-2023 by Ord. No. 4262]
(13)
Any stormwater management facilities required or regulated
by this chapter shall be designed to meet the performance standards
presented in this chapter. Compensatory stormwater management facilities
shall not be permitted outside of the drainage area developed. Applicants
shall utilize the Pennsylvania Stormwater Best Management Practices
Manual (PA BMP Manual), as amended, or other sources acceptable to
the Township Engineer, for testing and design standards for BMPs,
and where there is a conflict with the provisions of this chapter,
the most restrictive applies.
[Amended 3-15-2023 by Ord. No. 4262]
(14)
Any stormwater facilities shall be based upon the
following downstream impact evaluation requirements. Hydraulic capacity
analysis shall be conducted in accordance with the following criteria
to determine if adequate hydraulic capacity exists:
(a)
The developer shall submit to the Township an
analysis of the impacts of the value and timing of stormwater flows
from the proposal downstream in the watershed. The analysis shall
include the hydrologic and hydraulic calculations necessary to determine
the impact of the stormwater discharge on a downstream point to be
one of the following:
[1]
The first downstream tributary whose drainage
area equals or exceeds the drainage area contributing to the detention
basin.
[2]
The first downstream tributary whose peak discharge
exceeds the largest designed discharge rate of the detention basin.
[3]
The confluence of the receiving stream with
the Schuylkill River.
(b)
If the designated release rate for the proposed
stormwater system results in an increase in flooding or stream channel
erosion at the downstream point, the discharge rate of the detention
facility shall be reduced to a level that will prevent any increase
in flooding or stream channel erosion at the downstream control point.
(15)
Any facilities that constitute water obstructions (e.g., culverts, bridges, outfalls or stream enclosures), and any work involving wetlands as described in PA DEP Chapter 105 regulations (as amended or replaced from time to time by PA DEP), shall be designed in accordance with Chapter 105 and will require a permit from PA DEP. Any other drainage conveyance facilities that do not fall under Chapter 105 regulations must be able to convey, without damage to the drainage structure or roadway, runoff from the twenty-five-year design storm with a minimum 1.0 foot of freeboard measured below the lowest point along the top of the roadway. Roadway crossings located within designated floodplain areas must be able to convey runoff from a one-hundred-year design storm with a minimum 1.0 foot of freeboard measured below the lowest point along the top of the roadway. Any facility that constitutes a dam as defined in PA DEP Chapter 105 regulations may require a permit under dam safety regulations. Any facility located within a Pennsylvania Department of Transportation (PADOT) right-of-way must meet PADOT minimum design standards and permit submission requirements.
(16)
Storm sewers must be able to convey post-development
runoff from a twenty-five-year design storm without surcharging inlets.
(17)
Storm sewer inlet spacing and road cross section design
must ensure that post-development runoff resulting from a ten-year
design storm floods not more than 1/2 of a driving land, or as required
by the municipal engineer.
(18)
Easements along open channels shall be provided. The
minimum width of the required easement shall be equal to the width
of the one-hundred-year water surface (for post-development conditions),
including a minimum of 1.0 foot of freeboard.
(19)
For development sites that would be located in two
or more subareas, the applicable release rate for the portions of
the site located in different subareas shall be based on natural subareas'
drainage boundaries. The natural drainage boundaries between subareas
shall not be modified, nor shall drainage from a development site
be diverted or otherwise conveyed from one subarea to another subarea,
except where runoff naturally crosses subarea drainage boundaries.
(20)
No harm option. For any development site, the developer
has the option of discharging post-development runoff at a higher
rate than predevelopment runoff if the developer can provide that
no harm would be caused to any person or property located upstream
or downstream of the development site. The developer must assume that
the entire subarea in which the site is located is developed. The
type and amount of development that the developer must consider shall
be either based on current zoning or established by the Township,
whichever results in a greater amount of imperviousness. Proof of
no harm must demonstrate conformance with the hydraulic capacity criteria
specified in this chapter to demonstrate that no increase of erosion
results and demonstrate no degradation of the quality of downstream
waterways to the satisfaction of the Township Engineer. Areas that
drain through documented drainage problem areas would be precluded
from any no-harm-based peak runoff increases, except where hydraulic
capacity improvements would be provided, consistent with this chapter.
(21)
Regional or subregional stormwater management facilities.
For certain areas within the watershed, it may be more cost effective
to provide one stormwater management facility for an entire subarea,
group of subareas or portion of a subarea incorporating more than
one development site than to provide an individual stormwater management
facility for each development site. The initiative and funding for
any regional or subregional stormwater management alternatives are
the responsibility of prospective developers. The design of any management
alternatives are the responsibility of prospective developers. The
design of any regional stormwater management facilities must assume
development that the developer(s) must consider shall be either based
on current zoning or established by the Township, whichever results
in a greater amount of imperviousness. The peak outflow from a regional
stormwater management facility would be determined on a case-by-case
basis using TR 20 (the SCS Computerized Hydrological Model), as amended.
When regional or subregional stormwater management facilities are
utilized, the effect of phased growth on stormwater runoff flows must
be considered. At no time from the initial phase through ultimate
development shall the peak runoff flows exceed the predevelopment
peak multiplied by the applicable release rate.
(22)
Capacity improvements. If the developer could prove
that it would be feasible to provide capacity improvements to relieve
the capacity deficiency in the existing drainage network, then adequate
capacity improvements could be provided by the developer in lieu of
stormwater management facilities on the development site. Any capacity
improvements would be designed based on development of all areas tributary
to the improvement and the capacity criteria specified in this chapter,
or as required by the Township Engineer. The type and amount of development
that the developer must consider shall be either based on current
zoning or established by the Township, whichever results in a greater
amount of imperviousness. It shall be assumed that all new development
upstream of a proposed capacity improvement would implement applicable
stormwater management techniques, consistent with this chapter.
(23)
Adequate erosion protection which meets the regulations
of Chapter 102 of the Clean Streams Law shall be provided along all
open channels and at all points of discharge. If a perennial or intermittent
stream passes through the site, the applicant shall create a stream
buffer extending a minimum of 50 feet to either side of the top-of-bank
of the channel. The buffer area shall be maintained with appropriate
native vegetation (refer to Appendix H of the Pennsylvania Handbook
of Best Management Practices for Developing Areas for plant list).
If the applicable rear or side yard setback is less than 50 feet,
the buffer width may be reduced to 25% of the setback to a minimum
of 10 feet. If an existing buffer is legally prescribed (i.e., deed,
covenant, easement, etc.) and it exceeds the requirements of this
section, the existing buffer shall be maintained.
[Amended 7-18-2007 by Ord. No. 3821]
(24)
Riparian buffers.
[Amended 3-15-2023 by Ord. No. 4262]
(a)
In order to protect and improve water quality, a riparian buffer
easement shall be created and recorded as part of any subdivision
or land development that encompasses a riparian buffer.
(b)
Except as required by Chapter 102, the riparian buffer easement
shall be measured to be the greater of the limit of the 100-year floodplain
or a minimum of 35 feet from the top of the stream bank (on each side).
(c)
The riparian buffer easement shall be enforceable by the Township and shall be recorded in the Montgomery County Recorder of Deeds office, so that it shall run with the land and shall limit the use of the property located therein. The easement shall allow for the continued private ownership and shall count toward the minimum lot area a required by Chapter 155, Zoning, unless otherwise specified in the Zoning Ordinance.
(d)
Minimum management requirements for riparian buffers.
[1]
Existing native vegetation shall be protected and maintained
within the riparian buffer easement.
[2]
Whenever practicable, invasive vegetation shall be actively
removed and the riparian buffer easement shall be planted with native
trees, shrubs and other vegetation to create a diverse native plant
community appropriate to the intended ecological context of the site.
(e)
Any permitted use within the riparian buffer easement shall
be conducted in a manner that will maintain the extent of the existing
100-year floodplain, improve or maintain the stream stability, and
preserve and protect the ecological function of the floodplain.
(f)
The following conditions shall apply when public and/or private
recreation trails are permitted within riparian buffers:
(g)
Septic drainfields and sewage disposal systems shall not be
permitted within the riparian buffer easement and shall comply with
setback requirements established under 25 Pa. Code Chapter 73.
(25)
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 occurrence or perpetuation of adverse hydrologic or
hydraulic conditions within the watershed.
B.
Stormwater management districts.
[Amended 7-18-2007 by Ord. No. 3821]
(1)
To implement the provisions of the Lower Merion Drainage
Area and Darby-Cobbs Drainage Area Stormwater Management Plans, the
Township is hereby divided into stormwater management districts. These
districts are consistent with the Lower Merion Drainage Area Release
Rate Map (Appendix A) and Darby-Cobbs Watershed Map (Appendix D) presented
in the plans.[1] The boundaries of the stormwater management districts
are shown on watershed maps which are available for inspection at
the Township building.
[1]
Editor's Note: Said Appendixes A and D are
on file in the Township's offices.
(2)
Four types of stormwater management districts are
applicable in the Township. Different BMPs may be used to achieve
the required detention. The actual proposed method will be reviewed
by the Township and the Township Engineer for compliance.
(a)
Lower Merion Act 167 Drainage Area release rate district. A three-tiered control strategy is employed in the Lower Merion Drainage Area for all applicable activities as defined in § 121-2C of this chapter.
[1]
Whenever feasible, 100% of the stormwater flows
generated from the new development shall be infiltrated on site. Where
not feasible, the release rate criteria as specified for the subwatershed
shall be used to control post-development runoff. At a minimum, the
increased volume of stormwater generated by the proposed development
for the twenty-five-year storm shall be recharged.
[2]
Where soils are determined to be unsuitable
for infiltration of stormwater, control of all stormwater generated
from proposed development shall be achieved according to the release
rate criteria specified for the subwatershed(s) in question.
[3]
Subwatersheds with direct discharge listed as
the release rate criteria can release directly into the receiving
system, provided that water quality and capacity issues are satisfied
as required by this chapter.
[4]
Appendix B contains the release rates for the various subwatershed areas depicted on a watershed map, which is available for inspection at the Township building. Rates shall be controlled as listed and in accordance with § 121-4A(1)(b) of this chapter.
(b)
Lower Merion Act 167 Drainage Area provisional
infiltration only district. Within this district, control of stormwater
from the five-year storm shall be achieved through infiltration, wherever
feasible. Beyond this infiltration requirement, no further control
is necessary, provided that the capacity of the local runoff conveyance
system is determined to be adequate to convey the flows from the site
to the Schuylkill River without flooding or increased stream bank
erosion. Adequate capacity shall be determined by:
[1]
Calculating the peak flows assuming that the
site is developed as proposed and that the remainder of the local
watershed is in the existing condition.
[2]
Calculating the peak flows assuming that the
entire local watershed is developed per current zoning and that all
new development would use the runoff controls specified by this chapter.
[3]
The larger of the two peak flows calculated shall be used in determining if adequate capacity exists. If the adequate capacity exists to transport runoff from the site to the Schuylkill River safely, the developer may discharge postdevelopment peak runoff without detention facilities. If the calculations show that the local conveyance facilities lack adequate capacity to convey the flows without flooding or increased stream bank erosion, the developer shall either use a one-hundred-percent release rate control for the two- through one-hundred-year storm, or provide for additional capacity of downstream elements to convey the increased peak flows, consistent with § 121-4C. Before either option is approved, no harm to downstream properties shall be documented to the satisfaction of the Township Engineer. Any capacity improvements must be designed to convey runoff from development of all areas tributary to the improvement consistent with the capacity criteria specified in § 121-4C(3). By definition, a storm drainage problem area associated with the local runoff conveyance system indicates that adequate capacity does not exist in the conveyance system.
(c)
Non-watershed plan district. All other watersheds
not specifically identified on the subwatershed maps.
(d)
Darby-Cobbs Act 167 Drainage Area Release Rate
District:
[1]
At a minimum, the increased volume of stormwater
generated by the proposed development for the twenty-five-year storm
shall be recharged.
[2]
In addition, it shall also be demonstrated that
the following recharge requirements have been met:
[a]
The retention (infiltration) volume (Rev) must be demonstrated to be equivalent to capturing
one inch of rainfall over all proposed impervious surfaces.
|
Rev = I * Proposed impervious
area (square feet) ÷ 12 (inches) = cubic feet (cf)
| |
|
An asterisk (*) in equations denotes multiplication.
| |
|
Where: I=1.0 inch
|
[b]
Where the Rev volume
is not feasible due to site-specific limitations, then the retention
(infiltration) volume Rev required shall be
as much as physically accomplished with a minimum of 0.50 inch from
all impervious surface, depending on demonstrated site conditions.
|
Rev = I * total impervious
area (square feet) ÷ 12 (inches) = cubic feet (cf)
| |
|
An asterisk (*) in equations denotes multiplication.
| |
|
Where: I = The maximum equivalent infiltration
amount (inches) that the site can physically accept or 0.50 inch,
whichever if greater.
|
[3]
Field tests are to be provided in accordance
with the PA BMP Manual, such as double ring infiltrometer or hydraulic
conductivity tests, and shall be performed at the level of the soil
infiltration in order to determine the hydraulic conductivity rate.
[Amended 3-15-2023 by Ord. No. 4262]
[4]
A minimum depth of 24 inches between the bottom
of the BMP and the top of the limiting zone shall be provided.
[5]
The infiltration facility shall be capable of
completely infiltrating the retention volume (Rev) within four days (96 hours).
[6]
All developed areas must provide adequate storage
and treatment facilities necessary to capture and treat stormwater
runoff. The retention volume (Rev) may be a
component of the water quality volume if both are managed in a single
facility. If the retention volume is less than the water quality volume,
the remaining water quality volume may be captured and treated by
methods other than infiltration BMPs. The required water quality volume
(WQv) is the storage capacity needed to capture and treat a portion
of stormwater runoff from the developed areas of the site.
[7]
The following calculation formula is to be used
to determine the water quality storage volume (WQv) in acre-feet of
storage in the Darby-Cobbs Release Rate District:
|
WQv = [(P)(Rv)(A)] ÷ 12
| |
|
WQv = Water quality volume (acre-feet)
| |
|
P = 1 inch
| |
|
A = Area of the project contributing to the
water quality BMP (acres)
| |
|
Rv = 0.05 + 0.009(I), where I is the percent
of the area that is impervious surface [(impervious area/A)*100]
|
[a]
This volume requirement can be accomplished
by the permanent volume of a wet basin or the detained volume from
other BMPs. Where appropriate, wet basins shall be utilized for water
quality control.
[b]
Release of water can begin at the start of the
storm (i.e., the invert of the water quality orifice is at the invert
of the facility). The design of the facility shall provide for protection
from clogging and unwanted sedimentation.
C.
Stormwater management district implementation provisions.
(1)
Any stormwater management controls required by this
chapter and within the release rate district shall meet the applicable
rate criteria.
[Amended 7-18-2007 by Ord. No. 3821]
(2)
The exact location of the stormwater management district
boundaries as they apply to a given development site shall be determined
by mapping the boundaries using the two-foot topographic contours
provided as part of the drainage plan. The district boundaries as
originally drawn coincide with topographic divides, or, in certain
instances, are drawn from the intersection of the watercourse and
a physical feature, such as the confluence with another watercourse
or a potential flow obstruction. The physical features act as the
downstream limit of the watercourse and the downstream subarea boundary
is drawn from that point upslope to each topographic divide along
the path perpendicular to the contour lines.
(3)
Any downstream capacity analysis conducted in accordance
with this chapter shall use the following criteria for determining
adequacy for accepting increased peak flow rates:
(a)
Natural or constructed channels or swales must
be able to convey the increased runoff associated with a two-year
return period storm within their banks at velocities consistent with
protection of the channels from erosion. Acceptable velocities shall
be based upon criteria included in the DEP Erosion and Sediment Pollution
Control Program Manual (April 1990).
(b)
Natural or constructed channels or swales must
be able to convey the increased runoff peak rate from the twenty-five-year
return period storm without creating any hazard to persons or property.
(c)
Storm sewers or any other facilities which must pass or convey flows from the tributary area must have sufficient capacity to pass or convey the increased flows associated with the twenty-five-year return period storm runoff. Culverts, bridges and other facilities which constitute stream enclosures per DEP Chapter 105 regulations shall be designed to meet the requirements of DEP Chapter 105 and must be sized to pass the one-hundred-year storm.
(4)
For a proposed development site located within only
one release rate category area, the total runoff leaving the site
shall meet the applicable release rate criteria. For development sites
with multiple points of concentrated runoff discharge, individual
drainage points may be designed for up to a one-hundred-percent release
rate so long as the total runoff leaving the site is controlled to
the applicable release rate.
(5)
For a proposed development site located within two
or more different rate category areas, the peak rate of runoff that
may be discharged at any point is limited to the predevelopment peak
rate of runoff at that point multiplied by the applicable release
rate. The control rates shall apply regardless of any grading modifications
which may change the drainage area which discharges at a given point.
In no event shall a significant portion of the stormwater runoff generated
from a subwatershed with a more restrictive release rate be diverted
to a subwatershed with a less restrictive release rate.
(6)
For proposed development sites located partially within
a release rate category area and partially within a provisional infiltration
only district, in no event shall a significant portion of the site
area subject to the release rate control be drained to the discharge
point(s) located in the detention area.
(7)
For any proposed development site located outside of a provisional infiltration only district, the developer has the option of using a less restrictive runoff control (including no detention) if the developer can prove conclusively that no harm would be caused by discharging at a higher runoff rate than that specified by the plan. Proof of no harm must be shown from the development site through the remainder of the downstream drainage network to the confluence of the creek with the Schuylkill River. Proof of no harm must be shown using the capacity criteria specified in § 121-4C(3), if downstream capacity analysis is a part of the no harm justification. Attempts to show no harm based on downstream peak flow versus capacity analysis shall adhere to the following provisions:
(a)
The peak flow values to be used for downstream
areas for the design return period storm shall be the values from
the calibrated Penn State Model for the Lower Merion Drainage Area.
(b)
Any available capacity in the downstream conveyance
system as documented by a developer may be used by the developer only
in proportion to his development site acreage relative to the total
upstream undeveloped acreage from the identified capacity. For example,
if the site is 10% of the upstream undeveloped acreage, the developer
may use up to 10% of the documented downstream capacity.
(c)
Runoff controls proposed by the developer which would increase peak flow rates at a downstream problem area(s) shall be precluded from attempts to prove no harm, except in conjunction with proposed capacity improvements for the problem area(s) consistent with § 121-4C(9). Any no-harm justifications shall be submitted by the developer as part of the drainage plan submission per § 121-4H.
(d)
Recharge volume requirements of this chapter
must be met.
(8)
Regional or subregional detention alternatives may
be more cost effective for certain areas of the Lower Merion Drainage
Area. The initiative and funding for any regional or subregional runoff
control alternatives are the responsibility of the developer(s) involved.
The design of any regional control basins must incorporate reasonable
development of the entire upstream watershed. The peak outflow of
a regional or subregional basin shall be determined on a case-by-case
basis using the calibrated version of the Penn State Runoff Model
as developed for the plan, consistent with protection of the downstream
watershed areas.
(9)
In some instances, including but not limited to the provisional infiltration only district, local drainage conditions may dictate more restrictive levels of runoff control than those based on protection of the entire watershed. In these instances, if the developer shows that it is more feasible to provide capacity improvements to relieve the capacity deficiency in the local drainage network, then these capacity improvements can be provided by the developer in lieu of runoff controls on the development site. The design of any capacity improvements would be based on development of all areas tributary to the proposed improvement and the capacity criteria specified in § 121-4C(3). In addition, all new development upstream of a proposed capacity improvement shall be assumed to implement the applicable runoff controls consistent with this chapter. Capacity improvements may also be provided as necessary to implement any regional or subregional detention alternatives. They may also be provided to implement a modified no-harm option which proposes specific capacity improvements in support of a less restrictive discharge control.
(10)
Any proposed regulated activity for which a permanent
stormwater quality control detention basin is required under the NPDES
regulations shall use the more restrictive runoff control criteria
between this chapter and the NPDES requirements.
(11)
The required stormwater management criteria may be
met through off-site stormwater management measures, provided that
the proposed measures are in the same watershed and downgrade properties
are adequately protected to the satisfaction of the Township Engineer.
[Added 7-18-2007 by Ord. No. 3821]
(12)
Off-site areas. Off-site areas that drain through
a proposed development site are not subject to release rate criteria
when determining allowable peak runoff rates. However, on-site drainage
facilities shall be designed to safely convey off-site flows through
the development site.
[Added 7-18-2007 by Ord. No. 3821]
(13)
Site areas. Where the site area to be disturbed by
a proposed development activity differs significantly from the total
site area, only the proposed disturbed area utilizing stormwater management
measures shall be subject to the management district criteria. Thus,
undisturbed areas bypassing the stormwater management facilities would
not be subject to the management district criteria.
[Added 7-18-2007 by Ord. No. 3821]
D.
Calculation methodology.
(1)
Stormwater runoff from all development sites shall
be calculated using either the rational method or a soil-cover-complex
methodology, according to the direction of the Township Engineer.
(2)
The design of any detention basin intended to meet
the requirements of this chapter shall be verified by routing the
design storm hydrograph through the proposed basin. For basins designed
using the rational technique, the detention volume shall, at minimum,
equal the volume derived from the routing process as contained in
SCS Technical Release Number 55 (TR55, 1986).
(3)
All surface stormwater detention facilities shall provide a minimum one-foot freeboard above the maximum pool elevation associated with the two- through twenty-five-year storm. The two- through one-hundred-year storm runoff shall be controlled by the primary outlet structure. An emergency spillway for each basin shall be designed to pass the entire one-hundred-year return frequency storm peak flow rate with a minimum of 0.5 foot of freeboard. The spillway should function to control overflows in the event of a blockage of the basin outlet system. If this detention facility is considered to be a dam as per DEP Chapter 105, the design of the facility must be consistent with the Chapter 105 regulations and may be required to pass a storm greater than a one-hundred-year storm.
(4)
All calculations using the soil-cover-complex method
shall use the Soil Conservation Service Type II twenty-four-hour rainfall
distribution. The twenty-four-hour rainfall depths for the various
return periods to be used consistent with this chapter are:
[Amended 7-18-2007 by Ord. No. 3821]
|
Return Period
(years)
|
Twenty-four-Hour Rainfall Depth
(inches)
|
|---|---|
|
1
|
2.8
|
|
2
|
3.2
|
|
5
|
4.2
|
|
10
|
5.0
|
|
25
|
5.8
|
|
50
|
6.5
|
|
100
|
8.4
|
(6)
(7)
Runoff coefficients for use in the rational method
shall be based on the table presented in Appendix C, with concurrence
from the Township.
(8)
The Manning equation shall be used to calculate the
capacity of watercourses. Manning "n" values used in the calculations
shall be consistent with the table presented in Appendix C. Pipe capacities
shall be determined by methods acceptable to the Township Engineer.
(9)
The PADEP Chapter 105 Rules and Regulations shall apply to the construction, modification, operation or maintenance of both existing and proposed dams, water obstructions and encroachments throughout the watershed, despite any possible differences with the criteria that are used in permitting of dams under the Dam Safety Program.
(10)
Evapotranspiration
may be quantified and credited towards meeting volume requirements
according to the PADEP Post Construction Stormwater Management (PCSM)
Spreadsheet and Instructions (December 2020) or the most recent guidance
from PADEP.
[Added 3-15-2023 by Ord. No. 4262]
E.
Stormwater management facility standards.
(1)
Surface detention basins.
(a)
Any stormwater management facility required or regulated by this chapter shall be designed to provide an emergency spillway to handle flow up to one-hundred-year post-development conditions. The height of the embankment must be set to provide a minimum 1.0 foot of freeboard from the flow depth elevation when stormwater is discharging via the emergency spillway to the top of the berm when the facility functions for one-hundred-year post-development inflow. Should any stormwater management facility require a dam safety permit under PA DEP Chapter 105, the facility shall be designed in accordance with Chapter 105 and meet the regulations of Chapter 105 concerning dam safety.
(b)
Wherever possible, discharge from stormwater
management basins shall convert a concentrated flow into a sheet flow
at a noneroding velocity.
(c)
Stormwater drainage system design shall safely
route all runoff, especially that which cannot find its way into a
storm sewer pipe.
(d)
The riser pipe or pipe spillway shall have a
red line painted at the maximum level to indicate when cleanout of
sediment storage is required. If the riser is perforated, the hole
diameter shall be a minimum of one-half-inch size.
(e)
Wherever feasible, roof drainage shall be connected
to the stormwater management system.
(f)
For retention basins, a fence four feet in height
with a self-closing gate surrounding the basin may be required as
directed by the Lower Merion Director of Building and Planning.
[Amended 1-19-2002 by Ord. No. 3631]
(2)
Infiltration seepage beds.
(a)
Seepage beds shall be designed to recharge the
increased volume of runoff generated by the twenty-five-year storm
wherever the percolation rates will allow.
(b)
Seepage beds shall be designed to empty the
design storm volume for rate control in 24 hours or less, and the
full design storm volume in 96 hours or less.
[Amended 3-15-2023 by Ord. No. 4262]
(c)
Design consideration shall not include infiltration
rates in computing storage volume of a seepage bed. However, the infiltration
rate shall be used to determine the drain time requirements for the
stormwater management facility.
[Amended 3-15-2023 by Ord. No. 4262]
(d)
The seepage bed shall have a minimum of 12 inches
of cover and be located at least 10 feet from a basement wall. It
shall be located downhill from a building in the direction of surface
runoff where feasible.
(e)
Compaction of the seepage bed subgrade shall
not reduce the soil infiltration rate.
(f)
All seepage beds shall contain sediment traps
accessible for maintenance, as required by the Township Engineer.
(g)
An overflow system shall be provided for a storm
exceeding the volume of storage and safe discharge at a noneroding
velocity, converting a concentrated flow into a sheet flow.
(h)
A high-permeability filter fabric shall surround
the stone seepage bed to prevent migration of soil fines into the
bed.
(i)
Soil permeability tests (percolation rates)
shall be made to a depth below the bottom of the seepage bed as necessary
in order to demonstrate that the basin will function properly. The
test must be certified by a professional engineer or Sewage Enforcement
Officer. Where seepage beds are to be placed in fill, geotechnical
investigations of the subgrade stability must be performed as directed
by the Township Engineer.
[Amended 7-18-2007 by Ord. No. 3821]
(j)
Both horizontal and vertical seepage pits constructed
of perforated pipes may be used. Perforations shall be not less than
5/16 inch in diameter and provide an opening area not less than 3.31
square inches per square foot of pipe surface unless documentation
qualifying different dimensions is submitted by the design engineer
and approved by the Township Engineer. Provision shall be made for
maintenance of each seepage pit.
(k)
Infiltration seepage beds may be combined with
detention/retention surface basins.
(l)
Seepage beds shall not receive runoff without
the prior approval from the Township Engineer until the entire drainage
area contributory to the infiltration BMP has achieved final stabilization.
[Added 7-18-2007 by Ord. No. 3821]
(m)
Infiltration facilities are encouraged to be
used in conjunction with other innovative or traditional BMPs, stormwater
control facilities, and nonstructural stormwater management alternatives.
[Added 7-18-2007 by Ord. No. 3821]
(n)
Extreme caution shall be exercised where salt
or chloride would be a pollutant since soils do little to filter this
pollutant, and it may contaminate the groundwater. The qualified design
professional shall evaluate the possibility of groundwater contamination
from the proposed infiltration facility and perform a hydrogeologic
justification study if necessary or as directed by the Township Engineer.
[Added 7-18-2007 by Ord. No. 3821]
(o)
An impermeable liner will be required in a stormwater
management facility where the possibility of groundwater contamination
exists. A detailed hydrogeologic investigation may be required by
the Township Engineer.
[Added 7-18-2007 by Ord. No. 3821; amended 3-15-2023 by Ord. No. 4262]
(p)
The applicant shall provide safeguards against
groundwater contamination for land uses that may cause groundwater
contamination should there be a mishap or spill.
[Added 7-18-2007 by Ord. No. 3821]
(q)
Stormwater hotspots. Below is a list of examples
of designated hotspots. If a site is designated as a hotspot, it has
important implications for how stormwater in managed. First and foremost,
untreated stormwater runoff from hotspots shall not be allowed to
recharge into groundwater where it may contaminate water supplies.
Therefore, recharge requirements shall not be applied to development
sites that fit into the hotspot category. Water quality treatments
will still be required. Second, a greater level of stormwater treatment
shall be considered at hotspot sites to prevent pollutant wash off
after construction. The Environmental Protections Agency's (EPA) NPDES
stormwater program requires some industrial sites to prepare and implement
a stormwater pollution prevention plan.
[Added 7-18-2007 by Ord. No. 3821]
[1]
Examples of hotspots:
|
Vehicle salvage yards and recycling facilities
| |
|
Vehicle fueling stations
| |
|
Vehicle service and maintenance facilities
| |
|
Vehicle and equipment cleaning facilities
| |
|
Fleet storage area (bus, truck, etc.)
| |
|
Industrial sites based on Standard Industrial
Codes
| |
|
Marinas (service and maintenance)
| |
|
Outdoor liquid container storage
| |
|
Outdoor loading/unloading facilities
| |
|
Public works storage areas
| |
|
Facilities that generate or store hazardous
materials
| |
|
Commercial container nursery
| |
|
Other land uses and activities as designated
by the Township Engineer
|
[2]
The following land uses and activities are not
normally considered hotspots:
|
Residential streets
| |
|
Residential development
| |
|
Institutional development
| |
|
Office developments
| |
|
Nonindustrial rooftops
| |
|
Pervious areas, except golf courses and nurseries
[which may need an integrated pest management (IPM) plan]
|
(3)
Access and maintenance easements. An access and maintenance
easement that is of sufficient width to provide access and egress
from a public right-of-way shall be provided around all stormwater
management facilities if required by the Township Engineer.
[Added 7-18-2007 by Ord. No. 3821]
(4)
Maintenance and operation plan.
[Added 7-18-2007 by Ord. No. 3821]
(a)
A description of how each undedicated permanent
stormwater control and BMP will be operated and maintained shall be
submitted by the design professional. In addition the plan shall include
the following information:
[1]
The frequency of any required inspection.
[2]
The identity and contact information associated
with the person(s) responsible for operation and maintenance.
[3]
A statement signed by the owner of the property
acknowledging that the stormwater controls and BMPs are fixtures that
can be altered or removed only after approval by the Township.
(b)
The plan shall be presented in recordable form
to the Township as a covenant running with the land, shall be approved
by the Township Solicitor, and shall be recorded in the Office for
the Recording of Deeds of Montgomery County.
F.
Storm sewer standards.
(1)
Headwalls. Standard headwalls shall be installed on
all pipes and, additionally, trash bars shall be installed on all
pipes equal to or greater than 24 inches diameter.
(2)
Pipeline design.
(a)
Storm sewer pipelines shall be designed by either of the two methods described in Subsection F(2)(a) and (b) below. Both shall be based on the Manning equation and shall utilize the friction factors which have been established. Some of these values are as follows:
|
n
|
=
|
0.015 concrete pipe.
| |
|
n
|
=
|
0.021 corrugated metal pipe.
| |
|
n
|
=
|
0.010 PVC pipe.
| |
|
n
|
=
|
0.020 helical metal pipe.
| |
|
For other materials, an appropriate value must
be approved by the Township Engineer.
| |||
|
The minimum allowable pipe size is 15 inches.
| |||
(b)
Pipeline submerged.
[1]
This method is based on the assumption that,
when the storm sewer system is under maximum load, the hydraulic gradient
will be at or above the crown of the pipe and that flow in the lines
will be controlled by head differentials between structures or other
locations where the system is open to the ground surface, such as
inlets or outlets, manholes and stream inlets and outlets. The head
of water above the crown of any pipe can range from zero feet to a
point which will not cause surface flooding.
[2]
The slope of the hydraulic gradient in any section
of storm sewer between opening to the surface shall be calculated
on the assumption that the pipe is flowing full at a constant velocity
and at the required capacity.
[3]
The elevation of the hydraulic gradient at any
point in the pipe shall be no lower than the crown of the pipe and
no higher than the surface of the ground.
[4]
The elevation of the hydraulic gradient at any
point where the system opens to the surface, such as an inlet or manhole,
shall be three feet below the surface of the ground.
[5]
At all structures such as manholes, inlets,
etc., where the pipe size does not change and unless otherwise permitted
by the Township Engineer, the elevation of the hydraulic gradient
shall be dropped 0.2 foot to allow for losses therein. Where the inlet
and outlet pipe sizes are not the same, the elevation of the hydraulic
gradient shall be dropped an amount based on the following formula:
|
H = 0.2 inches + 0.8 (D2-D1)
| ||
|
Where
| ||
|
D1 = The diameter of inlet pipe.
| ||
|
D2 = The diameter of outlet pipe.
| ||
|
If D1 is larger than D2, head loss equals 0.2
foot.
| ||
[6]
The minimum slope of any pipe shall be such
that a minimum velocity of 2.5 feet per second shall be maintained
when the pipe is flowing 1/4 full.
(c)
Pipeline flowing full.
[1]
This method is based on the assumption that
the hydraulic gradient will match the inside top of the pipe when
the system is under maximum hydraulic load.
[2]
For this method, head losses through manholes,
inlets, etc., shall be ignored.
[3]
The minimum slope of any pipe shall be such
that a minimum velocity of 2.5 feet per second shall be maintained
when the pipe is flowing 1/4 full. When the pipe sizes change, the
inside tops of the pipes shall be matched.
(3)
Profiles. Continuous profiles for each reach of pipe
shall be plotted along with the location of the hydraulic gradient
and the hydraulic information pertinent to each reach within the system.
This information shall include the pipe size and type, the n-factor,
the slope of the hydraulic gradient, slope of the pipe, the design
capacity and the velocity at the design capacity.
(4)
Inlets. Unless adequate analysis is submitted with
differing requirements:
(a)
Single inlets shall be used on streets with
grades of 4% or less. Double inlets shall be used on streets of more
than 4%.
(b)
Sufficient inlets shall be located and constructed
so as to collect all of the stormwater flow in the drainage area tributary
to the inlet.
(c)
The gutter of all inlets shall be set not less
than two inches nor more than four inches below the gutter grade.
The surface of the paving adjacent to the inlets shall be constructed
to blend into the lowered gutter grade at the inlet in such a manner
that sudden dropoff or dip at the inlet will not be created.
(d)
Where surface water is collected from two directions
at one street corner, inlets shall be placed at, or near, the tangent
points of both ends of the radius. The use of an inlet in the radius
shall not be allowed unless no other location is feasible as determined
by the Township Engineer.
(5)
Manholes. Manholes shall be located at intervals of
approximately 400 feet where pipe sizes of 24 inches or less are used
and 600 feet apart for larger sizes. Inlets should be substituted
for manholes where they will serve a useful purpose. Manhole and inlet
castings may be obtained from the Lower Merion Department of Public
Works where inventory permits.
(6)
Open channel design. Open channel design should be
based on the following hydraulic considerations:
(b)
Allowable velocity.
|
Excavation Material
|
Velocity
(feet per second)
| |
|---|---|---|
|
Fine sand and firm loam
|
2.50 to 3.5
| |
|
Stiff clay and hardpan
|
3.75 to 6.0
| |
|
Concrete-lined ditch
|
15
|
(c)
Ample freeboard should be provided on all channels
as discussed in this chapter.
(d)
The channel should be designed to conform, wherever
possible, to the adjacent ground conditions. This means that it should
not be projecting excessively above the surrounding ground or placed
excessively below the surrounding ground.
(e)
Continuous profiles for each reach of open channel
shall be plotted along with the adjacent average ground and the hydraulic
information pertinent to each reach within the system. This information
shall include the type of channel lining, in the n-factor, the width
of the channel bottom, the side slopes, the water depth, the design
capacity and the velocity at the design capacity.
(f)
Open channels shall have a maximum side slope
of three to one and shall have adequate slope protection as required
by this chapter.
(7)
Bridges and culverts. Single-opening culverts are
desirable. The design of culverts shall be such as to minimize the
probability of debris accumulation. Bridges and culverts shall be
designed to meet current Pennsylvania Department of Transportation
standards for expected loads. They shall be constructed for the full
right-of-way.
(8)
Permits may be required from the Department of Environmental
Resources, Bureau of Dams and Waterway Management, for the construction
of dams and any activity within a watercourse, floodway or wetland.
No open watercourses shall be permitted within the right-of-way of
any street.
(9)
Location. Wherever practicable, storm drains shall
be located behind the curb and within the right-of-way of the street.
They shall be protected by a cover of at least 18 inches. The plan
shall note that the continued maintenance of storm drains outside
the street right-of-way shall be the continued responsibility of the
property owner.
(10)
Change in direction. Structures shall be used where
abrupt changes are made in alignment.
(11)
Stormwater roof drains. Stormwater roof drains and
pipes immediately adjacent to a sidewalk shall not discharge water
over a sidewalk but shall extend under the sidewalk to the gutter.
Where accessible, the roof drain shall be connected with the storm
drainage system of the Township.
(12)
Underground utilities in the floodplain. All utilities
shall be floodproofed to an elevation two feet above the one-hundred-year
design storm.
(13)
Runoff computations. Computation of the rate of flow
at any given location shall be based on the rational formula:
|
Q = CIA
| ||
|
Where
| ||
|
Q = Volume in cubic feet per second.
| ||
|
C = Runoff factor.
| ||
|
I = Intensity of rainfall in inches per hour.
| ||
|
A = Watershed area in acres.
| ||
|
In setting the value of the runoff coefficient
C, consideration will be given to the physical features of the drainage
basin and the best available data on the future density of development
of the drainage basin. In no case shall it be less than 0.40 in a
developed area.
| ||
(14)
Storm intensity. The intensity of the storm shall
be based on the following:
(a)
As a minimum, a ten-year storm shall be used
at low points with a relief swale, or a twenty-five-year storm where
carried in the pipe.
(b)
As a minimum, all inlet and culvert designs
shall be based on a twenty-five-year storm.
(c)
As a minimum, a one-hundred-year storm will
be used in connection with this chapter. The Rainfall-Intensity-Duration-Frequency
Curve for Philadelphia presented in Technical Paper No. 25, prepared
by the United States Department of Commerce Weather Bureau, a copy
of which is available for public inspection at the office of the Township
Engineer, shall be used.
(15)
Special drainage problems.
(a)
Unnatural drainage. Whenever construction stops
or concentrates the natural flow of storm drainage in such a way as
to affect adjoining properties, approval of the owners must be obtained
in writing. Approval of plans by the Township does not authorize drainage
affecting adjoining properties.
(b)
Water originating from other than natural sources,
such as swimming pools, air-conditioning units, sump pumps or other
dry-weather flow, wherever practicable, shall be discharged into natural
watercourses on the property or connected to the storm drainage system
of the Township. These facilities may not be used for toxic drainage.
G.
Landscaping of stormwater management facilities.
(1)
Landscaping shall be required in and around all stormwater
management facilities with a minimum surface area of 1,000 square
feet for the purposes of:
(2)
A planting plan shall be submitted in accordance with
the following:
(a)
Wet meadows, including basin floors.
[1]
Wet meadows and basin floors shall be planted with wildflowers and nonagressive grasses, the intent being to create a mixed meadow of such plantings, where appropriate. Selection of plantings should be based on whether the area in question is usually well drained or permanently wet and whether the area will be used for recreation purposes in accordance with Chapter A177 of the Appendix attached hereto. No woody plants shall be planted within the saturated zone (phreatic line) of a stormwater management basin.
[2]
Seeding by drills, corrugated rollers, Cyclone
or drop seeders or hand seeding of such areas is preferred; however,
hydroseeding followed by hydromulching can be used on wet ground and
steep slopes.
[3]
Fertilizers, as a nutrient supplement, shall
not be used unless it is documented that soil conditions warrant such
use. Soil for planting of wildflowers shall contain not less than
3% nor more than 10% organic matter, as determined by an agricultural
chemist, with certification of test before planting.
[4]
Seeding should take place either between April
1 and May 1 or September 1 and October 15. Planting areas shall be
soaked to maintain a consistent level of moisture for at least four
to six weeks after planting.
[5]
Once established, a single annual mowing when
plants are dormant should be sufficient to maintain a wet meadow and/or
basin floor.
(b)
Wet edges which remain wet all or most of the year shall be planted with wildflowers, grasses and shrubs. Plants to be located on rims or banks which remain dry most of the year should be planted with species tolerant of dry soil conditions in accordance with Chapter A177 of the Appendix attached hereto.
(c)
Wooded areas.
[1]
Where stormwater management facilities adjoin wooded areas, trees and shrubs shall be selected and planted so as to blend with existing surroundings in accordance with Chapter A177 of the Appendix attached hereto.
[2]
Plantings in such areas shall be of sufficient
density to eliminate the need for mowing.
[3]
It is recommended that clusters of trees and
shrubs be planted around stormwater management facilities, where applicable,
to provide for wildlife habitat, wind control and buffering and screening.
[4]
Vegetation shall be planted during appropriate
times of the year, predominantly between late March and mid-May from
early October until evidence of ground freezing, depending upon the
species selected. Most deciduous trees and shrubs can be planted in
either spring or fall. Evergreens are best planted in late summer
or early fall.
(d)
Slopes.
[1]
Where slopes are gentle, a mixture of meadow grasses and wildflowers (for wet meadows) shall be planted in accordance with Chapter A177 of the Appendix attached hereto.
[2]
On steep slopes, dense spreading shrubs (shrubs
tolerant of dry soils) shall be planted. A heavy mat mulch shall be
used during the period of establishment.
[3]
No woody plant materials or trees shall be located
on a constructed or natural berm acting as the impoundment structure
of a detention/retention basin. Trees shall be located on the downstream
side of an impoundment berm a sufficient distance from the toe of
the constructed slope to assure that the toe of the slope is outside
the dripline of the mature species planted.
(e)
Screening and buffering.
[1]
Stormwater management facilities shall be appropriately
screened in a manner which compliments the existing landscape and
provides sufficient access for maintenance. In many cases, complete
perimeter screening may not be necessary; however, it shall be the
applicant's responsibility to demonstrate in the submitted planting
plan why this is so.
[2]
Plantings shall consist of clusters of evergreens interspersed with groupings of deciduous trees and shrubs in accordance with Chapter A177 of the Appendix attached hereto. Single species planting is prohibited.
[3]
It is encouraged that plantings be used which
provide food and shelter for wildlife.
H.
Water quality.
(1)
In addition to the performance standards and design
criteria requirements of this chapter, the land developer shall implement
the following water quality requirements unless otherwise exempted
by the provisions of this chapter. To control post-construction stormwater
impacts from regulated activities and conform to state water quality
requirements, BMPs which replicate predevelopment stormwater infiltration
and runoff conditions must be provided in the site design such that
post-construction stormwater discharges do not degrade the physical,
chemical, or biological characteristics of the receiving waters. The
green infrastructure and low-impact development (LID) practices provided
in the PA BMP Manual, as well as the guidance on green infrastructure
and LID provided in Appendix E,[4] shall be utilized for all regulated activities wherever
possible. This may be achieved by the following:
[Amended 3-15-2023 by Ord. No. 4262]
(a)
Infiltration: replication of preconstruction
stormwater infiltration conditions;
(b)
Treatment: use of water quality treatment BMPs
to provide filtering of chemical and physical pollutants from the
stormwater runoff; or
(c)
Stream bank and streambed protection: management
of volume and rate of post-construction stormwater discharges to prevent
physical degradation of receiving waters (e.g., from scouring).
[4]
Editor's Note: Appendix E is on file in the Township offices.
(2)
BMP selection criteria.
(a)
In selecting the appropriate BMP's or combinations
thereof, the land developer shall consider the following:
(b)
The following additional factors should be considered
when evaluating the suitability of the BMP's used to control water
quality at a given development site:
[1]
Peak discharge and required volume control.
[2]
Stream bank erosion.
[3]
Efficiency of the BMP's to mitigate potential
water quality problems.
[4]
The volume of runoff that will be effectively
treated.
[5]
The nature of the pollutant being removed.
[6]
Maintenance requirements.
[7]
Creation/protection of aquatic and wildlife
habitat.
[8]
Recreational value.
[9]
Enhancement of aesthetics and property values.
I.
Waiver procedure.
(1)
The Township may hear requests for waivers where it
is alleged that the provisions of this chapter inflict unnecessary
physical constraint on the applicant. The waiver request shall be
in writing, or on an application promulgated by the Township and accompanied
by the requisite fee, based on a fee schedule adopted by the Township.
A copy of the completed application form shall be provided to each
of the following: Township, Township Engineer, Township Solicitor
and Montgomery County Planning Commission. The application shall fully
document the nature of the alleged hardship. The Township may grant
a waiver provided that all of the following findings are made in a
given case:
(a)
There are unique physical circumstance or conditions,
including irregularity of lot size or shape, or exceptional topographical
or other physical conditions peculiar to the particular property and
that the unnecessary hardship is due to such conditions and not the
circumstances or conditions generally created by the provisions of
the ordinance for the stormwater management district in which the
property is located.
(b)
That because of such physical circumstances
or conditions, there is no possibility that the property can be developed
in strict conformity with the provisions of this chapter, including
the no-harm provision, and that the authorization of a waiver is therefore
necessary to enable the reasonable use of the property.
(c)
That such unnecessary hardship has not been
created by the applicant.
(d)
That the waiver, if authorized, will represent
the minimum waiver that will afford relief and will represent the
least modification possible of the regulation in issue.
(e)
Financial hardship shall not constitute grounds
for granting of a waiver.
(2)
In granting any waiver, the Township may attach such
reasonable conditions and safeguards, as it may deem necessary to
implement the purposes of Act 167 and this chapter.
J.
Liability disclaimer.
(1)
Neither the granting of any approval nor the compliance
with the provisions of this chapter or with any condition imposed
by the Township, its officials, employees or designated representatives
hereunder shall relieve any person from any responsibility for damages
to persons or property resulting therefrom, or as otherwise imposed
by law nor impose any liability upon the Township, its officials,
employees or designated representatives to the maximum extent permitted
by law.
(2)
The granting of any permit which includes any stormwater
management requirements shall not constitute a representation, guarantee
or warranty of any kind by the Township, the officials, employees
or designated representatives thereof of the practicability or safety
of any stormwater structure or facility, use or other plan proposed
and shall create no liability or cause of action upon the Township,
its officials, employees or designated representatives for any damage
that may result pursuant thereto to the maximum extent permitted by
law.
[Amended 6-15-1988 by Ord. No. 3098]
A.
General requirements.
(1)
Minimum grading. The maximum bare area (without vegetative
cover) shall not exceed 25% of the total area at any one time. This
requirement shall apply only if the total area, before subdivision
or land development, is greater than four acres.
(2)
The maximum time of exposure for bare areas shall
be 20 days before stabilization measures must be implemented.
(3)
Fording of live streams or travel in their beds by
mechanized equipment is not permitted; bridges or culverts shall be
used when crossing streams is necessary.
(4)
Roads and parking areas, in active construction areas,
shall be completed (except for wearing course) before building construction
is permitted. The binder course shall be repaired and regraded to
grade before installing wearing course near the end of the project
construction. Any regulated activity which would create 1,500 square
feet or less of additoinal impervious cover is exempt from the stormwater
management provisions of this ordinance. This criteria shall apply
to the total proposed development, even if development is to be phased.
Additonal impervious cover shall include any roof, parking area, driveway
area and any new streets and sidewalks constructed as part of or for
the proposed regulated activity. Any areas initially to be gravel,
crushed stone, etc., shall be assumed to be impervious for the purposes
of the waiver criteria.
[Amended 11-17-1999 by Ord. No. 3546]
(5)
Temporary erosion control measures may be removed
only after the construction area and contained silt is stabilized
and the lawn area established.
(6)
All newly graded slopes four to one (25%) or over
shall be sodded or stabilized with erosion control netting.
(7)
A routine end-of-day check shall be made during construction
to make sure that all control measures are working properly. All persons
engaged in land disturbance activities shall design, implement and
maintain control measures which prevent accelerated erosion and sedimentation.
There shall be no adverse discharge of the sediment or other solid
materials from the site as the result of stormwater runoff. This should
be accomplished by scheduling the construction in such a way as to
utilize new plantings and properly installed erosion control fencing
and hay bales to minimize erosion damage. This process shall be accomplished
to the satisfaction of the Township Building and Planning Department
and the Township Engineer and be shown on the plan in an outline form
to attain environmental protection.
[Amended 1-19-2002 by Ord. No. 3629]
B.
Permit requirements.
(1)
No person shall commence or perform any grading, excavation, fill or clearing of trees without first having obtained a grading or runoff and erosion control permit from the Township Engineer. A separate permit shall be required for each site. One permit may cover both an excavation and any fill made on the same site. A runoff and erosion control permit will be required when stormwater management calculations and plans are required pursuant to § 121-6 hereof. A grading permit will not be required for the conditions set forth below. Where neither a runoff and erosion control permit nor a grading permit are required, all other provisions of this chapter shall nevertheless apply.
[Amended 12-17-1997 by Ord. No. 3470; 7-18-2007 by Ord. No. 3821; 7-18-2012 by Ord. No. 3980]
(a)
An excavation that does not exceed three feet in vertical depth
at its deepest point, measured from the natural ground surface, or
that covers an area of more than 1,000 square feet. This exception
shall not affect the applicability of this chapter to or the requirement
for a permit for any fill made with the material from such excavation.
(b)
A fill that does not exceed 10 cubic yards of material on any
one site, or a fill that exceeds three feet in vertical depth at its
deepest point, measured from the natural ground surface, or that covers
an area of more than 1,000 square feet, provided that the surfaces
of such fills do not have a slope at any point steeper than five horizontal
to one vertical.
(c)
An excavation below the finished grade for basements and footings
of a building, swimming pool or underground structure authorized by
a building permit where the area limitations set forth in this section
have not been exceeded. This exception shall not affect the applicability
of this chapter to or the requirement for a permit for any fill made
with the material from such excavation. A permit shall not be required
for the temporary stockpiling on the same site of the material from
such excavation.
(2)
Permits issued pursuant to this chapter do not relieve
the owner of the site of responsibility for securing required permits
for work to be done which is regulated by any other applicable code,
rules, statutes, regulations or ordinances. This chapter shall not
preclude the inclusion in such other permit of more stringent requirements
concerning regulation of runoff and erosion.
(3)
Application for permit. A written application from
the owner of the site or his authorized representative, in the form
prescribed by the Township Engineer, shall be required for each permit.
No permit shall be transferable without the written consent of the
Township Engineer. The permit application shall be accompanied by
six copies of the runoff and erosion control plans and specifications,
which shall meet as a minimum the standards and specifications required
by the Montgomery County Soil and Water Conservation District and
the Township Design and Construction Standards Manual, which shall
be available to the public for inspection at the office of the Township
Engineer, from whom copies of all or any part thereof may be obtained
at the rate of $0.10 per page. The Erosion and Sediment Control Design
Standards are hereby incorporated by reference in this chapter and
shall be as much a part of this chapter as if it were fully set forth
herein; provided, however, that said handbook may be amended from
time to time by ordinance of the Township of Lower Merion.
[Amended 7-18-2007 by Ord. No. 3821]
(4)
Any regulated activity which would create 1,500 square
feet or less of additional impervious surface is exempt from the stormwater
management provisions of this chapter. This criteria shall apply to
the total proposed development even if it is to be phased.
[Added 11-17-1999 by Ord. No. 3546]
(a)
This exemption shall not apply to any property
where more than 75% of the principal building area is being demolished;
or, for properties with no principal building, where more than 50%
of existing impervious coverage is removed and replaced with new impervious
surfaces. In such cases the existing impervious surface on the property
shall be considered as meadow for the purpose of calculating stormwater
runoff required by this chapter.
[Added 1-19-2005 by Ord. No. 3734; amended 3-15-2023 by Ord. No. 4262]
(b)
The Township Engineer may authorize a reduction
in the size of the stormwater management system if the installation
of the stormwater management system will result in the removal of
large trees or a large number of smaller trees, or where the Township
Engineer determines that it is not feasible due to site constraints
that would limit compliance.
[Added 1-19-2005 by Ord. No. 3734]
(c)
Exemption criteria for permits.
[Added 7-18-2007 by Ord. No. 3821]
[1]
Exemption responsibilities. An exemption shall
not relieve the applicant from implementing such measures as are necessary
to protect public health, safety, and property. Adequate erosion controls
must always be provided.
[2]
Emergency exemption. Emergency maintenance work
is that required to be performed immediately for the protection of
public health, safety, and welfare. A written description of the scope
and extent of any emergency work performed shall be submitted to the
Township Building Department within two days of the commencement of
the activity. If the Township finds that the work is not an emergency,
then the work shall cease immediately and the requirements of this
article shall be addressed as applicable.
[3]
Maintenance exemption. This exemption is provided
for any routine maintenance to an existing stormwater management system
made in accordance with plans and specifications approved by the Township
Engineer. Any replacement of an existing seepage bed or stormwater
basin must be coordinated with the Township so that necessary inspections
can be performed.
[4]
Even though a property owner or developer is
exempt from permit requirements, he is not relieved from complying
with other applicable regulations and codes.
[Amended 10-18-1978 by Ord. No. 1833; 6-15-1988 by Ord. No.
3098]
The applicant shall submit stormwater management
by Ord. No. 3098 and erosion and sediment control plans which shall
contain but are not limited to the following:
A.
The date, name, address and telephone number of the
preparer(s) of plans or each portion thereof; the owner of the site;
and lot, block, tract or street address or similar description which
identifies and locates the proposed work.
B.
Certification from the preparer(s) of the plan, who shall be registered professional engineers or other registered professional(s) qualified by background and experience in runoff and erosion control design and computation, attesting to the completeness and correctness of existing conditions as shown, to the compliance of all proposed earthmoving and other work with all of the requirements of this chapter, except for specific waivers or modifications as listed, and that the proposed measures will meet, as a minimum, the standards and specifications contained in the Erosion and Sediment Control Handbook as incorporated in this chapter by § 121-5B(3).
C.
A clear and definite delineation of the limits of
work and the areas to remain undisturbed, along with a statement of
the area, in square feet, of the total site and of the areas to remain
undisturbed which currently have an effective runoff- and erosion-resistant
ground cover or surface.
D.
A narrative report describing the project and giving
the purpose. The engineering assumptions and calculations for control
measures and facilities shall include a hydrologic data sheet. The
hydrologic data sheet shall include but not be limited to:
(1)
Watershed area in acres and slope in percentages.
(2)
Weighted soil cover complex numbers.
(3)
Peak discharge rates in cubic feet per second and
velocities in feet per second for emergency spillways and outlet structures.
(4)
Total storage capacity in cubic feet.
(5)
A plan showing the effective watershed.
(6)
Inflow and outflow hydrographs for all detention/retention
facilities.
E.
A clear and definite delineation, with dimensions
and locations, for all proposed fill.
F.
Existing topography and proposed finished grade, including
that for restoration of borrow pits and open-pit mines, and site work,
with contours at two-foot intervals, drawn at a scale of one inch
to 20 feet to 40 feet of the entire site, plus a minimum twenty-foot
adjacent peripheral strip or as might otherwise clearly reflect existing
conditions and proposed earthmoving and other work, provided that
such other interval and scale has the Township Engineer's approval
in advance of plan preparation.
G.
Established or, in the case of new subdivisions, approved
tentative street grades (elevations).
H.
Size, location and construction details of all existing
and proposed surface structures and storm drainage facilities; the
location, species and size in diameter of trees two feet above the
ground, alone and in groves, or street shade trees; and other site
work.
J.
A timing schedule and sequence indicating the anticipated
starting and completion dates of the development sequence. The schedule
must include:
(1)
Tree removal.
(2)
Stripping, stockpiling and stabilizing of topsoil
to a depth of six inches.
(3)
Rough grading and construction.
(4)
Final grading and vegetative establishment.
(5)
Exposure time of each area prior to the completion
of effective runoff and erosion control measures.
(6)
Maintenance to time of occupancy permit.
(7)
A description and clear and definite delineation of
the interim and permanent runoff and erosion control facilities and
measures to be provided.
(8)
The estimated cost of the earthmoving and/or filling
and the cost of the required runoff and erosion controls.
(9)
A planting schedule.
[Added 5-17-1989 by Ord. No. 3155]
A.
No excavation shall be made with a cut face steeper
in slope than one horizontal to one vertical, except under one or
more of the following conditions:
(1)
The excavation is so located that a line having a
slope of one horizontal to one vertical and passing through any portion
of the cut face will be entirely inside the property lines of the
property on which the excavation is made.
(2)
The material in which the excavation is made is sufficiently
stable to sustain a slope of steeper than one horizontal to one vertical,
and a written statement of a civil engineer, licensed by the Commonwealth
of Pennsylvania and experienced in erosion control, to that effect
is submitted to the Township Engineer and approved by him. The statement
shall state that the site has been inspected and that the deviation
from the slope specified above will not result in injury of persons
or damage to property.
(3)
A retaining wall or other approved support is provided
to support the face of the excavation.
B.
The Township Engineer may require an excavation to
be made with a cut face flatter in slope than one horizontal to one
vertical if he finds that the material in which the excavation is
to be made is unusually subject to erosion, or if other conditions
exist which make such flatter cut slope necessary for stability and
safety.
C.
Excavations shall not extend below the angle of repose
or natural slope of the soil under the nearest point of any footing
or foundation of any building or structure unless such footing or
foundation is first properly underpinned or protected against settlement.
D.
Before commencing any excavation which can in any
way affect an adjoining public or private property or structures thereon,
the person making or causing the excavation to be made shall, in writing,
notify the owners of adjoining buildings and the Township Engineer,
not less than 30 days before such excavation is to be made, that the
excavation is to be made. Adjoining properties and structures shall
be protected as provided in the then-current Building Code of the
Township of Lower Merion.[1]
A.
No fill shall be made which creates any exposed surface
steeper in slope than 1 1/2 horizontal to one vertical, except
under one or more of the following conditions:
(1)
The fill, in the opinion of the Township Engineer,
is located so that settlement, sliding or erosion of the fill material
will not result in property damage or be a hazard to adjoining property,
streets, alleys or buildings.
(2)
A written statement from a civil engineer, licensed
by the Commonwealth of Pennsylvania and experienced in erosion control,
certifying that he has inspected the site 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 Engineer.
B.
The Township Engineer may require that the fill be
constructed with an exposed surface flatter than 1 1/2 horizontal
to one vertical if he finds that under the particular conditions such
flatter surface is necessary for stability and safety.
C.
Whenever a fill is to be made of materials other than
clean soil or earth, the permit shall be subject to the following
additional limitations and requirements:
(1)
The fill shall be completed within a reasonable length
of time, said time limit to be determined by the Township Engineer
and to be specified on the permit.
(2)
Clean soil shall be placed over the top and exposed
surfaces of the fill to a depth sufficient to effectively conceal
all materials other than clean soil or earth within the fill. Where
the nature of the fill requires, the Township Engineer may require
clean soil or earth to be placed over the top and exposed surfaces
of the fill to a depth sufficient to conceal all materials at the
end of each day's operations.
All fills shall be compacted to provide stability
of material and to prevent undesirable settlement. The fill shall
be spread in a series of layers, each not exceeding 12 inches in thickness,
and shall be compacted by a sheepsfoot roller or other approved method
after each layer is spread. The Township Engineer may require tests
or other information if, in his opinion, the conditions or materials
are such that additional information is necessary.
The top or bottom edge of slopes shall be at
least three feet from property or right-of-way lines of streets in
order to permit the normal rounding of the edge without encroaching
on the abutting property. At property lines where walls or slopes
are steeper than one to one and are six feet or more in height, they
shall be protected by a substantial fence three feet or more in height.
Before a permit is issued, a bond may be required to guarantee the
protection of steep slopes.
A.
Portions of tree masses or trees with caliper of four
inches or greater shall not be cleared unless approved by Township
Engineer. The Township Engineer will not approve clearing of tree
masses that will result in substantially increased runoff.
B.
During construction, care shall be exercised to prevent
damage to trees which are to remain. The following procedures shall
be followed:
(1)
Where existing ground levels are changed, drainage
tile will be placed at the old soil level and open into a well built
around the base of the tree. Such well may be left open or can be
filled with coarse stones or gravel. Tiles may be installed in a radiating
pattern or laid in parallel lines.
(2)
Trees within 25 feet of a building site or bordering
entrances or exits to building sites shall be protected by wiring
wooden slats around such trees.
(3)
No boards or other materials shall be nailed to trees
during construction.
(4)
Heavy equipment operators shall be warned to avoid
damaging existing tree trunks and roots. Feeder roots shall not be
cut closer than 25 feet from tree trunks.
(5)
Tree trunks and exposed roots damaged during construction
shall be protected from further damage by being treated immediately
with tree paint.
(6)
Tree limbs damaged during construction shall be sawed
flush to tree trunks and treated immediately with tree paint.
(7)
Leaf trees shall be given a heavy application of fertilizer
to aid in their recovery from possible damage by construction operations.
(8)
Construction debris shall not be disposed of near
or around the bases of such trees.
C.
Trees with a minimum caliper of 1 1/2 inches
shall be provided where deemed advisable by the Township Planning
Commission and/or Board of Commissioners.
[Amended 7-18-2007 by Ord. No. 3821]
A.
The following permit requirements from other agencies,
as applicable, must be met prior to commencement of regulated earth
disturbance activities in the Township:
(1)
All regulated earth disturbance activities subject
to permit requirements of DEP under regulations at 25 Pennsylvania
Code, Chapter 102.
(2)
Work within natural drainageways subject to permit by DEP under 25 Pennsylvania Code Chapter 105.
(3)
Any stormwater management facility that would be located in or adjacent to surface waters of the commonwealth, including wetlands, subject to permit by DEP under 25 Pennsylvania Code Chapter 105.
(4)
Any stormwater management facility that would be located
on a state highway right-of-way or require access from a state highway
shall be subject to approval by PennDOT.
B.
No regulated earth disturbance activities shall commence
until the Township receives the required approval from the responsible
agency.
C.
No building permit will be issued on a site requiring
an NPDES construction permit until evidence is provided to the Township
that the permit for the regulated activity has been issued. If an
NPDES permit for stormwater discharges associated with construction
activities was required for the regulated activity, approval of the
notice of termination (NOT) must be obtained upon completion of construction
prior to final approval of the project by the Township.
[Amended 3-15-2023 by Ord. No. 4262]
Major modifications of the approved application
and plans shall be submitted to the Township Engineer and reprocessed
in the same manner as the original application and plans. Field modifications
of a minor nature may be authorized by the Township Engineer, provided
that written authorization is given to the person performing work
pursuant to this chapter, with a copy forwarded to the Montgomery
County Soil and Water Conservation District.
[Amended 9-21-1977 by Ord. No. 1802]
The applicant shall pay a fee for each site
as set forth in the Township Schedule of Fees, as adopted and amended
from time to time by the Board of Commissioners.[1]
[Amended 12-17-1997 by Ord. No. 3470]
In granting any permit, the Township Engineer
may attach such conditions thereto as he may deem reasonably necessary
to prevent danger to public or private property or any sewer, storm
drain or watercourse, or to prevent the operation from being conducted
in a manner hazardous to life or property, or in a manner likely to
create a nuisance. Such conditions may include, but are not limited
to, the erection or installation of walls, drains, dams and structures,
plantings and vegetation, runoff and erosion control measures or devices,
furnishing necessary easements and a specified method of performing
work. No runoff and erosion control permit shall be issued until a
runoff and erosion control plan is approved by the Township Engineer
and the developer certifies that all earthmoving, grading and filling
shall be performed pursuant to the runoff and erosion control plan.
The runoff and erosion control plan shall be a condition of the permit.
No person shall violate any such conditions so imposed.
Every permit shall expire by limitation and
become null and void if the work authorized by such permit has not
been commenced within six months or is not completed within one year
from the date of issue, provided that the Township Engineer may, if
the permit holder presents satisfactory evidence that unusual difficulties
have prevented work from being started or completed within the specified
time limits, grant a reasonable extension of time, and provided further
that the application for the extension of time is made before the
date of expiration of the permit.
Any permit issued under this chapter may be
revoked or suspended by the Township Engineer, after notice, for:
A.
Violation of any condition of the permit.
B.
Violation of any provision of this chapter or any
other applicable law, ordinance, rule or regulation relating to the
work.
C.
Existence of any condition or the doing of any act
constituting or creating a nuisance or hazard or endangering human
life or the property of others.
