A.
For any of the activities regulated by this chapter, unless preparation
of a stormwater management (SWM) site plan is specifically exempted,
the preliminary or final approval of subdivision and/or land development
plans, the issuance of any building or occupancy permit, the commencement
of any earth disturbance activity may not proceed until the property
owner or applicant or his/her agent has received written approval
from the municipality of a SWM site plan that demonstrates compliance
with the requirements of this chapter, and a written approval of an
adequate erosion and sediment (E&S) control plan from the municipality
or County Conservation District when required.
B.
SWM site plan approved by the municipality shall be on site throughout
the duration of the regulated activity.
C.
The municipality may, after consultation with the Department of Environmental
Protection (DEP), 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, 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. DEP regulations require an erosion
and sediment control plan for any earth disturbance activity of 5,000
square feet or more under 25 Pa. Code § 102.4(b). In addition,
under 25 Pa. Code Chapter 92, a DEP NPDES construction activities
permit is required for regulated earth disturbance activities. A copy
of the erosion and sediment control plan and any required permit,
as required by DEP regulations, shall be available on the project
site at all times. Various BMPs and their design standards are listed
in the Erosion and Sediment Pollution Control Program Manual (E&S
Manual), No. 363-2134-008 (April 15, 2000), as amended and updated.
E.
For all regulated activities, implementation of the water volume controls in § 190-23 is required.
F.
Impervious areas.
G.
Stormwater discharges onto adjacent property shall not be increased
without written notification of the adjacent property owner(s). Such
stormwater discharges shall be subject to the requirements of this
chapter.
H.
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)
Maintain 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 by directing runoff to pervious
areas, wherever possible.
(3)
To the maximum extent practicable, incorporate the techniques for
low-impact development practices described in the Pennsylvania Stormwater
Best Management Practices Manual (BMP Manual).
I.
The design of all facilities over karst shall include an evaluation
of measures to minimize adverse effects.
J.
Infiltration BMPs should be dispersed on site, made as shallow as
practicable, and located to maximize use of natural on-site infiltration
features while still meeting the other requirements of this chapter.
K.
Storage facilities should completely drain both the volume control
and rate control capacities over a period of time not less than 24
and not more than 72 hours from the end of the design storm.
L.
The design storm precipitation amounts to be used in the analysis
of peak rates of discharge shall be those from the upper limits of
the 90% confidence intervals for the twenty-four-hour precipitation
events in the Precipitation-Frequency Atlas of the United States,
Atlas 14, Volume 2, Version 3.0, U.S. Department of Commerce, National
Oceanic and Atmospheric Administration (NOAA), National Weather Service
(NWS), Hydrometeorological Design Studies Center (HDSC), Silver Spring,
Maryland. NOAA's Atlas 14 can be accessed at http://hdsc.nws.noaa.gov/hdsc/pfds/.
M.
N.
Various BMPs and their design standards are listed in the BMP Manual.
O.
Any stormwater facility located on state highway rights-of-way shall
be subject to approval by the Pennsylvania Department of Transportation
(PennDOT).
P.
Stormwater facilities shall not only handle the anticipated peak
discharge from the property being subdivided or developed in accordance
with the applicable release rate as set forth in this chapter, but
also the existing runoff being contributed from all land at a higher
elevation in the same watershed. The future use of undeveloped areas
upstream shall be taken into account in the calculation of pipe sizes
for storm sewer system designs. The capacity and maximum anticipated
present flow of the body or system receiving the proposed system(s)
discharge shall be calculated to verify its capability of receiving
any additional flow caused by the development or subdivision. The
runoff from any proposed development shall be subject to an evaluation
that includes the anticipated runoff from other existing or proposed
developments with the same watershed. Stormwater management facilities
designed to serve more than one property or development in the same
watershed are encouraged; consultation with the Township is required
prior to design of such facilities.
Q.
Requirements for watercourses, streams, or intermittent streams.
(1)
Whenever a watercourse, stream, or intermittent stream is located
within a development site, it shall remain open in its natural state
and location and shall not be piped. It shall be the responsibility
of the developer to stabilize eroded or erodible stream banks that
can cause sedimentation in downstream waterways.
(2)
The Township may require a developer to provide a permanent easement
along any watercourse located within or along the boundary of any
property being subdivided or developed. The purpose of any such watercourse
easement shall be for maintenance of the channel of any watercourse;
the terms of the easement shall prohibit excavation, the placing of
fill or structures, and any alterations that may adversely affect
the watercourse. The easement shall conform to the line of the watercourse
and shall be of sufficient width to protect the natural drainage system
which shall be calculated using a one-hundred-year storm event; however,
in no case shall the easement be less than 30 feet in width. The developer
will retain the easement until such time as one of the following is
accomplished:
(a)
If an easement acceptable to the municipality is established,
the maintenance shall then be the responsibility of the individual
lot owners over whose property the easement passes. For land developments,
the maintenance shall then be the responsibility of the owner. The
record plan shall contain description of such easement(s) and notation
indicating the maintenance responsibility.
(b)
A homeowners' association or other approved legal entity, approved
by the Township, assumes responsibility for the maintenance of the
development, including the watercourse easement.
(3)
Horsham Township will not accept dedication of watercourse easements;
however, it shall, upon satisfactory installation of improvements
as specified in a land development agreement with Horsham Township,
maintain and repair only the structural improvements within the easement,
if any, such as piping; inlets; headwalls/endwalls; and energy dissipation
structures or facilities. The Township will not be responsible for
any other repair/maintenance within the easement such as lawn/ground
cover maintenance; vermin control; removal of obstructions and accumulated
sediment or vegetation; removal of litter or garbage; repair of erosion;
and restoration of vegetation. The record plan and development agreement
for the approved land development shall contain a provision permitting
access to such easement(s), at any reasonable time, for inspection
and/or emergency repair/maintenance by Horsham Township or its designee,
of all facilities deemed critical to the public's welfare. In the
event the lot owner or homeowners' association responsible for maintenance
fails to honor the responsibility set forth herein, in any manner,
Horsham Township shall have the right of entry upon and within the
area of the easement to undertake any required corrective or maintenance
effort. The total cost of such, including administrative, engineering,
and legal costs for enforcement, may be imposed upon the responsible
party as determined by Horsham Township. Failure to remedy all associated
costs described above may be subject to the imposition of a lien by
the Township against the owner(s) in question, in the same manner
as the Township might otherwise be empowered by law to assess or impose
a lien against a property for municipal improvements.
R.
The existing points/patterns and character of natural drainage discharge
onto and off of adjacent property shall not be altered without the
approval of the affected adjoining property owners. Verification of
such approval must be obtained, in writing, and a copy filed with
Horsham Township. Approval of plans by the Township does not authorize
or sanction drainage affecting adjoining properties.
S.
When directed by the Township Engineer and/or where the finished
roadway grade is within one foot of the seasonal high water table
as designated for the site-specific soils type as designated by the
Soil Survey for Montgomery County, storm sewer shall be designed as
a combination storm sewer and underdrain. An acceptable alternative
to the combination of storm sewer and underdrain would be a six-inch
perforated corrugated polyethylene pipe. As much as practical, all
storm sewers, as required, shall be placed in the right-of-way, parallel
to the roadway. When located in undedicated land, storm sewers shall
be placed within an easement not less than 30 feet in width or as
otherwise approved by the Township Engineer.
T.
Where applicable, stormwater management facilities or programs shall
comply with the requirements of Pa. Code Title 25, Rules and Regulations,
Chapter 102 (Erosion Control), Chapter 105 (Dam Safety and Waterway
Management), and Chapter 106 (Floodplain Management) of the Department
of Environmental Protection.
U.
The developer may be required to participate in off-site storm drainage improvements in the drainage basin within which the proposed development is located. The specific contribution/improvements required shall be as specified by the Township Council and as specified in Article VIII of the Subdivision and Land Development Ordinance.[4]
V.
Stormwater roof drains from nonresidential and multiresidential buildings
(where applicable) shall not discharge water directly onto a sidewalk
or a street and shall be constructed to discharge overland prior to
eventual discharge into a storm sewer system and/or detention basin.
W.
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 discharged overland towards natural watercourses
on the property. These facilities shall not discharge water under
the sidewalk, through the curb, into the gutter.
X.
Where applicable, stormwater management plans/reports shall document
downstream effects of detention and delayed or altered storm peaks,
including necessary steps to avoid downstream combination of peak
flows.
Y.
All stormwater runoff and floodplain calculations and stormwater
management facilities design shall be prepared by a professional engineer
licensed in the State of Pennsylvania.
Z.
Flood protection. No stormwater runoff or natural drainage shall
be so diverted as to overload existing drainage systems, or create
flooding or the need for additional drainage structures on other private
properties or public lands, without proper and approved provisions
being made for resolving flooding conditions.
AA.
Low-point drainage. The developer shall grade and install all necessary
drainage facilities to insure drainage of all low points on subdivided
or developed land areas (except in areas of protected wetlands).
BB.
No person, corporation, or other such entity shall block, impede
the flow of, alter, construct any structure, or deposit any material
or thing, or perform any work that will affect the normal runoff onto
or off of an adjacent parcel.
CC.
All plans wherein a sump pump or basement drain is proposed to be
installed shall provide for discharge of the sump pump/drain overland
to a natural watercourse, drainage swale, stormwater easement, or
storm sewer system.
DD.
Where stormwater or surface water will be gathered within the subdivision
or land development and discharged or drained in volume over lands
within or beyond the boundaries of the subdivision or land development,
the subdivider, developer, or builder shall reserve or obtain easements
over all lands affected. The easements shall be adequate for such
discharge of drainage and for carrying off of such water and for the
maintenance, repair, and reconstruction of the same, including the
right of passage over, including vehicles, machinery and other equipment
for such purposes, and which shall be sufficient width for such passage
and work. If requested by the Township, the subdivider, developer,
or builder shall convey, at no cost, the easements to the Township.
Otherwise, all such easements shall be owned and maintained by individual
lot owner(s) or another approved entity. Ownership and maintenance
responsibilities shall be noted on the record plan and agreements
for such shall be as approved by the Township Solicitor.
EE.
Storm sewers and drainageways.
(1)
General provisions. All storm drains and drainage facilities
such as gutters, catch basins, bridges, inlets, and culverts shall
be installed and the land graded for adequate drainage as shown on
the grading plan submitted and approved with the final plan. Construction
of these facilities shall generally conform with PennDOT Specifications
Publication 408, latest version.
(2)
When required. Storm drains and appurtenances shall be required
to be constructed by the developer to take surface water from the
bottom of vertical grades to lead water away from springs, and to
avoid excessive use of cross gutters at street intersections and elsewhere.
(a)
Open watercourses will be permitted where they exist naturally
and where, in the opinion of the Township Engineer, they will not
interfere with public convenience or safety, but in fact will provide
comparable or superior drainage capabilities of piped drainage. Where
necessary, stream corridor improvements, including rock stabilization
and gabions, shall be performed.
(b)
When submitting a plan for approval involving the construction
of storm drainage facilities, the designer's computations shall be
submitted in duplicate to facilitate the checking of design.
(c)
Design of storm drainage facilities shall be completed in accordance
with accepted engineering practices subject to approval by the Township
Engineer.
(d)
Where practical, inlets shall be situated in grassed swales
or depressions.
(3)
Unnatural drainage. Wherever construction stops or concentrates
the natural flow of storm drainage in such a way to affect adjoining
properties, approval of the owners shall be obtained, in writing,
and a copy filed with the Township. Approval of plans by the Horsham
Township does not authorize or sanction drainage affecting adjoining
properties.
(4)
Design criteria.
(a)
Open channels should have a parabolic or trapezoidal cross section
in compliance with accepted engineering practices.
(b)
Permissible channel velocities, slopes, and cover shall be in
accordance with the Natural Resource Conservation Service Engineering
Field Manual, Chapter 7, Grassed Waterways and Outlets."
(c)
Existing stream channels shall be maintained in their natural
state. Only under unusual circumstances will it be permitted to line,
straighten, or relocate an existing stream, with approval of the Department
of Environmental Protection (DEP) and the Montgomery County Conservation
District.
(d)
Acceptable energy dissipation devices shall be installed to
bring discharge velocities down to limits specified in the DEP Erosion
and Sediment Pollution Control Program Manual, latest edition.
(e)
Rip-rapping and/or gabions may be required by the Township Engineer
where erosion potential is great.
FF.
Storm sewer design.
(1)
Design flow rate.
(a)
The storm sewer system shall be designed to carry the one hundred-year
peak flow rate. The design one-hundred-year peak flow rate into each
inlet shall be indicated in storm sewer calculations, and the drainage
area to each inlet shall be indicated on the stormwater management
plan. The one-hundred-year flow rate shall be determined by the Rational
Method formula Q = CIA, where:
|
Q
|
=
|
Peak runoff rate measured in cubic feet per second (cfs).
| |
|
C
|
=
|
Runoff coefficient. The coefficient of stormwater runoff includes
many variables, such as ground slope, ground cover, shape of drainage
area, etc.
| |
|
I
|
=
|
Intensity. average rainfall intensity in inches per hour for
a time equal to the time of concentration.
| |
|
A
|
=
|
Area. drainage area in acres.
|
(b)
Appropriate values for the rainfall intensity can be found in
the Pennsylvania Department of Transportation Design Manual, Part
2. Appropriate values for the runoff coefficient shall be as presented
below:
|
Rational Method Runoff Coefficients ("C") for the Design
of Storm Sewer
| ||
|---|---|---|
|
Description of Area
|
Runoff Coefficient ("C")
| |
|
Residential
| ||
|
Two-acre lots
|
0.4
| |
|
One-acre lots
|
0.45
| |
|
One-half-acre lots
|
0.5
| |
|
One-fourth-acre lots
|
0.6
| |
|
Townhouses, multifamily
|
0.7
| |
|
Commercial
|
0.75
| |
|
Industrial
|
0.8
| |
|
Parks, cemeteries
|
0.35
| |
|
Meadow
|
0.3
| |
|
Unimproved (forest, brush, meadow, mixed)
|
0.25
| |
(c)
Where the above table is not applicable, the following factors
may be used for smaller drainage areas:
|
Rational Method Runoff Coefficients ("C") for the Design
of Storm Sewer (Smaller Watersheds)
| ||
|---|---|---|
|
Description of Area
|
Runoff Coefficient ('C')
| |
|
Impervious
|
0.95
| |
|
Grass
|
0.35
| |
|
Forest
|
0.25
| |
(d)
More specific runoff coefficients, as stated in the Pennsylvania
Department of Transportation Design Manual, Part 2, or as stated in
the Pennsylvania Department of Environmental Protection publication,
"Recommended Hydrologic Procedures for Computing Urban Runoff from
Small Watersheds in Pennsylvania," may be used upon consultation with
the Township Engineer.
(2)
Consideration shall be given to future land use changes in the
drainage area in selecting the "C" coefficient. For drainage areas
containing several different types of ground cover, a weighted value
of "C" shall be used. In no case shall a weighted value of "C" be
less than 0.45 for an area to be changed from its natural state.
(3)
In determining the peak flow rate to individual storm sewer
inlets (or other collection structures) the time of concentration
method shall be used for inlet drainage areas in excess of 1.25 acres,
unless otherwise specified by the Township Engineer, and a five-minute
time of concentration shall be used for all other drainage areas,
unless otherwise approved by the Township Engineer.
(4)
In determining the required design flow rate through a storm
sewer piping system, a five-minute time of concentration (storm duration)
shall be used if the five-minute time of concentration does not result
in a maximum expected discharge that exceeds the capacity of a thirty-inch-diameter
pipe (or equivalent flow area of 4.9 square feet).
(5)
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 shall be used in determining storm duration.
(6)
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.
GG.
Required stormwater management facilities.
(1)
Storm sewer easements.
(a)
Easements shall be a minimum of 30 feet in width and shall be
provided in undedicated land to accommodate required storm sewer facilities
and drainage swales. Such easements shall be offered for dedication
to Horsham Township and shall be provided where storm sewer, drainage
swales, culverts, or other structures traverse, enter or discharge
onto private property. On private property, the property owner shall
maintain ground cover and landscaping within the easement. The Township
shall not maintain and/or repair any improvements within that easement
unless stormwater runoff from public roads or public park land crosses
through the easement. If stormwater runoff from public roads or public
lands crosses through the easement, the Township shall, upon satisfactory
installation of improvements as specified in a land development agreement
with Horsham Township, maintain and repair only the structural improvements
within the easement, if any, such as piping; inlets; stormwater manholes;
headwalls/endwalls; and energy dissipation structures or facilities.
Horsham Township will not be responsible for any other repair/maintenance
within the easement, such as lawn/ground cover maintenance; vermin
control; removal of obstructions from drainage swales; removal of
accumulated sediment or vegetation from drainage swales; removal of
litter or garbage; repair of erosion; and restoration of vegetation/ground
cover.
(b)
The record plan and development agreement for the approved land
development shall define easement ownership, maintenance responsibilities,
and access. Specifically, the record plan shall contain a provision
permitting access to such easement(s), at any reasonable time, for
inspection and/or emergency repair/maintenance, by Horsham Township
or its designee, of all facilities deemed critical to public welfare.
In the event the lot owner or homeowners' association fails to honor
their maintenance responsibilities set forth herein, in any manner,
Horsham Township shall have the right of entry upon and within the
area of the easement to undertake any required corrective or maintenance
effort. The total cost of such, including administrative, engineering,
and legal costs for enforcement, may be imposed upon the responsible
party as determined by Horsham Township. Failure to remedy all associated
costs described above may be subject of the imposition of a lien by
the Township against the owner(s) in question in the same manner as
the Township might otherwise be empowered by law to assess or impose
a lien against a property for municipal improvements.
(2)
Piping.
(a)
The capacity of all pipes shall, as a minimum, provide the required
carrying capacity as determined by the U.S. Department of Commerce,
Hydraulic Design Series No. 5, Hydraulic Design of Highway Culverts.
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.
(b)
The top of storm sewer pipes shall be at least six inches below
the subgrade elevation of bituminous pavements, except for reinforced
concrete pipe which may be at subgrade elevation, provided the applicant
verifies the pipe will withstand current PennDOT HS-25 loading standards.
For pipes in pervious areas, the top of pipe elevation shall be a
minimum of one foot below the finished surface.
(3)
Overflow system. An overflow system shall be provided to carry
all bypass flow and/or flow in excess of storm sewer design capacity
to the detention basin (or other approved outlet point) when the capacity
of the storm sewer system is exceeded. Stormwater runoff will not
be permitted to surcharge from storm sewer structures.
(4)
Manholes.
(a)
Storm sewer manholes shall be spaced as necessitated by the
proposed piping alignment; however, in no case shall manhole spacing
exceed 300 feet, except between manholes connected by pipe with a
diameter of 48 inches or greater, where spacing shall then not exceed
450 feet.
(b)
Manhole castings and covers shall conform to requirements of
Pennsylvania Department of Transportation Publications 72 and 408,
latest revisions. Manhole covers shall have the word "STORM" cast
in two-inch-high letters on the top of the cover.
(5)
Inlets.
(a)
Storm sewer inlets shall be spaced in accordance with this chapter;
however, in no case shall inlet spacing exceed 250 feet, except between
inlets connected by pipe with a diameter of 48 inches or greater,
where spacing shall then not exceed 450 feet.
(b)
Any inlet to be utilized in a storm sewer system shall conform
to the design standards of Pennsylvania Department of Transportation
(PennDOT) Publications 408 and 72 (Road Construction Standards). Specifically,
inlets to be utilized within a public right-of-way shall be Type "C"
with the "Alternate" inlet concrete top unit (which includes a Type
"C" structural steel frame) and Type "M" with standard concrete top
unit. All other inlets shall be Type "C" and "M" with standard concrete
top units. All inlet grates shall be bicycle-safe structural steel.
(c)
At street intersections, inlets shall be placed in the tangent
and not in the curved portion of the curbing. The gutter adjacent
to and immediately upgrade from the inlet shall be so warped as to
direct the water into the inlet. Inlets shall be required on the upslope
side of all approaches to an intersection.
(d)
The crowns of all pipes tying into an inlet or manhole shall
be set at equal elevations. A minimum of two inches shall be provided
between the inlet pipe invert elevation and the invert elevation of
the outlet pipe.
(e)
The capacity of inlets shall be based on a maximum surface flow
to the inlet of four CFS (cubic feet per second). The maximum flow
to inlets located in low points of paved areas (such as sag vertical
curves) shall include the overland flow directed to the inlet, as
well as the cumulative bypass surface runoff from inlets upstream.
The bypass runoff from each upstream inlet shall be calculated using
inlet efficiency curves (or other analysis method approved by the
Township Engineer) found in the Pennsylvania Department of Transportation
Design Manual, Part 2, latest revision. An 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, unless otherwise approved by the Township
Engineer. Double inlets will not be permitted where additional pipe
and inlets can be placed upstream to intercept the excessive surface
flow. Under special circumstances, a maximum of 12 CFS shall be permitted
to be collected by an inlet located in an isolated pervious area,
provided the designer can prove such an inlet would not cause stormwater
to accumulate on any adjacent public or private property, outside
of the associated proposed storm sewer easement, and that the depth
of the accumulated stormwater would not exceed 12 inches. Double four-foot
or six-foot inlets separated by 20 linear feet of pipe shall be required
if adequate efficiency is not realized with four-foot inlets.
(f)
Inlet spacing in paved areas shall be arranged so that a minimum of 80% of the gutter flow tributary to the inlet will be captured. The designer shall be required to verify that bypass surface runoff from the one-hundred-year design storm will enter the storm sewer piping system at some point, prior to discharge into a detention basin or other approved outlet point. Inlets shall be spaced so that the accumulation of surface bypass runoff and surface runoff tributary to an individual inlet will not exceed other design requirement specified in this chapter. Inlet capacity shall be based on inlet efficiency curves provided in Pennsylvania Department of Transportation, Design Manual, Part 2.
(g)
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).
(h)
Inlet castings, together with their covers or gratings, shall
conform to PennDOT or Horsham Township standards, as may be in effect
at the time the design of the sewer is submitted.
(i)
All inlets shall be stenciled by the developer to indicate that
"No Dumping Allowed — This inlet flows to . . . Creek," or other
approved wording by Horsham Township. Specifications for stenciling
are to be provided by Horsham Township.
(j)
Inlets in paved areas shall incorporate grit chambers.
(6)
Headwalls and endwalls.
(a)
An endwall shall be provided at the end of all pipe runs. Headwalls
shall be provided at the beginning of all pipe runs. A headwall shall
also be required (instead of an inlet) where calculated surface flows
exceed the requirements for inlets in pervious areas. Headwalls/endwalls
with a height greater than three feet may be required to have a safety
fence or guiderail if so determined by the Township Engineer. Rock
apron (rip-rap) or other approved velocity dissipation device shall
be placed at all pipe outlets (endwalls) to reduce flow velocity and
prevent erosion.
(b)
All headwalls and endwalls shall be stenciled by the developer
to indicate that "No Dumping Allowed — This inlet flows to .
. . Creek," or other approved wording by Horsham Township. Specifications
for stenciling are to be provided by Horsham Township.
(7)
Curb and roadside swales.
(a)
Roadside swales adjacent to shoulders. When swales are provided
in cut areas, the water shall not encroach upon the roadway area during
a twenty-five-year design storm of five-minute duration. Storm sewer
inlets shall be provided to control the roadway encroachment and water
velocity.
(b)
Shoulder swales (in cut areas - no roadside swale). Water flowing
in the shoulder shall not encroach more than 2/3 the shoulder width
during a twenty-five-year design storm of five-minute duration (time-of-concentration).
Storm sewer inlets shall be provided to control the shoulder encroachment
and water velocity.
(c)
Curbed sections. The maximum encroachment of runoff on the roadway
pavement shall not exceed 1/2 of the traveled lane during a twenty-five-year
design storm of five-minute duration. Inlets shall be provided to
control the encroachment of water on the traveled lane.
(8)
Bridges, culverts, and drainage channels.
(a)
Bridges and culverts shall be designed and constructed to meet
current Pennsylvania Department of Transportation construction and
loading standards (HS-25 loading standards). They shall be constructed
to the full width of the right-of-way or to an adequate dimension
to accommodate special grade conditions. Approval of the Pennsylvania
Department of Environmental Protection is required for all improvements
in and along all waters of the commonwealth. The applicant shall provide
verification of Department of Environmental Protection approval for
all such proposed bridges and culverts.
(b)
Approval of the Pennsylvania Department of Environmental Protection
is required for all improvements in and along waters of the commonwealth.
Where applicable, stormwater management facilities or programs shall
comply with the requirements of Chapter 102 (Erosion Control), Chapter
105 (Dam Safety and Waterway Management), and Chapter 106 (Floodplain
Management) of Pennsylvania Code, Title 25, Rules and Regulations
of the Department of Environmental Protection.
(c)
All bridges, culverts, and drainage channels shall be designed
to convey a flow rate equal to a one-hundred-year, twenty-four-hour
storm as defined by the U.S. Department of Agriculture, Soil Conservation
Service, Technical Release No. 55. All bridges and culverts shall
be designed to pass the one-hundred-year design storm without increasing
the extent and depth of the one-hundred-year floodplain.
(d)
Drainage channels shall be designed to convey the design discharge
at a stable, nonerosive velocity. Channel linings that are assumed
in the design must be established before the channel is utilized for
its stated objective. Diversions or alternate channel linings may
be necessary to provide channel conditions in the field to match design
assumptions. If the design channel lining cannot be developed before
the design discharge is introduced, then the designer must check the
stability of the channel assuming a bare earth condition.
(e)
Maximum channel velocities for vegetated and rock-lined channels
shall be as set forth in the Pennsylvania Department of Environmental
Protection, Bureau of Soil and Water Conservation Publication, Erosion
and Sediment Control Program Manual, latest revision.
(f)
The outfall of drainage channels shall be stable under the design
discharge. Energy dissipaters (rock lining/rip rap) shall be utilized
to return drainage channel discharge to a stable velocity.
(g)
Any vegetated drainage channel requiring mowing of the vegetation
shall have a maximum grade (side slopes) of four horizontal to one
vertical (4:1 - 25%) on those areas to be mowed.
(9)
Stormwater detention/retention basins.
(a)
Detention or retention basin easements, ownership, and maintenance.
Prior to granting of final approval of any subdivision or land development
plan, the developer shall provide written assurance, satisfactory
to the Township, that the detention or retention basin and associated
stormwater management facilities will be properly maintained. Such
assurances shall be in a form to act as a covenant that will run with
the land, and shall provide 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. If all or a portion of the
facilities are on property which will be conveyed to an individual,
homeowners' association or any other eventual owner, the guarantees
must be in such a form that they will carry through to the new owners.
Horsham Township will not accept dedication or maintenance responsibilities
for any detention or retention basin or similar facility located on
private grounds, unless otherwise approved by Township Council.
[1]
When a private entity (such as a homeowners' association) retains
ownership of any stormwater management facility, such entity shall
be responsible for maintenance of the facility. In such case, approval
of the stormwater management facility plans shall be conditioned upon
the private entity agreeing to be responsible for all maintenance
of the stormwater management facility. This agreement shall be in
writing, shall be in recordable form and shall, in addition to any
other terms deemed necessary by the Township, contain a provision
permitting access to such facilities deemed critical to the public
welfare, for inspection at any reasonable time by Horsham Township
or its designee.
[2]
When any stormwater management facility is located on an individual
lot, and when maintenance thereof is the responsibility of that landowner,
a description of the facility or systems and the terms of the required
maintenance shall be incorporated on a plat of the property. The plat
shall be recorded with the Montgomery County Recorder of Deeds within
90 days following Township approval. The plat of the property shall
also contain a provision permitting access to such facilities deemed
critical to public welfare, for inspection at any reasonable time
by Horsham Township or its designee. The Township may also require
as a condition of approval that any deed conveying any interest in
such lot contain language indicating that the conveyance is subject
to an express covenant by the grantee that the grantee will maintain
the stormwater management facility.
[3]
Upon presentation of proper credentials, duly authorized representatives
of Horsham Township may enter at any reasonable time upon any property
within the Township to investigate or to ascertain the condition of
the subject property with regard to any matter regulated by this chapter.
[4]
The failure of any person, individual lot owner or private entity
to properly maintain any stormwater management facility shall be construed
to be a violation of this section and is declared to be a public nuisance,
subjecting the violator to any and all penalties provided by law.
Specifically, in the event the entity responsible for maintenance
fails to honor those responsibilities set forth herein, in any manner,
Horsham Township shall have the right of entry upon the area of such
facilities to undertake any required corrective or maintenance effort.
The total cost of such, including administrative, engineering, and
legal costs for enforcement, may be imposed upon the responsible party
as determined by Horsham Township. Failure to remedy all associated
costs described above may be subject of the imposition of a lien by
the Township against the owner(s) in question, in the same manner
as the Township might otherwise be empowered by law to assess or impose
a lien against a property for municipal improvements.
(b)
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. Detention
basins shall be designed to facilitate regular maintenance, mowing
and periodic de-silting and reseeding. In residential developments,
shallow broad basins shall be provided for recreational use.
(c)
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 Engineer finds
that downstream conditions may warrant other design criteria for stormwater
release.
(d)
Emergency overflow facilities shall be provided for basins to
handle runoff in excess of design flows, or in the event the outlet
structure becomes blocked and passes no flow.
(e)
If the land of the proposed subdivision or land development
will be conveyed to two or more separate owners, the developer shall
provide written assurance and deed restrictions to the Township that
the owners will properly maintain the basin(s).
(f)
Basins shall not be located within floodplains; nor within areas
of floodplain soils with the exception that areas of alluvial soils
may be utilized, if approved by the Pennsylvania Department of Environmental
Protection and Army Corps of Engineers (where applicable), if proof
is accepted that the area is not subject to flooding.
(g)
The use of regional basins to combine and eliminate numerous
smaller basins is encouraged. Consultation with the Township is required
prior to design of a regional basin.
(h)
Landscaping and planting in and around the perimeter of basins
shall be provided. It shall be aesthetically pleasing and compatible
with surrounding land uses, and require minimum maintenance. Proposed
planting shall also be in accordance with the provisions of this chapter
and as recommended by the Township Landscape Architect. A plan of
basin planting shall be submitted to the Township for review and approval.
(i)
If a stormwater management basin will serve as a temporary sediment
control device, the temporary sediment control measures shall be shown
including perforated riser pipes or standboxes, filter berms, cleanout
stakes and other measures as may be required by Pennsylvania Department
of Environmental Protection Chapter 102, Regulations. Plans for such
facilities shall require Montgomery County Conservation District approval
prior to implementation. Sedimentation basins shall be in place prior
to any earthmoving activities within their tributary drainage areas.
A note summarizing the above shall be required on the record plan
of subdivisions and land developments as well as in land development
agreement with the Township.
(j)
Stormwater management basins shall be in place before the creation
of any additional impervious surfaces on the site. Runoff shall not
be directed to an infiltration structure until all tributary drainage
areas are stabilized.
(k)
Where permanent retention (pond) facilities are proposed, there
shall be a safety ledge, three feet wide at the maximum water surface
level. If the pond is to be stocked, the Pennsylvania Fish and Game
Commission shall approve the stocking plan.
(l)
All basins shall have slopes of three horizontal to one vertical
(3:1 - 33.3%), or less on the basin's outer berm and four 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.
(m)
All portions of the detention basin bottom shall have a minimum
slope of 2%. The Township, on the recommendation of the Township Engineer,
may accept a basin bottom slope of less than 2% provided vegetative
plantings consistent with current best management practices are provided.
(n)
All basin embankments shall be placed in lifts not to exceed
one foot in thickness and each lift shall be compacted to a minimum
of 95% of Modified Proctor Density as established by ASTM D-1557.
Prior to proceeding to the next lift, the compaction shall be checked
by a Soils Engineer employed by the applicant/developer. Compaction
tests shall be run on the leading and trailing edge of the berm along
with the top of the berm. Verification of required compaction shall
be submitted to the Township Engineer prior to utilization of any
basin for stormwater management.
(o)
Emergency overflow facilities/spillway shall be provided with
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 lined. Lining for emergency spillways shall incorporate
native colors and materials where possible including mono slab revetments,
grass pavers and native stone. Mono slab revetments and grass pavers
shall be backfilled with topsoil and seeded. The 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) if the designer can demonstrate that flow velocities
through the spillway will not cause erosion of the spillway. A dense
cover of vegetation shall be rapidly established in such spillways
by sodding or seeding with a geotextile anchor. Such a 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 (incoming
to the basin) with the primary outlet structure blocked. All spillway
velocities shall be based upon flows during an assumed clogged primary
outlet condition.
(p)
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, will
not be acceptable for permanent basins.
(q)
The minimum top of basin berm width (at the design elevation)
shall be 10 feet. A cutoff trench (keyway) of impervious material
shall be provided under all embankments that require fill material.
The cutoff trench shall be a minimum of eight feet wide, three feet
deep and have side slopes of one horizontal to one vertical.
(r)
The minimum freeboard through the emergency spillway shall be
two feet. "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.
(s)
Antiseep collars shall be installed around the pipe barrel and
shall be centered within the normal saturation zone of the berm. The
antiseep collars and their connections to the pipe barrel shall be
watertight. The antiseep collars shall be cast-in-place and extend
a minimum of two feet beyond the outside of the principal pipe barrel.
Precast collars may be permitted when approved by the Township Engineer.
A minimum of two collars shall be installed on each basin outlet pipe.
(t)
A perforated riser, 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, Risers shall not be removed until such time the entire area
tributary to the basin has been permanently stabilized and until approved
by the Montgomery County Conservation District and/or the Township
Engineer.
(u)
All basin outlet pipes shall be watertight reinforced concrete
having "O-ring" joints. All joints shall be mortared. To minimize
clogging and to facilitate cleaning, outlet pipes shall have an internal
diameter of at least 18 inches and a minimum grade of 1/2%.
(v)
Energy dissipating devices (rock lining/rip-rap, or other approved
means) shall be provided at all basin outlets and shall be sized in
accordance with Pennsylvania Department of Environmental Protection,
Bureau of Soil and Water Conservation Publication, Erosion and Sediment
Control Program Manual, latest revision.
(w)
Stone gabion baskets shall not be permitted for use in construction
of detention/retention basins.
(x)
Access easement and stabilized drive to stormwater detention
facilities shall be provided for maintenance and operation. This access
easement shall be cleared and, when possible, be at least 30 feet
in width. Proximity of detention facilities to public right-of-way
shall be encouraged in order to minimize the length of accessways.
Multiple accesses shall be encouraged for major facilities. The developer
shall provide access easements and drives of reinforced concrete checker-block
(backfilled with topsoil and seeded) or other similar paver acceptable
to the Township Engineer, over a six-inch bed of compacted PennDOT
type 2A coarse aggregate (or approved equivalent). Accessways to basins
shall be a minimum of 12 feet in width and be no steeper in slope
than 12 feet horizontal to one-foot vertical. In addition, depressed
curb should 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. Access
easement shall be owned and maintained by the individual lot owner(s)
or homeowners' association but shall be established to permit access
by Horsham Township or its designee, for emergency inspection and/or
maintenance, at any reasonable time.
(y)
The minimum distance between a proposed basin discharge point
and a downstream property boundary shall be 50 feet.
(10)
Best management practices. The Pennsylvania Handbook of Best
Management Practices for Developing Areas (1998) shall serve as a
design guide for structural and nonstructural best management practices.
Additional design guidance may also be obtained from other related
references prepared by the U.S. Environmental Protection Agency, the
Washington Metropolitan Council of Governments, the Natural Resource
Conservation Service, and the Pennsylvania Department of Environmental
Protection. All BMP designs are subject to approval by the Township
Engineer.
Approvals issued and actions taken under this chapter do not
relieve the applicant of the responsibility to secure required permits
or approvals for activities regulated by any other code, law, regulation
or ordinance.
A.
Evidence of any necessary permit(s) for regulated earth disturbance
activities from the appropriate DEP regional office or County Conservation
District must be provided to the municipality.
B.
Additional erosion and sediment control design standards and criteria
are recommended to be applied where infiltration BMPs are proposed.
They shall include the following:
(1)
Areas proposed for infiltration BMPs shall be protected from sedimentation
and compaction during the construction phase to maintain maximum infiltration
capacity.
(2)
Infiltration BMPs shall not be constructed nor receive runoff until
the entire drainage area contributory to the infiltration BMP has
achieved final stabilization.
The design of all regulated activities should include the following
to minimize stormwater impacts:
A.
The applicant should 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.
B.
An alternative is practicable if it is available and capable of implementation
after taking into consideration existing technology and logistics
in light of overall project purposes and other municipal requirements.
C.
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.
A.
Infiltration best management practices (BMPs) shall meet the following minimum requirements unless the site qualifies for an exemption from the infiltration requirements of this chapter as listed in § 190-6:
(1)
Infiltration BMPs intended to receive runoff from developed areas
shall be selected based on suitability of soils and site conditions
and shall be constructed on soils that have the following characteristics:
(a)
A minimum soil depth of 24 inches between the bottoms of the
infiltration BMPs and bedrock or other limiting zones.
(b)
An infiltration rate sufficient to accept the additional stormwater
load and dewater completely as determined by field tests conducted
by the applicant's qualified person.
(c)
All open-air infiltration facilities shall be designed to completely
infiltrate the recharge (infiltration) volume (Rev) within three days (72 hours) from the end of the design storm.
(d)
All subsurface and contained facilities such as capture-and-reuse
systems must have storage available equivalent to the water volume
control amount within three days (72 hours) from the end of the design
storm.
(2)
The size of the infiltration facility shall be based upon the following
volume criteria:
|
Where practicable and appropriate the recharge volume shall
be infiltrated on site. The recharge volume shall be equal to one
inch of runoff (I) over all proposed impervious surfaces.
|
|
The Rev required shall be computed as:
| ||||
|
Rev = (1/12) * (I)
| ||||
|
Where:
| ||||
|
Rev = Recharge Volume (cubic feet)
| ||||
|
I = Impervious Area within the limits of earth disturbance (square
feet)
| ||||
|
An asterisk (*) in equations denotes multiplication.
| ||||
B.
Soils. A detailed soils evaluation
of the project site shall be required to determine the suitability
of infiltration facilities. The evaluation shall be performed by a
qualified person, and at minimum address soil permeability, depth
to bedrock, and subgrade stability. The general process for designing
the infiltration BMP shall be:
(1)
Analyze hydrologic soil groups as well as natural and man-made features
within the site to determine general areas of suitability for infiltration
practices. In areas where development on fill material is under consideration,
conduct geotechnical investigations of subgrade stability; infiltration
may not be ruled out without conducting these tests.
(2)
Provide field tests such as double ring infiltrometer or hydraulic
conductivity tests (at the level of the proposed infiltration surface)
to determine the appropriate hydraulic conductivity rate. Percolation
tests are not recommended for design purposes.
(3)
Design the infiltration structure for the required recharge volume
(Rev) based on field determined capacity at
the level of the proposed infiltration surface.
(4)
If on-lot infiltration structures are proposed by the applicant's
qualified person, it must be demonstrated to the municipality that
the soils are conducive to infiltrate on the lots identified.
(5)
An impermeable liner will be required in detention basins where the
possibility of groundwater contamination exists. A detailed hydrogeological
investigation may be required by the municipality.
The low-impact development practices provided in the BMP Manual
shall be utilized for all regulated activities to the maximum extent
practicable. Water volume controls shall be implemented using the
Design Storm Method in Subsection A or the Simplified Method in Subsection
B below. For regulated activity areas equal to or less than one acre
that do not require hydrologic routing to design the stormwater facilities,
this chapter establishes no preference for either methodology; therefore,
the applicant may select either methodology on the basis of economic
considerations, the intrinsic limitations on applicability of the
analytical procedures associated with each methodology, and other
factors. All regulated activities greater than one acre must use the
Design Storm Method.
A.
The Design Storm Method (CG-1 in the BMP Manual) is applicable to
any size of regulated activity. This method requires detailed modeling
based on site conditions.
(1)
The post-development total runoff volume for all storms equal to
or less than the two-year, twenty-four-hour storm event shall not
be increased.
B.
The Simplified Method (CG-2 in the BMP Manual) provided below is
independent of site conditions and should be used if the Design Storm
Method is not followed. This method is not applicable to regulated
activities greater than one acre, or for projects that require design
of stormwater storage facilities. For new impervious surfaces:
(1)
Stormwater facilities shall capture at least the first two inches
of runoff from all new impervious surfaces. [Note: An asterisk (*)
in equations denotes multiplication.]
Volume (cubic feet) = (2/12) * Impervious Surfaces (square feet)
(2)
At least the first one inch of runoff from new impervious surfaces
shall be permanently removed from the runoff flow, i.e., it shall
not be released into the surface waters of the commonwealth. Removal
options include reuse, evaporation, transpiration, and infiltration.
Volume (cubic feet) = (1/12) * Impervious Surfaces (square feet)
(3)
Wherever possible, infiltration facilities should be designed to
accommodate infiltration of the entire permanently removed runoff;
however, in all cases, at least the first 0.5 inch of the permanently
removed runoff should be infiltrated.
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 established and maintained with appropriate native vegetation.
(Refer to Appendix B of the BMP Manual for plant lists.) 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 chapter, the existing buffer
shall be maintained. This does not include lakes or wetlands. applicants
shall adhere to the following stream bank erosion/channel protection
requirements:
A.
In addition to the control of water quality volume (in order to minimize
the impact of stormwater runoff on downstream stream bank erosion),
the primary requirement is to design a BMP to detain the proposed
conditions two-year, twenty-four-hour storm event to the existing
conditions one-year flow using the SCS Type II distribution. Additionally,
provisions shall be made (such as adding a small orifice at the bottom
of the outlet structure or a sand filter) so that the proposed conditions
one-year, twenty-four-hour storm event takes a minimum of 24 hours
to drain from the facility from a point when the maximum volume of
water from the one-year, twenty-four-hour storm event is captured
(i.e., the maximum water surface elevation is achieved in the facility).
Release of water can begin at the start of the storm (i.e., the invert
of the water volume control orifice is at the invert of the facility).
B.
The minimum orifice size in the outlet structure to the BMP shall
be three inches in diameter where possible, and a trash rack shall
be installed to prevent clogging. On sites with small drainage areas
contributing to this BMP that do not provide enough runoff volume
to allow a twenty-four-hour attenuation with the three-inch orifice,
the calculations shall be submitted showing this condition. Orifice
sizes less than three inches can be utilized, provided that the design
will prevent clogging of the intake. It is recommended that the design,
to accommodate maintenance, include a sand or porous media filter.
A.
In addition to the requirements specified in Table 408.1 below, the erosion and sedimentation control (§ 190-20), the nonstructural project design (§ 190-21), the groundwater recharge (§ 190-22), the water volume control (§ 190-23), and the stream bank erosion (§ 190-24) requirements shall be implemented.
Standards for managing runoff from each subarea in Horsham Township
for the two-, five-, ten-, twenty-five-, fifty-, and one-hundred-year
storm events are shown in Table 408.1. Development sites located in
each of the management districts must control proposed condition runoff
rates to existing condition runoff rates for the design storms in
accordance with Table 408.1.
|
Table 408.1
| |||
|---|---|---|---|
|
Peak Rate Control Standards for All Sites in Horsham Township
| |||
|
Proposed Condition Design Storm
|
Existing Condition Design Storm
| ||
|
Reduce To
| |||
|
Two-year
|
One-year
| ||
|
Five-year
|
Two-year
| ||
|
Ten-year
|
Five-year
| ||
|
Twenty-five-year
|
Ten-year
| ||
|
Fifty-year
|
Twenty-five-year
| ||
|
One-hundred-year
|
Fifty-year
| ||
|
NOTES:
| |
|---|---|
|
Projects that are required to obtain a NPDES permit for stormwater
discharges associated with construction activities are required to
show no increase in peaks from existing conditions.
|
B.
General. Proposed conditions rates of runoff from any regulated activity
shall not exceed the peak release rates of runoff from existing conditions
for the design storms specified in this section of this chapter.
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. However, on-site drainage facilities shall be designed
to safely convey off-site flows through the development site.
D.
Site areas. Where the site area to be impacted by a proposed development
activity differs significantly from the total site area, only the
proposed impact area utilizing stormwater management measures shall
be subject to the management district criteria. In other words, unimpacted
areas bypassing the stormwater management facilities would not be
subject to the management district criteria.
E.
Alternate criteria for redevelopment sites. For redevelopment sites,
one of the following minimum design parameters shall be accomplished,
whichever is most appropriate for the given site conditions as determined
by Horsham Township;
(1)
Meet the full requirements specified by Table 408.1 and § 192-25A
through E.
A.
Stormwater runoff from all development sites with a drainage area
of greater than two acres shall be calculated using a generally accepted
calculation technique that is based on the NRCS soil cover complex
method. The qualified person must consult with the municipality to
gain approval of design methods prior to design.
Table 409-1 summarizes acceptable computation methods and the
method selected by the qualified person shall be based on the individual
limitations and suitability of each method for a particular site.
The municipality may allow the use of the Rational Method to estimate
peak discharges from drainage areas that contain less than two acres.
The Soil Complex Method shall be used for drainage areas greater than
two acres.
|
Table 409.1
| |||
|---|---|---|---|
|
Acceptable Computation Methodologies For Stormwater Management
Plans
| |||
|
Method
|
Method Developed By
|
Applicability
| |
|
WINTR-20
|
USDA NRCS
|
Applicable where use of full hydrology computer model is desirable
or necessary
| |
|
WINTR-55
|
USDA NRCS
|
Applicable for land development plans within limitations described
in TR-55
| |
|
HEC-HMS
|
US Army Corps of Engineers
|
Applicable where use of full hydrologic computer model is desirable
or necessary
| |
|
Rational Method (or commercial computer package based on Rational
Method)
|
Emil Kuichling (1889)
|
For sites less than two acres or as approved by the municipality
and/or Municipal Engineer
| |
|
Other methods
|
Varies
|
Other computation methodologies approved by the municipality
and/or Municipal Engineer
| |
|
NOTES:
| |
|---|---|
|
Successors to the above methods are also acceptable. These successors
include WinTR55 for TR-55 and WinTR20 for TR-20.
|
B.
If a hydrologic computer model such as HydroCAD or HEC-HMS is used
for stormwater runoff calculations, then the duration of rainfall
shall be 24 hours. The rainfall distribution should reference NRCS
Type II.
C.
For the purposes of existing conditions flow rate determination,
undeveloped land shall be considered as "meadow" in good condition,
unless the natural ground cover generates a lower curve number or
Rational "C" value (i.e., forest).
D.
For Montgomery County only, all calculations using the Rational Method
shall use rainfall intensities from the NOAA 14 Precipitation-Frequency
Atlas of the United States (2004, revised 2006). 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 flow velocities
as determined by Manning's equation.
E.
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 rain data corresponding
to the Doylestown rain gage. These rainfall depths are: 2.7 inches
(one year), 3.3 inches (two year), 4.1 inches (five year), 4.8 inches
(ten year), 5.8 inches (twenty-five year), 6.7 inches (fifty year),
and 7.6 inches (one-hundred year). This data may also be directly
retrieved from the NOAA Atlas 14 website: hdsc.nws.noaa.gov/hdsc/pfds/orb/pa_pfds.html.
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.
F.
The Manning equation is preferred for 1-D, gradually varied, open
channel flow. In other cases, appropriate, applicable methods should
be applied, however, early coordination with the municipality is necessary.
G.
Outlet structures for stormwater management facilities shall be designed
to meet the performance standards of this chapter using the generally
accepted hydraulic analysis technique or method of the municipality.
H.
The design of any stormwater detention facilities intended to meet
the performance standards of this chapter shall be verified by routing
the design storm hydrograph through these facilities using the Storage-Indication
Method. For drainage areas greater than two acres in size, the design
storm hydrograph shall be computed using a calculation method that
produces a full hydrograph.
I.
In performing the stormwater calculations, all those areas to be
disturbed during construction and subsequently returned to open space
will be assumed to be reduced one hydrologic soil group category level
for post-development runoff calculations (i.e., hydrologic soil group
B is reduced to hydrologic soil group C and so forth).
