[Ord. No. 703, 11/14/2018]
1. All computations used in conjunction with the analysis and design
of stormwater management facilities shall be based on one or more
of the following methods, or as otherwise approved in advance by the
municipality:
A. TR-55-Soil Conservation Service (SCS) Technical Release No. 55.
B. TR-20-Soil Conservation Service (SCS) Technical Release No. 20.
C. Modified Rational Method.
E. Virginia Tech/Penn State Runoff Model.
2. These methods for determining peak discharge shall be used to:
A. Determine pre-development runoff conditions;
B. Analyze the impact of development; and
C. Perform calculations in the design of any detention/retention facilities
used in controlling runoff.
3. The SCS, Type II Rainfall Distribution shall be used for all analyses.
The design storm frequencies for the watershed are:
Design Storm
|
24-Hour Rainfall Depth
|
---|
1-Year
|
1.97
|
2-Year
|
2.35
|
5-Year
|
2.88
|
10-Year
|
3.30
|
25-Year
|
3.90
|
50-Year
|
4.40
|
100-Year
|
4.92
|
4. Pre-development Conditions. The cover type for all sites will be
considered to be the dominant land cover during the five-year period
immediately preceding a proposed regulated activity:
A. For the purposes of pre-development peak flow rate and volume determination,
existing nonforested pervious areas conditions shall be considered
as meadow (good condition). Forested land areas shall be considered
in good condition.
B. For the purposes of pre-development peak flow rate and volume determination,
20% of existing disturbed impervious area, when present, shall be
considered meadow (good condition).
5. Post-development Conditions. The hydrologic parameters used to develop
peak flow rates shall be reflective of anticipated soil runoff characteristics
following grading and development of the site.
6. Time of Concentration. The minimum time of concentration for any
watershed shall be six minutes. A minimum six-minute time of concentration
may be assumed for any post-development watershed. Calculations must
be provided for all pre-development times of concentrations. A minimum
time of concentration cannot be assumed for pre-development conditions.
7. The use of the Basic Rational Method in estimating runoff may be
employed in the design of the storm sewer conveyance system within
the development. The storm sewer system shall be interpreted as the
conduits, culverts, inlets and appurtenant features for the conveying
of stormwater to, through or from a development site to the point
of final discharge or control facility. The Rational Method shall
not be used in the analysis of stormwater runoff from the development
in its entirety or in conjunction with the design of any retention/detention
facilities or other runoff control measures.
8. Points of interest for analysis of pre- and post-development runoff must be selected to demonstrate compliance with the requirements of §§
23-303 and
23-304 is achieved at all points where runoff exits the perimeter of the property.
9. Stormwater runoff shall not be transferred from one watershed to
another unless the watersheds are subareas of a larger watershed that
are tributary to a common point of interest within or near the perimeter
of the property. Transfer of runoff from one watershed to another
under any other circumstances shall only be approved at the discretion
of the municipality. Documentation shall be provided that peak flow
rates are not increased following development and there will be no
detrimental impact in downstream areas.
10. The stormwater management plan shall consider all of the stormwater
runoff flowing over the project site. Runoff calculations shall be
made to ensure that runoff from the upstream watershed area can be
accommodated by the pipes, drainage easements and watercourses, etc.,
on the site.
[Ord. No. 703, 11/14/2018]
1. All stormwater collection and conveyance facilities (pipes, swales,
and structures) shall be designed for a 100-year design storm event,
unless the runoff would naturally drain overland to a stormwater detention
facility, in which case a twenty-five-year design storm event may
be used. The hydraulic grade line must be a minimum of two feet below
the surface elevation and within the pipe. Swales and channels shall
provide at least one foot of freeboard above the energy grade line.
Backwater effects of pipes discharging under surcharge conditions
shall be included.
2. Runoff calculations shall include complete hydrology and hydraulic
analysis of all downstream swales and pipe facilities to permanent
stream discharge point, to the municipal boundary or connection to
a state- or county-owned conveyance system.
3. Manholes shall not be spaced more than 300 feet apart for pipe sizes
up to 24 inches in diameter and not more than 450 feet apart for larger
pipe sizes.
4. No public stormwater pipe shall be less than 15 inches in diameter
that conveys surface runoff. The minimum pipe slope shall be 1.0%
grade or maintain velocity of two feet per second. For public storm
sewer systems, only pipes related to construction of stormwater BMPs
may be less than 15 inches in diameter.
5. All workmanship and materials shall conform to the municipality's
construction standards. In addition, all workmanship and materials
shall conform to the latest edition of PennDOT Form 408 and be supplied
by manufacturers or suppliers listed in PennDOT's Bulletin 15.
6. Manhole and inlet castings shall conform to the Pennsylvania Department
of Transportation Form 408 and PennDOT Standards for Roadway Construction.
Inlet grates shall be bicycle safe. Frames and grates shall be cast
iron or structural steel. Concrete frames shall not be permitted.
7. All connections to existing storm sewer pipes shall be made by construction
of a suitable junction box (inlet or manhole) to provide access for
cleanout. No blind connections shall be permitted.
8. The discharge of stormwater runoff shall be to a well-defined drainage
course which has a defined bed and bank. If stormwater runoff cannot
be discharged to a defined drainage course, documentation of written
permission from each downstream property owner shall be provided for
all properties between the source of discharge and the defined drainage
course. The use of level spreaders or similar BMPs that promote sheet
flow may be permitted if the applicant provides documentation that
the peak post-development 100-year discharge is no greater than pre-development
two-year discharge and a letter from the geotechnical engineer of
record regarding slope stability of the downstream area is provided.
Use of a level spreader requires a waiver to be granted by the municipality.
[Ord. No. 703, 11/14/2018]
1. All retention/detention facilities shall be equipped with multistage
outlet structures to provide discharge control for each designated
storm frequency. Provisions shall also be made to safely pass the
post-development 100-year storm runoff in the event of an outlet structure
failure without damaging or impairing the continued function of the
facilities (i.e., impairing the continued function of the facilities).
The facility(ies) must have an emergency outlet which is able to discharge
the post-development 100-year peak flow. Should any stormwater management
facilities qualify as a dam under PA DEP Chapter 105 criteria, the
facility shall be designed in accordance with those regulations and
meet the regulations concerning dam safety.
2. Any stormwater management facility designed to store runoff shall
provide an emergency spillway designed to convey the unattenuated
100-year post-development peak rate flow with a blocked primary outlet
structure and an inundation stormwater to the invert elevation of
the spillway, with a minimum one-foot freeboard to the crest of the
embankment.
3. Woody vegetation shall not be permitted on the embankments or within
25 feet of the emergency spillway.
4. The water depth of a storage facility which is not fenced shall be
limited to two feet unless approved by the municipality. Otherwise,
appropriate fencing at least four feet in height shall be required.
5. Side slopes of storage facilities shall not exceed a ratio of three
to one (3:1) horizontal to vertical dimension. The crest of the embankment
shall have a minimum width of not less than 10 feet.
6. The facility shall be equipped with an access road at least 10 feet
wide and with a maximum of grade of 15%. Access roads greater than
12% grade must be paved. Access roads 12% or less shall have a minimum
six-inch-depth noneroding aggregate surface. A gate with a minimum
opening of 10 feet shall be provided for maintenance access. An access
easement with a minimum width of 20 feet to all stormwater detention
facilities shall be provided. The access easement shall include a
statement on the recorded plan from the owner/operator of the facility
granting access to the municipality.
7. All stormwater facility outlet structures shall have suitable gaskets
to prevent leakage and piping of water through the facility embankment.
All storm pipe installed through the facility embankment must be constructed
of reinforced concrete pipe. Use of high-performance polypropylene
pipe may be permitted with documentation of the pipe manufacturer's
backfill requirements.
8. A geotechnical investigation report for the construction of the stormwater
detention/retention and infiltration facilities must be provided,
including design recommendation for embankment construction, interior
and exterior slopes, drainage swales and infiltration areas.
9. Basin outlet structures shall have nonclogging trash racks over all
design openings. Periodic cleaning of debris from trash racks shall
be included in the operation and maintenance plan.
10. Inlet structures and outlet structures shall be separated to the
greatest extent possible in order to maximize the flow path through
the basin.
11. BMPs must be designed to protect and maintain existing uses (e.g.,
drinking water use; cold water fishery use) and maintain the level
of water quality necessary to protect those uses in all streams, and
to protect and maintain water quality in "special protection" streams,
as required by statewide regulations at 25 Pa. Code Chapter 93 (collectively
referred to herein as "state water quality requirements").