The Lewisburg Borough Stormwater Management
Areas are as follows and appear on a map in Appendix A[1] of this chapter.
A.
Buffalo Creek Stormwater Area. Activities within the
Buffalo Creek Watershed shall comply with the requirements of this
chapter and the Buffalo Creek Act 167 Stormwater Management Plan,
adopted by the County of Union and approved by the Pennsylvania Department
of Environmental Protection.
B.
Bull Run (Limestone Run) Stormwater Area. Activities
occurring within the Bull Run Watershed shall comply with the requirements
of this chapter and the original Bull Run Act 167 Stormwater Management
Plan of 1993 and the Plan Update of 2002 adopted by the County of
Union and approved by the Pennsylvania Department of Environmental
Protection.
C.
West Branch Susquehanna River Stormwater Area. Activities
occurring within the West Branch Susquehanna River Watershed shall
comply with the requirements of this chapter and the West Branch Susquehanna
River Act 167 Stormwater Management Plan, adopted by the County of
Union and approved by the Pennsylvania Department of Environmental
Protection.
[1]
Editor's Note: Appendix A is on file in the
Borough offices.
A.
The management of stormwater on the site, both during
and upon the completion of construction, and the design of any temporary
or permanent facilities or structures and the utilization of a natural
drainage system shall be in full compliance with this section.
B.
Site designs shall minimize impervious surfaces and
shall promote the infiltration of runoff through seepage beds, infiltration
trenches, etc., where soil conditions permit to reduce the size or
eliminate the need for detention facilities.
C.
Stormwater runoff generated from development discharged
directly into a wetland or other waters of the Commonwealth shall
be done in accordance with Federal and State regulatory requirements
and shall be adequately treated to prevent degradation.
D.
Annual groundwater recharge rates shall be maintained
by promoting infiltration. At a minimum annual recharge from the post-development
site shall mimic the annual recharge from the predevelopment site
conditions.
E.
Applicants may select runoff control techniques, or
a combination of techniques, which are most suitable to control stormwater
runoff from the development site. Lewisburg Borough encourages applicants
to consider alternative solutions, including best management practices
(BMP's) for stormwater management. Applicants are urged to consult
the Pennsylvania Handbook of Best Management Practices for Developing
Areas and with the Municipal Engineer and the County Conservation
District. All stormwater designs are subject to the approval of the
Municipal Engineer. The Borough may request specific information on
design and/or operating features of the proposed stormwater controls
in order to determine their suitability and adequacy in terms of the
standards of this chapter.
F.
All stormwater management facilities including detention
basins, BMP's, sewers, and culverts shall be designed by an individual
qualified and/or experienced in their design. These qualifications
should be listed on the front cover of the plan narrative.
G.
The anticipated peak rate of stormwater runoff from
the site during and after full development shall not exceed the peak
rate of runoff from the site prior to development activities, measured
in accordance with the standards and criteria of this chapter.
H.
Roof drains shall not be connected to streets, sanitary
or storm sewers or roadside ditches but shall be allowed to drain
to the land surface to promote overland flow and infiltration of stormwater.
I.
Existing points of concentrated drainage that discharge
onto adjacent property shall not be altered without permission of
the affected property owner(s) and shall be subject to any applicable
discharge criteria specified in this chapter.
J.
If existing diffused stormwater flow is proposed to
be concentrated the applicant must document that adequate downstream
conveyance facilities exist to safely transport the concentrated discharge,
or otherwise prove that no erosion, sedimentation, flooding or other
harm will result from concentrated discharge.
K.
Storm sewers, swales, culverts and bridges.
(1)
Storm sewers, swales, culverts, bridges, and related
facilities shall be provided to:
(a)
Permit the unimpeded flow of natural watercourses;
(b)
Insure the drainage of all points along the
line of streets;
(c)
Intercept stormwater runoff along streets at
reasonable intervals related to the extent and grade of the area drained,
and to prevent the flooding of intersections and the undue concentration
of Stormwater; and
(d)
Insure unrestricted flow of stormwater under
driveways, and at natural watercourses or drainage swales.
(2)
Consideration should be given to anticipated up slope
development.
L.
All natural streams, channels, swales, drainage systems
and/or areas of surface water concentration shall be maintained in
their existing conditions unless an alteration is approved by the
Borough due to topographic conditions.
M.
Easements shall be provided conforming to the line
of all natural streams, channels, swales, and drainage systems and
other stormwater infrastructure installed to comply with this chapter.
The terms of such easements shall prohibit excavation, the placement
of fill or structures and any other alterations that may adversely
affect the flow of stormwater.
N.
All stream encroachment activities, including work
in and adjacent to waters of the Commonwealth or wetlands, shall comply
with applicable PA DEP requirements.
O.
Any stormwater facility located on a state or Borough
highway right-of-way shall be subject to the approval of the Pennsylvania
Department of Transportation or Lewisburg Borough.
Q.
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.
R.
The design of all stormwater management facilities
shall incorporate sound engineering principles and practices. The
Borough shall reserve the right to disapprove any design that would
result in the occupancy or continuation of an adverse hydrologic or
hydraulic condition.
A.
For water quality, the objective is to provide adequate
storage to capture and treat the runoff from 95% of the average rainfall
in accordance with the following where P represents the depth of rain
associated with 95% of the total rainfall events over 0.11 inches.
[Amended 11-15-2022 by Ord. No. 1069]
(1)
The size of the water quality facility shall be based
upon the following equation:
[Amended 11-15-2022 by Ord. No. 1069]
WQv
|
=
|
(1.2) (Rv)(A)
| ||
---|---|---|---|---|
|
P = 1.53 inches of rainfall
| |||
12
|
Where:
| ||
---|---|---|
WQv = water quality volume
(in ac-ft)
| ||
Rv = 0.05 + 0.009(I)
where I is percent impervious cover
| ||
A = area in acres*
|
*Treatment of the WQv for off-site areas and areas not disturbed is not required.
|
(2)
Treatment of the WQv shall
be provided at all developments where stormwater management is required.
A minimum WQv of 0.2 inches per acre shall
be met at sites or in drainage areas that have less than 15% impervious
cover.
(3)
The WQv shall be based on the
impervious cover for the proposed site. Off-site existing impervious
areas may be excluded from the calculation of the water quality volume
requirements.
(4)
When a project contains or is divided by multiple
drainage areas, the WQv shall be addressed
for each drainage area.
(5)
Drainage areas having no impervious cover and no proposed
disturbance during development may be excluded from the WQv calculations. Designers are encouraged to use these
areas as nonstructural practices for WQv treatment.
(6)
Where structural practices for treating the recharge
volume (Rev) are employed upstream of a BMP,
the Rev may be subtracted from the WQv used for design.
(8)
The design of the facility shall consider and minimize
the chances of clogging and sedimentation potential. Orifices smaller
than three inches diameter are not recommended. However, if the design
engineer can provide proof that the smaller orifices are protected
from clogging by use of trash racks, etc., smaller orifices may be
permitted.
B.
To accomplish adequate water quality treatment the
final WQv shall be treated by an acceptable
BMP from the list presented in Appendix M[3] or an equivalent practice approved by the Municipal Engineer.
The applicant may submit original and innovative designs to the Municipal
Engineer for review and approval. Such designs may achieve the water
quality objectives through a combination of BMPs.
[3]
Editor's Note: Appendix M is on file in the
Borough offices.
C.
The water quality requirement can be met by providing
a twenty-four-hour draw down of a portion of the (WQv) in conjunction with a stormwater pond or wetland system. Referred
to as “extended detention” (ED), this is different than
providing the extended detention of the one-year storm for the channel
protection volume (Cpv). The ED portion of
the WQv may be included when routing the Cpv.
D.
In selecting the appropriate BMPs or combinations
thereof, the applicant shall consider the following:
(1)
Total contributing area.
(2)
Permeability and infiltration rate of the site soils.
(3)
Slope and topography.
(4)
Seasonal high water table.
(5)
Depth to bedrock.
(6)
Proximity to building foundations and wellheads.
(7)
Erodibility of soils.
(8)
Subgrade stability and susceptibility to sinkhole
formation
(9)
Land availability and configuration of the topography.
(10)
Peak discharge and required volume control.
(11)
Stream bank erosion.
(12)
Efficiency of the BMPs to mitigate potential
water quality problems.
(13)
The volume of runoff that will be effectively
treated.
(14)
The nature of pollutants being removed.
(15)
Creation and protection of wildlife habitat.
(16)
Enhancement of aesthetic and property value.
(17)
Maintenance requirements.
E.
Stormwater hotspots. If a site is designated as a
stormwater hotspot as per Table 302-1, it has important implications
for how stormwater is managed.
(1)
A greater level of stormwater treatment is required
at hotspot sites to prevent pollutant wash off after construction.
(2)
For areas designated as hotspots, a Stormwater Pollution
Prevention Plan may be required to be designed and implemented that
contains operation practices at the site to reduce the generation
of pollutants by preventing contact with rainfall.
(3)
Stormwater Pollution Prevention Plans shall follow
the requirements of the U.S. EPA NPDES stormwater program.
(4)
The following land uses and activities are not normally
considered hotspots:
(a)
Residential streets and rural highways, residential
development, institutional development, commercial and office developments,
non-industrial rooftops, pervious areas except for golf courses and
nurseries. Large highways and retail gasoline outlet facilities are
not designated as hotspots although it is important to ensure that
stormwater plans for these facilities adequately protect groundwater.
Table 302-1: Stormwater Hotspots
| |
---|---|
Vehicle salvage yards and recycling facilities*
| |
Vehicle service and maintenance facilities
| |
Vehicle and equipment cleaning facilities*
| |
Fleet storage areas (bus, truck, etc.)*
| |
Industrial sites
| |
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
| |
Golf courses
| |
Other land uses and activities as designated
|
NOTES:
| |
---|---|
*Stormwater Pollution Plan implementation may
be required for these land uses or activities under the U.S. EPA NPDES
stormwater program.
|
A.
Design of the infiltration/recharge stormwater management
facilities shall give consideration to providing groundwater recharge
to compensate for the reduction in the percolation that occurs when
the ground surface is paved and roofed over. These measures are encouraged,
particularly in hydrologic soil groups A and B and shall be utilized
wherever feasible.
B.
The criteria for maintaining recharge is based on
the USDA average annual recharge volume per soil type divided by the
annual rainfall in Union County (40 inches per year) and multiplied
by 90%. This keeps the recharge calculation consistent with the WQv methodology. Thus, an annual recharge volume requirement
shall be specified for a site as follows:
(1)
Percent volume method.
Rev = [(S)(Rv)(A)]/12
|
Where:
| ||
---|---|---|
Rv = 0.05 + 0.009(I)
where I is percent impervious cover
| ||
A = site area in acres
|
(2)
Percent area method.
Rev = (S)(Ai)
|
Where:
| ||
---|---|---|
Ai = the measured impervious
cover
|
Hydrologic Soil Group
|
Soil Specific Recharge Factor (S)
| |
---|---|---|
A
|
0.40
| |
B
|
0.27
| |
C
|
0.14
| |
D
|
0.07
|
(3)
The recharge volume is considered part of the total
WQv that must be provided at a site and can
be achieved either by a structural practice (e.g., infiltration, bioretention),
a nonstructural practice (e.g., buffers, disconnection of rooftops),
or a combination of both.
(4)
Drainage areas having no impervious cover and no proposed
disturbance during development may be excluded from the Rev calculations. Designers are encouraged to use these
areas as nonstructural practices for Rev treatment.
(5)
The Rev and WQv are inclusive. When treated separately, the Rev may be subtracted from the WQv when sizing
the water quality BMP.
(6)
Recharge/infiltration facilities may be used in conjunction
with other innovative or traditional BMPs, stormwater control facilities,
and nonstructural stormwater management alternatives.
C.
Basis for determining recharge volume.
(1)
If more than one hydrologic soil group (HSG) is present
at a site, a composite soil specific recharge factor shall be computed
based on the proportion of total site area within each HSG. The recharge
volume provided at the site shall be directed to the most permeable
HSG available.
(2)
The "percent volume" method is used to determine the
Rev treatment requirement when structural practices
are used to provide recharge. These practices must provide seepage
into the ground and may include infiltration and exfiltration structures
(e.g., infiltration, bioretention, dry swales or sand filters with
storage below the under drain). Structures that require impermeable
liners, intercept groundwater, or are designed for trapping sediment
(e.g., forbays) may not be used. In this method, the volume of runoff
treated by structural practices shall meet or exceed the computed
recharge volume.
(3)
The "percent area" method is used to determine the
Rev treatment requirements when nonstructural
practices are used. Under this method, the recharge requirements are
evaluated by mapping the percent of impervious area that is effectively
treated by an acceptable nonstructural practice and comparing it to
the minimum recharge requirements.
(4)
Acceptable nonstructural practices include filter
strips that treat rooftop or parking lot runoff, sheet flow discharge
to stream buffers, and grass channels that treat roadway runoff.
(5)
The recharge volume criterion does not apply to any
portion of a site designated as a stormwater hotspot or any project
considered as redevelopment. In addition, the Municipal Engineer may
alter or eliminate the recharge volume requirement if the site is
situated on unsuitable soils (e.g., marine clays), karst or in an
urban redevelopment area. In this situation, nonstructural practices
(percent area method) shall be implemented to the maximum extent practicable
and the remaining or untreated Rev included
in the WQv treatment.
(6)
If Rev is treated by structural
or nonstructural practices separate and upstream of the WQv treatment, the WQv is adjusted
accordingly.
D.
Soils evaluation.
(1)
A detailed soils evaluation of the project site shall
be performed to determine the suitability of recharge facilities.
The evaluation shall be performed by a qualified professional, and
at a minimum, address soil permeability, depth to bedrock, susceptibility
to sinkhole formation, and subgrade stability. Advanced testing methods
such as the double ring test are encouraged. The municipal engineer
reserves the right to require an additional soils evaluation when
it is believed that test results are not reasonable.
(2)
Extreme caution shall be exercised where infiltration
is proposed in geologically susceptible areas such as strip mine or
limestone areas. Extreme caution shall also be exercised where salt
or chloride would be a pollutant since soils do little to filter this
pollutant and it may contaminate the groundwater. It is also extremely
important that the design professional evaluates the possibility of
groundwater contamination from the proposed infiltration/recharge
facility and recommends a hydrogeologic justification study be performed
if necessary. Whenever a basin will be located in an area underlain
by limestone, a geological evaluation of the proposed location shall
be conducted to determine susceptibility to sinkhole formations. The
design of all facilities over limestone formations shall include measures
to prevent ground water contamination and, where necessary, sinkhole
formation.
(3)
The Borough may require the installation of an impermeable
liner in detention basins underlain by limestone or in areas of karst
topography. A detailed hydrogeologic investigation may be required
by the Borough. The developer may also be required to provide safeguards
against groundwater contamination for uses which may cause groundwater
contamination, should there be an accident or spill.
E.
All recharge/infiltration facilities shall be designed
to completely drain within 72 hours of reaching maximum capacity.
A.
Stream channel protection shall be considered in implementing the standards of § 290-20 of this chapter. If a stormwater storage facility needs to be constructed then, to protect channels from erosion, the outflow structure shall be designed to provide twenty-four-hour extended detention of the one-year twenty-four-hour storm event. The method for determining the Cpv requirement
is detailed in Appendix N[1] of this chapter.
[1]
Editor's Note: Appendix N is on file in the
Borough offices.
B.
For discharges to streams having verified naturally
reproducing wild trout or that are stocked with trout (based upon
the most recent resource classification or other appropriate documentation
of the Pennsylvania Fish and Boat Commission or other appropriate
agency), only 12 hours of extended detention shall be provided. The
rationale for this criterion is that runoff will be stored and released
in such a gradual manner that critical erosive velocities during bankfull
and near-bankfull events will seldom be exceeded in downstream channels.
C.
Basis for determining channel protection storage volume.
(1)
The models HEC-HMS, TR-55 and TR-20 (or an equivalent
approved by the Municipal Engineer) shall be used for determining
peak discharge rates.
(3)
Off-site areas shall be modeled as present land use
in good condition for the one-year storm event.
(4)
The length of overland flow used in time of concentration
(tc) calculations is limited to no more than
150 feet.
(5)
The Cpv storage volume shall
be computed using the detention lag time between hydrograph procedures
outlined in Appendix N of this chapter. The detention lag time (T)
for a one-year storm is defined as the interval between the center
of mass of the inflow hydrograph and the center of mass of the outflow
hydrograph.
(6)
Cpv is not required at sites
where the one-year post-development peak discharge (qi) is less than or equal to 2.0 cfs. A Cpv orifice
diameter (do) of less than 3.0 inches is subject
to approval by the Municipal Engineer and is not recommended unless
an internal control for orifice protection is used.
(7)
Cpv shall be addressed for
the entire site. If a site consists of multiple drainage areas, Cpv may be distributed proportionately to each drainage
area.
(8)
Extended detention storage provided for the Cpv does not meet the WQv requirement
(i.e., Cpv and WQv shall
be treated separately).
(9)
The stormwater storage needed for the Cpv may be provided above the WQv storage in stormwater ponds and wetlands, thereby meeting all storage
criteria except Rev in a single facility with
appropriate hydraulic control structures for each storage requirement.
(10)
Infiltration is not recommended for Cpv control because of large storage requirements.
[Amended 11-15-2022 by Ord. No. 1069]
For a site located within two or more districts
the peak discharge rate from any sub-area shall be the predevelopment
peak discharge for that sub-area. The calculated peak discharges shall
apply regardless of whether the grading plan changes the drainage
area by sub-area. Pre-development and post-development runoff for
specific sites shall be computed using an approved method as per Table
307-1 for the two- , ten- , twenty-five- , fifty- , and one-hundred-year
storm. In addition to the other requirements provided in this section,
all sites must provide the following peak flow rate reductions for
all subareas within the site:
Two-year post-development peak flow rate
|
<=
|
One-year pre-development peak flow rate
|
Five-year post-development peak flow rate
|
<=
|
Two-year pre-development peak flow rate
|
A.
Bull run watershed. The Bull Run Watershed Release
Rate Percentage Maps in Appendix I[1] of this chapter illustrate the release rate subareas and
define the percentage of pre-development peak rate of runoff that
can be discharged from an outfall on the site after development. The
release rates are also shown in Table 305-1. The release rates apply
uniformly to all land development or alterations within a subarea.
Procedures for applying the release rate percentage are contained
in Appendix J.[2]
Table 305-1: Bull Run Watershed Release
Rates
| |||
---|---|---|---|
Subareas
|
Release Rate
(%)
| ||
Bull Run Watershed
|
1
|
80
| |
2
|
50
| ||
3
|
100
| ||
4
|
100
| ||
5
|
60
| ||
6
|
60
| ||
7
|
100
| ||
8
|
70
| ||
9
|
50
| ||
10
|
60
| ||
Miller Run
|
37-40
|
100
| |
42-45
|
100
| ||
46
|
75
| ||
47
|
100
| ||
49-51
|
100
|
B.
Buffalo Creek Watershed. The Buffalo Creek Watershed
contains three runoff control districts, "A," B," and "C." Development
sites in the Buffalo Creek Watershed must control post-development
runoff rates to pre-development runoff rates as shown in Table 305-2.
Lewisburg Borough is completely within the Buffalo Creek Runoff Control
District "C." See Appendix K[3] for the Buffalo Creek Runoff Control District and subwatersheds
within the Borough.
Table 305-2: Buffalo Creek
| ||||
---|---|---|---|---|
Subareas
|
Post-Development Design Storm
|
Pre-Development Design Storm
| ||
Runoff Control Districts
| ||||
A
|
1-4
|
2-year
|
1-year
| |
16
|
10-year
|
10-year
| ||
28-39
|
50-year
|
50-year
| ||
58-59
| ||||
B
|
5-15
| |||
17-27
|
2-year
|
1-year
| ||
40-55
|
10-year
|
5-year
| ||
60-65
|
50-year
|
25-year
| ||
70-75
| ||||
C
|
79-81
| |||
56-57
|
2-year
|
1-year
| ||
66-69
|
10-year
|
10-year
| ||
76-78
|
50-year
|
50-year
| ||
82-92
|
[3]
Editor's Note: Appendix K is on file in the
Borough offices.
C.
West Branch Susquehanna River. The release rate within
the West Branch Susquehanna River Watershed within Lewisburg Borough
is 100%. For sites that discharge directly to the Susquehanna River
the Municipal Engineer may waive the requirements of this section.
However, safe conveyance of runoff must be demonstrated for the two-
, ten- , twenty-five- , fifty- and one-hundred-year design storms.
The Borough reserves the right to require that the release rates be
met. Procedures for applying the release rates are in Appendix J.[4]
[4]
Editor's Note: Appendix J is on file in the
Borough offices.
A.
All storm sewers shall be able to convey the post-development
runoff from a ten-year design storm without surcharging inlets and
shall be constructed using Penn DOT Form 408 Specifications, Standard
Details, unless otherwise directed by the Municipal Engineer. When
connecting to an existing storm sewer system the applicant must demonstrate
that the proposed system will not exacerbate any existing stormwater
problems.
B.
Stormwater roof drains shall not discharge into any
municipal sanitary sewer line or over a sidewalk.
C.
Inlets shall be placed at the curb line where a curbed
section is installed. Inlets required for parallel or cross drainage
without a curbed section shall be set at the centerline of the ditch.
D.
Structures shall be Penn DOT Type M pre-cast concrete
or cast-in-place Class A concrete. Brick or block structures shall
not be permitted. Solid concrete block or brick may be incorporated
into a structure only for grade adjustment of the casting.
E.
All water obstructions (bridges, culverts, outfalls
or stream enclosures) shall have ample waterway opening to carry expected
flows, based on a minimum post-development peak storm frequency of
25 years and shall have a minimum of one foot of freeboard measured
below the lowest point along the top of the roadway.
F.
Bridge and culvert construction shall be in accordance
with the Pennsylvania Department of Transportation specifications
and shall meet the requirements of the Pennsylvania Department of
Environmental Protection.
G.
Any drainage conveyance facility and/or channel that
does not fall under PA DEP Chapter 105 Regulations must be able to
convey, without damage to the drainage structure or roadway, runoff
from the ten-year design storm. Conveyance facilities to or exiting
from stormwater management facilities (i.e., detention basins) shall
be designed to convey the design flow to or from that structure.
H.
Roadway crossings located within designated floodplain
areas shall be able to convey runoff from a one-hundred-year design
storm.
I.
Any stormwater management facility designed to store
runoff and requiring an earthen berm or embankment shall be designed
to provide an emergency spillway to handle flow up to and including
the one-hundred-year post-development conditions. The height of the
embankment must be set as to provide a minimum of one foot of freeboard
above the maximum pool elevation computed when the facility functions
for the one-hundred-year post-development inflow.
J.
Stormwater management facilities that require a dam
safety permit under PA DEP Chapter 105 shall meet the applicable dam
safety requirements that may require the facility to pass storms larger
than the one-hundred-year event.
K.
Adequate erosion protection shall be provided along
all open channels and at all points of discharge.
L.
Detention basins for stormwater peak discharge storage
shall comply with the following criteria:
(1)
Basins shall be installed prior to any earthmoving
or land disturbance that they will serve. The phasing of their construction
shall be noted in a narrative and on the plan.
(2)
Basins located in an area underlain by limestone may
require a geologic evaluation of the proposed location to determine
susceptibility to sinkhole formations. The design of all facilities
over limestone formations shall include measures to prevent ground
water contamination and, where necessary, sinkhole formation. The
Borough may require basins located over limestone to have an impermeable
liner.
(3)
Soils used for the construction of basins shall have
low erodability factors ("K factors").
(4)
Energy dissipators and/or level spreaders shall be
installed at points where pipes or drainage ways discharge to or from
basins. Discharge from basins shall be into a natural waterway or
drainageway.
(5)
Exterior slopes of compacted soil shall not exceed
one foot vertical per three feet horizontal and may be further reduced
if the soil has unstable characteristics.
(6)
Interior slopes of the basin shall not exceed one
foot vertical per three feet horizontal except with the approval of
the Municipal Engineer. Where concrete, stone, or brick walls are
used for steeper interior slopes, the basin shall be fenced with a
permanent wire fence at least 42 inches in height and a ramp of durable,
non-slip materials for maintenance vehicles shall be provided for
basin access.
(7)
Outlet structures within basins which will control
peak discharge flows and distribute the flows by pipes to discharge
areas shall be constructed of concrete, polymer-coated steel or aluminum
and shall have childproof, non-clogging trash racks over all design
openings exceeding 12 inches in diameter, except those openings used
to carry perennial stream flows. Small outlet structures may be constructed
of Schedule 40 PVC.
(8)
Where spillways will be used to control peak discharges
in excess of the ten-year storm, control weirs shall be constructed
of concrete of sufficient mass and structural stability to withstand
the pressures of impounded waters and outlet velocities.
(9)
Concrete outlet aprons shall be designed as level
spreaders and shall extend at a minimum to the toe of the basin slope.
The incorporation of any large stone found on the site into the concrete
apron to provide a more natural appearance is encouraged.
(10)
Inlet and outlet structures shall be located
at maximum distance from each other. The Borough may require a rock
filter berm or rock-filled gabions between inlet and outlet areas
when the distance is deemed insufficient for sediment trappings.
(11)
Temporary and permanent grasses or stabilization
measures shall be established on the sides of all earthen basins within
15 days of initial construction.
A.
Stormwater calculations to determine runoff, peak
flow rates, peak discharge, hydrographs and to design stormwater runoff
rate reduction facilities shall use a generally accepted calculation
technique that is based on the Natural Resource Conservation Service
(NRCS) Soil-Cover Complex method. Table 307-1 summarizes acceptable
methods.
B.
It is assumed that all methods will be selected by
the design professional based on the individual limitations and suitability
of each method for a particular site. The Borough may allow the use
of the Rational Method to estimate peak discharges from drainage areas
that contain 200 acres or less; however, the Rational Method shall
not be used to generate pseudo-hydrographs for drainage areas greater
than 10 acres.
C.
For predevelopment flow rate determination, it shall
be assumed that all undeveloped and pervious 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) as listed
in Appendix C or Appendix D of this chapter.[1] Twenty percent of all existing impervious land that will
be disturbed shall be considered meadow in good condition.
[Amended 11-15-2022 by Ord. No. 1069]
[1]
Editor's Note: Appendixes C and D are on file
in the Borough offices.
D.
All calculations using the Soil Cover Complex method
shall use the appropriate design rainfall depths for the various return
period storms as presented in the table in Appendix E[2] of this chapter. If a hydrologic computer model such as
PSRM or HEC-HMS is used for stormwater runoff calculations then the
duration of rainfall shall be 24 hours. The SCS "S" curve shown in
Appendix F[3] of this chapter shall be used for the rainfall distribution.
E.
All calculations using the Rational Method shall use
rainfall intensities consistent with appropriate times of concentration
for overland flow and return periods from the Design Storm Curves
from PA Department of Transportation Design Rainfall Curves (1986)
shown in Appendix G[4] of this chapter. Times of concentration for overland flow
shall be calculated using the methodology presented in Chapter 3 of
Urban Hydrology for Small Watersheds, NRCS, TR-55 (as amended or replaced
from time to time by NRCS). Times of concentration for channel and
pipe flow shall be computed using Manning's Equation.
[4]
Editor's Note: Appendix G is on file in the
Borough offices.
H.
Where uniform flow is anticipated, the Manning's Equation
shall be used for hydraulic computations and to determine the capacity
of open channels, pipes, and storm sewers. Manning's Equation shall
not be used for analysis of pipes under pressure flow or for analysis
of culverts. Values for Manning's roughness coefficient (n) shall
be consistent with Appendix H[7] of this chapter.
[7]
Editor's Note: Appendix H is on file in the
Borough offices.
I.
When existing storm sewers, streets, roadside ditches
or drainage swales are accessible, the applicant shall not connect
the stormwater drainage system to the existing facilities provided
without the approval of the Municipal Engineer and the facility owner.
J.
Routing of hydrographs through detention/retention
facilities for the purposes of designing those facilities shall be
accomplished using the Storage-Indication method or other recognized
routing method subject to approval of the Municipal Engineer. For
drainage areas greater than 200 acres in size, the design storm hydrograph
shall be computed using a calculation method that produces a full
hydrograph. The Municipal Engineer may approve the use of any generally
accepted full hydrograph approximation technique that shall use a
total runoff volume that is consistent with the volume from a method
that produces a full hydrograph.
K.
Any method approved by the Pennsylvania Department
of Transportation or the Pennsylvania Department of Environmental
Protection may be used to design the waterway areas of bridges.
Table 307-1: Acceptable Stormwater Management
Computation Methodologies
| |||
---|---|---|---|
Method
|
Method Developer
|
Applicability
| |
TR-20 (or commercial package based on TR-20)
|
USDA NRCS
|
Where use of full hydrologic computer model
is desirable or necessary.
| |
TR-55 (or commercial package based on TR-55)
|
USDA NRCS
|
For plans within limitations described in TR-55.
| |
HEC-1, HEC-HMS
|
US Army Corps of Engineers
|
Where use of full hydrologic computer model
is desirable or necessary.
| |
PSRM
|
Penn State University
|
Where use of full hydrologic computer model
is desirable or necessary.
| |
Rational Method
|
Emil Kuichling (1889)
|
For sites less than 10 acres, or as approved
by the Municipal Engineer.
| |
Other Methods
|
Varies
|
Other computations approved by Municipal Engineer.
|
A.
For any proposed development site not located in a provisional direct discharge district, the developer has the option of using a less restrictive runoff control (including no detention) if the developer can prove that "no harm" would be caused by discharging at a higher runoff rate than that specified by the Plan. The no-harm option is used when a developer can prove that the post-development hydrographs can match pre-development hydrographs, or if it can be proved that the post-development conditions will not cause increases in peaks at all points downstream. Proof of no-harm would have to be shown based upon the following Downstream Impact Evaluation which shall include a downstream hydraulic capacity analysis consistent with § 290-23B of this chapter to determine if adequate hydraulic capacity exists. The land developer shall submit to the Municipality this evaluation of the impacts due to increased downstream stormwater flows in the watershed.
(1)
The Downstream Impact Evaluation shall include hydrologic
and hydraulic calculations necessary to determine the impact of hydrograph
timing modifications due to the proposed development upon a dam, highway,
structure, natural point of restricted streamflow or any stream channel
section, established with the concurrence of the Municipal Engineer.
(2)
The evaluation shall continue downstream until the
increase in flow diminishes due to additional flow from tributaries
and/or stream attenuation.
(3)
The peak flow values to be used for downstream areas
for the design return period storms (two- , ten- , twenty-five- ,
fifty- and one-hundred-year) shall be the values from the calibrated
model used for the analysis and preparation of the particular Act
167 Stormwater Management Plan. These flow values can be obtained
from the applicable watershed plan.
(4)
Developer-proposed runoff controls that would generate increased peak flow rates at storm drainage problem areas would, by definition, be precluded from successful attempts to prove no-harm, except in conjunction with proposed capacity improvements for the problem areas consistent with § 290-21G.
(5)
A financial distress shall not constitute grounds
for granting a no-harm exemption.
(6)
Capacity improvements may be provided by the developer
as necessary to implement the no-harm option, which proposes specific
capacity improvements to provide that a less stringent discharge control
would not create any harm downstream.
B.
Downstream hydraulic capacity analysis. Any downstream
capacity hydraulic analysis conducted in accordance with this chapter
shall use the following criteria for determining adequacy for accepting
increased peak flow rates:
(1)
Natural or man-made channels or swales must be able
to convey the increased runoff associated with a two-year return period
event within their banks at velocities consistent with protection
of the channels from erosion. Acceptable velocities shall be based
upon criteria included in the DEP Erosion and Sediment Pollution Control
Program Manual.
(2)
Natural or man-made channels or swales must be able
to convey increased twenty-five-year return period runoff without
creating any hazard to persons or property.
(3)
Culverts, bridges, storm sewers or any other facilities
which must pass or convey flows from the tributary area must be designed
in accordance with DEP Chapter 105 regulations (if applicable) and,
at minimum, pass the increased twenty-five-year return period runoff.