A.
Applicants proposing regulated activities in Brodhead Creek Watershed which do not fall under the exemption criteria shown in § 425-20 shall submit a drainage plan consistent with the Brodhead Creek Watershed Stormwater Management Plan to the municipality for review. These criteria shall apply to the total proposed development even if development is to take place in stages.
B.
The applicant is required to perform an alternatives analysis to
find practicable alternatives to the surface discharge of stormwater,
the creation of impervious surfaces and the degradation of waters
of the commonwealth, and must maintain as much as possible the natural
hydrologic regime.
C.
The drainage plan must be designed through an alternatives analysis consistent with the sequencing provisions of § 425-11 to ensure maintenance of the natural hydrologic regime and to promote groundwater recharge and protect groundwater and surface water quality and quantity. The drainage plan designer must proceed sequentially in accordance with Article III of this chapter and comply with construction and maintenance responsibilities in accordance with Article VII of this chapter.
D.
Stormwater drainage systems shall be provided in order to permit
unimpeded flow along natural watercourses, except as modified by stormwater
management facilities or open channels consistent with this chapter.
E.
The existing points of concentrated drainage that discharge onto
adjacent property shall not be altered in any manner which could cause
property damage without permission of the affected property applicant(s)
and shall be subject to any applicable discharge criteria specified
in this chapter.
F.
Areas of existing diffused drainage discharge shall be subject to
any applicable discharge criteria in the general direction of existing
discharge, whether proposed to be concentrated or maintained as diffused
drainage areas, except as otherwise provided by this chapter. If diffused
drainage discharge is proposed to be concentrated and discharged onto
adjacent property, the applicant must document that adequate downstream
conveyance facilities exist to safely transport the concentrated discharge,
or otherwise prove that no erosion, sedimentation, flooding or other
impacts will result from the concentrated discharge.
G.
Where a development site is traversed by existing watercourses, drainage easements shall be provided conforming to the line of such watercourses. The terms of the easement shall conform to the stream buffer requirements contained in § 425-12I(7) of this chapter.
I.
Any stormwater management facilities regulated by this chapter that
would be located in or adjacent to waters of the commonwealth or wetlands
shall be subject to approval by PaDEP through the joint permit application
process, or, where deemed appropriate by PaDEP, the general permit
process. When there is a question whether wetlands may be involved,
it is the responsibility of the applicant or his agent to show that
the land in question cannot be classified as wetlands; otherwise,
approval to work in the area must be obtained from PaDEP.
J.
Any stormwater management facilities regulated by this chapter that
would be located on state highway rights-of-way shall be subject to
approval by the Pennsylvania Department of Transportation (PennDOT).
K.
Infiltration of runoff through seepage beds, infiltration trenches, etc., where soil conditions permit, and minimization of impervious surfaces to the extent permitted by Chapter 525, Zoning, are encouraged to reduce the size or eliminate the need for detention facilities or other structural BMPs.
L.
Roof drains shall not be connected to streets, sanitary or storm
sewers or roadside ditches in order to promote overland flow and infiltration/percolation
of stormwater where advantageous to do so. Considering potential pollutant
loading, roof drain runoff in most cases will not require pretreatment.
A.
The design of all regulated activities shall include the following
steps in sequence to minimize stormwater impacts.
(1)
The applicant is required to find practicable alternatives to the
surface discharge of stormwater, the creation of impervious surfaces
and the degradation of waters of the commonwealth, and must maintain
as much as possible the natural hydrologic regime of the site.
(2)
An alternative is practicable if it is available and capable of being
done after taking into consideration cost, existing technology and
logistics in light of overall project purposes.
(3)
All practicable alternatives to the discharge of stormwater are presumed
to have less adverse impact on quantity and quality of waters of the
commonwealth unless otherwise demonstrated.
B.
The applicant shall demonstrate that they designed the regulated
activities in the following sequence to minimize the increases in
stormwater runoff and impacts to water quality:
(1)
Prepare an existing resource and site analysis map (ERSAM), showing environmentally sensitive areas including, but not limited to, steep slopes, ponds, lakes, streams, wetlands, hydric soils, vernal ponds, floodplains, buffer areas, hydrologic soil groups A and B (areas conducive to infiltration), any existing recharge areas and any other requirements outlined in chapter 450, Subdivision and Land Development..
(3)
Prepare a draft project layout avoiding earth disturbance in sensitive areas identified in § 425-11B(1) and minimizing total site earth disturbance as much as possible. The ratio of the disturbed area to the entire site area and measures taken to minimize earth disturbance shall be included on the ERSAM.
(4)
Identify site-specific predevelopment drainage areas, discharge points,
recharge areas to be preserved and hydrologic soil groups A and B
to be utilized for recharge.
(7)
Satisfy groundwater recharge (infiltration) objective (§ 425-13) and provide for stormwater treatment prior to infiltration.
(8)
Determine what management district the site falls into (Appendix
D of this chapter)[1] and conduct a predevelopment runoff analysis.
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
(9)
Prepare final project design to maintain predevelopment drainage
areas and discharge points, to minimize earth disturbance and impervious
surfaces, and to reduce runoff to the maximum extent possible.
(10)
Conduct a post-development runoff analysis based on the final design and to meet the release rate and, in turn, the overbank flow and extreme event requirements (§ 425-14).
(11)
Manage any remaining runoff through treatment prior to discharge,
as part of detention, bioretention, direct discharge or other structural
control.
In addition to the performance standards and design criteria
requirements of this chapter, the applicant shall comply with the
following water quality requirements of this article.
A.
For water quality and streambank erosion, the objective is to design
a water quality BMP to detain the proposed condition's two-year, twenty-four-hour
design storm to the existing condition's one-year flow using the NRCS
Type II distribution. Additionally, provisions shall be made (such
as adding a small orifice at the bottom of the outlet structure) so
that the proposed condition's one-year storm takes a minimum of 24
hours to drain from the facility from a point where the maximum volume
of water from the one-year storm is captured, (i.e., the maximum water
surface elevation is achieved in the facility.) At the same time,
the objective is not to attenuate the larger storms in no-detention
areas (District C) only. This can be accomplished by configuration
of the outlet structure not to control the larger storms, or by a
bypass or channel to divert only the two-year flood into the basin
or divert flows in excess of the two-year storm away from the basin.
Where practicable, wet basins shall be utilized for water quality
control and shall meet the requirements found in the PA Stormwater
BMP Manual, as revised. Release of water can begin at the start of
the storm (i.e., the invert of the water quality orifice is at the
invert of the facility). The design of the facility shall consider
and minimize the chances of clogging and sedimentation. Orifices smaller
than three inches diameter are not recommended. However, if the design
professional can provide proof that the smaller orifices are protected
from clogging by use of trash racks, etc., smaller orifices may be
permitted.
B.
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 depth to bedrock.
(4)
Seasonal high water table.
(5)
Proximity to building foundations and well heads.
(6)
Erodibility of soils.
(7)
Land availability and configuration of the topography.
(8)
Peak discharge and required volume control.
(9)
Stream bank erosion.
(10)
Efficiency of the BMPs to mitigate potential water quality problems.
(11)
The volume of runoff that will be effectively treated.
(12)
The nature of the pollutant being removed.
(13)
Maintenance requirements.
(14)
Creation/protection of aquatic and wildlife habitat.
(15)
Recreational value.
C.
For areas within defined special protection subwatersheds, which
includes exceptional value (EV) and high quality (HQ) waters, the
temperature and quality of water and streams shall be maintained through
the use of temperature-sensitive BMPs and stormwater conveyance systems.
D.
The applicant shall consider the guidelines found in the references
specified in Appendix G[1] for constructed wetlands, where proposed,
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
F.
Streambank restoration projects shall include the following:
(1)
No restoration or stabilization projects may be undertaken without
examining the fluvial geomorphology of stable reaches above and below
the unstable reach.
(2)
Restoration project design must then consider maintenance of stability
in the adjacent stable reaches of the stream channel.
(3)
An erosion and sediment control plan approved by the Conservation
District must be provided by the applicant.
(4)
All applicable state and federal permits must be obtained.
G.
Biology shall be incorporated into the design of all wet basins in
accordance with the West Nile virus guidance found in Appendix E.[2]
[2]
Editor's Note: Said appendix is included as an attachment to this chapter.
H.
To accomplish the above, the applicant shall 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 (best management practices).
I.
Buffers.
(2)
Where resource buffers overlap, the more restrictive requirements
shall apply.
(3)
Preexisting lots or parcels/development in outer buffers. In the case of legally preexisting lots or parcels (approved prior to the effective date of this chapter) where the usable area of a lot or parcel lies within an outer buffer area, rendering the lot or parcel unable to be developed in accordance with the allowable use per municipal zoning, the development may only be permitted by variance as provided in § 525-57 of Chapter 525, Zoning.
(4)
Improvements to existing structures in outer buffers. The provisions of this § 425-12I do not require any changes or improvements to be made to lawfully existing structures in buffers. However, when any substantial improvement to a structure is proposed which results in a horizontal expansion of that structure, the improvement may only be permitted by variance as provided in § 525-57 of Chapter 525, Zoning.
(5)
Wetlands and vernal ponds.
(a)
Wetland identification. Wetlands shall be identified in accord
with the 1987 U.S. Army Corps of Engineers Manual for Identifying
and Delineating Wetlands, and properly flagged and surveyed on site
to ensure they are protected.
[1]
Wetlands in an artificial watercourse. Wetlands contained within
the banks of an artificial watercourse shall not to be considered
for buffer delineation purposes.
[2]
Wetlands in a natural watercourse. Wetlands contained within
the banks of a natural watercourse, only the stream buffer shall apply.
(b)
Wetland and vernal pond buffer delineation. A fifty-foot inner
buffer and 100-foot outer buffer, measured perpendicular to and horizontally
from the edge of the delineated wetland or vernal pond for a total
distance of 150 feet, shall be maintained for all wetlands and vernal
ponds.
[1]
Inner buffer. Measured perpendicular to and horizontally from
the edge of the delineated wetland or vernal pond, for a distance
of 50 feet.
[a]
Stormwater conveyance required by the Township,
buffer maintenance and restoration, the correction of hazardous conditions,
stream crossings permitted by DEP and passive unpaved stable trails
shall be permitted. No other earth disturbance, grading, filling,
buildings, structures, new construction, or development shall be permitted.
[b]
The area of the inner buffer altered by activities permitted in accord with § 425-12I(5)(b)[1] shall be minimized to the greatest extent practicable.
[2]
Outer buffer. Measured perpendicular to and horizontally from
the outer edge of the inner buffer for a distance of 100 feet, resulting
in a total buffer of 150 feet.
[a]
Stormwater conveyance required by the Township, buffer maintenance and restoration, the correction of hazardous conditions, stream crossings permitted by DEP, roads constructed to existing grade, unpaved trails, and limited forestry activities that do not clear cut the buffer (e.g., selective regeneration harvest) in accord with a forestry management plan shall be permitted, provided no buildings are involved, and those activities permitted under § 425-12I(3) and (4).
[b]
No more than 20% of the cumulative outer buffer on the subject parcel shall be altered by the activities permitted in accord with § 425-12I(5)(b)[2].
(6)
Lakes and ponds.
(a)
There is no outer buffer around lakes and ponds.
(b)
Lake and pond buffer delineation A fifty-foot buffer measured
perpendicular to and horizontally from the edge of any water body,
shall be maintained around any water body.
(c)
Permitted activities/development. Stormwater conveyance required
by the Township, buffer maintenance and restoration, the correction
of hazardous conditions, lakefront views, boat docks and unpaved trails
shall be permitted, provided no buildings are involved.
(d)
The area of the buffer impacted by activities permitted in § 425-12I(6)(c) shall not exceed 35% of the buffer on the subject parcel.
(7)
Streams.
(a)
Stream buffer delineation. A fifty-foot inner buffer and 100-foot
outer buffer, measured perpendicular to and horizontally from the
top-of-bank on both sides of any stream for a total distance of 150
feet, shall be maintained on both sides of any stream. See Figure
425-12.1.
[1]
Inner buffer. Measured perpendicular to and horizontally from
the top-of-bank of the stream for a distance of 50 feet.
[a]
Stormwater conveyance required by the Township,
buffer maintenance and restoration, the correction of hazardous conditions,
stream crossings permitted by DEP, wildlife sanctuaries and boat launch
sites constructed so as not to alter the floodplain cross section,
and unpaved trails shall be permitted, providing no buildings are
involved. No other earth disturbance, grading, filling, buildings,
structures, new construction, or development shall be permitted.
[b]
The area of the inner buffer altered by activities permitted in accord with § 425-12I(7)(a)[1] shall be minimized to the greatest extent practicable.
[2]
Outer buffer. Measured perpendicular to and horizontally from
the outer edge of the inner buffer for a distance of 100 feet resulting
in a total buffer of 150 feet.
[a]
Stormwater conveyance required by the Township,
buffer maintenance and restoration, the correction of hazardous conditions,
wildlife sanctuaries, boat launch sites, roads constructed to existing
grade, stream crossings permitted by DEP and unpaved trails and limited
forestry activities that do not clear-cut the buffer (e.g., selective
regeneration harvest) in accord with a forestry management plan shall
be permitted, provided no buildings are involved.
[b]
In areas of the outer buffer which are not wetlands,
vernal ponds or slopes of more than 15%, stormwater management facilities
which improve water quality of stormwater discharge shall be permitted
unless prohibited by other Township or state requirements. No other
earth disturbance, grading, filling buildings, structures, new construction,
or development shall be permitted.
[c]
No more than 20% of the cumulative outer buffer on the subject parcel shall be altered by the activities permitted in accord with § 425-12I(7)(a)[2].
 |
|
Figure 425-12.1 Stream Buffer
|
Maximizing the groundwater recharge capacity of the area being developed is required. 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 disturbed or impervious surface is created. It is recommended that roof runoff be directed to infiltration BMPs which may be designed to compensate for the runoff from parking areas. These measures are required to be consistent with § 425-2, and take advantage of utilizing any existing recharge areas.
A.
Infiltration BMPs shall meet the following minimum requirements:
(1)
Maximum infiltration requirements:
(a)
Regulated activities will be required to recharge (infiltrate), where practicable, a portion of the runoff created by the development as part of an overall stormwater management plan designed for the site. The volume of runoff to be recharged shall be determined from § 425-13A(2)(a) or (b), depending upon demonstrated site conditions.
(2)
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 depth of 24 inches between the bottom of the BMP and
the limiting zone.
(b)
An infiltration and/or percolation rate sufficient to accept
the additional stormwater load and drain completely as determined
by field tests conducted by the applicant's design professional.
(c)
The recharge facility shall be capable of completely infiltrating
the recharge volume within four days.
(3)
The size of the recharge facility shall be based upon the following
volume criteria:
(a)
NRCS Curve Number equation.
[1]
The NRCS runoff shall be utilized to calculate infiltration
requirements (P) in inches.
|
Eqn: 425-13.1
|
|
For zero runoff: P = I (Infiltration) (in.) = (200/CN) 2
|
|
Where:
|
|
CN-SCS (NRCS) curve number of existing conditions contributing
to the recharge facility.
|
[2]
This equation is displayed graphically in, and the infiltration
requirement can be determined from Figure 425-13.1.
[3]
The recharge volume required would therefore be computed as:
|
Eqn: 425-13.2
| |
|
Rev=I*% impervious area
| |
|
Where: I- infiltration requirements (in.)
|
[4]
Figure 425-13.1, Infiltration requirement based upon NRCS Curve
Number.
 |
(b)
Annual recharge - water budget approach.
[1]
It has been determined that infiltrating 0.6 inches of runoff from the impervious areas will aid in maintaining the hydrologic regime of the watershed. If the goals of § 425-13A(2)(a) cannot be achieved, then 0.6 inches of rainfall shall be infiltrated from all impervious areas, up to an existing site condition curve number of 77. Above a curve number of 77, Equation 425-13.1 or the curve in Figure 425-13.1 should be used to determine the Infiltration requirement.
[2]
The recharge volume (Rev) required would
therefore be computed as:
|
Rev=0.6 or I, whichever is less * % impervious
area.
|
B.
Soils. A detailed soils evaluation of the project site shall be required
where practicable to determine the suitability of recharge facilities.
The evaluation shall be performed by a qualified design professional,
and at a 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 watershed to determine general areas of suitability for
infiltration practices.
(2)
Provide site-specific infiltration test results (at the level of
the proposed infiltration surface) in accord with ASTM Guide No. D
5126 to determine the appropriate hydraulic conductivity rate.
(3)
Design the infiltration structure for the required storm volume 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
design professional, it must be demonstrated to the municipality that
the soils are conducive to infiltrate on the lots identified.
C.
Stormwater hot spots. A stormwater hot spot is defined as a land
use activity that generates higher concentrations of hydrocarbons,
trace metals or toxicants than are found in typical stormwater runoff,
based on monitoring studies. Table 425-13.1 provides samples of designated
hot spots. If a site is designated as a hot spot, it has important
implications for how stormwater is managed. First and foremost, untreated
stormwater runoff from hot spots cannot be allowed to infiltrate into
groundwater where it may contaminate water supplies. Therefore, the
Rev requirement is not applied to development
sites that fit into the hot spot category (the entire WQ, must still
be treated). Second, a greater level of stormwater treatment may be
needed at hot spot sites to prevent pollutant washoff after construction.
EPA's NPDES stormwater program requires some industrial sites to prepare
and implement a stormwater pollution prevention plan.
|
Table 425-13.1 - Classification of Stormwater Hot Spots
| |
|---|---|
|
The following land uses and activities are samples of stormwater
hot spots:
| |
|
•
|
Vehicle salvage yards and recycling facilities
|
|
•
|
Fleet storage areas (bus, truck, etc.)
|
|
•
|
Public works storage areas
|
|
•
|
Facilities that generate or store hazardous materials
|
(1)
Extreme caution shall be exercised where salt or chloride would be
a pollutant since soils do little to filter this pollutant and it
may contaminate the groundwater. The qualified design professional
shall evaluate the possibility of groundwater contamination from the
proposed infiltration/recharge facility and perform a hydrogeologic
justification study if necessary. The infiltration requirement in
high quality/exceptional value waters shall be subject to the Department's
Chapter 93 antidegradation regulations.[1] The municipality may require the installation of an impermeable
liner in detention basins where the possibility of groundwater contamination
exists. A detailed hydrogeologic investigation may be required by
the municipality.
[1]
Editor's Note: See 25 Pa. Code Chapter 93.
(2)
The municipality shall require the applicant to provide safeguards
against groundwater contamination for uses which may cause groundwater
contamination, should there be a mishap or spill. (See Appendix E.)[2]
[2]
Editor's Note: Said appendix is included as an attachment to this chapter.
D.
Extreme caution shall be exercised where infiltration is proposed
in source water protection areas or that may affect a wellhead or
surface water intake.
E.
Recharge/infiltration facilities shall be used in conjunction with
other innovative or traditional BMPs, stormwater control facilities,
and nonstructural stormwater management alternatives.
A.
The Brodhead/McMichaels Watershed has been divided into stormwater
management districts as shown on the Watershed Map in Appendix D.[1]
(1)
Standards for managing runoff from each subarea in the Brodhead/McMichaels
Watershed for design storms are shown in Table 425-14.1 Development
sites located in each of the A, B, or C Districts must control proposed
conditions runoff rates to existing conditions runoff rates for the
design storms in accord with Table 425-14.1.
(2)
In addition to the requirements specified in Table 425-14.1 below, the water quality and streambank erosion (§ 425-12), groundwater recharge (§ 425-13), and erosion control (§ 425-17) requirements shall be implemented.
|
Table 425-14.1 - Water Quantity Requirements
| |||
|---|---|---|---|
|
District
|
Proposed conditions
|
(reduce to)
|
Existing conditions
|
|
A
|
2-year
|
1-year
| |
|
5-year
|
5-year
| ||
|
10-year
|
10-year
| ||
|
25-year
|
25-year
| ||
|
50-year
|
50-year
| ||
|
100-year
|
100-year
| ||
|
B-1
|
2-year
|
1-year
| |
|
5-year
|
2-year
| ||
|
10-year
|
5-year
| ||
|
25-year
|
10-year
| ||
|
50-year
|
25-year
| ||
|
100-year
|
100-year
| ||
|
B-2
|
2-year
|
1-year
| |
|
5-year
|
2-year
| ||
|
25-year
|
5-year
| ||
|
50-year
|
10-year
| ||
|
100-year
|
50-year
| ||
|
B-3
|
50-year
|
10-year
| |
|
100-year
|
50-year
| ||
|
C
|
Provisional Direct Discharge District - Development sites which can discharge directly to the main channel or major tributaries or indirectly to the main channel through an existing stormwater drainage system (i.e., storm sewer or tributary) which meets the "Downstream Hydraulic Capacity Analysis" in § 425-14H and is shown by the design professional to not cause a downstream problem, may allow an increase in flow as long as no downstream harm is demonstrated. However, sites in District C shall comply with the criteria for water quality and streambank erosion (§ 425-12); and groundwater recharge (§ 425-13). If the proposed conditions runoff is intended to be conveyed by an existing stormwater drainage system to the main channel, assurance must be provided that such system has adequate capacity to convey the increased peak flows or will be provided with improvements to furnish the required capacity. When adequate capacity of the downstream system does not exist and will not be provided through improvements, the proposed conditions peak rate of runoff must be controlled to the existing conditions peak rate as required in District A provisions (i.e., ten-year proposed conditions flows to ten-year existing conditions flows) for the specified design storms.
| ||
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
B.
General. Proposed conditions rates of runoff from any regulated activity
shall not exceed the peak release rates of runoff prior to development
for the design storms specified on the Stormwater Management District
Watershed Map (Ordinance Appendix D)[2] and § 425-11, of this chapter.
[2]
Editor's Note: Said appendix is included as an attachment to this chapter.
C.
District boundaries. The boundaries of the stormwater management
districts are shown on an Official Map that is available for inspection
at the municipal office. A copy of the Official Map at a reduced scale
is included in the Ordinance Appendix D.[3] The exact location of the stormwater management district
boundaries as they apply to a given development site shall be determined
by mapping the boundaries using the two-foot topographic contours
(or most accurate data required) provided as part of the drainage
plan.
[3]
Editor's Note: Said appendix is included as an attachment to this chapter.
D.
Sites located in more than one district. For a proposed development site located within two or more stormwater management district category subareas, the peak discharge rate from any subarea shall be the existing conditions peak discharge for that subarea as indicated in § 425-11. The calculated peak discharges shall apply regardless of whether the grading plan changes the drainage area by subarea. An exception to the above may be granted by the municipalities if discharges from multiple subareas recombine in proximity to the site. In this case, peak discharge in any direction may be a 100% release rate, provided that the overall site discharge meets the weighted average release rate.
E.
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.
F.
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.
G.
No-harm option. For any proposed development site not located in a Provisional Direct Discharge District, the applicant has the option of using a less restrictive runoff control (including no detention) if the applicant can prove that no harm would be caused by discharging at a higher runoff rate than that specified by the stormwater management plan. The no-harm option is used when an applicant can prove that the proposed hydrographs can match existing hydrographs, or if it can be proved that the proposed conditions will not cause increases in peaks at all points downstream. Proof of no harm must be shown based upon the following "downstream impact evaluation" which shall include a "downstream hydraulic capacity analysis" consistent with Subsection H to determine if adequate hydraulic capacity exists. The applicant shall submit to the municipality this evaluation of the impacts due to increased downstream stormwater flows in the watershed.
(1)
The hydrologic regime of the site must be maintained.
(2)
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 stream flow or any stream channel
section, established with the concurrence of the municipality.
(3)
The evaluation shall continue downstream until the increase in flow
diminishes due to additional flow from tributaries and/or stream attenuation.
(4)
The peak flow values to be used for downstream areas for the design
return period storms (two-, five-, ten-, twenty-five-, fifty-, and
100-year) shall be the values from the calibrated model for the Brodhead/McMichaels
Watershed. These flow values can be obtained from the original Act
167 Watershed Stormwater Management Plans.
(5)
Applicant-proposed conditions runoff controls which 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 Subsection H.
(6)
A financial distress shall not constitute grounds for the municipality
to approve the use of the no-harm option.
(7)
Downstream capacity improvements may be provided as necessary to
achieve the no-harm option.
H.
"Downstream hydraulic capacity analysis." Any downstream hydraulic
capacity analysis conducted in accordance with this chapter shall
use the following criteria for determining adequacy for accepting
increased peak flow rates:
(1)
Existing 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)
Existing 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[4] (if applicable) and, at minimum, pass the increased twenty-five-year-return-period
runoff.
[4]
Editor's Note: See 25 Pa. Code Chapter 105.
I.
Hardship option. The Stormwater Management Plan and its standards
and criteria are designed to maintain existing conditions peak flows
and volumes throughout the Brodhead/McMichaels watershed as the watershed
becomes developed. There may be certain instances, however, where
the standards and criteria established are too restrictive for a particular
applicant. The existing drainage network in some areas may be capable
of safely transporting slight increases in flows without causing a
problem or increasing flows elsewhere. If an applicant cannot meet
the stormwater standards due to lot conditions or if conformance would
become a hardship to an applicant, the hardship option may be applied.
A financial distress shall not constitute grounds for the municipality
to approve the use of the hardship option. The applicant would have
to plead his/her case to the governing body with the final determination
made by the municipality. Any applicant's pleading the hardship option
will assume all liabilities that may arise due to exercising this
option. A financial distress shall not constitute grounds for the
municipality to approve the use of the no-harm option.
A.
Stormwater runoff from all development sites with a drainage area
of greater than 200 acres shall be calculated using a generally accepted
calculation technique that is based on the NRCS soil cover complex
method. Table 425-15.1 summarizes acceptable computation methods and
the method selected by the design professional 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
200 acres. The Soil Cover Complex Method shall be used for drainage
areas greater than 200 acres.
|
Table 425-15.1
Acceptable Computation Methodologies For Stormwater Management
Plans
| ||
|---|---|---|
|
Method
|
Method Developed by
|
Applicability
|
|
TR-20 (or commercial computer package based on TR-20)
|
USDA NRCS
|
Applicable where use of full hydrology computer model is desirable
or necessary
|
|
TR-55 (or commercial computer package based on TR-55)
|
USDA NRCS
|
Applicable for land development plans within limitations described
in TR-55
|
|
HEC-1/HEC-HMS
|
US Army Corps of Engineers
|
Applicable where use of full hydrologic computer model is desirable
or necessary
|
|
PSRM
|
Penn State University
|
Applicable where use of a hydrologic computer model is desirable
or necessary; simpler than TR-20 or HEC-1
|
|
Rational Method (or commercial computer package based on Rational
Method)
|
Emil Kuichling (1889)
|
For sites less than 200 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
|
B.
All calculations consistent with this chapter using the soil cover
complex method shall use the appropriate design rainfall depths for
the various return period storms according to the region in which
they are located as presented in Table 13-1 in Appendix B of this
chapter.[1] If a hydrologic computer model such as PSRM or HEC-I is
used for stormwater runoff calculations, then the duration of rainfall
shall be 24 hours. The NRCS S-curve shown in Figure B-1, Appendix
B of this chapter shall be used for the rainfall distribution.
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
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, as listed in Table B-2 or B-3 in Appendix B of
this chapter.[2]
[2]
Editor's Note: Said appendix is included as an attachment to this chapter.
D.
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) (Figures B-2 to B-4).
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.
E.
Runoff Curve Numbers (CN) for both existing and proposed conditions
to be used in the soil cover complex method shall be obtained from
Table B-2 in Appendix B of this chapter.[3]
[3]
Editor's Note: Said appendix is included as an attachment to this chapter.
F.
Runoff coefficients (c) for both existing and proposed conditions
for use in the Rational method shall be obtained from Table B-3 in
Appendix B of this chapter.[4]
[4]
Editor's Note: Said appendix is included as an attachment to this chapter.
G.
The designer shall consider that the runoff from proposed sites graded
to the subsoil will not have the same runoff conditions as the site
under existing conditions, even after topsoiling or seeding. The designer
may increase his proposed condition "CN" or "e" to better reflect
proposed soil conditions.
H.
Where uniform flow is anticipated, the Manning equation shall be
used for hydraulic computations, and to determine the capacity of
open channels, pipes, and storm sewers. Values for Manning's roughness
coefficient (n) shall be consistent with Table B-4 in Appendix B of
this chapter.[5]
[5]
Editor's Note: Said appendix is included as an attachment to this chapter.
I.
Outlet structures for stormwater management facilities shall be designed
to meet the performance standards of this chapter using any generally
accepted hydraulic analysis technique or method.
J.
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 200 acres in size, the design
storm hydrograph shall be computed using a calculation method that
produces a full hydrograph. The municipality 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.
A.
Any stormwater facility located on state highway rights-of-way shall
be subject to approval by the Pennsylvania Department of Transportation
(PennDOT).
C.
All wet basin designs shall incorporate biologic controls consistent
with the West Nile guidance found in Appendix E.[1]
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
D.
Any stormwater management facility (i.e., BMP, detention basin) designed
to store runoff and requiring a berm or earthen embankment required
or regulated by this chapter shall be designed to provide an emergency
spillway to handle flow up to and including the 100-year proposed
conditions. The height of embankment must provide a minimum 1.0 foot
of freeboard above the maximum pool elevation computed when the facility
functions for the 100-year proposed conditions inflow. Should any
stormwater management facility require a dam safety permit under PaDEP
Chapter 105,[2] the facility shall be designed in accordance with Chapter
105 and meet the regulations of Chapter 105 concerning dam safety
which may be required to pass storms larger than the 100-year event.
[2]
Editor's Note: See 25 Pa. Code Chapter 105.
E.
Any facilities that constitute water obstructions (e.g., culverts,
bridges, outfalls, or stream enclosures), and any work involving wetlands
governed by PaDEP Chapter 105 regulations[3] (as amended or replaced from time to time by PaDEP), shall
be designed in accordance with Chapter 105 and will require a permit
from PaDEP.
[3]
Editor's Note: See 25 Pa. Code Chapter 105.
F.
Any other drainage conveyance facility that does not fall under Chapter
105 regulations[4] must be able to convey, without damage to the drainage
structure or roadway, runoff from the twenty-five-year design storm
with a minimum 1.0 foot of freeboard measured below the lowest point
along the top of the roadway. Any facility that constitutes a dam
as defined in PaDEP Chapter 105 regulations may require a permit under
dam safety regulations. Any facility located within a PennDOT right-of-way
must meet PennDOT minimum design standards and permit submission requirements.
[4]
Editor's Note: See 25 Pa. Code Chapter 105.
G.
Any drainage conveyance facility and/or channel not governed by Chapter
105 regulations[5] must be able to convey, without damage to the drainage
structure or roadway, runoff from the twenty-five-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. Roadway crossings located within designated
floodplain areas must be able to convey runoff from a 100-year design
storm. Any facility located within a PennDOT right-of-way must meet
PennDOT minimum design standards and permit submission requirements.
[5]
Editor's Note: See 25 Pa. Code Chapter 105.
H.
Storm sewers must be able to convey proposed conditions runoff from
a twenty-five-year design storm without surcharging inlets, where
appropriate.
I.
Adequate erosion protection shall be provided along all open channels,
and at all points of discharge.
J.
The design of all stormwater management facilities shall incorporate
sound engineering principles and practices. The municipality reserves
the right to disapprove any design that would result in the construction
of or continuation of a stormwater problem area.
A.
Any earth disturbance must be conducted in conformance with 25 Pa
Code Chapter 102, Erosion and Sediment Control.
B.
Additional erosion and sediment control design standards and criteria
that must be or are recommended to be applied where infiltration BMPs
are proposed 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 contributory drainage area to the infiltration BMP has
achieved final stabilization.
A.
Submissions. All regulated activities shall submit a consumptive
use tracking report (CUTR) which shall be developed in accordance
with Appendix F[1] as follows:
(1)
Residential development or redevelopment. The CUTR shall be submitted
to the Monroe County Conservation District along with the erosion
and sedimentation control plan.
(2)
Commercial/industrial development — redevelopment. The CUTR
shall be submitted to the municipality during the preliminary plan
approval process. The CUTR shall be forwarded by the municipality
to Monroe County Conservation District along with a copy of the occupancy
permit.
[1]
Editor's Note: Said appendix is included as an attachment to this chapter.
