A.
Applicants proposing regulated activities in Brodhead/McMichaels Creek Watershed which do not fall under the exemption criteria shown in § 92-18 shall submit a drainage plan consistent with the Brodhead/McMichaels 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 § 92-9 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.
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 owner(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 § 92-10I 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 the municipality's
Zoning Ordinance 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 he or she 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 the municipal Subdivision and
Land Development Ordinance.[1]
(3)
Prepare a draft project layout avoiding earth disturbance in sensitive areas identified in Subsection B(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 (§ 92-11) and provide for stormwater treatment prior to infiltration.
(8)
Determine what management district the site falls
into (Appendix D[2]) and conduct a predevelopment runoff analysis.
[2]
Editor's Note: Appendix D is included at the end of 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 postdevelopment runoff analysis based on the final design and to meet the release rate and in turn the overbank flow and extreme event requirements (§ 92-12).
(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.
Requirements.
(1)
For water quality and streambank erosion, the objective
is to design a water quality BMP to detain the proposed conditions
two-year, twenty-four-hour design storm 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) so that the proposed conditions 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.
(2)
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.
(3)
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)
Streambank 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 include 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.
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: Appendix E is included at the end of 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.
(1)
A project designer shall provide a standard buffer for streams, wetlands, vernal ponds and lakes in § 92-10I(2).
(a)
Where resource buffers overlap, the more restrictive
requirements shall apply.
(b)
Preexisting lots or parcels/land development plan in outer buffers. In the case of legally preexisting lots or parcels or land development plans (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 modification as provided in § 98-38 of the municipality's SALDO.
(c)
Improvements to existing structures in outer buffers. The provisions of this section 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 modification as provided in § 98-38 of the municipality's SALDO.
(2)
Standard buffers for water quality.
(a)
Wetlands and vernal ponds.
[1]
Wetland identification. Wetlands shall be identified
in accord with the 1987 U.S. Army Corps of Engineers Manual for Identifying
and Delineating Wetlands, as amended, and properly flagged and surveyed
on site.
[a]
Wetlands in an artificial watercourse. Wetlands
contained within the banks of an artificial watercourse shall not
to be considered for buffer delineation purposes.
[b]
Wetlands in a natural watercourse. Where wetlands
are contained within the banks of a natural watercourse, only the
stream buffer shall apply.
[2]
Wetland and vernal pond buffer delineation.
A fifty-foot inner buffer and one-hundred-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.
[a]
Inner buffer: measured perpendicular to and
horizontally from the edge of the delineated wetland or vernal pond,
for a distance of 50 feet.
[i]
Stormwater conveyance required
by the Township, buffer maintenance and restoration, the correction
of hazardous conditions, wetland 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.
[ii]
The area of the inner buffer altered
by activities permitted in accord with § 92-10I(2)(a)[2][a][i]
shall be minimized to the greatest extent practicable.
[b]
Outer buffer: measured perpendicular to and
horizontally from the outer edge of the inner buffer for a distance
of 100 feet.
[i]
Provided no buildings are constructed,
stormwater conveyance required by the Township, buffer maintenance
and restoration, the correction of hazardous conditions, wetland crossings
permitted by DEP, parking lots constructed to existing grade, roads
constructed to existing grade, stable 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.
[ii]
No more than 20% of the cumulative
outer buffer on the subject parcel shall be altered by the activities
permitted in accord with § 92-10I(2)(a)[2][b][i] above.
(b)
Lakes and ponds.
[1]
There is no outer buffer around lakes and ponds.
[2]
Lake and pond buffer delineation. A fifty-foot
buffer measured perpendicular to and horizontally from the edge of
any lake or pond shall be maintained.
[3]
Permitted activities/development. Stormwater
conveyance required by the Township, buffer maintenance and restoration,
the correction of hazardous conditions, lake front views, boat docks
and passive unpaved stable trails shall be permitted provided no buildings
are involved.
[4]
No more than 35% of the cumulative pond or lake buffer on the subject parcel shall be altered by the activities permitted in accord with § 92-10I(2)(b)[3] above.
(c)
Streams.
[1]
Stream buffer delineation. A fifty-foot inner
buffer and one-hundred-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. See Figure
1 below.
Figure 1. Stream Buffer
|
[a]
Inner buffer: measured perpendicular to and
horizontally from the top-of-bank of the stream for a distance of
50 feet.
[i]
Stormwater conveyance required
by the Township, buffer maintenance and restoration, the correction
of hazardous conditions, stream crossings permitted by DEP, fish hatcheries,
wildlife sanctuaries and boat launch sites constructed so as not to
alter the floodplain cross-section, and passive unpaved stable trails
shall be permitted provided no buildings are constructed. No other
earth disturbance, grading, filling, buildings, structures, new construction,
or development shall be permitted.
[ii]
The area of the inner buffer altered
by activities permitted in accord with § 92-10I(2)(c)[1][a][i]
shall be minimized to the greatest extent practicable.
[b]
Outer buffer: measured perpendicular to and
horizontally from the outer edge of the inner buffer for a distance
of 100 feet.
[i]
Provided no buildings are constructed,
stormwater conveyance required by the Township, buffer maintenance
and restoration, the correction of hazardous conditions, agricultural
activities, plant nurseries, parking lots constructed to existing
grade, temporary fairs and carnivals, accessory uses for residential
purposes, private sportsmen's club activities, athletic facilities,
orchards, wildlife sanctuaries, boat launch sites that do not alter
the floodplain cross-section, fish hatcheries, stream crossings permitted
by DEP and passive unpaved stable 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.
[ii]
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.
[iii]
No more than 20% of the cumulative
outer buffer on the subject parcel shall be altered by the activities
permitted in accord with § 92-10I(2)(c)[1][b][i] above.
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 § 92-2, 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 Subsection A(3)(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. This equation is displayed
graphically in, and the infiltration requirement can be determined
from Figure 2 below.
|
For zero runoff: P = I (Infiltration) (inches)
= (200 / CN) – 2
|
Eqn: 304.1
|
|
Where: CN = SCS (NRCS) curve number of existing
conditions contributing to the recharge facility.
|
[2]
The recharge volume required would therefore
be computed as:
|
Rev (cubic feet) = I
* impervious area (square feet) / 12
|
Eqn: 304.2
|
|
Where: I = infiltration requirements (inches)
|
 Figure 2. Infiltration requirement based
upon NRCS Curve Number
|
(b)
Annual recharge: water budget approach. 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. Areas in the Township that fall outside of the Broadhead/McMichaels watershed will need to meet a two-year, twenty-four-hour net infiltration rate. If the goals of Subsection A(2)(a) cannot be achieved, then 0.6 inch 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 304.1 or the curve in Figure 2 should be used to determine the Infiltration requirement. 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 hotspots.
(1)
A "stormwater hotspot" is defined as a land use activity
that generates higher concentrations of hydrocarbons, trace metals
or toxic materials than are found in typical stormwater runoff, based
on monitoring studies. The list below provides samples of designated
hotspots. If a site is designated as a hotspot, it has important implications
for how stormwater is managed. First and foremost, untreated stormwater
runoff from hotspots 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 hotspot category (the entire WQv must still be treated). Second, a greater level of stormwater treatment
may be needed at hotspot 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.
(a)
Classification of stormwater hotspots. The following
land uses and activities are samples of stormwater hotspots:
(2)
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. 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.
(3)
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 of the complete Act 167 Study.)
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] Standards for managing runoff from each subarea in the Brodhead/McMichaels Watershed for design storms are shown in the table below. 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 the table. In addition to the requirements specified in the table below, the water quality and streambank erosion (§ 92-10), groundwater recharge (§ 92-11), and erosion control (§ 92-15) requirements shall be implemented.
|
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 § 92-12H 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 (§ 92-10); and groundwater recharge (§ 92-11). 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: Appendix D is included at the end of 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 (Appendix D[2]) and § 92-9, of this chapter.
[2]
Editor's Note: Appendix D is included at the end of 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 Appendix D. 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.
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 § 92-9. 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 one-hundred-percent 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 § 92-12H 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 streamflow 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 one-hundred-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 § 92-12H.
(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 (if applicable) and,
at minimum, pass the increased twenty-five-year return period runoff.
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. The table below summarizes acceptable computation
methods; 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.
|
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
|
U.S. 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 B-1 in Appendix B
of this chapter.[1] If a hydrologic computer model such as PSRM or HEC-1 is
used for stormwater runoff calculations, then the duration of rainfall
shall be 24 hours. The SCS 'S' curve shown in Figure B-1, Appendix
B of this chapter shall be used for the rainfall distribution.
[1]
Editor's Note: Appendix B is included at the end of 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.
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.
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.
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 c 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.
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: Appendix E is included at the end of 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 one-hundred-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 one-hundred-year proposed conditions
inflow. Should any stormwater management facility require a dam safety
permit under PaDEP Chapter 105, 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 one-hundred-year event.
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
(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.
F.
Any other drainage-conveyance facility that does not
fall under Chapter 105 regulations must be able to convey, without
damage to the drainage structure or roadway, runoff from the twenty-five-year
design storm with a minimum 1.0 foot of freeboard measured below the
lowest point along the top of the roadway. 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.
G.
Any drainage conveyance facility and/or channel not
governed by Chapter 105 regulations 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 one-hundred-year design storm. Any facility located
within a PennDOT right-of-way must meet PennDOT minimum design standards
and permit submission requirements.
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 PA Title 25, 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 Monroe County Conservation District
by the applicant along with a copy of the occupancy permit.
[1]
Editor's Note: Appendix F is included at the end of this chapter.
