A.
Applicants proposing regulated activities in the municipality which do not fall under the exemption criteria shown in § 316-6 shall submit a drainage plan consistent with this chapter and the respective stormwater management plan to the municipality for review. The stormwater management criteria of this chapter shall apply to the total proposed development even if development is to take place in stages.
B.
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.
C.
The drainage plan must be designed consistent with the sequencing provisions of § 316-19 to ensure maintenance of the natural hydrologic regime, to promote groundwater recharge, and to protect groundwater and surface water quality and quantity. The drainage plan designer must proceed sequentially in accordance with Article IV of this chapter.
D.
Stormwater drainage systems shall be designed in order to permit
unimpeded flow along natural watercourses, except as modified by stormwater
management facilities or open channels consistent with this chapter.
E.
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, whether proposed to
be concentrated or maintained as diffused drainage areas, shall be
subject to any applicable discharge criteria in the general direction
of existing discharge, 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 streams, drainage easements shall be provided conforming to the line of such streams. The terms of the easement shall conform to the stream buffer requirements contained in § 316-21G of this chapter.
H.
Any stormwater management facilities regulated by this chapter that
would be located in or adjacent to waters of the commonwealth or delineated
wetlands shall be subject to approval by DEP through the joint permit
application or the environmental assessment approval process, or where
deemed appropriate, by the DEP general permit process. When there
is a question as to 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 DEP.
I.
Any proposed stormwater management facilities regulated by this chapter
that would be located on state highway rights-of-way shall be subject
to approval by PennDOT.
J.
Minimization of impervious surfaces and infiltration of runoff through
seepage beds, infiltration trenches, etc., is encouraged where soil
conditions permit in order to reduce the size or eliminate the need
for detention facilities or other structural BMPs.
K.
All stormwater runoff shall be pretreated for water quality prior
to discharge to surface water or groundwater.
L.
All regulated activities within the municipality shall be designed,
implemented, operated and maintained to meet the purposes of this
chapter through these two elements:
M.
No regulated earth disturbance activities within the municipality
shall commence until the requirements of this chapter are met.
O.
Operations and maintenance of permanent stormwater BMPs shall be addressed as required by Article VII.
P.
All BMPs used to meet the requirements of this chapter shall conform
to the state water quality requirements and any more stringent requirements
as set forth by the municipality.
R.
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's soils.
(3)
Slope and depth to bedrock.
(4)
Seasonal high-water table.
(5)
Proximity to building foundations and wellheads.
(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.
The following permit requirements may apply to certain regulated
earth disturbance activities and must be met prior to commencement
of regulated earth disturbance activities, as applicable:
A.
All regulated earth disturbance activities subject to permit requirements
by DEP under regulations at 25 Pa. Code Chapter 102.
C.
Any stormwater management facility that would be located in or adjacent to surface waters of the commonwealth, including wetlands, subject to permit by DEP under 25 Pa. Code Chapter 105.
D.
Any stormwater management facility that would be located on a state
highway right-of-way or require access from a state highway shall
be subject to approval by PennDOT.
A.
No regulated earth disturbance activities within the municipality
shall commence until the municipality receives an approval from the
Conservation District of an erosion and sediment control plan for
construction activities.
B.
DEP has regulations that require an erosion and sediment control
plan for any earth disturbance activity of 5,000 square feet or more,
under 25 Pa. Code § 102.4(b).
D.
Evidence of any necessary permit(s) for regulated earth disturbance activities from the appropriate DEP regional office or County Conservation District must be provided to the municipality. The issuance of an NPDES construction permit [or permit coverage under the state wide general permit (PAG-2)] satisfies the requirements of Subsection A.
E.
A copy of the erosion and sediment control plan and any required
permit, as required by DEP regulations, shall be available on the
project site at all times.
F.
Additional erosion and sediment control design standards and criteria
are recommended to be applied where infiltration BMPs are proposed.
They shall include the following:
(1)
Areas proposed for infiltration BMPs shall be protected from sedimentation
and compaction during the construction phase to maintain maximum infiltration
capacity.
(2)
Infiltration BMPs shall not be constructed nor receive runoff until
the entire drainage area contributory to the infiltration BMP has
achieved final stabilization.
A.
The design of all regulated activities shall include the following
to minimize stormwater impacts.
(1)
The applicant shall find practicable alternatives, such as those
listed in Appendix F, Table F-5,[1] 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.
[1]
Editor's Note: Appendix F is on file in the Borough offices.
(2)
An alternative is practicable if it is available and capable of implementation
after taking into consideration existing technology and logistics
in light of overall project purposes and other municipal requirements.
(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 the regulated activities were
designed in the following sequence. The goal of the sequence is to
minimize the increases in stormwater runoff and impacts to water quality
resulting from the proposed regulated activity.
(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 pools,
stream buffers and hydrologic soil groups. Land development, any existing
recharge areas and other requirements outlined in the municipal SALDO
shall also be included.
(4)
Identify site-specific existing conditions drainage areas, discharge
points, recharge areas and hydrologic soil groups A and B (areas conducive
to infiltration).
(6)
Satisfy the groundwater recharge (infiltration) objective (§ 316-20) and provide for stormwater pretreatment prior to infiltration.
(8)
Provide stream bank erosion protection in accordance with § 316-22, Stream bank erosion requirements.
(10)
Prepare final project design to maintain existing conditions
drainage areas and discharge points, to minimize earth disturbance
and impervious surfaces and, to the maximum extent possible, to ensure
that the remaining site development has no surface or point discharge.
(11)
Conduct a proposed conditions runoff analysis based on the final design that meets the management district requirements (§ 316-23).
(12)
Manage any remaining runoff prior to discharge through detention,
bioretention, direct discharge, or other structural control.
Maximizing the groundwater recharge capacity of the area being developed is required. Design of the infiltration facilities shall consider groundwater recharge to compensate for the reduction in the recharge that occurs when the ground surface is disturbed or impervious surface is created. It is recommended that roof runoff be directed to infiltration BMPs that may be designed to compensate for the runoff from parking areas. These measures are required to be consistent with § 316-3 and to take advantage of utilizing any existing recharge areas. Infiltration may not be feasible on every site due to site-specific limitations such as soil type. If it cannot be physically accomplished, then the design professional shall be responsible to show that this cannot be physically accomplished. If it can be physically accomplished, then the volume of runoff to be infiltrated shall be determined from Subsection A(2) depending on demonstrated site conditions and shall be the greater of the volumes.
A.
Infiltration BMPs shall meet the following minimum requirements:
(1)
Infiltration BMPs intended to receive runoff from developed areas
shall be selected based on suitability of soils and site conditions
and shall be constructed on soils that have the following characteristics:
(a)
A minimum depth of 24 inches between the bottom of the BMP and
the top of the limiting zone.
(b)
An infiltration rate sufficient to accept the additional stormwater
load and dewater completely, as determined by field tests conducted
by the applicant's design professional.
(c)
The infiltration facility shall be capable of completely infiltrating
the retention (infiltration) volume (Rev) within
four days (96 hours).
(d)
Pretreatment shall be provided prior to infiltration.
(2)
The size of the infiltration facility shall be based upon the following
volume criteria:
(a)
Net two-year volume approach. In HQ/EV watersheds, the retention
(infiltration) volume (Rev) to be captured
and infiltrated shall be the net two-year volume. The net two-year
volume shall be determined by plotting the two-year project site post-development
hydrograph, drawing a straight line from the point-of-inflection of
the rising limb of the hydrograph to the predevelopment two-year storm
and measuring the volume under the curve as shown in the figure below.
|
(b)
One inch from impervious surface. In other portions of the watershed
that are not classified as HQ/EV, the retention (infiltration) volume
(Rev) will be equal to capturing one inch of
rainfall over all proposed impervious surfaces.
|
Rev = I * impervious area (square feet)
÷ 12 (inches) = cubic feet (cf)
| |
|---|---|
|
An asterisk (*) in equations denotes multiplication.
|
(c)
Obtain Rev volume:
[1]
Obtaining the Rev volume in Subsection A(2)(a) (above) may not be feasible on every site due to site-specific limitations such as soil type. If it cannot be physically accomplished, then the design professional shall be responsible for showing that this cannot be physically accomplished. If it cannot be physically accomplished, then the retention (infiltration) volume Rev required shall be as much as can be physically accomplished, with a minimum of 0.50 inch, depending on demonstrated site conditions. It has been determined that capturing and infiltrating 0.50 inch of runoff from the impervious areas will aid in maintaining the hydrologic regime (baseflow) of the watershed. If the goals of Subsection A(2)(a) or (b) cannot be achieved, then 0.50 inch of rainfall shall be retained and infiltrated from all impervious areas.
[2]
The minimum recharge volume (Rev) required
would, therefore, be computed as:
|
Rev = I * impervious area (square feet)
÷ 12 (inches) = cubic feet (cf)
| |
|---|---|
|
An asterisk (*) in equations denotes multiplication.
|
|
Where:
| ||||
|---|---|---|---|---|
|
I
|
=
|
The maximum equivalent infiltration amount (inches) that the
site can physically accept or 0.50 inch, whichever is greater.
| ||
[3]
The retention volume values derived from the methods in Subsection A(2)(a), (b) and/or (c) is the minimum volume the applicant must control through an infiltration BMP facility. However, if a site has areas of soils where additional volume of retention can be achieved, the applicant is encouraged to infiltrate as much of the stormwater runoff from the site as possible.
B.
Soils. A detailed soils evaluation of the project site shall be required
to determine the suitability of infiltration facilities. The evaluation
shall be performed by a qualified 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 site to determine general areas of suitability for infiltration
practices. In areas where development on fill material is under consideration,
conduct geotechnical investigations of subgrade stability; infiltration
may not be ruled out without conducting these tests.
(2)
Provide field tests such as double-ring infiltrometer or hydraulic
conductivity tests (at the level of the proposed infiltration surface)
to determine the appropriate hydraulic conductivity rate. Percolation
tests are not recommended for design purposes.
(3)
Design the infiltration structure for the required retention (Rev) 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.
(1)
Below is a list of examples 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 shall not be allowed to recharge into groundwater where it may
contaminate water supplies. Therefore, the Rev requirement shall not be applied to development sites that fit into
the hot spot category (the entire WQv must
still be treated). Second, a greater level of stormwater treatment
shall be considered at hot spot sites to prevent pollutant washoff
after construction. The Environmental Protection Agency's (EPA)
NPDES stormwater program requires some industrial sites to prepare
and implement a stormwater pollution prevention plan.
(2)
Examples of hot spots:
(a)
Vehicle salvage yards and recycling facilities.
(b)
Vehicle fueling stations.
(c)
Vehicle service and maintenance facilities.
(d)
Vehicle and equipment cleaning facilities.
(e)
Fleet storage areas (bus, truck, etc.).
(f)
Industrial sites based on Standard Industrial Codes.
(g)
Marinas (service and maintenance).
(h)
Outdoor liquid container storage.
(i)
Outdoor loading/unloading facilities.
(j)
Public works storage areas.
(k)
Facilities that generate or store hazardous materials.
(l)
Commercial container nursery.
(m)
Other land uses and activities as designated by an appropriate
review authority.
(3)
The following land uses and activities are not normally considered
hot spots:
(4)
While large highways [average daily traffic volume (ADT) greater
than 30,000] are not designated as stormwater hot spots, it is important
to ensure that highway stormwater management plans adequately protect
groundwater.
D.
Extreme caution shall be exercised where infiltration is proposed
in SWPAs as defined by the local municipality or water authority.
E.
Infiltration facilities shall be used in conjunction with other innovative
or traditional BMPs, stormwater control facilities and nonstructural
stormwater management alternatives.
F.
Extreme caution shall be exercised where salt or chloride (municipal
salt storage) 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 facility and perform
a hydrogeologic justification study if necessary.
H.
An impermeable liner will be required in detention basins where the
possibility of groundwater contamination exists. A detailed hydrogeologic
investigation may be required by the municipality.
I.
The municipality shall require the applicant to provide safeguards
against groundwater contamination for land uses that may cause groundwater
contamination should there be a mishap or spill.
The applicant shall comply with the following water quality
requirements of this article:
A.
No regulated earth disturbance activities within the municipality
shall commence until approval by the municipality of a plan which
demonstrates compliance with post-construction state water quality
requirements.
B.
The BMPs shall be designed, implemented and maintained to meet state
water quality requirements and any other more stringent requirements
as determined by the municipality.
C.
To control post-construction stormwater impacts from regulated earth
disturbance activities, state water quality requirements can be met
by BMPs, including site design, which provide for replication of preconstruction
stormwater infiltration and runoff conditions so that post-construction
stormwater discharges do not degrade the physical, chemical or biological
characteristics of the receiving waters. As described in the DEP Comprehensive
Stormwater Management Policy (No. 392-0300-002, September 28, 2002),
this may be achieved by the following:
(1)
Infiltration: replication of preconstruction stormwater infiltration
conditions;
(2)
Treatment: use of water quality treatment BMPs to ensure filtering
out of the chemical and physical pollutants from the stormwater runoff;
and
(3)
Stream bank and streambed protection: management of volume and rate
of post-construction stormwater discharges to prevent physical degradation
of receiving waters (e.g., from scouring).
D.
Volume requirements.
(1)
Developed areas shall provide adequate storage and treatment facilities necessary to capture and treat stormwater runoff. The retention volume computed under § 316-20 may be a component of the water quality volume if the applicant chooses to manage both components in a single facility. If the retention volume is less than the water quality volume, the remaining water quality volume may be captured and treated by methods other than infiltration BMPs. The required water quality volume (WQv) is the storage capacity
needed to capture and treat a portion of stormwater runoff from the
developed areas of the site.
(2)
To achieve this goal, the following criterion is established. The
following calculation formula is to be used to determine the water
quality storage volume (WQv) in acre-feet of
storage required by this chapter:
|
WQv = [(P)(Rv)(A)]
÷ 12
|
|
Where:
| ||||
|
WQv
|
=
|
Water quality volume (acre-feet)
| ||
|
P
|
=
|
1 inch
| ||
|
A
|
=
|
Area of the project contributing to the water quality BMP (acres)
| ||
|
Rv
|
=
|
0.05 + 0.009(I), where I is the percent of the area that is
impervious surface [(impervious area/A) * 100]
| ||
(3)
This volume requirement can be accomplished by the permanent volume
of a wet basin or the detained volume from other BMPs. Where appropriate,
wet basins shall be utilized for water quality control and shall follow
the guidelines of the BMP manuals referenced in Ordinance Appendix
G.[1]
[1]
Editor's Note: Appendix G is on file in the Borough offices.
(4)
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 provide for protection from clogging
and unwanted sedimentation.
E.
For areas within defined special protection subwatersheds that include
EV and 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.
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 different BMPs.
G.
If a perennial or intermittent stream passes through the site, the
applicant shall create a stream buffer extending a minimum of 25 feet
to either side of the top-of-bank of the channel. The buffer area
shall be maintained with and encouraged to use appropriate native
vegetation (refer to Appendix H of the Pennsylvania Handbook of Best
Management Practices for Developing Areas for plant lists[2]). If the applicable rear or side yard setback is less
than 25 feet, the buffer width may be reduced to 25% of the setback
to a minimum of 10 feet. If an existing buffer is legally prescribed
(i.e., deed, covenant, easement, etc.) and it exceeds the requirements
of this chapter, the existing buffer shall be maintained. This does
not include lakes or wetlands.
[2]
Editor's Note: Appendix H of the Pennsylvania Handbook
of Best Management Practices for Developing Areas is on file in the
Borough offices.
H.
Evidence of any necessary permit(s) for regulated earth disturbance activities from the appropriate DEP regional office must be provided to the municipality. The issuance of a NPDES construction permit [or permit coverage under the state-wide general permit (PAG-2)] satisfies the requirements of Subsection A.
A.
In addition to the control of water quality volume (in order to minimize
the impact of stormwater runoff on downstream stream bank erosion),
the primary requirement is to design a BMP to detain the proposed
conditions two-year, twenty-four-hour 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). 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).
B.
The minimum orifice size in the outlet structure to the BMP shall
be three inches in diameter where possible, and a trash rack shall
be installed to prevent clogging. On sites with small drainage areas
contributing to this BMP that do not provide enough runoff volume
to allow a twenty-four-hour attenuation with the three-inch orifice,
the calculations shall be submitted showing this condition. Orifice
sizes less than three inches can be utilized, provided that the design
will prevent clogging of the intake.
C.
In Conditional Direct Discharge Districts (District C) only (see § 316-23), the objective is not to attenuate the storms greater than the two-year recurrence interval. This can be accomplished by configuring the outlet structure not to control the larger storms or by a bypass channel that diverts only the two-year stormwater runoff into the basin or, conversely, diverts flows in excess of the two-year storm away from the basin.
A.
Division of watershed into districts.
(2)
In addition to the requirements specified in Table 316-23A below, the erosion and sedimentation control (§ 316-18), the nonstructural project design (§ 316-19), the groundwater recharge (§ 316-20), the water quality (§ 316-21), and the stream bank erosion (§ 316-22) requirements shall be implemented.
(3)
Standards for managing runoff from each subarea in the Darby and
Cobbs Creeks watershed for the two-, five-, ten-, twenty-five-, fifty-
and one-hundred-year design storms are shown in Table 316-23A. Development
sites located in each of the management districts must control proposed
conditions runoff rates to existing conditions runoff rates for the
design storms in accordance with Table 316-23A.
|
Table 316-23A
Peak Rate Control Standards by Stormwater Management District
in Darby-Cobbs Creek Watershed
| |||
|---|---|---|---|
|
District
|
Proposed Condition Design Storm
|
Existing Condition Design Storm
| |
|
A
|
2-year
5 year
10-year
25-year
100-year
|
1-year
5-year
10-year
25-year
100-year
| |
|
B-1
|
2-year
10-year
25-year
50-year
100-year
|
1-year
5-year
10-year
25-year
100-year
| |
|
B-2
|
2-year
5-year
25-year
50-year
100-year
|
1-year
2-year
5-year
10-year
100-year
| |
|
C
|
Conditional Direct
Discharge District
| ||
(4)
In District C, development sites that can discharge directly to the
Darby-Cobbs Creek main channel, major tributaries or indirectly to
the main channel through an existing stormwater drainage system (i.e.,
storm sewer or tributary) may do so without control of the proposed
conditions peak rate of runoff greater than the five-year storm. Sites
in District C will still have to comply with the groundwater recharge
criteria, the water quality criteria and stream bank erosion criteria.
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 flows greater than the two-year existing conditions peak flow
or will be provided with improvements to furnish the required capacity.
When adequate capacity in 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.
B.
General. Proposed conditions rates of runoff from any regulated activity
shall not exceed the peak release rates of runoff from existing conditions
for the design storms specified on the Stormwater Management District
Watershed Map (Appendix A[2]) and this section of the chapter.
[2]
Editor's Note: Appendix A is on file in the Borough offices.
C.
District boundaries. The boundaries of the stormwater management
districts are shown on an Official Map that is available for inspection
at the municipal and county planning offices. A copy of the Official
Map at a reduced scale is included in Appendix A.[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: Appendix A is on file in the Borough offices.
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 meet the
management district criteria for which the discharge is located. 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 if discharges from multiple subareas recombine in proximity
to the discharge site. In this case, peak discharge in any direction
may follow Management District A criteria, provided that the overall
site discharge meets the management district criteria for which the
discharge is located.
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.
(1)
For any proposed development site not located in a Conditional 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 conditions
hydrographs can match existing conditions hydrographs and if it can
be proved that the proposed conditions will not cause increases in
peaks at all points downstream.
(2)
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.
(a)
The hydrologic regime of the site must be maintained.
(b)
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.
(c)
The evaluation shall continue downstream until the increase
in flow diminishes due to additional flow from tributaries and/or
stream attenuation.
(d)
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 respective watershed. These flow values can be obtained from
the original Act 167 watershed stormwater management plans.
(e)
Applicant-proposed 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.
(f)
Financial distress shall not constitute grounds for the municipality
to approve the use of the no-harm option.
(g)
Capacity improvements to conveyance facilities or obstructions
may be provided as necessary to implement the no-harm option as long
as it can be demonstrated through the downstream hydraulic capacity
analysis that the improvements would not create any harm downstream.
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)
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.
I.
Hardship option. The standards and criteria outlined in this section of the chapter are designed to maintain existing peak flows and volumes throughout the municipality's watersheds as they become developed. There may be certain instances, however, where the standards and criteria established are too restrictive for a particular landowner or 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. This must be demonstrated as per Subsection H above in order for the hardship option to be considered. If an applicant or landowner cannot meet the stormwater standards due to lot conditions or if conformance would become a hardship to an owner, the hardship option may be applied. The applicant would have to plead his/her case to the governing body with the final determination made by the municipality. Any landowners pleading the hardship option will assume all liabilities that may arise due to exercising this option. Cost or financial burden cannot be considered a hardship. The applicant may consider off-site management controls or contributing to the Municipal Stormwater Control and BMP Operation and Maintenance Fund (§ 316-37) as long as the stormwater management controls are within the same subwatershed as shown in Appendix A.[4]
[4]
Editor's Note: Appendix A is on file in the Borough offices.
J.
Alternate criteria for redevelopment sites. For redevelopment sites,
one of the following minimum design parameters shall be accomplished,
whichever is most appropriate for the given site conditions as determined
by Borough of Sharon Hill:
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 316-24A 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 316-24A
Acceptable Computation Methodologies for Stormwater Management
Plans
| |||
|---|---|---|---|
|
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 where limitations described
in TR-55.
| |
|
HEC-1/HEC-HMS
|
U.S. Army Corps of Engineers
|
Applicable where use of a full hydrologic computer is desirable
or necessary.
| |
|
PSRM
|
Penn State University
|
Applicable where use of a hydrologic 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.
| |
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 F-1 in Appendix F[1] of this chapter. If a hydrologic computer model such as
PSRM or HEC-1/HEC-HMS is used for stormwater runoff calculations,
then the duration of rainfall shall be 24 hours. The Alternating Block
Method shown in Figure F-1 or the SCS Type II S Curve, Figure F-3
in Ordinance Appendix F,[2] shall be used for the rainfall distribution.
C.
The following criteria shall be used for runoff calculations.
(1)
For development sites not considered redevelopment, the ground cover
used in determining the existing conditions flow rates shall be as
follows:
(a)
Wooded sites shall use a ground cover of "woods in good condition."
Portions of a site having more than one viable tree of a DBH of six
inches or greater per 1,500 square feet shall be considered wooded
where such trees existed within three years of application.
(b)
The undeveloped portion of the site including agriculture, bare
earth and fallow ground shall be considered as "meadow in good condition,"
unless the natural ground cover generates a lower curve (CN) number
or Rational "c" value (i.e., woods) as listed in Table F-2 or F-3
in Appendix F[3] of this chapter.
[3]
Editor's Note: Appendix F is on file in the Borough offices.
(2)
For development and redevelopment sites, the ground cover used in
determining the existing conditions flow rates for the developed portion
of the site shall be based upon actual land cover conditions.
D.
All calculations using the Rational Method shall use rainfall intensities
consistent with appropriate times-of-concentration for overland flow
and return periods presented in the Region 5 Curves from the PennDOT
Storm-Duration-Frequency Chart (Figure F-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.
G.
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 F-4 in Appendix F[6] of the chapter.
[6]
Editor's Note: Appendix F is on file in the Borough offices.
H.
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.
I.
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. 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 PennDOT.
C.
Any stormwater management facility (i.e., detention basin) required or regulated by this chapter designed to store runoff and requiring a berm or earthen embankment 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 one 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 DEP Chapter 105, the facility shall be designed in accordance with Chapter 105 and meet the regulations of Chapter 105 concerning dam safety. Chapter 105 may be required to pass storms larger than the one-hundred-year event.
D.
Any facilities that constitute water obstructions (e.g., culverts, bridges, outfalls or stream enclosures) and any work involving wetlands governed by DEP Chapter 105 regulations (as amended or replaced from time to time by DEP) shall be designed in accordance with Chapter 105 and will require a permit from DEP.
E.
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 one foot of freeboard measured below the lowest point along the top of the roadway. Any facility that constitutes a dam as defined in DEP 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.
F.
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.
G.
Storm sewers must be able to convey proposed conditions runoff from
a ten-year design storm without surcharging inlets, where appropriate.
H.
Adequate erosion protection shall be provided along all open channels
and at all points of discharge.
I.
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 construction
in or continuation of a stormwater problem area.
