A.
Applicants proposing regulated activities in the municipality that do not fall under the exemption criteria shown in § 134-106 shall submit a drainage plan consistent with this chapter and the respective Act 167 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 § 134-404 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 § 134-406G 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 the 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 the 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)
Effectiveness 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 habitats.
(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 the DEP under regulations at 25 Pa. Code, Chapter 102.
B.
Work within natural drainageways subject to permit by the DEP under
25 Pa. Code, Chapter 105.
C.
Any stormwater management facility that would be located in or adjacent
to surface waters of the commonwealth, including wetlands, subject
to permit by the 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.
E.
Culverts, bridges, storm sewers, or any other facilities which must
pass or convey flows from the tributary area and any facility which
may constitute a dam subject to permit by the DEP under 25 Pa. Code,
Chapter 105.
A.
No regulated earth disturbance activities within the municipality
shall commence until the municipality receives an approval from the
Conservation District or the Municipal Engineer of an erosion and
sediment control plan for construction activities.
B.
The 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).
C.
In addition, under 25 Pa. Code, Chapter 92, a DEP permit for stormwater discharges associated with construction activities is required for land disturbances greater than one acre.
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.
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. Additional measures, such as placement
of orange construction fencing around proposed infiltration BMPs during
construction to minimize or eliminate traffic overtop of these areas,
and temporary sealing off of pipes and inlet connections to infiltration
BMPs to prevent sediment clogging, should be given consideration.
(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 to the surface
discharge of stormwater, such as those listed in Appendix E, Table
E-4,[1] 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 E is included as an attachment
to this chapter.
(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, hydrologic soil groups, wooded areas, and potential
infiltration areas. Land development, any existing recharge areas,
and other requirements outlined in the municipal SALDO[2] shall also be included.
(3)
Prepare a draft project layout avoiding sensitive areas identified in § 134-404B(1).
(4)
Identify site-specific existing conditions' drainage areas,
discharge points, recharge areas, and Hydrologic Soil Groups A and
B (areas conducive to infiltration). Infiltration should still be
considered in well-draining soils listed as Hydrologic Soil Group
C, but additional soils testing should be performed to verify on-site
conditions and placement of these BMPs.
(6)
Satisfy the groundwater recharge (infiltration) objective (§ 134-405) and provide for stormwater pretreatment prior to infiltration.
(8)
Provide stream bank erosion protection in accordance with § 134-407, 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 (§ 134-408).
(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 § 134-103 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. Appropriate soils testing and/or geotechnical evaluation should be included as part of any documentation for infiltration BMPs. If it can be physically accomplished, then the volume of runoff to be infiltrated shall be determined from § 134-405A(2).
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, preferably 36 inches, between
the bottom of the BMP and the top of the limiting zone (e.g., SHWT,
groundwater, bedrock, etc.).
(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 recharge (infiltration) volume (Rev) within
three days (72 hours) or less.
(d)
Pretreatment shall be provided prior to infiltration.
(2)
The size of the infiltration facility shall be based upon the
Net Two-Year Volume Approach, where the recharge (infiltration) volume
(Rev) to be captured and infiltrated shall
be the volume difference between the predevelopment two-year, twenty-four-hour
storm event and post-development two-year, twenty-four-hour storm
event.
The recharge volume calculated using this section 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 recharge
can be achieved, the applicant is encouraged to infiltrate as much
of the stormwater runoff from the site as possible.
(3)
Simplified volume computation: Capture 1.56 inches of rainfall
for all impervious surfaces — for projects with less than 5,000
square feet of impervious and less than 5,000 square feet of disturbance.
A simpler method of volume computation is permitted for projects
that meet all of the following criteria:
| |
(a)
|
More than 1,000 square feet and less than 5,000 square feet
of new impervious surface area is proposed;
|
(b)
|
Less than 5,000 square feet of total area is to be disturbed;
|
(c)
|
Soil and site conditions are suitable for infiltration as described in § 134-405A(1).
|
For such projects, the required infiltration volume may be computed
by the following equation:
| |
Recharge Volume (cubic feet) = Area Impervious (square feet)
x 0.13
|
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 recharge
(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 hotspots.
(1)
Below is a list of examples 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 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 hotspot category (the entire WQv must still
be treated). Second, a greater level of stormwater treatment shall
be considered at hotspot 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 hotspots:
(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
hotspots:
(4)
While large highways [average daily traffic volume (ADT) greater
than 30,000] are not designated as stormwater hotspots, 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. Specific consideration
should be given to the particular type of salt or deicing material
to be used within this watershed in regards to its potential long-term
effects on the soils, especially in areas that contain clay soil.
G.
The infiltration requirement in HQ or EV waters shall be subject
to the Department's Chapter 93 Antidegradation Regulations.
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 that 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.
Developed areas shall provide adequate storage and treatment facilities necessary to capture and treat stormwater runoff. If site conditions allow for infiltration, the water quality volume and the recharge volume are the same volume and may be managed in a single facility. If infiltration cannot be physically accomplished, the water quality volume should be calculated using the Net Two-Year Volume Approach described in § 134-405A(2). In this case, the water quality volume may be captured and treated by methods other than infiltration BMPs.
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 Appendix
F.[1]
The water quality volume shall take a minimum of 24 hours to
be discharged from a BMP facility. Release of the water quality volume
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.
[1]
Editor's Note: Appendix F is included as an attachment
to this chapter.
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 10 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 B of the Pennsylvania Stormwater Best
Management Practices Manual, latest version, for plant lists). 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.
H.
Evidence of any necessary permit(s) for regulated earth disturbance
activities from the appropriate DEP regional office must be provided
to the municipality.
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.
A.
Within the Swamp Creek watershed, the criteria for peak runoff control
are designed to reduce the post-development peak flow to 50% of the
predevelopment peak flow. Development sites must control proposed
conditions' runoff rates to 50% of the existing conditions'
runoff rates for the two-year, five-year, ten-year, twenty-five-year,
fifty-year, and one-hundred-year storm events.
B.
For lands not within the Swamp Creek watershed, the criteria for
peak runoff control are designed to reduce the post-development peak
flow to 100% of the predevelopment peak flow. Development sites must
control proposed conditions' runoff rates to no greater than
100% of the existing conditions' runoff rates for the two-year,
five-year, ten-year, twenty-five-year, fifty-year, and one-hundred-year
storm events.
C.
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 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.
D.
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.
E.
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 peak rate control standards noted above. In other
words, unimpacted areas bypassing the stormwater management facilities
would not be subject to the peak rate control standards.
F.
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 Lower Frederick Township:
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 409.1 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. Note that successors to the methods listed in Table 409.1 are
also acceptable, such as WinTR55 for TR-55 and WinTR20 for TR-20.
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 409.1
| ||
---|---|---|
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 model 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
|
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 E-1 in Appendix E of this chapter.[1] 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.
[1]
Editor's Note: Appendix E is included as an attachment
to this chapter.
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."
A site shall be considered to be a wooded site where a biological
community dominated by trees and other woody plants exists that covers
an area of 10,000 square feet or more and contains at least 100 trees
with at least 50% of those trees having a dbh of two inches or greater.
(Duerksen, Christopher J., with Suzanne Richman, Tree Conservation
Ordinances, Planning Advisory Service Report Number 446, American
Planning Association, Chicago, Illinois, and Scenic America, Washington,
D.C., August, 1993.)
(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
E-2 or E-3 in Appendix E of this chapter.[2]
[2]
Editor's Note: Appendix E is included as an attachment
to this chapter.
(c)
Off-site land use conditions used to determine storm flows for
designing storm facilities shall be based on existing land uses assuming
winter or poor land cover conditions.
(2)
For development considered redevelopment sites, 20% of existing
impervious area, when present, shall be considered meadow (good condition)
in the model for existing use conditions for redevelopment. Otherwise,
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 appropriate curves from the PennDOT
Storm-Duration-Frequency Chart, (Region 4 is included in Figure E-3.[3] The user should refer to the Atlas 14, Volume 2, Storm-Duration-Frequency
Chart.) 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.
[3]
Editor's Note: Appendix E is included as an attachment
to this chapter.
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 accepted published values.
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.
B.
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 1.5 feet
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.
C.
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 the DEP.
D.
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.
E.
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.
F.
Storm sewers must be able to convey proposed conditions' runoff
from a twenty-five-year design storm without surcharging inlets, where
appropriate.
G.
Adequate erosion protection shall be provided along all open channels
and at all points of discharge.
H.
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.