[Ord. 2010-3, 12/16/2010, § 1]
1. General Design Guidelines.
A. Stormwater shall not be transferred from one watershed to another,
unless (1) the watersheds are sub-watersheds of a common watershed
which join together within the perimeter of the property; (2) the
effect of the transfer does not alter the peak rate discharge onto
adjacent lands; or (3) easements from the affected landowner(s) are
provided.
B. Consideration shall be given to the relationship of the subject property
to the drainage pattern of the watershed. A concentrated discharge
of stormwater to an adjacent property shall be within an existing
watercourse or confined in an easement or returned to a pre-development
flow type condition.
C. Stormwater BMPs and recharge facilities are encouraged (e.g., rooftop
storage, drywells, cisterns, recreation area ponding, diversion structures,
porous pavements, holding tanks, infiltration systems, stream channel
storage, in-line storage in storm sewers, and grading patterns). They
shall be located, designed, and constructed in accordance with the
latest technical guidance published by PADEP, provided they are accompanied
by detailed engineering plans and performance capabilities and supporting
site specific soils, geology, runoff and groundwater and infiltration
rate data to verify proposed designs. Additional guidance from other
sources may be accepted at the discretion of the Borough Engineer
(a pre-application meeting is suggested).
D. All existing and natural watercourses, channels, drainage systems
and areas of surface water concentration shall be maintained in their
existing condition unless an alteration is approved by the appropriate
regulatory agency.
E. No outlet structure from a stormwater management facility, or swale,
shall discharge directly onto a Borough or state roadway.
F. The invert of all stormwater management facilities and underground
infiltration/storage facilities shall be located a minimum of two
feet above the seasonal high groundwater table or other soil limiting
zone. The invert of stormwater facilities may be lowered if adequate
sub-surface drainage, which does not alter the existing water table
level, is provided.
G. Any stormwater management facility may be required to be fenced with
a minimum four-foot high fence of material acceptable to Hummelstown
Borough. Gates with a minimum opening of 10 feet shall be provided
for access.
H. Stormwater management facilities excavated to carbonate rock must
either be fitted with an impervious clay liner, or over-excavated
four feet and refilled with a suitable material mix. Suitable backfill
material is subject to the approval of the Borough Engineer.
I. The type, location, and number of landscaping and planting specification
shall be provided for all stormwater management facilities and be
specific for each type of facility.
2. Stormwater Management Facilities. (With a Depth of Water Equal to
or Greater than Three Feet Measured from the Lowest Point Inside a
Facility to the Crest of the Emergency Spillway).
A. Any stormwater management facility designed to store runoff and requiring
a berm or earthen embankment, shall be designed to provide an emergency
spillway to handle peak rate of stormwater runoff up to and including
the one-hundred-year post-development flow, with a blocked primary
outlet structure. The height of embankment must be set as to provide
a minimum one foot of freeboard through the spillway, above the maximum
water surface elevation, computed when the spillway functions for
the one-hundred-year post-development inflow, with a blocked outlet
structure. The primary outflow structure must be designed to pass
all design storms (up to and including the one-hundred-year event)
without discharging through the emergency spillway. The maximum water
depth within any stormwater management facility shall be no greater
than eight feet when functioning through the primary outlet structure.
B. Emergency spillways shall be armored to prevent erosion during the
one-hundred-year post-development flow, with blocked primary outlet
structure. Synthetic liners or rip-rap may be used, and calculations
sufficient to support proposed armor must be provided. An earthen
plug must be used to accurately control the spillway invert if rip-rap
is the proposed armoring material. Emergency spillway armor must extend
up the sides of the spillway, and continue at full width to a minimum
of 10 feet past the toe of slope.
C. A stormwater management facility berm cross sections must be at least
five feet wide at the top, and eight feet wide through the emergency
spillway. For fill embankments, the side slopes shall be no steeper
then 3:1 on the inside of the facility and 2:1 on the outside of the
facility. For cut slopes, the side slopes shall be no steeper than
2:1.
D. A cutoff and key trench of impervious material shall be provided
under all embankments four feet or greater in height.
E. Soils used for the construction of stormwater management facilities
shall have low-erodibility factors ("K" factors) (refer to E&S
manual) and be identified on the SWM site plan.
F. Trash racks must be provided to prevent clogging of primary outflow
structure stages for all orifices equivalent to 12 inches or smaller
in diameter.
G. Anti-seep collars must be provided on all outflow culverts in accordance
with the methodology contained in the latest edition of the E&S
manual. An increase in seepage length of 15% must be used in accordance
with the requirements for permanent anti-seep collars.
H. Conventional, non-BMP stormwater management facilities (i.e., dry
detention basins) must empty over a period of time not less than 24
hours and not more than 72 hours from the end of the facility's inflow
hydrograph. Infiltration tests performed at the facility locations
and proposed basin bottom depths, in accordance with the BMP manual,
must support time-to-empty calculations if infiltration is a factor
is the sizing of the stormwater management facility.
I. Impervious low-flow channels are not permitted within stormwater
management facilities to promote water quality and groundwater recharge
for frequent storm events. Facilities designed as water quality/infiltration
BMPs may have a bottom slope of zero. Minimal maintenance, saturation
tolerant vegetation must be provided in basins designed as water quality/infiltration
BMPs. Conventional, non-BMP stormwater management facilities must
have a minimum slope of 1% extending radially out from the primary
outlet structure. Water storage below the lowest outlet structure
stage (i.e., dead storage) is permitted in stormwater management facilities
designed as water quality/infiltration BMPs.
J. Stormwater management facilities bottom elevations must be greater
than adjacent floodplain elevations (FEMA or HEC-RAS analysis). If
no floodplain is defined, bottom elevations must be higher than existing
ground elevations 50 feet from top of stream bank in the facilities
vicinity.
K. Basin outflow culverts discharging into floodplains must account
for tailwater. Tailwater corresponding to the one-hundred-year floodplain
elevation may be used for all design storms, or the applicant may
elect to determine flood elevations of the adjacent watercourse for
each design storm. The floodplain is assumed to be 50 feet from top
of stream bank in areas where a floodplain is not designated, or where
no other evidence is provided.
L. Exceptions to these requirements may be made at the discretion of
Hummelstown Borough for BMPs that retain or detain water, but are
of a much smaller scale than traditional stormwater management facilities.
3. Storm Sewer Facilities.
A. Storm sewers must be able to convey post-development runoff from
a ten-year design storm without surcharging inlets where appropriate.
When connecting to an existing storm sewer system, the applicant must
demonstrate that the proposed system will not exacerbate any existing
stormwater problems and that adequate downstream capacity exists.
B. A minimum pipe size of 15 inches in diameter shall be used in all
roadway systems (public or private) proposed for construction. Pipes
shall be designed to provide a minimum velocity of 2 1/2 feet
per second when flowing full, but in all cases, the slope shall be
no less than 0.5%. Arch pipe of equivalent cross-sectional area may
be substituted in lieu of circular pipe where cover or utility conflict
conditions exist.
C. In proposed curbed roadway sections, the maximum encroachment of
water on the roadway pavement shall not exceed half of a through travel
lane or one inch less than the depth of curb during the ten-year design
storm of five minute duration. Gutter depth shall be verified by inlet
capture/capacity calculations that account for road slope and opening
area. The maximum distance between inlets in curbed roadway sections
shall be no more than 600 feet, however access to underground pipes
shall be provided every 300 feet.
D. Standard type "C" inlets with eight-inch hoods shall be used along
vertical concrete curbs roadway networks. Type "C" inlets with ten-inch
hoods that provide a two inch sump condition may be used with approval
of the Borough Engineer when roadway longitudinal slopes are 1% or
less.
E. For inlets containing a change in pipe size, the elevation for the
crown of the pipes shall be the same or the smaller pipe's crown shall
be at a higher elevation.
F. All inlets shall provide a minimum two-inch drop between the lowest
inlet pipe invert elevation and the outlet pipe invert elevation.
G. On curbed sections, a double inlet shall be placed at the low point
of sag vertical curves, or an inlet shall be placed at the low point
and on each side of the low point at a distance not to exceed 100
feet, or at an elevation not to exceed 0.2 feet above the low point.
H. At all roadway low points, swales and easements shall be provided
behind the curb or swale and through adjacent properties to channelize
and direct any overflow of stormwater runoff away from dwellings and
structures.
I. Inlets shall be placed so drainage cannot cross intersections or
street center lines.
J. All inlets in paved areas shall have heavy duty bicycle safe grating
consistent with PennDOT Publication 72M. A note to this effect shall
be added to the SWM site plan or inlet details therein.
K. Inlets must be sized to accept the specified pipe sizes without knocking
out any of the inlet corners. All pipes entering or exiting inlets
shall be cut flush with the inlet wall. A note to this effect shall
be added to the SWM site plan or inlet details therein.
L. Inlets shall have weep holes covered with geotextile fabric placed
at appropriate elevations to completely drain the sub grade prior
to placing the base and surface course on roadways.
M. Inlets, junction boxes, or manholes greater than five feet in depth
shall be equipped with ladder rungs and shall be detailed on the SWM
site plan.
N. Inlets shall not have a sump condition in the bottom (unless designed
as a water quality BMP). Pipes shall be flush with the bottom of the
box or concrete channels shall be poured.
O. Inlets, manholes, pipes, and culverts shall be constructed in accordance
with the specifications set forth in PennDOT's Publication 408, latest
edition, and as detailed in the PennDOT's Publication 72M - Standards
for Roadway Construction (RC), latest edition, or as approved by the
Borough Engineer. All material and construction details (inlets, manholes,
pipe trenches, etc.), must be shown on the SWM site plan, and a note
added that all construction must be in accordance with PennDOT's Publication
408 and PennDOT's Publication 72M, latest edition. A note shall be
added to the plan stating that all frames, concrete top units, and
grade adjustment rings shall be set in a bed of full mortar according
to Publication 408.
P. Accessible drainage structures shall be located on continuous storm
sewer system at all vertical dislocations, at all locations where
a transition in storm sewer pipe sizing is required, at all vertical
and horizontal angle points exceeding 5°, and at all points of
convergence of two or more storm sewer pipes.
Q. All storm drainage piping (equal to or greater than 12 inches) discharging
to the ground surface shall be provided with either reinforced concrete
headwalls or plastic and metal pipe end sections compatible with the
pipe size involved in accordance with PennDOT Publication 408 and
Publication 72M.
R. Outlet protection shall be provided at all surface discharge points
with storm drainage piping (equal to or greater than 12 inches) in
order to minimize erosion consistent with the E&S manual.
S. Pavement base drain shall be provided at all low points in cut areas,
toe of slope areas, and other areas as dictated by proven engineering
principles and design judgment. All base drain shall be in accordance
with PennDOT Publication 408.
4. Swale Conveyance Facilities.
A. Swales must be able to convey post-development runoff from a ten-year
design storm with six inches of freeboard to top of the swale.
B. Swales shall have side slopes no steeper than 3:1.
C. All swales shall be designed, labeled on the SWM site plan, and details
provided to adequately construct and maintain the design dimension
of the swales.
D. Swales shall be designed for stability using velocity or shear criteria.
Velocity criteria may be used for channels with less than 10% slope.
Shear criteria may be used for all swales. Documentation must be provided
to support velocity and/or shear limitations used in calculations.
E. Where swale bends occur, the computed velocities or shear stresses
shall be multiplied by the following factor for the purpose of designing
swale erosion protection:
(1)
One-and-three-quarters-when swale bend is 30° to 60°.
(2)
Two-when swale bend is 60° to 90°.
(3)
Two-and-one-half-when swale bend is 90° or greater.
F. Swales must be designed for both temporary and permanent conditions
in accordance with the latest E&S manual.
[Ord. 2010-3, 12/16/2010, § 1]
1. All calculations shall be consistent with the guidelines set forth
in the BMP manual.
2. Stormwater runoff from all development sites shall be calculated
using either the Rational Method or a Soil Cover Complex methodology.
Methods shall be selected by the qualified professional based on the
individual limitations and suitability of each method for a particular
site.
3. Rainfall Values.
A. Rational Method. The Pennsylvania Department of Transportation Drainage
Manual, Intensity-Duration-Frequency Curves, Publication 584, Chapter
7A, latest edition, shall be used in conjunction with the appropriate
time of concentration and return period.
B. Soil Cover Complex Method. The design storm volumes to be used in
the analysis of peak rates of discharge should be obtained from the
latest version of the Precipitation-Frequency Atlas of the United
States, National Oceanic and Atmospheric Administration (NOAA), National
Weather Service, Hydrometeorological Design Studies Center, Silver
Spring, Maryland (NOAA Atlas 14).
[Amended by Ord. No. 2022-4, 10/13/2022]
4. Peak Flow Rates.
A. Rational Method. May be used for drainage areas up to 20 acres. Extreme
caution should be used by the qualified professional if the watershed
has more than one main drainage channel, if the watershed is divided
so that hydrologic properties are significantly different in one verses
the other, if the time of concentration exceeds 60 minutes, or if
stormwater runoff volume is an important factor. The combination of
Rational Method hydrographs based on timing shall be prohibited.
(1)
The use of the Modified Rational Method to design stormwater
management facilities must be approved by the Borough Engineer.
B. Soil Cover Complex Method. May be used for drainage areas greater
than 20 acres. This method is recommended for design of stormwater
management facilities and where stormwater runoff volume must be taken
into consideration.
C. For comparison of peak flow rates, flows shall be rounded to a tenth
of a cubic foot per second (cfs).
5. Runoff Coefficients.
A. Rational Method. Use Table C-1 (Appendix C).
B. Soil Cover Complex Method. Use Table C-2 (Appendix C).
C. For the purposes of pre-development peak flow rate and volume determination,
existing non-forested pervious areas conditions shall be considered
as meadow (good condition).
D. For the purposes of pre-development volume determination, 20% of
existing impervious area, when present on the project site, and contained
within the new proposed limit of disturbance, shall be considered
meadow (good condition) for pre-development hydrologic calculations
for redevelopment.
6. Design Storm.
A. All drainage facilities (inlets, pipes, and swales) shall be designed
to safely convey the ten-year storm.
B. All stormwater management facilities shall be verified by routing
the proposed one-, two-, ten-, twenty-five-, fifty-, and one-hundred-year
hydrographs through the facility using the storage indication (Modified
Puls) method. The design storm hydrograph shall be computed using
a calculation method that produces a full hydrograph.
C. The stormwater management and drainage system shall be designed to
safely convey the post-development one-hundred-year storm event to
stormwater detention facilities, for the purpose of meeting peak rate
control.
D. All structures (culvert or bridges) proposed to convey runoff under
a Borough road, shall be designed to pass the fifty-year design storm
with a minimum one foot of freeboard measured below the lowest point
along the top of the roadway.
E. All design within state or federal right-of-ways or that falls under
the design criteria of any higher authority must meet the requirements
of that agency in addition to meeting the minimum requirements of
this Part.
7. Time of Concentration.
A. Time of concentration shall be computed using the NRCS Segmental
Method as described in TR-55 (SCS 1986 or most current update). The
length of sheet flow shall be limited to 100 feet. The Manning's "n"
Roughness Coefficient for TR-55 sheet flow can be found in Table C-4
(Appendix C). Time of concentration for channel and pipe flow shall
be computed using Manning's equation.
B. For sites with insignificant channelized flow and less than 20% imperviousness
coverage, the time of concentration may be computed using the NRCS
equation for lag time:
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Time of Concentration = Tc = [(Tlag/.6)
* 60] (minutes)
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Tlag =
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L0.8
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(S + 1)0.7
1900√Y
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Where:
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Tlag
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=
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Lag time (hours)
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L
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=
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Hydraulic length of watershed (feet)
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Y
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=
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Average overland slope of watershed (percent)
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S
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=
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Maximum retention in watershed as defined by:
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CN
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=
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NRCS Curve Number for watershed as defined by the NRCS Loss
Method
|
C. Additionally, the following provisions shall apply to calculations
for time of concentration:
(1)
The post-development time of concentration shall never be greater
that the pre-development time of concentration for any watershed or
subwatershed.
(2)
The minimum time of concentration for any watershed shall be
five minutes.
(3)
The designer may choose to assume a five minute time of concentration
for any post-development watershed or subwatershed without providing
any computations.
(4)
The designer must provide computations for all pre-development
time of concentration paths. A five minute time of concentration can
not be assumed for pre-development.
(5)
Undetained fringe areas (areas that are not tributary to a stormwater
facility but where a reasonable effort has been made to convey runoff
from all new impervious coverage to best management practices) may
be assumed to represent the pre-development conditions for purpose
of time of concentration calculations.
8. Drainage areas tributary to sinkholes or closed depressions in areas
underlain by limestone or carbonate geologic features shall be excluded
from the modeled point of analysis defining pre-development flows.
If left undisturbed during construction activities, areas draining
to closed depressions may also be removed from peak runoff rates in
the post-development analysis. New, additional contributing runoff
shall not be directed to existing sinkholes or closed depressions.
9. Where uniform flow is anticipated, the Manning's equation shall be
used for hydraulic computations and to determine the capacity of open
channels, pipes, and storm sewers. The Manning's equation should not
be used for analysis of pipes under pressure flow or for analysis
of culverts. Manning's "n" values shall be obtained from Table C-3
(Appendix C). Inlet control shall be checked at all inlet boxes to
ensure the headwater depth during the ten-year design event is contained
below the top of grate for each inlet box.
10. Hummelstown Borough 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.
11. Hummelstown Borough has the authority to require that computed existing
runoff rates be reconciled with field observations, conditions and
site history. If the designer can substantiate, through actual physical
calibration, that more appropriate runoff and time of concentration
values should be utilized at a particular site, then appropriate variations
may be made upon review and approval of Hummelstown Borough.