•
|
Discharges from firefighting activities
|
•
|
Potable water sources including dechlorinated water line and
fire hydrant flushing
|
•
|
Irrigation drainage
|
•
|
Routine external building wash-down (which does not use detergents
or other compounds)
|
•
|
Air conditioning condensate
|
•
|
Water from individual residential car washing
|
•
|
Springs
|
•
|
Water from crawl space pumps
|
•
|
Uncontaminated water from foundation or from footing drains
|
•
|
Flows from riparian habitats and wetlands
|
•
|
Lawn watering
|
•
|
Pavement wash waters where spills or leaks of toxic or hazardous
materials have not occurred (unless all spill material has been removed)
and where detergents have not been used
|
•
|
Dechlorinated swimming pool discharges
|
•
|
Uncontaminated groundwater
|
•
|
Diverted stream flow
|
•
|
Uncontaminated pumped groundwater
|
*
|
The Borough Engineer will make the determination relative to
what constitutes these classifications on a case-by-case basis. The
Pennsylvania DEP wellhead protection contaminant source list shall
be used as a guide in these determinations. Industrial manufacturing
site and hazardous material storage areas must provide NPDES SIC codes.
|
•
|
Pre-development - NRCS's Lag Equation.
|
•
|
Post-development; commercial, industrial, or other areas with
large impervious areas (>20% imperviousarea) - NRCS's Segmental Method.
|
•
|
Post-development; residential, cluster, or other low impact
designs less than or equal to 20% impervious area - NCRS's Lag Equation.
|
The time of concentration is to represent the average condition
that best reflects the hydrologic response of the area. For example,
large impervious areas bordered by small pervious areas may not consider
the effect of the pervious areas in the Tc computation. If the designer
wants to consider the effect of the pervious area, runoff from the
pervious and impervious areas must be computed separately with the
hydrographs being combined to determine the total runoff from the
area.
|
Under no circumstance will the post-development Tc be greater
than the pre-development Tc for any watershed or sub-watershed modeling
purposes. This includes when the designer has specifically used swales
to reduce flow velocities. In the event that the designer believes
that the post-development Tc is greater, it will still be set by default
equal to the pre-development Tc for modeling purposes.
|
[Refer to Subsection a(29) regarding impervious area flashing
(IAF).]
|
One-year return period
|
Qpmin = 0.018 (DA) + 0.2
|
Two-year return period
|
Qpmin = 0.03 (DA) + 0.4
|
Ten-year return period
|
Qpmin = 0.09 (DA) + 1.0
|
Where:
| |||
DA
|
=
|
the drainage area in acres
| |
Qpmin
|
=
|
minimum allowable peak runoff rate in cfs
|
For return periods greater than 10 years, the minimum discharge
shall be equal to the computed pre-development peak runoff rate.
|
The minimum discharge criteria above are not appropriate for
use with the Rational Method. This is because these values were developed
based on NRCS model corrections and do not actually represent a true
physical process or discharge. However, common sense should be used
by both the designer and reviewer in the evaluation of acceptable
minimum discharges for use with the Rational Method.
|
The intent of the minimum discharge is to allow reasonable runoff
release from a site when a hydrologic model has produced a pre-development
runoff rate close to zero. The method is NOT permitted for areas that
previously drained completely to sinkholes in order to bypass the
sinkhole after development.
|
These minimum discharge values include the total of all stormwater
management facilities discharges and undetained area discharges. Peak
runoff rates for undetained fringe areas (where the designer has made
a realistic effort to control all new impervious areas) will be computed
using the pre-development time of concentration for the drainage areas
tributary to them. Undetained areas are those portions of the site
that cannot be routed to a stormwater management facility due to topography
and typically include lower pond berms, or small areas around entrance
drives. The site drainage areas used shall represent the pre-development
condition, even if drainage areas are altered following development.
|
To qualify for a recharge volume credit, constructed filter
strips shall be designed to the following standards:
| |
•
|
The minimum filter strip width shall be five feet or 1/2 of
the impervious area drainage length immediately tributary to the constructed
filter strip, whichever is greater. This width is measured parallel
to the direction of sheet flow.
|
•
|
The total impervious area tributary to a constructed filter
strip area can not exceed twice the constructed filter strip area.
|
•
|
The surface slope of the area tributary to the constructed filter
strip area, and the surface slope of the constructed filter strip
area itself may not exceed 5% and 3%, respectively. In special cases
steeper slopes may be used if specifically authorized by the Borough
Engineer.
|
•
|
The filter strip surface shall consist of a minimum of inches
of natural or reconstructed topsoil with a stable grass surface treatment.
Reconstructed topsoil designs must be approved by the Borough Engineer
prior to application. Reconstructed topsoil consists of soils augmented
by tillage and the addition of soil amendments such as compost, lime,
animal manures, crop residues, etc.
|
•
|
To minimize erosion of the topsoil layer during construction,
it is recommended that these areas be sodded. However, the Borough
Engineer may permit the use of an acceptable erosion control seeding
application. In this later case, any loss of topsoil and seed must
be replaced until a permanent vegetative stand is achieved.
|
The Borough Engineer may waive the recharge requirement in the
following situations:
| ||
•
|
The Borough Engineer may waive the recharge requirement in highly
developed areas or areas undergoing redevelopment where the Borough
Engineer has determined that forced recharge could have adverse impacts
on adjacent landowner structures, property, or municipal infrastructure.
These waivers should be limited to small land areas (generally less
than five acres in size), where the ability to place recharge beds
may be limited or may hinder redevelopment.
| |
•
|
The Borough Engineer may waive the recharge requirement in areas
where a qualified soils scientist or geologist has determined that
none of the site soils are suitable for recharge, or that the location
of the suitable soils is such that harm to adjoining properties could
occur as stated under item one above.
| |
•
|
The Borough Engineer may waive the recharge requirement in areas
where recharge cannot physically occur as documented by a qualified
soil scientist, geologist, or hydrologist. These areas include:
| |
•
|
Exfiltration areas commonly found at the base of wooded hillsides
where clay pans or fragipans exist; and
| |
•
|
Saturation areas near major streams or floodplains.
| |
•
|
Development and redevelopment within the Commercial District
is exempt from the Recharge requirement if there is less than 5% increase
in proposed impervious area.
|
As identified above, recharge analysis and/or waiver requests
must be supported by a geotechnical report sealed by a qualified professional
(soil scientist, geologist, hydrogeologist, or geotechnical engineer).
The intent of this report will be to establish the suitability of
a particular parcel of land or area for recharge, and to identify
areas on a development site appropriate for recharge. It is recommended
that the geotechnical/soils consultant discuss the extent and approach
to the analysis with the Borough Engineer prior to initiating the
field investigation. At a minimum this report should include the following
information:
| ||
•
|
A description of the geotechnical site investigation performed
including the methods and procedures used;
| |
•
|
Data presentation;
| |
•
|
Analysis results including the following minimum information:
| |
•
|
A map identifying site areas inappropriate for recharge along
with supporting justification. In addition to illustrating topographic
features, significant geologic and hydrologic features should be identified
(rock outcrops, sinkholes, closed depressions, etc.
| |
•
|
Determination of the permeability coefficient for potential
recharge areas.
| |
•
|
Determination of the infiltration capacity of natural site soils.
| |
•
|
Location, depth, and permeability coefficient for any restrictive
layers identified.
| |
•
|
Soil uniformity.
| |
•
|
Depth to bedrock in potential recharge areas, and a statement
reflecting the uniformity of the depth to bedrock across the site.
| |
•
|
A statement relating to the site's proximity to fracture zones
within the bedrock.
| |
•
|
Additional information deemed pertinent by the geotechnical
engineer.
| |
•
|
Recommendations for any special design considerations necessary
for the design of recharge systems on the site. For example, required
soil depth over bedrock, appropriate surface grades over recharge
areas, appropriate hydraulic head over recharge areas, etc.
| |
•
|
Justification as to why the site should be developed to a high
impervious density if the site has adverse soil and geotechnical limitations,
which prohibit the ability to induce natural recharge. Explain how
these limitations will not create the potential for undue harm to
the environment and the Spring Creek Watershed when the site is developed.
| |
Where it has been shown that recharge cannot be performed and
a waiver of the recharge requirements is being requested, it shall
be required that the first one inch of runoff from all new impervious
areas be treated through under-drained facilities. These facilities
may include under-drained basins, rain gardens, and infiltration trenches.
Treatment is to include use of an amended topsoil to provide filtration
of the stormwater. All under-drain outlets are to include a restrictor
plate to prevent the underdrain system from functioning as a primary
outlet.
| ||
The following guidelines are provided relative to the use of
subsurface exfiltration BMP's (often incorrectly referred to as engineered
infiltration BMPs):
| ||
•
|
Soils should have a minimum percolation rate of 50 min/cm for
effective operation of subsurface exfiltration BMPs. If no site soils
have percolation rates of 50 min/cm, subsurface exfiltration BMPs
should not be used.
| |
•
|
A minimum of 30 inches of soil must be maintained between the
bottom of a subsurface exfiltration BMP and the top of bedrock or
seasonally high groundwater table. This statement is subject to the
recommendation of a qualified Geotechnical Engineer.
| |
•
|
If the minimum percolation rate is not met and/or the minimum
soil depth can not be maintained on a site, recharge should be accommodated
by directing shallow sheet flow from impervious areas across surface
filter strips and/or undisturbed natural areas, or some other innovative
surface infiltration feature should be used. Limiting subsurface percolation
rates and/or depth to bedrock shall not by themselves warrant a recharge
waiver.
| |
In addition, since recharge is intended as a volume control,
innovative or new methods that address the significant increase in
the volume of runoff from sites having large impervious areas are
encouraged. These volume control alternatives can be used only if
they can be shown to function with the original intent through sound
engineering and science. The final determination of "original intent"
shall always be the right of the Borough Engineer.
|
Up to 200 acres in size
|
Rational Method
|
Between 200 acres and 1.5 square miles
|
HEC-1
PSRM
TR-20
|
Over 1.5 square miles in size
|
PSU-IV with the carbonate adjustment factor at the discretion
of the Borough Engineer
Other methods as approved by the Borough Engineer such as SWMM,
SWIRM-ROUTE, etc.
|
Local, residential, residential access
|
25-year
|
Residential collector, collector
|
50-year
|
Arterial
|
100-year
|
Runoff events in excess of the indicated design event must be
conveyed safely downstream.
|
WQdepth = 0.25+(0.012)2.9(0.044(SIA))
WQv = WQdepth(A)/12
|
Where:
| |||
WQv
|
=
|
water quality volume in acre-feet
| |
WQdepth
|
=
|
depth in inches that must be captured for impervious areas
| |
SIA
|
=
|
percent of site impervious area (all paved areas and roof with
asphalt-based roofs)
| |
A
|
=
|
total of all paved areas and asphalt-based roofs on site in
acres
|
The required water quality volume that must be treated for any
WQS development, on sites in sensitive areas underlain by carbonate
rock, and all areas not underlain by carbonate rock is to be computed
within the entire Spring Creek Basin as:
|
WQdepth = the larger of 0.5 inches or
0.25+(0.012)2.9(0.044(SIA))
|
WQv = WQdepth(A)/12
|
Where:
| |||
WQv
|
=
|
water quality volume in acre-feet
| |
WQdepth
|
=
|
depth in inches that must be captured for impervious areas
| |
SIA
|
=
|
percent of site impervious area (all paved areas and roof with
asphalt-based roofs)
| |
A
|
=
|
total of all paved areas and asphalt-based roofs on site in
acres
|
For designs in which the final roof material is unknown, the
Design Engineer must assume an asphalt-based roof.
|
The water quality volume must be captured and treated through
a water quality BMP over an extended period of time as per the specific
requirements of each structure. Credits to reduce the effective impervious
area are applicable as presented in Chapter 4 of the Stormwater Management
Plan.
|
Non-Structural Technique
|
Water Quality Credit
|
---|---|
Drainage-way Protection (DWP)
|
Subtract Drainage-way Protection Areas from impervious site
area in WQv computation.
|
Natural Area Conservation (NAC)
|
Subtract Conserved Natural Areas from impervious site area in
WQv computation.
|
Filter/Buffer Area
|
Subtract impervious areas discharged over pervious areas from
impervious site area in WQv computation.
|
IAc = WIA LIA
|
Where:
| |||
IAc
|
=
|
Impervious area recharge credit (L2).
| |
LIA
|
=
|
Length of impervious area measured perpendicular to the sheet
flow direction (L).
| |
WIA
|
=
|
Width of impervious area (L). Maximum width permitted for credit
is the smaller of 100 feet or twice the width of the vegetated filter
strip.
|