Approvals issued and actions taken under this chapter do not
relieve the applicant of the responsibility to secure required permits
or approvals for activities regulated by any other code, law, regulation
or ordinance.
Peak rate controls for large storms, up to the one-hundred-year
event, is essential in order to protect against immediate downstream
erosion and flooding. The following peak rate controls have been determined
through hydrologic modeling of the Neshaminy Creek Watershed.
A. Standards for managing runoff from each subarea in the Neshaminy
Creek Watershed for the two-, five-, ten-, twenty-five-, fifty-, and
one-hundred-year design storms are shown in Table 304.1. Development
sites located in each of the management districts must control proposed
development condition runoff rates to existing condition runoff rates
for the design storms in accordance with Table 304.1 below.
|
Table 304.1
Peak Rate Runoff Control Standards by Stormwater Management
Districts in the Neshaminy Creek Watershed
|
---|
|
District
|
Design Storm Post-Development
(Proposed Conditions)
|
Design Storm Predevelopment
(Existing Conditions)
|
---|
|
A
|
2-year
|
1-year
|
|
|
5-year
|
5-year
|
|
|
10-year
|
10-year
|
|
|
25-year
|
25-year
|
|
|
50-year
|
50-year
|
|
|
100-year
|
100-year
|
|
B
|
2-year
|
1-year
|
|
|
5-year
|
2-year
|
|
|
10-year
|
5-year
|
|
|
25-year
|
10-year
|
|
|
50-year
|
25-year
|
|
|
100-year
|
50-year
|
B. General. Proposed condition 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
Map (see Appendix D of this chapter). As shown on this map, portions
of the Borough are located in both District A and B.
C. District boundaries. The exact location of the stormwater management
district boundaries as they apply to a given development site shall
be determined by mapping the boundaries as part of the SWM site plan.
D. Sites located in more than one district. For a proposed development
site located within two or more stormwater management district category
subareas, the peak discharge rate from any subarea shall meet the
management district criteria for the district in which the discharge
is located.
E. Off-site areas. When calculating the allowable peak runoff rates,
developers do not have to account for runoff draining into the subject
development site from an off-site area. On-site drainage facilities
shall be designed to safely convey off-site flows through the development
site.
F. Site areas. The stormwater management site area is the only area
subject to the management district criteria. Nonimpacted areas or
nonregulated activities bypassing the stormwater management facilities
would not be subject to the management district criteria.
G. 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 the Borough:
(1) Meet the full requirements specified by Table 304.1 and §
338-12A through
F; or
(2) Reduce the total impervious surface on the site by at least 20% based
upon a comparison of existing impervious surface to proposed impervious
surface.
[Added 12-16-2015 by Ord.
No. 2015-1881]
Additional erosion and sediment control standards and criteria
are recommended to be applied where infiltration BMPs are proposed.
They shall include the following:
A. These areas shall be protected from sedimentation and compaction
during the construction phase.
B. BMPs shall not be constructed, nor the areas receive runoff, until
the entire drainage areas tributary to the infiltration BMPs have
achieved final stabilization.
[Added 12-16-2015 by Ord.
No. 2015-1881]
The design of all regulated activities should include the following
to minimize stormwater impacts:
A. The applicant should find practicable alternatives to the surface
discharge of stormwater, the creation of impervious surfaces, and
the degradation of waters of the commonwealth and must maintain, as
much as possible, the natural hydrologic regime of the site.
(1) An alternative is practicable if a determination is made by the Borough
that 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.
[Added 12-16-2015 by Ord.
No. 2015-1881]
A. Infiltration best management practices (BMPs) may be required by
the Borough if they can be reasonably implemented based upon site
conditions and will not be required if the site qualifies for an exemption
from the infiltration requirements of this chapter.
(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 soil depth of 24 inches between the bottoms of the
infiltration BMPs and bedrock or other limiting zones.
(b)
An infiltration rate sufficient to accept the additional stormwater
load and dewater completely as determined by field tests conducted
by the applicant's qualified professional.
(2) All open-air infiltration facilities shall be designed to completely
infiltrate the recharge (infiltration) volume (Rev) within three days (72 hours) from the end of the design storm.
(3) All subsurface and contained facilities such as capture-and-reuse
systems must have storage available equivalent to the water volume
control amount within three days (72 hours) from the end of the design
storm.
(4) Pretreatment shall be provided prior to infiltration.
(5) The size of the infiltration facility shall be based upon the following
volume criteria:
Where practicable and appropriate, the recharge volume shall
be infiltrated on site. The recharge volume shall be equal to one
inch of runoff (I) over all proposed impervious surfaces.
The Rev required shall be computed as:
|
Where:
|
|
|
Rev
|
=
|
Recharge volume (cubic feet)
|
|
|
I
|
=
|
Impervious area within the limits of earth disturbance (square
feet)
|
|
|
An asterisk (*) in equations denotes multiplication.
|
B. Soils. A detailed soils evaluation of the project site shall be developed
by the applicant to determine the suitability of infiltration facilities.
The evaluation shall be performed by a qualified professional, and,
at a minimum, address soil permeability, depth to bedrock, and subgrade
stability. The general process for designing an 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
must be implemented if these tests are not completed.
(2) Perform 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 accepted for design purposes.
(3) Design the infiltration structure for the required recharge volume
(Rev) based on field tests at the elevation
of the proposed infiltration surface.
(4) If on-lot infiltration structures are proposed by the applicant's
qualified professional, the applicant must demonstrate to the Borough
that the soils are conducive to infiltrate on the lots identified.
(5) The applicant must install an impermeable liner in detention basins
where the possibility of groundwater contamination exists. A detailed
hydrogeologic investigation may be required by the Borough.
[Added 12-16-2015 by Ord.
No. 2015-1881]
A. If a perennial or intermittent stream passes through the site, the
applicant shall create a riparian buffer extending a minimum of 50
feet to either side of the top of bank of the channel. The buffer
area shall be established and maintained in an undisturbed state.
This buffer area may be maintained as a meadow with minimal mowing
of the grassed area, or as a forested buffer, being planted with appropriate
native vegetation (refer to Appendix B of the BMP Manual for plant
lists). If the applicable rear or side yard setback is less than 50
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. The Borough may
allow a smaller buffer width of not less than 10 feet based upon site-specific
conditions. This does not include lakes or wetlands.
B. The Borough may require the following stream bank erosion/channel
protection requirements if they can be reasonably implemented based
upon site conditions:
(1) 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 storm event 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 or a sand filter) so that the proposed conditions
one-year, twenty-four-hour storm event takes a minimum of 24 hours
to drain from the facility from a point when the maximum volume of
water from the one-year, twenty-four-hour storm event 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 volume control orifice is at the invert of the facility).
(2) The minimum orifice size in the outlet structure of 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. It is recommended that the design,
to accommodate maintenance, include a replaceable and/or porous media
filter cartridge.
[Added 12-16-2015 by Ord.
No. 2015-1881]
A. Peak rate requirements.
(1) The Wissahickon Creek Watershed has been divided into stormwater
management districts as shown on the attached Management District
Map. The District A peak rate requirements specified in Table 312.1
below shall be implemented in the Borough in addition to all other
applicable requirements.
(2) Standards for managing peak rates of runoff from subareas of the
Borough located in the Wissahickon Creek Watershed for the two-, five-,
ten-, twenty-five-, fifty-, and one-hundred-year storm events are
shown in Table 312.1. Development sites located within the management
district must control proposed condition runoff rates to existing
condition runoff rates for the design storms in accordance with Table
312.1.
|
Table 312.1
Peak Rate Control Standards by Stormwater Management District
A
In The Wissahickon Creek Watershed
|
---|
|
District
|
Proposed Condition Design Storm
|
Existing Condition Design Storm
|
---|
|
A
|
2-year
|
Reduce to
|
1-year
|
|
|
5-year
|
|
5-year
|
|
|
10-year
|
|
10-year
|
|
|
25-year
|
|
25-year
|
|
|
50-year
|
|
50-year
|
|
|
100-year
|
|
100-year
|
(3) Projects that are required to obtain a NPDES permit for stormwater
discharges associated with construction activities are required to
show no increase in peaks from existing conditions.
(4) 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 predevelopment conditions
peak rate provisions for the specified design storms. The predevelopment
condition for new development is the existing condition.
B. General. Proposed condition 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.
C. District boundaries. The boundaries of the stormwater management districts are shown on an Official Map that is available for inspection at the Montgomery County Planning offices. A copy of the Official Map at a reduced scale is
attached to this chapter. The exact location of the stormwater management district boundaries as they apply to a given development site shall be determined by mapping the boundaries using the two-foot topographic contours (or most accurate data required) provided as part of the drainage plan.
D. Sites located in more than one district. For a proposed development
site located within two or more stormwater management district, the
peak discharge rate from any subarea shall meet the management district
criteria in 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. 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 the Borough:
(1) Meet the full requirements specified by Table 312.1 and §
338-20A through
F; or
(2) Reduce the total impervious surface on the site by at least 20% based
upon a comparison of existing impervious surface to proposed impervious
surface.
H. Stormwater control measures which, as determined by the Borough,
increase storage or infiltration volume, and which are not associated
with new land development or redevelopment activity that increases
runoff volume above existing levels, are exempt from the peak rate
requirements of this chapter, so long as peak outflow is not increased.
[Added 12-16-2015 by Ord.
No. 2015-1881]
A. Stormwater runoff from all development sites with a drainage area
of greater than 200 acres shall be calculated using a generally accepted
calculation technique that is based on the NRCS Soil Cover Complex
Method. The qualified professional must consult with the Borough to
gain approval of design methods prior to design.
Table 313.1 summarizes acceptable computation methods and the
method selected by the qualified professional shall be based on the
individual limitations and suitability of each method for a particular
site. The Borough 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 313.1
Acceptable Computation Methodologies for Stormwater Management
Plans
|
---|
|
Method
|
Method Developed By
|
Applicability
|
---|
|
WINTR-20
|
USDA NRCS
|
Applicable where use of full hydrology computer model is desirable
or necessary
|
|
WINTR-55
|
USDA NRCS
|
Applicable for land development plans within limitations described
in TR-55
|
|
HEC-HMS
|
U.S. Army Corps of Engineers
|
Applicable where use of full hydrologic computer model is desirable
or necessary
|
|
Rational Method or commercial computer package based on Rational
Method)
|
Emil Kuichling (1889)
|
For sites less than 200 acres and with times of concentration
less than 60 minutes (tc < 60 min), or as approved by the Borough
and/or Municipal Engineer
|
|
Other methods
|
Varies
|
Other computation methodologies approved by the Borough and/or
Municipal Engineer
|
|
*
|
Note: Successors to the above methods are also acceptable.
|
B. If a hydrologic computer model, such as HydroCAD or HEC-HMS, is used
for stormwater runoff calculations, then the duration of rainfall
shall be 24 hours. The rainfall distribution should reference NRCS
Type II.
C. For the purposes of existing conditions flow rate determination,
undeveloped land shall be considered as "meadow," unless the natural
ground cover generates a lower curve number or Rational "C" value
(i.e., forest).
D. 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 flow velocities as determined by the Manning Equation.
E. The Manning Equation is preferred for 1-D, gradually-varied, open
channel flow. In other cases, appropriate, applicable methods should
be applied; however, early coordination with the Borough is necessary.
F. Outlet structures for stormwater management facilities shall be designed
to meet the performance standards of this chapter using the generally
accepted hydraulic analysis technique or method of the Borough.
G. The design of any stormwater detention facilities intended to meet
the performance standards of this chapter shall be verified by routing
the design storm hydrograph through these facilities using the Storage
Indication Method. For drainage areas greater than 200 acres in size,
the design storm hydrograph shall be computed using a calculation
method that produces a full hydrograph. The 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.