The following permit or other regulatory requirements may apply
to certain regulated activities and shall be met prior to (or as a
condition of) final approval by the municipality of the SWM site plan
and prior to commencement of any regulated activities, as applicable:
A. All regulated activities subject to permit or regulatory requirements
by PADEP under regulations at 25 Pa. Code Chapter 102, or erosion
and sediment control requirements of the municipality.
B. Work within natural drainageways subject to permit by PADEP under 25 Pa. Code Chapter
105.
C. Any BMP or conveyance that would be located in or adjacent to surface waters of the commonwealth, including wetlands, subject to permit by PADEP under 25 Pa. Code Chapter
105.
D. Any BMP or conveyance that would be located on or discharge to a
state highway right-of-way or require access to or from a state highway
and 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 PADEP under 25 Pa. Code Chapter
105.
The applicant shall design the site to minimize the disturbances
to land, site hydrology, and natural resources and to maintain the
natural hydrologic regime, drainage patterns and flow conditions.
For regulated activities with 10,000 or more square feet of proposed
earth disturbance or two thousand (2,500) or more square feet of proposed impervious surfaces, the applicant shall demonstrate in its SWM site plan (as required in §
106-401) that the design sequence, objectives and techniques described below were applied to the maximum extent practicable in the site design of the regulated activity while complying with all other requirements of this chapter. The site design shall:
A. First, identify and delineate all existing natural resources and natural and man-made hydrologic features listed in §
106-403I and
J that are located within the site or receive discharge from or may be impacted by the proposed regulated activity.
B. Second, provide a prioritized listing of these resources and features
to identify:
(1)
Those to be incorporated into the site design in a manner that
provides protection from any disturbance or impact from the proposed
regulated activity;
(2)
Those to be protected from further disturbance or impact but
for which the proposed regulated activity will provide improvement
to existing conditions;
(3)
Those that can be incorporated into and utilized as components
of the overall site design in a manner that protects or improves their
existing conditions while utilizing their hydrologic function within
the limits of their available capacity (e.g., for infiltration, evapotranspiration,
or reducing pollutant loads, runoff volume or peak discharge rates,
etc.) to reduce the need for or size of constructed BMPs; and
(4)
Those that may be considered for alteration, disturbance or
removal.
C. Third, develop the site design to achieve the following:
(1)
Recognize and incorporate the priorities identified in Subsection
B as the basis for the proposed site layout, grading, construction, and permanent ground cover design;
(2)
Minimize earth disturbance (both surface and subsurface);
(3)
Maximize protection of or improvement to natural resources and
special management areas;
(4)
Minimize the disturbance of natural site hydrology, in particular
natural drainage features and patterns, discharge points and flow
characteristics, natural infiltration patterns and characteristics,
and natural channel and floodplain conveyance capacity;
(5)
Incorporate natural hydrologic features and functions identified in §
106-304B into the site design to protect and utilize those features and their hydrologic functions to reduce the need for or size of constructed BMPs;
(6)
Maximize infiltration and the use of natural site infiltration
features, patterns and conditions, and evapotranspiration features;
(7)
Apply selective grading design methods to provide final grading
patterns or preserve existing topography in order to evenly distribute
runoff and minimize concentrated flows;
(8)
Minimize the cumulative area to be covered by impervious surfaces
and:
(a)
Minimize the size of individual impervious surfaces;
(b)
Separate large impervious surfaces into smaller components;
(c)
Disconnect runoff from one impervious surface to another; and
(d)
Utilize porous materials in place of impervious wherever practicable;
(9)
Minimize the volume and peak discharge rates of stormwater generated;
(10)
Avoid or minimize stormwater runoff pollutant loads and receiving
stream channel erosion;
(11)
Locate infiltration and other BMPs:
(a)
At or as near to the source of generation as possible; and
(b)
At depths that are as shallow as possible;
(12)
Prioritize the selection and design of BMPs as follows:
(a)
Nonstructural and vegetation BMPs; then
(b)
Structural (surface and subsurface) BMPs;
(13)
For flow volumes requiring conveyance from the source of generation
to a BMP for management, give preference to open channel conveyance
techniques that provide infiltration and water quality benefits, and
landscaped-based management in common open space areas, where practicable;
and
(14)
Consider additional guidance for incorporating natural hydrology into the site and BMP designs, methods and techniques that support the objectives of Subsections
B and
C. Appendix B presents additional discussion of natural hydrology site design and sources of information for conservation design, low-impact design, and sustainable design.
D. The procedures set forth above shall be utilized to the maximum extent practicable for the overall site design and selection, location and design of features and BMPs to be used to comply with the requirements of §§
106-305,
106-306,
106-307 and
106-308.
Volume control BMPs are intended to maintain existing hydrologic
conditions for small storm events by promoting groundwater recharge
and/or evapotranspiration as described in this section. Runoff volume
controls shall be implemented using the Design Storm Method described
in Subsection A below or through continuous modeling approaches or
other means as described in the BMP Manual. Small projects may use
the method described in Subsection B to design volume control BMPs.
A. The Design Storm Method is applicable to any size of regulated activity.
This method requires detailed modeling based on site conditions.
(1)
Do not increase the post-development total runoff volume for
all storms equal to or less than the two-year twenty-four-hour storm
event.
(2)
For modeling purposes:
(a)
Existing (predevelopment) nonforested pervious areas must be
considered meadow in good condition.
(b)
When the existing project site contains impervious area, 20%
of existing impervious area to be disturbed shall be considered meadow
in good condition in the model for existing conditions.
(c)
The maximum loading ratio for volume control facilities in Karst
areas shall be 3:1 impervious drainage area to infiltration area and
5:1 total drainage area to infiltration area. The maximum loading
ratio for volume control facilities in non-Karst areas shall be 5:1
impervious drainage area to infiltration area and 8:1 total drainage
area to infiltration area. A higher ratio may be approved by the municipality
if justification is provided. Hydraulic depth may be used as an alternative
to an area-based loading ratio if the design hydraulic depth is shown
to be less than the depth that could result from the maximum area
loading ratio.
B. Volume control for small projects. At least the first one inch of
runoff from new impervious surfaces or an equivalent volume shall
be permanently removed from the runoff flow, i.e., it shall not be
released into the surface waters of this commonwealth. Removal options
include reuse, evaporation, transpiration and infiltration.
C. A detailed geologic evaluation of the development site shall be performed in areas of carbonate geology to determine the design parameters of recharge facilities. A report shall be prepared in accordance with §
106-405A of this chapter.
D. Storage facilities, including normally dry, open-top facilities,
shall completely drain the volume control storage over a period of
time not less than 24 hours and not more than 72 hours from the end
of the design storm. Any designed infiltration at such facilities
is exempt from the minimum twenty-four-hour standard, i.e., may infiltrate
in a shorter period of time, provided that none of this water will
be discharged into waters of this commonwealth.
E. Any portion of the volume control storage that meets the following
criteria may also be used as rate control storage;
(1)
Volume control storage that depends on infiltration is designed according to the infiltration standards in §
106-301.
(2)
The volume control storage which will be used for rate control
is that storage which is available within 24 hours from the end of
the design storm based on the stabilized infiltration rate and/or
the evapotranspiration rate.
F. Volume control storage facilities designed to infiltrate shall avoid
the least permeable hydrologic soil group(s) at the development site.
Rate control for large storms, up to the one-hundred-year event,
is essential to protect against immediate downstream erosion and flooding.
A. Match pre-development hydrograph. Applicants shall provide infiltration facilities or utilize other techniques which will allow the post-development one-hundred-year hydrograph to match the pre-development one-hundred-year hydrograph, along all parts of the hydrograph, for the development site. To match the pre-development hydrograph, the post-development peak rate must be less than or equal to the pre-development peak rate, and the post-development runoff volume must be less than or equal to the pre-development volume for the same storm event. A shift in hydrograph peak time of up to five minutes and a rate variation of up to 5% at a given time may be allowable to account for the timing affect of BMPs used to manage the peak rate and runoff volume. "Volume control" volumes as given in §
106-305 above may be used as part of this option.
B. Where the pre-development hydrograph cannot be matched, one of the
following shall apply:
(1)
For areas not covered by a release rate map from an approved Act 167 plan: Post-development discharge rates shall not exceed the predevelopment discharge rates for the two-, ten-, twenty-five-, fifty-, and one-hundred-year twenty-four-hour storm events. (NOTE: A twenty-four-hour SCS Type II storm or an IDF Curve Rational Method storm. See Table III-1 in §
106-308.) If it is shown that the peak rates of discharge indicated by the post-development analysis are less than or equal to the peak rates of discharge indicated by the predevelopment analysis for two-, ten-, twenty-five-, fifty-, and one-hundred-year, twenty-four-hour storms (NOTE: A twenty-four-hour SCS Type II storm or an IDF Curve Rational Method storm. See Table III-1 in §
106-308.), then the requirements of this section have been met. Otherwise, the applicant shall provide additional controls as necessary to satisfy the peak rate of discharge requirement.
(2)
For areas covered by a release rate map from an approved Act 167 plan: For the two-, ten-, twenty-five-, fifty-, and one-hundred-year storm events (NOTE: A twenty-four-hour SCS Type II storm or an IDF Curve Rational Method storm. See Table III-1 in §
106-308.), the post-development peak discharge rates will follow the applicable approved release rate maps.
C. Normally dry, open-top storage facilities shall completely drain
the rate control storage over a period of time less than or equal
to 24 hours from the peak one-hundred-year water surface design elevation.
D. A variety of BMPs should be employed and tailored to suit the development
site. The following is a partial listing of BMPs which can be utilized
in SWM systems for rate control where appropriate:
(1)
Decreased impervious surface coverage.
(3)
Grassed channels and vegetated strips.
(4)
Bioretention areas (rain gardens).
(5)
Concrete lattice block or permeable surfaces.
(6)
Seepage pits, seepage trenches or other infiltration structures.
(9)
Cisterns and underground reservoirs.
(13)
Other methods as may be found in the BMP Manual.
E. Small projects are not required to provide for rate control.