Green infrastructure and low-impact development practices provided
in the BMP Manual4 shall be utilized for
all regulated activities wherever possible. Water volume controls
shall be implemented using the Design Storm Method in Subsection A
or the Simplified Method in Subsection B below. This ordinance establishes
that only the Design Storm Method may be used for regulated activities
that add greater than 1/2 acre of new impervious area or disturb greater
than one acre of total area.
A. The Design Storm Method (CG-1 in the BMP Manual4) 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 duration
precipitation.
(2) For modeling purposes:
(a)
Existing (predevelopment) nonforested pervious areas must be
considered meadow in good condition.
(b)
Twenty percent of existing impervious areas to be disturbed
by the project, when present, shall be considered meadow in good condition
in the model for existing conditions. Regulated activities not requiring
an NDPES permit are exempt from this requirement.
B. The Simplified Method (CG-2 in the BMP Manual4) provided below is independent of site conditions and should be
used if the Design Storm Method is not followed. This method is not
applicable for regulated activities that add greater than greater
than 1/2 acre of new impervious area or for projects that require
routing of stormwater storage facilities. For new impervious surfaces:
(1) Stormwater facilities shall capture at least the first two inches
of runoff from all new impervious surfaces.
(2) At least the first one inch of runoff from new impervious surfaces
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.
(3) Wherever possible, infiltration facilities should be designed to
accommodate infiltration of the entire permanently removed runoff;
however, in all cases, at least the first 0.5 inch of the permanently
removed runoff should be infiltrated.
(4) This method is exempt from the requirements of §
400-16, Rate controls.
In areas of carbonate geology, a geologist shall certify to
the following:
A. No stormwater management facility will be placed in, over, or immediately
adjacent to the following features:
(1) Closer than 100 feet from sinkholes.
(2) Closer than 100 feet from closed depressions.
(3) Closer than 100 feet from caverns, intermittent lakes, or ephemeral
streams.
(4) Closer than 50 feet from lineaments in carbonate areas.
(5) Closer than 50 feet from fracture traces.
(6) Closer than 25 feet from bedrock pinnacles (surface or subsurface).
B. Stormwater resulting from regulated activities shall not be discharged
into sinkholes.
C. If the developer can prove through analysis that the project site is an area underlain by carbonate geology, and such geologic conditions may result in sinkhole formations, then the project site is exempt from recharge requirements as described in §
400-16, Volume control. However, the project site shall still be required to meet all other standards found in this chapter.
D. It shall be the developer's responsibility to verify if the
project site is underlain by carbonate geology. The following note
shall be attached to all stormwater management plans and signed and
sealed by the developer's geologist: "I, __________, certify
that the proposed stormwater management facility (circle one) is/is
not underlain by carbonate geology."
E. Whenever a stormwater management facility will be located in an area
underlain by carbonate geology, a geological evaluation of the proposed
location by a geologist shall be conducted to determine susceptibility
to sinkhole formation. The evaluation may include the use of impermeable
liners to reduce or eliminate the separation distances listed in the
BMP manual. Additionally, the evaluation shall, at a minimum, address
soil permeability, depth to bedrock, seasonally high groundwater table,
susceptibility for sinkhole formation, suitability of stormwater management
facilities, subgrade stability, and maximum infiltration capacity
in depth of water per unit area.
F. A detailed soils evaluation of the project site shall be performed
to determine the suitability of recharge facilities. The evaluation
shall be performed by a qualified professional, and at a minimum,
address soil permeability, depth to bedrock, susceptibility to sinkhole
formation, and subgrade stability. The general process for designing
the infiltration BMP shall be:
(1) Site evaluation to determine general areas of suitability for infiltration
practices.
(2) Provide field percolation tests throughout the area proposed for
development to determine appropriate percolation rate and/or hydraulic
conductivity. At least one infiltration test must be included in each
soil group and at least one infiltration test must be conducted for
each five lots proposed for development. Infiltration tests must be
taken at the location and depth of all proposed infiltration structures.
(3) Design infiltration structure for required storm volume based on
all available data.
G. Extreme caution shall be exercised where infiltration is proposed
in geologically susceptible areas such as strip mine or limestone
areas. It is also extremely important that the design professional
evaluate the possibility of groundwater contamination from the proposed
infiltration/recharge facility and recommend a hydrogeologic study
be performed if necessary. Whenever a basin will be located in an
area underlain by limestone, a geological evaluation of the proposed
location shall be conducted to determine susceptibility to sinkhole
formations. The design of all facilities over carbonate formations
shall include measures to prevent groundwater contamination and, where
necessary, sinkhole formation. The infiltration requirement in the
high-quality/exceptional waters shall be subject to the Department's
25 Pa. Code Chapter 93, and anti-degradation regulations. A detailed
hydrogeologic investigation may be required by the Township, and where
appropriate, the Township may require the installation of an impermeable
liner in detention basins.