Green infrastructure and low-impact development practices provided
in the BMP Manual 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 chapter 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 Manual) 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 Manual) 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 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 §
107-15, Rate controls.
Stormwater runoff from all development sites shall be calculated
using the Rational Method, Modified Rational Method, or a soil cover
complex methodology.
A. Any stormwater runoff calculations involving drainage areas greater
than 200 acres, including on- and off-site areas, shall use generally
accepted calculation technique that is based on the NRCS Soil Cover
Complex Method. It is assumed that all methods will be selected by
the design professional based on the individual limitations and suitability
of each method for a particular site.
B. The municipality may allow the use of the Rational Method or Modified
Rational Method to estimate peak discharges from drainage areas that
contain less than 200 acres. When using the Rational Method, an ascending
and descending limb factor of three and seven shall be used, respectively.
C. All calculations consistent with this chapter using the Soil Cover
Complex Method shall use the appropriate design rainfall depths. If
a hydrologic computer model such as PSRM or HEC-RAS is used for stormwater
runoff calculations, then the duration of rainfall shall be 24 hours.
The SCS Rainfall Type II curve shall be used for the rainfall distribution.
Those projects proposing infiltration structures shall utilize this
method.
D. When routing a detention basin, the Rational Method or SCS method
may be used to determine peak rates through the primary outlet structure.
Only the SCS method may be used to determine the peak water surface
elevation during the 100-year, twenty-four-hour design storm, dictating
the height of the embankment.
E. Underground storage facilities that solely receive stormwater and
are designed to capture and infiltrate the entire 100-year, twenty-four-hour
SCS runoff volume may have their drainage areas removed from the overall
post-development drainage area(s).
F. For the purposes of predevelopment flow rate determination, undeveloped
land, including disturbed areas, shall be considered as "meadow" in
good condition, unless the natural ground cover generates a lower
curve number or rational "C" value (i.e., forest), as listed in Tables
1 and 2, respectively.
G. All calculations using the Rational Method shall use rainfall intensities
consistent with appropriate times of concentration for overland flow
and return periods. 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). Time of concentration for channel and
pipe flow shall be computed using Manning's Equation.
H. Runoff curve numbers (CN) for both existing and proposed conditions
to be used in the Soil Cover Complex Method shall be obtained from
Table 1.
I. Runoff coefficients (c) for both existing and proposed conditions
for use in the Rational Method shall be obtained from Table 2.
J. Where uniform flow is anticipated, the Manning Equation shall be
used for hydraulic computations such as the capacity of open channels,
pipes, and storm sewers. Values for Manning's roughness coefficient
(n) shall be consistent with Table 3.
K. 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 either
manual methods or computerized routing. Routing shall be based upon
the modified PULS method; other routing methodologies shall be subject
to the approval of the Municipal Engineer.
L. The stormwater collection system shall be designed using the peak
discharge computed using the rational formula.
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 §
107-14, Volume controls. 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 registered professional: "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
every 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 is 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 antidegradation regulations. A detailed hydrogeologic investigation may be required by the municipality, and, where appropriate, the municipality may require the installation of an impermeable liner in detention basins.