Water volume controls shall be implemented using the Design Storm Method in Subsection
A or the Simplified Method in Subsection
B below for all regulated activities not otherwise exempted by §
170-14. For regulated activity areas equal to or less than one acre that do not require hydrologic routing to design the stormwater facilities, this chapter establishes no preference for either methodology; therefore, the applicant may select either methodology. Volume control by way of infiltration facilities may be required to incorporate a pretreatment facility capable of separating grit and other fine materials from the stormwater where accessibility and maintenance ability is restricted.
A. The Design Storm Method (see Section 8.7 of the most current version
of the Pennsylvania Stormwater Best Management Practices Manual) is
applicable to any size of regulated activity. This method requires
detailed modeling based on site conditions.
(1) The post-development total runoff volume for all storms equal to
or less than the two-year, twenty-four-hour duration precipitation
shall not be increased.
(2) For modeling purposes:
(a)
Existing (predevelopment) nonforested pervious areas must be
considered meadow in good condition or its equivalent.
(b)
Twenty percent of existing impervious area, when present, shall
be considered meadow in the model for existing conditions.
(3) Field infiltration tests shall be required in accordance with §
170-19.
B. The Simplified Method (see Section 8.7 of the most current version
of the Pennsylvania Stormwater Best Management Practices 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
to regulated activities greater than one acre or for projects that
require design of stormwater detention or rate-control facilities.
For new impervious surfaces:
(1) Stormwater facilities shall be sized to capture at least the first
two inches of runoff from all new impervious surfaces.
(2) 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 the commonwealth. Removal options include
reuse, evaporation, transpiration, and infiltration.
(3) Infiltration facilities shall be designed to accommodate infiltration
of the entire permanently removed runoff; however, in all cases at
least the first 1/2 inch of the permanently removed runoff should
be infiltrated.
(4) The second one inch of runoff from new impervious surfaces shall
be detained using structural and nonstructural BMPs (as outlined in
the most current version of the Pennsylvania Stormwater Best Management
Practices Manual) and released at a controlled rate.
(5) Regulated activities eligible under this method are exempt from the requirements of §
170-16, Rate controls.
(6) Field infiltration tests shall be required in accordance with §
170-19.
Infiltration testing shall be required at all infiltration BMP
locations in accordance with the following:
A. 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 BMPs shall be:
(1) Site evaluation to determine general areas of suitability for infiltration
practices.
(2) Provide field test throughout the area proposed for development to
determine appropriate infiltration rate and/or hydraulic conductivity.
For subdivision and land development creating between one and five
proposed lots, one infiltration test must be conducted for each lot.
For any subdivision and land development creating five or more proposed
lots, one infiltration test must be completed for every five lots.
For cases where existing lots are proposed for new land development,
an infiltration test must be completed in each area where infiltration
facilities are proposed. Additional infiltration testing may be required
where directed by the Township Engineer due to existing site conditions.
Infiltration tests must be taken at the location and depth of all
proposed infiltration structures.
(3) Design infiltration structures for the required storm volume based
on all available data.
(4) A double-ring infiltrometer test shall be used for all infiltration
tests in accordance with the current edition of the BMP Manual.
B. Plans for infiltration must show the locations of existing and proposed
septic tank infiltration areas and wells. A minimum twenty-five-foot
separation from on-lot disposal system (OLDS) infiltration areas,
including replacement areas, is desired and will be evaluated by the
Township on a case-by-case basis. However, the separation shall not
be less than the PADEP required 10 feet.
C. Alternative BMPs.
(1) An alternative BMP shall be utilized where the applicant can demonstrate
that infiltration is not possible due to the following reasons:
(a)
Seasonal high-water table.
(b)
Permeability rate below 0.5 inch per hour based on county soil
inventory or infiltration testing.
(c)
Soil depth less than three feet.
(d)
Separation distances as follows:
[1]
One hundred feet from water supply wells;
[2]
Ten feet downgradient or 50 feet upgradient from building foundations;
[3]
Fifty feet from septic system drain fields; and
[4]
Fifty feet from a geologic contact with limestone bedrock.
(2) Examples of alternative BMPs include rooftop disconnection and establishing
sheet flow in conjunction with the use of a bioretention area, forested
buffer area or other acceptable BMP measure in accordance with the
BMP Manual or as determined by the Township Engineer.