The stormwater management submission shall include, but not
be limited to, the following:
A.
A plan indicating the pre-developed drainage areas and time of concentrations
and flow paths used. This plan shall indicate the pre-developed contours
of the site in such detail as to be capable of being field verified.
B.
A plan indicating the post-developed drainage areas, time of concentration
and flow paths, and all drainage facilities. This plan shall indicate
the post-developed contours of the site. All facilities shall be labeled
in an easily understandable manner consistent with the stormwater
management report.
C.
A stormwater management report including all assumptions, methods,
references, and calculations for all stormwater drainage facilities.
The stormwater calculations shall be performed as specified
in the current edition PA DEP Erosion and Sediment Pollution Control
Program Manual, including the following:
A.
The methodology for calculating stormwater runoff shall be the rational
method or other method as approved by the Township Engineer.
B.
All pre-developed calculations shall be based upon existing land
use features. The conditions of the existing land uses shall, however,
be considered "good" in all instances regardless of actual conditions.
C.
Pre-developed and post-developed runoff shall be calculated for the
two-, ten-, twenty-five-, fifty-, and one-hundred-year storm events.
D.
All stormwater conveyance facilities shall be designed based on the
twenty-five-year storm event peak discharge to the facility.
E.
All stormwater pipes shall be smooth flow polyethelene pipe (ADS
N-12) reinforced concrete pipe, with a minimum diameter of 15 inches.
Changes in horizontal or vertical direction of pipes shall be accomplished
by installing a catch basin or a manhole. The minimum slope of pipe
shall be 0.35%. The crown of the pipe shall be at least six inches
below the subgrade elevation. Pipes under roads shall meet with requirements
set forth in PennDOT Design Manual Part 2 or any successor PennDOT
document on this subject. No riprap-lined areas shall be permitted
within existing Township right-of-way areas unless specifically authorized
by the Township.
F.
Catch basins shall be utilized at all inlet ends of pipes and shall
have bicycle-proof inlet grates.
G.
All catch basins, manholes, endwalls, etc., shall be as shown in
PennDOT Standard Details for Roadways.
H.
All swales shall be at least one foot deep, and the bottom of the
swale shall be at least six inches below subgrade elevation.
I.
Stormwater velocities in swales shall be calculated for both the
prestabilized and stabilized conditions. The type of swale lining
to be utilized shall be specifically indicated. Verification that
the velocity in the swale does not exceed the permissible velocity
for the design swale lining (prestabilized and stabilized) shall be
provided.
J.
Stormwater velocities at pipe outlets shall be calculated. Outlet
protection consisting of a riprap apron or other suitable control
measure shall be provided.
K.
The apron dimensions and riprap size shall be calculated and provided.
L.
The capacities and spacing of the catch basin inlets shall be indicated
to assure that they are capable of intercepting the design runoff.
M.
Post-developed runoff shall be restricted as follows for each pre-developed
drainage area:
(1)
The post-developed peak discharge for the ten-, twenty-five-, fifty-,
and one-hundred-year storm events shall not exceed 90% of the peak
discharge for the corresponding pre-developed storm events.
(2)
In order to achieve this, it is recommended that methods be considered
that will direct runoff from impervious areas to pervious areas or
infiltration trenches that will retain/detain the runoff, or, by some
other means suitable to the Township Engineer.
(3)
The post-developed twenty-five-year storm event peak discharge shall
not exceed the capacity of downstream stormwater facilities.
N.
Retention/detention facilities shall be designed such that the post-
to pre-development requirements set forth herein are met. Further,
the facility shall be designed with any emergency spillway, capable
of handling the entire one-hundred-year post-developed flow without
causing property damage. The emergency spillway shall be at an elevation
of at least two feet below the top of the impoundment embankment and
at least six inches above the highest principal outlet. Adequate evidence
that the facility(ies) will not present a public safety or health
hazard problem must be submitted in detail. This shall include, but
not be limited to the following:
