Stormwater runoff from all development sites shall be calculated
using either the Rational Method or a Soil-Cover-Complex Methodology.
A. 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 the Storage-Indication
Method. For drainage areas greater than 20 acres in size, the design
storm hydrograph shall be computed using a calculation method that
produces a full hydrograph. The Borough may approve the use of any
generally accepted full hydrograph approximation technique and shall
use a total runoff volume that is consistent with the volume from
a method that produces a full hydrograph.
B. All calculations consistent with this chapter using the Soil-Cover-Complex
Method shall use the appropriate design rainfall depths for the various
return period storms. If a hydrologic computer model such as PSRM
or HEC-1 is used for stormwater runoff calculations, then the duration
of rainfall shall be 24 hours.
C. 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, SCS, TR-55 (as amended or replaced
from time to time by SCS). Times of concentration for channel and
pipe flow shall be computed using Manning's equation.
D. Where uniform flow is anticipated, the Manning equation shall be
used for hydraulic computations and to determine the capacity of open
channels, pipes and storm sewers.
E. Outlet structures for stormwater management facilities shall be designed
to meet the performance standards of this chapter using any generally
accepted hydraulic analysis technique or method.
The methodology for determining required stormwater controls
for a regulated activity is outlined below:
A. Compute.
(1) Predevelopment hydrograph at the site discharge point for the two-
and ten-year, twenty-four-hour storm.
(2) Post-development hydrograph at the site discharge point incorporating
any nondetention techniques such as pervious areas, swales, infiltration
trenches, etc.
(3) Note: Hydrographs may be obtained from SCS methods such as TR-55
or from use of the modified rational formulas.
B. Compare. Post-development hydrographs with predevelopment hydrographs. If the peak rate of runoff and the shape of the hydrographs are nearly identical, stormwater management has been achieved. Detention will not be required. If not, proceed as per Subsection
C.
C. Design. Detention/retention facilities, in conjunction with any nondetention
techniques, such that post-development peak rates from the site will
not exceed predevelopment levels for both the two- and ten-year storms.