§ 190-215 § 190-215
D.
(1) Stormwater management systems must be designed so that
the ten-year, twenty-four-hour postdevelopment peak
discharge rate does not exceed the ten-year, twenty-four-hour
predevelopment peak discharge rates.
(2) In order to meet this standard, controls must be developed for
the two-year, ten-year and fifty-year, twenty-four-hour storm
events. The hundred-year, twenty-four-hour storm event must
be evaluated to demonstrate that there will not be increased
flooding impacts off site. [Amended 6-26-2018 by Ord. No.
O-18-015]
(3) Measurement of peak discharge rates shall be calculated
using point of discharge or the downgradient property
boundary. The topography of the site may require evaluation
at more than one location if flow leaves the property in more
than one direction. An applicant may demonstrate that a
feature beyond the property boundary is more appropriate as
a design point.
Recharge to groundwater. Annual groundwater recharge rates
shall be maintained by providing infiltration by the use of
structural and nonstructural methods. The annual recharge from
post development site conditions shall mimic the annual recharge
from predevelopment site conditions. Best management practice
(BMP) techniques to achieve recharge requirements include
infiltration, bioretention, dry swale, and _ nonstructural
techniques. Alternative techniques may be used if they meet the
performance criteria stated herein and are approved by the
Planning Board.
Water quality.
(1) For discharges to the Conservation Zone within the Water
Supply Protection District as defined in § 190-24, the runoff
volume to be treated for water quality is calculated as 1.0
inch of runoff multiplied by the total impervious area of the
postdevelopment project site.
(2) For all other discharges, the runoff volume to be treated for
water quality is calculated as 0.5 inch of runoff multiplied by
the total impervious area of the postdevelopment project site.
(3) Removal of 80% of the total suspended solids (TSS),
floatables, greases, and oils. For new developments,
stormwater management systems shall be designed to
remove 80% of the average annual load of total suspended
:2
21
Hic it Discharge Detection and Eimination Plan
21
