O-46-97 08/11/97VILLAGE OF LEMONT
ORDINANCE NO. O - -q7
AN ORDINANCE AMENDING ORDINANCE 955
ENTITLED "AN ORDINANCE REGULATING DEVELOPMENT
IN SPECIAL FLOOD HAZARD AREAS" FOR THE VILLAGE OF LEMONT
ADOPTED BY THE
PRESIDENT AND BOARD OF TRUSTEES
OF THE VILLAGE OF LEMONT
This Ii day of /4vgvsl- , 1997.
Published in pamphlet form by
authority of the President and
Board of Trustees of the Village
of Lemont, Cook, Will & DuPage
Counties, Illinois, this 1 ! fh
day of Awl us I , 1997.
AN ORDINANCE AMENDING ORDINANCE 955
ENTITLED "AN ORDINANCE REGULATING DEVELOPMENT
IN SPECIAL FLOOD HAZARD AREAS" FOR THE VILLAGE OF LEMONT
BE IT ORDAINED by the President and Board of Trustees of the Village of Lemont
that Ordinance No. 955 is hereby amended as follows:
Section 300.00 " Definitions" shall be amended to include:
300.42 " Depressional Storage ". Site depressional storage refers to the ability
of a site in predevelopment conditions to detain or retain stormwater runoff.
It's presence on site is indicated by the existence of areas characterized by
closed elevation contours. Depressional storage areas may be outleted in the
predevelopment condition and these outlets may consist of open channels,
culverts, storm sewers, filed tiles, seeps or other natural or manmade
drainage outlets. Depressional storage will be considered significant if the
volume is such that it has a hydrologic impact on the flow rate out of the area
for rainfall frequencies up to a 100 -year frequency. As a minimum, the
hydrologic affect of depressional storage shall be checked for rainfall
frequencies of 10 years and 100 years.
Depressional storage will be considered as having a hydrologic impact if for
those flood frequencies checked, the depressional storage attenuates the peak
discharge off -site by more than 10 percent when compared to the peak inflow
or the discharge for a 100 year frequency flood event is at a lower rate than
the maximum peak discharge allowed for 100 year flow rates for the
developed condition.
Section 700 "Occupation and Use of Flood Fringe Areas" shall be amended to include:
The applicant shall be responsible for demonstrating through the use of
hydrologic and hydraulic models the existence and effect of predevelopment
depressional storage. Hydrologic models shall generate flood hydrography for
design storms based on Illinois State Water Survey Bulletin 70. Rainfall
depths for frequencies and durations up to 100 years and 24 hours respectively
shall be checked. The volume of depressional storage shall be considered as
the volume contained on -site below the highwater elevation achieved by a
design storm of 100 -year frequency 24 hour duration routed through the
depressional storage. When this volume is found to be significant, it shall be
preserved in addition to any required volume of detention.
The applicant shall demonstrate that the site discharge in predevelopment
conditions is not less than the allowable peak discharge from the detention
ordinance considering the affect of depressional storage. If the site discharge
in the predevelopment condition is found to be less than the ordinance
allowable release rate, then the predevelopment peak site discharge shall be
used as the allowable release rate in computing the detention volume required
for development.
In those cases where detention for development is not required, then site
grading and drainage systems shall preserve the hydrologic function of the
existing depressional storage.
For purposes of meeting this ordinance requirement the ordinance will be
assumed to be met if the computed volume of existing predevelopment
depressional storage is added to the computed detention volume require in the
site storage facility, attained below the computed high water elevation.
This Ordinance shall be in full force and effect from and after its passage, approval
and publication in the manner provided by law.
All Ordinances or parts of Ordinances in conflict herewith shall be and the same are
hereby repealed.
The Village Clerk of the Village of Lemont shall certify to the adoption of this
Ordinance and cause the same to be published in pamphlet form.
PASSED AND APPROVED BY THE PRESIDENT AND BOARD OF TRUSTEES
OF THE VILLAGE OF LEMONT, COUNTIES OF COOK, WILL AND DUPAGE,
ILLINOIS, on this
Barbara Buschman
Keith Latz
Connie Markiewicz
Richard Rimbo
Ralph Schobert
Mary Studebaker
day of
, 1997.
AYES NAYS PASSED ABSENT
V
Approved by me this // day of
ATTEST:
ENE SMOLLEN, Village Clerk
1997.
Atank
A. - Village President
CHARLENE S OLLEN, Village Clerk
B: \VILLAGE \FLOOD.AMD