FLOOD04795

• Designation Number: 98
• County: Garfield
• Community: Glenwood Springs
• Stream Name: Roaring Fork River & Colorado River
• Basin: Colorado Mainstem
• Title: Floodplain Information Report - Roaring Fork River & Colorado River, Glenwood Springs, CO
• Date: 11/1/1977
• Designation Date: 3/1/1978
• Prepared For: Glenwood Springs, Garfield County, CWCB
• Prepared By: Gingery Associates, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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The locations of the field surveyed cross sections and
the cross sections from the topographic maps are shown by
ref.erence points on the flooded area maps (Plates 3-101
and the flood profiles (Plates 11-19). The reference points,
flood elevations and discharges are also listed in Table 3,
pages 17 through 19.

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INTERPRETATION AND USE OF REPORT DATA

FlOOd Frequency and Discharge
The 10, SO, 100 and 500 year flood events were used as the
flood frequencies for this flood plain analysis. Thus the
data developed in this report will be compatible not only for
regulation purposes, and H.B. 1041 designations, but also for
Federal Insurance Administration Flood Insurance Rate Studies.

Ch.mnel roughness factors (Manning's "n") for these computa-
tions were assigned on the basis of field inspection of the
flood plain areas.
During floods, debris collecting on bridges could decrease
their flow carrying capacity and cause greater water
depths (backwater effect) upstream. Since peak floods
on both the Colorado River and Roaring Fork River ate pri-
marily sno~elt derived, they would dcvclo~ gradually,
allowing ti~e for maintenance crews to dislodge accunu-
lating debris, Since the occurrence and amQunt of debris
are indeterminate factors, only the physical characteristics
of the structures were considered in ptcparing the maps,

These various flood events have an average occurrence of once
in the number of years as indicated. For example, the 100
year flood occurs on the average, once in a 100' year period,
and has a one percent chance of being equaled or exceeded in
any given year.

The particular uses for the various flood events in addition
to those stated above are as follows.

10 Year and SO Year Flood Events

Information regarding these lower frequency flood is especially
useful for future engineering studies and land use planning
purposes related to minor road systems, minor channel improve-
ments, the location of parks and recreational facilities, ag-
ricultural lands, and appurtenant structures. For structures
and uses of this type in areas where the high risk of struc-
tural failure is economically feasible and the hazard to life
and property nonexistent, the use of the lo~er frequency floods
may be consideren.

A detailed description of the hydraulic analysis is included
in the Technical Addendum.

100 Year Flood Event

The 100 year flood event may also be used for engineerinq design
purpose!:: where a lower risk or railur'~ th.J.n the 10 c.r 50 year
flood is d<:siretl. HOI"ever, the most imuort<:lnt use of the 100
year flood event lies in flood plain dcsi~nation and land u~e
regulation as set forth in the state statutes. The State of
Colorado con~iders the 100 Yt:<lr frequency flood as the flood
event to be used in designing and protecting structures and
awellings for human occupation. Therefore, <Ill flood plain
regulations are based upon the 100 year flood. Also, the area
which would be inundate~ by the 100 ye<lr flood may be delinated
as an area of state interest as set forth in H.B. 1041 -- the
state's land use bill.

500 Year Flood Event

The SOO year flood event is llseful in making the public ,J....arc
that floods larger than the 100 year flood can and do occur.
Just because a person is livin'J above th<-' 100 year ~lood bound-
ary docs not muan that he is completey safe fro~ flooding. Thp