FLOOD07531

• County: Ouray
• Community: Ouray
• Stream Name: Uncompahgre River
• Basin: Gunnison
• Title: Floodplain Information Report
• Date: 4/1/1978
• Prepared For: Ouray County
• Prepared By: A & S Consultants
• Contract/PO #: &&
• Floodplain - Doc Type: Floodplain Report/Masterplan

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I~TERPRETATION A~D USE OF REPORT DATA

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Flooded Areas

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Flood Frequency and Dlschar~e
The 10-, 50-, 100-, snd SOa-year flood events wero used ss the flood
frequencies for this floodplain 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 tate studies.

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The IOO-year frequency flood has been selected by the State of Colorado
as the flood event to be used for floodplain delineation snd regulation.
Thus the Flooded Area plates show only the boundary of the laO-year flood-
plain. Since the base map for the Flooded Area plates is a 5' contour map
the outlines of the other flood events, 10-, 50-, and 500-ye~r, cannot be,
in same areas, readily plotted using the information in Table 3.

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These various flood events have an average occurrence of once in the
number of years as indicated. For example, the lOa-year flood occurs,
On the average, once in a 100 year period, aad has a one percent chance
of being equaled or exceedcd in any given year.
The particular uses for the various flood events in addition to those
stated above are as followa'

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The area delineated on the Flooded Area plates as the laO-year floodplain
meets the requirements of H.B. 1041 as an area of state interest. Also
upon official approval of this report by the Colorado Water Conservation
Board the area outlined by the lOa-year flood boundary will be designated
a,floodplain area and may be regulated accordingly by the local offiCials.

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A portion of the laO-year floodplain bet~een stations 160 and 540 is sub-
ject to flooding due to upstream conditions, caused by topographical char-
acteristics which divert floodwaters to said lands a~ explained in the hy-
draulic section of this report. These areas have a different designation
than the normal floodplain.

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la-Year and 50-Year Flood Events
Information regarding these lower frequenoy floods is especially useful
for future engineering studies and land use planning purposes related to
minor road systems, minor channel improvements, the location of parks
and recreatonal facilities, agricultural lands, and appurtenant structures.
For structures and uses of this type On the smaller tributaries and in
areas where the high risk of structural failure is economically feasible
and the har.ard to life and property noncxistent, the use of the lower
frequency floods may be considered.

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Due to the 5-foot contour intervsl there may be areas included within the
laO-year floodplain which are above the lOa-year ~ater surface elevation.

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Flood Elevations

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lOO-Yeor Flood Ev~nt

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Flood crest elevstions for the 10-, 50-, 100-, and 500-year floods may be
found in four separate locations in this report. TheFloodFrequency_Ele_
viltionandDischargeD.ltaTable,Table3,listtheseelevatioLls"tsel<,cted
reference points. The flooded Area Plates 3 to 18 give the plan view of
the flooded area on a Contour base map and the high watcr elevations for
th~ lOa-year flood ean be interpolated from this. The Flood Profile plates,
Plates 19 through 32 sho~ the stream-bed elevation and the high water eleva-
tions for all four frequency floods. Also the Cross Section plates, Plates
33 through 35 show a graphical representation of the high water elevations
at typical valley cross sections throughout the study rexch.

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The 100-year flood ev"nt may also be used for cngin"cr:tng d"sign purpos"s
...here a lower rtakof failure than the 10- or 50-year flood Ie deSired.
However, the nost important use of rhe laO-year flood event lies :tn flood-
plain designation and land use regulation as sct forth in the state stat-
1Jtc~. ThC' St"tc of Colorndo considers the 100,">,en.. frc'll.\cncy flood n~ thc
flood event to be used in desiening and protecting structures and d...ellines
for human occupation. Therefore, all floodplain regulations are based up-
On the lOa-year flood. Also, the area which would be inundated by the
100-yearfloodm.'l.ybedel1nc.atednsanarc.:>ofstateinterestassetforth
in H.B. 10ql -- the state's land use bill.

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The Flood Profiles may be used in areas where controversy arises 0 verthe
laO-year floodboundnryon the Flooded Area plates. Since the Flood Pro-
file platcs give the elevations ,,,,d distance or stationing from a known
point the high water elevattons can be surveyed on the ground to all eviate
any discrepancies on the base map.

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SOO-Year Flood Event -
The 500-year flood eVent is useful in making the public aware that floods
larger than the laO-year flood can and do occur. Just because a person is
living above the lOa-year flood boundary does not menn that he is complete-
ly safe fromflnoding. The 500-yesr flood event csn also be used forregu-
laUnshishdskdevelopments'"'ithin the flOOdplain.
Table 3 lists the dischargcs and water surface elevations for the 10-, 50-,
100-,and500-ycarfloodevents<l.tselect<.>drcfereneepoints.

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