FLOOD01680

• Designation Number: 126
• County: Boulder
• Community: Unincorporated Boulder County
• Stream Name: St. Vrain Canyon upstream of Lyons
• Title: Floodplain Information Report - South St. Vrain Canyon upstream of Lyons, Boulder County, CO
• Date: 2/1/1978
• Designation Date: 1/1/1979
• Floodplain - Doc Type: Floodplain Report/Masterplan

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INTERPRETATION AND USE OF REPORT DATA

the state statutes. The State of Colorado considers the 100-year fre-

quency flood as the flood event to be used in designing and protecting

structures and dwellings for human occupation. Therefore, all flood-

plain regulations are based upon the 100-year flood. Also, the area

which would be inundated by the 100-year flood may be delineated as an

area of state interest as set forth in H.B. 1041 -- the state's land

use bill.

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Flood Frequency and Discharge

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The 10-, 50-, 100- and SOD-year flood events were used as 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 rate studies.

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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. It is interesting to

note that, statistically, chances that the 100-year flood will never

occur in the next 100 years are 37 percent.

SOD-year flood event --

The SOD-year flood event is useful in making the public aware that

floods larger than the 100-year flood can and do occur. Just because a

person is living above the 100-year flood boundary does not mean that

he is completely safe from flooding. The SOD-year flood event can also

be used for regulating high risk developments within the floodplain

such as nuclear power plants, or the storage or manufacture of toxic or

explosive materials.

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

Table 3 lists the discharges and water surface elevations for the 10-,

50-, 100-, and SOD-year flood events at selected reference points.

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10-year and 50-year flood events

Information regarding these more frequent 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 recreational facilities, agricultural lands, and appurtenant

structures. For structures and uses of this type on the smaller trib-

utaries and in areas where the high risk of structural failure is eco-

nomically feasible and the hazard to life and property nonexistent, the

use of the more frequent floods as design criteria may be considered.

Flooded Areas

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As indicated, the 100-year frequency flood has been selected by the

State of Colorado as the flood event to be used for floodplain delinea-

tion and regulation. Thus, the flooded area plates show only the

boundary of the 100-year floodplain. Since the base map for the

flooded area plates is a two-foot contour map, the outlines of the

other flood events, 10-, 50-, and SOD-year, can be readily plotted

using the information in Table

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100-year flood event --

The 100-year flood event may also be used for engineering design pur-

poses where a lower risk of failure than the 10- or 50-year flood is

desired. However, the most important use of the 100-year flood event

lies in floodplain designation and land use regulation as set forth in

The area delineated on the flooded area plates as the 100-year flood-

plain meets the requirements of H.B. 1041 as an area of state interest.

Also, upon official approval, the water surface elevation in this

report by the Colorado Water Conservation Board, the area outlined by

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