FLOOD08504

• County: El Paso
• Community: Calhan
• Stream Name: Calhan area rivers and streams
• Basin: Arkansas
• Title: Floodplain Management Study
• Date: 8/1/1995
• Prepared For: Calhan
• Prepared By: USDA
• Floodplain - Doc Type: Floodplain Report/Masterplan

B. Study [,h,,;tl!l . .
The study includes all of the drainage ways w~th~n and adjacent to
the corporate limits of the Town of Calhan. The study areas will
extend from the southern border of Section 2, T12S, R62W to
approximately SO ft from the downstream side of McClasky Road.
This includes the following reaches:

Reach

Lenath

0.80 Miles
0.40 .
0.30 Miles
1.50 Miles

Main Tributary
East Tributary
Fairaround Tributarv
Total Reach Length :
An index map showing the location of flood p~ain mapping is
included as figure 2. The flood plain maps themselves are shown
as sheets 1 through 2 following the index map.
RELATED FLOOD STO'DIES

No detailed flood stuaies have been made previously for this study
location. The Federal Emergency Management Agency, and Colorado
Water Conservation Soard prepared an approxi,;,ate flood plain map
to serve as intermediate guidelines until th~s more detailed data
was completed.

FLOOD HISTORY

No significant flooding had ever occurred in the ~own of Calhan.
According to one local town resident, whose home ~s loca~ed by ~he
Eight. Street bridge, the highest flow that he.had ever w~tnessea
since 1965, came within 3 to 4 feet of the br~dge's bottom rung
and this flow did not breach the char~el banks.

~STIGATIONS AND ANALYSIS

A. InterPretation and Use of Renort
1. Freauenev and Discharae: The 10-, 25-, SO-, 100-, and SOO-
year flood events are used as the flood fr7quen7ies for th~s flood
plain analysis. Thus, the data developed 1n th1s report w1Il be
suitable not only for regulation purposes, and H.B. 1041
designation but is also consistent with Federal Insurance
Administration flood insurance studies conducted by t?e Federal
Emergency Management Agency.
These various flood events have an average occurrence of once in
the nu~ber of years as i~dicated. For ex~mple, the lOa-year flood
occurs, on the average, once in a lOa-year period, and has a one
percent chance of being equaled or exceeded in any given year.
The particular uses for the var~ous flood events in addition to

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those stated above are as follows:
10-year, 2S-year and SO-year Flood Events
Information regarding these lower frequency 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. The use of the
lower frequency floods may be considered in planning flood
prevention projects to protect agricultural areas, or other
property where risk to life is not a factor.

100-year Flood Event
The 100-year flood event may be used in lieu of lower frequencies
for engineering design purposes where greater security from
structure failure is desired.
However, the most important use of the lOO-year flood event lies
in flood plain management and land use planning as set forth in
the state statutes. The State of Colorado and the Federal
Government consider the 100-year frequency 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 lOO-year flood.
soo-year Flood Event
The SOO-year flood event may be used to show the upper level of
flood plain management, or for a certain critical type of
construction such as one that involves water or sewage treatment
plant, the SOO-year flood event can be ueed to provide an upper
level of safety.
2. Flood Elev<<tion. Wdter surface elevations for the 10-, 2S-,
SO-, 100-year and SOO-year floods, as determined at each cross
section, may be found in Table 1 "Flood Frequency-Elevation and
Oischarge Data". The flood profile data (sheets lP through lSP)
show a graphical relationship of water surface elevations along
the stream reaches for the given frequencies. Selected typical
cross sections from different reaches within the study area are
shown on sheets lX and 2X.
The flood profiles may be used in areas where controversy arises
over the 100-year flood boundary shown on the Flood Plain Maps.
Since the flood profile exhibits give the water surface elevation
at a specific poi~t on the refere~ce line, the flood elevations
can be surveyed on the ground to alleviate any discrepancies on
t1".9 base map.

J. Plooded Areas, Flood plain maps, sheets 1 and 2,
boundaries of the 100-year and SOO-year flood plains.

show the
The flood

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