FLOOD04882

• Designation Number: 194
• County: Rio Grande
• Community: Monte Vista
• Basin: Rio Grande
• Title: Flood Insurance Study - Monte Vista, Colorado
• Date: 3/1/1982
• Designation Date: 8/1/1982
• Floodplain - Doc Type: Floodplain Report/Masterplan

expected to be equalled or exceeded once on the average during any 10-,
50-, 100-, and SOD-year floods, have1l10, 2, 1, and 0.2 percent chance,
respectively, of being equalled or exceeded during any year. Although the
recurrence interval represents the long term average peri od between fl oods
of a specific magnitude, rare floods could occur at short interval s or even
within the same year. The risk of experiencing a rare flood increases when
periods greater than 1 year are considered. For example, the risk of
having a flood which equals or exceeds the 100-year flood (1 percent chance
of annual occurrence) in any 50-year period is approximately 40 percent (4
in 10), and, for any 90-year period, the risk increases to approximately
60 percent (6 in 10). The analyses reported here reflect flooding poten-
tials based on conditions existing in the county at the time of completion
of this study. Maps and flood elevations will be amended periodically to
reflect future changes.

3.1 Hydrologic Analyses

Hydrologic analyses were carried out to establ ish the peak discharge-
frequency relationships for floods of the selected recurrence inter-
vals for each stream studied in detail and to compare with results of
previous studies for the flooding sources studied here.

Peak discharges for the various recurrence interval s were calculated
by the study contractor for the Rio Grande at Monte Vista, Colorado.

The peak discharges at Monte Vista were calculated using the method-
ology for ungaged sites as described in "Technical Manual No.1,
Manual for Estimating Flood Characteristics of Natural-Flow Streams in
Colorado," (TMl) by the USGS and CWCB (Reference 3). Streamflow data
from the gage on the Rio Grange River at Del Norte, Colorado, were
used to calculate a discharge-frequency relationship, using a Log-
Pearson Type II I di stributi on (Reference 4). Usi ng the area propor-
tion analysis as described in TMl, peak discharges were determined at
Monte Vi sta .

Table 1 - Summary of Discharges

Dra i nage Area
Flood Source and Location (sq. mi.)

Rio Grande

Peak Discharge (cfs)
10-year 50-year 10 -year SOO-year

At Monte Vi sta

1,320

9,320

11 ,850

12,880

15,150

3.2 Hydraulic Analyses

Analyses of the hydraul ic characteri stics of the fl ooding sources
studied in detail in the cOTllllunity were carried out to provide esti-
mates of the elevations of floods of the selected recurrence interval s
along each of these flood sources.

Water surface elevations for the 10-, 50-, 100- and 500-year floods
for the Rio Grande were canputed using the Corps of Engineers HEC-2
Water Surface Profiles Program (Reference 5). Cross sections for this

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