FLOOD08014

• Designation Number: 274
• County: Rio Grande
• Community: Unincorporated Rio Grande County
• Basin: Rio Grande
• Title: Flood Insurance Study - Rio Grande County, Colorado, Unincorporated Areas
• Date: 5/1/1987
• Designation Date: 5/1/1987
• Floodplain - Doc Type: Floodplain Report/Masterplan

3.0 ENGINEERING METHODS

For the flooding sources studied in detail in the community, standard
hydrologic and hydraulic study methods were used to determine the flood
hazard data required for this study. Flood events of a magnitude which
are expected to be equaled or exceeded once on the average during any
10-, 50-, 100-, or 500-year period (recurrence interval) have been select-
ed as having special significance for flood plain management and for
flood insurance rates. These events, commonly termed the 10-, 50-, 100-,
and 500-year floods, have a 10, 2, 1, and 0.2 percent chance, respectively,
of being equaled or exceeded during any year. Although the recurrence
interval represents the long term average period between floods of a
specific magnitude, rare floods could occur at short intervals 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 exceedence) 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 herein reflect
flooding potentials based on conditions existing in the community 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 establish the peak discharge-
frequency relationships for each flooding source studied in detail
affecting the community.

Peak discharges for the Rio Grande at Monte Vista were calculated
using the methodology for ungaged sites as described in the Manual
for Estimating Flood Characteristics of Natural Flow Streams in
Colorado (Reference 5).

Peak discharges for the Rio Grande at Del Norte were estimated
using a regional log-Pearson Type III analysis (Reference 6).

Peak discharges for the Rio Grande in the vicinity of the Town of
South Fork were developed from frequency analyses of peak discharges
for the Del Norte and Wason streamflow gages, which were active
for 62 and 40 years, respectively.

Peak discharges for the South Fork Rio Grande segment of this study
were based on the Town of South Fork stream gage No. 2195, with 53
years of data. Additional data from several other stream gages in
the region were used for checking and correlation purposes.

Information from the U.S. Water Resources Council (Reference 6),
the Colorado Water Conservation Board (Reference 5), and the USGS
(Reference 4) were also used as major references.

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