FLOOD10582

• County: Garfield
• Community: Silt
• Title: FIS - Silt
• Date: 8/2/2006
• Prepared For: Silt
• Prepared By: FEMA
• Floodplain - Doc Type: Current FEMA Regulatory Floodplain Information

In 1967, the Bureau of Reclamation completed the Silt Project, located near the Towns of
Rifle and Silt. The project stores the flows of Rifle Creek and pumps water from the
Colorado River to supply irrigation water for approximately 7,000 acres of land.
Principal features of the project are the Rifle Gap Dam and Reservoir, a pumping plant, and a
lateral system. The Bureau of Reclamation turned over the operation and maintenance of the
Silt Project to the Silt Water Conservancy District in 1968. The district also operates the
private Farmers Irrigation Company facilities as part of this project. Although no
specific reservoir capacity is assigned for flood control, the Silt Project has provided an
accumulated $150,000 in flood-control benefits from 1950 to 1999 (Reference 8).

3.0 ENGINEERING METHODS

For the flooding sources studied by detailed methods 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 that are expected to be equaled or exceeded once on the average during any
10-, 50-, 100-, or SOD-year period (recurrence interval) have been selected as having
special significance for floodplain management and for flood insurance rates. These events,
commonly termed the 10-, 50-, 100-, and SOO-year floods, have a 10-, 2-, 1-, and O.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 that equals or
exceeds the I-percent-annual-chance flood in any 50-year period is approximately 40 percent
(4 in 10); 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 peak discharge-frequency relationships for
each flooding source studied by detailed methods affecting the community.

Hydrologic analyses for the Colorado River were carried out by the CWCB (Reference 9).
Flood frequency curves for the 10-, 50-, 100-, and 500-year recurrence intervals were
developed based on records from five U.s. Geological Survey stream-gaging stations from
Dotsero, Colorado to Cameo, Colorado.

Peak discharge-drainage area relationships for the streams studied in detail are shown in
Table 1, Summary of Discharges.

3.2 Hydraulic Analyses

Analyses of the hydraulic characteristics of flooding from the sources studied were carried out
to provide estimates of the elevations of floods of the selected recurrence intervals.
Users should be aware that flood elevations shown on the FIRM represent rounded whole-foot
elevations and may not exactly reflect the elevations shown on the Flood Profiles or in the
Floodway Data tables in the FIS report. Flood elevations shown on the FIRM are primarily
intended for flood insurance rating purposes. For construction and/or floodplain management
purposes, users are cautioned to use the flood elevation data presented in this FIS report in
conjunction with the data shown on the FIRM.

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