FLOOD10483

• County: Alamosa
• Community: Alamosa County and Unincorporated Areas
• Title: FIS - Alamosa County and Unincorporated Areas
• Date: 4/21/1999
• Prepared For: Alamosa County
• Prepared By: FEMA
• Floodplain - Doc Type: Current FEMA Regulatory Floodplain Information

records have been kept. This flood was caused by melting snow and
increased by precipitation and has a recurrence interval of 250
years.

Scenes of the 1911 flood are shown in Figures 2 and 3.

Natural obstructions to floodflows along the Rio Grande include
trees, brus~1 and other vegetatio~ growing in and near the streaR
channel and between the levees. The meanders of the stream further
reduce the flow efficiency. Manrnaae obstructions include landfills
along the levee, two road bridges, and a railroad bridge. Addition-
ally, two diversion channels and their associated embankments are
obstructive to floodflows.

Flooding is also caused in some areas by street and runway drainage,
an increased water table, direct rainfall, and drainage from other
zones via rivulets.

Floods of approximately the 25-year or greater magnitudes will out-
flank the upper end of a levee system built along the Rio Grande and
become entrapped in the overbank areas.

2.4 Flood Protection Measures

No flood control structures exist on the Rio Grande to protect Alamosa
County. No space has been reserved in the irrigation reservoirs in
the upper watershed of the Rio Grande for storage of floodwater.
Local interests have constructed a levee system along the Rio Grande
to protect the City of Alamosa. The levee system begins just north
of the city and continues to the south of the community. As pre-
viously stated, floods of approximately 25-year or greater magnitude
will outflank the upper end of the levees and become entrapped in the
overbank areas.

3.0 ENGINEERING METHODS

For the flooding source studied in detail in the unincorporated areas of
Alamosa County, standard hydrologic and hydraulic study methods were used
to determine the flood hazard data required for this study. Floods having
recurrence intervals of la, 50, 100, and 500 years have been selected as
having special significance for flood plain management and for flood
insurance premium rates. The analyses reported here reflect current
conditions in the watersheds of the streams.

3.1 Hydrologic Analyses

Hydrologic analyses were carried out to establish the peak discharge-
frequency relationships for floods of the selected recurrence intervals
for the segment of the Rio Grande studied in detail.

6