FLOOD10487

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

3.0 ENGINEERING METHODS

For flooding sources studied in detail in the community, standard hydro-
logic and hydraulic study methods were used to determine the flood hazard
data required for this study. Floods having recurrence intervals of 10,
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 inter-
vals for the segment of the Rio Grande studied in detail.

Floodflow frequency data for the Rio Grande was based on statistical
analysis of stage-discharge records from the Del Norte gage, located
approximately 30 miles upstream from Alamosa, and the Alamosa gage
located in the City of Alamosa. The periods of record used were 83
years for the Del Norte gage and 60 years for the Alamosa gage
(Reference 2). This analysis followed the standard log-pearsoh Type
III method, as outlined by the Water Resources Council (Reference 3).

The discharge for the 500-year flood was determined by straight-line
extrapolation of a single-log graph of flood discharges computed for
frequencies up to lOa-years.

Frequency-Discharge, Drainage Area Curves for the Rio Grande are
shown in Figure 6.

3.2 Hydraulic Analyses

Analyses of the hydraulic characteristics of the Rio Grande we~e
carried out to provide estimates of the elevations of floods of the
selected recurrence intervals along the stream segment studied in
detail.

Because sheet flooding water-surface elevations in the overbank areas
of Alamosa are different from the water-surface elevations on the Rio
Grande, individual water-surface profiles were developed using the
U.S. Army Corps of Engineers HEC-2 step-backwater computer program
(Reference 4) for the Rio Grande and the left and right overbank
flows. Digitized cross sections were developed by photogrammetric
methods in a previous study (Reference 5), and were modified, repeated,
or adjusted for bridges, diversions, and other general features in
order to compute the significant backwater effects of all structures
and features. For cross section locations, refer to the Flood
Boundary Floodway Map.

For the overbank flows, profile stationing follows a right and left
overbank base line, which is shown on the maps.

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