FLOOD10167

• Designation Number: 299
• County: Costilla
• Community: Unincorporated Costilla County
• Basin: Rio Grande
• Title: Floodplain Information Report - for the Rio Grande, Trinchera Creek, Culebra Creek, Colstilla Creek, and Selected Tributaries in Costilla County
• Date: 3/1/1989
• Designation Date: 11/1/1989
• Floodplain - Doc Type: Floodplain Report/Masterplan

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4.0 HYDRAULIC ANALYSIS

In view of the size of Costilla County and the limited
amount of time and money available for preparing floodplain
delineations for the county. approximate methods of hydraulic
analysis were selected.

4.1 Base Maps

7 1/2 minute U.S.G.S. quadrangle maps were used as base
topographic maps to show the lOa-year floodplain. 21
quadrangle maps were used to provide base mapping for the study
area. Of these 21 quadrangles. 6 had a contour interval of 40
feet. 6 had a contour interval of 20 feet (with 1 of those
having selected intermediate 10 foot contours and 1 having
selected intermediate 5 foot contours). 8 had a contour
interval of 10 feet (with 2 of those having selected
intermediate 5 foot contours). and 1 had a contour interval of
5 feet.

In the San Luis study area field surveyed cross-sections
were superimposed on a blown-up version of the San Luis
quadrangle. Likewise. the CSU Maps of Floodprone Areas were
prepared by superimposing field-surveyed cross-sections on
quadrangle maps. In the case of the CSU maps. however. the
cross-sections were far less detailed and much more widely
spaced than those used for the San Luis study.

4.2 Flood DePths

The basic procedure for floodplain mapping was to determine
a representative depth of flooding for the lOa-year peak flow
based on an elevation-discharge relationship. That depth was
assumed to be constant for a given stream reach. Flood
contours were drawn on the maps. and the boundary was
determined by connecting the end-points of the flood contours.

The key step in this process was to determine appropriate
flood depths. The first sources of information for this
selection were the gage analyses performed by the various
governmental agencies. These analyses generally included flood
stages for the calculated laO-year flow. The second set of
data sources was a set of typical cross-sections that were
derived from the base maps (usually la' contour intervals were
used). Hydraulic calculations were performed for these
cross-sections. In the area around the Town of San Luis. the
elevation-discharge relationships determined in the December.
1988 report. Master Drainaqe Study. Town of San Luis. Colorado.
were used. From these analyses draft maps were prepared,
Final depths of flooding eould not be selected until a field
review of these draft maps had been conducted.

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