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

Peak discharge-drainage area relationships for the Rio Grande and
South Fork Rio Grande are shown in Table 1.

3.2 Hydraulic Analyses

Analyses of the hydraulic characteristics of flooding from the
sourceS studied were carried out to provide estimates of the eleva-
tions of floods of the selected recurrence intervals.

Water-surface elevations for the 10-, 50-, 100-, and 500-year floods
for the Rio Grande near Monte Vista and Del Norte were computed
using the U.S. Army Corps of Engineers HEC-2 computer program
(Reference 7).

Cross sections for this analysis were obtained from aerial photo-
graphs flown in May 1980 (References 8 and 9), together with appro-
priate field verification. All bridges and culverts were measured
in the field to obtain elevation data and structural geometry.

Water-surface elevations for the Rio Grande in the vicinity of the
Town of South Fork and the South Fork Rio Grande were computed
using the U.S. Soil Conservation Service WSP-2 computer program
(Reference 10). Cross section data and reach lengths were obtained
from photogrammetric maps prepared especially for this study. The
aerial photographs used in the preparation of maps were taken on
June 18, 1975 (Reference 3).

Locations of selected cross sections used in the hydraulic analyses
are shown on the Flood Profiles (Exhibit 1). For stream segments
for which a floodway was computed (Section 4.2), selected cross
section locations are also shown on the Flood Insurance Rate Map
(Exhibit 2).

Roughness factors (Manning's "n" values) were determined by field
inspection. Ground level photography aided in the selection of
roughness factors and aerial photographs were used to determine
the limits of each roughness zone. The roughness values for the
main channel of the Rio Grande ranged from 0.03 to 0.045; flood
plain values ranged from 0.02 to 0.1 for all floods. These figures
represent roughness factors observed at various points along the
drainages in the study area.

Starting water-surface elevations for the Rio Grande at Monte Vista
and Del Norte were computed using the slope-area method.

The hydraulic analyses for this study were based on unobstructed
flow. The flood elevations shown on the profiles are thus con-
sidered valid only if hydraulic structures remain unobstructed,
operate properly, and do not fail.

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