FLOOD08811

• Designation Number: 439
• County: Chaffee
• Community: Salida
• Basin: Arkansas
• Title: Flood Insurance Study - Salida, Colorado, Chaffee County
• Date: 3/1/1982
• Designation Date: 1/1/1997
• Floodplain - Doc Type: Floodplain Report/Masterplan

flow for the South Arkansas River was obtained from a set of corre-
lations that resulted in a relationship obtained frOOl the period
of COOlmon records between the South Arkansas River near Sal ida and
the Arkansas River at Salida.

The results of the regional analysi s were compared to the results of
the Floodplain Infonnation Report for the Lower South Arkansas River
(Reference 1). The lOa-year di sc harge agreed wi thi n 5 percent of
Reference 1, and therefore the hydrologic analysis as presented in
this previous study was adopted for use in estimating the flood
boundaries for the South Arkansas River. The peak discharges are
presented in Table 1.

Table 1 - Summary of Discharges

Flood Source and Location

Ora i nage Area
(sq. mi.) la-year

Peak Discharge (cfs)
50-year lOa-year 500-year

South Arkansas River
Confl uence wi th
Arkansas River

1,330

1,830
1,800

2,010

At County Road 107
3.2 Hydraulic Analyses

208
200

1,350

1,980

2,500
2,470

Analyses of the hydraulic characteristics of the flooding sources
studied in detail in the cOl1l11unity were carried out to provide esti-
mates of the el evati ons of floods of the sel ected recurrence
intervals along each of these flood sources.

Water surface elevations for the 10-, 50-, 100- and 500-year floods
for the South Arkansas River were cOOlputed using the Corps of Engi-
neers HEC-2 Water Surface Profil es Program (Reference 51. Topo-
graphic mapping and cross sections for this analysis were cOOlpiled
from aeri a 1 photographs fl own in May, 1978 (Reference 6), coupled
with appropriate field verification. All bridges and culverts were
measured in the field to obtain elevation data and structural
geometry.

Locations of selected cross sections used in the hydraul ic analyses
are shown on the Flood Profiles. For stream segments for which a
floodway is computed (Section 4.2J, selected cross section locations
are also shown on the Flood Boundary and Floodway Map.

Roughness factors (Mannings "n" valves) were detennined by fiel d
inspection. Ground level photography aided in the selection of
roughness factors and aerial photography was used to determine the
limits of each roughness zone. Roughness values for the main channel
of the South Arkansas River ranged from 0.038 to 0.040 with flood
plain values ranging from 0.032 to 0.085 for all floods.

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