FLOOD10136

• Designation Number: 114
• County: Chaffee
• Community: Poncha Springs
• Stream Name: South Arkansas River/Poncha Creek
• Basin: Arkansas
• Title: FIR - South Arkansas River/Poncha Creek, Poncha Springs, CO
• Date: 3/1/1978
• Designation Date: 11/1/1978
• Prepared By: WRC, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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TABLE 2

PEAK FLOWS (cfs)

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Return Period (Years)

Stream Reach

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10

500

50 100

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Poncha Creek

590

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South Arkansas River
Upstream of the
Poncha Creek Confluence

1020

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South Arkansas River
Downstream from the
Poncha Creek Confluence

1300

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730 870

1090

1240 1490

1850

1590 1900

2380

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CHAPTER V

HYDRAULIC ANALYSIS

The hydraulic analysis in this study entailed the computation of the

backwater within the study reaches for the four different frequency flows.

The topographic mapping, with a scale of 1, inch equal to 200 feet and

2 foot contour intervals, was provided by the Colorado Water Conservation

Board in cooperation with Chaffee County. Digitized cross section informa-

tion was also provided. Water surface profiles were determined using the

cross sections from the stream channel and valley, the channel and bank

roughness coefficients, the bridge hydraulic characteristics, and the flood

flows for the four frequencies presented in the hydrologic analysis phase

of this study.

It was necessary to adjust the digitized cross sections to truly reflect

the channel invert elevations. The minimum elevation given for the digitized

cross sections is that of the water surface elevation in the stream channel

at the time that the aerial photography was taken (November 4, 1975). The

flows in the study reaches on November 4, 1975, were obtained from the stream-

flow records for the South Arkansas River near the Salida gaging station; and

the ditch d,versions for the Harrington, Murry and Rosedale Ditches. Next,

the average depth of flow at each cross section was computed based on the

constructed streamflow for November 4, 1975. The computed values ranged from

0.1 to 0.6 feet with approximately 72% of the cross sections having an average

value of 0.2 to 0.3 feet. This depth was subtracted from the minimum channel

elevation given on the digitized cross sections to obtain the channel invert

elevations which were incorporated into the digitized cross section informa-

tion prior to its use for the computation of the water surface profile"

The details of these computations have been submitted to the Colorado Water

Conservation Board in the technical addendum

Two field investigations were conducted to determine the hydraulic

characteristics of the study reaches. During the initial field survey,

photographs of the typical channel and valley cross sections were obtained.

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