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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South Arkansas River
looking upstream at
the Limit Street bridge,

Station 84 + 80.

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Examples of such sections are presented on Pages 22 through 25. From these

photographs and the initial field survey, the roughness characteristics of

the channel and valley were assessed and determined. The values for the

Manning Roughness Coefficient used in this analysis ranged from 0.035 to

0.038 for the channel and 0.038 to 0.080 for the overbank of the South

Arkansas River. Similar figures for Poncha Creek ranged from 0.038 to 0.045

for the channel and 0.055 to 0.080 for the overbank.

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The water surface elevations for the 10-, 50-, 100-, and 500-year

recurrence interval floods were computed utilizing the Corps of Engineers

HEC-2 Step Backwater Computer Program. This program utilizes a solution to

the one-dimensional energy equation to determine the shape of the profile

between control sections where the water surface elevation is known or can

be assumed. The procedure for a steady flow profile calculation is called

the "Standard Step Method". In this method, the distance from a downstream

or upstream point, where the conditions are known, to the point, where the

backwater effects are to be determined, is divided into reaches by cross

sections at fixed locations along the river. Starting from one control

point, calculations of the water surface profile proceed, in steps, from

one cross section to the next. The HEC-2 program is also capable of handling

the effect of the various hydraulic structures which are located across the

river.

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The results of the 10-, 50-, 100-, and 500-year frequency flood compu-

tations are presented in Table-3. The flooded areas from a 100-year and

500-year flood frequency are presented on Plates 3 through 6. The flood

profiles are shown on Plates 7 through 11. Plates 12 through 20 show a

number of typical cross sections of the stream and valley in the study area.

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The water surface elevations presented in Table-3 are based on compu-

tations which assume no reduction in the channel or hydraulic structure,

conveyance capabilities. Such reductions, due to debris accumulation, are

likely to occur during flood flow periods resulting in higher water surface

elevations. It 1S recommended that the hydraulic capacities of both the

stream channel and the hydraulic structures (particularly bridges) be kept

clear of debris. This can be accomplished through a regular maintenance

program and onsite cleaning operations during flood periods.

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South Arkansas River
looking upstream
from the State Highway
285 bridge,

Station 57 + 00.

-22-

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South Arkansas River
looking downstream
from Station 68 + 40
in early winter 1977.

Note ice forming in
the channel.