FLOOD08336

• County: Chaffee
• Community: Poncha Springs, Salida
• Stream Name: South Arkansas River
• Basin: Arkansas
• Title: Floodplain Information Report - Lower South Arkansas River
• Date: 4/1/1979
• Prepared For: Chaffee County
• Prepared By: WRC Engineering, Inc.
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Two field investigations were conducted to determine the hydraulic charac-

teristics of the study reaches. During the initial field survey, photographs

of the typical channel and valley cross sections were obtained, Examples

of such sections are presented on pages 10 through 12. 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 Rough-

ness Coefficient used in this analysis ranged from 0.038 to 0.040 for the

channel and 0.032 to 0.085 for the overbank. Typically, each cross section

required five roughness coefficients to describe the variation of vegetation

across the valley.

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CHAPTER V

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HYDRAULIC ANALYSIS

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The hydraulic analysis in this study entailed the computation of the

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

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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

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

at the time that the aerial photography was taken May 9, 1978. The exact

flows in the study reach on May 9, 1978 were not recorded since no stream

gaging stations are active along this reach. Flows were approximated using

streamflow records for the Arkansas River confluence at Salida. Surveyed

cross sections were also obtained at various points along the study reach to

estimate the flow depth and discharge on the aerial photo date. Next, the

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

ted streamflow for May 9, 1978. The computed values ranged from 0.5 to 1.3

feet with approximately 74% of the cross sections having an average value

of 0.6 to 1.0 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.

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

rence 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 loca-

tions 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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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

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The results of the 10-, 50-, 100-, and SOO-year frequency flood computa-

tions are presented in Table-3. The flooded areas from a 100-year and' 500-

year flood frequency are presented on Plates 3 through 13. The flood pro-

files are shown on Plates 14 through 24. Plates 25 through 28 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 computa-

tions which assume no reduction in the channel or hydraulic structure convey-

ance capabilities. Such reductions, due to debris accumulation, are likely

to occur during flood flow periods resulting in higher water surface elevations.

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