FLOOD00374

• Designation Number: 146
• County: Chaffee
• Community: Unincorporated Chaffee County
• Stream Name: Lower South Arkansas River
• Title: Floodplain Information Report - Lower South Arkansas River
• Date: 4/1/1979
• Designation Date: 12/1/1979
• Floodplain - Doc Type: Floodplain Report/Masterplan

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CHAPtER IV

HYDROLOGIC ANALYSIS

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Flood Characteristics

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Flood flows in the Upper Arkansas River basin occur primarily during

the May-July period. These high flows are caused basically by snowmelt and

are augmented occasionally by rain.

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Methods of Analysis

Extensive hydrologic analysis was performed by WRC, Inc, for the adjoin-

ing upstream floodplain information report, The majority of that analysis is

also applicable to the downstream study reach. Basically, the previous data

was updated and the potential runoff from the additional 14 square miles of

drainage area to the river mouth Was evaluated, Flood flows were increased,

as necessary, to include the additional contributing area. The following

sections describe the entire method of analysis including tasks performed

previously.

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The hydrologic analysis entailed utilizing a number of different methods

to'compute the peak flow for the four different frequencies, Four such methods

were utilized inCluding the U.S. Geological Survey Regional Analysis, Regional

Statistics / Log-Pearson, Regional Log-Pearson type III Analysis (with and

without the 1957 data point), and Technical Manual No.1. The results for

the 1% flood which were obtained by these four methods compared closely with

the recorded historic peak flows and peak flows per unit area from nearby

watersheds. The detailed computations for the different methods were submit-

ted to the Colorado Water Conservation Board as part of the technical addendum.

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Hydrologic Procedures

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The hydrologic analysis was directed toward calculating the 10-, 50-,

100-, and 500-year frequency flood discharges in the study reaches. This

analysis is based on the existing basin development conditions, Further

development within the basin is assumed to be insignificant and thus it is

not considered in the peak flow analysis,

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Streamflow data from 11 measuring stations located within the Upper

Arkansas River basin was used in this analysis, A list of these stations

with their respective drainage areas and periods of recorded peak flows is

presented in Table-I. The source of this data is the official U.S. Geologi-

cal Survey water supply papers.

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The peak flow figures for the four different frequencies used in the

hydraulic analysis of this study were those obtained from a Regional Log-

Pearson Type III Analysis (including the 1957 data point). The 1957 peak

flow for the South Arkansas River was obtained from a set of correlations

that resulted in a relationship obtained from the period of common records

between the South Arkansas River near Salida and the Arkansas River at Salida.

A set of graphs were developed showing the drainage area in square miles

versus peak flow for the four different frequencies. The data from the

eleven gaging stations was used initially to plot such graphs. Based on

further analysis of the different drainage basins, it was determined that

the final plots of such graphs would be based on the data from the gaging

stations representative of the study area, The results were plotted and

are presented in Figure-I. A plot of discharge versus frequency is shown

in Figure-2 for Obtaining flood flows for various frequency intervals.

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Using Figure-l and the drainage areas of the reaches for the South

Arkansas River, peak flows for the four different frequencies were obtained.

The values for such flows at selected points are presented in Table-2. In

Figure-3, the variation of anticipated flows through the study area is

presented as a function of distance upstream from the river mouth,

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