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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CHAPTER IV

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HYDROLOGIC ANALYSIS

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Flood Characteristics

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Floodflows 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

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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 the nearby watershed. The detailed

computations tor the different methods were submitted to the Colorado

Water Conservation Board as part of the technical addendum.

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Hydrologic Analysis

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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. Future

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development within the basin is assumed to be insignificant and thus

it is not considered in the peak flow analysis.

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.

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 were 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.

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

Poncha Creek and the South Arkansas River, peak flows for the four different

frequencies were obtained. The values for such flows are presented in

Table-2.

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