WSP03817

• File Number: 8102
• Description: Arkansas River Basin Basic Hydrology
• State: CO
• Basin: Arkansas
• Water Division: 2
• Date: 5/7/1999
• Author: Inter Fluve Inc
• Title: Fluvial Geomorphic Assessment of Upper Arkansas River Final Report
• Water Supply Pro - Doc Type: Report/Study

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C1816

set of gage data, Upper Arkansas near Empire Gulch, 07083710, has a drainage area of
237 square miles and is located approximately 0.5 miles downstream from the Malta
gage. The Empire gage has four years of peak flow data from 1990 to 1993. Of the
combined total seventeen years of gage data, only twelve years of data could be used for
flood frequency analyses. Although these hydrologic records were utilized for a
comparison of flow duration values (Chapter 3), this was not considered a reasonable
record length for the dctermination flood flows, such that an alternate method was
applied.

Using the 87 years of record on the Arkansas gage at Granite, 07086000, comparisons of
flood flows were made between the Granite gage and the Malta and Empire gages.
Comparisons were made by plotting the ratio of peak discharge divided by watershed
area (cfs/mi2) for each flood event (Figure 7.1). Generally, the discharge-area ratio
correlates well between the downstream Granite gage and the upstream Malta and Empire
gages. Exceptions occur during the 1976 to 1980 period of record for the Malta gage
when peak discharges were heavily influenced by trans basin divcrsions (Chapter 3).
During this time period, the ratio for the Malta gage increases from approximately 80%
of the Granite gage ratios to values equal or larger than the Granite gage (Figurc 7.1).
Therefore, in order to assess the current hydrologic regime, a relationship was developed
between the gages excluding data from this time period.

Discharge-area ratios were plotted for the twelve remaining years of record with the
Granite gage ratios on the independent axis and the Malta and Empire ratios on the
dependent axis (Figure 7.2). Regression analysis revealed a strong correlation between
the discharge-area ratios, with a R2 value of 0.95. Therefore, the regression equation was
used to develop annual peak data for flows outside the period of record on both the Malta
and Empire gages using the extended period of record from the Granite gage.

Synthesized data were incorporated with actual flood data to develop flood frequency
analyses for the Malta and Empire gages. These analyses allow for the determination of
annual peak flood events and identification of the hydrologic characteristics of the stream
systcm. Using the synthetic gage data produced depressed annual peak events for all
flows as compared to the original gage data. Flood frequency analyses of the original
Malta and Empire gage data give high values for flood flows due to the limited amount of
data and the large augmented discharges conveyed past the gages during the short period
of record (Figure 7.3).

Annual peak discharges generated for the purposes of incipient motion analysis describe
a range of flows expected to currently influence the system. Annual peak discharges
used in the analysis are listed in Table 7.2.

May 7. 1999

Fluvial Geomorphological Assessment
Upper Arkansas River

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