9376

• UCREFRP Catalog Number: 9376
• Author: CH2MHill.
• Title: Duchesne River Hydrology and Water Availability Study.
• USFW Year: 1997.
• USFW - Doc Type: \
• Copyright Material: NO

The average flow is a useful measure of the central value of flows. The post-project average
flow was determined to be approximately 20 percent less than the pre-project flows. The
post-project average monthly flows were less than the pre-project flows in all months
except September and October, with the most significant differences in late winter and early
spring. Figure 4-1 shows the average monthly flow for the various periods.
Flow duration curves were developed to show the time variability of flows between the two
periods. A flow duration curve is a plot of flow versus percent exceedance. Percent
exceedance is a particular flow that can be expected to be exceeded that percentage of time.
For example, a flow at 75 percent exceedance means that 75 percent of the time the flows
exceeded this value and 25 percent of the time they were less than this value. The flow
d
uration curves were developed using average daily flow values to prevent averaging out
or masking the variations in flow that could be caused by using monthly averages. The
flow duration curves for each month are shown in Figures 4-2, 4-3, and 4-4. Observation of
h
t
e winter months reveals that as a general rule the pre-project flows were generally greater
and more constant. The flow duration curves for the spring months reveal that the
post-project flows were less than the pre-project flows for all but a short period in June.
Inspection of the flow duration curves for the summer months shows that in most cases the
post-project flows mimic the flow duration curve of the pre-developed flows, with the pre-
project flows actually having lower flows for percent exceedance values greater than
approximately 60 percent.
Tables 4-3, 4-4, and 4-5 show the calculated percent exceedance values for periods from
1943-1969, 1970-1994, and 1943-1994, respectively. Figures 4-5, 4-6, and 4-7 compare the
25, 50, and 75 percent exceedance flows for the different periods. The method used to
calculate percent exceedance in this report should also be noted. T'he method interpolates
the position of these percentages and is not an actual value, unless the percentage interval
of the total data series can be divided equally into the percentage desired.
Another parameter for showing the time variability of flows between the pre-project and
post-project period is plotting positions for the extreme records. Weibull's plotting was
applied to the minimum daily flows because it is the most commonly used method. The
results from Weibull's plotting are shown in Figures 4-8, 4-9, and 4-10. The results confirm
what the flow duration curves showed: the winter and spring months have a greater
frequency of lower flows for the post-project period, and the summer months have a greater
frequency of lower flows for the pre-project flows.
Methods for statistical distributions are available for estimating return periods of the high
flows. The Log-Pearson Type III method was applied to statistically evaluate high flows.
The maximum daily values were used from both periods to determine the return period of
high flows annually, and also for the months of June, July, and August. The results are
shown in Figures 4-11, 4-12, 4-13, and 4-14.
A flow evaluation was performed to determine frequency and magnitude of flows that have
been recommended by the USFWS for channel maintenance. The duration of the
recommended peak flows is 14 days. The 14-day moving average was calculated. The
percent exceedance was then calculated for the maximum 14-day average from each year.
Figure 4-15 shows the flow duration curve for both pre-developed and post-developed
flows. The 14-day average value should be used with some conservatism because the flow

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