WWRC 87-23
Assessment of Flushing Flow Recommendations
in a Steep, Rough, Regulated Tributary
Introduction
Alteration of stream flow regime and sediment loading from water
development activities can result in both short- and long-term changes in
channel morphology and conveyance capacity. Subsequently, the condition of
the aquatic habitat can be affected. In recent years, much research and
development effort has been directed toward the determination of suitable
instream flows to maintain fisheries habitat in regulated streams
(Stalnaker & Arnette 1976; Wesche & Rechard 1980). However, there are
several facets of the instream flow problem which have not been adequately
investigated, one involving the recommendation of flushing flows to
simulate the peak runoff hydrograph characteristics of most unregulated
streams (Reiser et al. 1985).
Limited research has been conducted to develop methodology for
determining the magnitude, timing, and duration of flushing flows needed
to maintain channel integrity and associated habitat characteristics
through the movement of sediment deposits. Of the 15 methodologies
identified by Reiser et al. (1985), a majority were not designed
specifically to assess flushing flows, but rather were approaches for
studying sediment transport problems. The several formal methodologies
currently available (Wesche et al. 1977; Environmental Research &
Technology, Inc. 1980; Rosgen 1982) were developed In response to immediate
management needs and are relatively untested in terms of accuracy and
reliability.
During 1984, the Wyoming Water Research Center initiated a research
project entitled, "Development of methodology to determine flushing flow
requirements for channel maintenance purposes." Objectives of this project
are to 1) document the rate of change of various channel characteristics
resulting from aggradation/degradation processes under altered flow
regimes; 2) quantify the physical and hydraulic properties needed to
transport deposited sediment through natural channels; 3) test the
predictive capabilities of existing sediment transport models against field
data; and 4) develop methodology to predict conditions of flow needed to
flush sediments to maintain given streams In prescribed hydraulic, physical
and biologic conditions.
One stream selected for study in response to these objectives was the
North Fork of the Little Snake River (North Fork), a steep, rough,
regulated, headwater stream. Wesche et al. (1977) recommended both
maintenance and flushing flow regimes for the North Fork in light of the
proposed expansion of water diversion facilities in the drainage by the
City of Cheyenne, Wyoming, as part of their Stage II water development
program. Construction of Stage II began in 1983. During the late summer of
1984, intense rainfall in the construction area resulted in the deposition
of a broad size range of sediments in that section of the North Fork where
flushing recommendations had been made. At the request of the Wyoming Game
and Fish Department and in cooperation with the United States Department
Agriculture Forest Service, the authors initiated a study of the North
Fork. The objectives of this paper are to 1) describe the methods used to
assess the extent of the 1984 sediment deposits; 2) present preliminary
results summarizing the response of the deposited sediment to the 1985
spring runoff flow regime; and 3) evaluate the effectiveness of the 1977
flushing flow recommendations in relation to the 1984 sediment deposits.
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