7910

• UCREFRP Catalog Number: 7910
• Author: Bovee, K. D. and R. T. Milhous.
• Title: Hydraulic Simulation In Instream Flow Studies
• USFW Year: 1978.
• USFW - Doc Type: Theory And Techniques, Instream Flow Information Paper No. 5.
• Copyright Material: NO

1. The reach should be highly sensitive to changes in stream flow.
This means that the rate of change of width, depth, and velocity with
respect to discharge, should be greater for the critical reach than for
other portions of the stream. Generally, the most sensitive stream
reaches with respect to discharge are elevated portions of the channel,
such as riffles and gravel bars. However, the shape of the channel also
effects sensitivity. Convex channel profiles exhibit a very rapid rate of
change of hydraulic variables with changes in discharge. Trapezoidal and
rectangular channels are fairly sensitive at low flows but relatively
insensitive to discharge at medium or high flows. Triangular or parabolic
channels are relatively insensitive to flow changes and the rate of change
is often difficult to interpret- Figure 14 shows several cross-sectional
profiles and the associated rate of change of a simple hydraulic parameter
(wetted perimeter) with discharge.
2. The critical reach must also act as a biological control. The
species for which flow-related interpretations are made must be directly
limited by the type of habitat present in the critical reach at some life
stage. For example, if spawning is limiting for a trout population, then
a convex gravel bar might best be selected as a critical reach. If
invertebrate food production is considered limiting, a rocky riffle area
would be an appropriate critical reach. In all cases, it is imperative
that the linkage between the limited life stage and the critical reach is
firmly established.
Representative Reach Concept
Application of the representative reach concept is, appropriate when
the limited life stage of a fish, population is not known with any cer-
tainty. It is also a useful tooT to analyze the interrelatedness of
different species and/or life stages at different times of the year.
Rather than assume a single type of habitat is controlling the population,
the representative reach concept assumes that the importance of a
particular area varies with time as well as with discharge. This approach
utilizes a series of cross sections to sample the relationships among the
flow regime and all the different types of habitat within a representative
reach of stream. In theory, the variance among the hydraulic parameters
of the study reach would be about the same as if another study reach
within the same stream segment had been used. In other words, the variance
among hydraulic parameters among study reaches within a river segment is
assumed less than the variance among hydraalic parameters among river
segments.
This assumption requires that the stream segment from which the study
reach is selected is fairly homogenous. Classification of these rather
large homogenous stream segments should consider topography, geology,
gradient, stream flow, and biological communities. Acts of man, such as
extensive channelization or diversions, may also delimit certain reaches
of streams. This process of delimitation of large homogenous reaches is
termed stratification, and is best completed in the office prior to any
visit to the field. The purpose of this stratified sampling process is to
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