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

Each transect represents a time commitment of 1 to 2 man-hours of field
work. Consequently, it is always advisable to describe the study area with
as few transects as possible. However, too few or improperly placed
transects will give a distorted view of the stream. Transect placement is
a critical determinant in the reliability of an instream flow model, and
is discussed in detail in this section.
When establishing a study area, the first task is the establishment
of a permanent (relative to the duration of the study) benchmark. The
benchmark serves as a reference point both for relocating transects and
for referencing elevations.. Typical benchmarks may be an X chipped in a
rock outcrop, a large distinctive boulder or bridge abutment, an iron pin
driven firmly into the ground, or a spike driven into a tree trunk. The
two criteria that a benchmark must meet are ease of relocation and
permanence. The investigator should be wary of establishing benchmarks
near trails or game paths, or on unstable ground (i.e. in areas which
might slump or slide). It is the responsibility of the notetaker to
clearly document the location and description of the benchmark. A map
sketch and photographs of the study area should be included in the
description.
The ends of cross sections (transects) should likewise be marked by
permanent markers, called headstakes. A headstake may be a length of pipe
or concrete reinforcing bar (rebar) driven flush to the ground surface or
buried. The headstake should be long enough to penetrate the frost zone
so that frost heave does not-change its elevation. Headstakes should also
be placed in such a way that disturbance by cattle, wildlife, or humans is
unlikely. Like the benchmark, the locations of headstakes must be
thoroughly documented by the person keeping the field notes.. If possible,
the positions of headstakes should be referenced by a bearing and distance
from the benchmark and. photographed. Several techniques for locating
headstakes are listed in Appendix A. Documentation of the study area is
important even if only one trip to the area is planned. In the event a
second trip is necessary, relocation of transects is difficult or
impossible without documentation. It is a rare instream flow study that
is completed with only one trip to the field sites.
The placement of transects is governed by two principles. Obviously,
transects should be placed where they are sampling a discrete type of
habitat: pools, runs', riffles, backwaters, etc. However, an overriding
principle to be foTlowed is that the downstream-most transect should be
placed at a hydraulic control, and all hydraulic controls within the
section should be crossed by a transect. A hydraulic control can be
defined as a physical feature, natural or man-made, which indicates a
stage-discharge relationship. The most notable attribute of a control is
its influence on the hydraulic slope. Controls are reflected by a break
or inflection in the slope of the water surface.
This rule is mandatory. Therefore, the first transects to be placed
within the study area are those which cross hydraulic controls. Figure 17
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