Environmental Engineering Reference
In-Depth Information
settings as appropriate. Their performances must be
compared with laboratory-control data and (or) fi eld
measurements by traditional techniques. In most
cases, performance comparisons should include col-
lection of concurrent data by traditional and new
techniques for a suffi cient period - probably years
- to identify potential bias and minimize differences
in precision between the old and new technologies.
However, with careful matching of surrogate-
monitoring technologies to selected river reaches and
objectives, it may be possible in the future to
remotely, continuously, and accurately monitor bed-
load discharges, possibly by particle-size class.
Qualifying the derived transport data with reliable
uncertainty assessments may also be possible.
These are revolutionary concepts in sedimentology
when considered from an operational perspective.
The benefi ts of such applied capability could be enor-
mous, providing for safer, more frequent and con-
sistent, arguably more accurate, and ultimately less
expensive fl uvial-data collection for use in managing
the world's sedimentary resources.
This chapter begins with an overview of tradi-
tional instruments and techniques used for measur-
ing bed load, against which the surrogate technologies
using hydroacoustics are evaluated. Descriptions of
the theory, applications, some advantages, limita-
tions, and costs of each surrogate technology are
presented and compared. A subjective evaluation of
the effi cacy of each technology concludes this chapter.
Use of fi rm, brand, or trade names are for identifi ca-
tion purposes only and do not constitute endorse-
ment by the US Government.
port rates remains a work in progress (Marr
et al
.
in press). No single apparatus or procedure has been
universally accepted as completely adequate for the
determination of bed-load discharges over the wide
range of sediment and hydraulic conditions found in
nature (ISO 1992).
Bed-load samplers fall under one or a combination
of the following four categories: Box or basket sam-
plers; pan, tray, or slot samplers; pressure-difference
samplers; and trough or pit samplers (Hubbell 1964).
Box or basket samplers retain sediment deposited in
the sampler owing to a reduction in the fl ow velocity
and (or) capture by the sampler screen (Hubbell
1964). Pan, tray, or slot samplers retain the sediment
that drops into one or more slots after the material
has rolled, slid, or skipped up an entrance ramp
(Hubbell 1964). Pressure-difference samplers are
designed so that the sampler's entrance velocity is
about equal to or somewhat larger than the ambient
stream velocity. They collect material that is small
enough to enter the nozzle but too large to pass
through the mesh collection bag. Figure 2.4 shows
selected pressure-difference bed-load samplers.
Trough or pit samplers are rectangular holes con-
structed in the streambed, into which bed-load par-
ticles drop. Troughs are usually continuous across
the channel, whereas pits cover only a part of the
streambed (Hubbell 1964). Troughs and pits tend to
provide the most reliable bed-load data (Federal
Interagency Sedimentation Project 1940; Hubbell
1964; Emmett 1980; Carey 2005).
There can be substantial differences in calibration
and deployment between the trough and other types
of sampler. The trough-type samplers are the most
diffi cult to construct and operate but the least chal-
lenging to calibrate. In contrast, no universally
agreed-upon method has been developed for cali-
brating portable bed-load samplers, but they are the
easiest to deploy (Carey 2005).
The effi ciency of a bed-load sampler is the ratio of
the sampled bed-load mass divided by the mass that
would have been transported in the same section and
time in the absence of the bed-load sampler. Unlike
FISP isokinetic suspended-sediment samplers which
are designed for isokinetic effi ciencies within about
10% of unity (Federal Interagency Sedimentation
Project 1940, 2008; Gray
et al
. 2008), known or
potential bias in effi ciencies of bed-load samplers can
cast doubt upon the reliability of their derivative
2.1.1 Background: traditional bed-load
sediment-sampling techniques
Published records of bed-load sampler use dates
back to at least the late 1800s, and published
attempts at bed-load sampler calibration date to at
least the early 1930s (Carey 2005). As with the
development of isokinetic suspended-sediment sam-
plers, the Federal Interagency Sedimentation Project
(FISP) endeavored to address problems and needs
related to bed-load data collection starting in the
later 1930s (Federal Interagency Sedimentation
Project 1940). However, development and calibra-
tion of reliable portable bed-load samplers capable
of sampling a wide range of particle sizes and trans-
