Environmental Engineering Reference
In-Depth Information
on Sedimentation (Gray
et al.
2007; Laronne
et al.
2007).
The 2002 workshop in Oslo, Norway, included 13
papers under the category, “bed-load monitoring
and transport processes.” The workshop paper by
Ergenzinger and DeJong (2003) listed and briefl y
described each of, “… the well known measuring
techniques of sediment trapping and sampling,
tracing, and surveying using both conventional
techniques and remotely sensed images.” Those
techniques that qualify as “bed-load surrogate
technologies” include passive hydroacoustics; visual,
radioactive, magnetic, and radiotracers; magnetic
detectors; underwater video cameras; load-cell
traps; and analyses of scanned or photographic
images.
Breakout session II from the 2003 workshop in
Flagstaff, Arizona, USA, was entitled, “Bedload-
Transport Measurements: Data Needs, Uncertainty,
and New Technologies” (Ryan
et al.
2005). Among
other information, the table in that report section
(reproduced herein as Table 2.1 without annotation)
lists eight bed-load surrogate technologies: active
and passive hydroacoustic sensors; gravel impact
sensors; magnetic tracers, and sensors; topographic
differencing with sonar; sonar-measured debris
basin; and underwater video cameras. The breakout
group identifi ed characteristics associated with the
ideal bed-load sampling device or technology, as
paraphrased below.
Surrogate technologies should:
•
provide accurate measurements and precise data
on the amounts and sizes of bed-load material over
a wide range of fl ow conditions;
•
be reliable, safe to operate, and used without
wading in streams at high fl ow;
•
be foolproof, easy to calibrate, and not disrupt the
local transport fi eld to the extent that it affects
measurements,
•
be rugged, durable, and able to withstand occa-
sional collisions with large grains;
•
have minimal and tractable power requirements
for use in remote environments;
•
automatically provide continuous record;
•
be scalable; and
•
be affordable.
The 2003 workshop summary (Gray, 2005)
included a matrix that compared and contrasted
selected characteristics of bed-load surrogate tech-
nologies to other types of sediment-surrogate tech-
nologies, and to related data-management and
fl ux-computation issues. This matrix is reproduced
herein as Table 2.2. About 50 participants from nine
countries attended the 2007 workshop in Minneapolis,
Minnesota, USA; others participated by video link.
The 25 papers submitted to the workshop identifi ed
passive- and active-hydroacoustic, magnetic-tracer
and magnetic-sensor, load-cell trap, topographic dif-
ferencing with sonar, particle-tracking, gravel-impact
sensors, and ground-penetrating radar technologies
to infer bed-load transport.
This chapter presents descriptions, progress in, and
examples of applications of active and passive hydroa-
coustics considered by the editors to be among the
most promising of the aforementioned bed-load sur-
rogate technologies. This observation is in part based
on the fact that no fewer than a combined 14 papers
presented at the three workshops listed above
described passive- and active-hydroacoustics research
results. In comparison, the next most prevalent topic
among these workshops was magnetic- and radio-
tracer studies, described in four of the papers. It was
also noted that in many cases hydroacoustic technolo-
gies are affordable, portable, and relatively robust.
Additionally, results from some techniques that are
not based on, or calibrated with integrated cross-
section bed-load measurements, such as some of the
tracer technologies and some impact sensors, can be
relatively diffi cult to interpret quantitatively. How-
ever, it is important to note that selected technologies
other than the hydroacoustics techniques presented
below have a potential monitoring niche, and should
not be ignored. Those interested in non-hydroacous-
tic bed-load surrogate technologies are encouraged to
peruse the relevant papers from these workshops and
from other publications on this subject.
The
in situ
technologies presented in this chapter
require periodic site-specifi c calibrations to infer the
bed-load transport characteristics representative of
the entire channel cross section or reach segment.
This requirement is expected to be substantial for
new river-monitoring applications, but may diminish
as comparative data accumulate.
None of the technologies represents a panacea
for bed-load monitoring in all rivers under all fl ow
and sediment-transport conditions. To make the
transition from research to operational monitoring
applications, these new technologies must be rigor-
ously tested with respect to accuracy and reliability
in different physiographic and (or) laboratory
