Environmental Engineering Reference
In-Depth Information
with data collected by physical bed-load samplers, it
should come as no surprise that careful calibration
with the most appropriate bed-load sampler is a
prerequisite for reliable bed-load transport-surrogate
monitoring in rivers.
advance. The following sections describe theoretical
principles, selected examples of fi eld or laboratory
applications, and advantages and limitations of two
bed-load surrogate technologies considered to be the
most promising by the USGS.
2.1.2 Information germane to surrogate
technology costs
2.2.1 Active hydroacoustics with a acoustic
doppler current profi ler
Janet Gaskin & Colin D. Rennie
After surrogate-technology effi cacy is resolved, cost
considerations are often of penultimate interest. The
cost of producing reliable, quality-assured bed-load
data can be separated into four categories:
•
the purchase price of the instrument;
•
other capital costs associated with installation,
and initial operation of the instrument;
•
operational costs to maintain and calibrate the
instrument;
•
analytical costs to evaluate, reduce, compute,
review, store, and publish the derivative data.
Of these four categories, only the current purchase
price is relatively straightforward to quantify. The
others are dependent on several factors, including site
location and physical characteristics, hydrological
and sedimentological regime, availability of electrical
power, limitations associated with accessibility,
safety considerations, and the time and complexity
associated with data analysis. Additionally, any such
information inevitably becomes obsolete due, in part,
to technological advances, marketing competition,
and changes in currency valuation. Costs referred to
in the ensuing sections might be placed in perspective
considering that the cost to compute, store, and
provide daily suspended-sediment-discharge data at
a United States Geological Survey (USGS) streamgag-
ing station in 2001 (adjusted for infl ation in 2008
dollars) ranged from US$24,000 to US$78,000 (Gray
2003). No comparable cost statistics were available
for acquisition of time-series bed-load data.
2.2.1.1 Background and theory
Active hydroacoustics refers herein to the use of an
acoustic emission and reception system to infer and
quantify the mobility of the riverbed. In this case, an
ADCP is used to perform a fast, non-intrusive meas-
urement of an apparent bed velocity, which yields a
spatial distribution of relative bed-load transport
when the ADCP is deployed from a boat. Apparent
bed velocity is defi ned as the difference between the
boat velocity measured by the bottom track pulse,
biased by near-bed sediment movement, and the
absolute boat velocity measured by a global position-
ing system (GPS). The bottom track boat velocity is
determined from the Doppler shift of the returning
acoustic echoes of the bottom track pulse. The meas-
urement realm comprises the locations of the conical
beams' “footprints” on the riverbed (Rennie
et al.
2002).
The technology generally requires manual deploy-
ment. The cost of a commercially available, manually
deployable ADCP is about US$20,000 in 2008.
Because quantifi cation of bed-load transport is typi-
cally diffi cult and problematic even in sand-bed
rivers, any surrogate means for providing quantifi a-
bly reliable sand bed-load data is desirable. Because
the technology is heretofore manually deployed,
there is no routine fi eld-maintenance cost.
An ADCP transmits sound pulses into the water
from either three or four transducers and measures
the Doppler shift of the echoes that refl ect off parti-
cles in the fl ow. The particles that scatter the acoustic
signal are assumed to be traveling at the speed of the
fi lament of fl ow in which they are suspended. The
Doppler shift is thereby related to the velocity of
the water relative to the instrument. The Doppler
shift is defi ned as:
FF
V
c
2.2 Technological advances in bed-
load surrogate monitoring
Unlike daily suspended-sediment records, which
have been collected and computed for the better part
of a century in the USA, bed-load transport is rarely
measured on a continuous basis. Hence, any technol-
ogy capable of providing a time-series of bed-load
transport, even with a relatively large coeffi cient of
variation, would represent a major technological
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