Environmental Engineering Reference
In-Depth Information
Double Bubbler data lacked consistency, as illus-
trated by Fig. 1.19. Although parts of the record
more or less show agreement between Double
Bubbler-derived SSC data and those from analyses
of physical samples, none of the sampled SSC values
on January 10-12, 2005, was among the dozens of
Double Bubbler values exceeding about 220 g/L.
However, the veracity of the larger Double Bubbler
measurements cannot be dismissed out-of-hand as
measurement artifacts; essentially all of the physical-
sample SSC values plot among Double Bubbler data,
and all but the largest Double Bubbler SSC value are
less than the historical maximum SSC of 1,080 g/L
reported by Beverage & Culbertson (1964) for the
Paria River streamgage.
It has been surmised that bed movement during
Paria River Double Bubbler tests caused the lower
orifi ce to become partly or fully blocked at times,
contributing to erroneous data. In their tests of an
in situ
densimeter (pressure-difference monitoring
system), Tollner
et al
. (2005) identifi ed the passage
of bed forms between the densimeter's orifi ces and
fl uid turbulence as potential complicating factors in
SSC computations. They conclude that densimeter
measurements, although feasible under laboratory
conditions, are unreliable in general fi eld conditions.
The USGS experience with the Double Bubbler
cannot unequivocally support or refute Tollner
et al
.'s (2005) conclusion. However, because of
its strong theoretical underpinnings, continuous
monitoring capability, and - not unimportantly - a
lack of any other proven surrogate technology for
providing SSC time-series data in highly concen-
trated and hyperconcentrated streamfl ow conditions,
the pressure-difference technique continues to be
evaluated.
simple and straightforward. Given a valid set of
temperature-compensated measurements at higher
SSC values that are adequately fi ltered and smoothed
to reduce the effects of turbulence, the technology
may provide a time series of SSC that is ultimately
superior to the periodic datasets obtained by tradi-
tional methods. The instrument can be calibrated
using single-vertical samples. The water-column
measurements are theoretically more represen-
tative of the mean cross-section SSC than point
measurements.
In spite of its sound theoretical underpinnings, the
fi eld performance of the Double Bubbler in Puerto
Rico and northern Arizona, USA, has yet to be fully
resolved. Research is continuing into whether devel-
opment and use of empirical relations from calibra-
tion data in lieu of the theoretical considerations are
warranted. The required computational scheme pre-
supposes that the SSC in the vertical profi le between
the sensors is more or less equal to that above the
higher sensor. This assumption is diffi cult to verify
and may not be valid. The technology is unreliable
for measuring SSC at less than about 10 g/L, and the
actual lower measurement threshold may be at a
somewhat larger SSC. The technology is incapable
of measuring SSC when the top orifi ce is out of
water. Spurious data are numerous and are believed
to be associated with fl ow turbulence or orifi ce
blockage by bedforms. Continuous pressure-
difference measurements may be useful in developing
a continuous SSC trace under some circumstances
but are not yet considered suffi ciently reliable to
replace traditional suspended-sediment-monitoring
techniques.
1.2.5 Acoustic backscatter
Jeffrey W. Gartner & Scott A. Wright
1.2.4.3 Summary: pressure difference as a
suspended sediment surrogate technology
1.2.5.1 Background and theory
The pressure-difference technology was tested to
ascertain if it could fulfi ll what may be a unique
niche in suspended-sediment monitoring because, at
least in theory, its performance improves as SSCs
increase. The technology is relatively robust, being
prone to neither signal drift nor biofouling, and is
comparatively inexpensive. The technology doubles
as a redundant stage sensor for the site. The theoreti-
cal underpinnings of the technology are relatively
Attempts to characterize SSC from
in situ
acoustic
backscatter sensors (ABS) have increased in recent
years. In contrast to traditional methods using analy-
ses of water samples utilizing gravimetric or other
techniques, use of ABS to estimate SSC is non-intru-
sive, far less labor intensive for the derived data
density, more or less unaffected by biofouling, and
results in a continuous time series of SSC. Use of ABS
is appealing because SSC profi les can be obtained in
