Environmental Engineering Reference
In-Depth Information
1.2.1.3 Summary: turbidity (bulk optics) as a
suspended-sediment-surrogate technology
clean and recalibrate the instrument (many sensors
offer an integrated wiper, considerably reducing
biofouling). A lack of consistency in measurement
characteristics among commercially available instru-
ments impinges on the comparability of turbidity
measurements (Landers 2003; Ziegler 2003).
Instrument response to grain size, composition,
color, shape, and coating can be variable, and hence,
can reduce the accuracy of derived SSC values.
Perhaps most importantly, saturation of the turbi-
dimeter signal can occur, resulting in constant,
erroneous SSC values above the saturation limit.
Saturation often occurs at high SSCs that tend to
occur concomitant with high fl ows, which are the
most infl uential in suspended-sediment-fl ux magni-
tudes. Hence, some knowledge of the turbidimeter
measurement range and site sedimentological char-
acteristics is desirable before deploying a continuous
turbidimeter for calculating SSC and sediment
transport.
Two types of bulk-optic instruments - turbidimeters
and optical-backscatter sensors - have been shown
to provide reliable data at several fi eld sites at which
the limitations of the instrument have not been
exceeded. Owing in part to the fact that bulk-optic
instruments are the most common and among the
most reasonably priced of the suspended-sediment-
surrogate technologies, results from a considerable
amount of research and evaluation associated with
the technology are available to improve and better
qualify the derived SSC data. One such outcome was
the USGSs development and endorsement of guide-
lines for converting continuous turbidity time-series
data (or continuous turbidity and water-discharge
time-series data) to SSC and SSL time-series data
(Rasmussen et al. 2009).
The primary advantage of regression-based esti-
mates using continuous turbidity measurements over
discrete sample collection is typifi ed by the SSC time
series for the Little Arkansas River near Sedgewick,
Kansas, USA. Regardless of fl ow conditions, SSC and
SSL values are obtained continuously at the interval
in which turbidity and water discharges are recorded
(Fig. 1.9).
Turbidity as an SSC surrogate, however, has draw-
backs. For example, turbidity time-series data derived
from a single point in the stream at the sensor loca-
tion may not be representative of the sedimentary
conditions of the river cross section. Biofouling of
optical windows may require frequent site visits to
1.2.2 Laser diffraction
Jeffrey W. Gartner & John R. Gray
1.2.2.1 Background and theory
Laser diffraction instruments exploit the principle of
small-angle forward light scattering to infer PSDs
and volume SSCs. These instruments measure scat-
tering over a suffi ciently wide range of small forward
scattering angles to allow determination of PSD
information over a wide range (typically 1 : 100 or
1500
1250
Regression-computed
hourly SSC
1000
750
Sampled SSC
500
250
Fig. 1.9 Hourly regression-computed
and sampled SSCs, Little Arkansas River
near Sedgwick, Kansas, USA, 2004.
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2004
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