Environmental Engineering Reference
In-Depth Information
mean particle size (the diameter of a sphere that has
the same volume/surface area ratio as the particle of
interest) in two size classes (2.5-63
1.2.2.2 Example fi eld evaluation
μ
m and 63-
Laser diffraction sensors are being investigated as an
alternative monitoring protocol for tracking reach-
scale suspended-sediment supply at a USGS stream-
gage on the Colorado River at Grand Canyon,
Arizona, USA, located 164 km downstream from
Glen Canyon Dam (Melis
et al
. 2003; Topping
et al
.
2004). A canyon wall-mounted LISST-100 provides
continuous PSD and SSC data for computing
suspended-sediment transport that may reduce
uncertainty in estimates of the transport of sand and
fi ner material.
An example of data collected by the LISST-100B
at the Colorado River at the Grand Canyon stream-
gage is shown in Fig. 1.11. Data were obtained by
averaging 16 measurements at 2-minute intervals
during a 24-hour deployment in July 2001. The time
series of 720 LISST-100B measurements obtained
from a single point in the river compare favorably
with cross-sectional data obtained concurrent with
some of the LISST-100B measurements using an iso-
kinetic bag sampler and techniques described by
Nolan
et al.
(2005). In addition, the LISST-100B also
recorded the increase of variance in the SSC of sand-
size particles expected with increasing fl ows (Melis
et al
. 2003); peak SSC values ranged between 0.06
and 0.14 g/L (60-140 mg/L).
500
m) (Sequoia Scientifi c, Inc. 2008). The LISST-
25 is based on the same principles as the LISST-100,
but, unlike the LISST-100, it determines SSC through
a weighted summation of the output of ring detectors
rather than the inversion of intensity distribution to
obtain size distribution. The weighted sum can be
affected by use of comet-like shaped focal plane
detectors (Yogesh Agrawal, Sequoia Scientifi c, Inc.,
personal communication 2008).
A cable-suspended, streamlined, isokinetic version
of the LISST-100, the LISST-SL (Fig. 1.10b), is being
developed for manual river deployment. The LISST-
SL is designed to address the potential problem of
fl ow disturbance associated with the size and shape
of the conventional LISST-100 instruments. The
LISST-SL features the capability of real-time velocity
measurement that is in turn used to control a pump
to withdraw a fi lament of water and route it through
the laser beam at the ambient current velocity (Gray
et al
. 2004; Agrawal & Pottsmith 2006). This isoki-
netic fl ow-through capability is a prerequisite for
reliably ascertaining the suspended-sediment proper-
ties in all but the shallowest or most sluggish rivers.
The performance of the LISST-SL is being evaluated
by the FISP (2008).
μ
500
0.20
500
140
0.19
400
400
0.18
120
0.17
100
300
300
0.16
80
0.15
200
200
0.14
60
0.13
100
100
40
0.12
0.11
0
0
20
0.10
0
0.09
24:00
12:00
24:00
12:00
24:00
24:00
12:00
24:00
12:00
24:00
7/19
7/20
7/19
7/20
Explanation
Date, 2001
Date, 2001
Calibrated LISST100B point measurement
Cross-sectionally integrated with D-77 sampler
Discharge of water
Fig. 1.11
Comparison of SSC (left) and median grain sizes (right) measured at the USGS streamgage, Colorado River at Grand
Canyon, Arizona, USA, using a LISST-100-B and a US D-77 bag sampler.
From Topping
et al
. (2004).
