Environmental Engineering Reference
In-Depth Information
pipes (William Fletcher, Design Analysis Associates,
Inc., personal communication 1999).
The specifi c weight of the water-sediment mixture
from measured pressure differences in a water
column between two pressure-transducer orifi ces
anchored at different depths can be calculated by the
following equation:
Analysis Associates, Inc., personal communication
2005) indicated that calculations based on a moving
average of the pressure-difference data tended to
provide a smoother time series of SSC that was more
comparable to SSC data derived from water-sedi-
ment samples obtained by methods described by
Nolan et al. (2005).
(
)
(
)
γ =−
pp z z
1
(1)
2
2
1
1.2.4.2 Example fi eld evaluations
where:
is the specifi c weight of the fl uid; p 1 and p 2
are the simultaneous pressure measurements at ori-
fi ces 1 and 2, respectively; and z 1 and z 2 are the
simultaneous measurements of the distances to the
water surface from orifi ces 1 and 2, respectively.
The difference in the distances from the fi xed ori-
fi ces to the water surface is a constant value. SSC is
calculated as the difference in the specifi c weights of
the water-sediment mixture and that of pure water
at the same temperature as the ambient streamfl ow.
Implicit assumptions in the method are that the
simultaneous pressure measurements represent
the same water surface, and that the density of the
water-sediment mixture above the lower sensor is
more or less equal to that above the higher sensor.
Exceptionally sensitive pressure transducers are
required. The technology has both laboratory and
fi eld applications (Lewis & Rasmussen 1999). The
purchase price of the technology is similar to that for
a fully equipped turbidimeter. In theory, the instal-
lation should require a minimum of maintenance
other than removal of debris from the in-stream
sensor assembly. The instrument-measurement realm
is a water column. Instrument calibrations can be
accomplished by sampling in or near the instru-
mented water column with a suspended-sediment
sampler, although they are often supplanted by
cross-section calibrations.
The technique has been applied in the laboratory
with promising results of better than 3% accuracy
(0.543
γ
Information on the fi eld performance of the pressure-
difference technology is available from USGS stream-
gages on the lower Río Caguitas in Puerto Rico
(Larsen et al . 2001) and near the mouth of the Paria
River in Arizona, USA. Continuous pressure-
difference data were collected during October-
December 1999 at the Río Caguitas streamgage
using a Double Bubbler Pressure Differential instru-
ment developed by Design Analysis Associates, Inc.
(2008) (Figs 1.16 and 1.17). Most of the annual
sediment discharge in the lower Río Caguitas occurs
in runoff from a few storms when SSC exceeds about
0.5 g/L. The maximum SSC measured at the stream-
gage during the Double Bubbler tests based on water
samples collected by an automatic pumping sampler
was 17.7 g/L.
The analytical procedure involved data smoothing
and removal of outliers. To calculate the weight
density of suspended sediment and dissolved solids
the weight density of pure water at 27 °C was sub-
tracted from the smoothed data values. Even with
these manipulations, this test of the Double Bubbler
instrument in Puerto Rico showed relatively poor
agreement among discharge, SSC, and the manipu-
lated water-density data measured by the Double
Bubbler (Fig. 1.18). The Double Bubbler data
contained a large amount of signal noise, making
interpretation diffi cult. Lacking a thermistor for tem-
perature compensation, 12 of 15 base-fl ow instru-
ment measurements inferred negative SSC values (an
impossibility) concomitant with in-stream measured
SSC values of 0.01-0.1 g/L (10-100 mg/L). However,
all but two of the samples collected during seven
high-fl ow periods showed concomitant increases in
inferred positive SSC values.
A complicating factor in the pressure-difference
method is in-stream turbulence, which introduces
noise about equal to the magnitude of the signal of
0.014 g/L) for determining mass concentra-
tion of suspensions of glass microspheres (Lewis &
Rasmussen 1999). However, application of this tech-
nique in the fi eld can be complicated by a low signal-
to-noise ratio associated with low-to-moderate SSC,
turbulence, large dissolved-solids concentrations,
and large water-temperature variations. Additionally,
analyses may be complicated by density variations in
the suspended material. William Fletcher (Design
±
Search WWH ::




Custom Search