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INVESTIGATING TECHNOLOGIES TO MONITOR OPEN-CHANNEL DISCHARGE BY
DIRECT MEASUREMENT OF CROSS-SECTIONAL AREA AND VELOCITY OF FLOW.
Abstract:
The
U.S. Geological Survey is investigating technologies that may enable the
direct, continuous, noncontact measurement of open-channel discharge.
Measurement of open-channel discharge could be achieved by monitoring bottom
and surface elevation and flow velocity of open channels. These parameters have
been individually measured using particle-image velocimetry, lasers, radar, and
acoustics in related applications. The U.S. Geological Survey is planning
research to analyze and refine the use of these technologies for direct
measurement of open-channel discharge. Direct measurement of discharge may
reduce streamflow-gaging costs, improve accuracy, and reduce hazards associated
with the traditional streamflow-gaging methods.
Keywords:
Stream gages, Discharge measurement, Average velocity, Surface velocity, Radar,
Lasers, Particle-image velocimetry, Acoustics
INTRODUCTION
The
U.S. Geological Survey (USGS) operates a network of about 7,000
streamflow-gaging stations that monitor open-channel discharge at selected
locations throughout the United States (Wahl and others, 1995). The traditional
method for determining open-channel discharge for these gaging stations was
developed during the 19th century and has been refined to its present state in
the past 100 years. The general equation for discharge at a selected cross
section is (Rouse, 1946):
(1)
where
|
Q |
= |
discharge, |
|
v |
= |
velocity, and |
|
A |
= |
cross-sectional area. |
Open-channel
discharge at gaging stations is determined by monitoring water-surface
elevation (stage) and defining a relation between the stage and discharge
(Rantz and others, 1982a,b).
Despite
its accuracy and near universal acceptance and use, the present method of
computing discharge has significant shortcomings. The extensive labor and
travel required to service gaging stations and to make current-meter
measurements of discharge are significant parts of the cost in compiling
discharge data. Most of the equipment used to monitor stage requires direct
contact with the channel and water, which exposes the equipment to a variety of
hazards such as sediment deposition, flood damage, and vandalism. Using stage
as an index of discharge requires current-meter measurements throughout the
full range of stage to adequately define the stage-discharge relation and
requires the use of expensive measuring equipment. Making current-meter
measurements under some conditions may expose personnel to hazardous
situations. Streamflow data may be needed at locations on a channel where the
stage-discharge relation is unstable; significant uncertainty may exist in discharge
data computed during the intervals between current-meter measurements. Many of
these shortcomings could be reduced or resolved if a method could be devised to
directly measure discharge.
DISCUSSION
OF POTENTIAL TECHNOLOGIES
In
1996, the USGS established a committee, Hydro 21, to identify and evaluate
technologies that might be used to more cost-effectively and safely monitor
open-channel discharge. The committee comprises personnel of the USGS that have
expertise on river measurements, hydraulics, and field instrumentation. The committee was charged with reviewing
the literature, gathering information about new surface-water methods and
technologies, and assessing their potential for measuring and (or) monitoring
discharge.
After
reviewing the present process, proposed methods, and relevant technologies, the
Hydro 21 Committee determined that the desired method for monitoring
open-channel discharge would be the direct measurement of open-channel
cross-sectional area and mean velocity from a noncontact sensor mounted on the
channel bank. Monitoring cross-sectional area would require monitoring channel
bottom, channel surface, and river width. Open-channel width would be
determined by the locations of points of intersection of channel bottom and channel
surface. Surface velocity could be used as a reliable indicator of mean
velocity for typical open-channel cross sections. After an extensive
evaluation, the committee determined that opportunities for measuring velocity
or cross-sectional area exist using particle-image velocimetry (PIV), lasers,
radar, and acoustics.
PARTICLE-IMAGE
VELOCIMETRY
Particle-image velocimetry (PIV) uses successive
images to track the motions of particles in suspension or certain features of
the water surface (Raffel and others, 1998). The motion of the particles or
features are assumed to be the water velocity. Discrete particle displacements
can be computed from cross-correlation between successive images taken over a
known lapsed time. The displacement divided by the lapsed time is the velocity.
The Iowa Institute of Hydraulic Research made successful experiments in
monitoring surface-water velocity of a small creek by this method (Anton
Kruger, Iowa Institute of Hydraulic Research, written commun., 1998) and made
measurements of mean velocity in a laboratory using water-contact sensors
(Muste and Patel, 1997). The feasibility of identifying sufficient particles to
determine a representative velocity by this method is largely untested, and the
method may only be suitable for monitoring surface velocity.
LASERS
Lasers
are being used by the U.S. Navy and other researchers for mapping the elevation
of the ocean bottom. Lasers can be designed to reflect off the water surface to
determine stage and penetrate to the channel bottom to determine channel-bottom
elevation. This capability could be used to monitor cross-sectional area.
Additionally, laser doppler may be suitable for measuring mean open-channel
velocity. Research is needed to determine the effects and limits of suspended
sediment on signal penetration, what incident angles are feasible for the
sensor, and whether needed data can be collected using lasers that are
optically safe.
RADAR
Radar
technology has been used to measure stage, channel-bottom elevation, and flow
velocity in specific applications. A number of commercially available
instruments use radar to monitor stage. Successful experiments have been made
by the USGS using low-frequency radar to measure channel-bottom elevation
during a discharge measurement (Spicer and others, 1997). Radar equipment has
been used by oceanographers to monitor ocean surface velocities from a fixed
point on shore. The Japanese are using radar to monitor flow velocity across a
river. Research is needed to determine the optimal radar frequencies, feasible
incident angles, and the effects of electrical conductivity on signal
dispersion.
ACOUSTICS
Acoustics
have been used to measure cross-sectional area and velocity for many
applications. This technology could be used to monitor channel discharge;
however, all known applications require a sensor with direct channel contact.
Consequently, acoustics would only be considered for conditions for which
noncontact methods were unworkable.
A
summary of some identified technologies and their potential for use in
measuring the needed parameters is shown in table 1. Note that for each of the
needed parameters, at least one successful field application has been
demonstrated.
[1, tested
(see example application); 2, possible; 3, not possible]
|
Technique |
Stage |
River bottom |
Mean velo- city |
Surface velo- city |
Example application |
Problems |
|
High-frequency
radar................... |
1 |
3 |
3 |
1 |
Tokyo
University |
Need
waves or surface roughness to return signal |
|
Low-frequency
radar................... |
2 |
1 |
2 |
3 |
Japan/Spicer
and others (1997) |
High
conductivity will attenuate signal. Limited distance |
|
Lasers................. |
2 |
1 |
2 |
2 |
Naval
Research |
High-sediment
concentration may affect penetration in water. Limited distance |
|
Particle-image
velocimetry (PIV) |
2 |
3 |
3 |
1 |
Iowa
Institute of Hydraulic Research |
Need
tracer/night use |
|
Acoustics............. |
1 |
1 |
2 |
2 |
Acoustic-
velocity meter/acoustic doppler current profile |
Water
contact |
RESEARCH
METHODS
Research
to determine the optimal method for direct measurement of cross-sectional area
and velocity will be done in three steps. The first step will determine the
optimal wavelengths and conditions for each parameter by reviewing test results
of experiments for similar conditions and through numerical analysis of various
wavelengths under various flow and channel conditions. These analyses should
provide preliminary information on depth limits and the effects of electrical
conductivity and sediment concentration. The numerical analysis should
establish the limits and conditions of each of the various methods and provide
data for a more refined field test. Upon completion of these analyses, specific
technologies will be selected for further testing.
The
second step will be controlled laboratory tests to determine the actual
performance of the selected technologies under controlled conditions for all
three parameters. This step will provide measurements of the accuracy of the
methods and the suitability under various conditions. If these tests indicate
that one technology or a combination of technologies can measure all three
parameters within acceptable accuracy bounds, a field test will be organized.
The
third step will be the development and testing of field equipment for the selected
technologies. The equipment will be developed in a modular form to permit easy
field modification and adaptation to various conditions. The field tests will
be done at a USGS gaging station that has a stable stage-discharge relation and
conditions that allow accurate discharge measurements in order for test results
to be compared to accurate baseline data. For the first test, the selected site
will represent typical conditions of river geometry, river velocity, channel
stability, and water chemistry, and a range of conditions at a
streamflow-gaging station on a natural watershed. If the initial test is
successful, subsequent tests can be done under various physical and
environmental conditions to determine the limits of the method.
BENEFITS
The
direct measurement of stage, channel-bottom elevation, and flow velocity to
monitor open-channel discharge will have significant advantages over the
traditional method, which principally depend on the relation between stage and
discharge. In the traditional method, uncertainty in the stage-discharge
relation is minimized using current-meter discharge measurements. During the 4-
to 6-week interval between current-meter discharge measurements, uncertainty
increases with time until the relation is validated or corrected by another
discharge measurement. This uncertainty will no longer be a factor if
cross-sectional area and flow velocity are measured continuously. As the
methods and equipment for direct measurement are perfected, the frequency of
visits to the gaging station and the overall costs to collect open-channel
discharge data likely will be reduced. Current-meter discharge measurements
could be reduced or eliminated, which would lessen exposure of personnel to
hazardous conditions. The accuracy of discharge data will not be as dependent
on site conditions, a stable channel, and a stable stage-discharge relations as
is the traditional method. As a result, the collection of accurate discharge
data will be possible under conditions presently considered unsuitable.
If
noncontact-equipment sensors can be designed, problems associated with exposure
to the environmental hazards and vandalism will be reduced significantly
because the sensor can be protected in the gaging-station structure. As this
method is refined, the projected reduction in costs and service could result in
a reduction in overall data-acquisition costs.
SUMMARY
A
critical need exists for more discharge data at more river locations throughout
the world. The traditional method for monitoring open-channel discharge uses
stage as an indicator of discharge. This method has been used throughout the
world for over 100 years and can produce a reliable record of discharge if site
conditions are favorable. The method is labor intensive because it requires frequent
direct measurements of discharge to define the stage-discharge relation for a
river. The use of new technologies may make the direct measurement of the
open-channel cross-sectional area and mean velocity of flow feasible. A
continuous record of surface-water discharge could be produced by continuously
monitoring these parameters. The U.S Geological Survey is researching the
suitability of selected technologies for this application. The combination of
reduced streamflow-gaging costs and more flexible site requirements should
facilitate an increase in the number of gaged sites and improved
streamflow-gaging networks.
REFERENCES
Muste, M., and Patel, V.C., 1997,
Velocity profiles for particles and liquid in open-channel flow with suspended
sediment: New York, American Society of Civil Engineers, Journal of Hydraulic
Engineering, v. 123, no. 9, September 1997, p. 742-750.
Novak, C.E., 1985, WRD data
reports preparation guide: U.S. Geological Survey publication, 199 p.
Rantz, S.E. and others, 1982a,
Measurement and computation of streamflow-Volume 1. Measurement of stage and
discharge: U.S. Geological Survey Water-Supply Paper 2175, 284 p.
1982b,
Measurement and computation of streamflow-Volume 2. Computation of discharge:
U.S. Geological Survey Water-Supply Paper 2175, 347 p.
Raffel, M., Willert, C., and
Kompenhans, J., 1998, Particle-image velocimetry-A practical guide: New York,
Springer-Verlag, 253 p.
Rouse, Hunter, 1946, Elementary
mechanics of fluids: New York, Dover Publications, Inc., 376 p.
Spicer, K.R., Costa, J.E., and
Placzek, G., 1997, Measuring flood discharge in unstable stream channels using
ground-penetrating radar: Geology, v. 25, no. 5, p. 423-426,
Wahl, K.L., Thomas, W.O., Jr., and
Hirsch, R.M., 1995, The stream-gaging program of the U.S. Geological Survey:
U.S. Geological Survey Circular 1123, 22 p.
Yamaguchi, T., and Niizato, K.,
1996, Flood discharge observation using radio current meter: University of
Tokyo, Hydraulic and Environmental Research report.