[Acoustics] OSW Technical Memo 2005.05-Guidance on the use of RD
Instruments StreamPro Acoustic Doppler Profiler
Kevin Oberg
kaoberg at usgs.gov
Wed Jun 29 10:49:50 CDT 2005
The following memo is also available at
http://hydroacoustics.usgs.gov/policy/OSW2005-05.pdf
Kevin
---------------------------------------------------
In Reply Refer To: June 27, 2005
Mail Stop 415
OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM 2005.05
Subject: Guidance on the use of RD Instruments StreamPro Acoustic Doppler
Profiler
RD Instruments (RDI) introduced a new acoustic Doppler current profiler
(ADCP) known as the StreamPro for shallow water discharge measurements in
the summer of 2003. The StreamPro is designed to make a moving boat
discharge measurement in streams with depths between 0.5 and 6.6 feet. The
profiler is deployed from a specially designed tethered boat (figure
1). Bluetooth radio telemetry is used for communication between the
StreamPro and a hand-held computer with the Pocket PC operating
system. Software on the hand-held computer saves the data transmitted by
the StreamPro and computes the discharge.
(Embedded image moved to file: pic08586.jpg)
Figure 1.--StreamPro acoustic Doppler current profiler.
In July 2004, Office of Surface Water (OSW) staff and other USGS employees
began testing the StreamPro. A plan for testing the StreamPro was
prepared and used for the field tests
(http://hydroacoustics.usgs.gov/policy/StreamProTestPlan.pdf). Evaluation
sites were chosen to test the StreamPro in a range of stream conditions.
Concurrent measurements of discharge were made with the StreamPro,
mechanical current meters (AA and pygmy), and other acoustical meters. In
some cases, comparison discharges were obtained from established USGS
streamflow-gaging stations with stable rating curves. Following USGS
policy for other ADCPs in OSW Technical Memorandum 2002.02, the mean of
four or more transects were used to obtain the measured StreamPro discharge.
Analysis of the test measurements made to date (May 2005) indicate that
discharges measured using the StreamPro compare favorably to the discharges
measured with the other meters when the mean channel velocity is greater
than 0.8 ft/s. Of the 13 measurements where the mean channel velocity was
greater than 0.8 ft/s, 10 of the StreamPro-measured discharges were within
5 percent of the comparison discharge and all 13 were within 7 percent of
the comparison discharge (figure 2).
(Embedded image moved to file: pic24137.jpg)
Figure 2.—Mean channel velocity and deviation of StreamPro discharges from
comparison discharges, in percent.
When the mean channel velocity is less than 0.8 ft/s, the StreamPro
discharge measurements for individual transects have much greater
variability than those StreamPro measurements where the mean channel
velocity is greater than 0.8 ft/s. Measurements with mean velocities less
than 0.8 ft/s had an average coefficient of variation for individual
transect discharges of 12 percent, whereas measurements with mean
velocities greater than 0.8 ft/s had an average coefficient of variation of
2.5 percent (figure 3). Despite this larger variation, there is no
indication that the measured discharges (the mean discharge for all the
transects) are biased, provided that enough transects are included in the
mean discharge. The results of testing showed that the variation can be so
great that even if eight or more transects are collected as specified in
OSW Technical Memorandum 2002.02, the measurement quality may be affected.
This result means that using the StreamPro to make discharge measurements
under these low velocity flow conditions may not be practical because of
the time required to obtain additional transects. More testing is needed
to determine how many transects are needed in low-velocity conditions to
make an accurate discharge measurement and whether an accurate measurement
under these conditions is even possible.
(Embedded image moved to file: pic26106.jpg)
Figure 3.--Mean channel velocity and coefficient of variation for StreamPro
discharge measurements.
RD Instruments is exploring the possibility of using a different
water-measurement mode in the StreamPro for low-velocity measurements. If
and when this water-measurement mode becomes available, it will be included
in the OSW testing program.
A number of StreamPro users have experienced problems with the StreamPro in
velocities greater than 4 ft/s, especially when surface waves are present.
At these high velocities, a wave may catch the bow of the boat and cause
the StreamPro to be pulled under by the current. The manufacturer has
redesigned the boat to help minimize this problem. However, it is
important that users set the bail properly (see figure 1). When the bail
is improperly set at a large angle relative to the water surface (greater
than 45 degrees), the nose of the StreamPro may be pushed down into the
flow. We have little information to-date regarding the accuracy of
discharge measurements made where the mean velocity is greater than 3.5
ft/s. However, there is no reason to believe that the StreamPro will not
accurately measure velocities greater than 3.5 ft/s and less than it's
maximum measurable velocity, 6.6 ft/s. Users should be aware that the
possibility of a moving bed condition affecting the measurement will be
greater at these higher velocities.
Based on our current knowledge, the OSW recommends the following practices
regarding the use of StreamPros.
1. Standard ADCP data collection techniques as outlined in OSW Technical
Memorandum 2002.02 should be followed. This includes such things as always
performing a moving bed test, minimizing erratic boat motion during
transects, accurately measuring edge distances, and keeping boat speed less
than or equal to water speed.
2. When the mean channel velocity is less than 0.8 ft/s, the StreamPro
may be used to make discharge measurements; however, the following guidance
should be observed.
· Eight transects or more may be necessary because of the high
coefficient of variation between transect discharges. Generally, as the
mean channel velocity approaches zero, more transects will be needed to
obtain an accurate measured mean discharge.
· If the coefficient of variation for individual transect discharges is
greater than 10 percent, OSW recommends that the discharge measurement
quality be considered poor. This recommendation is based on currently
available test data and may be revised when more test data are available.
· If the difference between the discharge measured with the StreamPro
and the rating discharge is excessive (much greater than five percent), OSW
recommends that a comparison discharge measurement be made using a
different instrument (for example, a mechanical current meter or Flowtracker).
3. Whenever feasible, OSW recommends that a temporary bank operated
cableway or rope-and-pulley system be used to move the StreamPro back and
forth across the stream during discharge measurements. Bank operated
cableways or rope-and-pulley systems allow for more uniform boat motion and
help reduce variability in measured discharges.
4. Although a discharge is reported on the hand-held computer as the
measurements are made, OSW recommends that the discharge measurement be
loaded into WinRiver software in order to review each StreamPro discharge
measurement for quality-assurance purposes. This process should include
such things as the review of the edge distances, types, and discharges;
velocity-profile extrapolations used to compute discharge in the unmeasured
top and bottom zones of the transect; and boat speed versus water speed.
5. The OSW recommends that the elapsed time for a single transect be
three minutes or more, whenever possible.
6. The OSW also suggests that StreamPro users make periodic comparison
measurements with mechanical current meters (AA and pygmy) or FlowTrackers
over the range of flow conditions the StreamPro measurements are
made. This action will provide quality-assurance and quality-control data,
and aid OSW's efforts to collect comparison measurements over a wide range
of conditions.
The OSW requests that StreamPro users share results of their experiences in
using the StreamPro. Please send information to the USGS Hydroacoustics
Work Group (hawg at simon.er.usgs.gov) or to Mike Rehmel, USGS Indiana Water
Science Center, (msrehmel at usgs.gov), 317-290-3333 ext. 158. OSW is
interested in receiving information regarding measurement comparisons,
problems identified with the instrument or software, possible hardware or
software enhancements, and other issues important to users. Tests of these
instruments are continuing and test results will be communicated in future
memorandums, on the OSW Hydroacoustics Web pages
(http://hydroacoustics.usgs.gov/), and through the Acoustics mailing list.
/signed/
Stephen F. Blanchard
Chief, Office of Surface Water
Distribution: Data Chiefs
District and Regional Surface-Water Specialists
Office of Surface Water
HydroAcoustic Work Group (HAWG)
Cc: A, B, District Chiefs, FO
----------
Kevin Oberg
USGS - Office of Surface Water
Voice: 217.344.0037 extension 3004
Cell: 217.840.9739
http://hydroacoustics.usgs.gov/
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