Interim Policy and Technical Guidance on Broadband Acoustic Doppler Current Profilers
In Reply Refer To: January 31, 1996
Mail Stop 415
OFFICE OF SURFACE WATER TECHNICAL MEMORANDUM 96.02
Subject: Interim Policy and Technical Guidance on Broadband Acoustic
Doppler Current Profilers
This memorandum supersedes Office of Surface Water Technical Memorandum
93.15, Interim Policy and Technical Guidance on Broadband Acoustic
Doppler Current Profilers (ADCPs). The purpose of this memorandum is
to provide information on the ADCP work group and to update policy and
technical guidance for the operation of broadband ADCP's.
The Office of Surface Water (OSW) has sponsored an ADCP work group to
provide operational guidance to users. The work group, which includes
members from Environment Canada and the Corps of Engineers, has put on
three ADCP workshops--two in the United States and one in Canada. They
have planned and commissioned the preparation of three reports: a
quality-assurance plan, an operational manual, and a report on a
verification study. The work group also has established an Internet
news group called usgs.adcp for dissemination and discussion of ADCP
topics.
The quality-assurance plan has been published as Open-File Report 95-
701, Quality Assurance Plan for Discharge Measurements Using Broadband
Acoustic Doppler Current Profilers, by Stephen W. Lipscomb. Copies are
available from OSW. The operational manual is in colleague review.
The verification study, comparing ADCP-produced discharges with stable-
rating discharges at 12 gaging stations, has been completed. The report
is approved and is being prepared for printing.
System performance and operation characteristics of ADCP's are changing
as hardware and software continue to improve. Frequent revisions of
software, firmware, and documentation revisions are expected. Policy
and technical guidance will thus be subject to change and some
information may be specific to a particular system (i.e. signal
frequency), or version, which may not be the most current.
POLICIES
The following policies apply to use of the ADCP for making velocity-
profile and discharge measurements from moving boats.
MEASUREMENT CONDITIONS
The ADCP can accurately measure discharge for only a limited range of
flow conditions. Based on manufacturer recommendations and preliminary
measurement-verification information, the following conditions must be
met for general use of the ADCP to measure velocity profiles or
discharge. Measurements made at or beyond these limits generally will
not produce accurate results and will require specific error analysis.
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| Average | Average | Maximum Unmeasurable Flow Due |
| | | |
| Depth | Velocity | To Limited Depth or Velocity |
| | | |
| between 4.5 and 66 ft | between 0.4 and 12 fps | 10 percent |
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These and other measurement conditions (site characteristics and
instrument operation) must be assessed to correctly set up the
instrument and software, to determine appropriate measurement
techniques, and to check the results for errors. The ADCP Discharge
Measurement Notes form or similar document should be used to record
measurement conditions. When a measurement condition appears
problematic, measurement documentation should include: (1) a brief
description of the condition; (2) methods used to determine if the
condition (i.e. mobile stream bed) is significant to the measurement;
and (3) how the effects of the conditions were accounted for (i.e.
appropriate instrument operating modes, configuration file settings,
tests, and error analyses). Measurement conditions to be assessed
include the following:
Mobile stream bed
Channel edge
Pitch and roll of boat platform
Measurement starting and stopping procedures
Side-lobe interference
Heading (compass) errors
Shallow-depth-caused errors not covered elsewhere
Vessel speed during measurement
Vessel course during measurement
A wide range of flow conditions may be encountered when using an ADCP.
Considerable judgement must be exercised by the ADCP operator. The
following general guidelines are provided:
1. Under steady flow conditions a minimum of four transects will be
made (two in each direction). The reported discharge will be the
average of the transected discharges. If any of the four transect
discharges differ more than 5 percent of the mean discharge, then at
least one more set (four transects) will be obtained.
2. Under rapidly varying flow conditions, it may be necessary to make a
single transect as a discharge measurement. The rationale for using
less than four transects should be documented and permanently stored
with the discharge measurement and documented in applicable station
analyses.
3. Use of an ADCP does not reduce the importance of site selection for
discharge measurements. The guidelines provided in Water Supply
Paper 2175 are still applicable.
4. When bed movement is suspected, the vessel should be anchored for
about 10 minutes in a section where the possibility of bed movement
exists, provided that this can be done safely. During this time ADCP
bed-tracking data should be collected and examined to reveal any
apparent upstream movement of the boat relative to the channel
bottom. If apparent upstream boat movement exists, then the ADCP
measured water velocity may be affected by bed movement, and the
ADCP indicated discharge may be lower than the true discharge.
5. As a general rule of thumb, boat speed across the transect should be
less than or equal to the average water velocity. In general, a
transect should not take less than about 3 minutes or
45 ensembles to complete.
TRAINING
Successful application of the ADCP requires considerable experience and
informed judgement based on an understanding of the physical and
computational factors which affect the accuracy of measurements or
individual components of measurements. These factors must be understood
and evaluated in order to optimize the set-up of the ADCP for a
particular measurement, and to evaluate the quality of that measurement.
Districts using or planning to use ADCP's are encouraged to develop
expertise in the use of this device. Parties making discharge
measurements using the ADCP should include a person who: (1) attended
basic training in the use of the ADCP's; (2) is familiar with the basic
principals of ADCP operation; (3) knows the principal sources of ADCP
velocity and discharge measurement errors and how to evaluate and
minimize them; and (4) is knowledgeable about the Transect Software.
This information can be gained by experience, manufacturer's manuals,
other relevant literature, and training at a U.S. Geological Survey
sponsored workshop. A practical alternative training method would be
working with experienced ADCP operators as on-the-job trainees in a
mentoring environment. OSW can provide information on qualified
mentors.
MEASUREMENT DOCUMENTATION
Measurements made with the ADCP should be documented on the standard
form 9-275 C or G, (all applicable information) and on the attached ADCP
Discharge Measurement Notes form or a modified ADCP form which
documents individual transect data. The ADCP configuration file and
the raw data file for each of the transects making up the measurement
must be archived. Districts may wish to archive additional files or
data. All ADCP discharge measurements must be reviewed by a qualified
person.
Thomas H. Yorke
Chief, Office of Surface Water
Attachment
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