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. --------------------------------------------------------------------------------- | 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 | --------------------------------------------------------------------------------- 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 WRD DISTRIBUTION: A, B, FO, PO