3.5 data retrieval modes real-time mode in which the ADCP can retrieve data 3.6 deploy ADCP initialized to collect data and propel the instrument across the section to record data 3.7 de
General
The Acoustic Doppler Current Profiler (ADCP) is an efficient, non-intrusive device designed to measure current velocity and direction throughout the water column It generates an instantaneous velocity profile while minimally disturbing only the upper few decimeters of water Utilizing the Doppler principle, an ADCP typically features a cylindrical design with a transducer head that consists of three or four acoustic transducers, strategically angled to the horizontal and to each other.
Figure 1 — Sketch illustrating typical ADCP with four sensors
Licensed copy: University of Auckland Library, University of Auckland Library, Version correct as of 02/07/2012 21:17, (c) The British Standards Institution 2012
Originally designed to study ocean currents by tracking them and generating velocity profiles, the instrument has evolved for applications in estuaries and rivers.
An Acoustic Doppler Current Profiler (ADCP) can be mounted on a boat, flotation collar, or raft, allowing it to traverse a river along various routes, not necessarily straight or perpendicular to the bank As it moves, the ADCP gathers data on velocity, depth, and position Additionally, it can take measurements at fixed locations across the measurement cross-section, akin to verticals used in traditional current meter gauging, a technique known as the “section-by-section method.”
3 path of boat on river bottom
Figure 2 — Sketch illustrating moving-boat ADCP deployment principles
Doppler principle applied to moving objects
The Acoustic Doppler Current Profiler (ADCP) employs ultrasound technology to assess water velocity, based on the Doppler effect discovered by Christian Doppler This principle states that sound waves reflected off a moving particle exhibit a change in frequency, known as the Doppler shift, which is the difference between the transmitted and reflected sound waves.
Licensed copy: University of Auckland Library, University of Auckland Library, Version correct as of 02/07/2012 21:17, (c) The British Standards Institution 2012
Only the components of velocity that are parallel to the direction of a sound wave result in a Doppler shift Therefore, particles moving perpendicular to the sound wave's direction, with no velocity components aligned with it, do not create a Doppler shift.
Figure 3 — Reflection of sound-waves by a moving particle results in an apparent change in the frequency of those sound waves
Doppler’s principle connects frequency changes to the relative velocities of the source and the observer In most Acoustic Doppler Current Profilers (ADCPs), transmitted sound reflects off particulates or air bubbles in the water column, returning to the transducer It is assumed that these particulates move at the same velocity as the water, allowing the frequency shift to be converted into a measurable velocity magnitude and direction.
Excessive air bubbles can distort or diminish the returned signal, as they tend to rise and may not accurately represent the magnitude and direction of the signal.
4.2.1 Speed of sound in water
The velocity calculated by Acoustic Doppler Current Profilers (ADCPs) is closely linked to the speed of sound in water, which is influenced by factors such as pressure, salinity, and sediment concentration, but is most affected by water temperature To ensure accurate measurements, most ADCP manufacturers measure the water temperature near the transducer and apply correction factors for temperature variations It is advisable to avoid ADCPs that lack temperature compensation features.
If the instrument is to be used in waters of varying salinity, the software used to collect data should have the facility to correct for salinity.
Figure 4 — Sound speed as a function of temperature at different salinity levels (left panel) and salinity at different temperature levels (right panel)
Licensed copy: University of Auckland Library, University of Auckland Library, Version correct as of 02/07/2012 21:17, (c) The British Standards Institution 2012
Figure 4 indicates the effect of temperature and salinity on the speed of sound As a general rule,
— a temperature change of 5 °C results in a sound speed change of 1 %,
— a salinity change of 12 ppt (parts per thousand) results in a change in sound speed of 1 %; freshwater is
0 ppt and seawater is in the region of 30 to 35 ppt), and
— the full range of typical temperature and salinity levels (−2 to 40 °C and 0 to 40 ppt) gives a sound speed range of 1 400 to 1 570 m/s (total change of 11 %).
Acoustic Doppler operating techniques
All ADCPs fit into one of three general categories, based upon the method by which the Doppler measurements are made:
Reference should be made to the instrument manual to determine the type of instrument being used.
An incoherent Doppler system transmits a long pulse of sound to measure the Doppler shift, which helps calculate the velocity of particles along the acoustic beam's path While these velocity measurements are robust across a wide range, they exhibit high short-term uncertainty with single pings To mitigate this uncertainty, multiple pulses (typically 9 to 20 per second) are transmitted and averaged before reporting the velocity The term "narrowband" refers to a pulse-to-pulse incoherent Acoustic Doppler Current Profiler (ADCP), where only one pulse is sent per beam per measurement In this system, the resolution of the Doppler shift occurs during the pulse's duration, and the narrowband acoustic pulse, being a simple monochromatic wave, allows for quick processing.
Coherent Doppler systems are the most precise among the three types, but they face notable range limitations These systems operate by transmitting a short pulse, capturing the return signal, and then sending a second pulse once the first return is no longer detectable By measuring the phase difference between the two returns, they calculate the Doppler shift, resulting in highly accurate velocity measurements with minimal short-term uncertainties However, coherent processing is restricted to specific depth ranges and has a significantly limited maximum velocity.
If these limitations are exceeded, velocity data from a coherent Doppler system are effectively meaningless.
Broadband Dopplers operate by transmitting two pulses and analyzing the phase change of the returns from successive pulses, with both acoustic pulses present within the profiling range simultaneously The broadband acoustic pulse features a complex waveform that incorporates a pseudo-random code achieved by reversing the phase This coding enables the collection of multiple independent samples from a single ping Although the processing speed is slower compared to narrowband systems due to the pulse complexity, the advantage lies in obtaining several independent samples from each ping.
Broadband processing offers a level of short-term uncertainty in velocity measurements that falls between incoherent and coherent systems Notably, broadband systems excel in measuring a broader range of velocities compared to their coherent counterparts.
The accuracy and maximum velocity range of a broadband system depend on the specific processing configuration employed; exceeding this range will result in meaningless velocity data.
Coherent processing offers precise velocity data in specific scenarios; however, it is not suitable for most contemporary profiling applications Instead, incoherent and broadband processing are the main techniques employed in Acoustic Doppler Current Profilers (ADCPs) for field use.
ADCPs divide the sampled water column into depth cells ranging from 0.01 m to over 1 m, following the blanking distance Each depth cell in each beam measures a center-weighted radial velocity Using these measurements and trigonometric relations, a 3-dimensional water velocity is calculated and assigned to each depth cell This process allows the ADCP to sample the entire measurable region of the water column, similar to a velocity profile obtained from a point velocity meter.
Figure 5 — ADCP depth cells or bins
Licensed copy: University of Auckland Library, University of Auckland Library, Version correct as of 02/07/2012 21:17, (c) The British Standards Institution 2012
To minimize measurement uncertainty, it is crucial to optimize the bin/cell size and blanking distance based on water depth, velocity, and measurement timing Longer lags enhance measurement accuracy, while larger bins improve the signal-to-noise ratio of the scatters in the pulse, thereby reducing uncertainty However, larger bins may restrict profiling in shallow depths, and using small bins with long lags can decrease the signal-to-noise ratio, resulting in increased uncertainty.
Larger bin sizes combined with longer measurement durations lead to reduced uncertainty in velocity measurements within each bin Additionally, increasing the number of bins in the water column enhances the overall accuracy of the velocity estimate for the ensemble Conversely, smaller bin sizes minimize the unmeasured areas in the water column.
Shallower streams and rivers necessitate the use of smaller depth cells, with at least two measured bins recommended at the edges For most of the cross section, a minimum of three cells in each ensemble is essential to effectively extend the velocity profile into the unmeasured areas of the water column.
The range-gating technique employed by Acoustic Doppler Current Profilers (ADCPs) generates center-weighted averages for each depth cell, utilizing overlapping bins An emitted pulse pair, with an overlap length matching the bin size, travels through the water column, capturing reflected signals from successive depth cells The strongest signal is detected when the entire overlap length of the pulse pair is contained within a depth cell, resulting in a weight of 1 at the cell's center, tapering to zero one bin size away This overlapping mechanism ensures that every section of the water column receives a weight of 1.
Figure 6 — Showing the effect of range-gating and bin size on velocity averaging as a pulse pair propagates down through the water column
Licensed copy: University of Auckland Library, University of Auckland Library, Version correct as of 02/07/2012 21:17, (c) The British Standards Institution 2012
The angle of ADCP transducers typically ranges from 20 to 30 degrees from the vertical, varying by manufacturer and instrument While ADCPs cannot measure all the way to the streambed, the acoustic transducers emit sound primarily in the main beam, with additional energy in side lobes Although these side lobes generally carry low energy and do not affect most of the water column, they can reflect off the streambed, which is a strong reflector of acoustic energy This reflection can lead to interference, as the energy from the main beam, which reflects off scatters in the water column, is relatively low compared to the energy returned from the side lobe Consequently, this interference creates a measurement gap near the bottom, where accurate data cannot be obtained due to side-lobe contamination.
In a 20-degree system, the range from the transducer is affected by 6% interference as the profile nears the boundary, caused by the reflection of side-lobe energy that takes a direct, shorter path to the boundary.
Licensed copy: University of Auckland Library, University of Auckland Library, Version correct as of 02/07/2012 21:17, (c) The British Standards Institution 2012
Figure 7 — Diagram illustrating depth zones within the water column: blanking distance, area of measured discharge and zone subject to side-lobe interference
To eliminate bias in velocity estimates, ADCP instruments and their software automatically disregard the water column section influenced by side-lobe contamination near the bed Users can find detailed information about this process in the user manual.
To prevent velocity bias, it is essential to ensure that the mean velocity at depth is only considered valid when all beams can measure to the same water depth Data obtained from shorter path lengths, which may occur due to boulders or other irregularities in the channel, should be disregarded.
As illustrated in Figure 8, the instrument is unable to make velocity measurements in three areas:
— near the surface (due to the depth at which the instrument is located in the water and, added to this, the instrument blanking distance);
Licensed copy: University of Auckland Library, University of Auckland Library, Version correct as of 02/07/2012 21:17, (c) The British Standards Institution 2012
— near the bed (due to sidelobe interference, channel undulations and acoustic reflections caused at the bed);
— near the channel edges(due to a lack of sufficient water depth or to acoustic interference from signals returned from the bank).
Movement monitoring techniques
ADCPs utilize the Doppler principle to measure discharge from a moving boat, employing a technique known as "bottom tracking." This method involves longer pings that reflect off the channel bed, allowing for the calculation of the instrument's velocity relative to the stable bed Additionally, ADCPs may include an onboard compass, which, when combined with bottom-tracking data, helps determine the direction and speed of the vessel.
Figure 9 — Velocity measurements taken during an ADCP gauging
4.4.2 Differential Global Positioning System (DGPS)
A Differential Global Positioning System (DGPS) can be attached to Acoustic Doppler Current Profilers (ADCPs) to provide movement data, serving as an alternative to bottom tracking in situations where the bed is unstable or when bottom tracking fails due to factors like weed growth or heavy suspended sediments For effective use, a sufficiently accurate DGPS is essential, and it is crucial to calibrate the internal compass of the ADCP and accurately estimate the local magnetic variation.
The instrument serves as a substitute for a current meter, such as a cableway-mounted device, with its horizontal position identified for conventional flow measurement When equipped with a built-in compass, it can be used accurately without introducing errors In the absence of a compass, it is essential to deploy the instrument perpendicular to the cross section and maintain its position during measurement If maintaining this position is not feasible, the instrument's orientation relative to the flow direction must be established, following principles similar to those used in conventional current meter gauging from a suspension cable.
Licensed copy: University of Auckland Library, University of Auckland Library, Version correct as of 02/07/2012 21:17, (c) The British Standards Institution 2012
While stationary operations resemble traditional current meter gauging and adhere to its fundamental principles, the use of Acoustic Doppler Current Profilers (ADCPs) presents unique challenges that must be addressed.
5 Principles of methods of measurement
Data retrieval modes
ADCPs can operate in two modes: real-time and autonomous In real-time mode, the device continuously communicates with a computer, allowing data to be processed and displayed instantly during the gauging process Conversely, the autonomous mode records measurements internally for later download, but this method is less common and generally not recommended, as real-time mode is suitable for most applications today.
A separate portable power source may be necessary to power the laptop when running the ADCP in real-time mode, as laptop batteries may not last a full day’s gauging.
Maintenance
Most Acoustic Doppler Current Profilers (ADCPs) can perform built-in diagnostic checks using a combination of firmware and software to ensure proper functionality It is essential to conduct these checks at the start and end of each field day, ideally before each discharge measurement or during site inspections for permanent installations Key diagnostics include tests for the CPU, DSP, beam operation, sensors, and battery condition.
Manufacturers advise regular servicing of Acoustic Doppler Current Profilers (ADCPs) to prevent undetected faults that can lead to inaccurate measurements Typically, ADCPs utilized for river discharge measurements do not require frequent servicing For instance, manufacturers suggest the routine replacement of O-ring seals, but since these devices are seldom submerged for extended periods, the need for frequent maintenance is minimized.
30 cm to 40 cm, this is not usually necessary.
Training
For effective ADCP gauging, it is essential that at least one team member has undergone formal training in the operation of the equipment and its software Additionally, other team members should possess a basic understanding of the equipment's field operation and the fundamental principles of ADCP gauging.
To stay current with the evolving ADCP technology, users should regularly coordinate with equipment suppliers for updates on software changes, bug fixes, and operational best practices Additionally, it is essential for practitioners to have access to initial and refresher training in field usage, as well as training focused on data analysis, processing, and quality control.
Flow determination using a vertically mounted ADCP
An Acoustic Doppler Current Profiler (ADCP) measures velocity in each depth cell, allowing for the computation of discharge based on depth cell size and the distance between profiles Velocities in unmeasured areas are extrapolated from the depth cells, and the discharge from these areas is calculated and combined with the measured discharge to determine the total discharge for each ensemble The total discharge for the measured cross section is the sum of the ensemble discharges, while the discharge in unmeasured sections is estimated using a suitable algorithm This unsampled discharge is then added to the total ensemble discharge to provide an overall estimate of the cross-sectional discharge.
Licensed copy: University of Auckland Library, University of Auckland Library, Version correct as of 02/07/2012 21:17, (c) The British Standards Institution 2012
Figure 10 demonstrates the process of determining discharge with Acoustic Doppler Current Profilers (ADCPs), where the discharge in each cell is calculated and summed to obtain the total measured discharge Discharges near the surface, bed, and banks are estimated using a suitable extrapolation technique (refer to Annex A) This process can be expressed mathematically.
The total discharge, represented as \$Q_{total}\$, is calculated by summing the incremental discharges (\$q_{n,j}\$) from each measured depth cell across the cross section, along with the extrapolated discharges (\$q_{estimate}\$) from the unmeasured areas In this equation, \$n\$ denotes the vertical cell number, while \$j\$ indicates the horizontal profile number.
Figure 10 — Showing the measured area of the channel cross section, divided into individual profiles and bins
To accurately estimate discharge, the Acoustic Doppler Current Profiler (ADCP) must traverse a river with its transducers submerged at a consistent depth, ideally mounted on a boat or flotation platform Various deployment methods are outlined in sections 5.4.1 to 5.4.6 For tethered deployments, the ADCP is affixed to a flotation platform, which varies by manufacturer It is crucial to select a platform that can withstand the anticipated water velocity, as excessive speeds may lead to capsizing.
When installing an Acoustic Doppler Current Profiler (ADCP) on a boat, it is essential to use non-ferrous materials for fittings that allow for vertical adjustment of the device This flexibility enables the transducers to be positioned at varying depths relative to the water surface, facilitating easy installation and secure attachment The ADCP does not need to be permanently affixed to the boat; however, it should be mounted forward of the engine to minimize noise and propeller wash Proper positioning is crucial to ensure that the ADCP measures velocities without interference from the vessel's hull, ideally placing the instrument at the bow of the boat.
The device can be easily repositioned from port to starboard based on the upstream direction When installed at the bow, it is crucial to minimize the bow wave to ensure accurate velocity measurements.
The ADCP can be mounted on a small floating platform that is tethered to a boat, facilitating its transport across a river This setup enables optimal positioning of the instrument during deployment, ensuring that the boat hull remains downstream of the ADCP.
5.4.2 Tethered deployment on a tow rope
Using a tethered boat and tow rope is the most straightforward and effective method for deploying equipment at various gauging sites The necessary equipment is minimal, requiring just two ropes to span the section and a flotation platform An operator can easily cross the river with one end of the rope, and a pulley system may be established with a single loop of rope Additionally, if deploying the ADCP from a bridge, it might be feasible to utilize a single rope, depending on the specific site conditions.
Towing an ADCP across a wide, navigable river can be impractical If this method is necessary, it's advisable to replace one of the ropes with a cable that can be lowered to the riverbed, allowing boat traffic to pass safely Additionally, operators should use a megaphone to alert boaters about the rope's presence and guide them on which side to navigate.
This method is suitable for smaller rivers or canals, and sites with lower velocities Very high velocities may cause the operators to be dragged into the water.
Existing cableways designed for conventional current meter flow measurement can effectively deploy Acoustic Doppler Current Profilers (ADCPs) without requiring additional equipment beyond a flotation platform To ensure optimal performance, the suspension cable must be slack enough for the platform to rest on the water surface, maintaining the transducers at a constant depth Additionally, the suspension weight should be positioned above the water surface to prevent turbulence around the ADCP.
The ADCP can be deployed from a bridge using a rope or handline, or with a bridge-gauging derrick or “A”-frame, similar to a conventional current meter It is essential to use a flotation collar during deployment to maintain the transducers at a constant depth When using the “A”-frame for lowering the instrument, it must be capable of safely supporting both the ADCP and the flotation platform.
5.4.5 Tethered deployment on a remote control craft
Deploying the ADCP on a remote control platform is ideal when there is no cableway or safe crossing for the operator Given the high cost of ADCP equipment, it is wise for practitioners to attach a light line for recovery in case of motor failure However, it is crucial to ensure that this line does not create drag or become entangled in the propellers.
Using ADCPs in self-contained mode is not advisable, as this method was primarily utilized when real-time operation was unfeasible It is mentioned here for completeness, particularly for users facing real-time communication issues with the ADCP Similar to real-time mode, determining flow requires multiple river transects; however, data will be recorded as a continuous set, making it challenging to pinpoint the end of each transect Therefore, it is crucial to document the time at both ends of each transect and to pause for 30 seconds at the end of each crossing to clearly mark the transect's conclusion, ensuring accurate measurements.
The ADCP must be synchronized with the timing device to accurately record the start and finish times of the transect, ensuring it can be identified and distinguished from other transects and pause times.
Discharge measurement process
Each ADCP used should be tested:
— when the ADCP is first acquired;
— after factory repair and prior to any data collection;
— after firmware or hardware upgrades and prior to any data collection; and
— at some periodic interval (for example, annually).
The primary goal of an instrument test is to ensure the accurate functionality of the Acoustic Doppler Current Profiler (ADCP) for discharge measurements Testing methods for ADCP accuracy include tow-tank tests, flume tests, and comparisons with conventional current meter measurements However, each method has its own limitations, as highlighted by Oberg (2002).
Misalignment of beams is a common source of instrument bias, which can be evaluated through a simple field test for instruments equipped with an internal compass This beam-alignment test compares the straight-line distance, known as the "distance made good," measured by bottom tracking against that measured by GPS While bottom tracking may have a small bias due to terrain effects, typically less than 0.2%, the USGS recommends that the ratio of bottom track made good for the Rio Grande ADCP should fall between 0.995 and 1.003 for acceptable beam alignment Although sufficient data for other ADCPs is lacking to validate this criterion, it is generally assumed to be applicable.
If the instrument fails to meet the beam-alignment criteria, it can be sent back to the manufacturer for the creation and installation of a custom transformation matrix.
Regular instrument checks are essential for maintaining consistency in discharge-measurement techniques and ensuring the accuracy of Acoustic Doppler Current Profilers (ADCP) These checks can be conducted at locations where the ADCP's discharge measurements can be validated against known discharges from reliable sources, such as stable stage-discharge ratings or independent measurements It is beneficial to utilize various water- or bottom-tracking modes during these checks, although it is not mandatory To capture a diverse range of hydrologic conditions and minimize site-specific biases, periodic checks should be performed at multiple sites Ideally, the discharge readings from the ADCP should align within 5% of the known discharge, and any persistent biases in annual records warrant further investigation.
When comparing a stable stage-discharge rating, if the ADCP measurement deviates by more than 5%, it may indicate a shift in the rating To verify the accuracy of the rating, a second measurement using either another ADCP or a conventional discharge method should be conducted before making any final conclusions about the ADCP instrument test.
Before conducting ADCP deployments in the field, it is essential to follow specific pre-field procedures These steps help prevent unnecessary trips and delays while ensuring the data collected is of high quality.
To optimize data collection and processing, it is essential to utilize the most recent software and firmware All field computers should have the latest software installed, and it is advisable to keep a backup of the software on separate storage media to safeguard against potential damage or loss of the computer.
Licensed copy: University of Auckland Library, University of Auckland Library, Version correct as of 02/07/2012 21:17, (c) The British Standards Institution 2012
Before heading into the field, it is essential to assemble and verify all equipment, including ancillary items like distance measurement devices Conducting a pre-field equipment check ensures that all necessary tools are ready for use For a detailed checklist, refer to Annex C.
— All cables, batteries and mounts should be checked.
— The ADCP should be connected to the field computer and all communications including radio modems, if these are to be used, should be checked.
— Any other ancillary equipment to be used, which will be connected to the ADCP in the field, such as echo sounders and DGPS, should also be connected and checked.
To ensure accurate measurements with the ADCP, follow these pre-measurement procedures: conduct required instrument diagnostic checks as per the manufacturer's guidelines from a stationary boat in still water After deploying the ADCP, measure and record the transducer depth, ensuring the flotation device's roll and pitch match those during discharge measurement to avoid significant errors Exercise caution when measuring transducer depth on a boat to maintain personal safety and accuracy Utilize a pre-calibrated mounting bracket to secure the equipment at a known transducer depth, accounting for any load changes in the boat Verify the instrument's temperature measurement with an independent sensor at the same depth, especially in areas with varying salinity, which should be recorded in the ADCP software Check and synchronize the ADCP’s clock with the gauging station recorder Ensure the built-in compass is calibrated correctly, particularly for loop or azimuth moving-bed tests, following the manufacturer's manual Finally, configure the ADCP to reflect site-specific hydraulic and hydrological conditions, setting essential parameters such as blanking distance, water mode, depth-cell size, and profiling range, while referring to the technical documentation for detailed configuration guidance.
Wind speed is a critical factor, particularly in areas with low velocities, as it can significantly impact surface velocities and the choice of top extrapolation method It is essential to document overall wind speed and direction, along with variations between transects, on all measurement field note forms to ensure accurate processing and review of measurements For users unfamiliar with the measurement section, conducting a trial transect across the river—whether recorded or not—can help identify key characteristics of the proposed measurement.
3) maximum water velocity and its location in the cross section;
5) effects of hydraulic structures, such as bridges, piers, and islands, on the flow;
6) unusual flow conditions, such as reverse or bi-directional flow;
To ensure accurate edge estimates, it is essential to identify approximate start-and-stop locations on both the left and right banks, where at least two depth cells with valid velocity measurements can be recorded Utilizing buoys to mark these locations can enhance consistency in the measurements.
It is essential to record the information obtained from the trial transect on the discharge-measurement notes, ensuring that data files adhere to a uniform convention An ADCP measurement field sheet should be utilized to document all relevant site information, configuration setups, and gauging details Any modifications to the configuration during measurements must be clearly noted, specifying the applicable transects Prior to conducting discharge measurements, a moving-bed test should be performed and documented, as a moving bed can lead to an underestimation of discharge due to the ADCP's velocity readings The results of this test will inform the monitoring method used and may necessitate adjustments to discharge measurements Various methods are available for conducting a moving-bed test.
1 File names for the data files collected (also called deployment names) should follow a uniform, documented convention developed by each organization involved in the ADCP operation.
Accurate measurement of the ADCP depth, defined as the vertical distance from the water surface to the transducer face, is essential and should be documented in the discharge-measurement notes and configuration file It is important that the boat's pitch-and-roll during depth measurement mirrors that during the discharge measurement If there are any changes in the ADCP depth throughout the measurement process, it is necessary to re-measure, record the new depth, and update the configuration file accordingly.
Most ADCP data-collection software features an automated configuration method that relies on user-provided site characteristics, including maximum water depth, bed-material properties, and anticipated maximum water and boat speeds.
It is essential to document the configuration parameters and site conditions in field notes when using an automated configuration program Any modifications to the ADCP configuration during measurements must also be recorded on the measurement field note forms, ensuring clarity regarding the changes made and the specific transects they pertain to.