Current, in terms of hydrology, refers to the movement of water that takes place in all natural surface waters, including streams, rivers, lakes, coastal waters, and oceans. Various factors influence water currents, such as wind patterns, temperature and salinity differences, the Earth’s rotation (Coriolis effect), the gravitational pull of the moon, the physical topography of the waterbed, and human activities such as the construction of dams and embankments.
Though various methods exist for measuring currents, most modern instrumentation uses acoustics (sound waves) and the Doppler effect for measurement. These instruments are commonly known as Doppler current meters for single-point measurement and Acoustic Doppler Current Profilers (ADCPs) for multi-depth profiling.
Water currents are generally reported in velocity units such as knots, meters per second (m/s), or centimeters per second (cm/s). They may be given as a single-point with velocity magnitude only, on a 2D plane with magnitude and direction in degrees, or in a 3D coordinate system such as XYZ or ENU (east-north-up).
Why Measure Water Currents?
Measurement of water currents is vital for understanding the dynamics of aquatic environments, both locally and globally. From ancient mariners navigating the seas to modern scientists studying climate change, understanding the movement of water has always been important to human civilizations.
There are a wide range of fields of study and practical applications that can benefit from current measurement:
- Stream and River Flow: As described in the section on water flow, ADCPs and other devices are often used to measure currents in rivers and streams. Current velocities are then used in total flow rate calculations, which are important for applications such as flood control, water resource management, and hydropower operations.
- Climate Patterns and Weather Prediction: Ocean currents such as the Gulf Stream and Kuroshio Current push warm waters towards the poles and drive global climate patterns. Tracing these currents is essential for predicting weather events and studying potential effects of climate change.
- Maritime Navigation: Ocean currents have long been important to ship captains for safe and efficient navigation of the seas. Accurate current measurements are vital for port security and the planning of maritime routes to optimize fuel consumption, reduce travel time, and avoid hazardous conditions at sea.
- Aquatic Ecosystem Assessments: Water currents influence the distribution of nutrients, pollutants, and sediments in aquatic environments. Monitoring currents helps in assessing the health of aquatic ecosystems, especially in rivers, estuaries, large lakes, and marine environments.
- Fisheries Management: Current patterns affect the migration, breeding, and feeding behaviors of many marine species. Fishermen and conservationists alike measure currents to track populations, sustainably manage fisheries, and protect endangered species.
- Search and Rescue Operations: Knowledge of local current patterns and quick deployment of instrumentation on emergency response buoys is essential for search and rescue operations at sea. Accurate current data enables first responders to quickly develop more efficient strategies to locate and rescue individuals in distress.
- Energy Generation: Measuring and modeling current flows is critical for the development of marine renewable energy sources like tidal and ocean current power, which offer potential solutions for sustainable energy generation.
How are Water Currents Measured?
ADCPs transmit high-frequency sound waves through the water and use the Doppler effect to measure water speed at various depths simultaneously, providing detailed profiles of current speed and direction over a range of depths.
The Doppler effect refers to the frequency shift of the sound waves when reflected off of particles in the water back to the ADCP instrument. The greater the current velocity, the greater the frequency shift will be.
ADCPs measure reflections of sound waves off of particles in the water to determine current velocities.
Sound waves are ideal for in-water measurements because they can travel great distances without significant signal loss, unlike light, which is scattered by particles in the water. Since sound waves reflect off of particles like zooplankton and suspended sediments, it is technically an indirect measurement of water currents, but it is assumed that the particles are moving at the same velocity as the water. Particles that deflect acoustic energy are known as scatterers.
Each beam of an ADCP measures one-dimensional (1D) motion parallel to the beam path. To measure currents in multiple dimensions, ADCPs integrate multiple acoustic beams slanted at different angles. The vectors of each beam are summed to calculate 2D or 3D velocities.
The vectors derived from the known beam angles are summed across multiple beams to calculate 2D and 3D velocities.
To profile at multiple depths, ADCPs are configured with cells or bins, which are layers of specified distance from the transducers. An acoustic signal, or ping, is sent, and then the instrument listens for echoes. The first echoes to arrive are from the first cell. A short time is allowed for the second echo to arrive, and so on, until the signal dies out and the returned echoes are too weak to calculate velocity.
The ADCP can determine the distance of each echo from the instrument because it can calculate sound speed in the water in which it is measuring. Sound speed is influenced by temperature, pressure, and salinity, but it is temperature that has the greatest influence. Therefore, ADCPs integrate thermistors near the transducers to measure the water temperature during sampling.
How to Select a Water Current Measurement Instrument?
There are a range of important factors that influence water current measurement instrument selection, including acoustic frequency, beam configuration, power consumption, deployment mode, and environmental conditions.
To select an instrument, first decide if a single-point measurement or multi-cell profile is required. For profiling ADCP instruments, the frequency influences the depth range and resolution. Lower frequencies are used for deeper water measurements due to their longer range, while higher frequencies provide better resolution in shallower waters.
Lower frequency instruments can reach greater depths, but higher frequency instruments provide better resolution in shallower applications.
ADCPs can have different numbers of beams, typically ranging from three to five. More beams can provide more detailed current profiles and better error estimation but also consume more power.
If the ADCP will be deployed on a moving platform such as a buoy, boat, or ROV (remotely operated vehicle), it should be equipped with tilt sensing. Bottom-mounted instruments must be rated for the depth at which they will be placed.
What to Consider When Preparing a Water Current Measurement Instrument?
Internal configuration plays a crucial role in achieving success with water current measurements. Besides configuration of depth cells, the settings will typically allow for inputs such as water salinity, deployment mode (fixed/buoy), orientation (up-looking/down-looking), sampling duration, and others. These will affect both the power consumption and the quality and accuracy of the measurements.
Power consumption is an important consideration whether an instrument will be deployed in a standalone configuration or as a part of an integrated system. For a standalone deployment, consider the required deployment duration, available battery pack sizes, and the effect of instrument settings on power efficiency. In integrated systems, a power budget analysis is an important step in system design.
Doppler current meters and ADCPs use an internal compass to determine heading for accuracy of current direction measurements. Compasses may be subject to interference from magnetic materials within the instrument or the mounting hardware.
Many instruments allow for user calibration prior to deployment, which should ideally be performed at the site and with the instrument already mounted. This helps to account for any offset and distortion errors introduced by surrounding materials.
Compass calibration can correct for error from magnetic materials near the ADCP and improve the accuracy of directional measurements.
Biofouling is another factor that can cause measurement interference and eventual instrument damage. The application of anti-fouling paints or patches can help to reduce biofouling and thereby extend maintenance intervals.
How to Deploy a Water Current Monitoring System?
For environmental field studies, water current measurement instruments are typically deployed from fixed platforms, subsurface buoys, or surface buoys.
Fixed platforms, such as a bottom tripod, offer a stable and secure method for instrument placement. They allow for ADCPs to measure nearly the entire water column, including surface measurements.
Bottom platforms such as tripods provide a stable method for deployment of ADCPs.
Data is typically logged and downloaded at retrieval. Real-time collection can be achieved via a data cable to a surface buoy or to shore, though this presents a potential point of failure. Acoustic modem systems can also be used for wireless transmission to the surface.
Subsurface buoys offer a more convenient method for instrument deployment in some situations, such as an uneven bottom surface or area subject to sedimentation that could interfere with a bottom-mounted instrument.
Provided that there are no large currents at the deployment depth, subsurface buoys can keep the instrument oriented close to vertical and directed upwards for near-surface measurements. They can also be used to place the instrument at a specific depth in the water column.
Subsurface buoys offer a practical deployment option for some applications and can capture surface currents.
Surface buoys offer a relatively simple option for supplying power and remote communications to a current meter or ADCP. Buoys can provide a substantial solar-charged power supply for the continuous operation of system electronics. An integrated data logger receives data output from the instrument and transmits in near real-time via onboard wireless communications.
Surface buoys can simplify deployment and maintenance during some long-term current monitoring applications.
Instrument mounting can also be facilitated by a surface buoy through various methods, as long as the acoustic beams are not blocked by either the buoy itself or the mooring system. Compass calibration and internal tilt sensing are of particular importance on surface buoy deployments.
Conclusion
Advancements in underwater acoustics have revolutionized water current measurements in natural surface waters like rivers and oceans. Instruments using Doppler technology allow for currents to be measured with high accuracy, at great depths, and in detailed, multi-cell, three-dimensional profiles.
Power-efficient electronics and processing, combined with anti-fouling measures, allow for extended deployments with minimal maintenance. Advanced tilt sensing enables deployment from both moving and stationary platforms. Instruments may also be connected to telemetry systems for data delivery in near real-time.
Additional Resources