Waves are periodic ridges or swells on the surface of bodies of water caused by energy passing through the water. Energy transfer from wind is the driving force behind most common surface gravity waves throughout the globe.
As air passes over the water, the friction at the air-water interface produces localized wave crests. Unimpeded wave crests can leave the area of generation and may combine with other waves. This produces swell in large lakes and oceans.
Other forces that impact wave formation and behavior include tides, weather events, and underwater disturbances.
- Tides are long waves caused by the gravitational forces of the moon and sun.
- Storm surge is the name given to water rising above the predicted astronomical tide due to low pressure areas associated with storm events like hurricanes.
- Tsunamis are large waves caused by disturbances like submarine earthquakes and landslides that quickly displace large amounts of water.
Wave data is used to classify the sea state, which is the overall surface wave condition of a large body of water. Wave heights are sometimes given on a scale such as the Douglas scale (WMO Code Table 3700), which classifies the sea state from 0 (calm) to 9 (phenomenal), or the Beaufort scale, which factors both wind and waves to rate the sea state from 0 to 12, with ratings such as calm, gentle breeze, gale, storm, and hurricane.
Why Measure Waves?
Waves impact surface waters throughout the globe, particularly in oceans, coastal waters, and large lakes. Measurement of waves is vital for understanding the dynamics of these aquatic environments and is useful for a number of practical applications such as:
- Coastal Engineering: Wave data is essential for designing coastal protection structures like seawalls, breakwaters, and jetties. Understanding wave dynamics helps engineers plan structures that can withstand extreme wave forces and prevent coastal erosion.
- Marine Navigation: Accurate wave data ensures safe maritime operations, particularly during extreme weather events. Large waves can affect ship stability, cargo safety, and overall navigation efficiency.
- Renewable Energy: Wave data are important in construction and management of facilities like offshore wind farms and are key in developing wave energy converters, which harness the kinetic energy of waves for electricity generation.
- Sedimentation and Erosion: Waves impact sediment transport and the morphology of coastlines, beaches, estuaries and harbors. Understanding wave action can help predict sediment buildup or erosion, crucial for maintaining navigable waterways and healthy ecosystems.
- Recreation and Beach Safety: Wave measurements are important for recreational water activities such as surfing, sailing, and swimming. Surfers rely on accurate wave forecasts to find optimal conditions, while coastal recreation areas benefit from wave data to ensure the safety of beachgoers.
- Climate and Weather Studies: Waves influence and are influenced by global weather patterns. Long-term monitoring. helps in understanding ocean-atmosphere interactions and contributes to studies of sea-level rise and climate change impacts.
- Tsunami Warning: Measuring waves is critical for tsunami detection and warning systems. Early detection allows authorities to issue warnings and evacuations, potentially saving lives and minimizing destruction.
How are Waves Measured?
Since waves are random and can vary significantly between successive crests, waves are typically analyzed over a given period of time. Statistical analysis of these data sets allows waves to be characterized by their height, period and direction. In general, the larger amount of raw data (i.e. number of individual waves) in a data set, the more precisely the wave conditions can be estimated.
Statistical analysis of wave data sets is used to determine the characteristics of the waves including the height, period, and direction.
Height, reported in distance units like feet or meters, is the vertical distance between the highest point (crest or peak) and the lowest point (trough) of a wave. It is denoted as H in reports and forecasts and further categorized based on statistical analysis:
- H is the most probable wave height, which is the wave height occurring most frequently during a period of time/wave data set.
- Mean H is the average height of all waves in the data set.
- Hs is the significant wave height, defined as the average of the highest one third of waves. Meteorological forecasts typically reference this value when reporting waves.
- H10 or H1/10 is the height of the highest 10% of waves.
- Hmax is the single highest wave height recorded.
Wave heights are categorized by statistical values such as the significant wave height (Hs), height of the highest 10% of waves (H10), and maximum wave height (Hmax).
Period is the amount of time that elapses between the passing of successive crests. It is generally denoted as T. The peak or dominant wave period is often the reported value. This is the period of the most energetic wave component and is denoted as Tp or DPD.
Common wave periods are on the order of:
- 1-20 seconds for wind-driven surface waves (surface gravity waves)
- 10-30 seconds for swell
- 10 minutes to over an hour for tsunamis
- 12 hours and 25 minutes for semidiurnal tides
- 24 hours and 50 minutes for diurnal tides
Lastly, wave direction is given in degrees or using cardinal points (e.g., north, south, east, west) and indicates the direction the waves are coming from (not where they are going).
For measurement of waves, a number of sensing technologies are available, each suited to specific environments and applications. These include accelerometers (inertial technologies), pressure-based sensors (such as PUV sensors), acoustic surface tracking (AST) instruments, and radar sensors.
Accelerometers detect wave motion with microprocessors that measure the acceleration and tilt of a floating platform such as a buoy in response to wave activity. These data are then processed with specialized algorithms to calculate wave height, period and direction, as well as heading, wave energy, and other wave data.
Accelerometer-based wave sensors are mounted on floating platforms to analyze wave motion.
Pressure-based sensors are deployed underwater and measure wave-induced pressure fluctuations, from which wave height and period are estimated. Measurement of the horizontal components of the wave’s orbital velocity by means of acoustic Doppler velocity sensors can add the directional wave data. This is sometimes called the PUV method, where P stands for pressure and U and V for the axial velocities.
Pressure-based wave sensors measure small changes in water pressure as waves pass above the sensor, while velocity measurements indicate direction.
Acoustic instruments like Doppler profilers equipped with acoustic surface tracking (AST) use acoustic waves rather than pressure to measure the distance between the sensor and the water surface. By monitoring changes in distance over time, AST devices can provide data on wave height and period. As with PUV instruments, the directional wave components are estimated through acoustic velocity measurements.
Acoustic-based wave sensors track wave motion at the surface using an acoustic beam, with velocity measurements to indicate direction.
Radar sensors offer a non-contact method for wave measurement where a downward-looking instrument transmits radar pulses to the water surface and measures the time it takes for the reflected signal to return. This is similar to radar water level measurement, but high-frequency measurements taken over a period of time and processed by specialized algorithms allows for advanced wave analysis.
Radar sensors measure wave motion from above the surface using high-frequency radar beams.
How to Select a Wave Measurement Instrument?
Selecting the appropriate wave measurement instrument depends on several factors, including the specific application, required accuracy, deployment depth, and environmental conditions.
Accelerometer-based wave sensors present a flexible solution that are ideal for any open water applications where surface data buoys are feasible. They can measure all types of surface wind waves (gravity waves) from short and choppy to swell, covering wave periods from about 1-30 seconds.
Since accelerometers are secured inside waterproof housings on data buoys, the sensors themselves require little to no maintenance. Regular buoy maintenance is required for cleaning, mooring hardware inspection, and other routine maintenance activities.
Buoys support data logging systems with telemetry for wave data processing and remote, near real-time data acquisition. In addition, they can support large power supplies with solar charging, making them a reliable, autonomous solution for long-term wave measurement applications like offshore construction monitoring, marine navigation, and climate research studies.
Pressure-based sensors and acoustic instruments are designed for underwater deployment. They are normally mounted on fixed structures in an upward-looking orientation for measurement of surface dynamics.
Pressure-based systems are limited by attenuation (reduced strength) as the water depth increases. They are, therefore, only applicable in shallow-water applications with water depths up to about 10-15 meters. In addition, they can only accurately analyze long waves like swell with periods from about 4 seconds or longer.
AST instruments increase the depth range for wave measurement up to about 150 meters depending on site conditions. They also improve the coverage of the wind-wave band to detect shorter period waves.
Being underwater instruments, both pressure-based and AST instruments are commonly deployed as standalone units with battery pack and internal logging for manual data collection. As such, they require periodic maintenance to replace batteries, download data, and clean the instrument. Integration into a real-time data acquisition system is possible but requires either running a power and communications cable to the bottom platform or some other form for data acquisition like inductive or acoustic telemetry.
Radar sensors have the primary advantage of being non-contact sensors. This simplifies both installation and maintenance as they are typically mounted onto bridges, docks, piers, or near-shore towers.
They are a good alternative when placing a buoy or submerged instrument is not feasible, such as busy harbor entrances and ports. However, site selection is challenging because they require suitable infrastructure for mounting that also does not interfere substantially with the waves to be measured.
Radar is generally resistant to environmental conditions, but heavy rain, dense fog, or other extreme conditions may potentially interfere with signals.
What to Consider When Preparing a Wave Measurement Instrument?
For accelerometers, the sensor’s internal configuration and physical mounting on the buoy platform are critical aspects of preparation. Most sensors have a preferred orientation, for example, with the circuit board or housing horizontal when the buoy is upright.
Some sensors have a heading mark that should be aligned with the North direction on the buoy’s wind sensor or along the bow-to-stern axis of the buoy, although the internal compass can track the heading and correct the directional data accordingly. For the compass to work properly, the sensor should be isolated from magnetic materials. Placing a needle compass near the sensor can help to indicate if there is potential interference.
Directional heading accuracy may be further optimized by accounting for magnetic declination. This is minimal in some parts of the world but can introduce a significant error in other regions, particularly in areas near the magnetic poles. Use a reference chart to determine the local declination at the monitoring site and update the wave sensor configuration accordingly.
Lastly, the mounting location in the buoy plays an important role. Ideally, the sensor should be located as close to the roll center of the buoy as possible. This is typically near the waterline and on the central axis of the buoy. If it is impractical to place the sensor near the roll center, then configuration offsets or other means should be implemented for the sensor to account for the off-center placement.
For submersible pressure sensors and acoustic instruments, heading and compass accuracy should also be confirmed. Some devices may allow for user calibration of the compass with the instrument secured in its mounting frame.
Since instruments are submerged and typically bottom-mounted, consider potential interference from sources such as biofouling and sedimentation. Anti-fouling measures like copper patches can extend deployment times and improve accuracy, while site selection should be carefully considered to avoid areas prone to sedimentation that can interfere with measurement beams.
Radar sensors are generally low maintenance given that they are non-contact instruments. Site selection is an important factor to avoid interference from vegetation, rocks, sand deposition, the mounting structure itself, or other obstructions that could potentially affect measurements. Although resilient towards environmental factors like heavy rain, fog, and wave splashes, placement away from constant exposure to these elements can help improve data accuracy.
How to Deploy a Wave Monitoring System?
Deployment of a wave monitoring system also depends on the type of sensor technology being used, the environmental conditions, and the application requirements.
The mooring system of a buoy with accelerometer-based wave sensor must allow the buoy to move freely with the waves for accurate measurement.
Accelerometers mounted on buoys are dependent on the motion of the buoy platform for accurate characterization of the waves. This depends heavily on the mooring system of the buoy, which must allow for free movement with the waves.
In general, an elastic mooring is desirable. The ideal mooring is one that provides a minor force that keeps a small, minimally varying (shock free) pressure on the buoy, keeping it in its desired location.
Multi-point moorings should be avoided if directional wave data is important because they restrict movements outside of the plane of the moorings. If there are uncertainties about the suitability of a mooring and quality of the data, many sensors log diagnostic parameters internally that can provide indication of any out-of-range values due to issues with the buoy mooring system.
Pressure-based sensors and acoustic instruments are typically deployed on fixed, bottom-mounted structures like tripods or frames in shallow to medium depth water. These devices must be securely anchored to withstand currents, tides, and any debris that may pass through. The frame or mounting structure should be designed to minimize vibrations or movements that could affect the sensor’s orientation and data accuracy.
Placement can be done by lowering the instrument from the surface, using divers if available to confirm the placement. If installing from the surface without divers, it is a good idea to inspect the frame or tripod with an underwater camera if possible. This ensures that the instrument is sitting with the correct orientation (looking upwards toward the surface without excessive tilt) and there are no objects in the immediate area that can cause interference.
Non-contact radar wave sensors must be securely mounted from a stable structure with clear view of a representative area of the water surface.
Radar sensors are mounted from structures such as bridges, piers, or observation towers. They must have an unobstructed view of the water surface to be measured, and the water surface must experience waves that are representative of the larger area of interest (such as a port or harbor). The sensor’s orientation should be precisely at a 90° angle towards the water surface unless configuration adjustments are made to account for the mounting angle.
Enough clearance above the highest expected water level must be maintained to account for the sensor’s minimum blanking distance and avoid instrument submersion. However, the distance to the water surface during the lowest water condition must also fall within the sensor’s measurement range.
The mounting structure must be sufficiently robust to minimize any vibration from wind, passing traffic, or any other sources that may affect the accuracy of the radar measurements. In addition, there should not be any other radar devices in the immediate vicinity operating on a similar frequency that could cause interference.
Conclusion
Waves are a phenomenon present throughout the globe, both in freshwater and saltwater surface water bodies. Wind is the most common cause of surface waves, but other forces can also cause waves in some cases. Waves have an impact in many practical applications, from routine shipping operations to marine construction projects, and even extreme cases like tsunami detection and warning.
Many technologies are available for wave measurement. Some of the most common types integrated into remote monitoring systems include accelerometers, pressure sensors, acoustic Doppler instruments, and radar sensors.
These devices are tuned to the specific wave characteristics of the waters they will monitor. They may be mounted on surface buoys, underwater platforms, or shoreline structures depending on the instrument type. With careful instrument and site selection, proper deployment, and regular maintenance, wave measurement systems can effectively deliver accurate data and contribute to sustainable management of marine and coastal resources.
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