Tides are the regular rise and fall of sea levels caused by the gravitational pull of the moon (and, to a lesser degree, the sun) on Earth’s oceans and seas. They are characterized by the difference between the highest and lowest points, known as tidal range.
There are two main types of tides: diurnal tides, which involve one high and one low tide per day, and semidiurnal tides, which involve two high and two low tides daily. Some regions experience mixed tides, with varying tidal ranges over each cycle.
Tides are actually very long-period waves that propagate throughout the globe. Semidiurnal tides have a period of 12 hours and 25 minutes, which is half the length of a lunar day. The period of diurnal tides is a full lunar day, 24 hours and 50 minutes.
Whether a site experiences semidiurnal, diurnal, or mixed tides depends on factors such as latitude, ocean basin geometry, the Coriolis effect (caused by Earth’s rotation), local topography, and wave resonance effects.
Why Measure Tides?
Tides influence many aspects of marine environments, including coastal ecosystems and human activities such as fishing, navigation, and coastal development. The study and measurement of tides are essential for managing coastal resources and ensuring safe navigation. Some common applications include:
- Navigation and Port Management: Accurate tidal data helps ships navigate shallow waters safely, plan docking times, and avoid grounding. Ports and harbors use tidal measurements to manage vessel traffic and ensure that channels are deep enough for safe passage.
- Marine Ecosystem Studies: In marine ecosystems, tides influence the distribution of nutrients, oxygen levels, and sediment in coastal ecosystems, affecting the health and sustainability of habitats such as salt marshes, mangroves, and coral reefs.
- Renewable Energy: The kinetic energy in tidal currents can be harnessed by tidal energy converters to produce electricity. These projects rely on accurate tidal data to optimize the placement and performance of tidal energy converters.
- Climate Change Studies: Long-term tidal measurements provide valuable data on sea-level changes, contributing to studies of climate change and its impacts on coastal regions.
How are Tides Measured?
Tides are primarily measured using tide gauges, which continuously record sea levels relative to a reference height called a datum. There are several types of tide gauges, each suited to different environments and applications.
One of the oldest methods for long-term, continuous measurement is the float tide gauge, which uses a float in a well that rises and falls with the tide. The float is connected to a recorder that tracks changes in water level over time.
Later, acoustic systems emerged as an alternative for tide measurement. These work by sending sound waves down a vertical tube called a sounding tube or stilling well and measuring the time it takes for the pulse to travel to the water surface and back.
However, most modern sensing systems rely on either pressure-based or radar sensors to measure the sea level. These may also be used in combination for redundancy and comparability of measurements.
Many modern tide gauge stations utilize pressure transducers and/or radar sensors to monitor tidal fluctuations.
Pressure-based tide gauges measure tides by detecting changes in water pressure, which varies with depth. These sensors have a thin membrane or diaphragm that, when submerged, exerts a force onto an internal pressure-sensing element. This changes the resistance of the internal electronics, which is used to calculate the pressure and corresponding water level.
Pressure transducers come in two types: vented and unvented. Vented sensors include a vent tube that equalizes the sensor with the atmosphere, thereby automatically accounting for changes in barometric pressure.
Unvented or absolute pressure transducers are completely sealed with no exposure to the atmosphere. They require a separate barometric pressure measurement to compensate for air pressure changes and accurately determine the water pressure.
Radar tide gauges are a non-contact, highly accurate method increasingly used in modern tide measurement stations. They transmit radar waves toward the water surface, measuring the time it takes for the signal to reflect back.
Being non-contact instruments, radar instruments are not subject to marine biofouling as with pressure transducers. Unlike acoustic instruments, their signals are not significantly affected by air temperature changes and are generally resistant to environmental influences, although extreme weather conditions such as heavy rain or fog may cause some interference.
Non-contact radar sensors provide high accuracy in variable conditions and minimize maintenance requirements.
How to Select a Tide Gauge System?
The installation location, required measurement range and accuracy, and maintenance requirements are some of the factors that influence sensor selection in tide gauge systems.
Radar sensors have emerged as the preferred measurement technology as they eliminate biofouling concerns and generally simplify maintenance. However, they require a robust mounting structure that prevents interference from movement or vibrations, while maintaining a clear view of the water surface even during low tide and extreme weather conditions like intense fog and rain.
The sensor must also be able to accurately determine the mean sea level from the surface area to be measured. If the site is subject to heavy waves, a barrier for dampening may be required, or the instrument must record for a longer time and/or over a larger area to get a representative reading of the sea level.
Pressure transducers require secure installation underwater. They must be able to withstand the forces of waves, currents, and storm surge without moving. Yet, they must also be accessible for periodic maintenance especially in areas prone to extreme biofouling and sedimentation.
Vented sensors work best with shorter cable lengths, so they are most appropriate for measurement in relatively shallow deployments. Absolute pressure transducers are better suited for placement in deeper waters or for measurement of relatively large tidal ranges.
Sites with heavy wave action can benefit from a deeper sensor placement that can more accurately determine the mean sea level. In either case, the pressure range of the sensor must be adequate for the deepest water (highest pressure) condition expected without being excessively high and compromising the measurement accuracy.
What to Consider When Preparing a Tide Gauge System?
Tide gauge preparation involves configuring and calibrating the sensor, ensuring it aligns with the environmental conditions for the site.
For radar systems, mounting height and angle are critical, as even slight tilts or movement from wind or waves can affect measurements. Since sensors measure the distance to the surface, they must be correlated to a local reference point (datum), such as a staff gauge.
In contrast, pressure-based sensors need to be calibrated or referenced based on local barometric pressure. Vented sensors should read at or near zero in air at sea level. They may optionally be zeroed prior to deployment, or the baseline reading should be noted to adjust the readings taken underwater.
Absolute pressure sensors typically do not require zeroing since factory calibration sets them relative to a perfect vacuum, but the water level and atmospheric pressure should be measured and noted at the time of installation.
How to Deploy a Tide Gauge System?
Deploying a radar tide gauge system requires securing the sensor to a stable structure, such as a pier or platform, to prevent movement and ensure accurate readings. Radar sensors should be mounted to maintain an unobstructed line of sight to the water surface, with minimal exposure to extreme environmental effects like sea spray or potential interference from nearby structures.
Radar sensors must be securely mounted on a stable structure to maintain measurement accuracy.
Consider the most extreme conditions the system will experience when selecting the mounting location and height. Radar sensors should have a sufficient mounting height to avoid submersion and always maintain a minimum blanking distance during high water events. Conversely, during the most extreme low tide conditions expected, they must have a sufficient range to maintain measurement accuracy.
For pressure-based gauges, the sensor needs to be positioned below the minimum low-tide level. A perforated deployment pipe or a bottom-mounted platform or frame are commonly used. Deployment pipes may offer the benefit of easier access for periodic maintenance. However, they must be securely deployed on a solid structure that will not move during storm events.
Care must be taken to avoid areas prone to excessive sedimentation, which can bury the sensor, especially if it is mounted near the seafloor. Using a diver or underwater camera to verify that the sensor is properly positioned is beneficial.
Proper cable management, particularly with vented sensors, is essential to avoid twists or damage that could affect the vent tube. Regular post-deployment checks ensure that the gauge remains stable and aligned with the reference datum, allowing for consistent and accurate tidal data collection.
Conclusion
Tide gauges are essential tools for accurately monitoring sea level changes and tidal patterns, supporting applications in coastal navigation, marine ecology, climate studies, and renewable energy. Modern tide measurement relies increasingly on radar and pressure-based sensors, each with specific deployment and maintenance needs that are determined by the site’s environmental characteristics.
With careful selection, installation, and maintenance, tide gauge systems provide vital, long-term insights into tidal dynamics and the broader implications of rising sea levels and climate change. These systems enable safer navigation, more efficient port management, and better planning for coastal resilience in the face of environmental changes.
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