Bathymetry: the seafloor changes the way surfers ride waves | Photo: Gold Coast/Creative Commons

Bathymetry is the science that studies the depth of water in oceans, rivers, streams, and lakes.

It involves the measurement and mapping of the underwater terrain, including the shape and depth of the seafloor.

Bathymetric maps are similar to topographic maps, which show the shape and elevation of land features, but instead of showing elevations, they show depths.

Bathymetry is important for various applications, including surf forecasting, navigation, ocean engineering, and oceanography.

It plays a crucial role in determining the behavior of waves and swells as they move through the water.

The shape and slope of the seafloor can affect the way that waves propagate and interact with the coastline.

For example, if the seafloor is steep and slopes rapidly, waves will become more energetic and may break more easily when they reach shallow water.

On the other hand, if the seafloor is more gradual and slopes more gently, waves will lose energy as they move through the water and may not break as easily when they reach the shore.

The depth of the water can also affect the height of the wave as it approaches the shore.

In deeper water, the wave may appear larger and have more energy, while in shallower water, the wave may appear smaller and have less energy.

Here's an example of the ocean bottom near Nazaré, in Portugal, where the underwater canyon channels swell energy toward the shore.

Nazaré: the underwater canyon is responsible for the creation of huge waves | Illustration: MaxSea

Assessing the Surf Zone

Bathymetry can help determine the size and shape of the surf zone, i.e., the area where the waves are breaking and where surfers ride.

But why and how?

The ocean floor is constantly changing due to various processes, such as tectonic plate movement, volcanic activity, and sedimentation.

These processes can alter the topography of the ocean floor, creating new features and changing existing ones.

Bathymetric measurements help to monitor these changes and provide insights into the processes that shape the ocean floor.

Bathymetric science can help determine the best places to surf by providing information and identifying areas with particular features, such as reefs or sandbars, which can create good wave-riding conditions.

Data is also be used in hydrodynamic models to predict currents, tides, water temperature, and other factors influencing wave quality.

Additionally, bathymetry can be used to study the habitats of marine life, which can help identify zones where there's good surf and a healthy ecosystem.

For instance, Google Earth's seafloor depth tool allowed the discovery of spectacular new surf spots.

Bathymetry is also essential in studying tsunamis, the giant waves caused by underwater earthquakes or landslides.

Tsunamis can travel great distances across the ocean and can have devastating impacts on coastlines.

Bathymetric data is used to model the movement and behavior of tsunamis, which helps scientists understand their potential impacts and predict where they might strike.

Overall, it is an important field of study that helps us understand the ocean environment and the forces that shape it.

Last, but not least, it is a valuable tool in constructing bridges, ports, and offshore structures, as well as in environmental studies and resource management.

Big waves: the result of swell energy and optimal bathymetric conditions | Photo: Lissenden/Creative Commons

Sandbars in Beaches and River Mouths

From a surfer's point of view, two other critical bathymetric features are beaches and river mouths.

Beach breaks need a particular shape of sandbars to provide a good arena for rideable waves.

If the sand under the waves is flat and featureless, it will almost certainly close out when swell arrives.

An ideal sandbar formation will be vaguely triangular, with slightly deeper water on either side of the bar.

This idyllic scenario occurs when a wave breaks on a bar and starts pushing water toward the beach, picking up sand along the way.

The water starts to get pushed sideways until it loses forward momentum and looks for a way back out to sea.

When this happens, rips and currents form, supporting the circulation of water and sand.

The rip gouges out a handy paddling channel and deposits more sand out towards the peak for more swell to focus upon.

River mouths work on the same principle whereby sand is constantly deposited at the sandbar and are far more reliable for a well-shaped seafloor.

Methods for Collecting Data

Bathymetry data is collected using a variety of methods, including:

  • Sonar;
  • Multi-beam and single-beam surveys;
  • Acoustic Doppler Current Profilers (ADCPs);
  • Sub-bottom profilers;
  • Autonomous Underwater Vehicle (AUV);

Sonar involves sending out a pulse of sound and measuring the reflected sound wave to determine the depth of the water.

Multi-beam and single-beam surveys use an underwater transducer to emit and receive sound waves to measure the depth of the water.

ADCPs measure the speed and direction of currents using sound waves.

Sub-bottom profilers use sound waves to create images of the layers of sediment and rock beneath the seafloor.

The AUV is an unmanned underwater vehicle that can be used for bathymetric surveys.

All this information is often displayed on maps using a system of contour lines similar to those found on topographic maps.

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