Both scientists and surfers have long debated how is wave height measured accurately.
This is because, as the wave approaches the shore, it becomes more vertical, loses its shape and breaks, so measuring its height at that point is very challenging.
When measuring the height of a breaking wave, a person measures from where the broken water meets the sea surface to the highest point of the wave. The main reason why wave is measured is because the waves cause erosion of the sea floor and the beachfront.
Let us learn in detail!
Important Terminologies on Wave Height Measurement
1. Wave Peak
As a swell approaches the shore, sandbar or other near-shore obstruction, the water becomes rougher as it moves up and pushes up against the bottom to a maximum height.
This is the wave peak. The highest peaks are often further away from the beach.
2. The Horizon
This is the furthest point one can see. The horizon is often relative to the wave height. In normal visibility, the horizon is approximately 19 to 22 miles.
Rainy and cloudy conditions may limit how far you can observe, which may also hinder wave height measurement.
3. Back Rush Curl
A back-rush curl or wave is the small, breaking wave that ripples outward from the main wash of water at the shoreline.
This gives a reasonable estimate of the current sea level, which helps distinguish it from historical sea levels.
4. The Crest
It’s a point on an ocean wave with the highest value of upward displacement in one wave cycle. Crests are high points on the surface of a wave. Crests finish the cycle of a wave by returning to the mean level, or baseline, of a medium.
5. A Trough
The trough is the lowest or minimum point in a wave cycle. When waves approach shallow water, the troughs of the waves assume different characteristics.
The trough of a wave is caused by many subtle factors and can vary greatly from one wave to the next.
Methodologies for Wave Height Measurement
The Bascom Method
The method was proposed by William Bascom in his book Waves and Beaches. With the Bascom method stand on the beach at a level where the top of a breaker is exactly in line with your eye and the horizon.
The height of a breaker can be estimated by the vertical distance between the curl of the crest and the leading edge of the trough, which is about equal to the average sea surface.
The Hawaiian Method
The second most preferred method on how is the height of an ocean wave measured is the Hawaiian Method.
The Hawaiian method of estimating wave size, in which measurements are taken from the back instead of the face, can result in underestimations of up to 50 percent.
This method came to be accepted in part due to the increased use of wave-buoys, whose measurements often corroborated the “Hawaiian method”.
The method which apparently was used as a form of psychological intimidation on new scientists and surfers, is said to have been in use during the 1960’s.
Fact: Because there is no fixed reference point, it is difficult to measure the maximum range of a wave - its face can be one foot high and disappear completely into foam from the back.
Modern meteorological forecasting uses information provided by buoys capable of gauging the swell in deep waters, potentially giving warning to shipping of oncoming swells and providing protection against wave damage to coastal land.
The faster the bottom contour changes, the larger the difference between a swell’s height and the height and duration of waves breaking on it.
Waves breaking in shallow water will be smaller than waves breaking on a steep, deep bottom.
Note: Wave buoys measure crest to trough oscillation of ocean waves. A breaker is a wave that has collapsed.
Using a Scale
In this method, the measurement is relative to the surfer or scientist’s height.
It’s the oldest method on how is the height of a wave measured as it also uses the imperial scale (feet). The imperial scale has often been used for both waves and surfboards.
Meteorological services now report wave heights in meters, a standardization that made it easier to compare data from different countries.
However, it’s important to note that it doesn’t matter if the measurement is one you don’t normally use, as long as it is reasonably accurate.
Fact: An example of conversion using the scale method: 1 Meter is approximately 3.25 feet.
Surfer on The Wave
When wave height is calculated in this method it’s either determined by the height of the skier in comparison to others or by an observer’s estimation of the rider’s size.
In the case of a video or photograph, by the height of whoever is viewing the recording.
To get an accurate estimate of wave size, an observer should be square to the wave face.
If the observer is looking down at waves, wave size will be overestimated; if the observer is looking up at waves, wave size will be underestimated.
Because surfboards are not uniform in terms of size or stance, it is very difficult to estimate the height or weight of someone standing on a board.
Note: This method is ideal for smaller wave sizes and is prone to significant errors with larger waves.
Alternative Methods to Measure Wave Height
With the major methods on how is wave height measured out of the way now let’s delve into alternative methods.
Measuring the height and period of waves is subjective and difficult to do accurately.
But even the alternative methods when done correctly will provide reasonably reliable usable results.
Here are some alternative methods:
1. Depth Sounder
A digital finder with a fast update speed is an accurate tool for measuring wave height.
If a depth measurement of the trough on an even keel is compared to a depth measurement at the crest of an even keel, it is easy to obtain accurate results of a wave height.
2. Comparison with a Fixed Structure
A fixed structure, such as a break-wall, pier, or jetty can make observing waves extremely accurate; it can also provide the wave period. This method is also called screening.
3. Comparison with a Vessel or Floating Structure
It is easiest to use when you are ashore. However, it can be applied at sea as long as the vessel is not moving too quickly nor in poor weather conditions.
If the distance between the waterline and the top of a buoy is known, then it is possible to estimate the height of waves passing by.
If you watch a vessel moving through the water and notice how much of the hull is exposed above the waves, you might be able to judge the wave height.
This method also helped captains judge whether there are heavy seas or smooth waters ahead.
4. Height of Eye
When you’re sitting in a boat that’s in an even keel, any wave that obscures your view of the horizon is greater than the height of the average person’s eye.
You can compare a wave to the deck edge or a structure such as a handrail.
You have to be above it and looking down at it to see its face, and you have to be below it and in a trough to look at it from an even keel.
When determining how is wave height measured experienced surfers, sea crews, and scientists have developed a number of different ways to estimate wave height, all of which are useful and fairly accurate.
The above methods are accurate and useful, but they require experience and practice.
For example, a sea crew usually observes the waves and then compares their estimates with information from the local Weather Service buoy.
Through experience, you should get better at determining wave height by the action of the waves themselves versus any instrumentation.