Watching And Measuring Wave Heights
When swimmers swim across a channel or lake participate in an ocean swim, they tend to most vividly remember the extreme moments of their swims. Swimmers are certainly asked about and share their traumatic experiences in the open water all day long: the painful jellyfish stings, the shark encounters, the huge ocean swells. Those instances of roughwater conditions and unpredictable turbulence are rarely forgotten.
Swimmers also talk about the beauty of a tranquil ocean, the brilliance of a rainbow or a coral reef, or a bright sunrise or colorful sunset.
The slight chop that lies somewhere in the middle of these two extremes – between turbulent and tranquil – is relegated somewhat to the deeper recesses of their memory.
They prefer to talk about those extremes where beauty and challenge are at their maximum. Waves and ocean swells are often indelible memories among open water swimmers. But can a listener really understand the definition of “huge waves” and “giant ocean swells”?
Knowing how to accurately report wave height in an observers’s report is a key skill for an escort boat crew of a channel swimmer or marathon swimmer.
How do mariners, surfers, observers and open water swimmers measure waves?
Traditionally, the wave height is measured from the trough of the wave to the the crest. While significant wave height is used in oceanography, sometimes swimmers can experience rogue waves or larger waves when wave fronts from two different waves intersect. These are what they remember most.
Captain Tim Johnson said, “However, there are issues in how to measure the height of just a single wave front. The easiest metric to obtain is the wave period – time from wave from crest to crest. There is a formula the allows you to calculate the speed of the wave C by dividing the Length L of the wave by the period, T. Deriving the height from this data is a bit more complicated and has been given over to computer software.
Mariners at sea use the Beaufort Wind Force Scale as it provides an estimate of the wind speed from a description of the state of the sea surface and gives you a height. This is a useful scale that has been developed and refined over the centuries since it was first developed. Marine stores sell card charts with this information and any well-equipped escort vessel should have some source that includes this information. Skippers usually practice their wind estimation and gauge their accuracy by the observed wave height. So channel swim observers can learn a lot by asking the skipper what they think the wind speed is. So their swim report record should include a spot to mark wind speed as estimated or instrument derived next to their wave height record.
In shallow coastal waters, the wave height differs from the Beaufort scale from open ocean where the depth is more than 100 fathoms (600 feet). The height become higher. But the wind doesn’t change, thus the Beaufort scale could in shallow, coastal water provide a minimum wave height.
Also, you need a wind of 100 knots to pick up a current of 1 knot.”
Copyright © 2008 – 2021 by World Open Water Swimming Association