The residents of the mid-Atlantic coast face the prospect of a prolonged period of east wind this week.
Surfers will be pleased; but owners of homes near the sea should be concerned.
What properties of the wind make this situation more worrisome than most?
Where Does An Ocean Wave Come From?
When wind travels over open water, it pushes the water into little elongated mounds that we call waves.
Wave height is dependent on three factors:
- Strength of the wind. The harder the wind blows, the higher the little mounds of water get.
- Duration of the wind. The longer the wind blows, the higher the little mounds get.
- Fetch — the distance the wind travels unobstructed. The longer the fetch, the higher the little mounds get.
A hurricane produces very high waves because the wind is very strong. But the other two factors can also generate powerful, damaging waves.
Lake Geneva, between Switzerland and France, is 45 miles long and nine miles across.
A modest 33 mile-per-hour wind (30 knots), which doesn’t even qualify as a gale, blowing across the lake for a long time, will make a wave five feet high.
A wind of the same speed and duration blowing down the lake will create a wave nine feet high.
But the same wind blowing over 1,000 miles of open ocean will form a wave 25 feet high.
The Motion Of Water And Energy In A Wave
The movement of water in an ocean wave is orbital — forward near the crest and backward near the trough. Energy is transferred in the direction of wave motion. The orbital motion decreases with depth, and the level at which it becomes negligible is called the wave base. The depth of the wave base is approximately one-half of the wavelength, the distance from one crest to the next.
A submarine or a fish in the ocean can avoid any effects of even the highest ocean wind-driven wave by diving below a few hundred feet. Swimmers understand the phenomenon of decreasing motion with depth instinctively; they dive beneath the breakers.
Where Does A Wave Break?
A wave can break in the open ocean if the wind gets strong enough, but it is certain to break when it meets an obstacle such as a shoreline.
In simplest terms, the breaking of an ocean wave on a beach represents the end result of the wave’s ‘feeling’ the bottom. This begins when the depth of the water is equal to the depth of the wave base. When the base of the wave encounters the bottom, its motion is slowed. As the wave enters shallower water, it loses its upright shape and the orbital motion becomes elongated. Eventually the top of the wave topples over — the wave breaks.
Developing The Sea
Waves increase with three variables: wind speed, duration, and fetch. If the wind continues to blow at a constant speed with a given fetch for a long time, it will produce a ‘fully developed sea.’ At this point, the waves cannot get any bigger because the energy expended in breaking is equal to the energy being supplied by the wind.
Waves Traveling Over Sea Where There Is No wind
The bane of many ocean travelers is mal-de-mer — seasickness. Even on a calm day with no wind and the surface of the ocean smooth, there is the phenomenon of up and down motion connected to passing waves; these waves are called swells.
A swell is simply a wave that has outrun its area of origination. The wind may continue to blow around a storm, but waves will eventually travel out of the area. They continue to transport energy until it is either dissipated by friction along the path of motion, or deposited by the breaking of the wave on a shoreline.
Since wave speed is a function of wavelength, the longest waves are the first to reach a distant shore from a storm that could be thousands of miles away. Surfers seek these waves for their uniformity and speed. In addition, since a wave begins to feel the bottom and start to crest when the water depth is half the wavelength, these waves give the longest rides.
Since waves of many wavelengths are traveling throughout the ocean, sometimes two or more can superimpose to create a gigantic ‘rogue’ wave. Ships at sea have measured waves over 100 feet high both during storms and from superimposed swells. In the open ocean, one wave out of every 23 will be twice the height of the average wave.
Where Are The Biggest Waves?
Because the southern hemisphere is virtually uninterrupted ocean from forty degrees to the edge of Antarctica, the west winds howl in this circumpolar band.
Wave heights average over fifteen feet in most of the region and this has given rise to the names Roaring 40s, Furious Fifties, and Screaming Sixties.
The Wave With Which We Are Most Familiar
Most people go to the beach, and wherever the interface of land and sea happens to be, they put their blankets on the land side – and their bodies in the water. Everyone is aware that the shoreline moves, reaching a maximum landward location twice a day. We know about the tide.
The tide is simply a wave that travels around the world at a speed that depends on latitude, but is generally about 500 miles per hour.
It is not a breaking wave, but simply a slow slosh up and back; in some places the tide can be enhanced by local features and reach heights of 20 feet or more, but in most locations, tide heights are in the single digits of feet.
Coastal Damage From Wind
The coastline is in a delicate balance between land and sea. The interface has moved great distances historically, and currently moves on time scales of centuries, decades and days. In the last twenty thousand years (since the start of the most recent interglacial period), the sea has risen about 110 feet and the boundary between land and sea has moved landward, in some places by many miles.
On a shorter-term basis, the primary reason for loss of waterfront property is storm waves. When a small wave strikes the shoreline, it rolls over the sand or rock and leaves things unchanged. But bigger waves pick up particles as large as rocks and rearrange the landscape.
The Wind And This Week’s Weather On The Atlantic Coast
The wind is caused by pressure gradient — change of pressure with distance. Because of the Coriolis force, a result of the earth’s spinning, air moves with high pressure on its right in the northern hemisphere.
On Monday, weak low pressure formed on an old cold front off the South Carolina coast. At the same time, high pressure built over northern New England.
This is a common scenario in the fall. The result is a moderate pressure gradient over the Atlantic waters for hundreds of miles.
The pattern will not change through the weekend, so winds around 30 miles per hour will blow over a long fetch for a long time.
Coastlines will take a beating as a fully developed sea pounds them for many days.
Decoding Science. One article at a time.