Photos by Charlie Levine
We frequently use the rather broad term "waves" to describe a series of vertical disturbances on the surface of water. The most common waves, found in all bodies of water from the smallest ponds to the largest oceans, are caused by the passage of wind across the water's surface. But there are other sources of waves as well. Boat wakes form a set of waves that move outward from the track line of the boat's passage through water, and objects falling in the water produce a set of waves that move outward from the point of impact in a series of circles. These waves are usually relatively small and of short duration. However, the impact of a large meteorite or avalanche can rival that of a tsunami (often incorrectly referred to as a tidal wave), which is caused by significant movements of the earth's crust - i.e. earthquakes or volcanic eruptions.
Still, the most common cause of big waves on oceans or large lakes such as the Great Lakes is wind. The height of wind-driven waves is determined by the strength of the wind, the duration and its "fetch," or the linear measurement of distance of open water over which the wind blows. A strong, long-lasting wind blowing over long stretches of open ocean can create waves large enough to sink the biggest of ships.
Recreational boaters should always check both the current weather - including wind strength and time of high and low tides - and the weather forecast for the time they plan on returning from sea. Dangerous waves build up amazingly fast, and being caught away from shelter in a safe harbor could end up costing someone his or her life.
Swells Become Breaking Waves
Even after a strong, long-lasting wind dies down, it may take some time for the waves generated to dissipate. Huge waves from a hurricane or violent winter storm hundreds, or even thousands, of miles away become seemingly benign low oceanic "swells" with a long "period" or time between waves. In the open sea, these long, low swells can be almost imperceptible.
Fishing off Kona, Hawaii, on the lee side of the volcano, the sea can be nearly glassy calm without even a ripple on the surface. Even a tourist with the weakest stomach remains undisturbed, and a sunburned newlywed charter would not even notice what could be impending danger.
The knowledgeable local crew, however, would become aware of any oceanic swell, even though it might only raise their boat a foot or so, with a hundred yards or more between the top of each of the waves. If the swell's vertical motion increased over time, becoming more noticeable, the crew would start to get nervous and probably seek additional information via radio or cellular telephone about the conditions back at the inlet.
A swell that passes easily across the open sea, and only raises the level of the ocean a little, also moves water deep beneath the surface. As the swell moves over a shallow sea floor, the bottom of the wave meets resistance first. The friction from the sea floor slows the speed of the moving wave and causes it to rapidly rise in height. The distance between waves becomes shorter, as later waves catch up to earlier waves, and the sea rises even higher. Its top becomes unstable and the crest, as the top of the wave is called, eventually falls down the smooth front, or face, of the wave. What started as a long, low oceanic swell quickly turns into breaking surf.
The very narrow entrance to Honokahau harbor on Hawaii's Big Island is lined with rocks on both sides, and large breaking surf makes entering or leaving the harbor nearly impossible. Plus, boats entering the harbor must make an immediate hard right turn to avoid another rock wall and successfully reach the safety of the inner harbor.
The Big Island will be affected several hours after the surfers in Waimea, and the charter boat's crew, probably surfers themselves, will be aware that the surf is rising. If they are prudent, they will run for home before the rising swell becomes dangerous and threatens the safety of the boat and its passengers.