Читать книгу Waves and Beaches - Kim McCoy - Страница 35

The calming effect of oil on the sea surface has been known for many centuries, long before the physics of the action was understood. All oils are not equally effective; experience has shown that fish oils or other viscous animal oils are best and that petroleum products have relatively little effect. Because the latter are much more likely to be available around a modern boat, there have been many attempts to use motor oils, without success. As a result, the idea of using oil to calm the sea surface has fallen into disrepute, to be regarded as an old seafarer’s tale without foundation in physics. But properly used, oil can be very helpful to the small boat operator under emergency conditions. However, it is “leaked” on rather than “poured” on the rough waters.

There is no doubt whatsoever that this method works, but one must not expect too much. A thin film of oil could hardly be expected to have any effect on large waves or swell, but it does quickly extinguish the small waves. Moreover, as the sea surface becomes slick, the wind has less effect on it, no spray is blown about, and the wave crests become more rounded.

Kelp beds protect the shoreline by smoothing out the shorter-period waves. This process creates the “glassy” areas inside the kelp. Leo Carrillo, California. Kyle Sparks

This smoothing is caused by increasing the surface tension of the nearby area of sea. The higher the surface tension of the liquid, the stronger this invisible membrane acts. But because engineering handbooks give a surface tension for water twice that of oil, it is not readily apparent how the addition of oil can help matters in times of distress.

One answer is that the surface tension of the oil increases as its thickness decreases. The thinner the film the better, for oil can act like an elastic membrane even when it is only a millionth of a millimeter thick. Thus, as the oil spreads away from the boat it becomes more effective, opposing any motion that tends to increase the surface area. At a distance from the boat, depending on the velocity of the wind, the elastic limit of the increasingly thin film is exceeded. It breaks up and blows away, making it necessary to continually add more oil at the center. Professor and mariner Charles “Chip” Cox published a paper in 2017 about this interesting yet poorly understood phenomena.

Other materials that are mixed in the water or are floating on it also tend to reduce wave action. Extremely muddy water, for example, will cause waves to decay rapidly, and so will masses of floating debris. Breakwaters have been constructed with buoyant subsurface objects tethered to the seafloor. Kelp (large brown algae) grows just offshore in the temperate seas of the world. The large waves coming from afar pass through the kelp beds almost unchanged, but the small waves caused by local winds are quickly dissipated, and the water surface is nearly always glassy just inside the kelp beds. Globally, kelp partially shields about a quarter of the Earth’s coastline from waves. Some areas have experienced a decrease in kelp bed coverage due to increasing seawater temperatures.

Lost in the Fog

Divers frequently consider wave height, direction, tidal currents, and visibility when planning a dive. I was freediving with a buddy in the kelp beds off Northern California; fish were plentiful, and all was going well. The kelp undulated with the peaks and troughs of the large swell as it passed through the kelp bed. The strengthening tidal current aligned the kelp with its flow. From the crest of each wave I could briefly view the coastline. Then the fog rolled in and we lost sight of land. After a few more dives I carelessly lost sight of my dive-buddy and drifted with the current out of the kelp. The thick fog had obscured the sun and reflected waves from afar made the swell complex. Soon I had no sense of direction. I was moving with the chaotic orbital wave motions and drifting at sea. The harrowing experience continued until a break in the fog allowed me to reorient myself and find my dive-buddy back in the kelp. He knew I was gone but was unaware that I had ever been adrift, confused by the swell and without any point of reference. A point of reference is good in a chaotic world. – KM

Fields of sea ice also reduce wave action; as waves move through the ice pack, the shorter ones are “damped out,” and there is an apparent increase in wave length. Ice, therefore, serves as a protective shield. Any ice cover reduces the amount of wind energy the ocean is willing to accept and reduces the undulating forces of waves upon the shores. The marginal ice zone, where the ice is churned by the waves, is a floating battleground. All the short-period waves die first, leaving behind interesting forms of what is called “pancake ice,” the remnant of the mixing of wave energy with the slush of ice crystals. Only the strongest and longest-period ocean waves survive to propagate into multiyear icefields. Long-period waves have been measured more than 60 miles (100 km) into polar ice. The cartographer Robert Perry produced one of the first detailed charts of the Arctic in 1985. It revealed many unknown features across the Greenland Abyssal Plain, Fram Strait, Yermak Plateau, Belgica Bank, Kane Basin, Beaufort Sea, and Northwind Ridge. Although submerged below, these bathymetric shapes determine both the propagation of waves and the circulation of the Arctic Ocean.

Waves in the Ice

In the 1980s, when sea ice coverage was much heavier, I was involved in many expeditions to the Arctic. Some of these expeditions were in the marginal ice zone off the northern coast of Greenland. Our research projects included icebreaker, helicopter, and ice camp operations during which we frequently encountered polar bears on the ice. An icebreaker served as our “mothership.” From the icebreaker we would venture out, walk, snowmobile, and helicopter to remote locations to collect wave, ice, and oceanographic data. When we were on the ice, away from ships and machinery, the subtle ebb and flow of energy became noticeable.

On one occasion, two of us ventured forth alone several miles from the icebreaker. The icebreaker had already been “made fast” to the thick multiyear ice with “ice anchors.” The main ship’s engines were shut down, and quietness reigned as we stepped on the ice. When we first stopped the snowmobile perhaps a half mile (1 km) from the icebreaker, the sound of the wind, our footsteps, and our breathing were all that we could hear. We stopped a second time, about 3 miles (5 km) from the breaker to drill through the ice and take some measurements. Whenever the squeaking, crushing noises from our footsteps ceased, there was almost silence, but not complete. A more subtle intermittent creaking sound began to emerge.

PANCAKE ICE Pancake ice is formed by waves passing through ice. Random wave motions cause the ice to bash into its neighbors and round off any square edges. Weddell Sea, Antarctica. Anne-Mari Luhtanen/Finnish Environment Institute SKYE

We continued to work on the frozen sea, isolated from the world, a thousand miles from any inhabited land. As we began to tire, our periods of rest increased. It was 20 degrees below zero. We listened more, and detected a rhythm to the creaking. We came to understand that it was the long-period swell fracturing the multiyear ice with its wave energy. We had not noticed the tiny fractures beneath us until the wind direction changed.

Larger fractures were appearing when suddenly the creaking sounds were replaced by the sound of a cannon. In the distance the shifting winds had caused one of the ship’s ice anchors to pull loose. The stretched mooring line connected to the anchor had stored enough energy to catapult a 400-pound (180-kg) anchor crashing into the bridge of the icebreaker. We watched as the 400-foot (122-m) Polar-class icebreaker slowly drifted away, blown by the wind. Stronger winds further relaxed the ice, and the fractures became larger gaps of exposed sea. The long-period swell had won.

We were adrift, marooned; floating on an ice floe. The liquid sea expelled a mist of sea smoke, which obscured our views. Amid the pandemonium aboard the icebreaker and the efforts to restart the main engines, we had been forgotten. Alone, we imagined polar bears in the mist and certain death. Eventually, the crew of the icebreaker refocused their attention to their drifting comrades and ended our chilling experience. Unseen waves are everywhere you look. – KM