Back to the Moon with Nuclear Rockets

Continued from page 42"Spilling breakers," in which the water from the crestslides down the inclined plane of the forward slope,are the prevalent surf where the sea floor slopes gentlyto beach over distances of a few hundred meters. Thebreaking action is far gentler than that of the "plungingbreaker," giving the surfer, canoer, and beginner along, slow "tour" as they approadh the beach. Such isthe surf off the famous Waikiki Beach of Honolulu,where tourists and beach boys alike enjoy surfing withrelative ease.Refracting Waves. When gravity waves reach waterthat is shallower than half their wavelength, and theircrests are not parallel to the contours of the sea floor,they will begin to refract (bend). This action comesabout as the forward speed of those portions of thewaves in shallower water slow in respect to those remainingin deeper water. Whilelthe wave frequencystays the same, the amplitude and the lengths changesubstantially. Approaching a straight shoreline over agently shoaling shelf, the wave crests continue to refract,until they break in the surf zone along a line parallelto the beach.Waves approaching coastal headlands are in deepwater almost to the shore, where they strike the cliffswith great force (see photo, top right). The wave crestsdo refract along either side of a Headland, causing divergingwave paths in the embayments adjacent to thecapes. The consequence is that most of the wave energyis concentrated at the headlands and dissipated alongthe beaches to the side. There is, therefore, a lot of turbulenceand mixing, and crashing waves at sea cape(see photo, bottom right).(As an aside, headlands are good places to put seweroutfalls so that the discharge is distributed a rapidly aspossible. Waters in embayments are just the opposite;there, mixing and wave action are mild leading to depositionof materials rather than distribution. As you mightguess, however, most sewage disposal locations havebeen built in quiet bays; not at turbulent headlands.)Submarine canyons that indent the shelf, with theirheads close to the shore, create refraction patterns thatare often disconcerting to the beach inhabitants. Thedeep water in the canyons permits the waves to approachthe beach with barely diminished forwardspeeds. On either side of the canyon, where the seafloor is shallower than that in the canyon, the wavefronts refract away from the canyon. The energy isconcentrated at the shore opposite the canyon's head,therefore, and dissipates along the beaches to eitherside.Those who are unaware of the location of a canyonhave built houses or piers on shore near its head. It is acommon situation in southern California, where suchowners often are surprised to find their house, or pier,under attack by destructive waves, while rather benignbreakers strike the beaches just a few hundred metersaway.The huge surf cr< shing against the 600-meter cliff of the westerncoast of the, \uckland Islands, was easily observed by thecrew aboard the Shuttle Endeavor, Jan. 26, 1998.Refracting ocear swell and currents around the southern tip ofthe island/head and Nosy Boraha on the northeast coast ofMadagascar we e visible to the crew of the Shuttle Atlantis,May 20, 1997, a 1 160 nautical miles altitude.21stCENTUFY Summer 1999 45