Waves reflect when they meet upright jetties, seawalls, or other vertical walls. Reflected waves rebound back into the path of oncoming waves, sometimes reinforcing an advancing wave and making it larger. They might also meet oncoming waves and halt them, turning them into standing waves that move up and down but make no horizontal progress. If waves meet a barrier at an angle they are reflected away at the same angle. When a wave meets a beach most of its energy is absorbed, but there is always some reflection.
Refraction occurs when waves are bent by shallow water. Waves rarely strike a beach straight on because one part of a wave always comes up against shallow water before other parts. The first part to touch bottom slows, forcing the wave to turn at an angle to shore, causing breakers to proceed from one end of the beach all the way down to the other rather than breaking everywhere at the same moment. When waves follow one after another at a consistent angle of refraction, they produce consistent longshore currents running parallel to the front of the beach. These currents carry a great deal of sediment along the shoreline and are an important force in building sandbars and other features of a beach.
Driftwood left stranded high on a beach has been deposited there by a somewhat different force than gravity waves or current. Objects floating in waves follow the oscillating pattern of the water particles, traveling slightly forward with each crest, returning slightly backward with each trough. They make progress, however, because they are subject to the power of currents beneath the surface and winds above it. Once a piece of driftwood reaches the area of breaking waves at shore it is propelled shoreward by waves of translation, or bores. These result when ordinary waves break in shallow water, dumping a sudden mass of water onto the shallow surface between breakers and sending a low, flat bore racing forward up the beach. Such shoreward-running waves have a crest but no trough, and are composed of water that actually passes forward as the wave progresses, making a "translation" of water from the breaker to the beach. Objects floating in waves near shore get caught in this translation and are pushed up high and dry into beachcomber country.
The writer Henry Beston, who spent a year living on the shore of Cape Cod before writing his nature classic, The Outermost House, proposed that the best time to be on a beach was when the surf was high.
COPYRIGHT 1996 GLENN WOLFF/JERRY DENNIS/HARPER COLLINS -- ALL RIGHTS RESERVED