| There are two great mysteries about the
beach. One is why human beings flock there by thousands, only to prostrate(俯卧)
themselves in dense packs of glistening flesh. The other is why the sand goes
there. Strange as it seems, oceanographers have never really understood why sand
piles up on the shore. Now Douglas Inman and Daniel Conley think they have
solved the puzzle. The puzzle had to do with waves. Though it might seem intuitive that waves carry water to shore, and sand along with it, it's not that simple. The crest(浪尖)of a passing wave lifts a given hit of water upward and landward, but the ensuing trough(波谷) pushes the water back down and Out to sea. Near the bottom, there the sand is, the water was always assumed to just slide back and forth—and the sand with it. "If you take a very aloof look at a beach," says Inman, "you'll realize that if the two motions move sand back and forth the same amount, then all the sand should end up in deep water.' So for beaches to exist, the crest's onshore flow must somehow move enough sand up the beach to counter the seaward tug of both the trough and gravity . The pressure changes in the sand bed, Inman and Conley think, are the key to beach creation. They found that sand doesn't just slide back and forth with each passing wave. Under a trough, it does slide seaward, in a thin layer just above the bottom. But under a crest its movement is often more elaborate. The higher pressure under a crest—higher because the water is piled higher—forces water into the porous(多孔的) sand. This creates strong whirlpools just above the sand, which help loosen it. As the crest passes overhead, the sand first rushes across the bottom; then it abruptly turns violent lifting off the bottom in large, boiling bunches. Finally, just after the crest passes, the sand explodes up into the great water column. The boiling and rushing move more sand than the backsliding under a trough, so there's a net movement of sand toward the shore. |