How Things Work: Beaches
Soon, summer will arrive. Many will spend their summer at the beach, tanning on towels, body-surfing the breakers, or enjoying a game of barefoot volleyball. We often take our beaches for granted, though: Before you curl your toes into the warm beach sand or run splashing into the water, take a moment to partake in the wonderful world of coastal formation!
Coastal formation occurs on several scales, ranging from the motions of the planets all the way down to the interactions of molecules of water. On the largest scale, coastlines are formed from two processes: coastal emergence and coastal submergence. Coastal emergence happens when land emerges out of the water. This can occur, for example, when a mountain range at a plate boundary is uplifted. Imagine the coastline of Chile, closely paralleled with the Andes Mountains. Millions of years ago the Andes were pushed up from the crust, forming a new coastline.
Far more coastlines, however, have been formed by submergence. Coastal submergence occurs when the water level of the ocean rises significantly, flooding formerly inland regions. The entire east coast of the U.S., for example, is a coast of submergence. The majority of today?s coastlines are of this type because of the rise in sea level following the last Ice Age.
Beach formation on a more familiar scale deals with actual aggregate movement. Put simply, the Sand Man?s day job is to shovel beach sand ? a lot of beach sand. The movement of beach sand happens in two ways ? the movement of sand perpendicular to the beach and the movement of sand along the beach. Both are caused by a combination of wave motion, water currents, and wind.
Sand movement toward and away from a beach is dependent largely on the type of wave. Waves known as spilling breakers, or constructive waves, tend to have elliptical pipes that surge water up on the beach, depositing sand and dissipating the wave?s energy. Plunging breakers, or destructive waves, tend to have a circular pipe that collapses in on itself, sucking sand away from the beach in a backwash and contributing energy to the following wave. The occurrence of these waves is dependent largely on seasonal variation and weather.
Currents also have a significant effect on the movement of sand in and out from the beach. If waves are responsible for the movement of sand on and off the beach, then currents are responsible for the movement of sand to and from the waves. Imagine a grain of sand being taken away from the beach. As this grain is swirled about in a tempestuous fury of foam and churning water, it is eventually picked up by a current known as a rip current or rip tide. These rip tides funnel outward between underwater sand bars until they die out with increasing depth.
The grain of sand will then drift towards the sandbar, settling down on it until it is picked up by an incoming wave. That wave will eventually deposit on the beach, repeating the cycle. Because rip tides only occur between breaks in under-water sandbars, they don?t have enough force to cause a net movement of sand away from the beach. (If you?re ever stuck in a riptide, swim parallel to the beach and you?ll soon escape its deathly grasp! If you want to avoid a tour of Neptune?s inky depths, remember my column!)
Another type of current, known as long-shore drift, is responsible for the movement of sand parallel to the beach. Long-shore drift occurs because waves often strike beaches at an angle. When they do this, the waves deposit sand along one direction and pull it away on another. This tends to form a zig-zag movement of sand along the edge of the beach, pushing sand in the direction that the waves are striking. This movement is responsible for the formation of several coastal structures, including spits, hooks, tombolos, and coastal bars. For more information on these terms I invite you to consult a dictionary.
This drift has important consequences for the man-made development of coastal regions. Because of this effect, many beaches worth billions of dollars in property value would disappear as sand is slowly moved down the coast. To counteract this, many coastal towns have built structures called ?groins? along their beaches. Groins are basically walls made of concrete, wood, or most commonly, piles of rocks. These walls project out into the water at regular intervals and stop the long-range movement of sand.
Water can be our friend, but it can also be our mortal enemy. The continuing development of many coastal regions is akin to building your house on a sea-side glacier. Sure, it may look stable now ? but eventually it?s going to fall into the sea.