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Beach and Shoreline Dynamics

Beach and shoreline dynamics

The coast and beach, where the continents meet the sea, are dynamic environments where agents of erosion vie with processes of deposition to produce a set of features reflecting their complex interplay and the influences of changes in sea level, climate , or sediment supply. "Coast" usually refers to the larger region of a continent or island which is significantly affected by its proximity to the sea, whereas "beach" refers to a much smaller region, usually just the areas directly affected by wave action.

The earth is constantly changing. Mountains are built up by tectonic forces, weathered, and eroded away. The erosional debris is deposited in the sea. In most places these changes occur so slowly that they are barely noticeable, but at the beach we can often watch them progress.

Most features of the beach environment are temporary, steady-state features. To illustrate this, consider an excavation in soil , where groundwater is flowing in, and being pumped out by mechanical pumps. The level of the water in the hole is maintained because it is being pumped out just as fast as it is coming in. It is in a steady state, but changing either rate will promptly change the level of the water. A casual observer may fail to notice the pumps, and erroneously conclude that the water in the hole is stationary. Similarly, a casual observer may think that the sand on the beach is stationary, instead of in a steady state. The size and shape of a spit, which is a body of sand stretching out from a point, parallel to the shore, is similar to the level of the water in this example. To stay the same, the rate at which sand is being added to the spit must be exactly balanced by the rate at which it is being removed. Failure to recognize this has often led to serious degradation of the coastal environment.

Sea level is the point from which we measure elevation, and for good reason. A minor change in elevation high on a mountain is undetectable without sophisticated surveying equipment. The environment at 4,320 feet above sea level is not much different from that at 4,310 feet. The same 10-foot change in the elevation of a beach would expose formerly submerged land, or inundate formerly exposed land, making it easy to notice. Not only is the environment different, the dominant geologic processes are different: Erosion occurs above sea level, deposition occurs below sea level. As a result, coasts where the land is rising relative to sea level (emergent coasts) are usually very different from those where the land is sinking relative to sea level (submergent coasts).

If the coast rises, or sea level goes down, areas that were once covered by the sea will emerge and form part of the landscape. The erosive action of the waves will attack surfaces that previously lay safely below them. This wave attack occurs right at sea level, but its effects extend from there. Waves may undercut a cliff, and eventually the cliff will fall into the sea, removing material from higher elevations. In this way the cliff retreats, while the beach profile is extended at its base. The rate at which this process continues depends on the material of the cliff and the profile of the beach. As the process continues, the gradual slope of the bottom extends farther and farther until most waves break far from shore and the rate of cliff retreat slows, resulting in a stable profile that may persist for long periods of time. Eventually another episode of uplift is likely to occur, and the process repeats.

Emergent coasts, such as the coast along much of California, often exhibit a series of terraces, each consisting of a former beach and wave cut cliff. This provides evidence of both the total uplift of the coast, and its incremental nature.

Softer rocks erode more easily, leaving resistant rock that forms points of land called headlands jutting out into the sea. Subsurface depth contours mimic that of the shoreline, resulting in wave refraction when the change in depth causes the waves to change the direction of their approach. This refraction concentrates wave energy on the headlands, and spreads it out across the areas in between. The "pocket beaches" separated by jagged headlands, which characterize much of the scenic coastline of Oregon and northern California, were formed in this way. Wave refraction explains the fact that waves on both sides of a headland may approach it from nearly opposite directions, producing some spectacular displays when they break.

If sea level rises, or the elevation of the coast falls, formerly exposed topography will be inundated. Valleys carved out by rivers will become estuaries like the Chesapeake Bay. Hilly terrains will become collections of islands, such as those off the coast of Maine.

The ability of rivers to transport sediment depends on their velocities. When they flow into a deep body of water they slow down and deposit their sediment in what will eventually become a delta . Thus, the flooding of estuaries causes deposition further inland. As the estuary fills in with sediment the depth of the water will decrease, and the velocity of the water flowing across the top of the delta will increase. This permits it to transport sediment further, and the delta builds out toward, and eventually into, the sea. The additional load of all the sediment may cause the crust of the earth to deform, submerging the coast further.

Wave action moves incredible amounts of sand. As waves approach shallow water, however, they slow down because of friction with the bottom, get steeper, and finally break. It is during this slowing and breaking that sand gets transported. When waves reach the shore they approach it almost straight on, so that the wave front is nearly parallel to the shore as it breaks. The wave front is not exactly parallel to the shore, however, and it is this difference which moves sand along the beach.

When a breaking wave washes up onto the beach at a slight angle it moves sand on the beach with it. This movement is mostly towards shore, but also slightly down the beach. When the water sloshes back, it goes directly down the slope, without any oblique component. As a result, sand moves in a

zigzag path with a net motion parallel to the beach. This is called "longshore drift." Although most easily observed and understood in the swash zone, the area of the beach which gets alternately wet and dry with each passing wave, longshore drift is active in any water shallow enough to slow waves down.

Many features of sandy coasts are the result of long-shore drift. Spits build out from projecting land masses, sometimes becoming hooked at their end, as sand moves parallel to the shore. At Cape Cod, Massachusetts, glacial debris, deposited thousands of years ago, is still being eroded and redistributed by wave action.

An artificial jetty or "groin" can trap sand on one side of it, broadening the beach there. On the other side, however, wave action will transport sand away. Because of the jetty it will not be replenished, and erosion of the beach will result.

The magnitude and direction of transport of longshore drift depends on the strength and direction of approach of waves, and these may vary with the season. A beach with a very gentle slope, covered with fine sand every July may be a steep pebble beach in February.

Long, linear islands parallel to the shore are common along the Atlantic coast. Attractive sites for resorts and real estate developments, these barrier islands are in flux. A hurricane can drive storm waves over low spots, cutting islands in two. Conversely, migration of sand can extend a spit across the channel between two islands, merging them into one.

Interruptions in sand supply can result in erosion. This has happened off the coast of Maryland, where Assateague Island has gotten thinner and moved shoreward since jetties were installed at Ocean City, just to the north.

Often, the steady-state nature of the beach environment has not been properly respected. At higher elevations, where rates of erosion and deposition are so much slower, man can construct huge hills to support interstate highways, level other hills to make parking lots, etc., expecting the results of the work to persist for centuries, or at least decades. But in a beach environment, modifications are ephemeral. Maintaining a parking lot where winds would produce a dune requires removal of tons of sand-every year. Even more significantly, because the flow of sediment is so great, modifications intended to have only a local, beneficial effect may influence erosion and deposition far down the beach, in ways that are not beneficial. Tossing a drain plug into a bucket of water raises the level of the water by just a tiny amount. Putting the same drain plug into the drain of a bathtub, into which water is flowing steadily, will change the level in the tub in very substantial ways. Similarly, it may be possible to protect the beach in front of a beach house by installing a concrete barrier, but this might result in eroding the supports to the highway that provide access to the beach house.

See also Continental shelf; Drainage basins and drainage patterns; Drainage calculations and engineering; Dunes; Ocean circulation and currents; Offshore bars; Wave motions

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wave velocity

wave velocity The velocity at which waves of energy are transmitted through a medium. It depends on the characteristic properties of the medium-in the case of seismic waves on its elastic properties and density. The wave velocity (v) is always related to frequency (f) and wave length (λ) by the expression v = fλ.

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