Читать книгу Waves and Beaches - Kim McCoy - Страница 20

Beaches are often of grand enough scale to be worthy of study as major coastal features. Although the comprehensive consideration of beaches in a physical geographic, or physiographic, sense is also beyond the scope of this book, it is important to briefly consider the three forms that beaches are most likely to take when they are treated as geologic units. A beach can be simply a narrow strip of sand separating the rocky cliffs of land from the sea; a spit or a baymouth bar; or a barrier island.

The first form—a beach that is narrow, of limited extent, and on which the sand is a shallow veneer over the rock—is indicative of a youthful shoreline. That is, not much time has passed, geologically speaking, since the most recent change in sea level. What little sand there is has been created in place by the undermining of the cliff and the grinding of the rocks by wave action. Many beaches of the California and Oregon coasts are in this category. These so-called pocket beaches (see photo on page 37) extend between rocky headlands and often have sheer cliffs behind; in the winter months, storms strip off most of the sand, exposing cobbles and the underlying rocks.

The second form, in which spits and baymouth bars are created by wave action, requires more time to develop. In it, rough coasts tend to be straightened by wave forces and ragged shorelines are smoothed. Headlands extending into the sea are attacked because wave energy is focused on them by the underwater topography. Waves striking the coast at an angle create longshore currents that transport sand, and seal off the mouths of relatively quiet bays.

The sequence of events in one form of coastal straightening is illustrated in figure 6 (page 38). At stage one, bold headlands project into the ocean where they are attacked by waves whose energy is concentrated there by the process of wave refraction. As the headland retreats and the cliffs are reduced to rocky fragments, currents caused by the waves striking the shore obliquely transport the smaller particles into the relatively quiet water at the head of the bay where they form a beach, stage two. Later, the headlands have been cut back and the bay becomes shallow, as in stage three. The longshore coastal currents, which were disorganized by turbulence around the headlands in the earlier stages, now become dominant and sweep sand along the coast, creating beaches and baymouth bars (see photo on page 39). With a straight shoreline, sand can be transported considerable distances, passing headlands and bays alike in its longshore migration. Eventually, at the land’s end, the water deepens and the transporting current spreads out and is reduced in velocity so that the sand it has been carrying drops to the bottom. These embankment-like deposits in which the outermost end is surrounded by water are called spits. They form wherever there is a supply of sand, a transporting current, and a dumping ground.

POCKET BEACHES Pocket beaches form between rocky headlands. Nusa Penida, Bali. Tommy Schultz

FIGURE 6: Waves straighten a rocky coast in stages. Top: Zones of equal wave energy in deep water are concentrated by wave refraction so that the headlands are attacked. Bottom: Eventually headlands are cut back to furnish enough sand to build beaches, bars, and spits.

There are two famous spits at the entrance to New York Harbor (see images on pages 40 and 41). Sandy Hook, to the south, was built by materials supplied by the erosion and retreat of the Navesink Highlands in New Jersey. It grew steadily until it reached an equilibrium situation in which the new sand added to the tip is just equal to that removed by the tidal currents at the harbor mouth.

Rockaway spit, northeast of the harbor entrance, was built with sand from the Long Island coast and grew at the rate of 200 feet (60 m) per year (1 mile in twenty-three years) for a long period until the present series of groins and jetties were built. Frequently these rivers of send flowing along a coast are supplied by the erosion of valuable property. This erosion creates one form of a beach problem; later the sand is deposited where it is not wanted, still another problem.

BAYMOUTH BAR AND SPIT Baymouth bars are formed by the deposition of sand, sometimes enclosing an embayment. A spit is similar, but is attached to land at only one end. Klamath River, California. Gary Crabbe

The beaches of the north Pacific coast are composed of an abundance of fine dark sand made from the disintegration of an inland basaltic plateau and brought to the sea by the Columbia River. The sand is distributed by wave and current action so that both north and south of the river mouth great spits have formed, straightening the coast by sealing off bays and headlands. These spits are continually widening, as evidenced by a series of sandy ridges or growth lines, and the underwater sandbars opposite bay entrances are constantly shifting.

NEW YORK BAY, 1860 Two large spits (Rockaway and Sandy Hook) at the New York Harbor entrance have changed due to the ongoing work of erosion and deposition since this chart was made in 1860. New York. US Coast Survey, Lindenkohl & P. Witzel

There are large meandering sandspits in Europe, at the mouth of the Bay of Arcachon, near Bordeaux, France. There the winds blow sandspits into Europe’s largest dunes—over 330 feet (100 m) high. The observer, feeling the shudder of the beach and hearing the roaring of the great winter breakers, gets an impression of natural forces in violent conflict, and one wonders why the changes are not more rapid.

The third major beach form, the barrier island, makes up a major part of the East and Gulf Coasts of the United States and much of the coasts of Holland and Poland. A half dozen major cities are built on these sandy strips, including Atlantic City, New Jersey; Miami Beach, Florida; and Galveston, Texas. Sometimes called barrier beaches or even offshore bars, these islands vary in width from a few yards to a mile (1.6 km). They may be dozens of miles long and are, in places, separated from the mainland by shallow bays many miles wide (see photo on page 42).

NEW YORK BAY, 2020 Rockaway Spit was ravaged by Hurricane Sandy in 2012. Today sand-nourishment and dune-restoration projects hope to resist the effects of climate change. New York Harbor, New York. NASA/Landsat

When sand is blown by the wind into dunes, as at Kitty Hawk, North Carolina (where the Wright brothers first flew in 1903), the hills on the islands may rise to a height of nearly 100 feet (30 m). Dunes act as a reservoir, a buffer of sand held in reserve for extreme events. When human activities compromise the dunes’ ability to serve this function, the natural supply of sand is decreased and the beach is starved of sand and coastal erosion advances. We must respect the dunes.

Frequently between the dunes and the main coast there is a chain of bays, marshes, and tidal lagoons, which in many places have been developed into an inland waterway where small craft can move safely along the coast. These large, sandy shoreline features are accumulated beach deposits which have grown so large and permanent that they no longer fit our definition that limits a beach to the area in which the sand is moved by ordinary wave action. In some areas such as in Namibia, on the west coast of Africa, coastal dunes rise to over 1,000 feet (300 m) and then extend their complexity almost endlessly into the desert.

BARRIER ISLANDS Barrier islands protect the mainland from storms and shelter inland bays, shallow marshes, and tidal lagoons. Cape Lookout, North Carolina. Steve Dunwell

Since beaches owe their existence to wave action, they have a dynamic yet chaotic quality. That is, beach materials are always in motion—as long as there are waves—although this complex mobility is not readily apparent to the casual observer. The motion of the beach material may be parallel to the shoreline, in which case it is transported by alongshore currents, or it may be moved toward or away from the land by wave action.

There are two major beach forms created by the waves: berms and bars. Berms are flat, above-water features that make up the familiar part of the beach. Bars are underwater ridges of sand that parallel the shoreline and are seldom seen except at unusually low tides. On most beaches there is a constant exchange of sand between these two features, and the direction of the transport depends on the character of the waves. When the waves are large and follow each other closely (as they do under storm conditions, for example) the berm is eroded and as a result, the offshore bar builds up. When calm conditions return, the small waves rebuild the berm at the expense of the bar. For this reason, the above-water part of a beach is generally much narrower in the stormy winter months than in the summer, which is convenient for the hordes of bathers who come to sun themselves on the wide summer berm and swim in the low surf.

BEACH SAND DUNES Dunes with vegetation hold sand in reserve for extreme events. Along the shore a sandbar is exposed at low tide. Dunes and bars work together to buffer the coastline. Jekyll Island, Georgia. Kim McCoy

The steeply sloping seaward side of the berm against which the waves are in constant contact is called the beach face. The face might also be described as the zone within which the shoreline wanders as the waves rush up the beach and wash back down it. This is the swash zone.

It is necessary to set limits on the extent of what is considered a beach, to keep the discussion of its properties within reasonable bounds. In the seaward direction, beaches extend outward as far as where ordinary waves move the sand particles. This limit has been found to be about 30 feet (10 m) below the low-tide level (see figure 5, page 34). This is an arbitrary but satisfactory limit that is generally accepted. Above water, the beach extends landward to the edge of the permanent coast. The latter may consist of a cliff, sand dunes, or human-made structures. In the geological sense these are not really permanent, but they endure far longer than the small-scale beach features that concern us here. The Earth’s coastlines have not always been the way they are now. Geologically speaking, most shorelines and beaches are very recent features. The present sea level has existed with little change, changing only a few feet over the past 4,000 years. This is because we have been living in an interglacial calm after a period when great ice sheets covered the Earth and sea level was much lower. Twenty thousand years ago the global sea level was also fairly stable—but about 400 feet (120 m) lower than today. The Mediterranean island nation of Malta was connected to Italy; now it is 60 miles offshore. However, by roughly 18,000 years ago, a warming period had been set in motion by the Milankovitch cycles. These are changing patterns in the Earth’s tilt and orbital movement around the Sun (roughly 20,000, 40,000, and 100,000 years) that influence the distribution of the Sun’s energy reaching the Earth. So, as ice sheets began to melt more rapidly, the ocean temperatures increased and sea levels rose (seawater expands with increasing temperature), changing the location of Earth’s water and the dynamics of Earth’s coastlines. During the Younger Dryas cold snap about 13,000 years ago, when pulses of cold meltwater went into the North Atlantic, the rate of sea-level rise slowed for a millennium, lessening the sea’s rate of assault on beaches. But the Earth continued to warm, glaciers continued to melt, and sea level rose for many thousands of years, creating new shorelines and beaches. The “old” beaches were left under hundreds of feet of water. No beach on Earth was left unchanged. Although sea level has been comfortably stable for most of the past four thousand years, previous sea-level rises provide earnest proxies for how today’s changing climate is already affecting the dynamics of coastal regions.

The coastal sand dunes of Namibia contain diamond-rich sediments from the Orange River that have been transported northward by the wave-driven longshore currents. Namib-Naukluft National Park, Namibia. Fabian von Poser/Getty Images

SAND BERMS Berms are generally flat, above-water features formed by waves during high tides. Berms eroded by steeper storm waves can reform into underwater bars. Salt House Beach, Norfolk, United Kingdom. Anthony Bennett

SANDBAR Bars are underwater ridges of sand that parallel the shoreline and are seen only at low tide. Water, elevated by waves, flows over the bar and then parallel to the shore, forming sand ripples on its way back to the sea. Shackleford Banks, North Carolina. Kim McCoy

In some areas, geological (or, tectonic) uplift and the post-glacial rebound have caused the land to rise relative to sea level. This means that few shorelines have reached steady-state conditions. An example of this post-glacial rebound is the city of Pisa, Italy, which was a port less than a thousand years ago. Even though the city was never covered by glaciers, it is now over 2 miles (3 km) from the sea. Excavations in the ancient city of Ephesus on the coast of Turkey have uncovered a colonnaded marble road from the harbor where distinguished visitors in Roman times could make a triumphal entry after landing. Now, the point of debarkation is more than a mile (2 km) from the sea. Such localized and regional post-glacial rebound changes will, of course, continue. Today, scientific measurements indicate that a new wave of global sea-level change began with the onset of the Industrial Revolution (see figure 4, page 28). It has sparked a new battle between the land and the sea.