Coastal erosion

As a result of the global rise in sea level and inappropriate development in the coastal zone, coastal erosion is becoming recognized as a serious national and worldwide problem. Coastal erosion is generally a more continuous, predictable process than other natural hazards, such as earthquakes, tropical cyclones, or floods, and large sums of money are spent in attempts to control it. If extensive development of coastal areas for vacation and recreational living continues, coastal erosion will certainly become a more serious problem.

Beach Budget 

The concept of littoral cell is important in coastal processes involving sand transport to, along and from a beach (that is a budget). A littoral cell is a segment of coast that includes an entire cycle of sediment delivery to the coast (mostly by rivers but also coastal erosion), long-shore sediment transport, and eventual loss of sediment from the near-shore environment (for examples, sediment may be flushed down a submarine canyon that heads in the surf zone or move inland to be part of sand dunes). An easy way to visualize beach erosion at a particular beach is to take a beach budget approach. An analogy to the budget is your bank account. You deposit money at regular or not so regular times. Some money is in storage, and that is your account balance; and you periodically withdraw funds, which is output. Similarly, we can analyse a beach in terms of input, storage, and output of sand or larger sediment that may be found on the beach. Input of sediment to a beach is by coastal processes that move the sediment along the shoreline or produce sand from erosion of a sea cliff or sand dunes on the upper part of a beach. The sediment that is in storage on a beach is what you see when you visit the site. Output of sediment is that material that moves away from the site by coastal processes similar to those that brought the sediment to the beach. If input exceeds output, then the beach will grow, as more sediment is stored and the beach widens. If input and output are relatively equal to one another, the beach will remain in a rough equilibrium at about the same width. If output of sediment exceeds input, then the beach will erode and there will be fewer grains of sediment on the beach. Thus, we see that the budget represents a balance of sand on the beach over a period of years. Short-term changes in sediment supply due to the attack of storm waves will cause seasonal or storm-related changes to the supply of sediment on a beach. Long-term changes in the beach budget caused by climate change or human impact cause long-term growth or erosion of a beach.

Erosion Factors

The sand on many beaches is supplied to the coastal areas by rivers that transport it from areas upstream, where it has been produced by weathering of quartz- and feldspar-rich rocks. We have interfered with this material flow of sand from inland areas to the beach by building dams that trap the sand. As a result, some beaches have become deprived of sediment and have eroded. Damming is not the only reason for erosion. For example, beach erosion along the East Coast is a result of tropical cyclones (hurricanes) and severe storms, known as Northeasters or Nor easters;3 a rise in sea level; and human interference with natural shore processes. Sea level is rising around the world at the rate of about 2 to 3 mm (0.08 to 0.12 in.) per year, independent of any tectonic movement. Evidence suggests that the rate of rise has increased since the 1940s. The increase is due to the melting of the polar ice caps and thermal expansion of the upper ocean waters, triggered by global warming that is, in part, related to increased atmospheric carbon dioxide produced by burning fossil fuels. Sea levels could rise by 700 mm (28 in.) over the next century, ensuring that coastal erosion will become an even greater problem than it is today.

Sea Cliff Erosion 

When a sea cliff (a steep bluff or cliff) is present along a coastline, additional erosion problems may occur because the sea cliff is exposed to both wave action and land erosion processes, such as running water and landslides. These processes may work together to erode the cliff at a greater rate than either process could alone. The problem is further compounded when people interfere with the sea cliff environment through inappropriate development. The rocks of the cliff are steeply inclined and folded shale. A thin veneer of sand and coarser material, such as pebbles and boulders, near the base of the cliff covers the wave-cut platform, which is a nearly flat bench cut into the bedrock by wave action. A mantle of sand approximately 1 m (3.3 ft) thick covers the beach during the summer, when long, gentle, spilling breakers construct a wide berm, while protecting the sea cliff from wave erosion. During the winter, plunging breakers, which have a high potential to erode beaches, remove the mantle of sand and expose the base of the sea cliff. Thus, it is not surprising that most erosion of the sea cliffs in southern California takes place during the winter.
A variety of human activities can induce sea cliff erosion. Urbanization, for example, results in increased runoff. If the runoff is not controlled, carefully collected, and diverted away from the sea cliff, serious erosion can result. Drainpipes that dump urban runoff from streets and homes directly onto the sea cliff increase erosion. Drainpipes that route runoff to the base of the sea cliff on the face of the beach result in less erosion. Watering lawns and gardens on top of a sea cliff may add a good deal of water to the slope. This water tends to migrate downward through the sea cliff toward the base. When water emerges as small seeps or springs from a sea cliff, it effectively reduces the stability of the sea cliff, facilitating erosion, including landslides. Landslides, from many small to few large, occur along a sea cliff as a result of wave erosion at the base of the sea cliff. Numerous small landslides produce the irregular shape (wavy) along the top of a sea cliff. Structures, such as walls, buildings, swimming pools, and patios, near the edge of a sea cliff may also decrease stability by adding weight to the slope, increasing both small and large landslides. Strict regulation of development in many areas of the coastal zone now forbids most risky construction, but we continue to live with some of our past mistakes. The rate of sea cliff erosion is variable, and few measurements are available. Near Santa Barbara, California, the rate of sea cliff erosion averages 15 to 30 cm (6 to 12 in.) per year. These erosion rates are moderate compared with those in other parts of the world. Along the Norfolk coast of England, for example, erosion rates in some areas are about 2 m (6.6 ft) per year. The rate of erosion is dependent on the resistance of the rocks and the height of the sea cliff.6 The rate of coastal erosion can be determined using remote sensing. Sea cliff erosion is a natural process that cannot be completely controlled unless large amounts of time and money are invested, and, even then, there is no guarantee that erosion will cease. Therefore, it seems we must learn to live with some erosion. It can be minimized by applying sound conservation practices, such as controlling the water on and in the cliff and not placing homes, walls, large trees, or other structures that contribute to driving forces close to the top edge of a cliff.


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