Notes-coasts - Notes on coastal hazards Coastal erosion is...

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Coastal erosion is the removal of coastal materials (sand, rock) due to wind and waves. Sometimes this material is removed from the coast system (a serious case of erosion), but more commonly it is merely relocated. When material is merely relocated the perception of erosion depends on what’s on the coast. Material is eroded from the shore when: (a) gravity pulls material off of high areas, a process known as mass wasting; and (b) the shore is attacked by relatively rare high-energy winds or waves. The common calm conditions that prevail most of the time do little to erode the shore, and in many cases cause it to enlarge. As a result, erosion of the shoreline tends to be episodic on the scale of weeks or months. Sediment can be both added to and removed permanently from the coast system. New sediment is produced by erosion of the coastal materials and supplied from rivers. Sediment is removed permanently when it is carried so far offshore during large storms that it cannot be returned during calmer times. Sometimes sediment falls into submarine canyons and is lost. Humans can induce beach erosion by robbing beaches of their sediment supply when dams are built on rivers. The dams trap the sediment that would otherwise be resupplying the beaches. This problem is especially severe in the western USA, especially along the California coast (i.e, Majilija Dam). Coasts characterized by steep slopes, such as are found on the Pacific and northern Atlantic coasts of the USA, are usually eroding. Erosion occurs as a process of: (a) slope undercutting; (b) mass wasting; (c) sand distribution. Coasts characterized by gentle slopes, such as found along the southern Atlantic and Gulf coasts, can be either eroding or accreting (building), and often do both seasonally within a single year. Along these coasts material is largely being relocated, not lost from the coast system. The sediment size on a beach depends on what kind of rock or sediment the coast is made of. Coasts made of hard rock tend to be rocky, while those made of soft sedimentary rocks or sediments tend to be sandy. The sediment size on a beach also depends on the wave energy. High-energy waves stir up, entrain, and remove finer sediment, leaving behind a beach made of pebbles and rocks. Low-energy waves remove only the finest silt and clay, and leave a sandy beach behind. Waves are characterized by a wavelength (λ), amplitude, and velocity. Geologists sometimes also use period, which is the time between wave crests, and is simply 1/λ. The height of a wave is the distance from the trough to the peak, and is twice the amplitude. As a wave passes the water on the surface and underneath is put into motion. Water below the waves follows circular paths with a radius that decreases with depth. At a depth of one-half the wavelength (λ/2) the radius of the circles 0 and the water stops moving. Waves in water deeper than λ/2 are called deep-water waves, while waves in water shallower than λ/2 are called shallow-water waves. Waves in the
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Notes-coasts - Notes on coastal hazards Coastal erosion is...

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