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Coastal Processes and Landforms

The shoreline is affected by waves (produced by wind at sea) and tides (produced by the gravitational effect of the moon and sun). Waves Waves are caused by wind. Wave height in the open ocean is determined by three factors. The greater the wind speed the larger the waves. The greater the duration of the wind (or storm) the larger the waves. The greater the fetch (area over which the wind is blowing - size of storm) the larger the waves. Waves have crests (high spots) and troughs (low spots). The wave height (amplitude) is the difference in height between the crest and the trough. The wavelength (L) is the distance between two crests (or troughs). The period (T) is the time between passage of successive wave crests (or troughs).

Wave velocity or celerity (C) As for any wave, C = L/T. In deep water: C = 1.56 (T)C = 1.25 (L)1/2In shallow water where bottom drag slows the wave, depth (D) becomes an important factor: C = 3.1 (D)1/2C = 0.49 (T)C = 0.7 (L)1/2(for distance in meters, time in seconds, and celerity in meters per second) Open Ocean Waves: As a wave passes, water molecules rise up and move forward (in the direction of wave motion) until the crest passes. After the crest the water molecules move down and backward. The result is that water molecules move in orbital paths as waves pass. This orbital motion is greatest at the sea surface and decreases with depth below the surface. At a depth of one-half the wavelength the orbital waver motion is nearly zero (actually 4% of the surface orbital diameter). This L/2 depth is considered wave base. Waves at the Shoreline: As a wave approaches the shore it slows down from drag on the bottom when water depth is less than half the wavelength (L/2). The waves get closer together (shorter wavelength), steeper, and taller. Orbital motions of water molecules become increasingly elliptical, especially on the bottom. There is a growing proportion of back and forth motion and less up and down motion as the wave moves through shallower and shallower water. Eventually the wave becomes too steep and it breaks. Breakers: Steep waves that roll over a very gradually shallowing seafloor form spilling breakers. Steep waves that come into a steeper shoerline or that encounter a sudden shallowing of water depth form the classic plunging breakers. In these, the bottom of the wave slows much faster than the top and the wave topples over. Low, flat waves that come into a steep shoreline form surging breakers. Collapsing breakers are midway between surging and plunging breakers. The dominant sense of motion is now forward and backwards resulting in the forward swash of water up the shoreface, followed by the backwash back down the shoreface. The swash and backwash move sediment up and down the shoreface, depositing some and eroding some. The balance depends on the nature of the waves (see below). Beach Profile Summer Growth: The beach is a depositional landform. During calm summer weather, waves deposit sediment on the shore and the beach grows in size. Waves surge up the shoreface carrying sediment. The swash slows due to gravity and friction, runs out of momentum, stops, then slides back down due to gravity but slowed by friction. Water also sinks into the porous beach meaning the backwash experiences more friction than the swash. More sediment is pushed up by the swash than pulled back down by the backwash. The top of the growing berm is the highest point that the waves reach.

Winter Erosion: During the winter months (or during tropical storms), storm waves carry much energy to the beach with extra energy to suspend sediments and redistribute them in the nearshore environment. Steady strong winds from a storm push water up on the shore raising water levels. Return flow from this wind setup helps to carry sediment away from the shore, especially since the strong wave energy mobilizes the sediments on the seafloor. The summer berm is eroded away and the sands deposited offshore. The winter berm is higher than the summer berm because storm waves can push sediments higher up. The winter beach profile is higher and narrower than the summer beach.

Wave Energy vs. Beach Slope vs. Sediment Size: The slope of the shoreface depends on the sedimentary grain size. Fine sand beaches have gentle slopes of around 3. Pebbly beaches have steeper slopes around 15. Cobble beaches have slopes of around 24. Coastal Sedimentation: In the wave-dominated shoreface and nearshore environment, fine sediments (silt and clay) remain suspended and are winnowed away, leaving behind the coarse grained sediments (sand and gravel). Suspended fine grained sediments are deposited offshore in deeper water below L/2 depth. Therefore sand and gravel dominates the beach, foreshore, near shore. Sands become finer the deeper the water, farther offshore. Sandy bottoms give way to silt-dominated and then clay-dominated bottom sediments (muds) farther offshore and out across the continental shelf.

This is an oversimplification because 1) wave size (wavelength) varies from day to day and season to season, and 2) sand can be transported fully across the continental shelf in places such as undersea canyons at the ends of rivers. Wave Refraction: When waves approach the beach at an angle, the part of the wave that reaches shallow water earliest slows down the most, allowing the part of the wave that is farther offshore to catch up. In this way the wave is refracted (bent) so that it breaks on the shore more nearly parallel to the shore. This wave refraction focuses wave energy around headlands and diffuses it in bays. Relatively large waves are found around headlands. Bays have quiet water (good for ship moorings) and are sites of deposition (nice sandy beaches). Rip Currents: Constantly breaking waves push water up against the shore. It is slower for that water to flow out underneath the waves (undertow) because of friction along the bottom. Sometimes to release water trapped shoreward of the breakers, narrow jets of outflowing water, rip currents, become established with a quasi-regular spacing. Water flows in with the forward wave momentum, flows laterally until it finds an outlet, then flows back out through a rip current. Beach Cusps: Beaches are commonly sculpted at high tide into a series of regularly spaced crescent-shaped beach cusps. Beach cusps range from a couple meters to several hundred meters wide. The size is proportional to the size of incoming waves and the height to which the swash rises up the shoreface. Incoming waves are split by the horns, meets in the swales and flows back out with the backwash; but this impedes the next incoming wave, so most incoming wave energy funnels in to the horns.Sediment on the horns is coarser due to deposition of the coarsest fraction of the sediment load. The outgoing water removes finer grained sediment from the swale and deposits it just offshore, making the seafloor shaloower there, further impeding incoming waves. Littoral Drift: As a wave breaks on the shore, the swash pushes sediment up the beach and then pulls it back down the beach as the water slides back down. If the waves do not come in parallel to the beach the swash pushes sand up and along the beach and the backwash pulls sand back down the beach. This occurs repeatedly with each wave resulting in sand moving along the beach. Waves approaching and breaking at an angle to the shore also causes longshore currents, primarily between the breakers and the swash zone. The longshore currents also transport sand along the shore. The longshore transport of sand by these two mechanisms is referred to as littoral drift. Sand Spits & Hooks: Littoral drift causes a sand bar to grow from the end of an island or where a shoreline turns into a bay. Wave action builds up a beach on the sand bar. This sand spit continues to extend as long as there is a continuous sand supply. It may completely cover a bay as a baymouth bar. Refraction of incoming waves around the end of the growing sand spit causes the end to take a curved, hook, shape. Tombolos: A small island just off the shore will cause incoming waves to refract around both sides of the island which, in turn, will cause littoral drift on the shoreline in the direction of the island. Sand builds up behind the island untill eventually there is a strip of sand connecting the mainland shore and the island.Coastal Type coasts of submergence Most of the U.S. east coast and Gulf coast is a low-lying coastal plain. The continental shelf is a portion of the coastal plain that is presently submerged below sea level because it was formerly an area of stretching when the supercontinent of Pangea was rifting apart. This is called a passive continental margin because it is no longer an active plate boundary. Stream networks flowed across the entire coastal plain (including the present continental shelf) around the time of the last glacial maximum, 21,000 years ago, when sea level was about 100 m lower than it is today. The coast was about 100 km (60 mi) farther offshore from Long Island at that time. - ria coasts: Since much of the continental ice sheets melted (except for Antarctica and Greenland), sea level rose and flooded former river valleys and drainage basins. The U.S. east coast has many deep embayments, especially Chesapeake Bay and Delaware Bay. Many coastal rivers are estuaries far upstream. For example, the Hudson river has tides as far upstream as New Paltz. Ideally, spits will gradually close off the bays and sediments will fill them in. Eventually the coastline will straighten out. - fjord coasts: During the last glacial maximum high continental ice sheets flowed from the interior all the way to the coast. Individual ice streams carved deep, wide glacial valleys. As the glaciers melted and the water drained back into the ocean, sea level rose, covering what we call the continental shelf and flooding the glacial valleys. Most are familiar with the beautiful fjords in places like Alaska and Norway. The Hudson River was carved by a glacial ice stream. It may be the lowest latitude fjord in the world. - barrier island coasts: The U.S. east and Gulf coast, with its very gently sloping coastal plain/continental shelf is fringed with barrier islands, low lying strips of sand with beach berm and dunes, backed by emergent scrub vegetation that grades into salt marsh and then a few kilometers of open lagoon separating the barrier island from the mainland. It is still debated how barrier islands originally form, but it is known that once formed they migrate landward with rising sea level.Offshore sediment coring shows former positions of the barrier islands when sea level was lower. Strong storm surges sometimes overtop and wash out a section of a barrier island, thereby pushing sand to the back (mainland) side of the island. This may be part of the mechanism of barrier island retreat. Inlets between the barrier islands form occassionally by these strong storms. The inlets are maintained by tidal currents produced by the daily rise and fall of the tides. coasts of emergence - tectonic coasts The U.S. west coast is an active coast because there are active tectonic plate boundaries all along the coast. The San Andreas and many related faults are part of the transform plate boundary separating the North American and Pacific plates. The Cascadia subduction zone runs along the coast of northern California Oregon, Washington, and British Columbia. It is the site of subduction of the Juan de Fuca plate under the North American plate. The coast line of these active plate boundaries typically rise steeply with little or no offshore continental shelf. Coastal sea cliffs are common. - wave-cut notch: Waves slamming against the base of sea cliffs weaken fractured rock. Waves do more damage if there are already loose boulders and cobbles to slam against the cliffs. The cliff becomes undercut, forming a wave-cut notch. Eventually the undercutting causes the cliff to collapse yielding a talus of broken rock at the base of the cliff. Wave action gradually further breaks and grinds up the talus and removes it. - wave-cut platform : As the seacliffs gradually retreat from continual undercutting and collapse, the sediment produced (boulders, gravel, sand) are constantly being moved by waves over the remaining rock under the water. The boulders, gravel, and sand grind away at each other and the underlying rock until they produce a flat rock platform just below sea level. The wave-cut platform continues to be carved landward as long as relative sea level remains constant. - marine terrace: Episodic tectonic activity can lift wave cut platforms to higher elevations where they then lie at the top of coastal sea cliffs. Along many sections of the California Coast there are multiple levels of marine terraces, uplifted former wave-cut platforms. - sea arches and sea stacks: In places along a receding seacliffs, rock may be locally more resistant to erosion, in which case it juts out into the waves beyond the cliffs. Aided by wave refraction, wave cut notches in a section of the rock that juts out, may erode together to form a sea arch. Eventually the arch will become too thin and it will collapse. The resistant rock still stands out in the waves off the seacliff shore. These remaining resistant rock are called sea stacks. References: Anderson, Robert S., and Suzanne P. Anderson, Geomorphology: The Mechanics and Chemistry of Landscapes, Cambridge University Press, New York, 637 pp., 2010. Easterbrook, Don J., Surface Processes and Landforms, 2nd Ed., Prentice Hall, Upper Saddle River, N.J., 546 pp., 1999. Kennett, James P., Marine Geology, Prentice-Hall, Englewood Cliffs, N.J., 813 pp., 1982.