Understanding Coastal Zones: Erosion, Sediment Transport, and Coastal Processes, Study notes of Geology

Download Understanding Coastal Zones: Erosion, Sediment Transport, and Coastal Processes and more Study notes Geology in PDF only on Docsity! This document last updated on 17-Jul-2012 Coastal Zones Coastal Zones A coastal zone is the interface between the land and water. These zones are important because a majority of the world's population inhabit such zones. Coastal zones are continually changing because of the dynamic interaction between the oceans and the land. Waves and winds along the coast are both eroding rock and depositing sediment on a continuous basis, and rates of erosion and deposition vary considerably from day to day along such zones. The energy reaching the coast can become high during storms, and such high energies make coastal zones areas of high vulnerability to natural hazards. Thus, an understanding of the interactions of the oceans and the land is essential in understanding the hazards associated with coastal zones. Tides, currents, and waves bring energy to the coast, and thus we start with these three factors. Tides Tides are due to the gravitational attraction of Moon and to a lesser extent, the Sun on the Earth. Because the Moon is closer to the Earth than the Sun, it has a larger affect and causes the Earth to bulge toward the moon. At the same time, a bulge occurs on the opposite side of the Earth due to inertial forces the explanation is beyond the scope of this course). These bulges remain stationary while Earth rotates. The tidal bulges result in a rhythmic rise and fall of ocean surface, which is not noticeable to someone on a boat at sea, but is magnified along the coasts. Usually there are two high tides and two low tides each day, and thus a variation in sea level as the tidal bulge passes through each point on the Earth's surface. Along most coasts the range is about 2 m, but in narrow inlets tidal currents can be strong and fast and cause variations in sea level up to 16 m. Coastal Zones 7/17/2012Page 1 of 14 Docsity.com The highest high tides occur Because the Sun also exerts a gravitational attraction on the Earth, there are also monthly tidal cycles that are controlled by the relative position of the Sun and Moon. when the Sun and the Moon are on the same side of the Earth (new Moon) or on opposite sides of the Earth (full Moon). The lowest high tides occur when the Sun and the Moon are not opposed relative to the Earth (quarter Moons). These highest high tides become important to coastal areas during hurricane season and you always hear dire predications of what might happen if the storm surge created by the tropical cyclone arrives at the same time as the highest high tides. Fluctuations in Water Level While sea level fluctuates on a daily basis because of the tides, long term changes in sea level also occur. Such sea level changes can be the result of local effects such as uplift or subsidence along a coast line. But, global changes in sea level can also occur. Such global sea level changes are called eustatic changes. Eustatic sea level changes are the result of either changing the volume of water in the oceans or changing the shape of the oceans. For example, during glacial periods much of the water evaporated from the oceans is stored on the continents as glacial ice. This causes sea level to become lower. As the ice melts at the end of a glacial period, the water flows back into the oceans and sea level rises. Thus, the volume of ice on the continents is a major factor in controlling eustatic sea level. Global warming, for example could reduce the amount of ice stored on the continents, thus cause sea level to rise. Since water also expands (increases its volume) when it is heated, global warming could also cause thermal expansion of sea water resulting in a rise in eustatic sea level. Changing the shape of the oceans occurs if volcanic output on the sea floor or at oceanic ridges increases substantially, thus raising the floor of the oceans. Oceanic Currents Oceanic circulation is three dimensional. Most visible to humans are the surface ocean currents that are mainly driven by the wind. Vertical currents and deep ocean currents are driven by upwelling and downwelling near the coasts and differences in density, temperature and salinity between the surface waters and the deep ocean waters. Coastal Zones 7/17/2012Page 2 of 14 Docsity.com Waves that crash onto the beach are called breakers. Wave energy is dissipated by turbulence, which creates frothy white water in the surf zone. A surge of water (swash) rushes up the beach face. Gravity pulls the backwash down the slope of the beach. Rip currents form where water is channeled back into ocean. Wave refraction - Waves generally do not approach shoreline parallel to shore. Instead some parts of waves feel the bottom before other parts, resulting in wave refraction or bending. Wave energy can thus be concentrated on headlands, to form cliffs. Headlands erode faster than bays because the wave energy gets concentrated at headlands. Coastal Erosion and Sediment Transport Coastlines are zones along which water is continually making hanges. Waves can both erode rock and deposit sediment. Because of the continuous nature of ocean currents and waves, energy is constantly being expended along coastlines and they are thus dynamically changing systems, even over short (human) time scales. Coastal Zones 7/17/2012Page 5 of 14 Docsity.com Erosion by Waves As we discussed previously, the motion of waves is only felt to a depth of 1/2 times the wavelength. Thus, waves can only erode if the water along a coastline is shallower than 1/2 times the wavelength. But, when the wave breaks as it approaches the shoreline, vigorous erosion is possible due to the sudden release of energy as the wave flings itself onto the shore. Rigorous erosion of sea floor takes place in the surf zone, i.e. between shoreline and breakers. Waves break at depths between 1 and 1.5 times wave height. Thus for 6 m tall waves, rigorous erosion of sea floor can take place in up to 9 m of water. In the breaker zone rock particles carried in suspension by the waves are hurled at other rock particles. As these particles collide, they are abraded and reduced in size. Smaller particles are carried more easily by the waves, and thus the depth to the bottom is increased as these smaller particles are carried away by the retreating surf. Furthermore, waves can undercut rocky coastlines resulting in mass wasting processes wherein material slides, falls, slumps, or flows into the water to be carried away by further wave action. Transport of Sediment by Waves and Currents Sediment that is created by the abrasive action of the waves or sediment brought to the shoreline by streams is then picked up by the waves and transported. The finer grained sediment is carried offshore to be deposited on the continental shelf or in offshore bars, the coarser grained sediment can be transported by longshore currents and beach drift. Longshore currents - Most waves arrive at the shoreline at an angle, even after refraction. Such waves have a velocity oriented in the direction perpendicular to the wave crests, but this velocity can be resolved into a component perpendicular to the shore (Vp) and a component parallel to the shore (VL). The component parallel to the shore can move sediment and is called the longshore current. Beach drift - is due to waves approaching at angles to beach, but retreating perpendicular to the shore line. This results in the swash of the incoming wave moving the sand up the beach in a direction perpendicular to the incoming wave crests and the backwash moving the sand down the beach perpendicular to the shoreline. Thus, with successive waves, the sand will move along a zigzag path along the beach. Coastal Zones 7/17/2012Page 6 of 14 Docsity.com Coastal Processes Coastlines represent a balance between wave energy and sediment supply. If wave energy and sediment supply are constant, then a steady state is reached. If any one of these factors change, then shoreline will adjust. For example, winter storms may increase wave energy, if sediment supply is constant, fine grained beach sand may be carried offshore resulting in pebble beaches or cobble beaches. Due to input of sediment from rivers, marine deltas may form, due to beach and longshore drift such features as spits, tombolos, and bay barriers may form Types of Coasts The character and shape of coasts depends on such factors as tectonic activity, the ease of erosion of the rocks making up the coast, the input of sediments from rivers, the effects of eustatic changes in sea level, and the length of time these processes have been operating. Coastal Zones 7/17/2012Page 7 of 14 Docsity.com level rise since the last glaciation. Tidal Flats - These are zones along the coast that are flooded during hight tides and form in the intertidal zones lacking strong waves. They are common behind barrier islands or in estuaries. Coastal Variability The shape of coast is controlled mainly by tectonic forces and climate, both of which act to determine the elevation of the coast. Plate tectonic setting governs the style of coastline.In general, along passive margins where not tectonic activity is currently taking place, broad low lying coastal plains dominate. Along active margins, uplifted rocky coasts dominate. Emergent coasts result from local tectonic or uplift or from a drop in eustatic sea level. Emergent coasts are characterized by rocky coasts with sea cliffs and raised wave cut benches (marine terraces). The west coast of the U.S. is a rugged emergent coast with many sea cliffs and raised wave cut benches (marine terraces). In this case is due to a recent episode of uplift of the land relative to the sea by tectonic forces. The coast of New England is also an emergent case, but in this case, the rise of the land surface has been due to removal of glacial ice which had depressed the land during the last glaciation which ended about 10,000 years ago. Submergent coasts result from either subsidence along the coast due to tectonic forces or eustatic sea level rise. These are characterized by gentle shorelines, flooded river valleys (estuaries and fjords), and barrier islands. Much of the East Coast and Gulf Coast of the U.S. has these characteristics which are attributed to the rise of eustatic sea level since last glaciation. Coastal Hazards Sea Level Rise - Sea level is presently rising and the rate of sea level rise may increase due to melting of continental ice sheets like now cover Greenland and Antarctica. Human habitation of low-lying coastlines may be in jeopardy in the near future. Higher sea level will make these coastal areas more susceptible to other hazards, like storms and tsunami. Storms - great storms such as hurricanes or other winter storms can cause erosion of the coastline at much higher rate than normal. During such storms beaches can erode rapidly and heavy wave action can cause rapid undercutting and mass-wasting events of cliffs along the coast, as noted above. Note that the El Niño driven storms on the west coast caused extensive coastal erosion in 1998. High winds blowing over the surface of the water during storms bring more energy to the coastline and can cause more rapid rates of erosion. Erosion rates are higher because: During storms wave velocities are higher and thus larger particles can be carried in suspension. This causes sand on beaches to be picked up and moved offshore, leaving behind coarser grained particles like pebbles and cobbles, and reducing the width of the beach. During storms waves reach higher levels onto the shoreline and can thus remove Coastal Zones 7/17/2012Page 10 of 14 Docsity.com structures and sediment from areas not normally reached by the incoming waves. Because wave heights increase during a storm, waves crash higher onto cliff faces and rocky coasts. Larger particles are flung against the rock causing rapid rates of erosion. As the waves crash into rocks, air occupying fractures in the rock becomes compressed and thus the air pressure in the fractures is increased. Such pressure increases can cause further fracture of the rock. As we have seen in our discussion of tropical cyclones, wind driven storm surge and can flood coastal areas and cause much death and destruction. Tsunami - a tsunami is a giant sea wave generated by earthquakes, volcanic eruptions, or landslides, as we have discussed before. Such waves can have wave heights up to 30 m, and have great potential to wipe out coastal cities. Landslides - On coasts with cliffs, the main erosive force of the waves is concentrated at the base of the cliffs. As the waves undercut the cliffs, they may become unstable and mass-wasting processes like landslides or rockfalls will result. Massive landslides can also generate tsunami. Mitigating Against Coastal Hazards Shoreline protection can be divided into two categories: hard stabilization in which structures are built to reduce the action of the waves and soft stabilization which mainly refers to adding sediment back to a beach as it erodes away. Hard Stabilization Seacliffs, since they are susceptible to landslides due to undercutting, and barrier islands and beaches, since they are made of unconsolidated sand and gravel, are difficult to protect from the action of the waves. Human construction can attempt to prevent erosion, but cannot always protect against abnormal conditions. Two types of hard stabilization are often used. One type interrupts the force of the waves. Seawalls are built parallel to the coastline to protect structures on the beach. A seawall is usually built of concrete or piles of large rocks. Waves crash against the seawall and are prevented from running up the beach. Breakwaters serve a similar purpose, but are built slightly offshore, again preventing the force of the waves from reaching the beach and any structures built on the beach. The other type interrupts the flow of sediment along the beach. These structures include groins and jetties, built at right angles to the beach to trap sand and widen the beach. Coastal Zones 7/17/2012Page 11 of 14 Docsity.com While hard stabilization does usually work for its intended purpose, it does cause sediment to be redistributed along the shoreline. Breakwaters, for example cause wave refraction, and alters the flow of the longshore current. Sediment is trapped by the breakwater, and the waves become focused on another part of the beach, not protected by the breakwater, where they can cause significant erosion. Similarly, because groins and jetties trap sediment, areas in the downdrift direction are not resupplied with sediment, and beaches become narrower in the downdrift direction. Soft stabilization is primarily accomplished by adding sediment to the coastline, usually by dredging sediment from offshore and pumping it onto the coastline. Adding sediment is necessary when erosion removes too much sediment. But, because the erosive forces are still Coastal Zones 7/17/2012Page 12 of 14 Docsity.com