CHAPTER 18 - COASTS AND OCEANS Overview Wind energy is transferred to water surface and forms waves. Wave height (distance between crest and trough) is determined by the amount of energy transferred. Wave length is the distance between crests. As a wave passes, water molecules describe circular orbits, equal to wave height at the surface, but decreasing with depth until there is essentially no motion at a depth equal to half the wavelength. Waves break because friction with the sea floor causes the top of the wave to move faster and it eventually collapses producing surf. As waves approach shore, they change direction as the leading edges are slowed by contact with the bottom and the waves become more parallel to shore. This wave refraction produces longshore currents that flow parallel to the shore. Rip currents are localized narrow currents that flow perpendicular to shore in areas with low wave height. They may either be in fixed positions over channels or shift because of wave interference. Tides involve the regular rising and falling of the sea surface and are caused by the gravitational attraction between the Earth, the sun and the moon. Two bulges are produced on the surface of the oceans that move around the Earth and cause water levels to rise (flood tide) or fall (ebb tide), usually on a semidiurnal (twice a day) cycle. Tidal range is the difference in water level between high and low tide and can range from less than 1 m to over 16 m. Areas of the shoreline exposed between high and low tides form an intertidal zone. Beaches are strips of sand or gravel along which waves break and extend inland from the low water line to a cliff or vegetated area. Beaches can be subdivided into an offshore marine terrace, a beach face, and an upper berm. The form and dimensions of beaches change seasonally, with winter beaches being narrow, and summer beaches wider. Tremendous amounts of sediment are transported along the beach face by swash and backwash and immediately off shore by longshore drift. Spits, baymouth bars and tombolos are the natural products of longshore drift. Jetties and groins constructed by humans can also interrupt sand transport along a beach. The bulk of beach sand is transported to the coast by rivers and streams. Damming this source cuts off sand supply and promotes beach erosion. Erosional coastlines exhibit features such as sea cliffs, arches, stacks, wave- cut platforms and undergo progressive coastal straightening. Depositional coastlines are characterized by barrier islands, deltas, and tidal deltas. Drowned coastlines are submergent and identified by estuaries or fiords. Uplifted coastlines are emergent and marked by uplifted marine terraces. Coastlines can also be shaped by organisms such as reefs and mangroves. The ocean floor consists of the gently sloping continental shelf, the steeply sloping continental slope and the deep ocean floor. Passive continental margins are those in areas of little tectonic activity, such as the eastern coast of the US, while active continental margins are associated with volcanic and earthquake activity, such as the margins of the Pacific Ocean. Continental shelves lie on the edge of continents and are mostly covered with land-derived sediment, although in areas of little sediment input, reefs may form. Continental slopes link continental shelves to the deep ocean and are characterized by steep slopes and turbidite deposition. Submarine canyons cut into the edge of continental shelves and the upper part of continental slopes and allow the transport of large amounts of sediment by turbidity and bottom currents to be deposited on submarine fans on the continental rise at the base of the slope. Turbidity currents are sediment-laden current that move rapidly downslope under the influence of gravity. Contour currents are slow moving and flow along (parallel to) the slope. Abyssal plains are flat areas at the base of the continental rise, although some areas of the deep ocean floor are characterized by rugged topography. Sediments accumulating on the sea floor are either terrigenous (derived from land) or pelagic (settle from suspension. Pelagic sediments include clays and the skeletons of microscopic organisms. Learning Objectives 1. Wind energy is transferred to water surfaces to form waves. Wave height (distance from crest to trough) reflects wind speed, duration and distance. Wavelength is the distance between crests. Waves passing a point move water particles in a circular orbit. At the surface, diameter of the orbit equals wave height. At depth, effects of wave passage are lost below depths equal to half the wavelength. 2. Wave refraction is the change in direction along a wave crest as it comes in contact with the bottom while approaching shore. Wave refraction helps develop longshore currents that are flow parallel to the shoreline and transport considerable amounts of sediment in the surf zone. Rip currents flow perpendicular to shore and carry fine sediment offshore. 3. Tides are caused by changes in the elevation of the sea surface, usually on a semidiurnal cycle. Tides are caused by a force resulting from the gravitational attraction between the Earth, sun and moon. 4. Beaches are strips of sand or gravel that extend from marine terraces offshore to cliffs or permanent vegetation zones onshore. The beach face is the steepest part and is exposed to wave action. The berm extends landward from the beach face. 5. Summer beaches have wide berms, while winter beaches have narrow berms and sandbars offshore. Most sediment on beaches was brought to the coast by rivers and streams. Damming free flowing rivers reduces sediment supply to beaches and promotes beach erosion. 6. Longshore drift is the movement of sediment parallel to shoreline by a combination of swash and backwash along the beach face, and longshore currents. Spits are fingerlike ridges of sediment deposited into open water. Baymouth bars extend from headland to headland cutting off bays from the ocean. Tombolos connect offshore islands to the mainland. All three depositional features are the result of longshore drift processes. Jetties and groins are structures made by humans to interrupt sand being transported along the shoreline by longshore drift. 7. Coastal areas are classified as erosional, depositional, drowned, uplifted, or shaped by organisms. Erosional coasts are subject to coastal straightening (erosion on headlands and deposition in bays), and exhibit sea cliffs, wave-cut platforms, stacks and arches. 8. Depositional coasts have barrier islands, deltas, tidal deltas, and may preserve glacial deposits such as moraines. Fiords and estuaries are typical of drowned (submergent) coasts. Uplifted coasts exhibit uplifted marine terraces. Reefs and mangroves may also shape coasts. 9. Beyond the shoreline, the ocean floor consists of a continental shelf, continental slope and abyssal plain. Passive continental margins are those formed as rifted margins, whereas active continental margins are those along which tectonic activity occurs. 10. Continental shelves are usually covered with sediment derived from land and moved by currents and waves. Reefs are resistant bodies formed of coral and other organisms and develop in areas where sediment supply is low. Continental slopes are steeply sloping and link shelves with the deeper ocean. Submarine canyons are erosional features that cut into the outer part of shelves and the upper slope and transport sediment by turbidity currents. The continental rise is a wedge of sediment at the base of the slope and passes into the flat abyssal plain. 11. Ocean crust on the ocean floor is covered with either terrestrial sediment derived from land or pelagic sediment consisting of clay and microscopic organisms. Boxes 18.1 ENVIRONMENTAL GEOLOGY – COMMUNITIES AND COASTAL EROSION Communities built on shorelines often face severe problems with coastal erosion, particularly when shorelines consist of loose and easily eroded sediments. Erosion rates are particularly high in areas of Atlantic Canada and along the Pacific coast of British Columbia and have resulted in damage to docks, roads, homes and lighthouses. Coastal erosion is not restricted to marine areas and shorelines of the Great Lakes also experience high erosion rates. Canadian Arctic coasts are often protected from wave erosion by sea ice although damage can be considerable during years of sparse sea ice development. 18.2 ENVIRONMENTAL GEOLOGY - THE EFFECTS OF RISING SEA LEVEL Long term sea level rise is caused by melting glaciers, while short term sea level rise is caused by storm surge. Sea level has risen 130 meters during the past 15,000 years. Initial sea level rise was 1.3 meters per 100 years, but declined to 4 centimeters per 100 years. Since 1930, however, the rate of sea level rise has increased six-fold to 24 cm per century. This rapid rise caused barrier island migration, increased erosion and allowed storm waves access further inland than before. Even small increases in sea level have the potential for catastrophic flooding of large portions of the coastal United States because these areas are so flat and low. Melting all glacial ice would raise sea level by 60 meters drowning significant coastal areas. Short-term sea level rise occurs from storm surges caused by hurricanes that produce a doming of sea level as they pass. This situation is compounded by high winds that drive the seawater onto shore. Storm surges can exceed 5 meters and have caused considerable damage and loss of life in coastal areas. A storm surge of 8 meters struck Galveston, Texas, in 1900, killing 6,000 people. A 5 meter storm surge produced by Hurricane Hugo in 1989 caused $10 billion damage to South Carolina. Short Discussion/Essay 1. How much would sea level rise if 1) the north polar ice cap melted; 2) the south polar ice cap melted; 3) all glacial on earth melted? 2. How do tides affect the operation of coastal processes? 3. Explain the relationship between wave refraction, longshore currents and longshore drift. 4. Why is beach erosion such a problem along the Atlantic coast? 5. Explain how jetties and groins work. Longer Discussion/Essay 1. If sea level is actually rising, how can both emergent and submergent coasts be found around the world? 2. How might a geologist use the concept of effective wave base to differentiate between rocks deposited in "shallow" and "deep" settings? 3. Why would beach dimensions change with season? 4. Explain the influence of waves and wind on the character of beach sand (i.e. sorting, roundness, size differences). 5. Why are beach replenishment programs ultimately doomed to failure? Selected Readings Businger, Steven. 1991. "Arctic hurricanes", American Scientist 79 (1): 18-33. Davis, R.E. and Dolan, R. 1993. "Nor'easters," American Scientist 81 (5): 428- 439. Horton, T. 1993. "Hanging in the balance: Chesapeake Bay" National Geographic 183: 3- 35. Pilkey, O. H. 1989. "The engineering of sand. Journal of Geological Education 37: 308 -311. Richardson, P.L. 1993. "Tracking ocean eddies," American Scientist 81 (3): 261-272. Schlee, J.S., Karl, H.A. and Torresoan, M.W.. 1995. "Imaging the Sea Floor" U.S. Geological Survey Bulletin 2079. Schneider, D. 1997. "The rising seas," Scientific America 276: 112-117. Trenhaile, A.S., 1998. Geomorphology: A Canadian Perspective. Oxford University Press Canada, 340pp Walden, D. 1990. "Raising Galveston." American Heritage of Invention and Technology (winter): 8-18.