Oceans & Coastlines Introduction Depth of the Ocean Floor Salinity & Temperature of the Oceans Oceanic Circulation Coastlines Wave Action Shorelines & the Sediment Budget Shoreline Protection Summary The oceans are the planet's last great living wilderness, man's only remaining frontier on earth, and perhaps his last chance to prove himself a rational species. John L.Culliney [The coastal zone is] rich in a variety of natural, commercial, recreational, industrial, and aesthetic resources of immediate and potential value to the present and future well-being of the nation. Coastal Zone Management Act Introduction • • • Over two-thirds of Earth’s surface is ocean. Global climate patterns are influenced by oceanic circulation which in turn is controlled by the physical characteristics of the oceans. The evolution of coastal landforms depends on the interaction of wave action with processes in the continental interior and human actions. Over two-thirds (~71%) of the planet is covered in seawater. The world’s oceans are unevenly distributed and lie mostly in the Southern Hemisphere (88% seawater). The three major oceans (Pacific, Atlantic, Indian Oceans) are connected together along their southern margins by the Southern Ocean that encircles Antarctica. Smaller, enclosed or partially enclosed water bodies are termed seas or gulfs (Fig. 1). All of these features are created by plate tectonic processes and many continue to increase or decrease in size as plates diverge or converge (see the Plate Tectonics chapter for more on plate tectonics and the seafloor). It is in the various small seas that we can observe the most obvious negative consequences of human activity on the ocean realm in the form of overfishing, pollution, eutrophication, and habitat destruction. These same consequences are visited on the margins of the major oceans. Figure 1. The distribution of the world's oceans and selected seas. Nearly 90% of the Southern Hemisphere is covered by oceans. 2 Today we appreciate the oceans for their direct and indirect impacts on human activity. This chapter is divided into two halves. The first part considers the physical characteristics of the oceans and begins with the depth of the ocean floor. The floor of the ocean is a few hundred meters below sea level along the margins of the continents but averages over 3 km depth between the major oceans and reaches a maximum depth of 11 km in the western Pacific. The ocean floor has the potential to be a great source of mineral wealth in the future and mining companies are already staking claims to potentially lucrative sections of subsea real estate. Average ocean area and depth: Pacific Ocean 165,250,000 km2 4.28 km Indian Ocean 73,440,000 km2 3.89 km Atlantic Ocean 82,440,000 km2 3.33 km Figure 2. Damage from Hurricane Fran along the coast of North Carolina. Note change in position of house indicated by arrow. Images Ocean currents are influenced not only by the extent and depth of the oceans but also the salinity and temperature of ocean waters. Ocean waters are thought to have originated from meteorites and icy comets colliding with the early Earth. Both temperature and salinity change with depth and latitude and each is influenced by atmospheric weather patterns. Each factor is examined in the section titled salinity and temperature of the oceans. The final section of the first half of the chapter examines how depth, salinity, and temperature combine to generate characteristic patterns of oceanic circulation. Surface currents are controlled by dominant wind patterns that are in turn linked to the rotation of the planet. Deepwater circulation patterns are controlled by the density of ocean waters and the distribution of landmasses. Both sets of currents redistribute the Earth's heat budget and play a crucial role in controlling climate. In the second half of the chapter we focus on how human activity impacts the ocean margins and enclosed seas. Coastlines represent the fragile strip of land that borders the ocean. Developed coastal areas are threatened with potential loss of life and billions of dollars in property damage as a result of storm impacts and long-term erosion. For example Hurricane Fran (Fig. 2) devastated parts of several eastern courtesy of USGS Recent HighlightsHazards. 3 states and was especially destructive in North Carolina where winds of over 100 mph generated more than $5 billion in damages. The evolution of landforms along a coastline depends upon the interaction of wave action with the shore with processes in the continental interior and human actions. The coastline is a dynamic environment that advances or retreats depending upon the balance between the supply of sediment and the material removed by erosion. The section on shorelines and the sediment budget examines this balance. The National Park Service completed the tricky task of relocating the historical Cape Hatteras lighthouse to a site further inland in July 1999, to protect the light from erosion that threatened to topple the structure. Twenty-six of the thirty states bordering an ocean or Great Lake are presently experiencing net loss of their shorelines. Well-intentioned efforts at shoreline protection often resulted in the construction of coastal structures such as jetties, groins, or breakwaters that altered the natural movement of sediment along the coastline and simply exacerbated existing erosion problems. We finish by examining the interaction of shoreline erosion and shoreline protection efforts along the southern shore of Lake Erie. Depth of the Ocean Floor • • • The average depth of the ocean floor is nearly 4 km and the maximum depth is a little over 11 km along the Mariana Trench. Four principal depth zones can be identified in the oceans; continental shelf (and rise), abyssal plain, oceanic ridge, and oceanic trench. The oceanic ridge is a submarine mountain range that occupies much of the floor of the Atlantic Ocean. The depth of the ocean floor varies from sea level to a maximum of over 11 km along the Mariana Trench in the western Pacific Ocean. In contrast, the highest landform on the continents is Mt. Everest, which is approximately 9 km (5.6 4 miles) in elevation. In addition, the average elevation of the land surface is less than a kilometer but the average depth of the oceans is approximately 3.8 km (2.3 miles). We could dump the continents in the ocean basins and still have plenty of room to spare. Recent analysis of originally classified satellite data at the National Oceanographic and Atmospheric Administration's (NOAA) National Geophysical Data Center (NGDC) has allowed scientists to use slight variations in the elevation of the ocean surface to determine the topography of the seafloor (Fig. 3). Figure 3. Characteristics of the seafloor for the eastern Pacific and northwestern Atlantic Ocean basins. Map of seafloor topography from satellite altimetry from NOAA's National Geophysical Data Center. Topography of the Ocean Floor Beginning at the edge of the continents we can recognize four principal depth zones in the oceans (Fig. 4). The first depth level is the continental shelf, the shallow ocean floor (0-150 meters) immediately adjacent to continental land masses. The shelf slopes gently toward the ocean from the coast with maximum depths of a few hundred meters. The shelf may be a relatively wide zone (hundreds of kilometers) adjacent to passive margins (e.g., U.S. Atlantic Coast) or a narrow strip 5 (tens of kilometers) landward of subduction zones (e.g., west coast of South America). The width of the shelf increases when sea level rises and decreases during times of sea level decline. Rapid seafloor spreading associated during the Early Cretaceous period (138100 million years ago) caused an expansion of the oceanic ridge system, displacing water and raising sea level. In contrast, sea level declined during the most recent Ice Age (2 million -10,000 years ago) when substantial volume of ocean water was locked up in ice sheets. The Atlantic shore of the North American continent was located near the present edge of the continental shelf during the Ice Age. Beyond the shelf break the ocean floor steps down across the continental slope and continental rise oceanward of the shelf. The slope and rise mark the transition from the relatively shallow shelf to the second depth level, the deep ocean floor known as the abyssal plain (Figs. 4, 5). As its name suggests, the continental slope represents an increase in the gradient of the ocean floor from the adjoining shelf. The slope is often dissected by submarine canyons formed during times of lower sea level. The canyons transported sediment from the shelf to the continental rise. The rise is that section of the slope that has been a site of sediment accumulation, resulting in an decrease in slope gradient toward the abyssal plain. There is an unbroken transition from slope to rise, to abyssal plain along passive margins where the continental and oceanic crust make up part of the same plate. These margins are not characterized by the volcanism and earthquake activity that distinguishes active margins located along plate boundaries. Much of the abyssal plain lies over four kilometers below the ocean surface and represents the flattest portions of the Earth's surface. The plains are covered by layers of sediment precipitated in the oceans and are dotted with submarine volcanoes (seamounts). The ocean floor rises to a third level approaching the oceanic ridge system, a submarine mountain 6 Figure 4. Diagrammatic view of the principal features of the ocean floor include an elevated oceanic ridge; deep, narrow trenches; and a gradual rise of the ocean floor to the continents along passive margins. Note narrow shelf along active margin, broad shelf along passive margin. Vertical and horizontal dimensions of some elements have been exaggerated. Figure 5. Principal topographic features of the floor of the southern Atlantic Ocean. The oceanic ridge occupies more than half the width of the ocean floor. Image modified from original at NOAA's National Geophysical Data Center. chain that can be traced around the world. The ocean floor is relatively shallow (less than 3 km) along the ridge system. The ocean ridge system dominates the floor of the Atlantic Ocean, occupying over half its width. The final depth level is apparent in the narrow oceanic trenches found along active margins marked by the boundary between two plates (see Plate Tectonics chapter). The trenches mark the locations of subduction zones where oceanic lithosphere descends into the mantle. Trenches, the deepest areas on the ocean floor, record depths of 7 to 11 kilometers (47 miles). Think about it . . . 1. Examine the maps and diagrams of ocean floor topography at the end of the chapter and answer the related questions. 2. Label as many features as you can in the image of the ocean floor adjacent to Monterey Bay, California, found at the end of the chapter. Salinity and Temperature of the Oceans • • • Salinity in surface waters is controlled by currents and temperature and averages approximately 35 parts per thousand. Salinity values are relatively uniform in well-mixed surface waters of the open oceans but are more extreme in restricted waters of coastal seas. Salinity and temperature change with depth with the most rapid change occurring in a depth zone labeled the halocline. 7 • • Ocean temperatures of 27oC are typical of tropical surface waters and temperatures of 2oC are typical for deep ocean waters. Cold water is more dense than warm water, but ice (frozen solid water) is less dense than liquid water. Salinity and Latitude Seawater contains dissolved salts. The concentration of salt in seawater is salinity. Salinity varies around the world's oceans depending on temperature and the mixing action of ocean currents. Salinity is measured in parts per thousand (ppt; 10 ppt = 1%) of salt in water. The salinity of the warm, wellmixed surface waters over much of the world's major ocean basins ranges from 33 to 37 parts per thousand (Fig. 6). Figure 6. Map of salinity at the ocean surface. Numbers represent salinity values in parts per thousand. Salinity in the open ocean is greatest in tropical regions and decreases in the isolated Arctic Ocean. Map generated at University of Tokyo website. Higher- and lower-salinity values are observed in smaller, restricted ocean basins and seas (Fig. 7). For example, salinity values of 20 to 30 ppt are recorded for the high-latitude Arctic Ocean and values of over 40 ppt occur in the narrow tropical Red Sea basin between north Africa and the Arabian peninsula. Salinity is higher at low latitudes because high temperatures at these locations promote evaporation which removes water but leaves the salt it contains behind. Salinity values are lower at high latitudes because of the lack of evaporation, high precipitation, and the influx of freshwater from melting ice sheets. The isolated Baltic Sea between Sweden and Finland has salinity values that approach freshwater along its northern shore. 8 Figure 7. High and low salinity in restricted seas in low (Red Sea) and high (Baltic Sea) latitudes. Maps generated at University of Tokyo website. Salinity and Depth Salinity values are variable in the shallow (e.g. 0-200 m) ocean but are much more uniform in deeper waters below 2,000 meters (6,600 feet; Fig. 8). Salinity may decrease with depth in the tropics but increases with depth at high latitudes (+60oN/S). The salinity in the Arctic Ocean (north of 70oN latitude) increases with depth from 30 to 35 ppt. Salinity in this relatively isolated ocean basin remains uniform below a depth of approximately 300 meters (1,000 feet). The change of salinity occurs over a depth zone known as the halocline. The depth range for the halocline is from approximately 200 to 1,000 meters (660-3,300 feet) but will show some variation with location. Salinity is uniform with a value of 34 to 35 ppt below the halocline. Temperature and Latitude Solar radiation strikes Earth more directly at the equator and tropics than in polar regions (Fig. 9). Radiation strikes Earth at a lower angle near the poles and the Sun’s rays must therefore penetrate a greater thickness of atmosphere. Some of the solar radiation is scattered in the atmosphere and more heat energy is lost near the poles as a result of scattering. Earth's surface at 9 Figure 8. Northsouth profile through the Pacific Ocean along the 155.5 meridian illustrating the range of salinity with depth and latitude. Numbers represent salinity values in parts per thousand. Cross section generated at University of Tokyo website. the equator receives 2.5 times more insolation, incoming solar radiation, than the atmosphere above the poles. The highest average annual ocean temperatures (~27oC) are present along the equator and temperatures decrease symmetrically to the north and south approaching 0oC at high latitudes (Fig. 10). Water has two relatively unusual thermal properties that make the oceans a great storage reservoir for heat energy and contribute to global oceanic circulation patterns. First, the heat capacity of a material is measured as the amount of heat required (in calories) to raise the temperature of 1 gram of the substance by 1oC. Materials with high heat capacity, such as water, can absorb substantial quantities of heat without any significant change in temperature. The ability of the oceans to store heat plays a crucial role in controlling global climate patterns. 10 Figure 9. Solar radiation is distributed over a wider area and must penetrate a greater thickness of atmosphere at the poles, reducing the amount of solar energy reaching Earth's surface. Consequently, ocean temperatures are greater near the equator. Second, cold water can be both less dense and more dense than warm water. Water density increases as water temperature decreases down to approximately 4oC. Below that temperature water density decreases, especially when water changes state from a liquid to solid (ice) form. Consequently, dense cold water can sink below less dense warm water but ice will float on the ocean's surface. Figure 10. Map of world's oceans illustrating the average annual range of temperature with latitude. Numbers represent temperature in degrees Celsius. Map generated at University of Tokyo website. Temperature and Depth The major oceans can be divided into layers of relatively warm waters at shallow depths and cold waters at greater depths (Fig. 11). Surface waters are warmed by solar radiation and currents cause thermal mixing that results in relatively uniform temperature distributions by latitude. Sunlight doesn't penetrate more than a few hundred meters below the ocean surface and the impact of current activity diminishes with depth. Temperatures exceed 20oC over much of the tropical ocean's surface but decline to a chilly 2oC below 2,000 meters (6,600 feet) depth. The depth zone in which temperature decreases rapidly is known as a thermocline. The base of the thermocline is at a depth of approximately 1,000 meters (3,300 feet). 11 Figure 11. Northsouth profile through the Pacific Ocean along the 155.5 meridian illustrating the range of temperature with depth and latitude. Cross section generated at University of Tokyo website. Think about it . . . Use the data in the thermocline exercise at the end of the chapter to plot two ocean temperature profiles and answer the questions that follow. Oceanic Circulation • • • • 12 Surface ocean currents are driven by winds and involve only 10% of ocean waters. Oceanic circulation patterns generate current systems known as gyres. Fast-flowing western boundary currents redistribute heat from the relatively warm tropics to cooler high latitudes The Coriolis effect is the name of the apparent deflection of ocean currents or winds to the right of their course in the Northern Hemisphere and to the left of their course in the Southern Hemisphere. • The global conveyer belt moves heat energy from the tropics to the poles in surface waters and transports cold waters to warmer location by deep ocean circulation. Ocean Currents Ocean surface currents are mainly controlled by climate (temperature, winds) but are also influenced by the distribution of continents and Earth's rotation. Surface currents involve approximately 10% of the world's ocean waters. Sea level is higher at the equator because of thermal expansion of warm waters and diminishes toward the poles. The contrast in the elevation of the ocean surface is about 15 cm (6 inches). In the absence of winds, water would simply flow away from the equator ("downhill") under the influence of gravity. Winds blowing over the ocean exert a frictional drag on surface waters and are the principal force in controlling oceanic circulation. Ocean currents follow wind directions except where wind blows onland. The continents represent barriers to currents, deflecting them to the north or south of their course (Fig. 12). Figure 12. Distribution of ocean currents. Note circular patterns (gyres) with clockwise pattern north of equator and counterclockwise pattern south of equator. Global atmospheric circulation patterns generate circular ocean current systems known as gyres that are centered on 30 degrees latitude in each of the major ocean basins (Fig. 12). Circulation of the gyres is clockwise in the Northern Hemisphere and counterclockwise in the Southern Hemisphere. Surface water might take several months to a few years to complete the circuit of a gyre. 13 Westerly winds cause water to pile up along the western sides of major oceans. These concentrations of surface water generate fast-flowing western boundary currents that redistribute warm tropical waters toward the poles (Fig. 12). These currents (e.g., Gulf Stream, Kuroshio, Brazil) can be thought of as marine rivers, relatively narrow (less than 100 km across) water masses that flow at speeds of 100 to 200 km/day for thousands of kilometers. The Gulf Stream can transport over 50 million cubic meters of water per second, hundreds of times more water than the Amazon, the world's largest river. In contrast, the eastern boundary currents (e.g., Canary, California, Peru) that complete the eastern leg of each gyre are wider, carry less water, and move more slowly. The Canary current, nearly 1,000 km (625 miles) wide, carries just a third of the volume of water in the Gulf Stream and travels at tens of kilometers per day. Coriolis Effect Currents are deflected to the right of their course in the Northern Hemisphere and to the left of their course in the Southern Hemisphere: this pattern is termed the Coriolis effect (Fig. 13). To an observer on earth, the path of a north- or south-directed wind or ocean current will appear to be deflected. Note that the wind or current doesn’t actually change direction, but the planet beneath it has changed position. An object (rocket, air mass, ocean current, etc.) that travels directly north or south in the Northern Hemisphere appears to be deflected to the right of its course when viewed from a location on the solid 14 Figure 13. Objects on Earth's equator travel further (and faster) than objects at higher latitudes. It is this contrast in velocity that results in the Coriolis effect. Objects moving north from the equator have a greater component of eastward motion than objects at higher latitudes and thus appear to deflect to the right of their course. Earth's surface. Objects are deflected to the left of their course in the Southern Hemisphere. The net result of these deflections is the circular path of ocean currents. Global Ocean Conveyer Belt Surface ocean currents carry warm water away from the equator and toward the poles. Deeper currents are driven by contrasts in water density and are dependent upon temperature and salinity contrasts below 1,000 meters. The pattern of deep currents is termed thermohaline circulation. Currents in the North Atlantic cool as they approach the northern latitudes. Cold, salty (dense) water sinks in the North Atlantic Ocean south of Greenland and moves southward as the North Atlantic Deep Water (NADW) current at depths of 2 to 4 km (1-2.5 miles; Fig. 14). When the NADW reaches Antarctica it is diverted to the Indian and Pacific Oceans by the Antarctic circumpolar current. The deep water current eventually comes to the surface (upwelling) in the northern Indian and Pacific Oceans before returning to the Atlantic Ocean by a series of surface currents (Fig. 14). A complete loop may take 1,000 years. Figure 14. Global oceanic circulation. Cold water sinks in northern Atlantic Ocean and travels southward in deep water before upwelling in the Indian and Pacific Oceans. Surface currents return warm water to Atlantic Ocean. The sinking of this cold, dense water in the North Atlantic is a key step in the global conveyer belt. This system moves energy from the tropics to the poles and back again and serves to moderate Earth's climate. 15 Think about it . . . 1. A shipment of rubber elephants falls overboard in the northern Pacific Ocean at location A on the map below. What path do the elephants subsequently follow? a) A to G to B to F to E to A c) A to G to C to E to A b) A to E to C to G to A d) A to E to F to B to G to A 2. How would the deflection of ocean currents be altered in the Northern Hemisphere if Earth’s rotation changed direction from west to east to east to west? Complete the statement below using one of the choices that follow. Ocean current directions would ____________ because currents would be deflected to the _______________. a) stay the same; right of their course b) stay the same; left of their course c) switch direction; right of their course d) switch direction; left of their course Coastlines • • 16 Over a quarter of the U.S. population lives in counties along the Atlantic or Gulf Coasts. Ten hurricanes have inflicted over a billion dollars of damages on sites along the East Coast since 1980 and Hurricane Andrew was the most expensive natural disaster in U.S. history. • • Coastal landforms are dependent on the interaction of wave action with fluvial processes in the continental interior and human activity. Long-term processes such as climate cycles (hundreds of years) and tectonic history (thousands to millions of years) can raise or lower sea level or coastlines, respectively. Author Henry Beston considered the sound of the ocean on a shore to be one of the three great elemental sounds of nature (the other two where the rain, and the wind in woods). Beston should have known, he built a small cabin and lived alone on Cape Cod, Massachusetts, for a year, recording his observations of the changing coastline in a highly regarded book, The Outermost House (1928). Our culture places a premium on living along the fringe of the continent facing the ocean. Over 70 million people live in counties along the Atlantic and Gulf Coasts and coastal property values in Florida rose by over 50% in the last 10 years. In an effort to ensure shoreline access to the public, Cape Hatteras, North Carolina, was designated (August 17, 1937) the nation’s first national seashore. Other public seashores followed, including Cape Cod (August 7, 1961). The establishment of Cape Cod represented an early effort to create parklands that were within easy access of millions of Americans. Figure 15. Hurricane Fran approaches the U.S. coastline. Image from Goddard Space Flight Center's Public Photographic Image Retrieval System. Developed coastal areas are threatened with potential loss of life and billions of dollars in property damage as a result of storm impacts and long-term erosion. The weather patterns that bring balmy breezes on warm summer days can also generate devastating hurricanes (Fig. 15). There were 30 U.S. weather disasters that resulted in over a billion dollars in damages between 1980 to 1997. A third of these events were hurricanes or tropical storms that battered the Atlantic or Gulf Coast 17 states. Hurricane Andrew (August 24, 1992) decimated southern Florida and was the most expensive natural disaster in U.S. history (58 deaths, $30 billion damages). Many small insurance companies went bust following Andrew. State Farm, the nation’s largest insurance company, shelled out $3.7 billion in claims. Many in the insurance industry are nervous of a financial meltdown following a cataclysmic hurricane that generates damages similar to those of an unnamed storm that killed over 8,000 people in Galveston, Texas, September 8, 1900. Figure 16. Rocky coastline with headlands and beaches characteristic of U.S. West Coast (top) and sandy coastline with lowlying beaches typical of the East Coast (bottom). Images courtesy of NOAA photo collection. The short-term evolution of landforms along a coastline represents the interaction of wave action with fluvial processes in the continental interior and human activity. The coastline is a dynamic environment that advances or retreats depending upon the balance between the supply of sediment and the material removed by wave erosion. This balance may be upset by geologic processes that act at a variety of time scales. Seasonal variations in stream flow and storm activity affect the volume of sediment supplied to the coast and the rate of erosion. Climate cycles that result in increasing or decreasing sea levels will have long-term effects measured in decades or centuries. Finally, tectonic cycles measured in hundreds or thousands of years may continually revitalize rugged coastlines by periodic uplifts. The role plate tectonics plays in influencing the physical character of the coastline is exemplified by the contrast between the sandy beaches of the Atlantic shore (passive margin) and the rocky headlands of the active margin represented by the Pacific Coast (Fig. 16). 18 Some areas in the Gulf of Mexico coastline are actively subsiding. Sediment deposited in a delta at the mouth of the Mississippi River is submerged below sea level during compaction. Subsidence rates are approximately 1 meter per century. In the past this subsidence was compensated by additional sediment supplied during flood events. However, the construction of levees along the river’s channel prevents the redistribution of sediment during flooding. Figure 17. High and low locations of the Atlantic coastline over the last 5 million years. Image from USGS Coasts in Crisis. Sea level has fluctuated considerably during the geological past. It was ~100 m (330 feet) lower during the last ice age when some of the water present in today’s oceans was locked up in ice sheets (Fig. 17). Today sea level is believed to be increasing at a rate of 10 to 15 cm (4-6 inches) per century in response to the global warming which causes melting of ice sheets and thermal expansion of ocean waters. Increasing sea levels will have a much more significant impact on the lowlying East Coast than the rugged West Coast. Some islands in Chesapeake Bay, between parts of Virginia and Maryland, that were inhabited by colonists four centuries ago are now submerged below the waters of the estuary. 19 Wave Action • • • • • Wave size, speed, and direction are controlled by winds. The wave shape moves but the water within it does not travel. Wavelength is the distance between adjacent waves. Waves increase height and break along the shore when the depth of water diminishes to less than the wave base. Waves are refracted toward headlands. Wave Motion Shoreline erosion is tied to the interaction between waves and the coastline. Wave action erodes, transports, and redistributes sediment along the shoreline. Wave size, speed, and direction are controlled by winds. Water does not travel with waves but simply moves vertically, tracing a circular path as a wave passes. It is the shape of the wave (the waveform) that moves across the ocean surface, not the water itself (Fig. 18). Consider the "wave" performed by a crowd at a sporting event. The wave passes around the stadium as each individual in turn stands up and sits down. The people in the stadium play the role of the water particles in ocean waves. Figure 18. Sequential wave motion in open water. The waveform moves while water particles follow a circular path and remain in place. The distance between adjacent wave crests is termed the wavelength. Wave motion only affects the surface waters. Wave motion decreases downward, with increasing distance from the winds, to the wave base. The depth of the wave base is approximately half the wavelength (Fig. 19). 20 Figure 19. Water particle motion decreases downward from waves on the ocean surface ending at the wave base. Waves and Coastlines Water piles up as waves approach shore because of the effect of friction between the wave and the seafloor above the wave base (Fig. 20). Water in contact with the seafloor is slowed by friction but water on the surface is unaffected and moves forward more rapidly, encroaching on preceding waves and piling up to form taller, steeper waves. Figure 20. Waves steepen and wavelength decreases as waves approach shore. Steepened waves eventually collapse (break) forming surf that surges up the slope of the shore. The wave eventually collapses (breaks) forming surf that washes up the shore before flowing back down to sea. Turbulent flow in the surf zone, between the line of breaking waves and the shore, can cause erosion by washing sand particles from beaches and breaking rocks from headlands. Some of this material may also be transported along the shore by currents in the surf zone. Waves approaching a coastline are reoriented to follow the slope of the seafloor, this process is termed wave refraction. The seafloor will shallow more rapidly toward headlands than adjoining bays and waves approaching a rugged coastline will be refracted toward the resistant headlands (Fig. 21). One result of this pattern is that wave erosion is concentrated on headlands while adjoining bays become areas of deposition. Sediment eroded from the headlands are deposited in the relatively quiet waters of the bays to form beaches. The coastline is straightened as erosion wears away the headlands and the bays are filled with sediment (Fig. 21). 21 Renewed tectonic activity may result in uplift of the coastline and another cycle of erosion. The rugged coastline of Oregon and Washington is constantly revitalized as part of an active plate boundary separating North America and the small Juan de Fuca Plate. The coastline is dominated by rocky headlands separated by small (pocket) beaches. Figure 21. Wave erosion is concentrated on headlands (top) and bays become areas of deposition (left). The coastline is straightened (bottom right) as erosion continues as the headlands are eroded back and the bays are filled with sediment. Wave erosion associated with large storms can remove large sections of beach in a single storm (Fig. 22). Recent hurricanes on the East Coast and El Nino-induced storms along the West Coast were responsible for substantial coastal erosion. Figure 22. Beach erosion near San Diego, California, following winter storms (1997) associated with the El Nino weather phenomenon. Top image taken in October 1997; bottom image from April 1998. Images courtesy of USGS Center for Coastal Geology. 22 Shorelines and the Sediment Budget • • • • Twenty-six of the thirty states bordering an ocean or Great Lake are presently experiencing net loss of their shorelines. Waves that strike a beach at an angle will generate a longshore current that transports sediment parallel to the beach in the surf zone. The sediment budget is the balance between material added to the shore by deposition and the material removed by erosion. Sediment entering the shoreline system is frequently reduced by human activity (dams, development) and is also subject to changing natural conditions (drought). Shoreline Erosion The short-term evolution of landforms along a coastline represents the interaction of wave action with fluvial processes in the continental interior, and human activity. The coastline is a dynamic environment that advances or retreats depending upon the balance between the supply of sediment and the material removed by wave erosion. Twenty-six of the thirty states bordering an ocean or Great Lake are presently experiencing net loss of their shorelines (Fig. 23). The most Figure 23. Coastal erosion rates along the Atlantic, Gulf, and Great Lakes shorelines. Image modified from original in Coasts in Crisis, an on-line USGS publication. 23 rapid erosion rates are along the Gulf Coast (Louisiana) and Atlantic shore (South Carolina, Maryland, New Jersey). Erosion rates in Louisiana are as high 20 meters per year. Much of the coastline of Louisiana is represented by sediment deposited in a delta at the mouth of the Mississippi River. The delta is submerged below sea level as the sediment becomes compacted. Subsidence rates are approximately 1 meter per century. In the past this subsidence was compensated by additional sediment supplied during flood events. However, the construction of flood control levees along the river’s channel across the delta has robbed the delta of its primary source of sediment. Erosion is accelerated by the actions of storm surges, violent waves associated with large storm events. Erosion is most effective on unconsolidated sediments of beaches or dunes. Rocky shorelines are less susceptible to erosion. Hurricanes generate waves that can destroy eastern beaches in a matter of hours. West Coast erosion (Fig. 24) is particularly severe in association with winter storms, especially those occurring during an El Nino year when sea level is higher than normal. Typical erosion rates for part of the California coastline near San Francisco are 0.2 meters per year. Yet, winter storms during 1997-1998 caused some cliff lines to recede over 10 meters (33 feet), more than 50 times faster than normal. 24 Figure 24. Retreating shorelines at Pacifica (top) and Monterey Bay (bottom left), California. Twelve homes were condemned when the cliff at Pacifica retreated 10 meters because of winter storms (1997-1998). The graph illustrates the magnitude of shoreline retreat (15 meters) at the Monterey Bay example. Lower image and original graph courtesy of USGS Center for Coastal Geology; upper image from NASA Airborne Topographic Mapper site describing West Coast projects. Sediment Transport Wave action causes the erosion, transportation and redeposition of sand along the Atlantic shore. In this environment, the relative orientation of the waves to the beach controls the distribution of erosion and deposition. Waves that strike a beach at an angle (Fig. 25) will generate a longshore current that transports sediment parallel to the beach in the surf zone (Fig. 26). In addition, sediment on the beach is also transported laterally parallel to the shoreline. Sediment is carried up the beach parallel to the direction of wave motion. Water washes back down slope carrying the sediment parallel to the slope of the beach. If the waves strike the beach obliquely the sediment is transported along the beach in a zigzag pattern. Both mechanisms ensure that sediment is transported along the length of the beach (Figs. 26, 27). Figure 25. Waves strike a Washington beach obliquely, generating a longshore current that transfers sediment from left to right along the shore. Image courtesy of USGS Center for Coastal Geology. Figure 26. Longshore currents are generated in the surf zone where waves strike obliquely against the shoreline. Sand particles are moved along the beach in a zigzag pattern and sediment is transported along shore. 25 Sediment transported along the beach can give rise to some characteristic landforms when it is eventually deposited in calm waters of adjoining bays (Fig. 28). Sediment may block the entrance to the mouth of a bay to form a baymouth bar or may only partially block a channel to form a landform termed a spit. Figure 27. Addition of sand to a beach as a result of longshore currents generated during winter storms, Tomales Bay, north of Point Reyes, California. Left: Prior to storms. Right: After storms, sand was transported along the shoreline from the left side of the image. Images courtesy of USGS Center for Coastal Geology. Figure 28. Matagorda Bay, Texas. The bay has been almost cutoff from the Gulf of Mexico by narrow baymouth bars formed from sediment transported along the coastline by longshore currents. Image courtesy of NASA's Earth from Space. Sediment Budget The beach (or shoreline) is not the final resting place for the sediment. It is an intermediate stop on a longer journey. Sediment is transported to the coast by streams, redistributed along the coast by longshore currents, and eventually deposited offshore. Shoreline processes are influenced by the sediment budget: the balance between material added to the shore by deposition and the material removed by erosion. Sediment added to the shore comes from headland erosion or is delivered to the coast by stream flow (Fig. 29). Much like a financial balance sheet, the sediment budget remains in a state of equilibrium as long as the sediment coming in is equal to the material that is lost. However, the material entering the shoreline system is 26 Figure 29. Sediment supply along a coastline can be disrupted by human activity such as dredging or building jetties to prevent in-filling of bays or stream channels; or by shoreline development that results in the construction of seawalls or breakwaters to prevent erosion (and sediment production) that threatens homes and other buildings. frequently reduced by human activity and is also subject to changing natural conditions (Fig. 29). The construction of dams on major rivers will reduce the volume of sediment reaching the coast, resulting in sediment starvation. Sediment that would once have been deposited along the shoreline is trapped in upstream reservoirs. Dams within the Mississippi River drainage basin have reduced sediment supply to the Mississippi delta by approximately half. Drought conditions may also reduce streamflow and thus diminish sediment transported to the shore by streams. Coastal development may result in the construction of structures designed to reduce erosion (e.g., breakwaters, seawalls) or to control the local depositional patterns to prevent infilling of navigation channels (Fig. 29). The loss of sediment from any of these sources may cause longshore currents to cannibalize existing beaches and cause a short circuit in the natural cycle of shoreline erosion, transport and deposition. The net result is the loss of beaches and increased erosion of the shoreline, especially during large storms. Think about it . . . Scientists surveyed the Californian coastline to evaluate erosion and/or accretion associated with winter storms of 1997-1998. Two images of the coastline around Ventura, California, are presented at the end of the chapter that show the coastline before and after the storms. Answer the related questions. 27 Shoreline Protection • • • • • • Structures built to protect coastlines may prevent erosion of part of the shoreline but can result in accelerated erosion elsewhere. Seawalls, groins, and breakwaters differ in their locations and orientations relative to the shoreline but all act to prevent erosion and/or encourage deposition. Artificial beach nourishment occurs when sand is dredged and pumped onto the beach from offshore. Lake Erie is the shallowest of the Great Lakes and is surrounded by the large population centers along U.S. and Canadian shores. Most of Lake Erie's shoreline is eroding and erosion rates are greatest where glacial deposits form the shoreline and are least where bedrock forms the coast. Presque Isle in the east basin of Lake Erie is eroding because of shoreline protection measures in the central and west basins. Techniques that attempt to prevent beach erosion revolve around methods to limit the removal of sediment along specific areas of the coast or involve adding material to areas undergoing erosion. Unfortunately, nearly all these methods have shortcomings. Most of these methods aim to prevent erosion but some recent regulations have recognized that erosion will inevitably occur and have instead focused on controlling construction adjacent to eroding coasts. Florida introduced strict regulations that required buildings constructed near the shoreline to meet rigorous standards to prevent destruction from storm surges or high winds. No buildings constructed to these standards failed when Hurricane Opel struck southern Florida in 1995. In contrast, 56% of all other habitable buildings in the storm's path were heavily damaged. Seawalls Sea walls are built to protect shoreline property owners from receding shorelines (Fig. 30). As such, they represent a barrier between waves and the shoreline. Waves are reflected back from the walls onto the adjoining beach and may promote beach erosion. Unfortunately, erosion is often exaggerated where the seawall ends, causing the shoreline to recede more rapidly on either side of the structure. 28 Figure 30. Seawall (left) at base of eroding cliff, north of Monterey, California. Note how erosion is exaggerated where the seawall ends. Image courtesy of USGS Center for Coastal Geology. Groins Groins are wall-like structures built along beaches to act as barriers to longshore currents (Fig. 31). A longshore current will lose velocity as it meets the groins, causing the current to deposit part of its sediment load on the upcurrent side of the groin, thus building up the adjacent beach. However, as the current passes the groin it picks up additional sediment on the downcurrent side of the structure causing local erosion. Figure 31. A groin adjacent to Cape Hatteras lighthouse, North Carolina, prior to relocation of lighthouse (1999). Image courtesy of USGS Center for Coastal Geology. Breakwaters Breakwaters are barriers built offshore to protect part of the shoreline (Fig. 32). They act as obstacles to waves, preventing erosion and allowing the beach to grow behind the structure. However, the beach behind the breakwater often grows at the expense of the adjacent unprotected shoreline. Artificial Beach Nourishment Artificial beach nourishment occurs when sand is dredged and pumped onto the beach from offshore (Fig. 33). The beach will 29 Figure 32. Breakwaters, south shore of Lake Erie, Maumee Bay State Park, Ohio. Image courtesy of the U.S. Army Corps of Engineers. grow if material is added to the beach faster than natural processes remove it. This is a temporary fix because the sand is eroded again and must be replaced. Material added to many East Coast beaches remained for less than two years before the beach returned to its prenourishment state. One successful effort was for Miami Beach, Florida, which spent $64 million in the 1970s to stabilize and expand its beaches to meet the needs of the booming tourism industry. Figure 33. Beach nourishment project on Ocean City Beach, Maryland. Note wider beach near bottom of image. Pipeline pumps sand collected offshore by a dredge onto beach. Images courtesy of the U.S. Army Corps of Engineers Digital Images Library. Example: Lake Erie Lake Erie is the shallowest of the Great Lakes and was formed when glaciers scoured out a depression in the bedrock during the last Ice Age. The lake is the 11th largest in the world and represents the "North coast" of Ohio, Pennsylvania, and western New York. It covers over 26,000 km2 and has a maximum depth of 64 meters (Fig. 34). The lake is divisible into three separate basins that increase in depth from west to 30 east. Regional flow in the lake carries water from west to east but local currents may reverse that direction. Figure 34. Map of Lake Erie. The map was modified from a bathymmetry map at the Great Lakes Forecasting System website. Figure 35. Below right: Erosion of the southern shore of Lake Erie at Painesville-on-theLake, Ohio. Note how roads end at top of receding cliff-line. Below left: The view from the lake (north) toward the shoreline (south). Image courtesy of Dr. Charles Carter. Ninety-five percent of the lake's shoreline in Ohio is eroding (Fig. 35). Average erosion rates are 10 to 80 cm per year (0.42.7 feet per year) but rates of up to 33 meters (100 feet) per year have been recorded. Rates are largely controlled by the geology of the coastline. More resistant rocks such as sandstone erode slowly whereas glacial sediments and weaker rocks erode more rapidly. Economic losses from damages to structures are estimated to be millions of dollars per year. Erosion of the shorelines of Lake Erie is evident in the image below that shows how the coastline receded southward, eroding the land along the northern edge of this subdivision. The white lines are roads. Notice how the east-west trending road near the center of the image is truncated by the cliff. Further information on the costs and effects of coastal erosion in Ohio are provided at the Ohio Geological Survey website. Development around the Great Lakes has covered much of the land area, reducing sediment sources. Population around Lake 31 Erie alone has climbed from 3 million to over 14 million people today. Erosion control measures in the western half of the lake have reduced sediment supply and resulted in increased erosion rates along the shoreline in the east basin. Presque Isle is an unusual sand deposit that built outward from the Lake Erie shoreline near Erie, Pennsylvania (Fig. 36). The construction of coastal structures in Ohio to the west blocked the eastward flow of sediment needed to replenish the deposit. The narrow neck that connects the island to the mainland is eroding as fast as 2.5 meters per year. The U.S. Army Corps of Engineers has the responsibility of coming up with a plan to protect Presque Isle. Figure 36. Top: View of Presque Isle, Pennsylvania, from the southwest looking along the shore of Lake Erie to the northeast. Erie, Pennsylvania, is to the right of the image. Current directions along the shoreline are from the bottom of the image toward the top. Below: Breakwaters and groins, Presque Isle. Deposition occurs behind breakwaters that form barriers to onshore currents. Think about it . . . Create a concept map that illustrates the characteristics of sediment erosion, deposition, and transport along the shoreline and the factors that affect these processes. 32 Summary 1. How much of Earth's surface is covered by oceans? Approximately 71% of the planet is covered by oceans. There are three major oceans (Indian, Pacific, Atlantic) that are connected along their southern margins by the Southern Ocean circling Antarctica. 2. How does the depth of the oceans vary? The average depth of the ocean floor is nearly 4 km and a maximum depth of a little over 11 km has been recorded along the Mariana Trench in the western Pacific Ocean. Four principal depth zones can be identified ranging from the shallow shelf along the continental margins (~100s meters), to the near horizontal floor of the abyssal plain (4-5 km), rising to the crest of the oceanic ridge (~3 km), and descending to the narrow depths of the oceanic trenches (7-11 km). 3. What factors control variations in the salinity of the oceans? Seawater contains dissolved salts. The concentration of salt in seawater is salinity. Salinity is measured in parts per thousand (ppt; 1 ppt = 0.1%, 10 ppt = 1%) of salt in water. Salinity varies depending on temperature and the mixing action of ocean currents. Salinity is higher at low latitudes because high temperatures at these locations promote evaporation which removes water but leaves the salt it contains behind. However, the mixing action of ocean currents ensures a consistent salinity range of 33 to 37 parts per thousand for much of the open ocean. 4. Where are salinity values highest and lowest? Salinity values are most extreme in restricted ocean basins where the effects of evaporation or stream inflow are exaggerated. Salinity values of over 40 ppt occur in the narrow tropical Red Sea basin between north Africa and the Arabian peninsula. Salinity is lower at high latitudes because of the lack of evaporation, high precipitation, and the influx of freshwater from melting ice sheets. Salinity values of less than 10 ppt are recorded from the Baltic Sea in northern Europe. 5. How does salinity change with depth? Salinity increases with depth in the restricted northern ocean waters (Arctic Ocean) but decreases slightly with depth in the tropical open ocean. Salinity is much more consistent at depth (2 km) below the halocline. 33 6. What factors control the temperature of the oceans? Solar radiation is distributed over a wider area and must penetrate a greater thickness of atmosphere at the poles, reducing the amount of solar energy reaching Earth's surface. Consequently, ocean temperatures are greater near the equator. The highest ocean temperatures (~27oC) are present along the equator and temperatures decrease symmetrically to the north and south approaching 0oC at high latitudes. 7. How does temperature vary with depth? Temperature decreases significantly with depth. The effects insolation and the surface mixing of currents diminish with depth. Temperature declines steadily to a depth of approximately 1,000 meters. Deeper waters have a uniform temperature of 1 to 2oC. 8. What controls the direction of ocean currents? Winds generated by atmospheric circulation patterns represent the principal control on ocean currents but the distribution of continents and the Coriolis effect also affect currents. Circulation patterns known as gyres control currents in the open oceans. Currents form a clockwise pattern in gyres of the Northern Hemisphere and a counterclockwise pattern south of the equator. 9. Which currents are important in global climate? Although all currents contribute to global climate patterns, the western boundary currents such as the Gulf Stream and Brazil currents have an especially significant role as they transport warm tropical waters to higher latitudes. 10. What is the Coriolis effect? The Coriolis effect represents the deflection of currents to the right of their course in the Northern Hemisphere and to the left of their course in the Southern Hemisphere. The Coriolis Effect results from the contrast in the Earth's rotation velocity with latitude. 11. What is thermohaline circulation? Thermohaline circulation occurs in deeper ocean waters and is driven by density contrasts related to differences in water temperature and salinity. Thermohaline circulation drives the global conveyer belt that causes surface waters to sink in the northern Atlantic Ocean and sends cold, deep currents through 34 the world's oceans before upwelling in the northern Pacific and Indian Oceans. 12. What factors influence the development of coastal landforms? The coastline is a dynamic environment that advances or retreats depending upon the balance between the supply of sediment and the material removed by wave erosion. Seasonal variations in stream flow and storm activity affect the volume of sediment supplied to the coast and the rate of erosion. Climate cycles that result in increasing or decreasing sea levels will have long-term effects measured in decades or centuries. Finally, tectonic cycles measured in hundreds or thousands of years may continually revitalize rugged coastlines by periodic uplifts. 13. How does water move in waves? The waveform is a shape that moves across the open ocean but the water particles don't move with the wave but instead trace out a circular path while remaining essentially in place. 14. What happens when waves approach the coast? Material is eroded and redeposited by turbulent flow that occurs in the surf zone as waves break along the shoreline. Wave refraction results in wave action being concentrated on headlands. Deposition occurs in the calmer waters of sheltered bays. 15. Where does coastal erosion occur? Coastal erosion occurs where erosion by wave action is not balanced by local deposition of the eroded material and the supply of sediment from streams. Erosion exceeds deposition, resulting in a loss of shoreline, along most of the U.S. coastline. 16. What is a longshore current? A longshore current is generated when waves strike the coast at an angle. The current forms in the surf zone and transports sediment laterally along the shoreline. Longshore currents can result in the formation of characteristic depositional landforms parallel to the coastline. 17. What is the sediment budget? The sediment budget is the balance between material added to the shore by deposition and the material removed by erosion. The construction of dams will reduce the volume of sediment 35 reaching the coast. Drought conditions may also reduce streamflow and thus diminish sediment transported to the shore by streams. Coastal development may result in the construction of structures designed to reduce erosion (e.g., breakwaters, seawalls) or to control the local depositional patterns to prevent infilling of navigation channels (jetties). 18. Why are shoreline protection structures regarded as a mixed blessing? Seawalls, groins, and breakwaters act to prevent erosion and/or encourage deposition. However, structures built to protect coastlines may prevent erosion of part of the shoreline but can result in accelerated erosion elsewhere. Artificial beach nourishment occurs when sand is dredged and pumped onto the beach from offshore but has a limited life span unless the processes that caused the original erosion are stopped. 36 Image Analysis: Continental Margins 1. The image below illustrates part of South America. The ocean floor is represented by the blue colors. The deeper depths are shown as darker blues. Label the following features on the image. active margin continental shelf abyssal plain Atlantic Ocean passive margin continental slope oceanic trench Pacific Ocean 2. Label three active continental margins on the map below with the letter A and label three passive continental margins with a P. 37 3. Which of the profile views below most accurately reflects the topography of the ocean floor along an east-west line from Australia to South America? 4. Sketch and label a profile of the ocean floor topography between North America and north Africa in the space below. 38 Image Analysis: Monterey Bay The image below illustrates the topography of the ocean floor adjacent to Monterey Bay, California. The landforms of California are shown on the top right side of the image in the green and gray colors. The blue colors represent the ocean floor. Label as many features as you can in the image. 39 Thermoclines The thermocline marks a zone of relatively rapid temperature change between the warm surface currents and deeper cold waters. This exercise will attempt to identify the depth range of this boundary by identifying the location of the thermocline. 1. Plot the data points from the table below on the graph on the next page and sketch a best fit line for the data. Depth (m) 0 100 300 500 1,000 1,500 2,000 3,000 Data 1 12 12 11 9 8 7 6 5 Data 2 30 24 21 17 13 10 9 8 2. These data come from subpolar and tropical oceans. Label the lines as subpolar and tropical. 3. Identify the approximate range of depths for the thermocline on each curve and label those parts of the curves accordingly. 4. Circle the data points on the graph where you expect to find the highest and lowest salinity values. Explain why you chose those points. 40 41 Doing Science: Measuring Rates of Coastal Erosion/Accretion Scientists with the U.S. Geological Survey devised a plan to evaluate erosion (removal of material along the shoreline) or accretion (addition of material) over 1,200 km (750 miles) of shoreline following the winter storms of 1997 to 1998. They flew aerial surveys of the coastline before and after the storm season and compared the images they acquired. Two images of the coastline around Ventura, California, are presented on page 43. One represents the coastline prior to the winter storms and the other illustrates the state of the coastline after the storms. The accompanying graph shows the relative positions of the beach before (blue squares) and after (red triangles) the storms. 1. Examine the graph. What happened to the beach as a result of the winter storms? 2. How much did the width of the beach increase/decrease over the winter? 3. Assuming that these changes occurred over a six-month period, the rapid rate of coastal erosion/accretion represented by the winter storms was approximately ________ m/yr? (Note: this was an exceptional cycle of erosion/accretion and we shouldn’t consider such rates to be “normal”). a) 20 b) 50 c) 120 d) 200 4. Which photograph represents the beach in October 1997? a) A 42 b) B x x marks the same location in each image A x B 43