Breaking Waves Provide the Energy that Changes the Shape and Texture of Beach Deposits As waves shoal (touch bottom) in shallow water: Wave speed decreases Wavelength decreases Wave height increases Waves refract Refraction is the bending of waves toward shallower water so that they break almost parallel to the shore. Waves become unstable and break in very shallow water. Waves generate: Longshore currents that flow parallel to the beach Rip currents that flow perpendicular to the beach
Longshore Currents Transport sand grains (and swimmers) parallel to the shore Figure 11.01a: Aerial view of oblique wave approach. Douglas Faulkner/Science Source. Figure 11.01b: waves that approach a beach at an angle produce a longshore drift of sand. Figure 11.01c: Where waves approach a shoreline at an angle, a shore-parallel current called a longshore current is generated in the surf zone. Figure 11.01d: Waves that are parallel to the beach can generate longshore currents, provided wave height varies along the waves crest.
Longshore Currents (Continued) Angle of wave approach is the acute angle (less than 90 ) between the wave crest and the beach. The direction of longshore current varies with the direction of wave approach. Longshore currents can also be generated by wave set-up, a process that creates (uneven) piles of water in the surf zone. When wave crests are parallel to shore, longshore currents can still develop due to wave set up Where two opposing longshore currents converge, a swift, narrow, seaward-flowing rip current forms.
Beaches The beach is the part of the land that touches the sea. It can be divided into the: Offshore Nearshore From breaker through surf, to swash zone. Breaker zone (waves begin to break) Surf zone (most wave energy expended) Swash zone (where beach is covered then uncovered by wave surge) Backshore The position of the beach zones varies with the tides, storms, lunar cycle, and the seasons (depending on climate zone!). The boundaries advance landward under high energy or elevated sea surface (tide, storm) and retreat seaward with low energy/low sea surface. Seasonal increases in wave energy also cause advance and retreat of shoreline zones.
The Coastal Zone The berm, a prominent wave-deposited feature of most beaches, is an accumulation of sand having a flat top surface and a relatively steep seaward slope. The offshore zone is the open water that lies seaward of the nearshore zone The backshore zone is the land that adjoins the nearshore zone.
Beach Sediments Are Moved by Currents and Breaking Waves A beach profile is a cross section of the beach along a line that is perpendicular to the shoreline. A swell profile is concave upward with a wide, broad berm (relatively flat backshore) and steep intertidal beach face. A storm profile displays erosion of the berm and a broad flat intertidal beach face, often with a submarine bar. Figure 11.03a: A swell profile (broad berm, steep beach face, concave profile) is compared with a storm profile (narrow beach, gentle beach face, longshore bars). Figure 11.03c: Following a storm, the eroded beach tends to undergo accretion.
Light wave activity Wide, sandy berm Steep beach face Swash dominates Longshore bars not present Generally milder storms Summertime Beach
Heavy wave activity Backwash dominates Sediment moved away from shore Narrower beach Flattened beach face Longshore bars are present Stormy weather Wintertime Beach
The sand budget is the balance between sediment added to and sediment eroded from the beach. (b) In this example of a hypothetical sand budget, sand inputs are less than sand outputs. The result is acute erosion of the beach at an estimated rate of 10,000 m3/yr. Sand budgets. (a) The state of a beach can be assessed by considering the credits (inputs) and debits (outputs) of sand to a stretch of shoreline. Beach accretion (buildup) results when sand inputs exceed outputs. Erosion results when sand outputs are greater than sand inputs. A balance between inputs and outputs produces no change in the volume of beach sand, a condition called steady state.
Coastal Cells (e.g. Southern California) The formation of a coastal cell: Sand is input from river. Longshore drift carries the sand down the shoreline. Sand drifts into the head of a submarine canyon. It is swept down the canyon into the deep-sea basin.
Beaches and Dunes Sand dunes are formed by winds blowing sand landward from the dry part of the beach. Well-developed dunes typically have a sinusoidal profile (like waves ) The primary dune is at the landward edge of the beach Possible secondary dunes are located farther inland Figure 11.06: Simple plants (grasses and shrubs) dominate the primary dune ridge; complex tall shrubs and trees dominate the secondary dune ridge and back dune.
Sand Dunes Vegetation on the dunes: Traps windblown sand on their downwind side Promotes dune growth and stability Blowouts are wind-scoured breaks in the dune or depressions in the dune ridge. They are common if vegetation is destroyed Dunes are best developed if: Sand is abundant Onshore winds are moderately strong and persistent The tidal range is large The beach is wide and gently sloping
Sand Dunes (Continued) Sand saltates (bounces) up the windward side of the dune. It collects in the wind shadow at the top. When the accumulation of sand becomes oversteepened, it slides down the leeward face of the dune. The result is dune migration. Figure 11.08: Wind causes sand grains to saltate up the dune s gentle windward slope.
Sand Dunes (Continued) Wave erosion of dunes: Supplies sand to the offshore Creates a steep scarp at the base of the dune Dunes act as a natural barrier and prevent or reduce inland flooding. Rooted plants that grow on dunes stabilize them and reduce rates of dune erosion. Human activity that damages vegetation leads to dune destruction by blowouts and washover by storm waves. Figure 11.09: A steep scarp has been cut into a large dune, exposing its internal cross-bedded structure. Timcaviness/Dreamstime.com Figure 11.10: Wooden foot ramps are used to direct foot traffic through dunes in order to minimize damage to grasses and shrubs. Courtesy of Sapelo Island National Estuarine Research Reserve/NOAA
Barrier Islands Composed of thick sediment deposits that parallel the coast. They form where: Sand supply is abundant The offshore bottom slopes gently seaward The islands are separated from the mainland by shallow bodies of water (estuaries and lagoons), which are connected to the ocean through tidal inlets. A series of distinct environments develop across the island parallel to the beach, including the: Nearshore zone Dune field Back-island flats Salt marshes
Barrier Islands (Continued) Landward migration Figure 11.11b: Several distinct land types comprise a barrier-island system. Adapted from Godfrey, P. J. Oceanus 19, (1976): 27 40.
Origin of Barrier Islands Formed in three ways, including: Sand ridges isolated by rising sea level Sand spits breached during a storm Vertical growth and emergence of longshore sand bars As sea level rises, barrier islands migrate landward. Washover transports sediments from the seaward side of the island to its landward side.
Barrier Islands Are Separated from One Another by Tidal Inlets These openings allow the exchange of seawater between the ocean and smaller bodies of water. Strong, alternating tidal current disrupt the longshore movement of sediment and store it in tidal deltas. If the longshore transport of sediment along a barrier island is great, a sand spit can form and grow into the downdrift inlet, causing it to constrict or shift its location.
Water Levels Storm surge is the high water created by: The accumulation of wind-blown water against the shore The uplift of the water surface generated by the low atmospheric pressure of the storm. The elevated water level allows waves to reach much farther inland than usual. Especially if the storm surge coincides with a high tide. Waves more easily breach the island and wash over lower areas. New tidal channels may form during a storm surge.
A Sea Cliff Is an Abrupt Rise of the Land from Sea Level A sea cliff is most vulnerable to erosion at its base. This is because waves that slam against the cliff compress air inside cracks, which expands violently. Also, sediment is hurled against the cliff by the waves. Sea water dissolves some rock types. Evaporated seawater leaves behind salt, which can chemically or physically break rocks apart.
Sea Cliffs When sufficient rock at the base of the cliff has been undercut, the upper part of the cliff eventually collapses. Collapsed material protects the base of the sea cliff from additional erosion until it is destroyed wave energy and removed by longhsore currents. Rate at which the cliff recedes is dependent upon: The composition and durability of cliff material Weaknesses in the cliff material Joints Fractures Faults, etc. The amount of precipitation The steepness of the cliff Wave energy, height, and direction
A Delta Is an Accumulation of Sediment Deposited at the Mouth of a River as it Flows into a Standing Body of Water Deltas were named after the Greek letter delta Δ. The three major areas of a delta are: Delta plain (exposed on land) Delta front (slopes down under water toward the seafloor) Prodelta (flat area at the base of the delta front, far offshore)
Deltas (Continued) As sediment accumulates, the delta expands seaward: Foreset beds bury bottomset beds Topset beds cover foreset beds Shape of the delta can be altered by: Tides Waves River deposition. Reduction in the supply of sediment to a delta results in: Delta erosion Retreat of the shoreline
Coastal Environment Determines Delta Shape Figure 11.17b: Depending on the relative effects of river, waves, and tides, deltas assume a variety of shapes. (Inset) Courtesy of NASA
Stabilizing the Coastline Humans try to stabilize the coastline in two ways: By interfering with longshore sand transport By redirecting wave energy to prevent erosion Preventing sand drift involves the construction of jetties and groins. Redirecting wave energy requires the construction of breakwaters and seawalls. (harbor or shoreline protection) Beach nourishment with sand is expensive and is a temporary solution to an erosion problem. An increase in sea level from global warming will cause more land to be flooded and threaten coastal buildings and infrastructure.
Coastal Engineering Structures Figure 11.18a: Groin Figure 11.18b: Jetties Figure 11.18c: Breakwater Figure 11.18d: Seawall
Oceans and the Planet Oceans are critical to the dynamic processes and the state of Earth s climate over short and extremely long time scales. Ocean water stores and redistributes immense quantities of heat. Climatic warming may occur because of the increased buildup of CO 2 in the atmosphere. Molecules of CO 2 permit sunlight to pass through the air and heat the Earth s surface, but absorb and trap heat that is radiated from the ground and ocean.
Oceans and the Planet (Continued) The amount of CO 2 in the atmosphere has increased 44% over the last 150 years. The oceans absorb 30% to 50% of CO 2 emissions created by burning fossil fuels. Figure 16.01: The 40-year record of CO 2 concentrations in the atmosphere measured at Mauna Loa Observatory in Hawaii. Adapted from Rhode, R. A. Atmospheric Carbon Dioxide, Global Warming Art, October 1, 2008, http://www.globalwarmingart.com/wiki/image:mauna_loa_carbon_dioxide_png
Global Warming Can Cause Polar Ice Caps to Melt, Resulting in Sea Level Rise Figure 16.02a: The recent global rise of sea level. Data from V. Gornits, S. Lebede, and J. Hausen, Science 215 (1982): 1611 1614.
Climate Change Global and regional wind and precipitation patterns can (and do) change. Effects of climate change will vary geographically: Some regions will experience longer growing seasons and more rainfall. Others will become hotter and drier.
Climate Change (Continued) Climate warming will affect oceans and ocean life in diverse and complex ways. Every aspect of ocean chemistry, circulation, heat content. Every facet of ecological, environmental, and biological activity. All will be impacted by changes in temperature and ph. Increased CO 2 changes both temperature and ph of the oceans. Figure 16.04: This diagram shows some direct (solid arrows), indirect (dashed arrows), and possible (dotted arrows) consequences of increasing atmospheric CO 2. Adapted from Kennedy, V.S., et al. Coastal and marine ecosystems and global climate change: Potential effects on U.S. resources, report of the Pew Center on Global Climate Change, Arlington, VA 2003.
Coastal Ecosystems Current estimates predict that sea level will rise 10 to 90 cm by the year 2100. Some inhabited islands and coastal areas will be submerged by the end of this century. Coastal delta plains are particularly vulnerable to seawater incursion. They are subsiding under a heavy sediment load, which accelerates the relative rise of sea level. Figure 16.07: Estimates of the contribution of the melting of ice sheets and mountain glaciers to the expected rise of sea level in the twenty-first century. Adapted from Haslett, S. K. Coastal Systems. Routledge, 2003.
Coastal Ecosystems (Continued) Storm surges are expected to be higher than usual. They will result in more flooding, erosion, and damage to coastal property. Intrusion of seawater into groundwater aquifers will contaminate the freshwater supplies of coastal communities. Anthropogenic structures interfere with ecosystems ability to adapt to environmental change. They prevent coastal ecosystems from shifting landward as sea level rises. Many salt marsh plants (Spartina sp, mangroves) rot after prolonged exposure to seawater.
Salt Marshes and Mangrove Swamps Provide natural protection from storm surges and coastal flooding. Serve as critical nurseries and refuges for many species of shellfish and finfish. Their demise: Opens the shoreline to greater erosion and damage Will have major negative impact on commercial fisheries
Possible Ways to Alleviate the Effects of Sea-level Rise Include Elevating buildings and infrastructure (accomodation) Engineering of coastal areas to offset/prevent erosion (protection) Planned relocation of coastal buildings and other infrastructure (relocation) Prohibiting future coastal development.(relocation)
Water Temperature Influences behavior and mortality of marine organisms. Changes in water temperature can affect: Predator/prey relations Ecological niches Resource allocations Species distribution Timing of reproduction/rate of development These alterations can be detrimental to the survival of populations and species.
Coastal Water Aquaculture in coastal areas is a rapidly increasing source of human food. Rising temperatures could mean that microbial infections of aquacultured organisms increase. Warmer coastal water may foster more frequent and larger algal blooms, such as red tides. This can devastate shellfish fisheries and cause human illness and even death.
Seawater The temperature and salinity of seawater cause dense water masses to sink. This helps drive a global conveyor belt of water movement. Climate change will influence deep-water flow. Atmospheric effects control seawater density. Major change in the redistribution of flow patterns, oxygen, and nutrients could result from increasing ocean temperatures
Salinity Melting of ice sheets decreases ocean salinity. Decreased salinity is expected to slow down the rate of downwelling in the North Atlantic. If prolonged, may shut it down entirely. Shutdown of circulation would cut off the supply of oxygen-rich water to the deep sea. This would cause hypoxia and anoxia in the deep ocean, inducing mass extinctions.
Polar Amplification Figure 16.02B: (b) Surface ocean temperature change for January 2010 relative to the mean temperature for 1951 1980. The largest positive temperature anomaly occurs in the Arctic, another in the Antarctic.
Arctic Sea Ice Is Melting at an Alarming Rate In terms of total cover In terms of seasonal cover Figure 16.13a: Inside of a decade (1992 2002), the area of the ice sheet that is melting rapidly has expanded markedly. Adapted from King, M. D., et al. Our Changing Planet: The View from Space. Cambridge University Press, 2007. Figure 16.13b: The floating terminus of a Greenland glacier called Jakobshavn Isbrae has retreated almost 50 kilometers since 1851. Figure 16.14a: This satellite photo shows the extent of melting of the sea-ice cover of the Arctic Ocean during the summer season. Courtesy of Josefino Comiso and NAA/Goddard Space Flight Center Scientific Visualization Studio. The sea-ice cover of the Arctic Ocean has been shrinking alarmingly during the past decades.
Sea Ice Melting sea ice affects mammals adapted to icecovered water. Arctic seals require extensive areas of ice for breeding and resting. These seals are essential prey for walruses and polar bears. Sea ice also affects plankton productivity, which is the basis of the food web. Spring phytoplankton bloom dependent on ice edge. Cycles of plankton and fish production will change, new food webs will emerge in ice free Arctic.
Sea Ice (Continued) The absence of sea ice during summer will allow open water to absorb more heat. This will accelerate seawater temperature increase. Increased water temperature: Delays onset of winter freezing Promotes an earlier spring breakup of sea-ice cover Figure 16.15a: Open water absorbs much more sunlight than ice-covered water. Figure 16.15b: Examples of positive feedback loops involving incident solar radiation and seaice expansion and contraction. Adapted from King, M. D., et al. Our Changing Planet: The View from Space. Cambridge University Press, 2007.
As Waters Warm: Warm-water species expand, displace coldwater species. Phytoplankton populations decrease. Marine food webs must readjust to these changes, sometimes causing collapse of populations.
CO 2 As CO 2 builds up in the lower atmosphere, more of it diffuses into the ocean. CO 2 complexes with water molecules to form carbonic acid (H 2 CO 3 ). This increases the acidity of the seawater. Reduces CO 3 2- omplications for organisms that secrete calcium carbonate shells (CaCO3) Figure 16.17: The relative levels of various carbonate ion species such as carbonate (CO 2 3 ) and bicarbonate (HCO 3 ) vary with the ph of the water. Adapted from Buddemeier, R. W., Kleypas, J. A., and Aronson, R. B. Coral Reefs and Global Climate Change: Potential Contributions of Climate Change to Stresses on Coral Reef Ecosystems. Report of the Pew Center on Global Climate Change, 2004.