Two are longshore currents and rip currents. Longshore currents move water and sediment parallel to the shore in the direction of the prevailing local winds. Rip currents are potentially dangerous currents that carry large amounts of water offshore quickly.
Each summer in the United States at least a few people die when they are caught in rip currents. Have you ever been to visit a beach? Some beaches have large, strong rolling waves that rise up and collapse as they crash into the shore.
All waves are energy traveling through some type of material Figure The waves that we are most familiar with travel through water. Most of these waves form from wind blowing over the water; sometimes steady winds that blow and sometimes from a storm that forms over the water. The energy of waves does the work of erosion when a wave reaches the shore. When you find a piece of frosted glass along a beach, you have found some evidence of the work of waves. What other evidence might you find?
As wind blows over the surface of the water, it disturbs the water, producing the familiar shape of a wave. You can see this shape in Figure The highest part of a wave is called the wave crest. The lowest part is called the wave trough. The vertical distance from the highest part of a wave to the lowest is called the wave height. The horizontal distance between one wave crest and the next crest, is called the wavelength.
Three things influence how big a wave might get. If the wind is very strong, and it blows steadily for a long time over a long distance, the very largest waves will form. Bigger waves do more work of erosion which changes our shorelines. Each day that waves break along the shore, they steadily erode away a minute bit of the shoreline. When one day, a really big storm like a hurricane arrives, it will do a lot of damage in just a very short time.
As waves come into shore, they usually reach the shore at some angle. This means one part of the wave reaches shallow water sooner than the parts of the wave that are further out. So the shallower parts of the wave slow down more than the parts that are further from the shore. The way that waves bend as they come into shore either concentrates wave energy or disperses it.
In quiet water areas like bays, wave energy is dispersed and sand gets deposited. Areas like cliffs that stick out into the water, are eroded away by the strong wave energy that concentrates its power on the cliff Figure Wave-cut cliffs form where waves cut into the bottom part of the cliff, eroding away the soil and rocks there.
First the waves cut a notch into the base of the cliff. If enough material is cut away, the cliff above can collapse into the water. Many years of this type of erosion can form a wave-cut platform Figure Figure If waves erode a cliff from two sides, the erosion produced can form an open area in the cliff called an arch Figure If the material above the arch eventually erodes away, a piece of tall rock can remain in the water, which is called a sea stack Figure Rivers carry the sand that comes from erosion of mountains and land areas of the continents to the shore.
A positive sediment supply produces beach accretion while when negative beaches erode Figure 7. Figure 7 a Greenmount Beach in Queensland, Australia widened by m as a result of beach nourishment. At the beach the three zones of wave transformation shoaling, breaking, and swash produce three morphologically distinct sub-systems Figure 5.
As they shoal they interact with the seabed, slowing down and increasing in steepness and height Figure 8. Figure 8 View of Makapu Beach, Hawaii, showing waves shoaling and steepening as they travel across and interact with the nearshore zone, then breaking across the surf zone.
The surf zone is the most dynamic part of the beach and extends from the breaker zone to the shore. Waves break when the water depth is approximately 1. They can break as a spilling breaker on low gradient slopes, a plunging wave on moderate gradients, or a surging wave on steep slopes. In breaking, waves transform their potential energy to kinetic energy, which is initially manifest as the broken wave of translation, or wave bore , which moves shoreward as broken white water.
At the shoreline the currents can be deflected longshore and water may return seaward as a rip current. Surf zone currents can transport sediment onshore, longshore and offshore and build the sand bars and troughs that occupy the surf zone Figure 5. The number and location of the bars is a product of infragravity waves , a low frequency greater 30 sec period wave produced by sets of higher and lower waves and which is enhanced by wave breaking across the surf zone.
The longer the infragravity wave period the more widely spaced the bar s. Another form of infragravity wave called edge waves also influence the longshore spacing of rip currents and channels, which are typically — m apart on ocean beaches. Rip currents are narrow, seaward moving currents that move seaward though the surf zone, often in a deeper rip channel Figure 9a.
They are a mechanism for returning the water back out to sea, and a conduit to transport seaward eroded beach sediment Figure 9b during high seas. They are also a major hazard to beach goers and responsible for most beach rescues and drowning Short Figure 9 a Low waves breaking on a shallow bar and flowing shoreward into a rip feeder channel.
The dye highlights the rip feeder current flowing along the base of the beach face, then turning to flow seaward in the deeper rip channel. When the broken wave reaches the base of the wet beach it collapses and runs up the beach face as swash or uprush in the swash zone Figure The uprush stops toward the top of the slope, some percolates into the beach, the remainder flows back down the beach as backwash.
As sediment is deposited in the swash zone it can build a berm , a near horizontal to slightly landward-dipping sand surface, the area where most people sit when they go to the beach. The swash zone may also contain beach cusps , spaced about every 20 to 30 m and produced by another form of edge wave Figure Figure 10 Wave runup on the steep beach face at Ke lli Beach, Hawaii.
Figure 11 A steep reflective beach with well developed high tide beach cusps at Hammer Head, Western Australia. Wave-dominated beaches have an RTR tide range less than three times the average wave height RTR Reflective beaches are produced by lower waves H Figure 12 A plot of breaker wave height versus sand size, together with wave period, that can be used to determine the approximate beach state for wave-dominated beaches.
To use the chart, determine the breaker wave height, period and grain size mm. They are characterized by a surf zone with one or two bars up to m wide.
The bar is usually cut by regular rip channels and currents Figure Figure 13 Well-developed intermediate beach containing transverse bars and rip channels along Lighthouse Beach, Australia. Note the waves breaking on the bars, with no waves breaking in the deeper darker rip channels. Also note the rhythmic shoreline protruding in lee of the bars and forming an embayment in lee of the rips.
Waves break on the outer then inner bar s , thereby dissipating their energy as the move across the surf zone Figure The swell breaks over the wide outer bar, reforms in the central trough, then breaks across the inner bar, resulting up to 10 lines of breakers and a m wide dissipative beach and surf zone. They usually have a steep, coarser-grained, cusped, reflective, high tide beach.
This is fronted by a wide, finer-grained, low gradient, often featureless, intertidal zone, up to m wide, then a low tide surf zone which may contain bars and rip channels Figure Figure 15 A steep reflective high tide beach face fronted by a m wide tide-modified low tide terrace crossed by shallow drainage channels at North Harbour Beach, Australia.
An additional beach type consists of a high tide reflective beach face fronted by intertidal rocks flats, and in the tropics a high tide beach fronted by a fringing coral reef flat Figure Furthermore any beach located in the high latitudes will be seasonally exposed to freezing air and water temperatures leading to the development of sea ice, shoreface ice, and a frozen snow covered beach Figure Figure 18 The beach at Pingok Island, north Alaska, shown a during summer, with floating ice against the shore; b during freeze-up, with snow and sea ice accumulating; and c the frozen winter beach and ocean.
Beach systems are an essential component of a larger scale coastal landform called barriers , which are long-term accumulation of wave, tide, and wind deposited marine sediment usually sand at the shore. When separated from the mainland by lagoons and marshes Figure 19 they are called barrier islands Figure 20 , which occur along the US East and Gulf coasts.
Some are backed by large dune systems as along the Oregon coast. Figure 19 A coastal sand barrier consisting of a beach and vegetated dunes, backed by a lagoon, at Big Beach, Queensland, Australia.
Figure 20 A series of low barrier islands separated by tidal inlets, at Corner Inlet, Victoria, Australia. Davis, R. Davis, Jr. Komar, P. Beach Processes and Sedimentation , 2nd ed. Masselink, G.
Introduction to Coastal Processes and Geomorphology. Short, A. Handbook of Beach and Shoreface Morphodynamics. Australian beach systems — Nature and distribution. Journal of Coastal Research 22 , 11—27 The Coast of Australia. Melbourne, Australia: Cambridge University Press, Woodroffe, C. Coasts — Form and processes.
Global Change: An Overview. Conservation of Biodiversity. Introduction to the Basic Drivers of Climate. Tropical Weather. Terrestrial Biomes. Causes and Consequences of Dispersal in Plants and Animals. Causes and Consequences of Biodiversity Declines. Disease Ecology. Coastal Dunes: Geomorphology. Coastal Processes and Beaches. Drip Water Hydrology and Speleothems. Earth's Earliest Climate. El Nino's Grip on Climate. The relatively simple shores of the Outer Banks of North Carolina have been a frequent focus for nearshore research.
It was assumed that waves, currents, and sand levels were uniform up and down these beaches. However, recent studies by Jeff List of the U.
Geological Survey have shown that even on these long, straight coastlines, one section of beach may recede shoreward by tens of yards during a storm, while a few miles down the coast the beach may be unaltered. Most of the eroded sand eventually returns after the storm, but that is no consolation to the owners of homes and structures destroyed by the shifting shoreline. Several hypotheses explain the divergent erosion rates along the same coast.
Perhaps there are differences in the underlying geology of the region or in the flow of groundwater to the ocean. Maybe something as subtle as the size of sand grains makes a difference. My research group is intrigued by a different theory: the location of underwater sandbars in the surf may cause variable erosion along the coast. Sandbars appear to protect beaches by causing increased breaking and dissipation of wave energy before the waves can attack the shoreline.
We found that sandbars affect the coastal water levels and flooding during storms. When a sandbar is near the beach, waves break in shallow water and drive more water onto the shore. This causes flooding and allows the surf to reach dunes and manmade structures. We believe that shallow sandbars may lead to increased erosion. But further research is needed to determine whether the feedback between sandbars and coastal water levels is important during storms.
In contrast to much of the East Coast of the United States, many continental shelves have abrupt, irregular changes in the seafloor that cause large changes in the waves beyond the surf zone. For instance, the steep topography of Scripps and La Jolla submarine canyons in Southern California produces dramatic changes in wave energy over distances of a few hundred meters.
As waves pass over the canyons, the seafloor acts like a magnifying glass, concentrating ocean wave energy into hot spots. These changes in wave heights along the coast result in complex flows of water and changes to sand levels on the beach. When these currents intersect with opposing currents—perhaps between the heads of the two canyons—strong offshore-directed flows, called rip currents, can form.
Rip currents are a danger to swimmers and have been observed to carry huge plumes of sand and pollutants offshore. Scientists from WHOI and 10 other institutions recently conducted a major field program in those complicated Southern California waters to determine how abrupt coastal-seafloor topography affects waves, currents, and changes to sand levels.
The team of more than three dozen scientists, engineers, students, and research assistants deployed instruments to measure the effect of the canyons on waves, and the resulting changes in the flows onshore. In the Nearshore Canyon Experiment NCEX , sponsored by the Office of Naval Research and the National Science Foundation, my colleagues and I deployed pressure gauges and current meters in the surf and swash zones to measure wave heights and directions, and the resulting movement of water and sand.
Beach surveys were conducted frequently to see how the seafloor and sand levels evolved under changing surf conditions. Other scientists bounced radar signals off the water surface to determine its speed, just as police use radar guns to track moving cars. Video cameras chronicled sea foam as it was carried along the coast by the complex currents. We have only begun to analyze our observations, but eventually we will be able to update and improve numerical models of the important physical processes, and to test the model predictions with real-world measurements.
Our long-term goal is to predict coastal wave heights and directions, nearshore currents, and changes to beach sand levels. We may not achieve this goal until we can study nearshore processes on a wide range of coasts for instance, rocky shorelines, peninsulas, and large bays , but it is achievable within our lifetime.
Surfers, sport-fishermen, and recreational boaters frequently use predictions of wave heights along the coast of Southern California, as generated by our colleagues from the Scripps Institution of Oceanography. The new NCEX measurements will improve the prediction models, helping not only those who play at the beach for the day, but also civic leaders who must manage our beaches and coasts for the long term.
One hundred years ago, we could not predict whether it would be sunny or rainy the day after tomorrow. Now we can predict the weather as much as 10 days in advance. By the middle of the 21st century, we ought to be able to predict the weather at the beach…both above and below the water line. He uses techniques that span isotope geochemistry, next generation DNA sequencing, and satellite tagging to study the ecology of a wide variety of ocean species.
He recently discovered that blue sharks use warm water ocean tunnels, or eddies, to dive to the ocean twilight zone, where they forage in nutrient-rich waters hundreds of meters down.
Born in New Zealand, Simon received his B. With much of his work in the South Pacific and Caribbean, Simon has been on many cruises, logging 1, hours of scuba diving and hours in tropical environs.
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