Presentation on theme: "SHORELINES The restless waters of the ocean are constantly in motion. Winds generate surface currents, the gravity of the moon and sun produces tides,"— Presentation transcript:
SHORELINES The restless waters of the ocean are constantly in motion. Winds generate surface currents, the gravity of the moon and sun produces tides, and density differences create deep-ocean circulation. Further, waves carry the energy from storms to distant shores, where their impact erodes the land.
Shorelines Shorelines are dynamic environments. Nowhere is the restless nature of the ocean’s water more noticeable than along the shore – the place where air, land, and sea intersect. The shore is where sediment is deposited in transition zones between marine and continental environments.
The Coastal Zone Many terms are used to describe the boundary between land and sea. Terms like shore, shoreline, coastal zone, coast, and beach are all common names. Let’s take a few minutes to clarify what these mean. The shoreline is the line that marks the contact between land and sea. Each day, as tides rise and fall, the position of the shoreline migrates. Over longer time spans, the average position of the shoreline gradually shifts as sea level rises or falls. The shore is the area that extends between the lowest tide level and the highest elevation on land that is affected by storm waves. By contrast, the coast extends inland from the shore as far as ocean-related features can be found. The coastline marks the coast’s seaward edge, whereas the inland boundary is not always obvious or easy to determine.
The shore is divided into the foreshore and the backshore. The foreshore is the area exposed when the tide is out (low tide) and submerged when the tide is in (high tide). The backshore is landward of the high-tide shoreline. It is usually dry, being affected by waves only during storms. The Coastal Zone
Two other zones are commonly identified. The nearshore zone lies between the low-tide shoreline and the lie where waves break at low tide. Seaward of the nearshore zone is the offshore zone. Although thought of as the sandy area along the water’s edge, a beach is technically an accumulation of sediment found along the landward margin of an ocean or lake.
The Coastal Zone Beaches consist of one or more berms, which are relatively flat platforms often composed of sand that are adjacent to coastal dunes or cliffs marked by a change in slope at the seaward edge. Sunbathers usually prefer the berm, but joggers prefer the wet, hard-packed sand of the beach face, which is the wet sloping surface that extends from the berm to the shoreline.
Beaches Beaches are composed of whatever material is locally abundant. The sediment for some beaches is derived from the erosion of adjacent cliffs or nearby coastal mountains. Other beaches are built from sediment delivered to the coast by rivers. Shell beach in FloridaVolcanic beach in Hawaii
Waves Ocean waves are energy traveling along the interface between the ocean and atmosphere, often transferring energy from a storm far out at sea over distances of several thousand miles. That is why even on calm days the ocean still has waves that travel across its surface.
Waves When observing waves, always remember that you are watching energy travel through a medium (water). If you make waves by tossing a pebble into a pond or by splashing in a pool or by blowing across the surface of a cup of coffee, you are imparting energy to the water, and the waves you see are just the visible evidence of the energy passing through.
Waves Most ocean waves derive their energy and motion from the wind. When a breeze is less than 2 miles per hour, only small wavelets appear. At greater wind speeds, more stable waves gradually form and advance with the wind.
Waves This diagram illustrates the basic parts of a wave as well as the movement of water particles with the passage of the wave. Negligible water movement occurs below a depth equal to one half the wavelength (the level of the dashed line).
Waves The tops of waves are called crests, which are separated by troughs. Halfway between the crests and troughs is the still water level, which is the level the water would occupy if there were no waves. The vertical distance between the trough and crest is the wave height, and the horizontal distance between successive crests or troughs is called the wavelength.
The height, length, and period eventually achieved by a wave depend upon three factors: (1) wind speed; (2) length of time the wind has blown; and (3) fetch, the distance the wind has traveled across the open water. As the energy transferred from the wind to the water increases, eventually a critical point is reached and the waves grow so tall that they topple over, forming ocean breakers called whitecaps. These whitecaps take energy away from the waves, stopping them from growing any taller. Waves
Wave Motion Waves can travel great distances across oceans. In one study, waves traveled over 6,000 miles from Antarctica to Alaska before the energy was finally expended! The water itself does not travel the entire distance, but the wave form does. As the wave travels, the water passes the energy along by moving in a circle. This movement is called circular orbital motion. This allows a waveform (the wave’s shape) to move forward through the water while the individual water particles that transmit the wave move around in a circle.
When a wave approaches the shore, the water becomes shallower and influences wave behavior. The wave begins to “feel bottom” at a water depth equal to its wave base. As a wave advances, the slightly faster waves farther out to sea catch up, decreasing the wavelength. As the speed and length of the wave diminish, the wave grows higher until it becomes too steep and collapses, or breaks.
The turbulent water created by breaking waves is called surf. On the landward margin of the surf zone the turbulent sheet of water from collapsing breakers, called swash, moves up the slope of the beach. When the energy of the swash has been expended, the water flows back down the beach toward the surf zone as backwash.
Wave Erosion In addition to erosion caused by wave impact and pressure, abrasion, the sawing and grinding action of water armed with rock fragments, is also important. In fact, abrasion is probably more intense in the surf zone than in any other environment.
Beaches: Rivers of Sand Energy from breaking waves often causes large quantities of sand to move along the beach and in the surf zone roughly parallel to the shoreline. Wave energy also causes sand to move perpendicular to (toward and away from) the shoreline. If you stand ankle deep in water at the beach, you’ll see the swash and backwash move sand toward and away from the shoreline. Whether there is a net loss or gain of sand depends on the level of wave activity. When waves are quiet (less energy), much of swash soaks into the beach, which reduces the backwash. When high-energy waves prevail, the strong backwash causes the berm to erode, allowing sand to move down the beach face. In the summer, light activity is generally normal, which builds up the berm. When rough winter seas come in, a wide berm that took all summer to build can be eroded in just a few hours.
Wave Refraction The bending of waves, called wave refraction, affects the distribution of energy along the shore and influences where and to what degree erosion, sediment transport, and deposition takes place. Headlands take the brunt of wave action, while wave attack is weakened in bays. The wave hits the headland first, eroding material, and bends into the bay area, where the sediment is then deposited.
Although waves are refracted, most still reach the shore at a slight angle. Consequently, the uprush of water from each breaking wave (the swash) is not head on, but oblique. However, the backwash moves straight down the slope of the beach. The effect of this pattern of water movement is to transport particles in a zigzag motion. This movement is called beach drift, and it can move sand and pebbles hundreds or even thousands of meters each day. The typical rate of movement is about 5 to 10 meters per day.
Oblique waves produce currents within the surf zone that flow parallel to the shore and move substantially more sediment than beach drift. Because the water here is turbulent, these longshore currents easily move sand and gravel particles along the bottom. Longshore currents and beach drift move tons of sediment each day. Longshore currents change direction because the direction that waves approach the beach changes seasonally. Even so, they usually flow southward along U.S. shores.
Shoreline Features Shoreline features found in a coastal region depend largely on the type of rocks exposed along the shore, the intensity of waves, the nature of coastal currents, and whether the coast is stable, sinking, or rising. Features that owe their origin primarily to the work of erosion are called erosional features; while deposits of sediment produce depositional features. Common erosional landforms found along the rugged and irregular New England coast and along the steep shorelines of the West Coast of the United States include wave-cut cliffs, wave-cut platforms, marine terraces, sea arches and sea stacks. Common depositional landforms (found all over, really, depending on the environment) include spits, bars, tombolos, and barrier islands.
Erosional Features Wave-cut platforms are caused by the surf cutting into the land, while marine terraces are created if a wave-cut platform is uplifted above sea level by tectonic forces. Marine terraces are often desirable spots for coastal roads, buildings, or agriculture.
Headlands extending into the sea are vigorously attacked by waves because of refraction. The surf erodes the softer or highly fractured rock at a faster rate, creating sea caves. (and eventually sea arches). Eventually the arch caves in, resulting in a sea stack. Erosional Features
Depositional Features Where beach drift and longshore currents are active, several features related to the movement of sediment along the shore may develop. A spit is an elongated ridge of sand that projects from the land into the mouth of an adjacent bay. The term baymouth bar is applied to a sandbar that completely crosses a bay, sealing if off from the open ocean. A tombolo, a ridge of sand that connects an island to the mainland or another island, forms in much the same manner as a spit.
The Atlantic and Gulf Coastal Plains are relatively flat and slope gently seaward. The shore zone is characterized by barrier islands. These low ridges of sand parallel the coast at distances from 3 to 30 kilometers offshore. Some barrier islands originate as spits that were severed from the mainland by wave erosion or a rise in sea level. Others are created from turbulent waters shifting sand from one area to another.
Stabilizing the Shore Unfortunately, people often treat the shoreline as if it were a stable platform on which structures can be built safely. But most coastal landforms are relatively fragile, short-lived features easily damaged by development. Humans use various forms of technology in an effort to stabilize coastlines. Although the same processes cause change along every coast, not all coasts respond in the same way. The effectiveness of stabilizing efforts depend on local factors, including (1) the proximity of the coast to sediment-laden rivers; (2) the degree of tectonic activity; (3) the topography and composition of the land; (4) prevailing winds and weather patterns; and (5) the configuration of the coastline and nearshore areas.
Structures built to protect a coast from erosion or to prevent the movement of sand along a beach are known as hard stabilization. Hard stabilization can take many forms and often results in predictable yet unwanted outcomes. Hard stabilization includes groins, jetties, breakwaters, and seawalls. A groin is a barrier built at a right angle to the beach to trap sand that is moving parallel to the shore. Groins are usually constructed of large rocks or wood. Unfortunately groins work too well, and often result in lack of sand movement down current, resulting in more groins being built.
A jetty confines water to a narrow passage, preventing sand buildup at the entrance of a harbor. It also prevents longshore transport of sand, so eventually dredging will be needed elsewhere.
Breakwaters help protect boats and marinas from wave activity, while seawalls act as armor for the coast. Both have unintended side effects. Breakwaters build up sand and have a need for dredging over time; while seawalls promote erosion on the seaward side of the wall. The dangers and need for hard stabilization remain a highly debated topic among engineers and environmentalists.
An alternative to hard stabilization is beach nourishment. This practice involves adding large amounts of sand to the beach in an effort to stave off erosion. This process is very expensive, however, and has to be repeated often. Also, if the wrong type of sand is added, it could have dangerous environmental effects on the plants and creatures living in or near the ocean environment.
Tides Tides are daily changes in the elevation of the ocean surface. Tides are caused by the gravitational attraction of the moon, and to a lesser extent, by the sun. The pull of gravity actually stretches the Earth a bit, elongating it and causing the water on it to bulge up and move in response. This is why you have two high tides on opposite sides of the Earth at the same time.
Near the times of the new and full moons, the sun and moon are aligned, and their gravitational forces are added together to produce especially high and low tides. These are called spring tides. At about the same times as the first and third quarters of the moon, when the gravitational forces of the moon and sun are at right angles, the daily tidal range is less. These are called neap tides.
Tidal currents are horizontal movements of water that accompany the ride and fall of tides. Tidal flats are the areas that are affected by the advancing and retreating tidal currents. When tidal currents slow after emerging from narrow inlets, they deposit sediment that many eventually create tidal deltas.