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Natural Disasters Coast Presentation Natural Disasters Coast Presentation Presentation Transcript

  • 1 Coastal Processes Chapter 13 Waves, Beaches And Coastal Erosion Rivers of Sand
  • 2 Wave Processes and Shore Erosion Waves form as wind blows over the ocean surface. Waves increase in size with the following factors: • greater wind velocity • greater amount of time the wind blows • greater fetch
  • Wave Processes and Shore Erosion 3 Wave height and wavelength increase with increasing energy. • wave height -- the vertical distance from low point (trough) to high point (crest) • wavelength -- the horizontal distance from one crest to the next crest
  • 4 In deep water the wave form itself moves, but the water does not have net forward motion -- instead the water travels in circlular orbits (right). The circular motion of the water fades downward to a depth equal to ~half the wavelength, called wave base (below). Wave Processes and Shore Erosion
  • 5 Wave Processes and Shore Erosion At a water depth equal to or less than half the wave length, the waves begin to ‘feel bottom’. The crest of the wave moves forward as the base drags on the bottom. This causes the waves to bunch up (wavelengths decrease) and grow taller (wave heights increase).
  • 6 Wave Processes and Shore Erosion Eventually the waves oversteepen, causing them to tumble toward the shore, or break, and the water moves landward
  • Beaches 7 Beaches are accumulations of sand or gravel supplied by Sea Cliff Erosion and by River Transport of Sediment to the Coast. The sand and gravel supplied to the coast is then moved by wave action.
  • 8 Wave Refraction and Cliffed Coasts Wave refraction is the bending of waves to conform to the shape of the coastline. Wave refraction concentrates wave energy on headlands and deposits the products of that erosion in bays, ultimately straightening the coastline.
  • 9 Erosional Remnants of Cliffed Coastlines Eroded by Wave Refraction
  • Longshore Drift 10 Longshore drift in parts of coastal California averages 750,000 m3/year, or the equivalent of 2,000 10m3 dumptrucks per day. Longshore drift along the East Coast averages 75,000 m3/year. The gradual migration of sand along the shoreline. Sand grains are pushed up onto the beach in the direction of wave travel. Gravity pulls it back directly down the slope of the beach. We measure longshore drift by the volume of sand moved in cubic meters per year.
  • 11 Seasonal Beach Winter/Summer Beach Profile
  • Storm Response Loss of Sand from the Beach 12 Top: This beach in California was eroded by waves during a storm event. Right: But was naturally rebuilt by 1998 Larger waves and storm surge erode sand from the shallow portion of the beach and transport much of it just offshore. Much of the eroded material comes from the surface of the beach, eroding it to a flatter profile.
  • Barrier Islands 13 Barrier islands are long, narrow, offshore deposits of sand that parallel the coast line, separated from the main land by a shallow sound, bay or lagoon. They are the products of dynamic coastline processes: erosion, deposition, longshore drift, and wind transport.
  • Barrier Islands 14 Barrier islands are often found in chains along the coast line and are separated from each other by narrow tidal inlets. A baymouth barrier is a barrier beach that is attached at both ends to the mainland and backed by a bay or lagoon. A spit is a protrusion or linear beach extending from the mainland.
  • Barrier Islands 15 Most barrier islands in North America are along the Atlantic Coast. The photograph at left shows the barrier islands off of Long Island and New Jersey. These islands extend almost continuously south to Georgia.
  • Barrier islands are places of change and motion. 16 Longshore drift moves sand from one beach to another. Islands grow and shrink depending on the amount of sediment available (dams, sand mining, hardening). Sea level rise causes them to erode and to shift toward the mainland over time. An island that is migrating toward the mainland is termed retrograding. In most cases an island that is retrograding undergoes a process known as "island rollover".
  • 17 Two forms of evidence that show an island is rolling over are washovers and exposed marsh mud and oyster shells on the beach. Barrier islands are places of change and motion.
  • 18 Barrier islands are places of change and motion. This figure shows how Hog Island, Virginia, has changed over the past 150 years, as a result of hurricanes and wave erosion. The dashed lines show the former shorelines of the island. Entire towns, such as the once- thriving community of Broadwater, have disappeared into the ocean as the barrier has shifted toward the mainland.
  • 19 Barrier islands maintain their equilibrium profile by migrating shoreward with rising sea level. Because many barrier islands are now covered with buildings, the island cannot migrate but is progressively eroded. Barrier islands are places of change and motion.
  • 20
  • Seawalls 21 Seawalls are structures constructed parallel to the coastline and are generally used as a last resort to protect property landward of an already eroded, small beach. They may be constructed of concrete, large stones (riprap), wood, or other material and are essentially built to repel the sea.
  • 22 Seawalls Seawalls accelerate erosion as water hits the sea wall with high energy and retreats with high energy. The beach narrows and becomes steeper and the water in front of the seawall deepens. As a result, bigger waves approach closer to shore, hastening erosion and removal of the beach. Seawalls require costly repair. Large storm waves can overtop sea walls and cause flooding.
  • Sea Cliffs 23 Some shorelines, particularly on the western coast of the U.S., are dominated by sea cliffs rather than beaches. The tops of these cliffs form prime real estate. But sea cliffs are subject to erosion by both wave undercutting and mass wasting.
  • Groins 24 Groins are built to trap sand being carried by longshore drift and build up the beach updrift of the structure. Unfortunately these structures starve downdrift beaches of sand, and erosion on those beaches is accelerated. The only way to save downdrift beaches is to construct more groins, and therefore groins tend to multiply into groin fields.
  • Jetties 25 Jetties are built to maintain an open inlet by blocking the movement of sand by longshore drift. The sand builds up on the upcurrent side of the jetties, and gets eroded on the downcurrent side. The result: severe shore erosion on the downcurrent side of the jetties.
  • Breakwaters and Artificial Reefs 26 Breakwaters, built offshore and parallel to the shore, have a similar effect, causing deposition in the protected area behind the breakwater and erosion on the downdrift side. An artificial reef is a man-made, underwater structure, typically built for the purpose of promoting marine life and the dissipation of wave energy. Objects that have been used include cars, ships, aircraft, concrete rubble, military tanks, and oil rigs.
  • Beach Replenishment 27 Beach Replenishment (Nourishment) is the addition of sand to a beach to replace that lost to the waves. Common sand sources are shore areas in which sand shows net accumulation (borrower areas) or sources well offshore and below wave base. Large scale replenishment projects are typically conducted by the US Army Corps of Engineers.
  • Beach Replenishment 28 Above: Miami beach before and after replenishment by the US Army Corp of Engineers. Above: Sea Bright before and after nourishment conducted by the US Army Corps of Engineers Left: Critical Zone on Sandy Hook filled by beach nourishment conducted by the US Army Corps of Engineers. • The cost of replenishment is largely funded by state and federal governments. • The actual beach extends well offshore. If that part of the beach is not also raised, waves will move much of the sand offshore to even out the slope of the beach. • Offshore sand used to replenish beaches is typical finer than natural deposited beach sand and therefore easily eroded. • A replenished beach must receive small scale renourishments at intervals of two to six years.