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Coastlines Beaches and Coasts • The coastal zone and beaches along the continental margin are among the most dynamic geologic setting on earth • Human settlements are concentrated along the coast and most of the world’s population lives along the coast • Human activities along the coast oftentimes accelerate coastal erosion processes Coastal Zone • World’s coastlines extend for 440,000km • Coastal zone comprises < 0.05% of landmass area • > 50% world’s population lives within < 100km of coastline • Most critical part of earth surface in terms of global economy Open Coasts • Dominantly Erosional (West Coast) – Narrow – High relief – Rocky • Dominantly Depositional (East Coast) – River deltas, barrier island systems – Strand plains, reef coasts, glaciated coasts – Complexity of depositional environments EROSIONAL COAST DEPOSITIONAL COAST Nearshore Circulation • Wave Propagation, Refraction, Defraction, Reflection • Longshore Currents • Rip Currents WAVES • Most water waves begin as wind (the generating force) blows over the water and friction causes wrinkles to form on the surface. • These are called ripples or capillary waves and their restoring force is surface tension. • Small areas of capillary waves can appear and disappear rapidly giving the impression that they are jumping from point to point over the surface. • These rapidly moving patches have been called cat’spaws. Waves • As more energy is transferred to the water, the waves will grow in size. • This will increase the roughness of the surface and make it even easier for the wind to transfer energy to the water so the waves will grow in size rapidly. • As the waves grow, gravity will become the restoring force and the waves will be called gravity waves. • These are the waves we typically see at the coast or any other water surface Ideal Wave Anatomy of a Wave: Definitions • The amount of time necessary for one wavelength to pass by a stationary point is called the period of the wave. • Period is usually measured in seconds/cycle (cycle is another term that can be used for wavelength in this case). L=1.56T2m • The number of wavelengths that pass a stationary point in a unit amount of time is the wave frequency. • Frequency is usually measured in cycles/second, the reciprocal of the wave period. Moving wave form sets water particles in circular motion Size of Waves • Actual transfer of energy from wind to water complex, but size of wave depends on: – How fast the wind blows (velocity) – The length of time the wind blows (duration) – Distance over the water that the wind blows (fetch) Wave Speed • The speed of a wave, its wavelength, and its period are all related to one another • Wave speed is usually represented with the symbol C which stands for celerity which is derived from a Latin word meaning swift. • The speed of a wave (C ) is equal to its wavelength (L ) divided by its period (T ). C =(L/T ) Wave Refraction • As waves approach the shore they will bend or refract because of their interaction with the seafloor • As the wave enters shallow water it will slow down as a consequence of friction or drag with the seafloor; however the wave portion in deep water still moves relatively faster • The wave front then turns or refracts Figure 9.12 Wave Diffraction • As a wave passes an impermeable object such as a jetty, breakwater or other type of structure, the wave energy is spread along the crest behind the obstacle Wave Reflection Reflection is dependent on the breaking state of the wave as well as the slope and hardness of the surface. It can vary from 0% for breaking waves on low-sloped, porous beaches to nearly 100% for non-breaking waves in deep water on a hard, vertical surface such as a sea wall. wave reflection. Longshore Drift- Littoral Transport • Sediment movement along the coastline is done by longshore currents and is termed longshore drift • Longshore current confined to surf zone Rip Currents • Rip currents are very strong currents that flow out to sea in the surf zone • These narrow currents flow at speeds of a few tens of centimeters per second and can transport sediment • Size, orientation, and spacing of rip currents are related to wave conditions and nearshore bathymetry Rip currents represent the primary circulation system in the nearshore zone The seaward extent of rip currents can vary from just beyond the line of breaking waves to hundreds of yards offshore, extending up to a maximum of 2.5 times the surf zone width. Video Animation.