Study Guide For EESA05, Chapter 9

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Environmental Science
Ingrid L.Stefanovic

Chapter 9: Coastal Hazards 9.1 Introduction to Coastal Processes Waves waves are generated by wind; wind blowing over the ocean or a lake transfers some of its energy to the water, producing waves waves travel through the water and eventually expend their energy at the shoreline they have a range of sizes and shapes the size of it depends on a combo of the following: o velocity of the wind; the stronger the wind, the larger the waves o duration of the wind; those tat last longer have more time to impart energy to the water, thereby producing larger waves o distance of the wind blows across the water, referred to as the fetch; longer fetch allows larger waves to form, thus waves in the ocean are generally larger than those in a lake as waves move away from their source, they become organized into groups, or sets, of similar size and shape; sets of waves may travel long distances across the ocean and arrive at distant shores with little loss of energy sets with different characteristics arrive at coastlines at different times alteration of small and large sets allow surfers to wait and take advantage of the sets of larger waves that they know will eventually arrive interactions among different sets from different sources produce a regular pattern, or surf beat unusual large rogue waves arrive at shore with disastrous results waves moving across deep water have a similar basic shape or wave form 3 parameters describe the size and movement of a wave: o wave height (H)- difference in height between the trough and crest of a wave o wavelength (L)- distance between successive wave crests o wave period (P)- time in seconds for successive waves to pass a reference point reference point used to determine wave period can be a pier or another object anchored to the seafloor one can understand how waves transmit energy thru the water by studying the motion of an object on the water surface and one below the surface o when a wave moves thru the area, an object 20 m below the surface moves up, forward, down and back in a circular orbit, always returning to the same place o an object nearer the water surface also moves in circles but the circles are large shape of orbital motions changes as waves enter shallow water o at depth of less than their wavelength the waves begin to feel the bottom, causing the circular motions to become ellipses o in very shallow water (depth0.05L), the motion at the bottom may be horizontal with only a small vertical component wave sets generated by storms far out at sea are called swell o wave height, period and velocity of swell can be predicted by using equations based on the fetch, wind velocity and length of time the wind blows over the water by knowing the velocity and height of the waves generated by a distant storm, we can estimate when the waves will strike the shore and how erosive they will be as waves approach the coast, they become unstable and break o wavelength and velocity decrease and wave height increases; only wave period remains constant o waves also change shape from rounded crests and troughs found in deep water to peaked crests with relative flat troughs in shallow water as waves approach their breaking point in shallow water, their height may increase to twice that in deep water the waves collapse, or break, toward the beach because the wave crests keeps moving fwd while the lower part of the wave slows down waves release a lot of energy as they rush ashore; the energy is ~ proportional to the square of wave heightwaves 5 m high expend ~ 25 times the energy of waves 1 m high Variation along a Coastline wave heights along a coat may increase or decrease as waves approach the shore o variations caused by irregularities in near-shore bathymetry and the shape of the coastline 1
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