GEOL110_Natural Hazards CH11
GEOL110_Natural Hazards CH11 GEOL 110
Long Beach State
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This 5 page Class Notes was uploaded by Doris M on Sunday April 24, 2016. The Class Notes belongs to GEOL 110 at California State University Long Beach taught by Ewa Burchard in Spring 2016. Since its upload, it has received 51 views. For similar materials see Natural Disasters in Geology at California State University Long Beach.
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Date Created: 04/24/16
Coastal Hazards Learning Objectives -how plate tectonics impact the coastal topography? -how coastal processes: waves, coastal forms, and processes, are impacted by rising sea level -why coastal erosion rates vary along different US coastlines -what is coastal erosion hazards -what is the link btwn coastal processes and other natural hazards -how use of the coastal zone affects the coastal processes Folly Island and submerging coast -barrier island south of Charleston , S Carolina -barrier to ocean waves that would strike the mainland -about 10 km long, less than 1 km wide -most of island has an elevation of 1.5-3 m -Typical Atlantic barrier island-eroding at a high rate Introduction to Coastal Hazards -ex. Sonoma County -Coasts-rapid change at interface (water, air, rock) -plate tectonics impact on coasts: -west coast US and Canada ACTIVE -its close to convergent and transform boundaries -have sea cliffs an rocky shorelines -east coast US and Canada, Canadian Arctic-PASSIVE -because they aren't close to convergent boundary -have wide continental shelves with barrier island and sandy beaches -most serious coastal hazards include -strong coastal currents -coastal erosion -storm surge (ch 10) -tsunamis (ch 4) -tidal waves -constantly changing Waves -waves are caused bu offshore producing friction over water -size of waves depends on… -speed of wind -duration of wind -fetch-dist wind blows over water surface -waves become sorted into groups as they move away from their origin -rogue waves are exceptions to these groups -unpredictable -still being studied because they are unpredictable -rogue waves -appear to form by constructive interface -multiple, similarly sized waves intersect to create a much larger wave -wave may be as high as the sum of the intersecting waves -can be extremely dangerous to the unsuspecting beachgoer -can appear out of nowhere -lives are lost-sweep individual ocean -can also appear out of nowhere in the open ocean -can threaten ships -waves can reach 30 m -ships are designed to withstand 15 m -wave form -height: dist from crest to trough -wavelength: dist from crest to crest -period: time btwn crests -Study the wave motion helps to understand t=wave energy transmission-study the wave motion -motion is circular in open ocean -circles decrease in diameter with inc depth -waves in shallow water become ellipses as waves "feel bottom" -when depth is 1/2 wavelength -may eventually become a narrow ellipse w/ a back and forth movement -when the ocean is transported to the coast, it becomes elongated -moves slower on the ocean floor, and faster at the surface -waves slow down but height inc -wave sets generated by storms are called swells -swells enter shallow water , become unstable and break -mathematical equations can be used to predict height, period, and velocity -wave steepness= H/L, unstable when H/ L > 1/7 -waves move in open ocean with little energy loss -energy is spent on reaching coastline -wave energy is proportional to square of wave height -Variations along a coastline -irregulations in topography ocean floor and coast cause variations in wave height as is approaches shore -wave front: a single wave front -irregular coastlines have headlands -the shape of the coast is similar underwater to that of the coastline -water gets progressively shallower close to shore -waves slow as it gets closer to shore -wave front: a single wave, can bend around headband known as refraction -emerging coast -uplifted -properties: cliffs -associated with tectonic movement -more wave erosion -submerging coast -large shells -lots of sand, more sand than immerging -submerging with the title wave -deposition of sand -more erosion -more protection than the emerging coast -if the topography of the ocean is shallow, the wave would be larger -wave refraction: if you have a coast, you can draw the lines that are parallel to the coast because the shape of the coast will be extending down to the ocean. Waves would behave from shallower to deeper. When there are gentler wave the waves would have lower heights. Discussion -Beach is the river of sand because it is moving -the sand at the shores one day will not be the same on another day -river transports sediment around the continent -ocean transport transports sediment to the shores -dams stop the flow of sand -more sediment larger beaches, more protection from the waves Waves cont. -Effects of wave refraction -effects of wave refraction -wave normal, perpendicular to wave fronts pointing toward shoreline -wave refraction causes normal to converge and diverge to depth change -convergence Types of Breakers -breaking waves -plunging breakers -waves that pick up quickly -typical on steep beaches -more erosive -spilling breakers -waves that spill gently -typical on wide, flat beaches -more likely to deposit sand -wave steepness = H/L , unstable when H/L > 1/7 Beach From an Processes -Beach consists of loose material which has accumulated by wave action on shoreline -type of beach material depends on source of sand -white beaches from shell and coral (Pacific Islands) -Black beaches from volcanic rock (Hawaii) -brown beaches from quartz and feldspar (Carolina) -The Beach onshore -landward extent of a beach on seashore or lakeshore -line of sand dunes -they protect the coast -line of permanent vegetation -sea cliff of bluff (lake) forms from erosion of rock or sediment -beaches are divided into -berm- deposited by waves -beach portion sloping landward -beach face- in swash zone (where waves swash and backwash) -beach portion tht slopes toward water -the beach offshore -Beach Form and Processes, -sand transport -littoral transport -sand movement parallel to shore -beach drift -sand moving in zigzag pattern in swash zones -longshore drift -transport of sand by longshore currents -longshore currents -current that flows parallel to shoreline as a result of up an back movement of water in swash zone -updrift and downdrift -indicate the direction in which sediment is moving or accumulating along the shore -updrift- towards the coast -downdrift- away from the coast Sea Level Change -The level of sea is constantly changing -relative sea level -position of the sea at the shore -influenced by movement of both the land and water -Eustatic sea level -global sea level -controlled by processes that affect overall volume of water in the ocean and shape of the basins -rises or falls when the amount of water in the worlds oceans inc or decreases -sea levels -if the temp causes expansion of the water, the wave height would change on the shoes of cold or warm water routes -wave cut notch: caused by erosion, found at the bottom of a sea cliff -if you look on a globe, you can see that passive coast have wider beaches with more sediment -where there is tectonic activity, there will we shelves -active coast will have deeper sea levels Relative Sea lv -glacier melt or earthquakes can cause uplifting of land -dec in sea lv Tides -changes in elevation of the ocean surface: extremely long period waves -caused by the gravitational forces exerted upon the earth by the -moon and to a lesser extent by the sun -the ocean water can be pulled by the gravity therefor some water stays behind can be affected differently from waters in another place -moon rotates around the earth -when tides from, the ocean forms there -the ocean wave impact the rotation of earth -Earth's rotation slows down -the period is the time it takes for the wave to move -Diurnal tidal- 1 high and low tide each trial day -Semidiurnal- 2 highs and lows of approximately equal heights during each tidal day A mixed- 2 highs and lows of unequal heights during each tidal day -Monthly tidal cycle -spring and neap tides -Tidal patterns -Many factors influence the tides -shape of the coastline configuration of the ocean basin -water depth -tidal currents -horizontal flow accompanying the rise and tall of tides -types of tidal currents -flood current- advances into the coastal zones -ebb current- seaward moving water -sometimes tidal deltas are created by tidal currents
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