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Ch 11 Coastal Hazards Notes

by: Nishita Patel

Ch 11 Coastal Hazards Notes 2301.001 Environmental Geology

Nishita Patel
Environmental Geology
Dr. Tom Brikowski

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These are the notes on Ch 11 Coastal Hazards
Environmental Geology
Dr. Tom Brikowski
Class Notes
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This 11 page Class Notes was uploaded by Nishita Patel on Wednesday October 21, 2015. The Class Notes belongs to 2301.001 Environmental Geology at University of Texas at Dallas taught by Dr. Tom Brikowski in Fall 2015. Since its upload, it has received 24 views. For similar materials see Environmental Geology in Environmental Science at University of Texas at Dallas.

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Date Created: 10/21/15
CH 11 Coastal Processes Case History Cape Hatteras Lighthouse controversy N Carolina Erosion began to threaten the Cape Hatteras Lighthouse Lighthouse 500 m when first built in late 19th cent early 1990 s only 100 m The ppl had 3 options 1 artificially control coastal erosion at the site and reverse state policy of yielding to erosion 2 do nothing 3 move lighthouse inland Nat Park Service decided to move it inland 111 Intro to Coastal Hazards Coastal areas vary in topography climate and vegetation Capable of rapid change E Coast of US passive margin coastline far from convergent plate boundary 0 Wide continental shelf w barrier islands w wide sandy beaches 0 Rocky coastlines restricted to New England coast where Appalachians merge w Atlantic W Coast close to convergent boundary btwn N American and Pacific plates 0 Mtn building has made a coastline w sea cliffs and rocky coastlines 0 Sandy beaches present but not as abundant as w coast Many populated areas located near coasts Nation s largest cities lie in coastal zone and approx 75 of pop live in coastal states 0 Global warming and global rise in sea level increasing costal erosion problem Most serious coastal hazards 0 Rip currents generated in surf zone 0 Coastal erosion o Tsunamis 0 Tropical cyclones hurricanes Atlantic typhoons Pacific 112 Coastal Processes Waves Waves that batter the coast are generated by offshore storms Wind blowing over water produces friction along the airwater boundary Air moves faster than water some E transferred to water resulting in waves 0 Size ofwaves depends on O O 0 Velocity speed of wind more velocity bigger waves Duration of wind longer duration bigger waves Distance that wind blows across surface fetch longer fetch longer waves Important parameters of waves are wave height difference in height btwn wave s trough and its peak and wave length distance btwn successive peaks Wave period time in seconds for successive waves to pass a reference pt Swell wave groups generated by storms far out at sea 0 In shallower water transformation in swell takes place and waves break on the shore There are equations to predict wave height wind velocity and length of time that wind blows over water for deep water conditions 0 0 We can estimate when waves w particular erosive capability generated by a distant storm will strike the shoreline Lots of E expended over coastline by waves Wave steepness ratio of wave height to wave length Waves are unstable when wave height is greater than 10 percent of wave length Shallower water wave period is constant but wave length and velocity decrease and height increases Waves change from rounded crests to peaked crests w at troughs Near shoreline wave steepness is unstable causes it to break Local wave height may increase or decrease when wave front reaches nearshore environment I b c of irregularities in offshore topography and shape of coastline I offshore topography is similar to that of coastline shape of front changes and becomes more parallel to coastline as it comes near b c the waves slow down where water is shallowest and the result is bending refraction of wave front 0 wave normal 0 convergence height and E expenditure increases of wave normal at headland rocky point and divergence at beaches or embayments More E expenditure more erosion Plunging breaker waves are highly erosive at shoreline Spilling breakers more gentle Beach Form and Beach Processes landform consisting of loose material sand gravel that has accumulated by wave action at shoreline Compostion of loose material depends on environment 0 Pacific Island beaches bits of shell and coral 0 Hawaii black sand b c of volcanic rock 0 S Cali bits of quartz feldspar Landward extension of beach terminates at a natural topographic and morphologic change such as sea cliff or a line of sand dunes Berms at backshore areas on beaches formed by deposition of sediment as waves rush up and expend the last of their energy Beach face sloping portion of beach below berm Swash zone the part that is exposed to uprushbackwash of waves Surf zone portion of the seashore environment where turbulent translational waves move toward the shore after incoming waves break 0 Breaker zone area where incoming waves become unstable peack and break 0 Longshore trough and longshore bar elongated depression and adjacent ridge of sand produced by wave action Transport of Sand Sand on beaches are not static Wave action keeps it moving 0 Longshore current produced by incoming waves striking coast at angle 0 If wave arrives parallel to beach no longshore current I Longshore current stream of water owing parallel to shore in surf zone can be strong 0 process that transports sand along the beach has 2 components 0 1 Sand is transported along the coast w the longshore current in surf zone 0 2 Upandback movement of beach sand in swash zone causes sand to move along the beach in a zigzag path 0 most sand transported by longshore current 0 Direction of sand transport is usually north to south Rip Currents When series of large waves arrive at a coastline ad breaks on beach the water tends to pile up on shore and does not return as it came in along the entire shore line but is concentrated in narrow zones known as riptides undertow They re not tides they don t pull ppl under water but can pull them offshore US up to 200 ppl are killed and 20000 ppl rescued from rip currents Serious coastal hazard to swimmers kill more than hurricanes on annual basis 0 Rip currents are usually narrow and dissipate outside surf zone 0 To safely escape current simmer must recognize current and then swim parallel to shore until he or she is outside current can t panic 113 Coastal Erosion global rise in sea level and inappropriate development in coastal zone erosion is a problem Coastal erosion continuous and easily predictable Beach Budget An easy way to visualize beach erosion is the approach Input of sediment to beach occurs by coastal processes that move sediment along the shore line or produce sand from erosion of a sea cliff or sand dunes Output of sediment is material that moves away from site by coastal processes similar to those that brought sediment to peach Input exceeds output more sediment on beach and beach grows Output exceeds input beach erodes Budget balance of sand on beach over a period of years Erosion Factors Sand on many beaches is supplied to coastal areas by rivers that transport it from areas up stream where it has been produced by weathering of quartz and feldspar rich rocks 0 We ve interfered by building dams that trap sand so some beaches have eroded East coast tropical cyclones cause erosion Increase in sea level greater coastal erosion prob Sea Cliff Erosion Sea cliff along a coastline additional probs may occur b c sea cliff is exposed to both wave action and land erosion processes running water landslides this erodes cliff at greater rate Southern cali sea cliff ex during winter plunging breakers high potential to erode beaches remove mantle of san and expose base of sea cliff Most erosion of sea cliffs in So Cal takes places during winter Urbanization increased runoff more erosion on sea cliff drainpipes that dump urban runoff onto sea cliffs watering lawns and gardens on top of sea cliff also erodes it structures walls buildings swimming pools patios decreases stability by adding weight to slope rate of sea cliff erosion variable dependent on resistance of rocks and height of sea cliff 114 Coastal Hazards and Engineering Structures Efforts to stabilize a beach can be generalized into 3 approaches Hard Stabilization Engineering structures to protect a shoreline from waves sea walls groins breakwaters jetties designed to improve navigation or retard erosion tend to interfere w littoral transport of sediment along beach can cause deposition and erosion in their vicinity Sea Walls Sea walls structures constructed parallel to coastline to help retard erosion Constructed at base of sea cliff may produce mass wasting processes on cliff itself and erosion of base Sea walls are vertical re ect incoming waves and the re ection may enhance erosion and produce a narrower beach over decades also reduce biodiversity of beach ecosystem Groins Groins linear structures placed perpendicular to shore usually in groups called groin fields 0 Designed to trap a portion of the sand that moves in the littoral transport system 0 Small accumulation of sand will develop updrift of each groin builds an irregular but wider beach 0 Erosion occurs in downdrift direction can make a zone of erosion o Erosion may be minimized by artificially filling each groin requires extracting sand from ocean or other sources and placing it onto beach Breakwaters Breakwaters and jetties protect limited stretches of shoreline from waes designed to intercept waves and provide a protected area or harbor for boat mooring may be attached to or separated from the beach 0 Blocks natural littoral transport of beach sediment causes configuration of coast to change locally as new areas of deposition and erosion develop erosion probs in downdrift direction 0 Sand accumulates sand in updrift direction trapped sand may fill or block entrance to harbor as a deposit called a sand spit or bar develops o Degrading program used w artificial bypass transport of sand from site where it s degraded to downdrift location often necessary to keep harbor open and clear of sediment Ietties Ietties often constructed in pairs at the mouth of a river or inlet to a lagoon estuary or bay 0 Designed to stabilize channel prevent or minimize deposition of sediment in channel and generally protect it from large waves 0 Block littoral transport of beach sediment updrift widens downdrift erodes o Impossible to build a breakwater or jetty that won t interfere w longshore movement of beach sediment 0 Sometimes hard solution is decided upon to stabilize a stretch of coast of particular value to people I Sea dikes Netherlands Soft Stabilization Adding sand to a beach Beach Nourishment An Alternative to Engineering Structures 0 Alternative to engineering structures 0 Beach nourishment artificially placing sand on beaches in the hope of constructing a positive beach budget 0 If sand that enters and leaves the beach is accounted for there is enough sand left to maintain the beach itself 0 Ideally presence of nourished beach protects coastal property from attack of waves O O 0 mid 1970s Miami Beach FL and US Army Corps of Engineers beach nourishment program to reverse beach erosion problems I beach had nearly disappeared by 1950s I program designed to widen the beach I 1980s dune restoration added involved establishing native vegetation on the dune more than 600 km of coastline in the US have received some sort of beach nourishment I not all are positive results Ocean City N 1982 nourished beach for 5 million series of storms eroded beach in 25 months Beach nourishment controversial b c considered as quotsacrificial sandquot that will eventually erode I Miami beach exception Managed retreat Living w beach erosion w perhaps a mixture of hard and soft stabilization 115 Human Activity and Coastal Erosion Some Examples Human interference considerable coastal erosion 0 Highly populated areas more problems The Atlantic Coast The Atlantic coast from N Florida to New York is characterized by barrier islands long narrow islands separated from the mainland by a body of water Most altered by humans 0 Barrier island Maryland MD 0 Demand for Atlantic oceanfront beach in MD is very high and limited Ocean City on Fenwick Island High rise condos and hotels built on its waterfront Serious beach erosion prob b c natural frontal dune system has been removed Compared to Assateague Island located south much more natural state used for passive recreation sunbathing swimming walking etc Both islands are in same littoral cell they share the same sand supply Ietties were built and costal erosion in northern few km of Assateague island has averaged abt 11 m per yr Observed changes on MD s Atlantic coast are clearly related to pattern of longshore drift of sand and human interference 0 Construction of Ocean city inlet jetties interfered w natural southward ow of sand and diverted it offshore rather than allowing it to continue southward to nourish the beaches on Assateague Island The Gulf Coast Erosion is serious prob along Gulf of Mexico 0 Human modifications most responsible for the accelerated erosion are coastal engineering structures subsidence as a result of groundwater and petroleum withdrawal and damming of rivers that supply sand to the beaches The Great Lakes Erosion is big prob along Great Lakes and also bad along Lake Michigan shoreline o Worse when there are prolonged periods of high lake levels 0 High water stage lots of erosionand many buildings roads retainng walls and other structures are destroyed by wave erosion beaches become narrow 0 Below average lake level wide beaches develop that dissipate E from storm waves and protect shore o Cliffs along the shores of lakes are referred to as coastal bluffs and are analogous to the sea cliffs of the ocean shoreline also face erosion at Lake Michigan site Severity of erosion at a particular site depends on many factors including the following 0 Presence or absence of frontal dune system dune protected bluffs erode at slower rate 0 Orientation of coastline sites exposed to high E storm winds and waves erode at an increased rate 0 Groundwater seepage Seepage along the base of a coastal bluff causes slope instability increases erosion rate 0 Existence of protective structures Structures be locally beneficial but often accelerate coastal erosion in adjacent areas 116 Tropical Cyclones known as in Pacific and in Atlantic 0 Ex Nov 1970 tropical cyclone in northern bay of Bengal in Bangladesh 6m rise in sea ooding killed lots of ppl lots of damage to coast 0 Hurricane Mitch Atlantic Hurrican Katrina 2005 also lots of death and damgage Hurricane Form and Process To be a hurricane storm must have sustained winds of at least 119 km hr 0 Hurricanes are a variation of tropical cyclone a huge complex series of thunderstorms that rotate around an area of low pressure forming over warm tropical ocean water 0 They begin as tropical disturbances which are large areas of unsettled weather w a diameter as large as 600 km 0 In this area there exists an organized mass of thunderstorms w a general low pressure in which there is initial rotation caused by movement of the storm and rotation of earth Tropical depression may grow in size and strength as warm moist air is drawn into depression and begins to rotate clockwise in Southern hemisphere Warm water evaporates and is drawn into storm increases intensity of storm bigger air mass of storm potential E in form of latent heat amt of heat req rd to change liquid water to water vapor enters storm moist air rises condensation rain occurs as latent heat is released warming the air and making it lighter Lighter air rises more warm water drawn in storm may increase in size strength and intensity If wind speeds in storm reach 63 km the depression is called a tropical storm and receives a name I Hurricane has bands of spinning storm and thunder cells w an eye where air is subsiding in center around what is called the eye wall I When hurricanes make landfall it weakens and eventually dies I Intense rainfall causes numerous landslides Hurricanes classified based upon size and intensity I SaffirSimpson Hurricane Scale I 5 categories one smalles 5 largest category one still dangerous satellites used to see when potential hurricane might come airplanes collect data on wind speed air temp and air pressure to verify satellite data hurricanes slow down near landfall 0 storm surge most dangerous aspect of hurricanes causes most ooding and loss of life it s a local rise in sea level that results when hurricane winds push water toward the coast I pp drown or are struck by solid objects w in surge 0 most hurricanes and typhoons form in belt btwn 8 degrees north and 15 degrees south of the equator where warm surface water temps exceed about 27 degrees Celsius 0 Avg yr 5 hurricanes will develop 0 One of 3 storm tracks may develop 0 1 Storm heads toward e coast of Florida or islands such as Puerto Rico before striking land moves out into Atlantic to n east 0 2 Storm travels over Cuba into Gulf of Mexico to strike Gulf Coast 0 3 A storm skirts along E Coast and may strike land form central orida to New York 0 hazards high winds ooding intense precipitation landward transport of winddriven waves of ocean waters surges 0 property damage is high costs lots of money 0 population increased along Atlantic and Gulf coasts but loss of lives have decreased b c of effective detection and warning property damage has increased however 117 Perception of and Adjustment to Coastal Hazards Perception of Coastal Erosion Ppl living close to coast know about the damage and are generally well informed Ppl living few hundred meters away are aware of hazard but know little about frequency of occurrence Farther inland know coastal erosion exists but have little perception of hazard Adjustment to Coastal Hazards Tropical Cyclones 0 ppl adjust by doing nothing and taking loss or taking some action to modify potential loss 0 ex Homes built to allow storm surge to pass under house 0 better warning procedures and evacuation Coastal Erosion o adjustments include 0 beach nourishment soft solution 0 shoreline stabilization hard solution 0 land use change that attempts to avoid problem by not building in hazardous areas or by relocating threatened buildings Coastal erosion is a natural process rather than natural hazard Any shoreline construction causes change beach environment is dynamic Stabilization of coastal zaone through engineering structures protects property of relatively few ppl at a large expense to general public Engineering structures designed to protect a beach may eventually destroy it Once constructed shoreline engineering structures produce a trend in coastal development that is difficult if not impossible to reverse


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