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Chapter 9.doc

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Rob Milne

1 Chapter 9 – Hurricanes Hurricane Katrina, 2005 • Formed over the Bahamas on August 23, 2005 and crossed southern Florida as a category 1 hurricane • Regained strength in the warm waters of the Gulf of Mexico – its peak had winds at 280km/h • Devastation for more than 160km from its center • The storm surge ranged from 4-12m high – caused catastrophic damage to Louisiana, Mississippi and Alabama • Caused $115 Billion in damage – costliest disaster in US history, 2140 dead 9.1 – Intro to Coastal Processes Wind • Waves, created by wind thousands of kms off shore have a variety of sizes and shapes. Size depends on the following: o Velocity of the wind o Duration of the wind o Distance the wind blows across the water – which is called a fetch • As waves move away from their source they are organized into groups or sets – based on size and shape • Unexpected large rogue waves arrive at shore with disastrous results • 3 parameters of waves size and movements o Wave height (H) – difference in height between trough and crest of wave o Wavelength (L)- distance between successive wave crests o Wave period (P)- time in seconds for successive waves to pass reference point • Wave sets generated by storms far out at sea are called swells See chart • Variations along a Coastline – the shape of the coastline effects wave heights as they approach shore o Irregular coastlines have small rocky peninsulas known as headlands o This causes waves to slow down and bend or refract • Effects of wave refraction – causes convergence (the height and energy of the waves increases) and divergence o Thus the largest waves along a shoreline are along rocky headlands • Breaking Waves – waves differ on how they break along the shore o Plunging breakers – form on steep beaches and are erosive o Spilling breakers – develop on wide sloping beaches and are less erosive than plungers 2 o Tidal Bores – inflowing tidal waters are slowed by outflowing river water to produce these. Tidal bores have steep fronts and surge forward like broken waves Beach Form and Processes • Beach - consists of loose material such as sand or gravel, that has accumulated due to wave action at the shoreline. All beaches are made from different materials (coral, shells, rocks) and differ in colour • Sea cliff - landward boundary of the beach can be a cliff along the seashore • Bluff - landward boundary of the beach can be a cliff along the lakeshore o These are both erosional landforms • Beaches are divided into two zones that parallel the shoreline o Berm - flat, slopes downward o Beach face – slopes seaward. Lies within the swash zone - experiences the repeated up-rush and backwash of waves • Surf zone - seaward of the swash zone and is the place where waves move turbulently towards the shore after they break • Breaker zone - incoming waves peak and break See diagram • Sand is carried parallel to the shore in the swash and surf zones by the process littoral transport – consists of 2 processes o Beach drift – repeated seaward movement of sand produces as zigzag path o Longshore drift – transport of sediments by currents that flow parallel to the shoreline • Landforms are also produced by littoral drift: o Spit - long narrow ridge of sand that extends parallel to the shore from a point of land off the coast o Tombolos – are spits that are attached to the coast at both ends, they enclose lakes o Barrier islands - similar in form to spits but are wider and extend for longer distances Rip Currents • Rip Currents – carry large amounts of water directly away from the shore. Develop when a series of waves pile up water between the longshore bare and the swash zone o Mistakenly called undertow 9.2 – Tropical Cyclones • Tropical Cyclone – large cells of moisture laden air that rotate around an area of low pressure. Form over tropical oceans and have a variety of names depending on intensity and location 3 o High intensity, Indian/pacific ocean - typhoons o Similar intensity, Atlantic – hurricanes • Hurricane Formation o “Evil spirit and big wind” o Must have sustained winds of 119 km/h and they only form in oceans warmer than 26.5 Celsius o Usually start out as tropical disturbances – large areas of unsettled weather and thunderstorms for more than 24 hours o Tough - elongated area of low pressure o A low pressure cell forms when winds rotate around disturbed weather  At this time it is called a tropical depression o Warm moist air is drawn into depression and rotates faster – 63lm/h = tropical storm • Hurricane characteristics o Need a supply of warm water, Atlantic hurricanes weaken as they move north o In the northern hemisphere winds circulate in a counter clockwise direction because of the Coriolis effect • Hurricanes are ranked according to the Saffir-Simpson Scale (5-pt) o See chart 9.3 – Tsunami • Tsunami is a Japanese word meaning ‘harbour wave’ • Can be triggered by several catastrophic events: o Rapid uplift of seafloor during earthquake o Underwater landslide o Collapse of flank of a volcano into ocean o Submarine volcanic explosion o Asteroid impact 2004 Indian Ocean Tsunami • 2 largest earthquake and most deadly tsunami of all time – 9.3 magnitude and 7.2 aftershocks, December 26, 2004 • Indo-Australian and Burma plates separated • Deaths reached 228,000 – 1/3 were Indonesian 9.4 – Geographic Regions at Risk from Coastal Hazards Coastal Erosion • Because the majority of the US population (75%) is along the coast – coastal erosion is a big issue  8m per year 4 • Great lakes shorelines retreat 15m per year • In the arctic where less people live this is not a severe issue – however coupled with the warming of the climate and thawing of permafrost it has become a bigger issue Hurricanes • The Atlantic and gulf coasts have the highest risk of hurricanes in North America o 5 hurricanes/year o They form off the west coast of Africa and take one of 3 tracks  Westward across eastern Caribbean, then northeast into Atlantic  Westward over to Cuba into the gulf of Mexico  Westward across eastern Caribbean and north-east along Atlantic coast Tsunami • Can occur in any of the world’s oceans • 85% occur in the Pacific ocean Hurricane Juan, 2003 • Struck Nova Scotia and PEI as category 2 • 200 million in damage and killed 6 people • Peak winds of 170km/h • The warning kept changing giving people little time to prepare 9.5 – Effects of Coastal Processes Coastal Erosion • Becoming a serious problem because of sea level rise and increased development on shore lines • Beaches gain and lose material – sand added from updrift sources (rivers, eroding cliffs) and is lost (littoral transport, storms, wind). The severity depends on the following: o Existence of coastal dunes – shorelines with dunes erode slower than without dunes
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