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Earth Science - Lecture 29 Notes.docx

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Earth Sciences
Sergei Basik

November 12, 2013 COASTAL PROCESSES: MITIGATION OF COASTAL CHANGE Living on Dangerous Coasts  By 2004: about 42% of population lived in coastal countries (this continues to increase)  Beaches and sea cliffs constantly change  Coastal processes do not stop  People build permanent structures at coast trying to hold back the sea and prevent natural changes  Effects of hurricanes are a normal part of the evolving landscape (seen as ‘nature on a rampage’)  Human activities can affect wav action, beach response and sea-cliff collapse Waves and Sediment Transport  Winds blowing across sea push water surface into waves (friction between air and water)  Wind-driven waves are described in terms of: o Wave height o Wavelength o Period  Height of wave increase by: o Fetch: length of water surface over which wind blows o Amount of time wind blows across water surface  Waves of different sizes and moving at different speeds can interfere with each other to increase/decrease the size of the combination (giant rogue waves)  Offshore: water has circular motion within the wave  Waves approaching shore: o Begin to ‘feel bottom’ where water depth is less than half the wavelength o Waves slow in shallow water but rise in height causing them to break  Wave energy is proportional to mass of moving water Wave Refraction and Long-shore Drift  Waves approach the shore at an angle  Part of wave in shallower water slows down  Part of wave still in deeper water moves faster  Crest of wave curves around toward shore  Wave refraction: waves bend or refract toward shore  Breaking wave pushes sand grains up the beach at an angle  Retreating waves flow down the beach slope (with sand) perpendicular to the water’s edge  Angled waves create long-shore drift (carrying sand along shore over time) November 12, 2013 Beaches  Beaches: o Accumulation of sand or gravels supplied by sea-cliff erosion o River transport of sediments to coast  Size and number of sediment particles depends on: o Energy of wave attack o Resistance to erosion of material o Particle size into which material breaks  Fine sand can be moved on a gentle slope  Coarse sand or pebbles can only be moved on a steeper slope  Whether sediment moves shoreward or not depends on balance (equilibrium profile) of: o Shoreward bottom drag by waves o Size of bottom grains o Downslope pull by gravity  Slope of bottom controlled by: o Energy required to move grains o Energy related to water depth, wave height and grain size  Most sediment moves at water depths of less than 10m Engineered Beach Protection Structures  Typical response to threat to beachfront or clif
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