Coastal Processes - Envir Sc 1G03 Started November 22nd,...

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Unformatted text preview: Envir Sc 1G03 Started November 22nd, 2011 Earth & the Environment On the Beach Shorelines and Coastal processes Outline 1. Why do we need to understand coastal processes? Reading: 2. Sediment transport on coastlines Chapter 18 Coasts & Oceans - waves, longshore drift, rip currents, beaches 3. Tides 1. Why do we need to understand coastal processes? coasts - Canada has the longest coastline in the world -- (over 200,000 km) - coastlines are dynamic; global sea level is rising ---- (23cm/100yrs) - need to carefully plan coastal land uses Hurricane Hugo 1989 South Carolina 5m storm surge $10 billion damage 500,000 people evacuated 29 killed Hurricane Katrina August, 2005 1322 lives lost $70 to $130 billion damage costliest storm in US history >1 million people displaced Extensive coastal damage Katrina coastal damage Lidar images - elevation gains (green) - losses (red) http://coastal.er.usgs.gov Katrina coastal damage Dauphin Island, AL. 3-D views looking along the island http://coastal.er.usgs.gov 2. Sediment transport on coastlines Waves and currents move sediment How do waves form? - induced by flow of wind across water surface Fig. 18.2 . - water molecules have circular orbits below waves - orbits become smaller with depth Wave base - depth at which water movement is negligible Fig. 18.3 * Depth of wave base = 1/2 wavelength Why is it important to know the depth of wave base? the impact on sediment movement Waves... - As waves approach shore... orbits become elliptical, develop into breakers Fig. 18.4 Longshore drift What can you tell about the movement of sediment here? - Sediment is moving parallel to coastline - LONGSHORE DRIFT What conditions do you need for longshore drift? Longshore When waves approach coast at an oblique angle; - sediment moves obliquely up the beach by swash - moves directly downslope in backwash - transports sediment along coast Land Beach face Sea Wind direction Net sediment movement Longshore drift... - Maintains supply of sediment to beach Fig. 18.11 Forms 'spits' (ridges of sediment that extend into open water) Forms 'bars' (ridges of sediment that cut o bays from open water) Fig. 18.12 Local example of a bar??? Burlington Bar, one in Cootes Paradise What happens if something is built out into longshore drift system? Fig. 18.14 'Groynes' - trap sediment - used to widen beaches Problem! May cause erosion downcurrent Jetties - May cause deposition & erosion of sand Fig. 18.14 Breakwaters - Santa Monica Protect areas of the shore from waves that can be deposited in protected area Rip currents - Narrow currents that flow out to sea - returning water seaward - Travel at water surface - transport sediment - and swimmers! Fig. 18.6 ** Shoreline environments Shoreface - below low tide mark - sediment gets sandier towards shore (coarser grain b/c of higher energy) Beach face - Steepest part of the beach - lots of wave action Backshore/dune - landward of beach - sand blown inland from dunes Backshore dune systems Along coast of Lake Erie over 75m high How do beaches develop? Beaches usually composed of Sand - shell fragments in tropical areas - gravel in colder areas Seasonal cycle - storm waves in winter erode beach face, build oshore sand bar - low energy waves in summer move sand onshore Canadian beaches Commonly affected by ice - lake or sea ice blown onshore in winter - boulders can be removed onshore to form 'boulder barricades' or 'ramparts' lake erie in january 3. What are tides? - Sea level on the earth's surface rises and falls once or twice daily as a 'tide' - Caused by tide-generating force created by gravitational attraction of 'moon' and 'sun' - Creates two bulges in the oceans –– one closest to the moon and the other on the opposite side of the Earth - As Earth rotates -- water levels rise ('flood tide') - water levels fall ('ebb tide') Tidal Range dierence in sea level between high tide and low tide Fig. 18.7 Where would you find the highest tidal range in the world? - Bay of Fundy N.S. - Open ocean < 1m - Increases toward coast Fig. 18.8 Bay of Fundy - Tidal range >16m - 'Tidal Bore' can reach speeds of 35 km/hr highest in the world Fig. 18.8 Bay of Fundy tidal power Annapolis Power Tidal Facility - Generates 20 megawatts/day - Water moves through barrage - need >5m tidal range - Environmental concerns ...
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