EESA05 - Environmental Hazards - Lec 10 (near-verbatim).docx

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Environmental Science
Mandy Meriano

EESA05 Lecture 10: Waves, Currents and Coastlines PY Date: Nov 20, 2012 Slide 1:  Impact of sea-level rise on coastlines  Remediation techniques to keep currents/waves back to prevent erosion Slide 2:  Slide 3:  Slide 4: Sea-level rise and Coastal Erosion  As sea level rises, the equilibrium position of sand bodies offshores moves inland o Here is the beach, water-level; there’s a mound here (if you go swimming at beach, all of a sudden it gets shallow because of mound) o Sandbar is just the way the water reaches the coast such as the current that’s going along the coast – you get these sandbodies o As the sea levels change, these sandbodies change (they aren’t transient); change in the morphology of the coastline  Barrier islands are strongly affected by sea level change o Barrier islands are like sandbars = form in front of the coast and usually between the sandbars, between barrier beach and the coast there are usually lagoons; these barrier islands separate the coast from the sea/lake – so have a bay/lagoon between the barrier beach and the coast o These barrier beaches are very affected by sea level change and can move/are transient o Don’t want to buy fast-moving real-estate on top of these barrier islands  When we build on barrier beaches = they are vulnerable to changes in sea level o Ex) fire hydrant; at some point, the beach was higher up but now can see quite a bit of erosion in the beach; the sand that’s eroded here is actually deposited somewhere else  A small rise in sea level leads to a large migration of the barrier island; movement of sand deposits/mound closer to the shore where it’s more shallower  Many of these have been extensively developed, particularly in the southern US o An example of coastal erosion – Chesapeake Bay, Virginia – can see that it has moved quite a bit over time o Back in 30s, there was a town on that little island but now all of that under water because it has moved o Not a smart place to build structures on  There’s a difference between local change in sea-level and global sea-level rise o Local mean sea level = average level of the water close to a land; can be lake/sea level; changes in that local sea level is usually in correspondence with any change in the level of the land o Global sea level = eustatic sea level – changes in global level, not local 1 EESA05 Lecture 10: Waves, Currents and Coastlines PY Date: Nov 20, 2012  How do we get changes in sea level? o There are smaller short-term changes in sea level but can also have longer term changes in sea level o We get longer term changes in sea level when there’s change in temperature o Water warmer = expands – on a global level, sea level rises after being warmed because of expansion of the water o Melting ice = increase in sea level – glacial ice at the poles; not talking about sea ice; it’s only ice that’s on LAND that if it melts that it will result in increase in sea level; if the ice already in sea, won’t make a difference in sea level if it melts o Subsidence = also results in increased sea level; when land subsides, sea level tends to transgress inland and your sea level goes up o A way to get localized changes in sea level is any kind of isostatic change:  If you have really heavy stuff sitting on continent or specific area – it depresses the ground and it actually depresses the mantle – isostatic change – water rebound after glaciers melted; initially quick isostatic rebound but now still continuing at slower, gradual rate Slide 5: Sea-level Rise and Coastal Erosion  During the last deglaciation, sea levels rose about 130 m, and this rise was so rapid that most offshore sand bodies are still moving inland to compensate  Developed barrier islands are a hazard for this reason  Hog Island is migrating inland, leading to the the loss of the town of Broadwater Slide 6: Coastal Erosion – Wave Action  Waves increase in size with the following factors: o Wind velocity o Length of time that the wind blows  Larger waves created by longer storms o Fetch: the distance of water over which the wind blows o So, if you have higher wind velocity, longer storm and longer fetch – will create mch larger wave  The wave is round/orbital; if on the wave, will bob up and move forward, go down and move back  ***The water itself isn’t moving, it’s the wave that’s moving along; if cork on a wave, it will just move up and down and rotate back on the surface of the water; whereas the wave will travel towards the coast; the potential NRG will travel with the wave  Waves are characterized by period, wavelength, velocity and amplitude  When the wave moves, that NRG is moving with the wave – kinetic NRG as it reaches closer to the shore, where it changes its height; the potential NRG changes and so it can’t maintain the height that it has 2 EESA05 Lecture 10: Waves, Currents and Coastlines PY Date: Nov 20, 2012  From potential NRG to kinetic NRG which is why it goes from small wave to wave with greater height as it reaches the shore – breakwater – if you’re surfing, this is where you want to be, where the wave actually picks up in height – where it’s hitting the shallower zone near the coast Slide 7: Coastal Erosion – Wave action  Rogue waves are unexpectedly large waves that arise by the combination of several smaller waves  Have waves travelling in one direction and then you have a wind/storm going against it – creates rogue wave – increases in height and then breaks  These are transient events, but are quite common  Over 200 super-carriers have been lost at sea in the past 2 decades, some due to waves as high as 25 m rising out of calm seas  Can be in a calm sea and all of a sudden there’s a rogue wave Slide 8: Waves  The water of a wave moves in a circle (prograde)  The wave form itself moves, but the water does not have net forward motion – the motion is the same for a point suspended in the water column  Wave circles are much larger and as it gets closer and closer to the shore, there’s a change in potential NRG – it goes up in height, it’s at the surf zone where you get the break water – where you can catch the surf and come to the shore  As you go deeper into the water column in sea or ocean, these orbital circles gets smaller and smaller until @ the bottom, it becomes more ellipitical and almost become linear; same thing happens as it gets closer to the shore as well Slide 9: Breaking Waves  The NRG contained within waves is delivered to shore o Not all beaches are gentle; some can be steep o 2 types of breakage:  On gentle slope – can get spilling breakers  On steep shores – plunging breakers – lot more erosive  As they slow down, their NRG is concentrated into a smaller area and the wave grows higher  Once its height is about 1/7 of its length, the wave front is steep enough for the wave to break Slide 10: Wave refraction  Wave refraction: as the wave travels near the coast, there’s a change in the direction of the mvt  Wave front and imagine a line that’s drawn normal to all these wave fronts and then put arrow at the end of it so that it becomes a vector; if you remove all these arrows and you remove this part of the coast and you’re asked to draw the coast line based on the waves – should be able to draw the coastline 3 EESA05 Lecture 10: Waves, Currents and Coastlines PY Date: Nov 20, 2012 o Because as the waves travel towards the headland, there’s a convergence of the waves and away from headlands, there’s divergence (desiccation of NRG)  Around convergence = much more erosive power/strong; divergence zone = more calm – usually called a bay  Waves refract around promontories and rocky headlands  Refraction may focus the NRG of the waves onto a smaller portion of the coast, leading to erosion  In bays, NRG is dispersed  Picture shows sea arch = shows the very erosive power of the waves @ the headland areas – very strong rocks but waves will eventually cut into it  So, we know about waves heading straight towards the shore; but there’s also another mvt of the waves – called longshore drift – separate mvt that moves parallel to the coast Slide 11: Coastal Erosion  The sea will erode the coastline directly Slide 12: Longshore Drift  There’s surf zone where longshore drift is taking place and then just along in the swash zone o Can see swash zone if you stood on the beach, the waves come up the beach and then they switch back o Along swash zone, get another kind of longshore drift; here, the waves are moving the sand, see how the mound was changing – this was closer to the surf zone o When you’re closer to the shore, this longshore drift is also moving sediments/sand along in the direction of the arrow  This is really impt in the way that beaches are created or islands are created (ex: Scarborough bluffs erosion sediments created the Toronto Islands and Woodbine Beach; this is Scarborough bluffs sand; we want to protect the property on Scarborough bluffs so doing all kinds of things to reduce erosive power of the waves; b/c we’re preventing erosion, we are starving the Queen St. beach and the Toronto Islands)  How do you get longshore drift? o We know that as waves get closer and closer to the shore, it doesn’t hit straight on to the shore; it hits at an angle of less than
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