Lecture 17-Coastal Oceanography II Flashcards Preview

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Flashcards in Lecture 17-Coastal Oceanography II Deck (30):

What is a groin?

-Groins are structures that extend from the beach into the water. They help counter erosion by trapping sand from the current. Groins accumulate sand on their updrift side, but erosion is worse on the downdrift side, which is deprived of sand.

-it is good for one bit of the coast but bad for another

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What is a seawall?

-Seawalls protect property temporarily

-increase beach erosion by deflecting wave energy onto the sand in front of and beside them.

-High waves can wash over seawalls and destroy them and property.

-rigid objects parallel to the coast -expensive

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What is beach nourishment/importing sand?

- Importing sand is considered the best response to erosion.

-The new sand often is dredged from offshore, can cost tens of millions of dollars, and can disturb aquatic biodiversity.

-Because it is often finer than beach sand, dredged sand erodes more quickly.

-has to be a particular type of sand

-there are limited sources of sand

-really expensive

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What is a breakwater?

-a structure off the coast, protect part of the coast -allows sediment accumulation behind it -protects against erosion -same as seawall!


What are the two areas where freshwater and saltwater mix?

-deltas and estuaries -when have a large river= delta -small= estuary


Why are deltas and estuaries important?

-Some of the most important environments for human habitation, often with densely populated adjacent land regions • As a result, they are the most polluted of all -very productive regions


What is needed for delta formation?

-large river -broad continental shelf where sediment can accumulate -if the sea level drops off quickly the sediment sinks to the depth and cannot accumulate to form a delta


What factors determine shape of deltas?

Shape determined by competing influences of waves, tides and the river flow – For example, river‐dominated deltas tend to be finger‐like and protrude into the ocean


What is an estuary?

A body of water partly surrounded by land,where fresh water mixes with river water -wave, tide or river dominated • Salinity varies over time depending on the - e.g. Port Phillip Bay, a tide‐dominated estuary as Yarra is too small and the narrow mouth protects it from waves


What are the 4 types of estuaries based on their evolution?

1. Drowned river mouths 2. Fjords 3. Bar built 4. Tectonic


What is a drowned river mouth estuary?

-used to be a valley of a river, then sea level rise

- eg. Sydney Harbour

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What is a fjord estuary?

-inlet carved out by glaciers

-then sea level rise

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What is a bar built estuary?

-barrier islands= in between islands it is narrow so it increases the influence of the river

-eg. Moreton Bay

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What is a tectonic estuary?

-land moves up and down(there the water is)

-eg. San Francisco Bay

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How are estuaries divided by relative influences or river and sea?

a)Salt wedge: small tides= river flows out, small mixing

b)Well mixed: bigger tides, more mixing

c)Partially mixed: when a deep estuary and big tide: river water is less dense so on top, get vertical stratification

d)Fjord: big river flow so stays on top and flows, the sea water on bottom, little mixing

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Are tides waves?

-yes -Tides are really long waves with periods of about 12 hours – Period and height varies tremendously from place to place along the coast eg.Mont St. Michel France: flat and an island, the speed with which the tide comes in is fast as the landscape is flat


What causes tides?

-the gravity pull of the Moon and Sun

- get a bulge of water being pulled by the Moon on one side and then another bulge at the opposite side

-the Earth is moving around its axis under the bulges= that is why we have about 1 tide every 12 hours

-have 2 bulges =aligned with the Moon and Sun

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What is the difference in lunar and solar influence on tides?

-the Sun also exerts a tidal force but has about 1/2 the influence of the Moon

-have solar and lunar tide

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What is a spring and neap tide?

1.spring tide: when Sun and Moon aligned= pull the most= biggest tide 2. neap tide: when Sun and Moon at a 90 degree angle= smallest tides due to the competing gravitational pulls


Why do tides vary locally?

-Since tides are waves, they can be affected by orientation of coastline and continents -they bounce off, interfere with each other, get trapped in bays etc.


Why is there such extreme tide in Bay of Fundy?

-the shape of the Bay is perfect for creating tidal resonance -the tide coming in and out amplify each other


What is a tidal bore?

- a tidal phenomenon in which the leading edge of the incoming tide forms a wave (or waves) of water that travels up a river or narrow bay against the direction of the river or bay's current. -V shaped region of the bay, as it narrows, water is in a smaller volume=amplitude goes up and more energetic per unit


What is isostatic rebound?

-the rise of land masses that were depressed by the huge weight of ice sheets during the last glacial period, through a process known as isostasy.


What is subsidence?

the motion of a surface (usually, the Earth's surface) as it shifts downward relative to a datum such as sea-level.


Is the mean global sea level constant?

-no -currently increasing a few mm every year


What are the drivers of change in global sea levels? (5)

-thermal expansion of the oceans as the planet warms(volume of warmer water goes up) -melting of ice sheets (Greenland in particular) -tectonic uplift or downlift -isostatic rebound -local subsidence


How has the sea level developed over the past 250 000 years?

-mostly below the level at present

-except for b) in interglacial period

-the changes are driven by coming and going of ice ages

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How has the sea level developed in the past 20 000 years?

-postglacial rise over this period

-the change is not smooth

-fast changes

-noticable in a lifetime in period 10 000 to 8000 years ago

-from 8000 years ago not as dramatic rise

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Will the sea levels continue to rise?

-yes -not clear how much as it is not clear how much warmer it will be -major driver of this is thermal expansion


Why will the sea level rise?

• Expansion of the oceans as they warm • Melting of Greenland, minor ice sheets • Little contribution (at first) from Antarctica – Too cold to melt • Little or no contribution from melting sea ice – Just replaces ice with water, so little net effect on sea level • Implications for coastal environment