GEOS1100 Lecture Notes - Lecture 9: Energy, Tonne, Longshore Drift
29 May 2018
School
Department
Course
Professor
WEEK 9
LECTURE 1
Coastal processes and Coastal management I: sea-level variation
What is the coastal zone?
The coastal zone includes coastal waters or all areas to the landward side of coastal waters in
which there are physical features, ecological or natural processes or human activities that
affect, or potentially, the coast or coastal resources.
Comment-
….arguably the most progressive in Australia;
... aligns poorly with agencies and governments that are traditionally used to dealing with
lines on maps.
Overview
There is no single definition of the coastal zone
Varies according to the problem being addressed
For some purposes it is necessary to include the continental shelf and hinterland
The coast is rarely in a steady state, but changes over time in response to forcing - from daily
(e.g. tides and precipitation- river flow), seasonal (e.g.g climatic patterns), annual (e,g,
fisheries yield), and decadal (e.g. ENSO) to millennial scales (e.g. sea level)
Why should we care?
The coastline
• Where most of us live
• Cultural icon
• Destination of internal migration
• Destination of tourism
• Destination for recreation
• Sources of resources
Schematic diagram of the east coast Australia
Three main morphological zones:
North- low energy, sheltered by reefs
Central- high energy, beaches & dunes
South- high energy, rock platforms and cliffs
North-South gradient in shelf width:
• Area available for reef development
• Shallow water away from mainland for reef development
• Source of sediments for coastal dunes
• Narrowing shelf to south creates rocky coasts
North: Coral reefs
True reefs limited to tropical/sub-tropical waters
Effect of coral reefs or other barriers - block energy
• Wave power is cut
• Fine sediment from the land cannot reach open ocean
• Creates muddy backwater areas
• Mangroves and cheniers
•
Factors influencing the coastal physical environment
• Vertical and horizontal movement of water
• Elevation differences caused by
• Tectonics
• Climate change (glacial cycles)
• Tides
• Climate change (climate systems; human induced changes)
• Horizontal motion - waves
Milankovitch and ice ages
• 3 Main Milankovitch cycles
• Eccentricity (100,000 years)** ice ages
• Obliquity/Tilt (40,000 years)
• Precession(21,000)
• During periods of low seasonality ice accumulates (ice age) - it melts when seasonality
is high (interglacial)
• 10% ice cover to 30% cover = 120m of sea level
Sea level variation (glacial cycles)
At long time scales global sea level varies:
• Cycles of sea level rise and fall
• Period of c. 100 000 years
• Range of c.120m and sea level rise is fast
Sea levels during glacial cycles
• Present sea-levels very high - only this high 5-10% of the time in the last 800,000 years
• Average is 50-90m lower
• Shoreline was much further out (Beyond Straddie at Brissie)
• Modern sea level achieved only 6500 years ago
Sea level variation II - TIDES
• Tides are caused by
• Gravitational attraction of moon, sun (and planets)
• Centrifugal force of rotation of earth-moon pair
• Earth's rotation within 'tidal bulge'
• Tidal range is vertical height from low tide to high tide
• Tidal range varies spatially - coastal configuration/geometry
• 0->15m;
• About 2m in SEQ
• Temporally - e.g. with phases of the moon
• Neap tides - Sun and moon at right angle
• Spring tides - Sun and moon in alignment
Sea level variation (tides)
• Range
• Current velocities
• Threat from storm surges
• Potential 'green' energy source but limited areas of application
• Currents
• Suspend and transport sediments
• Transport sediments suspended by waves
• Transport propagules e.g. Larvae (marine animals), Seeds (marine plants)
• Shipping safety
Sheltered coasts often tide dominated (e.g. Moreton Bay)
Characterized by interactions between:
• Land forms
• low energy waves (fetch)
• Coastal sediments (mud, sand)
• River inputs (mud, fresh water)
• Marine plants (saltmarsh, mangroves, seagrasses)
Tidal circulation influences
