EVSC20004 Lecture Notes - Lecture 9: Coastal Erosion, Stokes Drift, Wind Wave
18 May 2018
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LECTURE 9: OPEN COASTS – COASTAL OCEANOGRAPHY
• Coastal Zone: region that is affected by coastal processes that make the environment
different that the open ocean
• Distance from shore of boundary between open ocean and coastal region at a specific
location varies depending on ocean depth and relative importance of coastal processes
Waves
• Open ocean: mature, regular and consistent wind waves, swell
o Wind driven water waves (deep ocean) - main source of
energy is wind
o Don't transport mass horizontally (except Stoke's drift,
important for currents) - circular movement of particles
o Waves classified on depth of ocean - change towards
shallow water on the coast
▪ Deep-water waves: depth greater than half the
wavelength (L/2)
▪ Transitional water waves: depth less than half
the wavelength (L/2) but more than a twentieth of the wavelength (L/20)
▪ Shallow water waves: depth less than (L/20)
• Coast: Waves and wave breaking near the coast
• Changing in depth of the ocean which causes waves to break
o Swell feels bottom at less than L/2, waves energy is compressed vertically and waves
become more peaked, ocean floor places vertical constraint on circular motion, waves
slow down yet waves are closer together (wavelength shortens)
o Critical ratio: 1:7 (height : wavelength), or 3:4 (height: depth) - WAVES BREAK
o Wave breaks depend on slope, roughness etc of bottom
• Tsunamis: generated by strong vertical displacement of ocean floor (fault, tremor,
earthquake)
• Long wavelengths (move much faster than wind waves), small displacement distributed
over large area - hard to detect, can only detect when they approach coast
• Have small height in open ocean, but then get pushed up as they slow down
approaching land
• Tides: Longest of all ocean waves, periods of 12 hours
• Height varies depending on coastline and moon
o Sun exerts tidal force due to gravitational pull - pulls water towards sun, moon also
does this - interaction creates large or small tides
o Lunar tides - twice as large as those from the sun - large gravitational force
o Affected by orientation of the coastline and continents
Waves, Coasts and Erosion
• Long timescale changes in coastline due to refraction, reflection, diffraction and interference
of waves on the rock
• Beaches: sandbars causing wave breaking, offshore, nearshore, foreshore (effected by waves,
depends on physical characteristics), backshore
• Particle size: - both interacting and competition, affected by wave energy (low = high slope,
deposition) (high = erosion, less slope)
o Swash:
• Coarse particle size (steep slopes): water percolates, relatively less backwash, less
water running back downslope increases beach steepness
o Backwash:
