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Lecture 4

GEOL 106 Lecture Notes - Lecture 4: Lake Ontario, Estuary, Lidar

Geological Sciences and Geological Engineering
Course Code
GEOL 106
John Hanes

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Week 4: Coastal Hazards
Coastal Topography & Plate Tectonics
Tectonically passive coasts are characterized by:
o Wide continental shelves
o Barrier islands
o Sandy beaches
Tectonically active coasts are characterized by:
o Sea cliffs
o Rocky shorelines
o Intermittent beaches
Tectonically passive coasts:
o East coast of the USA and Canada
o Canadian Arctic
Tectonically active coats:
o West coast of USA and Canada
Coastal Topography & Climate
Coastlines are also influenced by climate and associated organisms
o Canada, Alaska, Great Lakes affected by seasonal ice, glaciers
o Temperature region coastlines affected by marsh vegetation
o Tropical and subtropical region coastlines affected by mangroves and coral reefs
Coastal Hazards
The most serious coastal hazards include:
o Strong currents (rip currents and tidal currents)
o Coastal erosion
o Storm surge
o Tsunamis
Coastal Processes: Waves
Waves are generated by offshore winds
Winds creates friction over the water surface, transferring energy to create waves
Wave size depends on:
o Velocity of the wind (up speed = up waves)
o Duration of the wind
o Distance that wind blows across the water surface, the fetch (up fetch = up waves)
As waves move away from source, they sort themselves into groups or sets
Creates a regular pattern of surf at a beach
Typically have the largest wave in the centre of the set
Waves can travel thousands of km across the open ocean with little loss of energy
Parameters describing the size and movement of a wave:
o Height (H): difference in height between the trough and crest of a wave
o Wavelength (L): the distance between successive wave crests
o Wave Period (T): time in seconds for successive waves to pass a reference point
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In the open ocean, water particles move in stationary circular orbits that get smaller to a depth of
ca. 0.5 times the wavelength
As waves enter shallow water, the circular orbits become ellipses
Wavelength decreases and wave height increases until they break
Irregularities of offshore topography of the seafloor and the shape of the coastline cause
variations in wave height
Headlands cause refraction of waves and concentration of energy at certain areas along a coast
Waves vary in how they break along a coastline
May peak quickly and plunge or surge, or they may peak and then gently spill
Depends on slope of coastline
Coastal Processes: Wave Energy
Wave energy is approximately proportional to the square of the wave height
o a 2 m high wave will expend 4 x the amount of energy of a 1 m wave
o a 5 m wave (typical for large storms) will expend 25 x the amount of energy of a 1 m
Coastal Processes: Rogue Waves
Rogue waves are the exception to regular breaking surf & wave sets
Rogue waves can be formed by constructive interference
o Multiple, similarly sized waves intersect to create a much larger wave
Underwater irregularities & currents can also influence formation of a rogue
Rogue waves are very dangerous on coastlines and in the open ocean
On the coast, appear out of nowhere and can overtop piers and cliffs
Lives are lost every year when people are swept out to sea by rogue waves
In the open ocean, they can threaten ships
Rogue wave may reach > 30m in stormy seas
Beaches are landforms consisting of loose material - sand or gravel - which accumulate by wave
action at a shoreline
Colour and composition are directly tied to the source of the sediment (coral, volcanic rock, quartz
and feldspar)
Beaches: Structure
Beach structure (onshore):
o Sea cliff/bluff, sand dunes, or permanent vegetation
o Berm (slopes landward)
o Beach face (slopes towards the water)
o Swash zone
Beach structure (offshore):
o Surf zone (turbulent water, after waves break)
o Breaker zone (incoming waves become unstable and break)
o Longshore bar
o Longshore trough
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Beaches: Sand Transport
Sand is transported in the swash and surf zone on and off shore, and parallel to shore
Littoral = shore
Movement includes two processes: beach drift and longshore drift
Beach drift: up and back movement of beach material in swash zone
Longshore drift: transport of sediment ca. parallel to shoreline
Sea Level Change: Relative
Relative sea level change is influenced by the level of the land and water
o Earthquakes
o Melting glaciers
o Net erosion or deposition
o Tides
o Weather conditions
o El Nino
Sea Level Change: Eustatic
Global sea level rise ad fall due to the aout of water i the world’s oeas ireasig or
decreasing or basins changing shape
Climate (air temperature) influences temperature of ocean and amount of water stored in ice
Global warming is responsible for thermal expansion
Present eustatic sea level rise is ca. 3 mm/year, half of that is due to thermal expansion because of
a warming atmosphere, half is due to glacier and icecap melt
Sea Level Change: Hazards
Relative sea level change contributes to coastal flooding and hazardous nearshore currents
Eustatic sea level change increases hazards from storm surge and coastal erosion
In the long term, rising sea level (eustatic) threatens coastal cities and islands
Coastal Hazards: Who Is At Risk?
Sea coasts and lake shores, particularly big lakes (Great Lakes, Great Slave Lake, Lake Winnipeg)
Certain areas are higher risk for certain hazards:
o Storm surge - coasts that experience hurricanes & tsunamis
o Coastal erosion - more universal, depends on longshore current
o Nearshore currents - coastlines with irregular, strong surf
o Astronomical tides - areas with particularly strong tides
o Sea level rise - low lying areas
Coastal Hazards: Erosion
Barrier islands along southeast coast of North America can be as high as 8 m/year (average closer
to 0.3 - 2 m/year)
Areas of the Great Lakes experience erosion rates up to 15 m/year (average is 0.3 - 1 m/year)
West coast rates are closer to 0.1 - 0.3 m/year
Averages are difficult to predict and impact is dependent on other factors
Coastal erosion is a slower and more predictable hazard than hurricanes, earthquakes, etc.
Threatens large numbers of people and huge areas of land
Occurs differently on a beach vs. a cliff
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