GEOL 106 Lecture Notes - Lecture 23: Cascadia Subduction Zone, Convergent Boundary, Coastal Hazards
25 May 2018
School
Course
Professor
11.1 Introduction to Coastal Hazards
- continental and oceanic processes converge to produce landscapes that are capable of rapid change
- coastal topography is greatly influenced by plate tectonics overall
o e.g. east coast of US and Canada, Canadian Arctic coast: tectonically passive because they are
not close to a convergent plate boundary
- tectonically passive coasts usually have wide continental shelves with barrier islands and sandy
beaches
o less typically, rocky shorelines: most restricted to New England, Atlantic Canada, Arctic Islands
(e.g. Appalachians) where ancient mountain ranges meet Atlantic and Arctic Oceans
- along Great Lakes, most shorelines are sandy with rock shores limited to northern areas that have
experienced greater uplift following retreat of Pleistocene glaciers
- in contrast, west coasts of US and Canada are tectonically active because they are close to transform
boundaries between NA and Pacific plates and to convergent plate boundaries between NA and Juan
de Fuca or Pacific plates (Cascadia subduction zone)
- tectonically active regions:
o mountain building produces coasts with sea cliffs and rocky shorelines
o long sandy beaches are not as abundant as eastern and Arctic coasts
- topography of coasts is significantly influenced by climate and organisms:
o e.g. shorelines in Alaska, Canada, Great Lakes → affected by seasonal ice or by movement of
present-day glaciers
o coastlines in temperate regions are influenced by marsh vegetation
o subtropical and tropical shores are affected by growth of mangroves and offshore coral reefs
- hazardous coastal processes → many populated areas located near coast
o 40% of US population lives in coastal counties
o population density for coastal counties is 6X than that of inland counties, larger with vacation
periods
- coastal problems will increase as more people live in coastal areas where hazards occur
o hazards along coasts like coastal erosion may become compounded by global warming which
is contributing to worldwide rise in sea level
- most serious coastal hazards:
1. strong coastal currents, including rip currents generated in surf zone and tidal currents in
narrow bays and channels
2. coastal erosion, which produces considerable property damage that requires human
adjustment
3. storm surge from tropical and extratropical cyclones which claims many lives and causes
enormous amounts of property damage
4. tsunamis, particularly hazardous to coastal areas of Pacific Ocean
11.2 Coastal Processes
WAVES
- wind blowing over water produces friction along the water surface
o air is moving faster than water → moving air transfers some of its energy to water and
produces waves
o waves eventually expend their energy at shorelines
- size of waves depends on combination of following:
1. velocity or speed of wind; stronger wind → larger waves
find more resources at oneclass.com
find more resources at oneclass.com
2. duration of wind; longer wind e.g. storms → more time to impart energy to water → larger
waves
3. distance that wind blows across water surface = fetch
a. longer fetch → larger waves; thus waves are generally larger in ocean than in lake
- as waves move away from origin, they become sorted out into groups or sets of waves that have
similar sizes and shapes → can travel long distances with little energy loss
- surf beat: regular pattern produced by interactions between different sets from different sources
- rogue wave: occasional wave that is much larger than the rest, created by number of factors
o form by constructive interference: multiple, similarly sized waves intersect to create much
larger wave
o crests and troughs match → new wave can be high as sum of intersecting waves
o undersea irregularities as well as currents can also influence formation of rogue wave
o can be dangerous to people on beach → appears out of nowhere, out of nowhere in open
ocean too → large enough to break in open water and threaten ships
o stormy seas: rogue waves may reach 30m in height
- waves moving across deep water have a similar basic shape or wave form
o wave height (H): different in height between trough and crest
o wavelength (L): distance between successive wave crests
o wave period (T): time in seconds for successive waves to pass a reference point
- if you were below water surface, you would move in circles but circles would be smaller: you would
move up, forward, down, back and would remain in same place
- shape of orbital motions changes as waves enter shallow water:
o at depth of ½ their wavelength, waves begin to feel bottom → lack of room causes circular
orbits to become ellipses
o motion at bottom may eventually be narrow ellipse with back and forth movement that is
essentially horizontal
-
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Wave energy is approximately proportional to square of wave height. Wave height along coast may increase or decrease as waves approach shore: variations caused by irregularities in offshore topography of seafloor and by changes in shape of coastline. Waves also vary in how they break along coastline. Breaking waves may peak up quickly and plunge or surge or they may gently spill depending on local conditions. Beach: landform consisting of loose material such as sand and gravel which has accumulated by wave action at shoreline. Bar and trough are usually elongate, parallel to breaking waves. Wide and gently sloping beaches may have several lines of breakers and longshore bars. Sand transport sand is not static on beaches, wave constantly keeps sand moving in surf and swash zones storms erode sand from beach and redeposit sand either offshore or landward from shoreline.
