Week 8: Tides and coastlines Flashcards
What is a tide?
Periodic short-term changes in height of ocean surface caused by the combination of the gravitational force of the Moon and the Sun and motion of the earth
Are tides a wave?
Yes, a huge one
- wavelength 1/2 of the earth’s circumference
What do tides need to keep moving?
Needs to remain under the influence of the force that created it gravity
What causes tides?
Gravitational pull of moon and sun acting on the ocean and the earth’s rotation
Sun vs moon
The sun has half the role of the moon on tides
What is the barycentre?
The centre of mass of two orbiting bodies
** earth-moon system revolves once a month (27.3 days) around centre of mass
Newton’s law of universal gravitation
Every object that has a mass in the universe is attracted to every other object
- if mass increases, gravitational force increases
- if distance increases, gravitational force decreases
Centripetal forces
Required to keep identical sized particles in identical-sized orbits as the result of the rotation of the earth-moon system around barycentre
**all arrows equal and same direction
Resultant forces
What generates tides
Two vectors (gravity and centripetal force)- diff btwn them creates 2 bulges of water
Tidal bulges
Earth rotates under the water bulges
How long does it take for a full tidal cycle?
24 hours and 50 min
Lunar bulges
Moon revolves around earth is same direction as earths rotation
Takes an extra 50 min for earth to catch up to moon
Lunar day
Time of day when the moon is highest in the sky and the next time of day when is is at its highest again
Role of the sun
Creates 2 solar bulges
Spring tide
Tide of max range
Occurs every 2 weeks when moon is in new or full moon phase
**greatest gravitational attraction bc moon, sun and earth are all in alignment
**coastal flooding and storm surge
Neap tide
Tide of minimal range
Occurs every 2 weeks when moon is in 1st quarter or 3rd quarter phase (7 day after spring tides)
**low gravitational attraction bc moon and sun are at right angles
What are the three tidal patterns?
- Diurnal
- Semidiurnal
- Mixed semidiurnal
Diurnal
1 high tide, 1 low tide
ex. gulf of mexico
Semidiurnal
2 high, 2 low (equal heights)
ex. atlantic coast
Mixed semidiurnal
2 high, 2 low (unequal heights)
ex. pacific coast
Tidal range
Vertical diff btwn consecutive high and low tides
Macrotidal
> 4 m
Mesotidal
2 to 4 m
Microtidal
<2 m
Greatest tidal range in the world and why?
Bay of Fundy (15-17m)
Basin shape is restricted
Two ways to explain tides
- Equilibrium theory of tides
- Dynamic theory of tides- Amphidromic circulation
Equilibrium theory of tides
Ignore ocean basins and coastlines, ocean that covers entire earth
Dynamic theory of tides
More complex situation- continents, coastlines, coriolis effect, water depth etc.
Amphidromic circulation
Water sloshes from left to right at side of basin
Tide as rotary wave- moves around a central part
**tide is a counterclockwise rotation in northern hemipshere
Amphidromic point
Locations where there are little or no tide in the ocean
Tidal flats- Mudflats
Coastal wetlands form in intertidal zone; submerges and emerges 2x day
Tidal currents
Caused by rise and fall in water as a tide crest approaches and passes
Flood current
Water moving towards land, rising water levels
Ebb current
Water moving away from land, falling water levels
Slack water
No movement either towards land or offshore
Tidal zones
- Subtidal
- Intertidal
- Supratidal
Subtidal zone
Always underwater, below tide line
Intertidal zone
Tidal flat, exposed then submerged then exposed
High biodiversity, cant live here
Supratidal zone
Almost never covered by water, except for when high tide w storm surge
Tidal bore
Flood tide creates a wave of water that travels upriver
Large tidal ranges
Incoming tide moves into narrow opening
Zones of a coastline
- Coast
- Shore
Coast
Extends inland as far as coastal processes reach
Shore
Low tide and highest point storm waves reach
Parts of the shore
- Backshore
- Foreshore
- Offshore/nearshore
- Shoreline
Shoreline
Position changes; at water’s edge
Backshore
Above high tide level
Foreshore
Exposed at low tide, covered at high tide
Nearshore
From low tide shoreline to breaking waves
Beach
Wave-worked sediment
Beach face
Wet sloping surface
Berm
Shore-parallel ridge, dry sediment, high tide mark, mobile
Shephard coastal classification
- Primary coasts
- Secondary coasts
Variables that effect the classification of coastlines?
Materials, processes, forms, developmental age, environment, human impact
Primary coasts
Formed by non-marine processes (such as volcanic activity) ; young, possible strong terrestial influence
Secondary coasts
Shaped by marine processes; erosional or depositional processes dominate
Shephard Coastal classification- primary
- Land erosional coast
- Subaerial depositional coast
- Volcanic coasts
- Fault/tectonic coasts
- Glacial/ice coasts
Land erosional coast
Subaerial coast
Post glacial sea level rise
Drowned coast
Subaerial depositional coast
Deposition by rivers, glaciers, wind, landslides
Shephard coastal classification- secondary
- Wave erosion coasts
- Marine deposition coasts (beaches and barriers)
- Coasts built by organisms (reefs)
Submergent coasts
Drowned coastal zones
Coastal land is sinking relative to sea level (transgressive)
Land subsidence or sea level rise
Emergent coasts
Wave-cut terraces and platforms are exposed
Coastal land is rising relative to sea level (regressive)
Erosional coast
Dominant processes are those that remove coastal material
EROSION>DEPOSITION
**more material being removed than added
Depositional coast
Material accumulated
EROSION<DEPOSTION
**More material added than removed
