Storms to pocket beaches Flashcards
What are tropical storms?
Collective draw of air towards thunder storms creates this low pressure bulb.
Increasingly strong low pressure system from the comparative high pressure systems on the outside.
Starts to draw in more air (regionally) more updraft = more suction and turns into an eye of the hurricane.
Why do cyclones spin?
The earth’s rotation sets up an apparent force (called the Coriolis force) that pulls the winds to the right in the Northern Hemisphere (and to the left in the Southern Hemisphere).
So when a low pressure starts to form north of the equator, the surface winds will flow inward trying to fill in the low and will be deflected to the right and a counter-clockwise rotation will be initiated.
The opposite (a deflection to the left and a clockwise rotation) will occur south of the equator.
What is angular momentum?
The rotational equivalent of linear momentum.
It’s a conserved quantity (the total angular momentum of a system remains constant unless acted on by an external torque)
As the mass moves towards the centre of rotation, the velocity of the rotation increases.
What is the equation for angular momentum?
L = rmv o angular momentum L o radius r o mass m o rotational velocity v
So: strong circulation + conservation of angular momentum -> (faster you spin = faster contracts)
o always some initial circulation
o contracting air spins faster (r^, v v)
o generates wind
o forms a vortex
o centrifugal “force” (not a real force, just a tendency tendency for things to fly outward)
When do you get an eye of a hurricane?
- vortex (low deepens)
- centrifugal “force” (not a real force, just a tendency tendency for things to fly outward)
- more contraction = more resistance to contraction
- eventually, inward flow prohibited and balanced out
Opposing force in the centre prevents it from contracting anymore
What are the two elements of a storm surge?
- low pressure under storm -> elevated water surface will grow until gravity starts pulling it down
- wind drag = onshore winds push water onshore -> creates a hill of water [BIG wave set-up]
- grows until gravity (pressure gradient) = drag (pushing)
In a storm surge, the lower the bed slope…
the more you can pile (the less water can leak out via a return flow -> another asymmetry)
o return flow still there, piling force very strong
o hard to pile up very high against a steep slope
o SO -> surge higher where nearshore water is shallower and coastal slope less steep
Where wind is offshore -> water pushed offshore (lower water level at beach)…?
Dynamic effects?
o water rushing onshore (water level rise)
o winds turn
o water rushes offshore
Not just flooding o flooding with a CURRENT o highly destructive o can cut inlets - winds reverse - pushes water out of bays - lots of morphology rearranged
What are pocket beaches also known as?
“Embayed beaches”
What is a pocket beach?
Small beach, between two headlands where there is very little or no exchange of sediment.
What is wave forcing?
Cross-shore (onshore) component
o Piles water against shore
o Until gravity (pressure gradient) = wave force
- Alongshore component
- Waves, even with refraction (bending), often look parallel but almost never are; even a small angle can have a major long-term effect
BUT wave breaking stirs up sediment, which is then advected alongshore
Over time, alongshore moves»_space; sediment than cross-shore
o Cross-shore piles against beach and stops
o Alongshore: as long as nothing is in the way, get a current
- Over time…
o always happening, so amounts add up
o net transport (vs daily transport, or isolated magnitudes)
o net transport -> long-term shoreline change
- through gradients in alongshore sediment flux
- more important than magnitudes
- longer time scales than storms and recovery
Alongshore transport
- Amount of transport depends on
o breaking wave height
- (more force, more momentum)
- breaking wave angle relative to shoreline
How are pocket beaches formed/shaped?
Pocket beaches are formed when material from adjacent headlands and bluffs is eroded by wave action, and transported by waves and currents.
Due to refraction, waves converge on headlands, causing erosion, and diverge in coves, causing deposition of sediment (see waves).
The shape of pocket beaches is determined by the form of the surrounding bedrock.
The forces of waves, currents and tides determine the type and amount of sediment deposited.
