Week 3 - Physical processes and external agents acting along coastal environment (TIDES AND SURGES) Flashcards
What are tides
Waves caused by gravitational interactions between Earth, Sun, and Moon
Always behave as shallow water waves
Water depth < 1/20 wavelength
what causes tides
the changes in the relative positions of the sun and moon have a direct effect on daily tidal heights and tidal current intensity
what are the differences between spring and neap tides
Spring tides:
High tides higher
Low tides lower
occur when there is a new or full moon
Neap tides:
High tides lower
Low tides higher
occur when there is 1/4 or 3/4 moon
describe tides in an estuary
Tides are always shallow water waves
Similar transformations when entering shallower water
But effects look different because they are very long waves
Depth gradually decreases -> distortion
Friction -> dissipation and distortion
Morphology -> funnel shape -> increased water level
Closed upstream boundary (e.g., dam or bay) -> reflection
what is a flood tide
As the tide rises, water moves toward the shore
what is an ebb tide
As the tide recedes, the waters move away from the shore
describe tidal propagation in estuaries
Because flood currents occur in deeper water, they experience less friction and are faster than ebb currents
This produces a quick rise and a slow fall in the tide in estuaries
There are many more examples of this tidal asymmetry in estuaries, and the effect becomes more pronounced with distance inland
describe tidal currents in estuaries
Currents in estuaries respond to the tidal asymmetry
Generates flow asymmetry
Currents transport (nearly) the same volume of water up the estuary as down it
Therefore, the flood currents are shorter and faster
And the ebb currents are weaker but last longer
Fast flood, slow ebb is the norm
what are storm surges
An abnormal, less predictable rise of water generated by a storm, over and above the predicted astronomical tides
Greatest threat for coastal area during hurricanes
What causes a storm surge?
Wind
Water pushed toward the shore by the force of the winds moving cyclonically around the storm
Horizontal action
Pressure
Low pressure -> higher water level
Vertical action
Inverse barometer effects
1mb drops = 1cm increase
describe how a storm surge forms
Low pressure near the eye of the storm pulls water higher
Winds pile up water and push it towards the shore
A shallow coastline slope produces a greater surge than a steeper slope
Waves push the water inland faster than it can drain off
The surge can begin before the storm hits, cutting off escape routes
Floating debris can act as battering rams
Battering waves may erode beaches and damage buildings
what are tropical storms
Small, very intense gyres
Generated at sea
Unpredictable track
Very high surges
E.g., hurricanes
Wind stress > pressure
describe a storm surge generated by a hurricane
Warm ocean causes evaporation
Warm, moist air rises
Surrounding air moves into area of low pressure left behind
New air becomes warmer and rises
Everything starts rotating -> cyclones
what are extratropical storms
large, slow moving
surround a depression
considerable duration
pressure ≈ wind effect
what is stratification
layering in the sea caused by temperature, density, and salinity
in tidal sea/estuaries there is a competition between the promotion of stratification by solar heating and the destruction of stratification by tidal currents
