Coastal processes
What happens when sediment reaches the ocean?
decrease water velocity
material deposited - delta. More information on deltas.
New transporting agents: waves and currents rework or remove sediment.
Waves are due primarily to winds. They result from the fact that water oscillates back and forth
waves act as agents of erosion and sediment transport
Near shore waves slow as the water gets shallower. Also the waves get higher.
Finally wave breaks --> waves of translation
either primarily uprush - spilling breaker - constructive
or primarily backwash - plunging breaker - destructive
Wave refraction –
Happens when water shallowing is not parallel to wave crests
- if the waves arrive at an angle to the shore - due to dominant winds – the waves are refracted toward the shore
- if the shore is irregular – the waves are refracted towards headlands
Longshore currents and transportation
-the current arises from a component of the wave energy that moves parallel to the coast
-individual waves - swash and back wash results in longshore drift of sediment - aided by the longshore currents
Because of on- and off-shore wave action and currents get sediment transport with coarse material concentrated along coast in beaches.
Marine erosion is aided by mass wasting. The erosion rate depends on rock resistance and wave strength. Processes operate only in narrow vertical range from slightly above to slightly below sea level, along edge of land. Foreshore and offshore surface ends up as combined product of erosion and deposition.
cliffs
wave cut platform
wave built terrace
Tendency to dynamic equilibrium profile
- if shoreline steep - get destructive waves
- if shoreline shallow - get constructive waves. These steepen the shore.
- may alternate destructive and constructive in winter and summer depending on wind strength variations.
Along shore - tendency to straighten coast
- any change in land level or sea level or tidal range will interrupt evolution of coast
Also an equilibrium distribution of sediment size - depending on
- sediment source
- whether the waves are primarily constructive or destructive
Given stable sea level relative to land, sediments fine outward: shingle-sand-silt-mud
Where gradual sea level change - prograde or retrograde get time transgressive sediment distribution
Causes of change in relative sea level:
world wide
- less or more room in oceans - due plate process as related mainly to mid-ocean ridge
- more or less water due ice cap size (see also this reference on sea level and climate)
- change in water volume due to temperature changes
- fill ocean basins with sediment
In the past 50 years steady sea level rise has averaged 1.2 mm/yr.
local