Marine processes and waves Flashcards
What generates waves?
Waves come from friction between wind and water.
Transfers energy from the wind to the water
The force of the wind blowing on the surface generates ripples, which grow into waves if the wind is sustained.
Waves in open sea
- Are just energy moving through water
- water moves up and down, not horizontally
- some orbital water particle movement but no net forward water particle movement.
What does wave size depend on?
- strength of wind
- duration wind blows for
- water depth
- wave fetch
Fetch
The uninterrupted distance across water over which wind blows, and therefore the distance waves have to grow in size.
What happens as water depth shallows towards the coastline?
- Waves break
- water depth approximately half wavelength, orbital motion of water within the wave touches the sea bed
- creates friction between water and sea bed, slows wave down.
- waves approach shore, wavelength decreases, wave height increases, waves bunch together
- wave crest moves forward much faster than wave trough.
- wave crest outruns trough, wave topples forwards, breaks.
What are tides?
Formed from gravitational pull of the moon acting on water on the earth’s surface. Bulge of water rotates around locations in the ocean. Bulge passes location on the coast, high tides occur.
Are tides important in terms of coastal processes?
- in some locations, large tidal ranges can produce waves capable of erosion, tidal bores. Eg, Severn Estuary UK
- Storms occur, at the same time as high tides, storm waves act on parts of the coastline which they normally wouldn’t.
- High waves on top of spring tide leads to increased risk of coastal flooding
What are features of constructive waves?
- Low wave height, under 1m
- Long wavelength, up to 100m
- flat waves, with strong swash but weak backwash
- strong swash, which pushes sediment up the beach, depositing it as a ridge of sediment (berm) at the top of the beach
- backwash which drains into sediment
What are the features of destructive waves?
- High wave height, over 1m
- short wavelength, under 20m
- strong backwash, which erodes beach material, carries it offshore, creating an offshore ridge or berm
In which conditions can beaches experience both constructive waves and therefore significant changes to beach morphology?
- over a day, as storm passes, destructive waves change to constructive ones, as wind drops
- Between summer and winter. Summer = constructive, winter = destructive
- Changes to climate, e.g. if global warming resulted in the UK climate becoming on average stormier, destructive waves and winter beach profiles would be more common.
How can human affect beach profiles?
- Sediment supply from rivers can be reduced by the construction of dams on rivers, traps sediment upstream and leads to less deposition
- Interfering in sediment supply along beaches, by coastal management in one place, which affects processes in another
Beaches and landforms which change constantly
- Storm beaches are high at the back, as the result of high energy deposition of coarse sediment during severe storms.
- Berm/ridges, typically shingle/gravel result from summer swell wave deposition
- low channels and runnels between berms
- offshore ridges/bars formed by destructive wave erosion and deposition of sand and shingle offshore
Swash
The flow of water up a beach with a breaking wave. Can transport sediment.
Backwash
The water draining down the beach back into the sea. Can transport sediment.
Beach Morphology
The shape of a beach, including its width and slope (the beach profile) and features such as berms, ridges and runnels. It also includes the type of sediment (shingle, sand, mud) found at different locations on the beach.
