8.1 Flashcards
Characteristics of a wave:
Crest
Trough
Wave height
Wavelength
Crest: highest point of the wave
Trough: lowest point of the wave
Wave height: vertical distance between crest and trough
Wave length: distance it takes for the wave to repeat (crest to crest)
Wave formation
- in the offshore zone waves have a circular orbit, with constant wave length and speed, as the depth is greater than 1/2 the wavelength, meaning it is unaffected by friction - waves of oscillation
- leads to a circular orbit
- as the seabed becomes shallower in the nearshore zone, the orbit of the water particles becomes more elliptical (waves of transition), leading to more horizontal movement of the waves
- wave height increases, but the wavelength and velocity both decreases (shoaling occurs)
- as the waves move from the breaker to the surf zone, the elliptical orbit increases, until the waves reach the foreshore, where they become waves of transition (plunging, surging or spilling) and eventually break
Factors affecting size/energy of waves
Strength of the wind: stronger wind means that more energy transferred to the water - frictional drag
Size of fetch: longer fetch, more time for wind to transfer its energy to water
Duration of wind: longer duration, more energy transferred
Waves of translation?
When waves break - constructive or destructive
Constructive waves
- depositional waves (spilling waves)
- elliptical orbit
- strong swash, weak backwash
- gentle slope - friction increases gradually, therefore shoaling occurs gradually
- waves therefore small and far apart when they reach shore
- an entire wave cycle is complete before next wave comes in due to long wavelength
- swash runs far up beach, due to low angle, so infiltration rates high and backwash weak
- built up beach
What is shoaling?
Wave shoaling occurs when ocean waves move from deeper water into shallower areas, causing them to slow down, decrease in wavelength, and increase in height.
Destructive waves
- erosional
- circular orbit
- surging/plunging waves
- steep slope = friction increases quickly and shoaling occurs quickly
- waves larger/higher and fall over on themselves (plunging)
- beach material can be eroded (scouring) when waves plunge over
- infiltration rates low, so backwash strong
- waves closer together so backwash often returned in next wave, so material often deposited in offshore bar
3 main types of breaking waves
spilling breakers - constructive
plunging breakers - destructive
surging breakers - destructive
spilling breakers
- constructive
- associated with gentle beach gradients and steep waves
- characterised by gradual peaking of wave until crest becomes unstable, resulting in gentle spilling forward of crest
Plunging wave
- destructive
- tend to occur on steeper beaches than spilling breakers, with waves of intermediate steepness
- distinguished by shore-ward face of wave becoming vertical, curling over and plunging forward and downward as an intact mass of water
Surging waves
- destructive
- found on steep beaches with low steepness of waves
- front face and crest of waves remain relatively smooth and wave slides directly up the beach without breaking
- large proportion of wave energy reflected at the beach
Wave refraction
- wave orthagonals are lines of wave approaching from deep water at right angles tp wave crest
- wave approaching a coastline will firstly encounter shallower water in front of headlands
- this shallower water will slow down the wave - friction
- section of the wave in front of adjacent bays will still be in deeper water and continue to travel, at a faster speed than section encountering headland
- results in wave crest line starting to bend/refract
- wave energy concentrated around headlands and dispersed in the bays
Tides
- regular movements in the sea’s surface, caused by the gravitational pull of the moon and sun on the oceans
- moon accounts for the larger share of the pull, despite being smaller than the sun, because its closer to the earth
semi diurnal: 2 high and 2 low tides every 24 hours
diurnal: one high and low tide every 24 hours
Spring and Neap tides
Spring:
- greatest difference between low and high tide
- gravity of sun and moon pull in the same direction
- occur at new moon and full moon
Neap tides:
- tide with least difference between high and low tide
- moon is at a right angle tot he sun
- occur at first and last quarter
Implications of tides on the coastline
- less steep the beach the larger the foreshore and so larger the impact of the tidal range
the tidal range will influence the following:
- vertical range of erosion and deposition
- weathering: more time the coastline is exposed the more time there is for weathering such as wetting/drying
- velocity of tidal flow is influenced by the tidal range and has an important scouring effect