Wind generated waves Flashcards
What is a wind wave?
A vertical displacement of the surface of a body of water from the crest to the trough from the normal water level, due to transferral of KE from wind and the friction between the 2 fluids resulting in potential and circular orbital energy in waves, resulting in a progressive wave.
What is a progressive wave?
This is a wave that advances across the water surface and the speed at which it travels is the wave celerity.
What is the wave celerity?
length/period
what is the wave frequency?
the number of wave crests passing per unit of time.
what are the effects of increasing wind speed, duration and fetch.?
increased wave length, height and period
What is the circular orbital energy?
within a wave the particles move in a circular motion that, at the surface, has a diameter equal to the wave height. the orbitals diameter and velocity decreases exponentially with depth till they are negligible at 0.5 wave length.
What happens to waves that are in water less than 0.5 wavelength?
wave celerity and length decreases whilst the wave height increases. This is a result of the compression of wave orbitals and shortening wave length, whose horizontal diameter becomes greater than vertical diameter (compressed)
What are the three types of breaking waves?
- plunging
- surging
- spilling
Why does a wave break?
a wave breaks because the base can no longer support the top. this results from the top of the wave travelling faster than the bottom, and at the point of breaking the horizontal velocity of the water particles is greater than the wave velocity, water particles leave the wave form and wave breaks.
How can you calculate the total energy of a wave?
E = p g h2 L
———–
8
Height is the most important factor, a doubling in height creates a 4 fold increase in energy
How is routine analysis of wave records completed?
using spectral analysis, which transforms the history of the surface elevation into the frequency domain.
How does the wind pressure instantly adjacent to the water surface affect formation of wand generated waves?
air pressure is at a maximum on the windward side and a minimum on the lee side. this causes the wave to be pushed down on the wind side and lifted on the lee slope this reinforces the upward movement as the crest approaches and the downward movement as it passes.
Becomes more effective with size of wave.
What controls the size of the wave?
wind speed, duration and fetch
what is the primary way waves lose their energy?
white capping, caused by wave over steepening
How does temperature differences between fluids effect wave size?
the rate of energy transfer is greater when the wind is cooler than the water surface.
What is wave shoaling?
wave shoaling is the term that is used to describe waves as the propagate into shallow water h/ l <0.5. at this depth they interact with the sea floor.
What is wave refraction?
Wave refraction describes the process of waves aligning themselves with the shore. This process results from the decreasing wave celerity of waves in shallow water whilst the section of the wave in deeper water continues to travel at the same speed. results in eaves bending and aligning with the shore.
What are the advantages of evaluating wave orthongonals?
allows you to determine the effects of refraction on the wave height and the energy distribution.
What is longshore drift?
the process where sediment is moved along a beach due to the acute angle of incoming swell moving material across and up the beach with swash.
what 2 forces does a current exert that can move sediment?
lift and shear forces
What is shear stress?
this is the product of viscosity and the change of velocity with depth .
= n dv
——–
dy
What is the boundary shear stress?
this is the energy of the water moving over the sea bed
= 0.5 p * Fw * U02
What is the critical shear stress?
This is the shear stress that must be overcome for the sediment to be moved.
= 0 g (ps - p ) D
What is equimobility?
This means that all grain sizes are equally mobile on a bed, because the larger grains on top of small grains are easier to move than otherwise and the smaller grains between larger grains are harder to move than otherwise.
What are the 4 kinds of load?
Wash load
dissolved load
bed load
suspended load
What type of load do alluvial environments consist of ?
bed and suspended load.
When is sediment deposited?
sediment becomes deposited when the bed shear stress falls below the critical shear stress, however then the particles most overcome bouyancy.
= (square root) 8gr (ps- p)
————–
3Cd p
How do flocs form?
they form due the electrical attraction between particles.
What is stokes law?
this calculates the settling velocity of fine particles.
How do bed forms form?
transport of bedload by waves (flow), bedforms then modify flow conditions so are again modified.
What are the three types of bed forms?
dunes, antidunes and ripples
what is the effect of water depth on wave periods?
in deep water long period waves travel faster than short period waves.
What limits wave growth in the open ocean?
Wave steepness H/L. limiting steepness = 1/7 - white caps form.
Why do long period waves have a greater energy than short period waves?
energy is defined per area, therefore because long period waves have a greater wavelength they have a greater energy.
What is wave dispersion?
this process describes how long period waves outrun short period waves, and wave sorting takes place.
What property remains constant as waves enter shallow water?
wave period.
What 2 factors influence energy lost due to bed friction?
width and gradient of continental shelf, roughness also effects
Where do spilling breakers, plunging breakers and surging breakers form?
- gentle gradient, steep wave
- steeper beach, intermediate steep wave.
- steepest beach, low steepness wave.
What is a standing wave?
this is the interaction between an incoming wave and a reflected wave.
What is wave set up?
the piling of water against the shoreline increasing the sea levels caused by waves, tides and onshore winds.
What is swash motion?
2 phases. onshore swash phase with decelerating flow, offshore backwash with an accelerating flow.
What are the 3 parts to sediment movement?
- entrainment,
- transport,
- settlement
How does grain shape affect movement?
entrainment and settling.e.g flat grains are more difficult to entrain and settle more slowly.
what is viscosity?
the resistance to deformation within a fluid.
what causes lift force?
arises due to the slightly faster flow over the top of the grain than the bottom, results in pressure differences.
How does sea water effect flocculation?
acts as an electrolyte, improving the process.
particles bought into close proximity thanks to brownian motion and fluid turbulence.
What is the difference between ripples and dunes/antidunes?
ripples are small scale bedforms, whilst anti/dunes are large scale.
Where do ripples/dunes/antidunes form?
Ripples and dunes are the product of subcritical flow and antidunes are the result of supercritical flow.
