Lec #2 Wind generated Waves and sediment transport Flashcards
Define a wind generated wave
A vertical displacement on the surface of a body of water which results in a transfer of energy from the wind to the water surface
What three factors determine a waves total energy
Wind speed, longevity of wind event and fetch
What type of waves are wind generated waves and what does this mean
They are progressive waves, which means the wave form advances across the surface of the water and grows
What is the wave celerity
Wave length divided by wave period
What is the wave frequency
The number of crests passing per unit in a period of time
What are wave orbitals
these are the circular movement of water under the surface of the wave
what is the diameter of the wave orbitals at the surface
equal to the wave HEIGHT
What happens as orbitals move down through the water colum
Their size decreases exponentially, until a value is so small it becomes negligible
What is the maximum the orbital depth will reach (depth and which wave energy ceases)
Half the total wave length
What happens as wave orbitals reach a shower area s
The orbitals become compressed and their length increases and their height decreases
What is a breaking wave
A wave whose base can no longer support the wave top which means it collapses
What causes a wave to break
When it runs into shallow water, or two opposing waves forces combining
What is used to calcuated the total engery for a wind generated wave
p = denisty g = gravity H = wave height L = wave lenght
What is the forumla for wave energy
8
Discuss the formation of a wind generated wave (in the form of a flow chart)
Sun - heats air - causes convection - differences in pressures - movement of air from high to low - adiabatic cooling - causing convection cell - movement of air across water (wind) - water sticks to wind due to friction - water mirrors movement of air
discuss the pressure differences acting on the two sides of a wave
the back side has high pressure causing the water to move downwards and the forward side has low pressure causing the wave to move up
What is wave shoaling (and at what depth does it happen)
Wave shoaling is the propagation of waves into decreasing water depths, as the waves enter the 0.5L depth they begin to act with the sea floor. Wave height increases while wave length decreases
What is refraction and why does it occour
Refraction is the bending of the wave, it occurs due to the underwater topography of the sea bed. In deeper water the wave would move faster than the wave in shallow water thus causing the wave to bend.
What causes long shore drift
Its caused by the wave refraction and prevailing wind on the beach, it moves sediment a long the beach
What forces must the current overcome in order to move material on the sea floor
Shear force (or stress) Lift force
Define shear force
The product of the viscosity and the rate of change in velocity with depth
What factors are needed to calculate To
dV change in verlocity
dY change in depth
What is the formula for shear stress
To = n dv
—-
dy
How does viscosity effect wave depth penetration
the high the viscosity the more energy is needed to penetrate the water column
What is the critical shear stress (T1)
The energy which is needed to move material on the sea floor
What is needed to calcuate critical shear stress (6)
D = grain size g = gravity 0- = shields number (not correct symbol) Ps = density of sediment Pw = density of water C = cohesvion value
What is the forumla for T1
T1 = Dg (ps-pw) 0- + C (not correct for shields number)
Explain the concept of equimobility
Its harder for both a small rock in between large rocks and a large rocks resting on small particles to move so therefore everything is of equal mobility
What is the forumla for amount of sediment moved
Qs = F (To - Tc)
What are the four types of load (material being moved by water)
Suspended
Disolved
Bedload
Washload
Where are suspended and bedload deposited?
Fulvial environments
What is unique about the wash and dissolved load?
once they are entrained in the system they will not leave the fluvial environment until the water has left the fluvial zone and reached the sea
What are bedforms?
Physical features of sediment accumulation. created by the deposition of bedload waves, having unique characteristics
What are the three main types of bedform
Ripples
dunes
antidunes
What is the relationship between bedforms and flow
flow conditions dictate the morphology of the bedform and the bedform then has effect on the flow
