Lec #2 Wind generated Waves and sediment transport

Question 1

Define a wind generated wave

Answer 1

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

Question 2

What three factors determine a waves total energy

Answer 2

Wind speed, longevity of wind event and fetch

Question 3

What type of waves are wind generated waves and what does this mean

Answer 3

They are progressive waves, which means the wave form advances across the surface of the water and grows

Question 4

What is the wave celerity

Answer 4

Wave length divided by wave period

Question 5

What is the wave frequency

Answer 5

The number of crests passing per unit in a period of time

Question 6

What are wave orbitals

Answer 6

these are the circular movement of water under the surface of the wave

Question 7

what is the diameter of the wave orbitals at the surface

Answer 7

equal to the wave HEIGHT

Question 8

What happens as orbitals move down through the water colum

Answer 8

Their size decreases exponentially, until a value is so small it becomes negligible

Question 9

What is the maximum the orbital depth will reach (depth and which wave energy ceases)

Answer 9

Half the total wave length

Question 10

What happens as wave orbitals reach a shower area s

Answer 10

The orbitals become compressed and their length increases and their height decreases

Question 11

What is a breaking wave

Answer 11

A wave whose base can no longer support the wave top which means it collapses

Question 12

What causes a wave to break

Answer 12

When it runs into shallow water, or two opposing waves forces combining

Question 13

What is used to calcuated the total engery for a wind generated wave

Answer 13

p = denisty
g = gravity
H = wave height
L = wave lenght

Question 14

What is the forumla for wave energy

Answer 14

      8

Question 15

Discuss the formation of a wind generated wave (in the form of a flow chart)

Answer 15

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

Question 16

discuss the pressure differences acting on the two sides of a wave

Answer 16

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

Question 17

What is wave shoaling (and at what depth does it happen)

Answer 17

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

Question 18

What is refraction and why does it occour

Answer 18

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.

Question 19

What causes long shore drift

Answer 19

Its caused by the wave refraction and prevailing wind on the beach, it moves sediment a long the beach

Question 20

What forces must the current overcome in order to move material on the sea floor

Answer 20

Shear force (or stress)
Lift force

Question 21

Define shear force

Answer 21

The product of the viscosity and the rate of change in velocity with depth

Question 22

What factors are needed to calculate To

Answer 22

dV change in verlocity

dY change in depth

Question 23

What is the formula for shear stress

Answer 23

To = n dv
—-
dy

Question 24

How does viscosity effect wave depth penetration

Answer 24

the high the viscosity the more energy is needed to penetrate the water column

Question 25

What is the critical shear stress (T1)

Answer 25

The energy which is needed to move material on the sea floor

Question 26

What is needed to calcuate critical shear stress (6)

Answer 26

D  = grain size
g = gravity
0- = shields number (not correct symbol)
Ps = density of sediment
Pw = density of water
C = cohesvion value

Question 27

What is the forumla for T1

Answer 27

T1 = Dg (ps-pw) 0- + C (not correct for shields number)

Question 28

Explain the concept of equimobility

Answer 28

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

Question 29

What is the forumla for amount of sediment moved

Answer 29

Qs = F (To - Tc)

Question 30

What are the four types of load (material being moved by water)

Answer 30

Suspended
Disolved
Bedload
Washload

Question 31

Where are suspended and bedload deposited?

Answer 31

Fulvial environments

Question 32

What is unique about the wash and dissolved load?

Answer 32

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

Question 33

What are bedforms?

Answer 33

Physical features of sediment accumulation. created by the deposition of bedload waves, having unique characteristics

Question 34

What are the three main types of bedform

Answer 34

Ripples
dunes
antidunes

Question 35

What is the relationship between bedforms and flow

Answer 35

flow conditions dictate the morphology of the bedform and the bedform then has effect on the flow