Important (Must Learn)

Question 1

For this question:

A regular wave train impinges on a plane beach and breaks where Hb = 2.16m.

What will be the wave set-up at the shore ?

Describe the step by step process to answer this:

Also what is an assumption we make to make the calculation simpler?

Answer 1

1 - Find Sxx, using the equations for Sxx and E from the formula book (assuming shallow condtions, kh —> 0 )

2 - Input into equation for mean water surface elevation, and integrate from xb to xs.

3 - Use equation for n (xb) from formula book to leave the only unknown in the equation as h (xb)

4 - Use Miche’s criterion to find value for h (xb)

5 - Calculate n (xs)

We approximate n(xs) to equal n’(xs), allowing for a much simpler calculation, as there is very small error.

Question 2

In beach nourishment, what is the point where h = h*?

Answer 2

Where the summer profile and winter profile join together

Question 3

If h > h*, what will no longer occur?

Answer 3

Sediment will no longer be transferred

Question 4

What is cg?

Answer 4

The speed at which energy is transported

Question 5

What is celentiy?

Answer 5

The speed at which the wave propagates

Question 6

Is E*cg constant?

Answer 6

Yes, no matter what height

Question 7

What happens to cg in shallow water?

Answer 7

Approximately equal to the celentiy, which is approx root gh

Question 8

What is the difference between an intersecting and non-intersecting beach profile?

Answer 8

Both are Df > Dn

However, an intersecting profile is steeper, and completes before hitting h*

Question 9

What does V / delta y equal, where V is the total volume of nourishment, and delta y is the alongshore extent of the nourishment ? (For an intersecting profile)

Answer 9

B*delta x + An * integral ( x^2/3) from xi to 0 minus Af * integral ( x - delta x) ^2/3

Question 10

How do you determine xi, using the dean profile equation ?

Answer 10

An*xi^2/3 = Af (xi - delta x) ^ 2/3

Question 11

What does V / delta y equal, where V is the total volume of nourishment, and delta y is the alongshore extent of the nourishment ? (For a non-intersecting profile)

Answer 11

Bdelta x + An * integral ( x^2/3) from x to 0 minus Af * integral ( x - delta x) ^2/3 from x* to delta x

Question 12

What is the 3-step process for soliving a beach noursihemnt question for a non-intersecting beach ?

Answer 12

1 - Determine An and Af

2 - Calculate xi, if h(xi) > h * then the profile is intersecting

3 - Calculate the volume

Question 13

What is the definition of Ecg ?

Answer 13

The rate of transport of wave energy in the theta direction per unit width of wave crest

Question 14

What is the definition of Ecg cos theta ?

Answer 14

The rate of transport of wave energy in the theta direction per unit length in the alongshore direction

Question 15

What is the definition of Ecg cos theta sin theta ?

Answer 15

The alongshore component of the rate of energy transport in the theta direction per unit length in the alongshore direction.

Question 16

What does c equal in shallow sections ?

Answer 16

root gh

Question 17

You are told that in a wave-structure interaction problem the Keulegan-Carpenter number is 12. What does this tell you about the balance of drag and inertial forces in the problem?

Answer 17

Kc = 12: neither drag or intertial forces can be ignored, so both must be considered

Question 18

In which two coastal zones does alongshore sediment transfer occur?

Answer 18

Occurs both in the surf zone (where the waves break) and the swash zone (where the waves periodically wet and dry the beach)

Question 19

You are asked to calculate the uplift force due to a progressive wave on a submerged structure. Would it be appropriate to use Morison’s equation to do this? Justify you answer briefly.

Answer 19

No, becuase Morison’s equation is developed for estimation of the net horizontal force.

Question 20

A surface gravity has a wavlength 100 m, what is the wavemnumner of the wave in radians per m, degrees per m, and oscillations per m?

Answer 20

k = 2*pi/100 radians per m

k = 360/100 = 3.6 degrees per m

k = 1 oscillation over 100m = 0.01 oscillations per m

Question 21

You are tasked with estimating mean overtopping rate at a sloping sea wall that protects a promenade behind a tourist beach, at high tide, and during a storm with onshore winds.

You have the high tide level and have wave and wind conditions at an offshore buoy, and bathymetry.

List briefly, in the correct order, the five tasks you would need to perform to accurately estimate the mean overtopping rate:

Answer 21

1 - Estimate storm surge at the shore.
2 - Transform wave height to the break point using the newly calculated mean depth
3 - Estimate the set up at shore, using the wave height at break point
4 - Transform that wave height to the toe of the newly estimate mean depth
5 - Finally, estimate mean overtopping rate from the wave condtions at the toe

Question 22

What does c equal? Therefore what can it be deduced that c^2 equals?

Answer 22

c = w / k, therefore c^2 = w^2 / k^2

Question 23

When waves propagate, what quantity is tranported at group velocity?

Answer 23

Wave energy is transported at the group velocity

Question 24

Storm surge is an elevation in the mean water level at the coast. State the two main forces that contribute to the size of the storm surge:

Answer 24

The two forces contributing to the storm surge size are the sear stress exterted by the wind on the sea surface, and the hydrostatic force that opposes this, due to gravity.

Question 25

If we know the keulegen carpenter number for a wave structure interaction problem, can we therefore estimate the size of the drag force on that structure? Briefly explain your answer:

Answer 25

No, we do not therfore know the size of the drag force. We only know its size relative to the intertia force

Question 26

If you were told that an armour unit has a Kd value of 62, what does it tell you about its performance to a tribar of the same mass (Kd of tribar = 10) ? State briefly why

Answer 26

An armour unit with Kd = 62 will resist movement more than the tribar of the same mass. 62 > 10, and the armour unit with a larer Kd will interlock better and therefore resist movement more

Question 27

Exlpain briefly what the vertical orbital excursion is:

Answer 27

The vertical orbital excursion is the difference between the highest and lowest positions of a water particle in the water column, during one wave period of a surface gravity wave.