Coastal Structures Design Questions and Answers

Answer: C

It expresses the submerged weight available to resist the hydraulic load. A denser rock gives a larger value and therefore a smaller required mass. Density enters the calculation cubed. A modest density gain gives a large mass saving.

Answer: D

Both are true and the reason gives the dependence, and density enters cubed in the mass calculation. The saving in volume can be substantial. Transport cost may offset part of the gain. Source and haul distance both enter the comparison.

Answer: A

Correct answer: flow accelerates and vortices form around the roundhead

The head is exposed to waves from a much wider range of directions, and it has no neighbouring units on the seaward side to support it. Currents accelerate around it and shed vortices that scour the bed at its foot. All three effects concentrate at the same place, which is why the head is armoured with units one and a half to two times the trunk mass, or on a flatter slope, and why head damage is the commonest form of breakwater failure.

Answer: B

Both are true and the reason gives the reason, and damage frequently begins there. A larger unit size or a flatter slope is used over the head section. The head is treated as a separate design case. Damage records consistently justify the practice.

Answer: C

Correct answer: 1 and 2 only

Statements 1 and 2 are correct and follow from one another: because the head is more vulnerable, it receives heavier armour or a flatter slope. Statement 3 reverses the comparison - conditions at the head are more severe than along the trunk, not less, through the combination of multi-directional attack, reduced support and flow concentration.

Answer: D

Correct answer: the cost of a zero damage design is disproportionate

Armour mass rises with the cube of wave height, so designing for no movement at all under the most extreme condition costs far more than designing for a few per cent of units displaced and repairing them afterwards. The damage level is chosen by comparing the extra capital cost against the expected cost and consequence of repair, which is a straightforward economic calculation once the damage can be predicted.

Answer: A

Correct answer: the eroded area divided by the square of the unit size

The damage parameter is

the cross-sectional area eroded from the profile, normalised on the square of the nominal unit diameter, so it expresses damage as the number of unit-sized volumes removed per unit length. Values of 2 to 3 mean the start of damage, 8 or more approaching failure, and the acceptable figure depends on the slope and on the consequence of repair.

Answer: B

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the basis of the choice. A breakwater protecting a nuclear intake or a busy port is designed to a lower damage level than one sheltering a fishing harbour, because the consequence of failure differs. So does the cost of repair - a remote site with no local quarry or plant may justify a heavier initial design simply because putting it right afterwards would be so expensive.

Answer: C

Partly built structures are far more vulnerable than completed ones to a storm. Construction risk is managed by sequencing the work so that the exposed states are as brief as possible. Sequencing is part of the design, not an afterthought.

Answer: D

Both are true and the reason gives the exposure, and the construction sequence is planned around it. A lower design condition is sometimes accepted for the construction period. Exposure duration justifies the lower condition.