Coastal Structures Design Questions and Answers

Answer: C

Both are true and the reason gives the mechanism, and the impulse rather than the peak governs the response. A caisson may survive a pressure that would fail a rigid element outright. Duration matters as much as the peak pressure.

Answer: D

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: impulsive breaking pressures last only milliseconds, and whether a caisson responds to them depends on the impulse - pressure integrated over time - relative to its inertia, not on the peak alone. Statement 3 is therefore wrong, and it is a dangerous simplification: a very high peak of very short duration may move a massive caisson hardly at all.

Answer: A

Correct answer: sliding, overturning and foundation bearing failure

A caisson is a gravity structure, so it fails as one: it slides horizontally on its rubble base, it overturns about its heel, or the foundation beneath it fails in bearing. All three are checked under the wave pressure distribution given by Goda, including the uplift, which reduces the effective weight and so worsens both sliding and bearing. The other options belong to rubble mounds, concrete armour and dunes.

Answer: B

Sliding as a rigid body is a caisson failure mode rather than a mound one. The other three are the characteristic ways a rubble mound deteriorates. Mound and caisson fail quite differently. Rigid body sliding has no analogue in a mound.

Answer: C

Undermining at the toe removes the support that the armour above depends upon. Toe failure is among the commonest initiating causes of breakwater damage. Many failures can be traced back to the toe. It is also the least accessible part to inspect.

Answer: D

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

Both are true and the reason gives the mechanism. The armour on a slope is supported from below, so if scour or settlement removes the toe the units above slide down into the hollow, the underlayer is exposed and the damage propagates upward. Toe failure is one of the commonest initiating causes of breakwater damage, which is why the toe is given its own berm of heavy stone and its own scour check.

Answer: A

Tolerable limits are quoted in litres per second per metre for each situation behind. Individual wave volumes are assessed separately where personal safety is at stake. Mean and individual volumes are specified separately.

Answer: B

Correct answer: A-2, B-4, C-1, D-3

The four quantities answer different questions. The mean discharge, in litres per second per metre run, is what limits are set against. The maximum individual volume matters for the safety of a person or a vehicle, since one large event can be lethal where the mean is acceptable. The number of overtopping waves indicates how often the crest is wetted. The velocity determines the force on anything standing behind.

Answer: C

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

Both are true and the reason states the relation. A structure strong enough to survive the design wave may still be too low to keep the overtopping discharge within what a promenade, a road or a building behind can tolerate, so the crest is raised beyond what stability requires. Where the land behind is open and can accept water, the reverse holds and the crest can be lower.

Answer: D

Correct answer: reduces both the runup and the overtopping discharge

A rough, permeable slope dissipates energy in turbulence and percolation within the armour, so the water climbs less far and less of it passes the crest - the roughness factor in the overtopping formulae is as low as 0.4 or 0.5 for rock armour against 1.0 for smooth concrete. The stability of the units is unaffected by their collective roughness; it is set by their individual mass and interlock.