Coastal Structures Design Questions and Answers

Answer: A

Excessive overtopping or minor armour displacement are the typical serviceability failures. The ultimate limit state by contrast concerns the loss of the structure itself. The two are assessed against different return periods.

Answer: B

Rubble mound design in particular accepts a defined level of damage rather than none at all. This makes the choice of an acceptable damage level a central design decision. Repair cost and accessibility both bear on the choice.

Answer: C

Both are true and the reason gives the economics, and the damage level is chosen explicitly. Stability formulae are written in terms of that accepted damage rather than of no damage at all. Zero movement would demand impractically heavy units.

Answer: D

A higher consequence of failure justifies a longer return period and a larger structure. Encounter probability over the whole design life is the quantity that actually matters. Consequence of failure drives the whole selection.

Answer: A

One minus 0.99 raised to the fiftieth power gives about forty per cent. Return period is therefore a poor guide to the risk over a structure's whole life. Encounter probability is the honest measure. A fifty year life sees the hundred year event often enough.

Answer: B

Both are true and the reason gives the reason, and a hundred year event is likely within fifty years. Modern guidance expresses the target as an encounter probability directly. The change makes the residual risk explicit to the client.

Answer: C

Waves larger than about 0.6 times the local depth cannot reach the structure at all. Water level therefore becomes as important as the offshore wave climate in setting the design condition. A higher water level admits a larger wave to the toe.

Answer: D

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: in shallow water the wave that can reach the toe is limited by the depth, so the breaking criterion rather than the offshore extreme governs, and a higher water level admits a larger wave. Statement 3 is the error the first two exist to prevent - the offshore extreme is transformed by shoaling, refraction and breaking before it arrives, and applying it directly at the structure is wrong in either direction.

Answer: A

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. Depth-limited breaking caps the wave height at roughly 0.6 to 0.8 of the water depth, so the design condition is the joint occurrence of a water level and a wave, not the worst of each taken separately. The largest wave at the toe usually arrives on a high surge tide, and it is that combination, at its own joint probability, that the design must be checked against.

Answer: B

Each layer must retain the one beneath it while remaining permeable enough to dissipate energy. Filter rules govern the permitted size ratios between successive layers. Permeability and retention pull against each other.