Coastal Structures Design Questions and Answers

Answer: A

Each stage makes the next more likely, so early intervention is decisive. Monitoring exists precisely to catch the sequence at its first stage. Each stage costs more to remedy than the last. Catching it early is the whole purpose of survey.

Answer: B

Correct answer: interlock between adjacent armour units

A caisson is a monolithic gravity structure with no armour units and therefore no interlock; that check belongs to a rubble mound. What is checked for a caisson is sliding on its rubble foundation, overturning about its heel, and bearing failure of the foundation beneath - all under the wave pressure distribution including the uplift on the base.

Answer: C

The overturning moment concentrates the bearing pressure towards one edge of the base. Foundation design must accommodate that concentration rather than the average pressure. Edge pressure governs the foundation design.

Answer: D

Correct answer: True

A caisson concentrates a very large load onto a narrow footprint, so the rubble base and the ground beneath must carry it without bearing failure or excessive settlement, and the base must be screeded level within tight tolerance or the caisson will not seat evenly. On soft or variable ground the foundation must be dredged out and replaced, or improved, and the cost of doing so frequently decides in favour of a rubble mound instead.

Answer: A

Drag dominates for slender piles in large waves and inertia for the larger diameters. Their relative importance is judged from the ratio of the pile diameter to the wavelength. Slenderness decides which term dominates.

Answer: B

Both are true and the reason gives the criterion, and a diameter above about a fifth of the wavelength is the usual threshold. Below it the structure does not appreciably disturb the wave. The threshold separates two distinct methods.

Answer: C

Above that value the member modifies the incident wave and diffraction theory is required. The criterion separates the two approaches cleanly. Beyond it the structure scatters the incident wave. Diffraction theory then replaces the Morison approach.

Answer: D

Attenuation requires the structure to be large relative to the incident wavelength. Long period swell passes almost entirely unaffected beneath a floating unit of practical size. Size relative to wavelength is the controlling ratio.

Answer: A

Correct answer: 1 and 2 only

Statements 1 and 2 are correct and define where a floating breakwater is useful: it attenuates short, steep waves whose motion is confined near the surface, and it is largely transparent to long period swell, whose orbital motion extends far deeper than the structure. Statement 3 is wrong - the shelter is partial, which is why floating breakwaters serve marinas and fish farms rather than commercial ports.

Answer: B

Both are true and the reason gives the demand, and anchor design is frequently the critical element. Mooring failure is the commonest cause of loss for such structures. Anchor design frequently governs the whole scheme.