Coastal Structures Design Questions and Answers

Answer: C

Too large a ratio allows the finer material beneath to wash out through the voids above. A ratio of roughly four to five between successive layers is the conventional guide. A geotextile substitutes where the step is too large.

Answer: D

Correct answer: 4 to 5

Successive layers of a rubble mound are conventionally sized within a ratio of about four or five to one in mass, so that the finer material beneath cannot wash out through the voids of the coarser above. Too large a step and the underlayer is lost through the armour, undermining it; too small a step and an unnecessary number of layers is required. The same filter reasoning governs the bottom layer against the sea bed.

Answer: A

Both are true and the reason gives the function, and it replaces one or more granular filter layers. Its durability under wave attack and construction traffic must be considered. Damage during placement is the usual failure mode.

Answer: B

Mass varies with the cube of the wave height, making the design wave estimate critical. It takes no account of the wave period, the storm duration or the permeability of the structure. Van der Meer added exactly those missing terms.

Answer: C

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: Hudson gives the armour mass as proportional to \( H^3 \), and it contains neither wave period nor storm duration. Statement 3 is wrong - permeability appears in the Van der Meer formulae, which were developed precisely to bring period, duration, damage level and permeability into a relation that Hudson lumps into a single empirical coefficient.

Answer: D

Separate expressions are given for the plunging and for the surging wave conditions. The additional parameters allow a far more discriminating design than the single Hudson coefficient. The extra inputs must themselves be established.

Answer: A

Both are true and the reason gives the reason for the distinction, and the surf similarity parameter decides which applies. A transition value separates the two regimes. Slope and steepness together decide which applies.

Answer: B

Correct answer: eight

The Hudson formula makes the required mass proportional to the cube of the wave height, so doubling the height multiplies the mass by \( 2^3 = 8 \). A unit of five tonnes becomes one of forty, which may exceed what a quarry can supply or a crane can place, and is often the point at which concrete units replace rock.

Answer: C

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

Both are true and the reason gives the leverage. With mass going as \( H^3 \), a ten per cent error in the design wave height changes the required armour mass by about a third, and the volume of the whole primary layer with it. That is a large sum on a major breakwater, and it is why the wave climate study and the extreme value analysis behind it receive the attention they do.

Answer: D

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

The four represent the development of armour. Rock is the natural material, limited by what the quarry will yield. The cube is the simplest concrete unit, relying on its own weight rather than any interlock. The tetrapod interlocks through slender projecting legs, which is both its strength and its weakness. The Accropode is a single layer interlocking unit of high stability, requiring accurate placement but far less concrete in total.