Coastal Structures Design Questions and Answers

Answer: A

Correct answer: breakage of the legs under repeated wave impact

The slender legs that give a tetrapod its interlock are also its weakest feature: unreinforced concrete in tension, they crack under repeated impact and rocking, and once broken the unit loses both its interlock and its mass. The failure is not chemical - marine concrete of good quality is durable - but structural, and it is what led to the development of bulkier single layer units.

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 mechanism. In an interlocking system each unit is held by its neighbours, so the loss of one transfers its share of the load to those around it, which are then more likely to move in turn. Damage therefore accelerates once it begins, in contrast with rock armour, which loses units one at a time and gives warning. It is the reason interlocking systems are designed for no damage rather than for a tolerated level.

Answer: C

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: a single layer of interlocking units achieves stability with substantially less concrete than two layers of cubes, and the interlock only develops if the units are placed to the pattern and density specified. Statement 3 is wrong and is the system’s principal risk - single layer armour fails progressively rather than gradually, so it is designed for essentially no damage.

Answer: D

Correct answer: wave height divided by the relative density and the unit size

The stability number is

with \( \Delta \) the relative buoyant density of the unit and \( D_{n50} \) its nominal diameter. It is the dimensionless group in which every armour stability formula is expressed, and it makes plain what can be traded: a denser unit or a larger one both reduce it, and a larger wave raises it.

Answer: A

Correct answer: is allowed to reshape into a stable profile under wave attack

A conventional mound is designed so that its armour does not move; a berm breakwater is built with a wide berm of smaller stone and allowed to be reshaped by the waves into an S-profile that is then stable. The advantage is that it can be built from what a quarry actually yields, without the very large blocks a conventional design demands, and often with simpler plant.

Answer: B

Both are true and the reason gives the advantage, and quarry yield often governs the choice. The reshaping must be predicted so that the final profile remains adequate. Reshaping is designed for rather than merely tolerated.

Answer: C

Correct answer: 1 and 2 only

Statements 1 and 2 are correct and are the whole point of the type: reshaping is intended, and smaller stone suffices because stability is achieved by the profile rather than by individual unit weight. Statement 3 contradicts the concept - a berm breakwater that did not move would simply be an over-designed conventional mound.

Answer: D

A vertical caisson uses far less material than a rubble mound in deep water. It requires a competent foundation and careful attention to the wave forces that it must resist. Depth and material availability decide the choice.

Answer: A

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

Each type suits a different situation. A rubble mound is the natural choice in shallow water where rock is available, since its cost rises steeply with depth. A caisson suits deep water, where a mound would need an enormous volume, and where rock is scarce or costly. A composite structure places a caisson on a rubble base, combining the two. A berm breakwater suits a site with ample small rock but no large armour stone.

Answer: B

Correct answer: wave pressures on a vertical caisson breakwater

The Goda formula gives the distribution of wave pressure over the face of a vertical caisson, including the uplift beneath it, from which the sliding, overturning and bearing checks follow. It is the standard method for composite and caisson breakwaters, and it was extended by Takahashi to account for the impulsive pressures that arise when a wave breaks directly on the wall.