Seawalls Groynes and Breakwaters Questions and Answers

Answer: A

Correct answer: stop the marine undercutting that drives the retreat

A cliff retreats because waves remove material from its base, oversteepening it until the face above fails. A toe revetment interrupts that mechanism by absorbing the wave energy before it reaches the cliff foot. It does not widen the beach, it cannot raise the cliff, and it does nothing about water within the cliff itself - which is why it is a partial remedy.

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 limitation. Cliffs also fail from within: groundwater raises pore pressures and lubricates bedding planes, frost and wetting cycles weather the face, and the material may simply be at an unstable angle. A revetment stops the marine cause and leaves the terrestrial ones untouched, so a cliff can and does continue to fail behind a perfectly sound revetment unless drainage and slope stabilisation accompany it.

Answer: C

A partial barrier stops short waves while allowing longer ones to pass. It offers a small footprint and good water exchange in sheltered conditions. Footprint and flushing are both improved. Exposure is what limits where they can be used.

Answer: D

The first two are correct and define their proper application. The third overstates them considerably, since long swell passes almost unimpeded. They are a partial barrier by design. Short period energy is what they are meant to stop.

Answer: A

Water and debris on a carriageway create a serious hazard at speed. The tolerable discharge for vehicles is far lower than that for open ground. Water on a carriageway is dangerous at speed. Tolerable limits differ greatly by use behind.

Answer: B

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

Each structure has the situation it fits. A rock revetment suits an exposed frontage where suitable rock can be obtained economically. A stepped seawall suits an urban promenade, combining defence with access. A timber groyne field suits a shingle beach of moderate exposure, where the loading is gentle and repairs are cheap. A detached breakwater suits a frontage where the problem is wave energy rather than drift.

Answer: C

Correct answer: it was aimed at the wrong cause of the problem

A structure that is well designed, well built and in the right place will still fail to achieve anything if it addresses a process that is not the one causing the erosion - groynes on a frontage losing sediment offshore, or a detached breakwater where the loss is a drift deficit. Diagnosis of the sediment budget and the wave climate therefore precedes the choice of structure, and it is the step most often skipped.

Answer: D

No structure adds sediment to a cell; only nourishment does that. The other three are the three genuine purposes structures are built for. Sediment supply comes only from nourishment. No structure creates material out of nothing.

Answer: A

Coastal processes carry the consequences well beyond the immediate site. Assessment at the scale of the sediment cell is now the normal expectation. Consequences travel well beyond the works. The cell is the appropriate unit of assessment.

Answer: B

Both are true and the reason gives the reason for the wider scope, and the sediment cell is the usual unit. Assessing only the immediate frontage misses the most important consequences. The cell is the appropriate unit of assessment.