Seawalls Groynes and Breakwaters Questions and Answers

Answer: A

Orientation is the simplest way to tell the families apart on a plan. Their purposes differ accordingly, and confusing one for another leads to the wrong structure being chosen. Plan orientation is the quickest way to tell them apart.

Answer: B

Correct answer: protect the land behind rather than to retain a beach

A seawall is a defence of the land: it forms a hard boundary that waves break against, protecting whatever lies behind it. It does nothing to keep sand or shingle in front of it, and often makes matters worse, since the reflected energy scours the toe. Retaining a beach is the groyne’s function, and enclosing sheltered water is the breakwater’s. Choosing between them begins with deciding which of those objectives actually matters at the frontage.

Answer: C

Both are true and the reason gives the distinction, and the two are frequently used together. Choosing between them means first deciding what is actually being protected. The two are often built together on one frontage.

Answer: D

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

The four profiles form a sequence from reflection to dissipation. A vertical face throws almost all the energy back, which gives the highest reflection and the worst toe scour. A recurve adds a lip that turns the uprush seaward at the crest, cutting overtopping. A stepped face breaks the uprush on successive treads. A sloping revetment, rough and permeable, dissipates most and reflects least - at the cost of the widest footprint.

Answer: A

Correct answer: dissipates energy while providing pedestrian access to the beach

The steps serve two purposes at once, which is why they are so common on promenades. Hydraulically each tread breaks up the uprush and dissipates some of its energy, so reflection is lower than from a plain vertical face. Practically they are a staircase, giving the public a way down to the sand. They do not reduce the material required, and the toe still needs protecting.

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 whole case. An urban promenade needs a defence that is compact, that people can walk down to the beach from, and that does not reflect so violently as to scour its own toe. The stepped profile answers all three, which is why it is found along seafronts from Brighton to Mumbai. Where access does not matter, a sloping revetment would dissipate more for less money.

Answer: C

Correct answer: redirect the climbing water outward and cut the discharge

The recurve is a lip curling seaward at the crest. Water running up the face follows it, is turned back out to sea, and never crosses the wall, so the overtopping discharge for a given wave and crest level falls sharply. The gain is paid for locally: the recurve intercepts the full momentum of the uprush and attracts a large impulsive load, so it must be designed and anchored for it.

Answer: D

Correct answer: 1 and 2 only

Statements 1 and 2 are correct: because the recurve reduces overtopping, the same performance can be had from a lower crest, which saves material and preserves the view; and the recurve itself takes a heavy local load. Statement 3 claims too much. No wall eliminates overtopping under all conditions - a sufficiently severe storm surge with large waves will overtop any practicable crest, and the design is set to an acceptable discharge, not to zero.

Answer: A

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

Rock armour is rough and permeable, dissipates well, and can be repaired by placing more rock. Concrete blockwork is regular and tighter, so it reflects more and is harder to repair once units are displaced. A gabion mattress is cheap but its wire baskets abrade and corrode quickly in the surf, giving a short life. Grouted rock is rigid and nearly impermeable, so it develops pressure behind it and cracks.

Answer: B

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

Both are true and the reason explains the mechanism. Grouting converts a flexible, permeable, self-healing layer into a rigid impermeable slab. Water that would have drained through now builds up pressure behind the face, and the layer can no longer settle to accommodate movement; when it does crack, the fragments are large, the underlayer is exposed at once, and the damage spreads rapidly. An ungrouted slope, by contrast, deforms and continues to work.