State whether the following is true or false.
Adding a rock toe to an existing vertical seawall reduces the scour in front of it.
Answer: A
The rock dissipates energy that would otherwise be reflected from the vertical face. It is among the commonest remedial works on an ageing seawall. It addresses the cause rather than the symptom. Reflection is what drove the scour in the first place.
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All of the following are common remedial works on an ageing seawall EXCEPT
Answer: B
Exposing the foundation is precisely what causes a seawall to fail. The other three each address a recognised mode of deterioration. Exposing a foundation is how seawalls are lost. Toe protection is never the thing to remove.
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A shore connected breakwater differs from a detached one in that it
Answer: C
Correct answer: joins the land, so drift cannot pass around its landward end
A shore-connected structure blocks the longshore transport completely on the updrift side, since there is no gap behind it. A detached breakwater stands offshore with water between it and the beach, so sediment continues to move along the shore behind it and only the wave shelter alters the pattern. The distinction determines the scale of the downdrift consequences.
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Assertion (A): A shore connected breakwater has a greater downdrift effect than a detached one.
Reason (R): Nothing can pass behind it, so the whole of the arriving drift is intercepted.
Answer: D
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason is the mechanism. Because nothing can pass behind it, a shore-connected breakwater intercepts the entire drift until it has filled, and every cubic metre trapped is one that does not reach the frontage beyond. A detached structure lets the drift continue past it, so its downdrift effect is limited to what the altered wave field does. This is why shore-connected works so often require a bypassing arrangement.
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A groyne built of rock rather than timber generally requires
Answer: A
Correct answer: a considerably wider footprint on the beach
Rock must be placed at a stable slope, so a groyne of any useful height needs a base several times its crest width - often ten metres or more across the beach. A timber groyne is a line of piles and planks a few hundred millimetres thick. The footprint matters practically: it consumes beach, obstructs access along the shore, and on a narrow frontage may be decisive against rock.
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Consider the following statements about groyne materials.
1. A rock groyne occupies a much wider footprint than a timber one.
2. Sheet pile groynes are slender but offer little permeability.
3. Material choice has no bearing on the footprint occupied.
Answer: B
Correct answer: 1 and 2 only
Statements 1 and 2 are correct: rock needs a wide stable base, while sheet piling is slender but solid, so it traps the whole of the drift rather than letting part through. Statement 3 is plainly wrong, and the footprint is often the deciding factor on a narrow beach where a rock groyne would occupy the whole width available.
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A terminal groyne at the end of a defended frontage is provided to
Answer: C
Correct answer: limit the loss of beach material past the end of the works
A terminal groyne closes the downdrift end of a scheme so that the beach retained within it is not simply drawn out at the far end. It holds the material the scheme has gathered or the nourishment it has placed, and it defines where the defended frontage stops. It does not increase the drift arriving; if anything it worsens conditions immediately beyond, which is the price of holding the line where it does.
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Assertion (A): A terminal groyne creates a sharp discontinuity at the end of a scheme.
Reason (R): It intercepts the drift completely at the point where the works stop.
Answer: D
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. Because the terminal groyne intercepts the drift completely, the beach steps down abruptly at its downdrift face: full within the scheme, starved immediately beyond it. That discontinuity is often where the next erosion problem appears, and it is why terminal groynes are sometimes tapered in height, or accompanied by nourishment beyond the works, to soften the transition.
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A breakwater built of a single row of interlocking concrete caissons differs from a rubble mound in that it
Answer: A
Correct answer: occupies a far smaller footprint on the sea bed
A caisson is a vertical box founded on a prepared bed, so its width is only what stability against sliding and overturning requires. A rubble mound must be laid at a stable slope on both faces, so in deep water its base may be five or six times the width. Where the bed is expensive to occupy, where dredged material is scarce, or where water is deep, that difference decides the choice.
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Consider the following statements comparing caisson and mound breakwaters.
1. A caisson occupies a much smaller footprint on the bed.
2. A mound requires a far greater volume of material.
3. A caisson can be founded on any ground without preparation.
Answer: B
Correct answer: 1 and 2 only
Statements 1 and 2 are correct and are the two chief advantages of the caisson. Statement 3 is quite wrong: a caisson concentrates a very large load onto a narrow strip of bed, so it requires a carefully prepared and levelled foundation, usually a screeded rubble bedding layer, and it is unsuitable on soft ground without treatment. A mound spreads its load and is far more tolerant of poor ground.
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