Seawalls Groynes and Breakwaters Questions and Answers

Answer: A

Tranquillity within the basin is the governing requirement rather than shoreline response. The consequences for the adjacent coast are nonetheless a major design consideration. Tranquillity governs, but the coast cannot be ignored.

Answer: B

Correct answer: 1 and 2 only

Statements 1 and 2 are the two standard devices for keeping waves out of a basin: orienting the entrance away from the dominant wave direction, and overlapping the arms so that energy must diffract twice to get in. Statement 3 is the reverse of the truth - a wider entrance admits more wave energy. It also eases navigation and improves flushing, which is precisely why the width is a compromise.

Answer: C

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

Both are true and the reason sets out both sides. Narrowing the entrance cuts wave penetration and improves tranquillity at the berths, but it makes the approach harder for large vessels in a cross wind or tide, and it restricts the tidal exchange on which water quality depends. The width finally chosen satisfies the design vessel and the flushing requirement, and the wave problem is then dealt with by orientation, overlap and internal absorption.

Answer: D

Correct answer: force waves to change direction before entering the basin

Overlapping the arms means there is no straight line from open sea to basin: energy must diffract round the head of one arm and then round the other, and each diffraction sheds a large part of it. The navigable width is unchanged, since it is measured across the channel rather than along it. The arrangement costs more breakwater length, not less, and it has no bearing on dredging.

Answer: A

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

Each feature answers one problem. Overlapping arms reduce the wave energy reaching the entrance. A wave-absorbing quay face stops that energy bouncing around inside the basin, where reflection would make berths untenable. A training wall confines a channel so that the flow maintains its own depth. Jetties at a river mouth hold the outlet in one place instead of letting it wander with the drift.

Answer: B

Reflections from vertical quays can produce troublesome vessel motions at the berth. A perforated or sloping absorbing face reduces the reflected energy considerably. Downtime at the berth is the practical consequence.

Answer: C

Both are true and the reason gives the mechanism, and long period energy is the usual culprit. Absorbing treatments are added where downtime at the berth becomes unacceptable. Long period energy is usually the culprit.

Answer: D

Concentrating the ebb flow increases the velocity and scours the channel. It reduces or removes the need for maintenance dredging at the entrance. The structure works with the natural flow. Self scouring channels need far less dredging.

Answer: A

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. Confining the ebb flow between walls concentrates it into a narrower section, so its velocity rises and it carries away the sediment that would otherwise settle. The channel scours itself. The saving in maintenance dredging is often what justifies the walls, though the sediment removed has to go somewhere and frequently forms a bar further offshore.

Answer: B

Correct answer: stabilise the position of the channel through the inlet

A natural tidal inlet migrates as longshore drift builds a spit across it, so the channel wanders and shoals unpredictably. Jetties fix its position and hold it open. They do trap drift on the updrift side, and that impoundment is often used as a bypassing source, but it is a consequence rather than the purpose - and the downdrift starvation it causes is one of their principal drawbacks.