Beach Erosion and Protection Questions and Answers

Answer: A

Correct answer: a salient or a tombolo of accreted sand in its lee

The structure shelters the water behind it, so waves diffracting round its ends carry sediment towards the quiet zone and deposit it there. The result is a bulge in the shoreline - a salient - and if the structure is close enough inshore or long enough, the bulge grows out to meet it and forms a tombolo. The sediment that builds it comes from the longshore drift, so accretion here means less arriving further along.

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. Once the accretion reaches the structure there is no longer a water path behind it, so the drift cannot pass and the breakwater acts like a shore-connected groyne. That is why a tombolo, though it looks like a success, is often the outcome a designer sets out to avoid: a salient that stops short of the structure gives the shelter without severing the drift.

Answer: C

Longer structures placed closer inshore favour a tombolo. Design charts express the outcome in terms of this ratio for a range of conditions. Longer and closer structures favour a tombolo. The ratio is the first thing checked in design.

Answer: D

Correct answer: the length of the groyne, typically two to three times it

The spacing is scaled to the groyne length because it is the length that determines how far offshore the drift is intercepted and therefore how wide a compartment the structure can hold in equilibrium. Spacing of two to three times the length is the usual starting point, adjusted for the obliquity of the waves. Tidal range, wave period and grain size all influence the design, but none of them sets the spacing directly.

Answer: A

Both are true and the reason gives the reason for the practice, and a field spreads the effect. Tapering the field at its downdrift end reduces the discontinuity further. A single groyne creates a sharp erosion step beside it.

Answer: B

Correct answer: True

A groyne field ends abruptly at its downdrift limit, and the beach steps down sharply beyond it because the last compartment is full and nothing passes. Reducing the height and length of the last few groynes lets an increasing share of the drift through, so the transition is spread over a distance instead of concentrated at one point. It does not remove the deficit, but it stops it being focused where the damage is worst.

Answer: C

The compartments are filled artificially so that the natural drift passes through. Without it the field fills at the expense of the coast downdrift. Pre-filling is now regarded as standard practice. It removes the downdrift penalty almost entirely.

Answer: D

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. A new groyne field fills its compartments by capturing the drift, and until they are full nothing passes to the frontage beyond - so the downdrift erosion is at its most severe in the first years. Filling the compartments artificially at construction removes that phase altogether: the field starts in equilibrium and the natural drift passes through from the outset.

Answer: A

Vegetation is what converts blown sand into a stable stored reservoir. Trampling and vehicle access are the principal threats to that vegetation. Access management is cheaper than restoration. Boardwalks and fencing are the usual measures.

Answer: B

Both are true and the reason gives the purpose, and blowouts develop where vegetation is lost. Restoring a breached dune ridge is far more costly than preventing the damage. Prevention is the cheapest stage to intervene at.