The Bruun rule relates shoreline retreat to
Answer: C
It predicts retreat many times the rise itself, depending on the profile slope. It is widely used and widely criticised for the assumptions it makes. Its predictions can be large for gentle slopes. The result is sensitive to the slope assumed.
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Consider the following statements about the Bruun rule.
1. It predicts retreat proportional to the rise divided by the profile slope.
2. It assumes a closed sediment budget and an equilibrium profile.
3. It accounts fully for longshore transport and sediment supply.
Answer: D
The first two are correct and include the assumptions on which it rests. The third is exactly what it does not do, and this is the principal ground on which it is criticised. A closed budget is rarely a realistic assumption.
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Assertion (A): The Bruun rule is applied with caution in practice.
Reason (R): It assumes a closed budget that few real coastal frontages actually have.
Answer: A
Both are true and the reason gives the main limitation, and longshore effects frequently dominate. It is best treated as an order of magnitude indication rather than a prediction. Site specific studies are needed for design.
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For a profile slope of 1 in 50, the Bruun rule implies a shoreline retreat of roughly how many metres per metre of sea level rise?
Answer: B
Correct answer: 50 metres
The Bruun rule gives the retreat as the sea level rise divided by the profile slope:
R = S / tan(beta) = 1 / (1/50) = 50 metres.
So a metre of rise on a 1 in 50 profile withdraws the shoreline by fifty metres. The rule is a simplification - it assumes a closed profile translating landward with no longshore effects - but it makes the essential point that a gentle profile amplifies a modest rise enormously.
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State whether the following is true or false.
A gentler beach profile slope implies greater shoreline retreat for a given sea level rise.
Answer: C
Correct answer: True
The Bruun retreat is the rise divided by the slope, so the flatter the profile the larger the retreat for the same rise - a 1 in 100 beach withdraws twice as far as a 1 in 50 one. This is why low-lying gently sloping coasts, which are also the most heavily populated, face the greatest shoreline change from sea level rise.
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Relative sea level change at a site combines
Answer: D
Correct answer: global sea level change with local vertical land movement
What matters at a site is the sea level relative to the land, so the global change in ocean volume must be combined with whatever the land itself is doing - post-glacial rebound raising some coasts, subsidence from tectonics, sediment compaction or groundwater extraction lowering others. Parts of Scandinavia see falling relative sea level while parts of the Gulf coast see several times the global rate.
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Three of the following contribute to relative sea level change at a site. Which is the odd one out?
Answer: A
The spring neap cycle is a periodic fluctuation rather than a change in mean level. The other three each alter the long term relative level. A periodic cycle is not a change in mean level. Only the first three shift the long term datum.
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Salt marshes and mangroves attenuate waves principally by
Answer: B
Stems and roots extract momentum from the wave as it passes over. Attenuation increases with the width of the vegetated belt and with stem density. Attenuation accumulates along the flow path. Belt width dominates the protection achieved.
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Assertion (A): A wide belt of mangrove offers more wave attenuation than a narrow one.
Reason (R): Attenuation accumulates along the path the wave travels through the vegetation.
Answer: C
Both are true and the reason gives the dependence, and the reduction is roughly exponential with distance. Belt width is therefore the key parameter in nature based defence. A narrow fringe offers comparatively little.
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Consider the following statements about nature based coastal defence.
1. Vegetated foreshores attenuate wave energy measurably.
2. They can adapt to sea level rise by accreting sediment.
3. They provide protection that never requires monitoring.
Answer: D
Correct answer: 1 and 2 only
Statements 1 and 2 are the case for nature based defence: vegetated foreshores measurably attenuate waves, and unlike a structure they can keep pace with sea level rise by trapping sediment and accreting. Statement 3 is wrong - a living system needs monitoring precisely because it changes, and it can be lost to disease, storm damage or a change in the sediment supply.
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