Assertion (A): Sediment travels along a beach in a zig-zag path.
Reason (R): Uprush runs obliquely up the face while backwash returns straight down it.
Answer: A
Both are true and the reason gives the geometry, and each wave advances the particle a small distance alongshore. Over a season the accumulated movement can amount to kilometres. Each individual wave moves a grain only centimetres.
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The longshore transport rate is most sensitive to which wave property?
Answer: B
Standard formulae make the transport proportional to wave height raised to a power near two and a half. A modest increase in height therefore produces a large increase in transport. A stormy year can carry several times an average year's drift.
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State whether the following is true or false.
A doubling of breaking wave height roughly quintuples the longshore transport rate.
Answer: C
With transport varying as height to the power of two and a half, doubling gives a factor near five and a half. This steep dependence means a few storms can dominate the annual total. Design must consider the distribution, not just the mean.
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The CERC formula relates longshore transport to
Answer: D
It is the most widely used first estimate despite taking no account of grain size. Later formulae such as Kamphuis add sediment and slope terms to improve the estimate. Grain size affects transport rates substantially.
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Consider the following statements about the CERC formula.
1. It uses the longshore energy flux at the breaker line.
2. It contains no explicit grain size term.
3. It is more accurate than formulae including sediment properties.
Answer: A
The first two are correct and describe its form. The third is not so, since the omission of grain size is precisely the weakness that later formulations set out to address. Its simplicity is both its strength and its limitation.
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A littoral cell is
Answer: B
Boundaries are usually headlands, submarine canyons or major structures. Managing a coast cell by cell recognises that intervention in one place affects the whole cell. Cell boundaries are usually headlands or canyons.
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Assertion (A): Coastal management is increasingly organised around littoral cells.
Reason (R): Sediment moves within a cell but only slightly across its boundaries.
Answer: C
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason gives the justification. A littoral cell is a length of coast within which the sediment circulates and across whose boundaries little passes, so it is the natural unit for a sediment budget - what is done at one point is felt at another within the same cell and hardly at all beyond it. Managing by administrative boundary would cut across those links and allow one authority to solve its problem at a neighbour’s expense.
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A coastal sediment budget accounts for
Answer: D
A surplus builds the shoreline seaward while a deficit causes erosion. Quantifying each term is difficult, and the budget is often closed by treating the residual as the least certain term. Closure errors can be a large fraction of the total.
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Match each sediment budget term with its category.
List I
A. River input
B. Cliff erosion
C. Submarine canyon
D. Dune deposition
List II
1. a sink removing sediment permanently
2. a source of sediment to the cell
3. a temporary sink landward of the beach
4. a source from within the cell itself
Answer: A
Correct answer: A-2, B-4, C-1, D-3
The budget divides into sources, sinks and stores. A river delivers sediment into the cell from outside it. Cliff erosion supplies sediment from within the cell itself, which is why protecting a cliff removes an input. A submarine canyon carries material into deep water beyond recovery, so it is a permanent sink. Dune deposition stores sediment landward of the beach, from which storms may later withdraw it.
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Three of the following are sources of sediment to a beach. Which is the odd one out?
Answer: B
Correct answer: loss into a submarine canyon offshore
A submarine canyon takes sediment out of the littoral system permanently, carrying it into deep water from which no wave will return it - so it is a sink, not a source. The other three all add material to the beach: rivers delivering from the catchment, cliffs releasing from within the cell, and onshore transport bringing sediment up from the shoreface.
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