Why can mangrove attenuation not simply be added?

Each strip removes a share of the energy still left, so the factors multiply: three strips of thirty per cent leave about thirty-four per cent, not ten.

Why does a dam narrow the beach downstream?

The dam traps river sediment while offshore loss and drift output continue, so the budget turns negative and the beach keeps narrowing.

S5 Geography | Coastal Landforms Paper 3 (Challenge)

2026-11-04

Tutor Prep Guide · Secondary Geography

Paper 3 is all multi-step work: energy must be compared with period included, attenuation multiplies rather than adds, and a budget has to be taken from surplus to deficit to see the change.

Attempt first, mark afterwards　Allow about thirty minutes; write the formula before substituting, keep the fraction remaining separate from the percentage lost, and check units and direction at the end.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10 items
Suggested time	About 30 minutes
Difficulty mix	Foundation 1 · Intermediate 2 · Challenge 6 · Error redo 1
Chapter	Chapter: S5 Geography \| Coastal Landforms
Level	S5 (Secondary 5) Geography

02　Questions (10 items)

FoundationTwo wave sets: the first has height 2 m and period 8.0 s, the second height 1 m and period 6.0 s. If energy varies with the square of height and with period, how many times more energy does the first carry?
IntermediateA cliff retreats 0.5 m a year normally, and one major storm every 10 years removes another 6 m at once. What is the total retreat over 50 years?
IntermediateLongshore drift moves 9,000 cubic metres a year eastwards and 3,500 westwards. What is the net drift?
ChallengeA beach holds 60,000 cubic metres (1200 m by 25 m by 2 m) and loses a net 3,000 a year. With no nourishment, when is only thirty per cent left?
ChallengeMangroves cut wave energy by thirty per cent for every 50 m of width. After 150 m, what fraction of the energy remains and what percentage has been lost?
ChallengeA sea wall must resist a high tide of 2.1 m, a storm surge of 0.8 m and wave run-up of 1.2 m. What crest level is needed, and how much higher than the present wall of 3.4 m?
ChallengeA cliff retreats 0.35 m a year over a planning horizon of 100 years, plus a margin of twenty per cent. How far from the edge should the setback line be?
ChallengeAfter a dam, river sediment falls from 20,000 to 6,000 cubic metres a year, while 5,000 is lost offshore and 4,000 leaves by drift. What is the budget before and after, and how big is the change?
ChallengeOption one: nourishment every 4 years at 300,000 dollars. Option two: a sea wall at 2,800,000 plus 25,000 a year. Over 40 years, what does each cost and which is cheaper?
Error redoA student writes that beach nourishment replaces the lost sand and therefore solves coastal erosion for good. What is wrong?

03　Answers and one-line notes

#	Answer	One-line note
1	About 5.33 times.	Four times 8.0 over one times 6.0 is 32 over 6, about 5.33; long-period swell from a distant fetch carries more energy, so height alone is not enough.
2	About 55 m.	Normal retreat is 0.5 times 50, or 25 m; the 50 years contain 5 storms adding 30 m, giving 55 m. A single average rate understates loss because collapse is episodic.
3	A net 5,500 cubic metres eastwards.	9,000 less 3,500 is 5,500 towards the larger side; the net direction determines which way a spit hooks and which side of a groyne needs nourishment.
4	About 20 years.	The volume is 1200 times 25 times 2, or 60,000; seventy per cent of it is 42,000, and dividing by 3,000 gives 20 years, since only the removable part counts.
5	About 34.3 per cent remains, so 65.7 per cent is lost.	Each 50 m leaves 0.7, so three sections leave 0.7 cubed, or 0.34; the loss is 65.7 per cent, since attenuation multiplies rather than adds.
6	It needs 4.1 m, so 0.7 m more than now.	Summing 2.1, 0.8 and 1.2 gives 4.1 m, which is 0.7 m above the wall; all three components count, or waves overtop in a storm.
7	About 42 m.	0.35 times 100 is 35 m, and times 1.2 gives 42 m; the margin absorbs rate error and sudden collapse, not appearance.
8	A surplus of 11,000 becomes a deficit of 3,000, a change of 14,000 cubic metres.	Before: 20,000 less 5,000 less 4,000 is 11,000; after: 6,000 less the same two is -3,000, a yearly loss of 3,000; the change is 14,000. Dams trapping sand are a common cause of beach narrowing.
9	Nourishment 3,000,000, the wall 3,800,000, so nourishment is cheaper by 800,000.	Nourishment runs 10 times at 300,000, giving 3,000,000; the wall is 2,800,000 plus 25,000 times 40, giving 3,800,000. The gap is 800,000, but what is protected and the risk of failure also matter.
10	The word for good is wrong: nourishment only tops up sand that keeps leaving, so it must be repeated every few years unless the sediment budget itself is fixed.	Nourishment treats a symptom, not the cause; unless upstream supply is restored and dredging stopped, the sand is lost again. A good answer notes the need for repeat cycles and the untouched sediment budget.

04　What to do after marking

After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
Repeat the same item two days later; move to the next paper only when both attempts are correct.
Parents only need the answer and note columns; there is no need to explain every item.

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