Why does a groyne cause erosion downdrift?

It traps longshore drift, so less sand reaches the far side while waves keep removing sediment, producing erosion.

When can a salt marsh keep pace with sea level?

Only when organic and mineral accretion together match or exceed the rate of sea-level rise; otherwise it drowns relatively.

S5 Geography | Coastal Landforms and Sea-Level Rise Paper 2 (Advanced)

2026-11-04

Tutor Prep Guide · Secondary Geography

Paper 2 mixes management with arithmetic: combined height and period effects, long-term deficits behind groynes, Bruun retreat, marsh accretion rates, engineering costs and return-period chance.

Separate rates from totals　Allow about thirty minutes; decide for each item whether a rate or a total is wanted before multiplying by years.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10 items
Suggested time	About 30 minutes
Difficulty mix	Foundation 4 · Intermediate 3 · Challenge 2 · Error redo 1
Chapter	Chapter: S5 Geography \| Coastal Landforms and Sea-Level Rise
Level	S5 (Secondary 5) Geography

02　Questions (10 items)

FoundationWave height rises from 1.5 m to 2.5 m and period from 8 s to 10 s. By what factor does energy rise?
FoundationA groyne traps 65% of the 40,000 m³ of sand drifting each year. How much passes downstream yearly, and what is the 20-year deficit?
FoundationSea level rises 4.1 mm a year. How much in 30 years, and how far does a shoreline with a 1/80 beach slope retreat?
FoundationA salt marsh accretes 2.5 mm a year while sea level rises 5.0 mm a year. How much relative drowning occurs in 50 years?
IntermediateA 2.8 km sea wall costs $4,500 per metre. What is the total cost?
IntermediateOne beach nourishment needs 120,000 m³ of sand at $95 per m³. What does one round cost, and how many rounds in 40 years at one every 5 years?
IntermediateWhat is the chance that a storm surge with a 20-year return period occurs at least once in the next 25 years?
ChallengeA deep-water wave has a period of 12 s. What are its speed and wavelength? (c = 1.56T, L = 1.56T².)
ChallengeA spit extends 3.5 m a year and the channel mouth is 60 m wide. How far does it grow in 12 years, and how wide is the channel then?
Error redoError retry: a student multiplies 5 mm a year by 50 years and answers 250 m of sea-level rise. What is correct?

03　Answers and one-line notes

#	Answer	One-line note
1	About 3.47 times ((2.5/1.5)² x (10/8)).	Energy is proportional to height squared times period, so both factors multiply; using one alone understates the change.
2	14,000 m³ a year passes; the 20-year deficit is 280,000 m³.	Trapping 65% leaves 35% passing, and the starved lower shore erodes as the shortfall accumulates year by year.
3	It rises 123 mm (0.12 m), so the shoreline retreats 9.84 m.	Retreat is rise divided by slope; a slope of 1/80 is 0.0125, and dividing by a small number magnifies the retreat.
4	A net loss of 2.5 mm a year, that is 125 mm (12.5 cm) in 50 years.	Survival depends on whether accretion keeps pace with sea-level rise, so the two rates must be compared directly.
5	$12,600,000.	The rate is per metre, so the length must be converted first; using kilometres understates the cost a thousandfold.
6	$11,400,000 per round, and 8 rounds in 40 years.	Nourishment is a recurring cost, so any comparison must multiply by the number of rounds rather than counting one.
7	About 72.3% (1 – 0.95^25).	A return period gives only a 5% annual chance; across years the chances compound instead of dividing.
8	Speed about 18.72 m/s and wavelength about 224.64 m.	Going from an 8 s to a 12 s period lengthens both speed and wavelength, with wavelength scaling as the square of period.
9	It grows 42 m, leaving a channel of 18 m.	A growing spit narrows the channel; growth is rate times years, and the remainder is the original width minus that growth.
10	The correct rise is 250 mm, that is 0.25 m.	Millimetres times years stay millimetres; dividing by 1,000 gives metres, and skipping that step is the usual error.

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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