Why does urbanisation make peaks higher and earlier?

Impermeable surfaces speed water to the channel and cut infiltration, so lag time shortens and the peak rises.

How do return period and probability relate?

Annual probability is the reciprocal of the return period; the chance of at least one event in n years is 1 minus (1 - 1/T) to the power n.

S5 Geography | River Landforms and Management Paper 2 (Advanced)

2026-11-04

Tutor Prep Guide · Secondary Geography

Paper 2 covers catchment hydrology and sediment budgets: storm runoff and mean discharge, urban effects on flood peaks, meander migration rates, reservoir trapping, aggradation, terrace age and channelisation.

From water volume to sediment budget　Allow about thirty minutes; watch the time unit when computing peak discharge, and remember the trapping percentage is taken from the incoming load.

01　Paper details (check answers afterwards)

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

02　Questions (10 items)

FoundationAn 85 km² catchment receives 90 mm of rain in six hours with a runoff coefficient of 0.6. What is the total runoff volume?
FoundationIf that volume drains evenly over six hours, what is the mean discharge?
IntermediateUrbanisation raises the runoff coefficient from 0.30 to 0.60 for the same storm, and cuts lag time from 8 hours to 3. What changes?
IntermediateA meander outer bank migrates 1.5 m a year and a field lies 75 m from the bank. Roughly when will it be eroded away?
IntermediateA reservoir traps 65% of the 2.4 million tonnes of sediment arriving each year. How much reaches the lower course, and what follows?
ChallengeA riverbed aggrades 0.12 m a year and the navigation channel needs 0.4 m of depth. How often must it be dredged?
ChallengeWhat is the chance that a 1-in-100-year flood occurs at least once in the next 30 years?
ChallengeA river incises 0.2 mm a year and a terrace now stands 8 m above the channel. Roughly how old is the terrace?
ChallengeStraightening a channel from 5.6 km to 4.0 km raises the gradient by what factor, and the velocity if velocity scales with the square root of gradient?
Error redoError retry: a student writes that a 50-year return period means one flood every 50 years. What is correct?

03　Answers and one-line notes

#	Answer	One-line note
1	4,590,000 m³ (0.090 × 85,000,000 × 0.6).	Divide millimetres by 1,000 and multiply square kilometres by 1,000,000; both conversions are needed.
2	212.5 m³/s (4,590,000 / 21,600 seconds).	Six hours is 21,600 seconds; using minutes or hours as the denominator gives an answer sixty times out.
3	Runoff volume doubles; the peak arrives 5 hours earlier.	Impermeable surfaces deliver water to the channel faster, so the peak is higher and earlier: the classic urban flood effect.
4	About 50 years (75 / 1.5).	Bank retreat rates vary year to year, but a mean rate is acceptable for planning estimates.
5	0.84 million tonnes; with too little sediment the lower channel incises and the coast erodes faster.	Trapping sediment has a cost: the reservoir keeps its storage but downstream loses its sediment supply.
6	About every 3.33 years (0.4 / 0.12).	Dredging only moves sediment on; unless the supply is cut the cost recurs forever.
7	About 26.0% (1 – 0.99^30).	A one-in-a-hundred event still has a one-in-four chance within thirty years, so planning cannot assume it will not happen.
8	About 40,000 years (8 m / 0.0002 m per year).	A terrace is an abandoned former floodplain; its height divided by the incision rate gives an age estimate.
9	Gradient 1.4 times; velocity about 1.18 times (the square root of 1.4).	Straightening delivers water downstream faster, raising peak discharge, so levees and bridges must be reassessed.
10	A 50-year return period is a 2% chance each year; it can happen in any year, and twice in a decade.	A return period is an annual probability, not a schedule; two big floods in successive years are perfectly possible.

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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