How to choose between hard and soft engineering?

Hard works hold water back but cost more and disrupt ecology; soft works are cheap with modest peak reduction, so schemes usually combine both.

Why track both full capacity and 60% storage?

Losing 40% of effective storage already harms supply and power generation, so usable life ends long before the reservoir is full of silt.

S5 Geography | River Landforms and Management Paper 3 (Challenge)

2026-11-04

Tutor Prep Guide · Secondary Geography

Paper 3 turns to planning and management decisions: levee cost, detention basin fill time, the effect of cut-offs on velocity, wetland storage share, bank erosion losses, reservoir life, payback and mixing of effluent.

Price the management options　Allow about thirty-five minutes; state your assumptions, since cost, payback and mixing all use simplified models.

01　Paper details (check answers afterwards)

Item	Detail
Questions	10 items
Suggested time	About 35 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)

FoundationA 4.2 km levee costs $2,800 per metre. What is the total cost?
FoundationA detention basin holds 120,000 m³ with inflow of 45 m³/s and outflow of 12 m³/s. How long to fill from empty?
IntermediateA cut-off shortens a channel from 6.8 km to 4.2 km. By what factor does the gradient rise, and the velocity if it scales with the square root of gradient?
IntermediateEighteen hectares of wetland can hold 0.15 m of water. What volume is that, and what share of the 4,590,000 m³ flood?
IntermediateA bank retreats 0.8 m a year along a 1,200 m reach. How much farmland is lost each year, and how long to lose 2.5 hectares?
ChallengeA reservoir of 60 million m³ silt up 2.4 million m³ a year. When is it full, and when has it lost 40% of its storage?
ChallengeA scheme cuts annual flood losses from $4.5M to $0.8M and costs $74M. What is the simple payback period?
ChallengeA 2-year flood peaks at 120 m³/s and a 50-year flood at 620 m³/s. What is the ratio, and what does it imply?
ChallengeA factory discharges 0.35 m³/s at 12 mg/L BOD into a river flowing at 1.4 m³/s with 2 mg/L. What is the mixed BOD?
Error redoError retry: a student argues that upstream tree planting cutting peak flow by 25% makes a levee unnecessary. On a 212.5 m³/s peak, what is left, and why may it still flood?

03　Answers and one-line notes

#	Answer	One-line note
1	$11,760,000 (4,200 × 2,800).	Work in metres: 4.2 km is 4,200 m, and using kilometres understates the cost a thousandfold.
2	About 60.6 minutes, on a net inflow of 33 m³/s.	Use the net rate, 45 minus 12; using the gross inflow understates how long the basin takes to fill.
3	Gradient 1.62 times; velocity about 1.27 times.	Velocity is not proportional to gradient; the square-root relation makes the velocity gain smaller than the gradient gain.
4	27,000 m³, about 0.59% of the flood.	Wetland storage is small against total flood volume; its value is in slowing the flow, not containing all of it.
5	960 m² or 0.096 hectares a year; about 26 years for 2.5 hectares.	Divide square metres by 10,000 for hectares; treating 960 as hectares is out by a factor of ten thousand.
6	Full in 25 years; 40% of storage is lost after 15 years (36 million m³).	Track both the full-capacity and the effective-storage thresholds; the second arrives far earlier.
7	20 years (74 / 3.7).	This counts direct losses only; avoided casualties and business interruption usually make the real return better.
8	About 5.17 times; discharge is not linear in return period, so rare floods are far more destructive than intuition suggests.	Designing on an average flow badly understates the load; design must use the discharge for a stated return period.
9	4.0 mg/L ((12×0.35 + 2×1.4) / 1.75).	Use a flow-weighted mean; simply averaging the two concentrations overstates river pollution.
10	159.4 m³/s; if the channel safe capacity is lower than that it still floods, so soft measures must be combined with hard ones.	Soft measures cut the peak modestly but cheaply and with ecological gains; hard engineering holds water back at high cost. It is not either-or.

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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