中文（繁體）
中文（简体）
English
S5 Geography | River Landforms and Management Paper 4 (Error Retry)
S5 Geography | River Landforms and Management Paper 4 (Error Retry)
Paper 4 targets the unit and ratio traps that cost most marks: gradient as a percentage, velocity units, daily sediment load, runoff coefficient, urbanisation factors, return-period chance, sediment trapping, levee cost and basin fill time.
Find the step that went wrong Allow about thirty minutes; write your own working first, then compare with the correct version. The faulty step is usually a unit or a denominator.
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 | River Landforms and Management |
| Level | S5 (Secondary 5) Geography |
02 Questions (10 items)
- FoundationA river runs 180 km with a fall of 900 m. How much does it drop per kilometre, and what is that as a percentage? Error: writing the gradient as 5%.
- FoundationA channel of cross-sectional area 25 m² carries water at a mean velocity of 2.4 m/s. What is the discharge? Error: reading 2.4 m/s as 2.4 m per minute.
- FoundationWith the same discharge and a suspended load of 0.6 kg/m³, what mass passes each second, and roughly how many tonnes a day? Error: giving a daily figure without multiplying by 86,400 s.
- FoundationA catchment of 120 km² receives 1,800 mm of rain a year with a runoff coefficient of 0.40. What is the annual runoff? Error: computing 120 x 1.8 x 0.4 = 86.4.
- IntermediateUrbanisation raises the runoff coefficient from 0.25 to 0.55 for the same storm, and lag time falls from 9 to 4 hours. By what factor does runoff volume rise, and how much earlier is the peak? Error: saying the volume is unchanged and only arrives sooner.
- IntermediateWhat is the chance that a 1-in-25-year flood happens at least once in the next 20 years? Error: answering 20 / 25, that is 80%.
- IntermediateA reservoir traps 70% of the 1.8 million tonnes of sediment arriving each year. How much passes downstream, and what follows? Error: answering that 70% gets through.
- ChallengeA 3.6 km levee costs $3,200 per metre. What is the total cost? Error: computing 3.6 x 3,200 = $11,520.
- ChallengeA detention basin holds 90,000 m³, with inflow of 40 m³/s and outflow of 10 m³/s. How long does it take to fill from empty? Error: using the gross inflow and getting 37.5 minutes.
- Error redoError retry: a student writes a fall of 5 m per kilometre as 5%, then computes a drop over 30 km as 1,500 m. What is the correct fall, and where is the error?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | 5 m per km, that is 0.5%. Divide 900 by 180 for metres per kilometre, and by 180,000 for the decimal fraction. | A 5% gradient would mean 50 m of fall per kilometre, ten times too much: metres per kilometre and per cent are different units. |
| 2 | 60 m³/s (25 x 2.4). The velocity unit is per second, not per minute. | If the velocity really were 2.4 m per minute the discharge would be only 1 m³/s, so a wrong unit makes the answer sixty times too small. |
| 3 | 36 kg each second; 3,110.4 tonnes a day (36 x 86,400 / 1,000). | Mass per second times the seconds in a day gives the daily total, and kilograms must be divided by 1,000 for tonnes. |
| 4 | 86,400,000 m³ (120,000,000 m² x 1.8 m x 0.4). | Square kilometres become square metres by multiplying by a million, so working in kilometres makes the answer a million times too small. |
| 5 | 2.2 times; the peak comes 5 hours earlier. | The coefficient multiplies the volume, so the factor is 0.55 / 0.25, not a difference; more water arriving sooner raises the peak as well. |
| 6 | About 55.8% (1 – 0.96^20). | A return period only gives an annual chance of 1/25, or 4%; across years the chances compound instead of dividing. |
| 7 | 0.54 million tonnes (1.8 x 0.30); the starved lower course incises and the coast erodes faster. | Trapping 70% leaves 30% passing; swapping the trapped and passed shares is the commonest slip here. |
| 8 | $11,520,000 (3,600 m x 3,200). | The rate is per metre so the length must be in metres; using kilometres understates the cost a thousandfold. |
| 9 | About 50 minutes, on a net inflow of 30 m³/s, that is 3,000 seconds. | Water leaves while it enters, so use the net rate; the gross inflow understates the fill time. |
| 10 | The correct fall is 150 m; 5% means 50 m per kilometre, so the conversion is already ten times too big. | Five metres per kilometre is 5 / 1,000 = 0.5%; a percentage compares metres with metres, not metres with kilometres. |
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.
Need a tutor for this chapter? Register hereTutor Registration →
Questions and answers are written originally by the TutorZone team and are not taken from any school, publisher or examination authority paper. Every value is checked by computer.