S4 Geography Paper: River Landforms, Hydrology And Flood Management (100 Marks)

S4 Geography Paper: River Landforms, Hydrology and Flood Management (100 marks)

2026-11-11

S4 Geography – rivers and flood management

S4 Geography paper on river landforms, discharge calculations, storm hydrographs, causes of flooding, flood management and coastal processes, with answers and marking notes.

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Paper details　S4 Geography practice paper with six questions on river erosion and the long profile, discharge and runoff calculations, storm hydrographs, causes of flooding, hard and soft flood management and coastal landforms. Full answers with working and marking notes are included.

Section A: River Erosion and the Long Profile (15 marks)

A river flows from its upper course to its lower course. Erosion works through hydraulic action, abrasion, attrition and solution. The long profile shows bed height falling with distance, and average gradient is measured in m/km or as a percentage.

#	Question	Answer and marking notes
1	State the four processes of river erosion and give one feature of each that can be observed in a Hong Kong upper course.	Answer: hydraulic action (fast water lifting loose bed material), abrasion (load scraping the bed), attrition (stones rounding as they collide) and solution (acidic water dissolving rock). (Marking: 1 mark per process, 1 mark for a reasonable feature.)
2	A river section falls from 320 m above sea level to 40 m over a horizontal distance of 28 km. Find the drop, the average gradient in m/km and the gradient as a percentage.	Answer: drop is 280 m; average gradient is 280 divided by 28, that is 10.0 m/km; as a percentage it is 10.0/1000 x 100, which is 1.0 per cent. (Marking: 1 mark drop, 2 marks m/km, 2 marks percentage.)
3	Explain how a meander develops into an oxbow lake, and state which bank of a meander is dominated by erosion.	Answer: on a bend, centrifugal force drives fast flow against the outer bank, which is eroded, while slower flow on the inner bank deposits sediment, so the neck narrows. A flood cuts the neck, leaving a straight channel and an abandoned loop whose ends silt up as an oxbow lake. Erosion is concentrated on the outer bank. (Marking: 2 marks erosion and deposition pattern, 1 mark neck narrowing, 2 marks cutoff and silting.)

Section B: Cross-section, Discharge and Runoff (16 marks)

Discharge equals channel cross-sectional area multiplied by mean velocity, measured in cubic metres per second. Runoff volume equals catchment area multiplied by rainfall, then by the runoff coefficient.

#	Question	Answer and marking notes
4	A channel is 18 m wide with a mean depth of 2.5 m and a mean velocity of 1.2 m per second. Find the cross-sectional area and the discharge.	Answer: area is 18 x 2.5, that is 45 square metres; discharge is 45 x 1.2, that is 54.0 cubic metres per second. (Marking: 2 marks area, 2 marks discharge.)
5	If the mean velocity at the same cross-section falls to 0.90 m per second, what is the new discharge? Find the percentage decrease.	Answer: new discharge is 45 x 0.90, that is 40.5 cubic metres per second; the decrease is 25 per cent. (Marking: 2 marks new discharge, 2 marks percentage.)
6	A catchment of 240 square kilometres receives 60 mm of rain. Find the total volume of rainfall in million cubic metres, and the runoff volume when the runoff coefficient is 0.35.	Answer: rainfall is 0.06 m; total volume is 14.4 million cubic metres; runoff volume is 5.04 million cubic metres. (Marking: 1 mark unit conversion, 2 marks total volume, 1 mark runoff.)
7	A river section is 12 km long with a drop of 48 m. Find the average gradient in m/km, as a percentage, and as a ratio of 1 to a number.	Answer: gradient is 4 m/km, that is 0.4 per cent, or 1 in 250. (Marking: 2 marks m/km, 1 mark percentage, 1 mark ratio.)

Section C: The Storm Hydrograph (17 marks)

A storm hydrograph shows how discharge changes after a rainfall event. Base flow is the flow before the storm, peak discharge is the highest flow, and lag time is the delay between peak rainfall and peak discharge.

#	Question	Answer and marking notes
8	At a station the base flow is 12 cubic metres per second and the peak discharge after a storm is 85 cubic metres per second. Find the flow above base flow and the ratio of peak to base flow, correct to two decimal places.	Answer: flow above base flow is 73 cubic metres per second, and peak is 7.08 times the base flow. (Marking: 2 marks difference, 3 marks ratio.)
9	Peak rainfall occurs at 2 pm and peak discharge at 8 pm. Find the lag time and explain how a short lag time affects flood risk downstream.	Answer: lag time is 6 hours. A short lag means water reaches the channel quickly, giving a high and steep peak, less warning time downstream and a higher flood risk. (Marking: 2 marks lag time, 2 marks effect.)
10	The catchment is 240 square kilometres. Find the peak discharge per unit area in cubic metres per second per square kilometre, correct to three decimal places.	Answer: peak discharge per unit area is 85 divided by 240, that is 0.354 cubic metres per second per square kilometre. (Marking: 2 marks formula, 2 marks value and unit.)
11	Compare the shape of the hydrograph for a round, steep catchment with that of a long, gentle catchment, and explain the difference.	Answer: the round, steep catchment gives a steep rising limb, a high peak and a short lag because water travels a short distance down a steep slope; the long, gentle catchment gives a gentle rise, a lower peak and a longer lag because the flow path is long and infiltration and evaporation have more time to act. (Marking: 1 mark per shape, 1 mark per explanation.)

Section D: Causes of Flooding (17 marks)

A flood occurs when river discharge exceeds channel capacity and water spills onto the floodplain. Natural causes include rainfall intensity, relief, vegetation and soil saturation; human causes include urbanisation, channel modification and overloaded drainage.

#	Question	Answer and marking notes
12	Before urbanisation the runoff coefficient of a catchment is 0.35 and afterwards 0.70. For 60 mm of rain, find the runoff volume in both cases in million cubic metres and the percentage increase.	Answer: before urbanisation the runoff is 5.04 million cubic metres, afterwards 10.08 million cubic metres, an increase of 100 per cent, that is 5.04 million cubic metres more. (Marking: 1 mark per volume, 2 marks percentage, 1 mark difference.)
13	Explain how four natural factors increase flood risk and state which is most common in Hong Kong summers.	Answer: intense rainfall delivers more water than infiltration can absorb; steep relief speeds overland flow and concentrates the peak; sparse vegetation reduces interception and infiltration; saturated or impermeable soil turns later rain into surface runoff. In Hong Kong summers the dominant factor is intense rainfall from typhoons and severe convective storms. (Marking: 1 mark per factor, 2 marks for the local judgement.)
14	Explain how three human factors linked to urbanisation worsen flooding, and why channelisation does not necessarily solve the problem.	Answer: impermeable surfaces raise the runoff coefficient and shorten lag time; infilling or narrowing of channels reduces capacity; drainage designed for a short return period overflows in an extreme storm. Channelisation only speeds water downstream, so if downstream capacity is unchanged the flood is transferred rather than removed. (Marking: 1 mark per factor, 3 marks for the channelisation discussion.)

Section E: Flood Management (20 marks)

Flood management includes hard engineering such as reservoirs, storage tanks, channel widening and embankments, and soft engineering such as warning systems, land-use planning, wetland restoration and planting. Evaluation must weigh cost, benefit and side effects.

#	Question	Answer and marking notes
15	A flood-control reservoir has a usable capacity of 2.4 million cubic metres and costs 4800 ten-thousand dollars. Find the cost per cubic metre of storage, correct to one decimal place.	Answer: the cost is 48000000 dollars, so each cubic metre of storage costs 20.0 dollars. (Marking: 2 marks unit conversion, 3 marks division and answer.)
16	A new development covers 1.5 square kilometres. The design storm is 80 mm with a runoff coefficient of 0.7, and the storage tank holds 60000 cubic metres. Find the runoff volume, decide whether the tank is adequate and give the shortfall.	Answer: the runoff volume is 84000 cubic metres; the tank holds 60000 cubic metres, so it is not adequate and the shortfall is 24000 cubic metres. (Marking: 3 marks runoff volume, 1 mark judgement, 1 mark shortfall.)
17	A channel is widened from a cross-section of 45 square metres to 63 square metres, with velocity unchanged at 1.2 m per second. Find the discharge before and after, and the percentage increase in capacity.	Answer: discharge rises from 54.0 to 75.6 cubic metres per second, an increase of 40 per cent. (Marking: 2 marks per discharge, 1 mark percentage.)
18	Compare the effectiveness of hard and soft engineering for flood control, and propose a combined scheme for a densely built old district, with reasons.	Answer: hard engineering acts quickly and copes with rarer storms but is costly, damages ecology and riverside amenity and may shift the peak downstream; soft engineering is cheaper and environmentally compatible but works slowly and mainly for moderate storms. For an old district, combine upstream storage, drainage upgrading and a storm warning and evacuation system: channels cannot be much widened, storage cuts the peak and warning reduces loss of life and property at lower cost than full reconstruction. (Marking: 2 marks hard engineering, 1 mark soft engineering, 1 mark scheme, 1 mark reasons.)

Section F: Coastal Processes and Landforms (15 marks)

Waves, tides and longshore currents are the main coastal processes. Wave speed equals wavelength divided by period, and the load moved by longshore drift can be expressed per unit length of coastline.

#	Question	Answer and marking notes
19	A wave has a wavelength of 60 m and a period of 8 s. Find its speed in metres per second.	Answer: speed is wavelength divided by period, that is 7.5 m per second. (Marking: 2 marks formula, 2 marks answer and unit.)
20	Longshore drift moves 20000 cubic metres of sand per year past a coastline 800 m long. Find the volume moved per metre of coast per year.	Answer: the volume per metre is 20000 divided by 800, that is 25 cubic metres per metre per year. (Marking: 2 marks formula, 2 marks answer.)
21	Put sea cliff, wave-cut platform, sea cave and sea arch in order of development from weakest to strongest erosion, and give one cause for each.	Answer: cliff, cave, arch, then platform. A cliff forms as waves attack and undercut the base; a cave is a joint enlarged by hydraulic action and abrasion; an arch forms when caves on two sides meet through a headland; a platform is the flat rock left as the cliff retreats. (Marking: 1 mark order, 0.5 mark per cause, 0.5 mark expression.)
22	Explain the formation of a spit and a tombolo, and state the difference between them.	Answer: longshore drift carries sand along the coast; where the coastline turns sharply or meets a river mouth the flow slows and sand accumulates below water, building above the surface and growing seawards as a spit. If it grows far enough to join an island to the mainland it is a tombolo. A spit is attached at one end only, while a tombolo links land to an island. (Marking: 2 marks spit, 1 mark tombolo, 1 mark difference.)

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