Form 4 Geography Practice Paper: River Processes, Landforms And Flood Management (12 Questions, 100 Marks, 75 Minutes)

Form 4 Geography Practice Paper: River Processes, Landforms and Flood Management (12 questions, 100 marks, 75 minutes)

2026-11-06

Form 4 Geography – 12 questions – 100 marks – 75 minutes

Form 4 Geography: 12 questions, 100 marks, 75 minutes on river processes, discharge and gradient calculations, landforms, hydrographs and flood management, with full answers.

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Paper details　Form 4 Geography practice paper: river processes, landforms, discharge and gradient calculations, storm hydrographs and flood management, 12 questions, 100 marks, 75 minutes, with full answers and marking points.

Section A: The water cycle, river processes and channel calculations (Q1-Q4)

Show the formula, substitute the given data and state units (m, m2, m3 s-1, percentages).

#	Question	Answer and marking notes
1	The water cycle moves water continuously between stores. (a) Name three processes that return water from the surface or from vegetation to the atmosphere, and describe each in one sentence. (3 marks) (b) Explain how urbanisation (widespread concrete paving and storm drains) changes infiltration and surface runoff during the same storm. (3 marks) (c) Distinguish between a drainage basin and a watershed. (2 marks)	(a) Evaporation (water surfaces such as rivers and lakes turn to vapour), transpiration (water vapour leaves plants through leaf stomata), sublimation (ice and snow turn directly into vapour). (b) Concrete is impermeable so infiltration falls sharply; rain is quickly carried away by storm drains, so surface runoff and runoff velocity increase, giving a higher peak discharge and a shorter lag time. (c) A drainage basin is an area surrounded by watersheds from which all precipitation drains through one outlet; a watershed is the high ground dividing two adjacent basins, so rain on either side flows into different systems.
2	Rivers erode and transport material in different ways. (a) Name the four processes of river erosion and give one example of each. (4 marks) (b) Explain why sediment in the lower course is generally rounder and finer than in the upper course. (2 marks) (c) Distinguish between suspended load and bed load. (2 marks)	(a) Hydraulic action (flow prises loose material from the bed), abrasion (load scrapes the bed and banks), solution (water dissolves soluble rock such as limestone), attrition (boulders collide and are worn round). (b) Load is worn smaller and rounder by collisions over a long distance, and because velocity falls downstream, coarse material has already been deposited upstream, so only fine particles travel far. (c) Suspended load is fine material (silt, clay) carried within the water; bed load is coarser material (gravel, pebbles) rolled, slid or bounced along the bed.
3	Measurements at a straight reach of a river are: channel width 12.0 m, mean depth 2.5 m, mean velocity 0.8 m s-1. The water surface here is 100 m above sea level, and 6.0 km upstream it is 220 m above sea level. (a) Calculate the cross-sectional area of the reach. (2 marks) (b) Calculate the discharge and state its unit. (3 marks) (c) Calculate the gradient of the reach and express it in the form 1:n. (2 marks) (d) In the flood season the mean velocity rises to 1.2 m s-1 while width and depth stay the same; calculate the new discharge and explain the effect of the higher discharge on bed erosion. (2 marks)	(a) Cross-sectional area = 12.0 x 2.5 = 30.0 m2. (b) Discharge = area x velocity = 30.0 x 0.8 = 24.0 m3 s-1. (c) Drop = 220 – 100 = 120 m; gradient = 120 / 6,000 = 0.02, i.e. 1:50. (d) New discharge = 30.0 x 1.2 = 36.0 m3 s-1; the higher velocity raises kinetic energy and shear stress, so hydraulic action and abrasion on the bed and banks increase and the channel may deepen and widen.
4	River landforms result from erosion and deposition. (a) Describe how a waterfall forms where hard and soft rock alternate. (3 marks) (b) Compare flow velocity on the outer and inner bends of a meander and describe the landform produced at each. (4 marks) (c) Explain how a flood plain is formed. (2 marks)	(a) The river erodes the soft rock faster, cutting a recess, while the hard rock above resists erosion and forms an overhanging lip; the soft rock beneath is undercut and the hard cap collapses, so the fall retreats upstream leaving a gorge. (b) On the outer bend flow is faster and lateral erosion cuts a steep river cliff; on the inner bend flow is slower, so competence falls and a gently sloping point bar of deposition builds up. (c) When the river overtops its banks, velocity falls abruptly and fine sand and silt are deposited on both sides; repeated flooding thickens the layer into a flat, fertile flood plain.

Section B: Sediment load, storm hydrographs and flood management (Q5-Q8)

Explanations must state cause and effect using correct terms (erosion, transport, infiltration, runoff, lag time).

#	Question	Answer and marking notes
5	A river reach carries 1,500 tonnes of sediment a year, of which 1,200 tonnes is suspended load and the rest is bed load. (a) Calculate the mass of the bed load. (1 mark) (b) Calculate the bed load as a percentage of the total sediment load. (2 marks) (c) Explain why the sediment load downstream of a reservoir falls sharply, and state two resulting changes to the channel or landforms. (5 marks)	(a) Bed load = 1,500 – 1,200 = 300 tonnes. (b) 300 / 1,500 x 100% = 20%. (c) The reservoir traps sediment on its bed and releases sediment-poor water, which then has extra erosive power (clear-water scour); the channel may incise downwards and the delta or estuary bars retreat because sediment supply is cut off.
6	Records for the same catchment under two identical storms: before urbanisation the peak discharge was 40 m3 s-1, occurring 6.0 hours after the rain began; after urbanisation the peak discharge was 95 m3 s-1, occurring 1.5 hours after the rain began. (a) Calculate the percentage increase in peak discharge after urbanisation. (2 marks) (b) Calculate how much the lag time has been shortened. (2 marks) (c) Explain why more impermeable surface both raises the peak and shortens the lag time. (3 marks) (d) Apart from impermeable surfaces, state one other urban factor that makes the rising limb of the hydrograph steeper. (2 marks)	(a) (95 – 40) / 40 x 100% = 137.5%. (b) 6.0 – 1.5 = 4.5 hours. (c) Impermeable surfaces stop infiltration so almost all rain becomes surface runoff in a short time, raising the peak; drains carry water quickly and shorten travel time, so the peak arrives earlier and the lag time is shorter. (d) Dense, smooth drainage networks, or straightening and channelling of the natural river, speed up the concentration of flow.
7	Flood management may be hard or soft. (a) Define hard engineering and soft engineering flood measures and give one example of each. (4 marks) (b) Compare embankments with restoring flood plain wetlands in terms of flood control and ecological impact. (3 marks) (c) Explain why continually raising embankments may actually increase flood risk downstream. (2 marks)	(a) Hard engineering uses structures to hold back, block or divert water, e.g. embankments, concrete bank protection, flood storage tanks; soft engineering works with natural hydrology and land use, e.g. restoring flood plain wetlands, planting trees, zoning of flood plain land use. (b) Embankments raise channel capacity immediately and protect a defined area, but they remove flood storage and damage bank habitats; wetlands store water temporarily, lower the peak and provide habitat, but need more land and take longer to take effect. (c) Embankments confine the flood, making the downstream peak higher and faster, and a breach or overtopping is far more destructive; silt trapped inside also raises the bed, reducing real capacity, so defences must keep being raised.
8	A catchment of 25 km2 receives 180 mm of rain in one storm and has a runoff coefficient of 0.60. (a) Calculate the total volume of rainfall in m3. (3 marks) (b) Calculate the volume of surface runoff produced. (2 marks) (c) If further urbanisation raises the runoff coefficient to 0.85, explain the effect on downstream peak discharge and on the timing of the peak. (3 marks)	(a) 180 mm = 0.180 m; volume = 25,000,000 m2 x 0.180 m = 4,500,000 m3. (b) Runoff = 4,500,000 x 0.60 = 2,700,000 m3. (c) A larger share of rain becomes runoff, so peak discharge rises markedly; water is delivered faster, so the peak arrives earlier and the lag time, and hence warning time, is reduced.

Section C: Comparing river courses, pollution and case evaluation (Q9-Q12)

Answer comparison questions point by point; a table is acceptable.

#	Question	Answer and marking notes
9	A river’s energy conditions change from source to mouth. (a) Compare the upper, middle and lower courses in terms of valley shape, channel gradient and dominant process. (6 marks) (b) Define base level and state the effect of a rise in sea level on the lower course. (2 marks)	(a) Upper: V-shaped valley, steep gradient, dominant vertical erosion; middle: wider valley, moderate gradient, lateral erosion and transport with meanders; lower: broad flat valley or plain, very gentle gradient, deposition dominant, forming flood plains and deltas. (b) Base level is the lowest level to which a river can erode, usually the surface of the water body it enters; a rise in sea level raises base level, so velocity falls further downstream, deposition extends upstream and the risk of silting and flooding increases.
10	River water quality is affected by human activity in the catchment. (a) State three common sources of river pollution. (3 marks) (b) Explain how fertiliser use on farmland causes eutrophication in a river, giving one link in the chain of events. (3 marks) (c) Explain how planting trees along river banks reduces silting of the channel. (2 marks)	(a) Untreated domestic and industrial sewage, farm animal waste, fertiliser and pesticide washed off fields, solid waste and oil from vessels. (b) Nitrates and phosphates wash into the river and cause an algal bloom; when the algae die, bacteria decompose them and use up dissolved oxygen, so fish die and water quality deteriorates. (c) The canopy intercepts rain and slows flow, and roots bind the soil, reducing slope erosion and the supply of sediment to the channel.
11	Some urban channels in Hong Kong have been improved and revitalised. (a) State two common environmental problems of urban nullahs. (2 marks) (b) State two benefits and one limitation of covering a nullah or revitalising it with vegetation. (3 marks) (c) Explain why urban areas in Hong Kong are prone to flash floods in the rainy season. (3 marks)	(a) Poor water quality with smell and accumulated rubbish; a uniform habitat of low ecological value, and fast flow in concrete channels that can worsen flooding downstream. (b) Benefits: cleaner water and less smell, more urban greenery and open space, higher biodiversity; limitation: high cost and the need to keep full flood discharge capacity while working in the dry season. (c) Most urban ground is impermeable, so the runoff coefficient is high; the drainage network delivers water quickly and the peak is concentrated; intense storms plus ageing drains of limited capacity cause rapid ponding.
12	A government plans a reservoir in the upper course of a river for both water supply and flood control. (a) Give two arguments for and two against building the reservoir. (4 marks) (b) Explain how the reservoir changes the sediment budget of the lower course. (2 marks) (c) Suggest one management measure to reduce reservoir silting and explain its principle. (2 marks)	(a) For: a reliable water supply and irrigation, flood storage that cuts the peak, and possible hydropower and recreation. Against: loss of farmland and villages, resettlement, blocking of fish migration, altered downstream hydrology and sediment load, and silting that shortens the reservoir’s life. (b) The reservoir traps sediment so the downstream load falls sharply; deposition downstream weakens, the bed incises and the delta retreats, affecting estuary ecology and navigation. (c) Plant trees and build terraces or check dams in the upper catchment to cut sediment inflow, and flush sediment periodically by sluicing to keep the storage volume.

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