S5 Geography Practice Paper: Rivers And Coastal Management (12 Questions, 100 Marks, 75 Minutes, With Answers)

S5 Geography Practice Paper: Rivers and Coastal Management (12 Questions, 100 Marks, 75 Minutes, with Answers)

2026-11-09

HKDSE Geography: Rivers and Coastal Management

S5 Geography paper on rivers and coasts: catchment water balance, discharge and hydrographs, meanders, flood control and coastal erosion, with answers and marking points.

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Section A: Data and Calculation Questions — Catchment Water Balance and Hydrological Data (32 marks)

Study the information in each question and answer as instructed.

#	Question	Answer and marking notes
1	Data: a catchment has an area of 128 square kilometres and a mean annual rainfall of 2400 millimetres; the runoff coefficient is 0.45. (a) Calculate the total volume of rainwater received by the catchment in an average year, in cubic metres. (2 marks) (b) Calculate the mean annual direct runoff, in cubic metres. (4 marks) (c) State one destination of the remaining rainwater. (2 marks)	(a) Convert the units: 128 square kilometres is 128000000 square metres and 2400 millimetres is 2.4 metres, so the total volume is 128000000 multiplied by 2.4, which is 307200000 cubic metres. (2 marks) (b) Direct runoff is the rainfall volume multiplied by the runoff coefficient: 307200000 multiplied by 0.45, which is 138240000 cubic metres. (4 marks) (c) The rest leaves the catchment mainly through interception by plants, evapotranspiration, storage in depressions and deep infiltration that recharges groundwater; part is held as soil moisture and does not become surface runoff at once. (2 marks)
2	Data: a river channel has a cross-sectional area of 42 square metres and a mean velocity of 1.8 metres per second. (a) Calculate the discharge of the channel, in cubic metres per second. (3 marks) (b) Calculate the volume of water passing the section in one day, in cubic metres. (3 marks) (c) If the discharge rises to 90 cubic metres per second while the cross-sectional area stays the same, what is the mean velocity? Give one decimal place. (2 marks)	(a) Discharge is cross-sectional area multiplied by mean velocity: 42 multiplied by 1.8, which is 75.6 cubic metres per second. (3 marks) (b) A day has 86400 seconds, so the volume is 75.6 multiplied by 86400, which is 6531840 cubic metres. (3 marks) (c) Velocity is discharge divided by cross-sectional area: 90 divided by 42, which is 2.1 metres per second, so the velocity must roughly double to pass the larger flow through the same section. (2 marks)
3	Data: a catchment of 36 square kilometres received a rainstorm that began at 14:00; the peak arrived at 20:30 with a peak discharge of 120 cubic metres per second and a baseflow of 15 cubic metres per second. (a) Calculate the lag time of the storm, in hours. (4 marks) (b) Calculate the peak of direct runoff, in cubic metres per second. (4 marks)	(a) The storm began at 14:00 and the peak arrived at 20:30, a gap of 6 hours 30 minutes, so the lag time is 6.5 hours. (4 marks) (b) The peak of direct runoff is the peak discharge minus baseflow: 120 minus 15, which is 105 cubic metres per second; baseflow is the groundwater-fed flow that existed before the storm, so it must be removed to show the storm response. (4 marks)
4	Data: a sea cliff retreats at a mean rate of 0.35 metres per year, and a house at the cliff top is now 28 metres from the cliff edge. (a) Calculate how far the cliff line will retreat in 50 years. (3 marks) (b) If the rate stays the same, how many years will pass before the house is threatened? (2 marks) (c) State two human factors that speed up cliff retreat. (3 marks)	(a) In 50 years the cliff line retreats 0.35 multiplied by 50, which is 17.5 metres. (3 marks) (b) The house is 28 metres from the cliff edge, so at 0.35 metres a year it is about 28 divided by 0.35, roughly 80 years, before the cliff reaches it. (2 marks) (c) Human factors include building houses and roads on the cliff top, which adds load and destabilises the face; removing vegetation so that roots no longer bind the soil; dredging sand at the cliff foot or building groynes that change wave energy; and drainage that concentrates surface water and weakens the rock mass. (3 marks)

Section B: Structured Questions — Fluvial Processes and Management (36 marks)

Answer in points with reference to geographical concepts and the data given.

#	Question	Answer and marking notes
5	(a) State one difference between erosion in the upper and in the lower course of a river. (3 marks) (b) Explain how erosion on the outer bank and deposition on the inner bank of a meander bend take place. (6 marks)	(a) In the upper course vertical downcutting dominates, giving a narrow and deep channel with gorges and waterfalls, while in the lower course lateral erosion dominates, giving a wide and shallow channel with an open valley and floodplain. (3 marks) (b) On entering a bend the flow is thrown outwards by centrifugal force, so surface water swings to the outer bank where the level is slightly higher and the velocity faster, giving strongest erosion and producing an undercut river cliff. Bottom water flows inwards to compensate, and its slower flow reduces its competence, so sand and gravel are deposited on the inner bank as a slip-off slope. The outer bank retreats and the inner bank advances, so the bend grows more pronounced and may eventually be cut off as an oxbow lake. (6 marks)
6	Data: in a catchment the impermeable surface rose from 12 per cent to 78 per cent, the lag time fell from 9.2 hours to 3.1 hours, and the peak discharge rose from 60 to 148 cubic metres per second. (a) Calculate the reduction in lag time, in hours. (4 marks) (b) Explain how urbanisation raises the peak discharge and shortens the lag time. (5 marks)	(a) The lag time falls by 9.2 minus 3.1, which is 6.1 hours, and peak discharge rises from 60 to 148 cubic metres per second, an increase of 146.7 per cent. (4 marks) (b) With impermeable surface rising from 12 to 78 per cent, rain cannot infiltrate and runs off roads and drains at once, so infiltration and depression storage fall sharply. Natural channels are often straightened and lined with concrete, reducing roughness and raising velocity, so water reaches the gauging point sooner and the lag time shortens. Large volumes arrive in the main channel within a short period, so the peak discharge rises and the hydrograph becomes more peaked. Less interception by vegetation also reduces evapotranspiration losses and raises the runoff total. (5 marks)
7	(a) Compare hard engineering with soft engineering in flood management, giving one difference. (4 marks) (b) Explain two possible negative effects of hard engineering. (5 marks)	(a) Hard engineering uses structures to control water directly, such as dams, detention basins, channel straightening and flood walls, with an immediate and concentrated effect, while soft engineering uses planning and management to reduce damage, such as land-use zoning, floodplain management, tree planting and flood warning, with a dispersed effect that takes time to establish. (4 marks) (b) Negative effects include: dams trap sediment, so downstream beds degrade and coastal erosion worsens; reservoirs and embankments take farmland and disrupt livelihoods, and resettlement causes social conflict; concrete channels destroy aquatic habitats and reduce biodiversity; the sense of safety encourages more development on the floodplain, so losses are larger when a flood exceeds the design standard; and upstream dams can cause transboundary water disputes. (5 marks)
8	(a) State one difference between a sea wall and beach nourishment as coastal management. (3 marks) (b) Explain how managed retreat reduces losses from coastal erosion. (3 marks) (c) State one factor that must be considered when siting a coastal management scheme. (3 marks)	(a) A sea wall is hard engineering that blocks wave attack on the cliff foot with a fixed structure, while beach nourishment is soft engineering that adds sand to widen the beach so wave energy is dissipated across the foreshore and must be repeated periodically. (3 marks) (b) Managed retreat moves buildings and infrastructure out of high-risk zones and sets a development setback, so waves and erosion can act naturally; losses are avoided at the planning stage, long-term maintenance costs are lowest, and the beach and dune system is kept as a natural buffer. (3 marks) (c) The scheme must consider the direction and rate of longshore sediment movement, so that a structure does not starve the downdrift coast and worsen erosion; it must also consider storm surge levels, geology and slope, ecological value, residents’ livelihoods and land ownership, and the cost-effectiveness of construction and long-term maintenance. (3 marks)

Section C: Essay Questions — Synthesis and Evaluation (32 marks)

Answer each question with reference to examples and concepts.

#	Question	Answer and marking notes
9	‘River flooding is mainly caused by natural factors, and human factors have only a limited influence.’ Do you agree? (8 marks)	For: flooding does have powerful natural causes. Rainfall intensity and duration beyond infiltration capacity, storm surge, steep channel gradients, a large and compact catchment, and thin or saturated soils can all make a flood peak exceed channel capacity, and tropical cyclones and the monsoon concentrate rainfall so that the natural setting already carries risk. Against: human factors often enlarge, or even dominate, the severity of flooding. Urbanisation greatly expands impermeable surface and shortens the lag time, so peaks are higher and arrive sooner; infilling of the floodplain, encroachment on channels and illegal structures narrow the flow section; deforestation upstream increases soil loss and raises bed levels; and inadequate or blocked drainage directly causes urban flooding. Natural factors therefore trigger floods, but human factors set their scale and the resulting losses, so both must be weighed. (8 marks)
10	‘Hard engineering is the most effective way to manage river flooding.’ Do you agree? (8 marks)	For: hard engineering is genuinely effective in the short term and at the local scale. Dams can regulate the flood peak, detention basins can store surplus water, and channel improvement can enlarge the flow section; design standards are explicit and benefits can be calculated, which is essential in dense urban areas. Against: effective is not the same as sustainable. Hard works are costly and need continuing maintenance, and they fail once a flood exceeds their design capacity; dams trap sediment, so downstream beds degrade and coasts erode; concrete channels destroy habitats; and the safety they create encourages further development of the floodplain, transferring risk. Soft approaches such as land-use zoning, floodplain management, warning and evacuation, and source control cost less and work with natural processes, but need long-term commitment and coordination across departments. Hard works suit the protection of high-value assets and soft works suit risk management, so the two are complementary and no single method is most effective. (8 marks)
11	‘Human factors now matter more than natural factors in accelerating coastal erosion.’ Do you agree? (8 marks)	For: much of the acceleration in coastal erosion is directly linked to human activity. Groynes and sea walls at the cliff foot cut off the supply of sediment to the downdrift coast, so beaches narrow and wave energy is concentrated; dams and sand mining in estuaries reduce river sediment; removal of seagrass, coral and dune vegetation destroys natural energy barriers; and groundwater abstraction and drainage cause subsidence, which is equivalent to a rise in sea level. Against: natural factors remain fundamental. Wave and storm surge energy, sea-level rise, geological structure and rock resistance, and the natural budget of longshore drift cannot be changed by people in the short term, and in some places retreat was already under way before human activity. Human factors can therefore dominate the rate of erosion locally, but the overall trend is still constrained by natural conditions, and each site must be assessed on its own. (8 marks)
12	‘In river management, economic development and environmental protection cannot coexist.’ Do you agree? (8 marks)	For: there is real conflict. Channel improvement, embankments and reclamation add developable land and transport capacity, but destroy riverbank habitats, reduce the flood storage of the floodplain and raise risk downstream, while rising water demand for tourism and agriculture can leave too little flow for ecology. Against: conflict is not inevitable. Detention basins that double as parks, wetland buffers that carry footpaths, river restoration with eco-friendly revetments can deliver flood control, recreation and habitat at once; land-use zoning, rainwater harvesting and green infrastructure reduce the hydrological impact of development; and catchment-wide management with public participation can share costs and benefits more fairly. The outcome therefore depends on how planning and institutions are designed, not on development and protection being necessarily opposed. (8 marks)

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