A-Level Geography Past Papers & Drilling (Paper 1): Rivers, Climate and Mass Movements

Question 1

(a) Cross-sectional area = 12.0 x 1.50 = 18.0 m2; discharge Q = 18.0 x 0.85 = 15.3 m3/s; wetted perimeter = 12.0 + 2 x 1.50 = 15.0 m, so the hydraulic radius R = 18.0 / 15.0 = 1.2 m. (6 marks)

(b) Load = 15.3 x 0.48 = 7.344 kg/s; over 86400 seconds this is about 634.5 tonnes per day. (4 marks)

(c) Faster flow raises kinetic energy and the shear stress exerted on the bed far more than proportionally, so particles are prised up by hydraulic action, the bed is scraped by the load in abrasion, stones wear each other down by attrition and soluble rock is dissolved – net erosion therefore increases sharply. (5 marks)

Question 2

(a) ELR = (27.4 – 22.0) / (1200 / 1000) = 4.5 C per km. This is below both the dry adiabatic lapse rate (about 10 C per km) and the saturated adiabatic rate (about 6 C per km), so the atmosphere is absolutely stable, rising air is suppressed and cloud and rainfall development is limited. (5 marks)

(b) Net absorption = 340 – 100 = 240 W/m2; albedo = 100 / 340 x 100 = about 29%. (4 marks)

(c) Increase = (420 – 280) / 280 x 100 = 50%. Carbon dioxide absorbs outgoing long-wave radiation and re-radiates part of it back to the surface, so the lower atmosphere gains net energy; a higher concentration thickens this blanket, surface temperatures rise and the knock-on effects include sea level rise, more extreme weather and shifting agricultural belts. (6 marks)

Question 3

(a) Slope angle = arctan(3.6 / 6.0) = about 31.0 degrees. (3 marks)

(b) Shear stress = 1800 x 9.81 x 1.5 x sin31 = about 13.64 kPa; normal stress = 1800 x 9.81 x 1.5 x cos31 = about 22.70 kPa.
Dry season strength = 5 + (22.70 – 6) x tan33 = about 15.85 kPa, so the factor of safety is 15.85 / 13.64 = 1.16 and the slope stands.
Wet season strength = 5 + (22.70 – 11) x tan33 = about 12.60 kPa, so the factor of safety falls to 12.60 / 13.64 = 0.92, below 1, and the slope fails. (7 marks)

(c) Infiltration raises pore water pressure, which reduces the effective normal stress (total stress minus pore pressure) and therefore the shear strength; water also adds weight and lowers cohesion, while toe erosion and the removal of vegetation are common triggers. Once the factor of safety drops below 1 the driving force exceeds the resisting force and the mass slides along the failure plane. (5 marks)

Question 4

Line of argument: hard engineering delivers clear short-term protection but can rarely be called the most effective option on its own; the sustainable answer combines hard and soft approaches.

(1) Case for: embankments and dredging raise channel capacity directly. In the data above the discharge is 15.3 m3/s, bankfull flow is 45 m3/s and a 51 m3/s peak exceeds capacity by 13.3%, so raising embankments or building storage areas cuts flood risk immediately.

(2) Case against: hard schemes transfer the flood peak downstream, prevent overbank deposition so the bed incises, damage habitats and landscape, and carry high maintenance costs with worse consequences when they fail.

(3) Soft engineering: tree planting and wetland restoration raise interception and infiltration and lower the runoff coefficient – in the example here 560 mm of runoff from 1240 mm of rainfall gives 0.45, and urbanisation pushes that coefficient up and amplifies the peak.

(4) Evaluation: the choice should follow catchment characteristics, population density and budget, with land-use planning, warning systems and insurance sharing the risk; treating hard engineering as the only answer ignores cost, ecology and downstream fairness. (15 marks)

Question 1

Return period = (30 + 1) / 1 = 31 years; annual exceedance probability = 1 / 31 x 100 = about 3.2%. A “31-year flood” does not occur once every 31 years: it is the chance in any single year of a flood of that size or larger.

Question 2

Runoff coefficient = 560 / 1240 = 0.45. Urbanisation replaces vegetation and soil with impermeable surfaces, cutting infiltration, interception and evaporation; the coefficient can rise above 0.7, producing a faster, higher flood peak and flushing pollutants into the channel.