A student investigating soil science set up a drainage experiment using five different soil types. For each sample, 100 ml of water was poured into a funnel...

(a) The soil type with the fastest drainage rate was coarse sand, which drained 100 ml of water in approximately 15 seconds. [1]

(b) Relationship between soil particle size and drainage time: [3]

• As particle size decreases from coarse sand to clay, the time for water to drain through the soil increases significantly. There is an inverse relationship between particle size and drainage time. [1]
• Coarse sand (the largest particles) drained in only about 15 seconds, while clay (the smallest particles) took approximately 480 seconds, which is about 32 times longer. Fine sand (45 seconds) and silt (120 seconds) fall between these extremes, following the expected trend. [1]
• Loam, which is a mixture of different particle sizes, has an intermediate drainage time of approximately 90 seconds. This is faster than silt or clay because the larger sand particles in the mixture create some wider drainage channels. [1]

(c) Why clay soil retains the most water: [3]

• Clay particles are very small (less than 0.002 mm), and they pack closely together, creating very small pore spaces between them. Water is held tightly within these tiny pores by capillary action and cannot drain easily under the force of gravity. [1]
• Water molecules are attracted to the surfaces of soil particles by adhesion. Because clay particles are so small, they have an extremely large total surface area relative to their volume. This means there is far more surface for water molecules to adhere to, greatly increasing water retention. [1]
• Surface tension within the narrow pore spaces further resists the downward movement of water. The combined effect of adhesion to particle surfaces and capillary forces in tiny pores means that clay holds water against gravity far more effectively than coarser soils. [1]

(d) Why this experiment is relevant to understanding flooding: [3]

• Areas with sandy soils have high infiltration rates, meaning rainwater soaks into the ground quickly. This reduces surface runoff and lowers the risk of flooding, even during heavy rainfall. [1]
• Areas with clay soils have very low infiltration rates, as shown by the long drainage time in the experiment. During heavy rain, water cannot penetrate the soil fast enough, so it runs off the surface and collects in streams and rivers, significantly increasing flood risk. [1]
• Understanding the soil type in a region helps planners predict flood vulnerability and design appropriate drainage systems or flood defences. For example, clay-rich catchments may need larger storm drains, detention basins or permeable surfaces to manage runoff. [1]