Precipitation Patterns: Intense, short-duration rainfall often leads to flash floods because the water volume exceeds the soil's infiltration capacity. Conversely, prolonged rainfall saturates the ground, meaning any additional water must flow as surface runoff into the river.
Geology and Soil: Impermeable rocks (such as granite) and clay-rich soils prevent water from soaking into the ground. This forces water to move quickly as overland flow, reaching the river channel much faster than it would through groundwater flow.
Relief and Basin Shape: Steep slopes accelerate the movement of water toward the valley floor via gravity. Circular drainage basins also tend to have higher flood risks because all points in the basin are roughly equidistant from the channel, causing water to arrive at the same time.
Urbanization: The construction of towns and cities replaces permeable vegetation with impermeable surfaces like concrete and tarmac. This eliminates infiltration and uses artificial drainage systems (gutters and pipes) to transport water rapidly into river channels.
Deforestation: Trees play a vital role in the hydrological cycle through interception (catching rain on leaves) and evapotranspiration. Removing forests reduces these processes and eliminates the root systems that help water infiltrate the soil, leading to higher runoff volumes.
Agriculture and Land Management: Intensive farming practices, such as ploughing up and down slopes or overgrazing, can compact the soil. Compacted soil acts like an impermeable surface, increasing the speed and volume of water entering the river system.
| Feature | Physical Causes | Human Causes |
|---|---|---|
| Origin | Natural meteorological or geological events | Anthropogenic land-use changes |
| Examples | Heavy rain, snowmelt, impermeable rock | Urbanization, deforestation, farming |
| Control | Largely unpredictable and uncontrollable | Can be mitigated through planning and management |
| Impact on Lag Time | Steep relief and saturated soil shorten lag time | Urban drains and lack of trees shorten lag time |
Analyze the Hydrograph: When presented with a graph, always look for the 'peak' and calculate the lag time. A 'flashy' hydrograph (steep rising limb, short lag time) usually indicates impermeable surfaces or steep relief.
Categorize Impacts: In long-answer questions, always structure your response by Social, Economic, and Environmental impacts to ensure comprehensive coverage and higher marks.
Check Scale: Distinguish between localized causes (like a specific housing development) and basin-wide causes (like regional climate patterns) to provide a nuanced argument.
Common Error: Do not assume all flooding is 'bad'. Mentioning the deposition of fertile alluvium (silt) shows a higher level of geographical understanding regarding environmental benefits.
