3.3.5 Hydrographs and Flooding

Hydrographs

Lag Time

Factors Influencing Lag Time

• Size of drainage basin

• Vegetation

• Valley side steepness

• Soil type

• Short or Long Lag Time:

  • Influenced by factors such as vegetation.
  • No Vegetation: Water runs off into the river quicker, leading to a short lag time.
  • Plenty of Vegetation: Plants intercept rainfall, leading to a longer lag time. A short lag time means water reaches the river quickly, increasing flood risk.

Human & Physical Factors Affecting Flood Risk

Factors Affecting Flood Risk

• Rainfall: After a long period of rain, the soil becomes saturated. Any further rain can't infiltrate, which increases runoff into rivers. This increases discharge, so flooding is more likely.

• Trees: Trees intercept and rainwater, and also take up water from the ground. Cutting down trees therefore by deforestation increased the risk of floods.

• Rock Type: Clay soils and some rocks are impermeable and don't allow water through and run off is increased.

• Relief: If a river has steep sides, water will reach the river much quicker, increasing discharge and making flooding more likely.

• Land Use: Buildings are often made from impermeable materials like concrete, as well as being surrounded by roads. Impermeable surfaces increase runoff and drains quickly take water to rivers.