flooding

• When does flooding occur?

  When a river's discharge exceeds its channel's volume, causing the river to overflow.
• What are the 3 physical factors that affect flood risk?

  Precipitation, geology and relief.
• How does precipitation affect flood risk?

  Prolonged, intense rainfall can saturate soil. Excess rainwater then flows to rivers as surface run-off and increases river discharge. Heavy rainfall and sudden snow can also cause high levels of surface run-off if the infiltration rate is too slow.
• How does geology affect flood risk?

  Water cannot pass through impermeable rock, such as clay, granite and slate. Instead, water runs over the rock quickly and into the river, increasing flood risk.
• How does relief affect flood risk?

  Areas with steep slopes have high levels of surface run-off because precipitation cannot infiltrate the soil. Low lying, flat floodplains are also highly susceptible to flooding.
• What human factor affects flood risk?

  Land use.
• How does land use affect flood risk?

  Impermeable materials and drainage systems used in urban areas increase surface run-off resulting in a greater flood risk. Deforestation reduces the amount of vegetation able to intercept rainwater.
• What is river discharge and what is it measured in?

  River discharge is the quantity of water that passes a given point in a river within a given time. It is measured in cumecs (m3/s), or cubic metres per second.
• What are hydrographs used to show?

  Hydrographs can be used to show how a river's discharge changes in response to precipitation.
• What are the different stages shown on a hydrograph?

  Base flow, lag time, peak rainfall, rising limb, peak discharge, falling limb and bankfull discharge.
• What is peak discharge?

  The highest level of discharge after rainfall.
• What is peak rainfall?

  The greatest rainfall during the period.
• What is rising limb?

  An increase in river discharge.
• What is falling limb?

  A decrease in river discharge.
• What is lag time?

  The time difference between peak rainfall and peak discharge.
• Why is lag time beneficial to people?

  It gives people time to prepare for floods.
• What do high levels of surface run-off lead to?

  High levels of surface run-off shorten the lag time and increase peak discharge, resulting in a high flood risk.
• What does high infiltration lead to?

  High infiltration rates increase the lag time and reduce peak discharge, lowering flood risk.