River flooding + management

• Precipitation


  Increases the chance of flooding because rain will contribute to surface run-off and groundwater flow
• Geology


  • Impermeable rocks do not absorb rainwater meaning soils become saturated, leading to increased surface run-off and a risk of flooding
• Relief

  • Steep hillsides allow water to run into rivers quickly, increasing the risk of flooding
  • Very flat land can also increase the risk of flooding because the water does not flow
• Urbanisation


  • Urban areas replace natural surfaces like fields with largely impermeable surfaces like concrete, meaning rainwater cannot soak into the soil and runs off quickly into rivers, increasing flood risk
• Deforestation


  • Trees intercept precipitation and use water as they grow, so removing trees means more water enters rivers, increasing the risk of flooding
• Agriculture


  • When trees, bushes, and grass are removed to grow crops there is less vegetation to intercept rainwater, especially when fields have bare soil, meaning more surface run-off and increased risk of flooding
• Flood hydrograph


  1. Rainfall as a bar graph in mm

  2. Discharge (how much water is in the river) as a line graph in m³ per second or cumecs

  3. Lag time - the time between peak rainfall and peak discharge

  4. Base flow - the 'normal' discharge of the river
• Flashy hydrographs


  • Have short lag times and high peaks as rainwater reaches the river quickly
  • Caused by heavy, intense rain
  • Impermeable rocks
  • Flat slopes
• Gentle hydrographs


  • Have long lag times and low peaks because rainwater reaches the river slowly
  • Caused by steady, light rain
  • Permeable rocks
  • Gentle slopes
  • Forested areas
• Flood management scheme was needed in Banbury due to a history of flooding, including in 1958 and 2007
• Banbury flood alleviation scheme


  1. Flood storage reservoir upstream of Banbury

  2. 2.9km embankment to create a storage reservoir for floodwater

  3. Two flow control structures to regulate the flow of water

  4. Raising the A361 road through the reservoir

  5. Local defences downstream of the reservoir
• The Borrow Pit (which provided material for the embankment) was converted into a country park with new trees, hedgerows, and ponds as part of the Biodiversity Action Plan
• Social, economic, and environmental issues of the Banbury flood alleviation scheme


  • Social: New recreation space created, A362 can remain open during floods, people have more confidence their houses won't flood
  • Economic: Cost of £18.5 million, 441 houses and 73 businesses (value of £100 million) protected, key funders were Environment Agency and Cherwell District Council
  • Environmental: Flood control structures are 'passive' and operate automatically, earth embankment built from material excavated locally, Borrow area converted to a reservoir and Country Park
• Hard engineering


  Structures built to prevent flooding
• Hard engineering


  • Dams and reservoirs
  • Channel widening
  • Straightening the river channel
• Dams and reservoirs


  The lake created behind a dam is called a reservoir
• Dams and reservoirs


  • The flow of water can be controlled, preventing floods
  • Can be combined with projects like hydroelectric power
  • expensive
  • A large area of land is flooded behind the dam, destroying natural ecosystems
• Channel widening


  • The river channel is widened so water flows more quickly, moved out of the area before flooding
  • Channel widening is expensive with ongoing maintenance costs
  • Flood risk increased downstream
• Straightening the river channel

  • The river channel is straightened to divert water from flood prone areas
  • expensive
• Soft engineering


  Management strategies working with natural processes to reduce flooding
• Soft engineering


  • Flood warning and preparation
  • Flood plain zoning
  • Afforestation
  • River restoration
• Flood warning and preparation

  • Potential flooding is monitored, with warnings and flood forecasts given out
  • People can prepare with sandbags or temporary flood gates, and move valuables upstairs
  • -Flood warning and preparation does not stop flooding
  • -Flood warning and preparation has high clean-up costs
  • -People might ignore warnings
  • -Flood warning and preparation relatively cheap
• Flood plain zoning


  • Land next to the river is organised into zones, with different land use in each zone moving away from the river
  • Valuable land and economic activity is located in areas unlikely to flood
  • -Flood plain zoning is difficult to do retrospectively
  • -Flood plain zoning does not stop flooding
• Afforestation


  • Trees are planted in the drainage basin
  • Trees intercept and absorb precipitation, reducing run-off
  • -Afforestation may not reassure people like hard engineering schemes do
  • -Afforestation takes a long time to have any effect
  • +Afforestation adds to biodiversity and creates new recreational spaces
  • +Afforestation is cheap
• River restoration


  • Rivers are restored to their natural state by removing artificial embankments and recreating meanders
  • The impact of human interference is removed, which can reduce flooding in some places
  • Natural habitats are recreated
  • -Land that used to flood will flood again
  • -expensive