CGP Urban Drainage

• Infiltration


  The process by which water on the surface enters the soil
• Surface runoff


  The flow of water over the land surface into rivers and streams
• Urban areas

  • Covered in impermeable materials like concrete and tarmac
  • Many urban structures are designed to shed water quickly, e.g. camber on roads funnels water to drains
• Infiltration is low in urban areas


  Replenishment of groundwater stores is slow
• Groundwater feeds rivers


  During drier periods, river discharge in urban areas is low
• Precipitation in urban areas

  • Higher than in rural areas
  • Storms are more intense
• Higher precipitation and more intense storms in urban areas

  Increases surface runoff
• Runoff in urban areas

  • Channelled through man-made pipes
  • Transported to rivers and streams
• Low infiltration, high surface runoff, and channelling of water in urban areas


  Water enters rivers quickly, giving a short lag time and high peak discharge
• Catchment management


  A way of managing rivers and improving drainage systems by looking at the whole river catchment and the interactions between water and land
• Aims of catchment management

  • Minimise issues such as flooding, drought, water pollution and erosion in sensitive areas
  • Improve river ecosystems
• Methods of catchment management

  • Hard engineering (e.g. dams, floodwalls, reservoirs)
  • Soft engineering (e.g. land use management, river restoration and conservation)
• Decisions about catchment management methods are made by experts in water management rather than residents
• Catchment management schemes can be disruptive, e.g. the Three Gorges Dam in China flooded 13 cities and 140 towns, forcing over 1.2 million people to move
• Sustainable Urban Drainage Systems (SUDS)

  Aim to imitate natural drainage systems, rather than channelling water through pipes and drains
• Methods used in SUDS

  • Vegetated trenches (swales) to increase interception and infiltration
  • Retention basins to store water
  • Vegetated roofs to intercept rainfall and increase evapotranspiration
  • Rainwater harvesting containers on buildings
• SUDS

  More sustainable than traditional urban drainage methods as they work with the natural environment, improve drainage and water quality, and provide new habitats
• River restoration and conservation aims to restore river systems to a more natural state
• Aims of river restoration and conservation

  • Reduce flood risk
  • Decrease water pollution
  • Create new habitats
  • Increase biodiversity
• Enfield, north London, is restoring its rivers to address issues such as regular flooding, increased surface runoff, and water pollution
• Sources of water pollution in Enfield

  • Surface runoff from roads
  • Domestic sources like toilets, showers, and dishwashers
• Over 5000 homes in Enfield are at risk of flooding
• SUDS used in Enfield's river restoration project

  • Vegetated trenches around roads
  • Rain planters in schools
  • Constructed wetlands around Salmons Brook
• Defra and Thames Water are providing funding for Enfield's river restoration project
• Local residents, volunteers, and schools have been involved in Enfield's river restoration project
• Enfield's river restoration project has had some successes, such as reduced flood risk, improved water quality, and increased populations of eels and some insect species
• Pollution and litter are still issues in many of Enfield's rivers