9.5 Urban Drainage

• Built up areas need to be drained to remove surface water run off

  This has traditionally been achieved using pipe systems. Water quality issues have become increasingly importan; pollutants from urban areas are being washed into rivers or into the ground and polluted.
• Rural - natural ground cover, more vegetation so more evapotranspiration. 40% evapotranspiration, 25% shallow infiltration and 25% deep infiltration.
• Suburban - 35-50% impermeable surfaces

  30% runoff, 35% evapotranspiration, 20% shallow infiltration and 15% deep infiltration
• Central urban - 75-100% impermeable surfaces, less trees less evapotranspiration

  55% run off, 30% evapotranspiration, 10% shallow infiltration and 5% deep infiltration
• Hard engineering

  • river straightening
  • embankments
  • diversion spillways
  • channelisation
• Soft engineering

  • afforestation
  • riverbank conservation
  • floodplain zoning
  • river restoration
• SUD's - designed to reduce the potential impact on new and existing developments with respect to surface water drainage discharges
• Advantages of SUD's:

  • improve water quality
  • increase green space
  • evapotranspiration reduces urban heat island effect
  • helps improve biodiversity
  • prevents water pollution
  • recharges groundwater to help prevent drought
• Disadvantages of SUD's

  • cities are short on space for ponds and wetlands
  • infiltration can spread pollutants when near industries
  • cost
  • maintenance
• SUD's examples:

  Swales/Retention basins - allow shallow, broad vegetated channels designed to store run off and remove water

  Permeable pavements - focus on diverting water into the ground, increasing infiltration. Slows down runoff and traps any pollutants into the soil

  Wetlands - shallow ponds covered in aquatic vegetation. Slows flow for an extended period to allow sediment to settle and to remove contaminants

  green roofs, water butts
• The Cheonggyecheon River Project, Seoul South Korea

  • decking posed safety risks
  • elevated freeway and concrete deck were dismantled
  • 22 bridges were built to improve movement north to south across the sea
  • car use discourage, raid bus lanes added
  • underground waterways were redirected to create a new stream bed with landscaped banks and foundation and stones of earlier bridges were included as decorative elements
  • recreational areas, waterfront decking and stepping stones
  • stream widens which is designed to look overgrown and untamed
• The Cheonggyecheon River Project, Seoul South Korea

  • has become popular with residents and visitors
  • traffic speeds slowed by 12%, more pedestrian activity
  • 2.5 degrees reduction in temperatures
  • reestablished lost habitats
  • not inclusive design
• Lamb drove, Cambourne

  Background:

  • development of 35 affordable homes
  • relatively low lying county where flooding in river valleys is a major concern
  • European funded project

  Aim:

  • showcase practical ad innovative sustainable water management techniques with new residential developments
  • demonstrates SUD's are a viable and attractive alternative
• Lamb drove, Cambourne

  Plan:

  • green roofs, filter strip, swale, retention pond, permeable paving and water butts
  • Swales collect all excess water from the site, further slowing the flow

  Outcome:

  project proven successful

  cost effective, both construction and ongoing maintenance costs have been 10% less than conventional pipe drainage systems

  substantial improvements in the biodiversity, ecology and subsequent quality of life

  resulted in the improvement of water quality
• River hydrographs - shows how the amount of discharge in a river and how it changes after a rainfall event
• Enfield

  • north London
  • regular floods
  • heavily urbanised
  • caused problem for drainage and water quality
• Enfield - project

  • 2012
  • reduce flood risk and reduce impact of flooding habitats
  • decrease surface run off from roads
  • provide more habitats for species
• Enfield - SUD's

  • vegetation and shrubs around road to absorb surface run off
  • wetlands
  • rain planters - in schools to collect rainwater
• Enfield - urbanised

  • increased surface run off and channel flow increased, increased intensity and duration of floods
  • over 9000 homes at risk
  • surface run off from roads have increased pollutants
• Enfield - outcome

  • flood risk seems reduced
  • sud's reduce ammonia levels
  • increased species population
  • pollution and litter in rivers still a problem