Urban hydrology

• Summary:

  • Built up urban areas need to be drained well to remove surface run-off water
  • Traditionally done with underground pipe systems
• The urban drainage basin hydrological cycle:


  Stemflow > water trickling through leaves

  Throughflow > through soil

  Groundwater flow (baseflow) > through rocks

  Interception > leaves catching water on leaves and temporarily storing it

  Infiltration > vertical movement of water from surface to soil

  Percolation > vertical movement from soil to bedrock

  
• Impacts of urbanisation on the urban hydrological cycle:


  • Increase in impermeable surfaces > reduce infiltration > promote surface runoff > flash floods
  • Pipeflow ~ subterranean water flow where water travels from the surface storage to the channel storage by travelling along cracks in the soil
  • Drains and sewers have smooth surfaces > water increase in velocity and erosional power as travels underground (pipeflow) > when water enters streams they fill up quickly and suffer from erosion > positive feedback (amplifying change)
• Impacts of urbanisation of drainage:

  1. Inputs increase
  2. Outputs unbalanced
  3. Transfers increase
  4. Stores decrease

  CBD sees biggest impacts > decrease further away towards rural-urban fringe > evapotranspiration and infiltration increase away from CBD, runoff decreases by 45%
• 1. Inputs increase:

  • Precipitation is increased due to pollution and temperature > more hygroscopic condensation nuclei > increase condensation and clouds and rain
  • Extra water is artificially imported into the system due to population demand and high water stress
• 2. Outputs unbalanced:

  • Less vegetation > less evapotranspiration
  • Impermeable surfaces increase surface runoff > poor quality > pick up litter particulates and atmospheric pollutants - less in rural areas but still poor quality due to farmland and fertilisers > eutrophication
  • Wastewater discharge > increases with higher population
• 3. Transfers increase:


  • Runoff contaminated via dust and pollutants
  • Artificial drainage systems added e.g. guttering, storm drains
• 4. Stores decrease:

  • Impermeable surface > decrease infiltration > reduce stores
  • Lakes and ponds in homes/ parks often drained and water channelled underground
  • Less water stored in soil (soil moisture storage) and rock (groundwater storage) as less infiltrates
• How do the sloping roofs influence stores/ transfers:


  • Increase surface runoff > increase channel flow in river
  • Increase pipeflow into drains, rivers etc
• How does tarmac/ concrete roads with gutters influence stores/ transfers?

  • impermeable surface > increase runoff and pipeflow
  • decrease surface storage
• How does draining wetlands for housing influence stores/ transfers?


  • Decrease soil water surface storage and soil moisture storage
  • Increase surface runoff and flooding
• How does reduced veg cover influence stores/ transfers?


  • Decrease veg storage and transpiration > increase surface runoff
  • Decrease interception > decrease evapotranspiration > decrease stemflow
• How do smooth storms drains and sewers (enable pipeflow) influence stores/ transfers?


  • Increase pipeflow and channel flow and storage
• Changes that have occurred to increase the risk of flooding and contaminated water in URBAN areas:

  • Greater inputs from precip due to more hydroscopic nuclei from industry etc
  • Greater water runoff due to impermeable surfaces > decrease infiltration and increase surface runoff
  • Less water stored in soil moisture storage and in rock via groundwater storage as less infiltrates
  • Evapotranspiration reduced as less plants
  • Water in drains (pipeflow) have higher velocity and erosive power > erode rivers > flood
  • Less water stored as lakes and ponds often drained and water channelled underground