Urban perception and drainage

• • Urban areas have less vegetation and therefore less evapotranspiration, which reduces moisture in the air (i.e. lower humidity).
  • Less vegetation also means less interception and, therefore, more precipitation lands on hard, urban surfaces.
  • The slate, tile, concrete and tarmac used in urban areas are impermeable and so the urban catchment is dominated by surface run-off, particularly in city centres.
  • Both shallow and deep infiltration is significantly reduced and drains are therefore needed to remove surface water quickly.