Urban Precipitation (AQA A Level Geography) : Revision Note

Characteristics of Urban Drainage

• Urban areas experience higher rates of precipitation compared to rural regions because of:

  • Pollution - urban air holds more condensation nuclei in the form of dust and dirt particulates

  • Warmer air - the warmer the air, the more moisture it holds

• Surface runoff (overland flow) is dominant as much of the surfaces are impermeable or designed to shed water - the camber of a road, funnels water to drains

• There is also less vegetation to intercept precipitation, which means less evapotranspiration (EVT) rates to cool the air and reduce humidity

• As a result, more drainage is needed to remove surface water quickly

Impacts of Urbanisation on Drainage

• The urban form impacts the drainage system in the following ways:

  • Inputs are increased

    • Precipitation is increased due to pollution and temperature

    • Extra water is artificially imported into the system

• Outputs are unbalanced

  • Less vegetation reduces EVT

  • Impermeable surfaces increases runoff

  • Wastewater discharge is an adjusted output into a river or sea (sewage discharge is treated but there will still be liquid waste added to the system)

• Transfers are increased

  • Runoff is contaminated through dust and pollutants from industry and transport

  • Artificial drainage systems are added above and below ground (e.g. guttering and storm drains etc.)

• Stores are decreased overall

  • Impermeable surfaces reduce ground infiltration and stores are reduced

  • Rivers are diverted, dredged or channelised which increases capacity 

  • Reservoirs and ponds are permanent stores but vulnerable to evaporation

  • Depression (puddles) stores are temporary

  • Lakes and ponds in parks and homes are often drained

• These changes become more pronounced across the urban pathway, with the urban CBD system seeing the greatest impacts, compared to the rural-urban fringe system

Urban Water Cycle

• Urban hydrographs are 'flashy'

• They show a rapid rise in discharge over a short period of time 

  • Short lag time

  • High peak discharge

  • Fast return to base flow

• Caused by:

  • Low infiltration rates

  • High surface runoff

• Flash floods in urban areas are increased through:

  • Heavier rates of precipitation

  • Increased thunderstorms

  • High rates of impermeable surfaces

  • Channelling of surface runoff to drains

  • Building on floodplains to meet demand for housing

  • Culverts, rivers and drains blocked with debris

  • Narrow channels under bridges constricting flow