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Urban Drainage Urban drainage

Urban precipitation, surfaces and catchment characteristics; impacts on drainage basin storage areas; urban water cycle: water movement through urban catchments as measured by hydrographs.

We have already seen that urban areas can drastically alter the climates of urban areas, but what of the drainage systems that exist in urban locations?

Precipitation is higher in urban areas due to increased presence of hygroscopic nuclei, and urban areas also experience more thunderstorms. These alter the INPUTS to the urban drainage system, but other changes also take place. When looking at the changes in the urban drainage system, it is useful to consider the Urban water cycle and balance;

A natural water balance displays the RELATIONSHIP between inputs (precipitation), stores (soils, rocks, surface, plants) and outputs (evaporation, transpiration, runoff) of a drainage basin system. This is significantly disrupted by the construction of urban areas

The urban water balance also displays the RELATIONSHIP between inputs (precipitation), stores (soils, rocks, surface, plants) and outputs (evaporation, transpiration, runoff) of a drainage basin system BUT tends to have less infiltration and evapotranspiration, greater runoff than natural systems and also has imported water and waste water losses not present in natural systems. These changes are shown in the diagrams below;

Contrasting drainage basins

The main changes include:

  1. Greater inputs from precipitation due to more hygroscopic nuclei
  2. Water is artificially imported into the system adding extra water as an INPUT
  3. Greater water runoff is evident due to the use of Impermeable surfaces (Any substance that will not allow water to pass through, such as concrete, used in urban areas to speed water away from areas of high property values)
  4. Runoff is of poor water quality in urban areas as it picks up dust and pollutants created by urban processes like industry or transport.
  5. Evapotranspiration is reduced in urban areas as there are fewer plants.
  6. Infiltration is reduced in urban areas as concreted and tarmacked areas are impermeable and don't allow water to soak in.
  7. Wastewater discharge is an adjusted OUTPUT to the system as liquid waste or sewage discharged into a river or the sea in urban areas (although much of this will be dealt with by a sewage system (the process of removing contaminants from wastewater, primarily from household sewage)
  8. An artificial drainage system is added via a network of underground pipes and rains used to get rid of rainwater in urban areas
  9. Guttering - channels water off buildings and into a drainage system
  10. Concrete driveways - Have increased in popularity in the UK but increase localised flood risk as they do not allow water to infiltrate and exaggerate surface runoff
  11. Less water is stored in urban areas as lakes and ponds are often drained and water is channelled underground. Less water is also stored in the soil and rock as less infiltrates.

These changes are more pronounced the more urban an environment, so as we progress from a rural urban fringe area into suburbs and on into the CBD the impacts on urban water budgets become more severe.

The Urban Hydrograph

A Hydrograph is a graph which shows the discharge of a river, related to rainfall, over a period of time. As a result of the changes to urban drainage basins, urban hydrographs tend to be flashy, where rivers are more likely to flood due to lower interception and infiltration rates. Although the base flow of rivers is lower in urban rivers that in rural rivers as less if fed into the system via soil and rock stores, these rivers fill quickly during rain events and are more likely to flood. The lag time of an urban river is shorter due to drainage systems which speed the flow of water through urban areas and a higher peak discharge is evident in urban rivers. These rivers also have steep rising and falling limbs as water reaches urban rivers quicker via drainage networks. The contrasts between a rural and urban hydrograph can be seen below.

Urban versus rural hydrographs

NEXT TOPIC - Catchment Management and SUDS

Posted by Rob Gamesby April 2020



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