CUE: Urban Drainage

Question 1

What is a drainage basin?

Answer 1

An area of land drained by a river, its edge is called a water shed

Question 2

Why is precipitation in urban areas more common?

Answer 2

• Urban areas have 5 – 15% more precipitation than rural areas
  —> Warmer air in cities (due to urban heat island effect) more convection (rising warm air) and more evaporation = more rainfall. Hold more moisture
  —> Dust and pollution make more condensation nuclei (particles of dust in the air that water forms around)

Question 3

How does Urban areas effect runoff and flooding?

Answer 3

• less vegetation and therefore less evapotranspiration reduces moisture in the air (humidity). Less vegetation also means less interception and more precipitation landing on hard urban surfaces. Many urban surfaces are impermeable and so the urban catchment is dominated by runoff (overland flow), particularly in city centres
• both shallow and deep infiltration is reduced, as is through flow
• drains are needed to shift all the surface water to prevent flooding

Question 4

What’s the difference in drainage between rural areas and urban areas?

Answer 4

• rural: 95% infiltration, 5% runoff
• urban: 5% infiltration, 95% runoff

Question 5

What are the impacts on the drainage basin storage areas?

Answer 5

• urban rivers are the primary exit for the water transferred through the drainage basin, but they are also important stores.
• management by dredging, embanking and channelisation will increase storage capacity
• reservoirs, lakes, ponds and swimming pools are permanent stores, but vulnerable to evapotranspiration. Depression storage, such as surface puddles after rain is temporary
• interception storage is reduced owing to built environment
• soil moisture storage will vary according to ground conditions (type of soil), but there is generally less storage capacity as urban development reduces exposed surfaces and vegetated areas.

Question 6

What is a hydrograph?

Answer 6

A record of river discharge over a period of time.
River discharge is the volume of water passing a particular point in a river at a given moment in time - measured in m3 per second.
- cross section area (m2) x rivers mean velocity (mps)
—> (at a particular point in its course)

Question 7

What do storm hydrographs show?

Answer 7

The change in discharge caused by a period of rainfall.
—> help predict flooding events

Question 8

What are some of the factors influencing storm hydrographs?

Answer 8

• type of surface
• seasons
• topography
• drainage density
• weather
• type of soil/rock
• temp
• land use
• vegetation

Question 9

How are flood hydrographs and the urban water cycle linked?

Answer 9

In urban areas, surface runoff can reduce lag time (the time between peak precipitation and peak discharge), therefore increasing the potential flood risk.

Question 10

What are some of the reasons for increased flood risk?

Answer 10

• water will flow rapidly along pavements and roads. Drains may be unable to cope with the sheer volume of water and culverts may become blocked with debris.
  Urban rivers themselves may become blocked by debris (particularly where narrow bridges are placed)

Question 11

What is some drainage management?

Answer 11

The rapid removal of water form urban surfaces is essential if day to day city life is to function normally.
- urban drainage is an engineering challenge.
—> hard engineering - high cost and maintenance means that increasingly soft engineering ideas are favoured.
- sewage and water treatment plants will alway be required for safe management of human waste and industrial effluent

Question 12

How can we prevent urban flooding?

Answer 12

• LA - river channel was converted in response to the flash floods in the 1930s.
  + has helped control flooding
• destroyed all the natural habitats and ecosystems
• if any section of the structure was to break (e.g. in an earthquake) then the entire system would be useless.
  —> an alternative to hard engineering techniques are SUDs