Urban Drainage

Question 1

Why is precipitation higher in urban areas

Answer 1

. Increased presence of hygroscopic nuclei
. Urban areas also experience more thunderstorms
. This alters the inputs to the urban drainage system

. Warmer air in cities can hold more moisture
. Dust and pollution make more condensation nuclei
. There is less vegetation in urban areas and therefore less evapotranspiration due to reduced moisture in the air
. Less evapotranspiration also means less interception and more precipitation landing on hard surfaces
. High surface run off due to impermeable surfaces

Question 2

Natural water balance

Answer 2

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

Question 3

Urban water balance

Answer 3

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.

Question 4

Urban drainage changes

Answer 4

2. Water is artificially imported into the system adding extra water as an INPUT
3. Runoff is of poor water quality in urban areas as it picks up dust and pollutants created by urban processes like industry or transport.
4. Evapotranspiration is reduced in urban areas as there are fewer plants.
5. Wastewater discharge is an adjusted OUTPUT to the system as liquid waste or sewage discharged into a river or the sea in urban areas
6. An artificial drainage system is added via a network of underground pipes and rains used to get rid of rainwater in urban areas
7. Guttering - channels water off buildings and into a drainage system
8. Concrete driveways
9. 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.

Question 5

How much more precipitation do urban areas get than rural areas?

Answer 5

5-10%

Question 6

Lag time

Answer 6

. Urbanisation reduces infiltration through impermeable surfaces and drains transport water quickly to the river channel
. Lag time is reduced and discharge is increased