Urban Drainage

Question 1

in urban areas, infiltration is low and surface runoff is high - why?

Answer 1

• impermeable surfaces reduce infiltration and increase surface runoff
• because infiltration is low, replenishment of groundwater stores is slow
• drainage structures direct runoff to rivers and streams
• precipitation is higher in urban areas, increasing surface runoff
• short lag time and high peak discharge

Question 2

what’s catchment management?

Answer 2

a way of managing rivers and improving drainage systems to reduce issues such as flooding, riverbank erosion, contamination of groundwater, water shortages, and water pollution. It also aims to improve river ecosystems

Question 3

methods of catchment management

Answer 3

hard engineering (man-made structures) - dams, flood defences and channel management, reservoirs

soft engineering (natural, uses knowledge of the river basin and processes) - dredging, land use management, river restoration and conservation

Question 4

hard engineering vs soft engineering

Answer 4

• hard engineering is expensive and disrupts natural systems and habitats
• soft engineering is generally cheaper and can improve the local environment, however it can be limited by planning restrictions

Question 5

what’s dredging?

Answer 5

• where sediments are removed from the river channel with machinery
• a way to reduce flooding as it deepens the river channel allowing more water to flow into it
• however, it can cause water to flow more turbulently and increase the speed of water flow, enhancing the risk of flooding further downstream

Question 6

what are dams?

Answer 6

• used to manipulate water flows and stores to reduce flooding
• dam construction can cause environmental damage e.g. valleys can be flooded to make reservoirs, dams change how sediment is transported which can affect erosion, settlements may need to be displaced

Question 7

what are reservoirs?

Answer 7

• used as a water supply
• however, valleys have to be flooded to make reservoirs and villages may need to be displaced

Question 8

what are flood defences and channel management?

Answer 8

• physically altering water courses through straightening channels, constructing flood walls, and creating drainage pipe systems
• forces water out of urban areas
• however, this may worsen flooding downstream

Question 9

what’s land use management?

Answer 9

planting trees to decrease flood risk and water pollution, preventing building on flood plains

Question 10

disadvantages of catchment management

Answer 10

• decisions are made by experts, residents may feel ignored
• catchment management schemes can be disruptive e.g. construction of the Three Gorges Dam in China flooded 13 cities and 140 towns, displacing over 1.2 million people

Question 11

what are SUDS?

Answer 11

• sustainable urban drainage systems
• aim to imitate natural drainage systems
• aim to decrease flooding, water pollution, and drought in urban areas

Question 12

examples of SUDS

Answer 12

• swales (vegetated trenches)
• ponds
• rainwater harvesting
• green roofs (intercept rainfall, increase evapotranspiration)
• introducing permeable surfaces e.g. grass verges
• containers

Question 13

how do swales work?

Answer 13

• increase interception of rainfall
• retention basins store water
• increase infiltration and storage
• decreases runoff and channel flow, lowering flood risk and reducing pollutants
• also provide habitats, increasing biodiversity

Question 14

how do containers work?

Answer 14

they catch and store rainwater for reuse, reducing drought risk