Urban Drainage

Question 1

Impact of urban areas on hydrographs

Answer 1

Shorter basin lag time due to impermeable surfaces
Steeper rising limb- water management systems allow water to flow into the river quicker
Steeper falling - less underground storage of water so the vast majority will have entered to river quicker so the base level will return quicker

Question 2

Two types of flood management

Answer 2

Fail safe - reducing problems that have occurred in the past (flooding)
Safe to fail - Sustainable Urban Drainage System

Question 3

Safe to fail management- Los Angeles

Answer 3

1930s - made river into concrete channel, increasing capacity of the river, destroying most of the floodplain
Restoration
- improve wildlife biodiversity
- aims to restore 11 miles of the LA river
- much of the LA river functions only as a flood channel
- aims to accommodate recreational and natural uses of the river while maintaining the city’s flood risk management levels

Question 4

SUDS

Answer 4

A relatively new approach to managing rainfall by using natural processes in the landscape to reduce flooding

Question 5

Examples of SUDs

Answer 5

Source control - tanks or butts to store rainfall, green roofs trap water, permeable pavements to decrease run-off - keeping rainwater where it falls
Site control - ponds, swells, keeping rainwater near where it fell
Regional control - large pond, permanent wetland - keeping water coming from a larger area.

Question 6

Benefits of SUDs

Answer 6

Improve water quality by filtering pollutants
Inc. green space in cities
Reduce UHI effect
Improve urban biodiversity
Control pollution - although infiltration drainage can spread pollutants in industrial areas

Question 7

Aims of SUDs

Answer 7

Store run-off and release it slowly - attenuation
Harvest and use the rain close to where it falls
Allow water to soak into the ground (infiltration)
Slowly transport (convey) water on the surface
Filter out pollutants
Allow sediments to settle out by controlling the flow of water

Question 8

Olympic Park- London - SUDs

Answer 8

Porous asphalt strips employed throughout pedestrian areas
Wetlands area containing swales, filter drains and small ponds - haven for wildlife as well
Rainwater Harvesting - used for toilets etc.
Traditional road drains and combined curb drainage collection system
Has the benefit of protecting the area from extreme flood events, protect groundwater and local watercourses from pollution as surface run-off is reduced
Challenge to ensure all systems are adoptable

Question 9

Flooding in Mumbai

Answer 9

2005 severe flooding :
-400 dead
-10000 homes destroyed
-$1.2 billion in losses
Causes:
-Urban development has consumed parks, wetlands, gardens, mangrove swamps and beaches - still uncontrolled
Outdated drainage system - 100 years old, 2000km of surface drains, 400km of underground drains - heavily silted and punctured in many places, capacity of 25mm per day at low tide (monsoon brings up to 1000mm of rain in a day
Mangroves are natural barrier for floods.
Following 2005 flood, the Greater Mumbai Disaster Management Plan - widened and deepened the river Mithi