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Urban workbook 3 (Hydrographs (peak precipitation- rainfall reaches it'…
Urban workbook 3
Hydrographs
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discharge- the amount of water originating as precipitation which reaches the channel by surface run-off, throughflow and baseflow
bankfull discharge- occurs when a river's water reaches the top of it's channel. Any further increase will result in flooding
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URBAN has a higher peak- better draining and sewers, steeper limbs = faster movement
RURAL- slow, longer lag time, infiltration and longer lifetime
URBAN VARIATIONS
air quality poorer in urban areas as more pollution from vehicles and industry in particular diesel engines (particulates)
urban temperature is higher as buildings give off heat
- there is a larger difference in winter due to central heating
- CBD- cafes and kitchens that release heat via vents and fans
- higher difference at night as temperature drops
URBAN HEAT ISLAND- urban areas are warmer than the countryside (2-4 degrees average)
- lower winds due to building heights
- pollution traps outgoing energy
- burning fossil fuels for domestic and commercial use
- buildings increase in their capacity to retain and conduct
- less albedo
- less heat diffusion due to changes on air flow
PRECIPITATION- increased in urban areas as higher temperatures encourage low pressure
- Urban Heat Island generates convection
- ground is heated
- high rise buildings create air turbulence
- city pollution increases cloud formation
- pollutants have hygroscopic nuclei which causes formation of water vapour droplets so more rain
FOG AND THUNDERSTORMS- particles act as condensation nuclei and increase fog under high pressure
- recent industrialisation has increased fog
- Thunderstorms develop in hot humid air
- when water droplets collide, the positive electrons create lightning
WIND- buildings have frictional drag on air and change speed
- turbulence due to uneven heights
- high rise buildings create low movement and make wind chanels
AIR QUALITY- poorer in urban areas as particulates released with noxious gases
- combustion of fossil fuels from vehicles
- decrease in health
- smoke and fog = smog
SUDS
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sustainable because:
- manage run-off volumes and reduce flooding
- protect and enhance water quality
- sympathetic to the environment and local community
- provide habitats for wildlife
- encourage natural groundwater recharge
e.g Milton Keynes New Town 1960s
Willen lake, artificial balancing lake
managed outflow to control flooding
reed beds- reduce pollution
Enfield- North London suffers from regular flooding
- heavily urbanised which increases run-off, floods and pollutants e.g oil and heavy metal
- in 2012 they aimed to reduce flood risk, reduce run-off, improve water quality and create more habitats
- used SUDS e.g vegetated trenches, rain planters (education) and wetlands
- DEFRA (£340,000), local residents and local schools all helped.
- Thames 21 project 2015- 1000 students litter picked and monitored wildlife
- decreased flood risks, reduced ammonia levels in Salmons Brook by 67% and nitrogen by 43%- levels of eels and dragonflies have increased
- pollution and litter are still an issue
Examples of SUDS
VEGETATED TRENCHES- increases interception and retains water store
- decreases run-off
- decreases channel flow
- decrease flooding
- decrease pollutants
VEGETATED ROOFS- intercept rainfall
- increase evapotranspiration
- decrease flooding
- decrease vegetation storage
- slows lag time
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