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AS Geography: River Environments - Urbanisation |
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Human modification of drainage basin
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Urbanisation
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| Factor |
Impact |
| Storm-water sewers |
- Reduce distance travelled by rainwater before reaching channel
- Smooth surface of sewers increase velocity
- Reduction in storage capacity as sewers drain quickly
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| Encroachment on the river channel |
- Reduction of river channel, leads to higher floods
- Bridges restrict river discharge
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| Pollution control problems |
- Storm-water washes over roads collecting chemicals and heavy metals
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| Water resource problems |
- Groundwater recharge lowered as sewers bypass mechanisms of percolation and infiltration
- Groundwater abstraction reduces local stores
- Irrigation depletes water resources and causes pollution
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| Flood control problems |
- Increase in peak flow and reduction in lag times increases flood risk
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| Rainfall climatology of urban areas |
- Urban heat islands created
- More rainfall especially in the summer
- Heavier and more frequent thunderstorms can increase flood risk
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| Building activity |
- Clears vegetation, exposing soil and increases overland flow
- Increase in amount of impermeable surface
- Farmland used for housing
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| Impermeable surfaces |
- Reduces storage and increase run-off
- Increases velocity of overland flow
- Decreases evapotranspiration because urban surfaces are usually dry
- Reduces infiltration and percolation
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Canon’s Brook basin, Harlow. UK
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Due to urbanisation, the hydrograph of the catchment has been altered
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Changes:
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Rainfall has not changed, but run-off has increased by 30%·
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Increased frequency of high flows due to storms over the developed catchment
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Flood frequency has been increased by the reduction of lag times and increase in peak discharges
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Area has additional inputs from homes and businesses, passed on through the sewage works
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D. Drake 2011
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