5B - Human Actions and Flooding
Human actions that can exacerbate flood risk (changing land use within the river catchment, mismanagement of rivers using hard engineering systems.)
A combination of economic and population growth during the 20th century has caused many floodplains to be built upon and many natural landscapes to be modified for agricultural, industrial and urban purposes. The impacts of human activities on the hydrological cycle were examined in 2C. These same activities, all related to changing land use within river catchments, frequently increase the flood risk, none more so than urbanisation.
E.g.:
- Impermeable areas of tarmac
- Wells sunk to supply settlements
- Sprinkling of groundwater onto arable crops
- Dams built to supply towns with water
- Streams channelled into culverts to aid rapid drainage of farmland
- natural streams meander and have marshy areas; but channelisation does not
- Ploughing compacts soil
- Grazing animals trample soil
- Woodlands intercept rain and transpire moisture; roots give good soil structure. Deforestation destroys this.
- Natural grasslands allow water to sink in, replaced by improved pasture
- Bridge supports built in rivers, ramps on floodplain
- Sewers feed water into channel
River mismanagement:
- channelisation: an effective way of improving river discharge and reducing the flood risk. The trouble is that it simply displaces the river downstream. Some other location may well be overwhelmed by the increased discharge
- dams: block the flow of sediment down a river, so the reservoir gradually fills up with silt; downstream there is increased river bed erosion
- river embankments: designed to protect from floods of a given magnitude. They can fail when a flood exceeds their capacity. Inevitably, when this happens, the scale of flooding is that much greater.
These examples of hard-engineering intervention serve as reminders that soft-engineering methods of reducing the flood risk are preferable. These include making greater use of floodplains as nature intended, as temporary stores of flood water, and using them only for nature conservation and perhaps agriculture and recreation.
