It is the combination of physical and human factors that make some places more prone to flooding than others. As people live near to the rivers, a need has developed to manage these floods.
Natural rivers shift position over time due to erosion and deposition, but increasingly rivers are required to flow within fixed parameters, to allow roads, bridges, and residential land to remain unaffected by these fluctuations in discharge. Human activity not only prevents rivers from spreading in order to respond to natural changes in state, but it increases runoff due to the change in land use and thus increasing the risk of flooding.
- Floods occur when large volumes of water enter a river system quickly. Discharge increases to the point where it can no longer be contained in the river channel and water spills out onto the floodplain.
Natural Causes of River Flooding:
- Primary causes ~ the result of climatic factors (for example heavy rains in Summer in the UK in 2007)
- Secondary causes ~ Drainage Basin specific (for example dependant on geology, soil, topography and vegetation).
Vegetation: Greater vegetation = greater interception, storage and evapotranspiration. This reduces rainfall and increases the lag time.
Slope: Steeper angled slopes means that less water is absorbed, meaning more runoff.
Rock Type: Permeable rock allows greater infiltration and ground storage, leaving less water to runoff.
Drainage Density: Where drainage density is low, there is a longer lag time and a reduced risk of flooding.
Soil Depth: Deeper soil absorbs more water and results in less runoff.
Human Causes of River Flooding:
The development of once natural landscapes for residential, industrial, and agricultural purposes tends to reduce infiltration and increase runoff. Modifications of natural drainage systems can put them out of balance. Urbanisation helps to increase the frequency and magnitude of flooding by:
- Creating impermeable surfaces (roofs, car parks)
- Speeding up the drainage of water in built up areas by artificial conduits (drains, sewers)
- Impeding Channel flow by building alongside the river (bridge supports)
- Changing land use…