11.6 Factors increasing flood risk Flashcards
What are the physical factors that lead to an increase in flood risk?
➢ Precipitation
➢ Geology
➢ Relief
How does precipitation affect the risk of flooding?
➢ Rainstorms and thunderstorms can lead to flash floods ar river channels cannot contain the sheer volume of water flowing into them
➢ Increases the risk of flooding
How does geology affect the risk of flooding?
➢ impermeable rocks ( that do not allow water to pass through them) such as shales and clays encourage water to flow overland and into river channels
➢ Speeds up water flow and makes flooding more likely
How does relief affect the risk of flooding?
➢ In mountain environments steep slopes encourage a rapid transfer of water towards river channels
➢ Increases the risk
What are the human factors that affect the risk of flooding?
➢ Urbanisation
➢ Deforestation
➢ Agriculture
How does urbanisation affect the risk of flooding?
➢ Building on a flood plain creates impermeable surfaces such as tarmac roads
➢ Water is transferred quickly to drains and sewers then into urban river channels
➢ Rapid movement of water makes flooding more likely
➢ Increases the risk of flooding
How does deforestation increase the risk of flooding?
➢ Much of the water that falls on trees is evaporated or stored temporarily on leaves
➢ When trees are removed much more water is suddenly available and transferred rapidly to river channels - increasing the flood risk
How does agriculture affect the risk of flooding?
➢ When growing crops in a field - soil is left unused and is exposed to the elements for a period of time
➢ This can lead to more surface run-off
➢ Increases the risk of flooding
What is discharge?
➢ The volume of water flowing along a river is its discharge
What is the purpose of a hydrograph?
➢ A graph that plots river discharge after a storm
➢ It shows how discharge rises after a storm - reaches its peak then returns to the normal rate of flow
What is the lag time?
➢ The time in hours between the highest rainfall and the highest peak discharge
➢ This shows how quickly water is transferred into a river channel
➢ The shorter the lag time the greater the risk of flooding
What are the characteristics of a flashy hydrograph?
➢ The basin size is small which leads to rapid water transfer
➢ The drainage density is high - which speeds up water transfer
➢ Impermeable rock encourages the rapid overland flow
➢ Urbanisation encourages rapid water transfer
➢ Steep slopes lead to rapid water transfer - flash hydrograph
➢ Saturated soil results in rapid overland flow
➢ Heavy rain will lead to rapid overland flow due to exceeding the capacity of the river
What are the characteristics of a low flat hydrograph?
➢ The basin size is large which results in a slow water transfer
➢ Drainage density is low and leads to a slower water transfer
➢ Permeable rocks encourage a slow transfer by groundwater flow
➢ Forest floor slow down water transfer because of interception
➢ Gentle slopes slow down water transfer
➢ Dry soil soaks up water and slows down its transfer
➢ Light rain will transfer slowly and most will soak into the soil
