L5- Causes of flooding

• precipitation

  any source of moisture reaching the ground eg rain, snow, frost
• interception

  water being prevented from reaching the surface by trees or grass
• surface storage

  water held on the ground surface eg puddles
• infiltration

  water sinking into soil/ rock from the ground surface
• soil moisture

  water held in the soil layer
• percolation

  water seeping deeper below the surface usually through the soil to the rock below
• groundwater

  water stored in the rock
• transpiration

  water lost through pores in the vegetation
• evaporation

  water lost from ground / vegetation surface
• surface run off

  water running on top of the ground
• throughflow

  water flowing through the soil layer parallel to the surface
• water table

  current upper level of saturated rock / soil where no more water can be absorbed
• input

  the water put in
• transfers/ flows

  the movement of water within the cycle
• stores

  places where water stops within the cycle (lakes, as snow / ice - 94% of all water is stored in the sea)
• outputs

  water leaving the land
• condensation

  the process of changing from a gas to a liquid
• factors affecting discharge

  • characteristics of a drainage basin
  • type and amount of precipitation
  • land use and human impact
  • present conditions of the drainage basin
  • temperature
• COF: impermeable rocks

  granite and soil will not allow water to pass through, resulting in large amounts of surface runoff and greater flood risk as rivers respond quickly
• COF: permeable rocks

  and soil have a high infiltration capacity and will absorb water quickly, reducing overland flow
• a drainage basin with a steep gradient will result in greater overland flow and where the gradient is less steep, more infiltration will occur and reach the river slower
• heavy rain results in rapid saturation of the upper soil layers and the excess water therefore reaches streams quickly as surface runoff
• slow light rain can be absorbed by infiltration and the river takes longer to respond to rainfall as water takes longer to pass through the drainage basin via throughflow and groundwater flow
• impermeable man made surfaces such as concrete and tarmac are impermeable and therefore rivers in urban drainage systems tend to have higher discharge due to higher amounts of surface area and drainage systems taking water to rivers quickly
• vegetated areas help to reduce flood risk by increasing the time it takes for water to reach a river by encouraging infiltration, intercepting water by their leaves and taking up water into their roots
• areas cleared by deforestation will respond quickly to rainfall due to the reduced interception
• if the soil has already been saturated by heavy rain its infiltration capacity will be reduced and further rain will go as surface run off
• if the soil is dry, it will be able to absorb more water during infiltration and therefore the discharge will increase slowly
• if it is very hot, there will be more evaporation and transpiration leading to water being recycled into the atmosphere before it even reaches the river, this then leads to lower discharge
• if it is very cold or in winter, then there will be few leaves on the trees and little transpiration happening. this will lead to increased discharge in the river, the ground may also be frozen and impermeable, leading to river discharge increase
• how does water get into and out of a river system?

  the drainage basin hydrological cycle explains how precipitation falling in a catchment gets into a river. the risk of flooding depends on how quickly precipitation gets into the river channel. surface run-off is the quickest route as it doesnt have much in the way to slow it down like as with throughflow and groundwater flow. if there is a lot of surface run off, then the discharge of the river will increase quickly
• what is flood risk increased by?

  bands of depressions passing over the uk at frequent intervals, especially in winter, result in continuous heavy rain, which may saturate the soil, which causes surface run off to increase and rainwater will therefore enter the river quicker, resulting in higher discharge and floods
• what is flood risk increased by?

  sudden bursts of heavy rain often results in the infiltration rate being too slow to cope. this may occur after a period of drought that has baked the soil hard. surface run off occurs , discharge increases quickly and flash floods may occur
• what is flood risk increased by?

  prolonged light rainfall may cause floods if there has been a lot of previous rainfall (known as antecedent rainfall) that has saturated the soil
• what is flood risk increased by?

  sudden snowmelt causes a release of stored water that flows over still frozen ground as surface run-off, increasing discharge
• how can geology and relief increase flood risk?

  the type of rock found in mountains is usually impermeable rock such as slate, which does not allow water to pass through it, the rock is often bare, with thin soils and little vegetation to intercept the rain
• how can geology and relief increase flood risk?

  low lying areas often contain an impermeable clay soil. it is usually vegetated, but the soil is so compacted that it is difficult for infiltration to occur
• how can geology and relief increase flood risk?

  flooding is much less likely in areas of permeable rock such as chalk and limestone, as water passes through these rocks
• relief

  the height and shape/ slope of the land. steep slopes mean that surface run off occurs on mountainsides before rain has had time to infiltrate the soils
• low lying, flat floodplains also have a high flood risk as there is not enough of a gradient to remove the water. floodplains are there for the river to flood onto and will flood extensively. flood risk is increased here if the soils are impermeable