Geography - Flooding + Flood Hydrograph definitions gcse 9-1

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  • Geography: Flooding
    • Flood Hydrographs
      • Recession limb: Falling flood water in the river
      • Peak flow: Maximum discharge in river
      • Basin lag time: Tine difference between the peak of the rain storm and the peak flow of the river
      • Rising limb: The rising flood water in the river
      • Base flow: Normal discharge of river
      • Discharge in m3/s, as a line graph
      • Overland flow + Through flow = Stormflow
      • Rainfall shown in mm, as a bar graph
    • Definitions
      • The more infiltration occurs the longer the lag time and shallower the rising limb
      • Infiltration is generally greater on thick soil, although less porous soils e.g. clay act as impermeable layers
      • Land use  
        • Afforestation - intercepts the precipitation, creating a shallow rising limb and lengthening the lag time
        • Urbanisation - concrete and tarmac form impermeable surfaces, creating a steep rising limb and shortening the lag time
      • Slope Channel flow can be faster down a steep slope therefore steeper rising limb and shorter lag time
      • Drainage density Drainage density is the total length of all the streams and rivers in a drainage basin
      • Slope Channel flow can be faster down a steep slope therefore steeper rising limb and shorter lag time
      • Tidal Conditions: High spring tides can block the normal exit for the water, therefore extending the length of time the river basin takes to return to base flow 
      • Shape Why does the drainage basin shape matter? An elongated basin will produce a lower peak flow and longer lag time than a circular one of the same size
      • Precipitation & Temperature
        • If there have been extreme temperatures, the ground can be hard (either baked or frozen) causing rapid surface run off
        • Short intense rainstorms can produce rapid overland flow and steep rising limb
        •  Snow on the ground can act as a store producing a long lag time and shallow rising limb.  Once a thaw sets in the rising limb will become steep
      • Area  Large basins receive more precipitation than small therefore have larger runoff Larger size means longer lag time as water has a longer distance to travel to reach the trunk river

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