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  1. 1. River Drainage Basins & Flooding
  2. 2. <ul><li>Learning Objectives: </li></ul><ul><li>To understand what a drainage basin is. </li></ul><ul><li>To understand what a flood hydrograph is. </li></ul><ul><li>To understand what factors influence the shape of a flood hydrograph. </li></ul>
  3. 3. river evapotranspiration precipitation throughflow surface runoff groundwater flow water table interception percolation infiltration Processes in the Drainage basin system
  4. 4. Drainage Basin – Key Definitions <ul><li>Precipitation: </li></ul><ul><li>Water in any form that falls from the atmosphere onto the surface of the earth. It includes rain, snow & hail </li></ul><ul><li>Infiltration: </li></ul><ul><li>Process of water soaking into the soil. </li></ul><ul><li>Interception: </li></ul><ul><li>Precipitation that is prevented from reaching the ground by vegetation. </li></ul><ul><li>Surface/Oveland Flow: </li></ul><ul><li>the movement of water down slope to the river over the surface of the ground. </li></ul><ul><li>Throughflow: </li></ul><ul><li>The down slope movement of water through the soil. </li></ul><ul><li>Evaporation: </li></ul><ul><li>Process in which water changes into water vapour (which is a gas). </li></ul><ul><li>Evapotranspiration: </li></ul><ul><li>the total amount of moisture removed from the drainage basin from evaporation & transpiration. </li></ul><ul><li>Percolation </li></ul><ul><li>Process by which water moves downwards through cracks, joints and pores rock. </li></ul><ul><li>The Water Table </li></ul><ul><li>The level below which the ground is completely saturated with water. </li></ul>
  5. 5. Processes in the Drainage basin system inputs precipitation outputs river runoff evaporation transpiration stores and flows interception surface runoff surface water storage soil water storage groundwater storage infiltration groundwater flow throughflow percolation
  6. 6. Rising limb Falling limb Peak discharge Peak rainfall storm flow normal (base) flow Hydrographs I = Lagtime Lagtime = difference between peak discharge and peak rainfall
  7. 7. The Flood Hydrograph – Key Words
  8. 8. Hydrograph are affected by the characteristics of the drainage basin What affects the shape of a hydrograph? Land use Type and amount of precipitation Geology and soil Gradient of the valley sides Drainage density
  9. 9. How do trees reduce flooding? trees intercept the rain evapotranspiration reduces the amount of water that reaches the river water is taken through the roots and so less enters the river
  10. 10. Using the data, draw your own flood hydrograph. Answer the following questions: 1) What is the amount of the peak discharge? 2) What is the length of the lag time? 3) Can you think what may produce a short lag time? 4) What do you think may produce a long lag time? <ul><li>On your hydrograph label: </li></ul><ul><li>Lag time </li></ul><ul><li>Rising limb </li></ul><ul><li>Peak discharge </li></ul><ul><li>Storm event </li></ul><ul><li>Peak rainfall </li></ul><ul><li>Storm runoff </li></ul><ul><li>Normal flow </li></ul><ul><li>Falling limb </li></ul>
  11. 13. Imagine that these two areas have had the same amount of rain over a 24-hour period. Draw a possible hydrograph for each of these areas. A B Hydrographs