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Flooding 1


Key idea:
The amount of water in a river
fluctuates due to a number of
reasons
Lesson objectives
• To know the terms:
 Flooding, hydrograph, rising limb, falling
 limb, lag time, drainage basin
• To be able to describe the
  characteristics of a hydrograph
Starter activity….
                     The area of land from which
A DRAINAGE BASIN     precipitation flows into a
                     stream.

DISCHARGE
                   The amount of water in a river
                   channel at a given time
VELOCITY           The speed of the river


WATERSHED              The boundary of a drainage
                       basin
A flood occurs when…
…..the discharge is so great that all the
  water can no longer be contained within
  the channel so that the river overflows
  its banks
Factors affecting discharge

•Amount and type of rainfall   • Soil

•Temperature                   • Land Use
                               • Slope
•Previous weather conditions
• Relief
•Drainage Density
•Rock Type
Hydrographs
                           Peak discharge


                                                  Falling
                       Rising
                                                  limb
                       limb
                                     storm flow

                                      normal (base) flow
I = Lagtime          Peak rainfall




Show the change in
discharge caused
by a period of
rainfall
Storm hydrograph
A graph which shows the relationship between
precipitation and the level of a river
APPROACH SEGMENT = base flow
• The discharge of the river before it rains.
RISING LIMB
• Results from a rapid increase in rainwater
  reaching the river.
LAG TIME
• The difference between the time of the
  heaviest rainfall and the maximum level and
  /or discharge of the river.
FALLING LIMB
• Some rainwater is still reaching the river, but
  in decreasing amounts – throughflow and
  groundwater flow
Why Construct & Analyse
Hydrographs ?
• To find out discharge patterns of
   a particular drainage basin


  Help predict flooding events,
   therefore influence implementation
   of flood prevention measures
                                        ©Microsoft Word clipart
ph
                                 g ra Basin lag time
                   d ro                                           Peak flow
              y                  3
          H
     od

                                                   limb



                                                                  Re
  lo




                                                                    ce
              Discharge (m3/s)


F
                                               Rising




                                                                      ss
                                                          Overland
                                 2




                                                                         io
                                                          flow




                                                                        nl
                                                                           im
                                              mm




                                                                              b
                                          4                Through flow
                                 1        3

                                          2
                                                               Base flow


                                     0         12         24     36      48       30   72

                                         Hours from start of rain storm
Discharge (m3/s)   3


                   2


                   1



                       0       12   24   36   48   30   72

                           Hours from start of rain storm
Rainfall shown
                   3                                  in mm, as a
                                                       bar graph
Discharge (m3/s)


                   2
                                mm
                            4

                   1        3

                            2




                       0         12   24   36   48    30   72

                           Hours from start of rain storm
Discharge in
                   3                                  m3/s, as a
                                                      line graph
Discharge (m3/s)


                   2
                                mm
                            4

                   1        3

                            2




                       0         12   24   36   48   30   72

                           Hours from start of rain storm
Rising limb
                                                               The rising
                   3                                         flood water in
                                                                the river


                                     li m b
Discharge (m3/s)




                                 Rising
                   2
                                mm
                            4

                   1        3

                            2




                       0         12           24   36   48   30   72

                           Hours from start of rain storm
Peak flow
                                                 Peak flow    Maximum
                   3                                         discharge in
                                                              the river


                                     limb
Discharge (m3/s)




                                 Rising
                   2
                                mm
                            4

                   1        3

                            2




                       0         12         24   36   48     30   72

                           Hours from start of rain storm
Recession limb
                                                 Peak flow
                                                                   Falling flood
                   3                                               water in the



                                     limb
                                                                       river




                                                  Re
Discharge (m3/s)




                                                      ce
                                 Rising




                                                      ss
                   2




                                                        ion
                                mm




                                                           li m
                                                            b
                            4

                   1        3

                            2




                       0         12         24   36    48         30   72

                           Hours from start of rain storm
Basin lag time
                                Basin lag time
                                                 Peak flow       Time
                   3                                             difference
                                                                 between the


                                     limb



                                                 Re
                                                   ce
                                                                 peak of the
Discharge (m3/s)




                                  Rising




                                                     ss
                   2




                                                        io
                                                                 rain storm and




                                                       nl
                                                        im
                                mm
                                                                 the peak flow




                                                             b
                            4

                   1        3
                                                                 of the river
                            2




                       0          12        24   36     48       30   72

                           Hours from start of rain storm
Base flow
                                Basin lag time
                                                    Peak flow
                                                                       Normal
                   3                                                discharge of



                                     limb
                                                                      the river




                                                    Re
                                                      ce
Discharge (m3/s)




                                  Rising




                                                        ss
                   2




                                                           io
                                                          nl
                                                           im
                                mm




                                                                b
                            4

                   1        3

                            2
                                                 Base flow


                       0          12        24     36      48       30   72

                           Hours from start of rain storm
Basin lag time
                                                    Peak flow

                   3




                                     limb



                                                    Re
                                                      ce
Discharge (m3/s)




                                  Rising




                                                        ss
                                            Overland
                   2




                                                           io
                                            flow




                                                          nl
                                                             im
                                mm




                                                                b
                            4                Through flow
                   1        3

                            2
                                                 Base flow


                       0          12        24     36      48       30   72

                           Hours from start of rain storm
Overland flow = surface run off

 Volume of water reaching the river from
 surface run off

    Through flow
Volume of water reaching the river through
horizontal movement of water through the
soil and underlying rock layers
• After a period of heavy rainfall, the
  discharge increases.


• The lagtime is the difference in time
  between the peak of the rainstorm and
  the peak of the discharge. The shorter
  this is and the steeper the rising limb
  the greater the risk of flooding.


• Complete task 1
Plenary
Homework
Worksheet - task 2 a - c

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Flooding 1 Discharge And Hydrograph

  • 1. Flooding 1 Key idea: The amount of water in a river fluctuates due to a number of reasons
  • 2. Lesson objectives • To know the terms: Flooding, hydrograph, rising limb, falling limb, lag time, drainage basin • To be able to describe the characteristics of a hydrograph
  • 3. Starter activity…. The area of land from which A DRAINAGE BASIN precipitation flows into a stream. DISCHARGE The amount of water in a river channel at a given time VELOCITY The speed of the river WATERSHED The boundary of a drainage basin
  • 4. A flood occurs when… …..the discharge is so great that all the water can no longer be contained within the channel so that the river overflows its banks
  • 5. Factors affecting discharge •Amount and type of rainfall • Soil •Temperature • Land Use • Slope •Previous weather conditions • Relief •Drainage Density •Rock Type
  • 6. Hydrographs Peak discharge Falling Rising limb limb storm flow normal (base) flow I = Lagtime Peak rainfall Show the change in discharge caused by a period of rainfall
  • 7. Storm hydrograph A graph which shows the relationship between precipitation and the level of a river
  • 8. APPROACH SEGMENT = base flow • The discharge of the river before it rains. RISING LIMB • Results from a rapid increase in rainwater reaching the river. LAG TIME • The difference between the time of the heaviest rainfall and the maximum level and /or discharge of the river. FALLING LIMB • Some rainwater is still reaching the river, but in decreasing amounts – throughflow and groundwater flow
  • 9. Why Construct & Analyse Hydrographs ? • To find out discharge patterns of a particular drainage basin Help predict flooding events, therefore influence implementation of flood prevention measures ©Microsoft Word clipart
  • 10. ph g ra Basin lag time d ro Peak flow y 3 H od limb Re lo ce Discharge (m3/s) F Rising ss Overland 2 io flow nl im mm b 4 Through flow 1 3 2 Base flow 0 12 24 36 48 30 72 Hours from start of rain storm
  • 11. Discharge (m3/s) 3 2 1 0 12 24 36 48 30 72 Hours from start of rain storm
  • 12. Rainfall shown 3 in mm, as a bar graph Discharge (m3/s) 2 mm 4 1 3 2 0 12 24 36 48 30 72 Hours from start of rain storm
  • 13. Discharge in 3 m3/s, as a line graph Discharge (m3/s) 2 mm 4 1 3 2 0 12 24 36 48 30 72 Hours from start of rain storm
  • 14. Rising limb The rising 3 flood water in the river li m b Discharge (m3/s) Rising 2 mm 4 1 3 2 0 12 24 36 48 30 72 Hours from start of rain storm
  • 15. Peak flow Peak flow Maximum 3 discharge in the river limb Discharge (m3/s) Rising 2 mm 4 1 3 2 0 12 24 36 48 30 72 Hours from start of rain storm
  • 16. Recession limb Peak flow Falling flood 3 water in the limb river Re Discharge (m3/s) ce Rising ss 2 ion mm li m b 4 1 3 2 0 12 24 36 48 30 72 Hours from start of rain storm
  • 17. Basin lag time Basin lag time Peak flow Time 3 difference between the limb Re ce peak of the Discharge (m3/s) Rising ss 2 io rain storm and nl im mm the peak flow b 4 1 3 of the river 2 0 12 24 36 48 30 72 Hours from start of rain storm
  • 18. Base flow Basin lag time Peak flow Normal 3 discharge of limb the river Re ce Discharge (m3/s) Rising ss 2 io nl im mm b 4 1 3 2 Base flow 0 12 24 36 48 30 72 Hours from start of rain storm
  • 19. Basin lag time Peak flow 3 limb Re ce Discharge (m3/s) Rising ss Overland 2 io flow nl im mm b 4 Through flow 1 3 2 Base flow 0 12 24 36 48 30 72 Hours from start of rain storm
  • 20. Overland flow = surface run off Volume of water reaching the river from surface run off Through flow Volume of water reaching the river through horizontal movement of water through the soil and underlying rock layers
  • 21. • After a period of heavy rainfall, the discharge increases. • The lagtime is the difference in time between the peak of the rainstorm and the peak of the discharge. The shorter this is and the steeper the rising limb the greater the risk of flooding. • Complete task 1