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Storm Hydrographs -Show the change in river
  discharge caused by a period of rainfall.
Construct & Analyse Hydrographs ?

  To find out discharge patterns
of a particular drainage basin

  Help predict flooding events,
  therefore influence
  implementation of flood
  prevention measures
                                                     Rainfall shown
                   3                                  in mm, as a
                                                       bar graph
Discharge (m3/s)


                   1        3


                       0         12   24   36   48   30   72
                           Hours from start of rain storm
                                                     Discharge in
                                                      m3/s, as a
                                                      line graph
Discharge (m3/s)


                   1        3


                       0         12   24   36   48   30   72
                           Hours from start of rain storm
The normal
(base) flow                                                The time it
   of the              Basin lag time
                                        Peak flow
                                                         takes for the
    river                                                water to find
 starts to    3                                          its way to the
 rise when
ground and    2
 soil water            mm
  reaches          4
                                                           shows that
 the river.   1    3                                         water is
                   2                                           still
    Maximum                                                  reaching
discharge in the
                                                             the river
 river, the time
               0         12    24       36   48     30    72
 when the river                                               but in
   reaches its                                             decreasing
  highest flow                                               amounts
The peak rainfall is the time of highest rainfall.
The peak discharge (the time when the river
reaches its highest flow) is later because it takes
time for the water to find its way to the river (lag
time – from max precipitation to peak discharge) .
Normal (base) flow of the river starts to rise
(rising limb) when run-off, ground and soil water
reaches the river.

The falling limb shows that water is still reaching
the river but in decreasing amounts.
          Factors influencing
          Storm Hydrographs

• Area                 • Land Use
• Drainage Basin Shape • Drainage Density
• Slope                • Precipitation / Temp
• Rock Type
• Soil
•rain water does not all fall directly into
the river
•water soaks into the ground
•run-off takes time to reach the river
•water can be intercepted by vegetation
•water can be stored in soil and rock
How are the following variables going to
     effect the runoff/discharge?
Were you right?!
•Channel flow can be faster down a steep slope
therefore steeper rising limb and shorter lag time
Land Use
•Urbanisation - concrete and tarmac form
impermeable surfaces, creating a steep rising limb
and shortening the time lag
Drainage Density
•A higher density will allow rapid overland flow and
therefore a steeper rising limb.
• Elongated basin will produce a lower peak flow and
longer lag time than a circular one of the same size
    Match up the correct land use/rock type with
              the correct hydrograph

•Permeable rocks mean rapid infiltration and little
overland flow therefore shallow rising limb
•   Infiltration is generally greater on thick soil

•The more infiltration occurs the longer the lag
time and shallower the rising limb
         Describing Hydrographs

 Remember, the main things you should be looking
 to describe are:

•Rising Limb
•Recession Limb
•Lag time
•Rainfall Intensity
•Peak flow compared to Base flow
•Recovery rate, back to Base flow

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