2. Flooding and flood mitigation

Syllabus link:  Hydrographs characteristics (lag time, peak discharge, base flow).

Key terms and components of the flood hydrograph

Flood hydrographs are graphs that show how a drainage basin responds to a period of rainfall.

  • Peak rainfall – time of the highest rainfall level.
  • Peak discharge – time of the highest river channel level.
  • Lag time – difference in time between the peak rainfall and peak discharge.
  • Rising limb – the increase in river discharge.
  • Falling limb – the fall in river discharge.
  • Base flow – normal river level (for a given season/time of the year).

Screen Shot 2018-08-27 at 8.07.15 pm

The hydrograph above shows a short lag time and high peak discharge.  The rising limb is very steep meaning rain that has fallen in the drainage basin has entered the river channel very quickly.  The graph shows the majority of the rainfall has travelled to the river channel via overland flow, this could be for a number of reasons including, the type of land use (if the surface is impermeable and rainfall has been intense then little infiltration can take place).  Relief (shape of the land could be steep, which also reduces infiltration rates.  

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Source:  Tutor2u

Bankfull discharge – meaning the channel cannot hold any more water, resulting in water running out over the surface.  The flood hydrograph above shows bankfull being exceeded around 43 cumecs.

Flood hydrographs can be used to collect data over a period of time which may tell planners a range of useful information that may help them ‘prepare’ for various types of floods.

Flood hydrographs can be used as models for future flooding, they can indicate and estimate both lag time (will tell us how much time we have before we may need to evacuate) and peak discharge, which provides vital information about evacuation times or the extent of possible flooding.

Additional information that is useful is knowing how much or how little rain is needed before bankfull discharge is reached.

Recurrence interval (also known as the return period)

Using flood hydrographs and recording recurrence intervals for particular magnitudes (sizes) can help governments plan, prepare and potentially predict floods of certain magnitudes.

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Source:  Author

The graph above shows the ‘recurrence’ interval (how often a specific event will take place given a particular magnitude).  If you look at the red dot, with a discharge below 1000 m3/sec, this event is likely to take place once every 2-3 years.  The blue dot with a much greater discharge of 4000 m3/sec has a return period of 50 years.  Hence, the greater the discharge (magnitude/size), the longer the return period.  

Examination question:

Explain how hydrographs are used to forecast floods (6 marks).

Syllabus link:  Natural influences on flood hydrographs.

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Source:  Author

Factors influencing discharge and the flood hydrograph (printable notes)

Examination question:

Discuss the relative importance of the factors affecting the characteristics of hydrographs (10 marks).


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