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Key words

  1. Discharge - The quantity of water that passes a given point on a stream or river‐bank within a given period of time.
  2. Flood - Occurs when river discharge exceeds river channel capacity and water spills out of the channel onto the floodplain and other areas.
  3. Hydrograph - A graph which shows the discharge of a river, related to rainfall, over a period of time
  4. Precipitation - Moisture falling from the atmosphere − as rain, hail sleet or snow.

A storm hydrograph is a way of displaying how the discharge of a river can change over time in response to a rainfall event. They show the RELATIONSHIP between precipitation events and the volume of water in a river. The discharge of a river is just the amount of water passing a certain point every second, and is calculated by multiplying the cross sectional area of the river by the velocity. Because the cross section is measured in metres2 and the velocity is measured in metres per second the discharge is measured in metres 3 per second. These units are known as CUMECs (CUbic Metres per sECond).  The key features of a hydrograph are shown below;

Model hydrograph

The storm hydrograph

The graph shows base flow which are the contributions made to the river via soil and ground water flows. These will be ever present on the graph unless there is a long extended period without any rainfall. The runoff or storm flow is the water that arrives in the river via surface runoff or rapid throughflow through the rock. The rising limb gives an indication of how fast water is reaching the channel and represents the level of water rising in the channel. The steeper the rising limb the more likely a flood is to occur; this is vital knowledge for flood forecasters. The falling limb shows the river as its level falls. Peak discharge is the maximum amount of water in a river after a rainfall event, if this level surpasses the bankfull discharge then a flood will occur where the river overtops its banks. The last item indicated on the hydrograph is the lag time, this is the amount of time between the peak amount of rainfall and the peak discharge in the river. Generally, the less the lag time the quicker the river rises, the more FLASHY the graph and the more likely a flood.  Graph A below is more likley to flood, whilst graph B is less likely


Hydrograph types

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