Flood hydrographs can be used to predict flooding by showing how different levels of precipitation affect a river during a storm.
A flood hydrograph shows the amount of rainfall in an area and the discharge of a river. The discharge of a river is the volume of water passing a point each second. It is expressed in cumecs (cubic metres per second). River discharge is displayed as a line graph. Precipitation is shown as a bar graph and is usually displayed in millimetres.
Hydrographs can be different shapes. The characteristics of the river and how likely it is to flood affects its shape.
A gentle hydrograph shows the river is at low risk of flooding. These types of hydrograph have a gentle rising limb and a long lag time which means it takes longer for the peak rainfall to reach the river channel, so the river discharge is increasing slowly.
A gentle hydrograph
Flashy hydrographs have a steep rising limb and a small lag time. This indicates the river discharge increases rapidly over a short period of time, indicating rainwater reaches the river very quickly. This means the river is more likely to flood.
A flashy hydrograph
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