What features on a hydrograph indicate a high risk of flooding?

A high risk of flooding on a hydrograph is indicated by a steep rising limb, high peak discharge and short lag time.

A hydrograph is a graph that shows the discharge of a river, in other words, the volume of water flowing in a river, over a period of time. It is a useful tool in predicting the likelihood of flooding. There are several features on a hydrograph that can indicate a high risk of flooding.

Firstly, the rising limb of the hydrograph, which represents the increase in river discharge following rainfall, can be a key indicator. If the rising limb is steep, it means that the river level is rising rapidly. This could be due to heavy rainfall or rapid snowmelt. A steep rising limb can indicate that the ground is already saturated, meaning it cannot absorb any more water, leading to a higher risk of flooding.

Secondly, the peak discharge, which is the highest point on the graph and represents the maximum amount of water flowing in the river, can also indicate a high risk of flooding. If the peak discharge is high, it means that the river is carrying a large volume of water. If the river's capacity is exceeded, it can lead to flooding.

Lastly, the lag time, which is the time difference between the peak rainfall and peak discharge, can also be a significant indicator. A short lag time means that the peak discharge occurs soon after the peak rainfall. This can indicate that the ground is already saturated or that the drainage basin responds quickly to rainfall, both of which can increase the risk of flooding.

In conclusion, by analysing the features of a hydrograph, such as the steepness of the rising limb, the height of the peak discharge and the length of the lag time, one can predict the likelihood of a flood event. Understanding these features can help in flood management and prevention.