Storm Hydrographs

A hydrograph shows the discharge of a river at a given point over a period of time.

The hydrograph is used to show how a particular river reacts to a storm.

Because very little of the precipitation that falls will fall directly into the river channel, the discharge will not increase immediately.

It is only when the water that has been affected by some of the processes below, that it will enter the river channel.

Processes:

Interception – the absorption of water by vegetation

Throughflow

Stemflow - the flow of water down the trunk or stem of a plant

Infiltration – the process where water enters the soil

Runoff

Groundwater

The different processes reach the river channel in differing amounts of time.

Surface runoff is the fastest process, then

Throughflow

Followed by groundwater

Infiltration

Stemflow

Interception

When the water reaches the river channel, the discharge of the river will INCREASE this is shown on a hydrograph by the RISING LIMB

The difference between the time of peak rainfall and peak discharge is shown as the LAG TIME

A river with a short lag time and a high discharge is more likely to flood, than a river with a longer lag time and a low discharge.

The lag time depends on many different factors;

• Relief

• Rock Type

• Soil

• Vegetation

• Land Use

• River Use

Storm Hydrographs

The discharge levels of the River Wyre changed dramatically between the 18 th and 19th of December. This is clearly the result of heavy rainfall affecting the region during that time.

Before the first fall of rain, the discharge levels remained quite low and steady, at approximately 4 cumecs, and remained like this for at least 9 hours, some 4 hours after the first fall of rain.

Continued heavy rainfall has created a steep rising limb on the hydrograph which reaches a peak at about 74 cumecs.

The river continues to flow at a high level for several hours more before falling steeply.

A later period of rainfall, although not as heavy as the previous spell, during the evening of the 18 th explains the secondary peak in discharge, this time reaching approximately 40 cumecs in the early hours of the 19 th.

It stopped raining around midnight on the 18th/19th. This is reflected in the gradual lowering of water levels which eventually even off and almost return to the level they were at before the rain started.

The reason for the differences between peak rainfall and peak discharge, may be due to a number of different factors and processes.

The lag time works out at approximately 4 hours, which means that it has taken at least 4 hours from the time that it started raining for the rainwater to reach the River Wyre. When precipitation reaches the ground, it can be affected by several processes which take differing amounts of time.

The precipitation may be infiltrated into the soil layer before eventually reaching the river channel, this is a slow process that could account for the time difference. As well as this the precipitation may have become intercepted by vegetation. The area around the River Wyre may be well vegetated and therefore much of the rainwater has been either intercepted or has been affected by stemflow. Also geological factors such as rock type may have played a role. If the rocks surrounding the river Wyre are impermeable, the water might percolate through the cracks in the rocks, or it may become part of the groundwater.

Relief is another important factor, if the land is very steep, then the precipitation should reach the river channel quicker than if the land was very flat. I would suggest that the surrounding land of the River Wyre is reasonably flat, and therefore this has affected the time taken for the rainwater to reach the river channel. All of these factors are accountable for explaining the difference in peak rainfall and peak discharge in the River Wyre during the 18th and 19th of December.