Storm hydrographs

Overview

Storm hydrograph, lag time and peak discharge.

What you need to know

Answer each one before you reveal it.

Questions from this subtopic

Each one with the model answer underneath.

1. 1

   What is a storm hydrograph, and what does each axis show?

   Model answer

   A storm hydrograph is a graph showing how the discharge of a river at a particular gauging point responds to a single rainfall event. Time in hours since the storm is plotted along the horizontal axis. Discharge in cumecs, cubic metres per second, is plotted on the left vertical axis as a curve. Rainfall in millimetres is usually plotted as vertical bars from the top of the graph downwards, against a second vertical axis on the right.

2. 2

   Describe the shape of a typical storm hydrograph and name its features.

   Model answer

   It begins with baseflow, the low, gently declining discharge before the storm. After the rain begins there is a short delay, then the rising limb climbs steeply as storm water reaches the channel. The curve reaches peak discharge, the highest flow recorded. It then declines along the falling or recessional limb, which is gentler and longer than the rising limb, until discharge returns to baseflow. Lag time is measured between peak rainfall and peak discharge.

3. 3

   Define lag time.

   Model answer

   Lag time is the interval between the time of peak rainfall and the time of peak discharge at the gauging station. It measures how long the basin takes to deliver storm water to the channel. A short lag time means water is reaching the river quickly, usually by overland flow, and warns of a rapid rise in level; a long lag time means water is arriving slowly, by infiltration, throughflow and groundwater flow.

4. 4

   What is peak discharge, and why does it not coincide with peak rainfall?

   Model answer

   Peak discharge is the maximum flow, in cumecs, recorded at the gauging point during the storm. It comes after peak rainfall because water needs time to travel to the channel - it must first be intercepted, infiltrate, move through the soil or run across the surface, and then flow down the channel from wherever it fell in the basin. Only rain falling directly on the channel arrives instantly.

5. 5

   Distinguish between the rising limb and the falling limb of a storm hydrograph.

   Model answer

   The rising limb is the section of the curve where discharge is increasing after the storm, and it is normally steep because storm water arrives rapidly by overland flow and throughflow. The falling limb, also called the recessional limb, is the section where discharge is declining after the peak, and it is gentler and takes longer because the water arriving now has travelled by slower routes and the basin stores are draining out gradually.

6. 6

   What is the difference between baseflow and stormflow?

   Model answer

   Baseflow is the steady, slow contribution of groundwater to the channel, which keeps a river flowing between storms and changes only gradually over weeks and months. Stormflow, or quickflow, is the additional water delivered rapidly during and after a rainfall event by overland flow and throughflow; it is what produces the rising limb, the peak and the falling limb on the hydrograph, and it disappears within days.

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