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Stage 1: Before you start

Stage 1: Background information | Questions to investigate

Background information

The storm hydrograph

The storm hydrograph (also known as the flood hydrograph) is a graph on which river discharge (y-axis) during a storm, or single precipitation event, is plotted against time (x-axis). They can be used to predict a flood risk.

 

The first stage of a storm hydrograph shows discharge before precipitation. From the beginning of the storm, river discharge rises from the approach segment until it reaches peak discharge.

The lag time is the time it takes between the peak of rainfall and the peak of discharge. The rising limb is the period during which discharge increases. The descending limb (some textbooks use the term falling limb) is the period during which discharge decreases. Note that the rising limb is always steeper than the descending limb.

Falling rain can reach the river channel in three main ways.

• Overland flow - rainwater which flows over the ground surface.
  
• Throughflow - rainwater which has infiltrated into unsaturated soil can move horizontally to the river channel. This process is slower than overland flow but faster than baseflow.
  
• Baseflow - rainwater which has percolated to the aquifer can seep into the river channel. This is the slowest process.

The level of discharge before the storm is called the antecedent discharge. This tends to be much higher in winter than summer.

Initially at the start of a rain storm, discharge hardly changes as very little rain actually falls into the channel itself. As the discharge increases, a larger proportion is throughflow. Near to peak discharge, a significant proportion of the water will have reached the channel by overland flow. After the peak discharge has passed, the descending limb is less steep because discharge no longer contains any element of surface runoff. The discharge returns to baseflow level.

Factor	Effect on shape of the storm hydrograph
Length of storm	Lag time is shorter and peak is higher after longer periods of rainfall. Antecedent moisture levels are higher.
Intensity of rainfall	Lag time is shorter and peak is higher after intense (rapidly falling) rain. Surface runoff starts more quickly that with less less intense rainfall.
Afforestation	Lag time is slower and peak is lower in a heavily forested basin. There is more interception. Intercepted rainfall is either evaporated from leaves, or reaches the ground slowly by stemflow and drip.
Soil texture	Lag time is slower and peak is lower in areas of sandy soil compared to clay soil. Surface runoff starts more quickly because clay soil has smaller pore spaces, and has a lower infiltration rate.
Rock type	Lag time is shorter and peak is higher in an area of impermeable rock type, like granite. Surface runoff starts more quickly.
Urbanisation	Lag time is faster and peak is higher in a heavily urbanised catchment. Impermeable surfaces (such as tarmac) have a much lower infiltration rate than naturally vegetated surfaces. Surface runoff starts more quickly.

Questions to investigate

• How do contrasting drainage basin characteristics influence the rainfall - runoff relationship of a river? (i.e. storm simulation experiments).
• How do urban and grassy surfaces differ in their response to floods / rainfall?
• How does land use impact on infiltration rates?

These could all be combined into a single investigation into flooding.

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