GCSE Geography | Flood Hydrographs (River Landscapes 9)

Water returns to rivers by surface run-off (overland flow), throughflow or groundwater flow - and the speed of this return depends on a number of physical and human factors.

River discharge is the volume of water flowing through a river channel at any given point, and is measured in cubic metres per second (cumecs). River discharge is determined by rainfall and this relationship is shown on a flood hydrograph, which shows how discharge changes after a storm event.

On a hydrograph the horizontal axis shows time (hours or days) and the vertical axis shows both rainfall (mm) and discharge (cumecs). Rainfall is shown by a bar graph and discharge is shown by a line graph.

Understanding hydrographs

The rising limb shows how fast the river level is rising, and is always steeper than the falling limb, which shows the how quickly the river level recedes after a flood event.

During a storm, water reaches the river by surface run-off over impermeable surfaces, which causes river discharge to rise quickly - hence the steep rising limb. Once the surface run-off has stopped river discharge can start to fall - this decreases at a steady rate which is why the falling limb on a hydrograph is less steep. This is because the water reaching the river is mainly flowing as throughflow (through the soil) which is slower than surface run-off. After a storm event the river will return to its base flow because the water flowing into it is from groundwater flow - this is through rocks and is a slow flow.

Hydrographs can either be 'flashy response' or 'slow flow'.

Slow flow hydrographs have a steady rising limb (actually a similar gradient to the falling limb), low peak discharge and a long lag time - this means that the flood risk here is low.

Flashy hydrographs often show sudden flood events - they have a steep rising limb caused by a high surface run-off, so water enters the river much more quickly with a short lag time - this means that the flood risk is high.

What increases the risk of a flash flood?

• prolonged rainfall leading to saturated soil, so cannot absorb anymore water
• drought conditions beforehand so the soil has become baked hard, too hard to absorb water
• soil/rock type that is impermeable, e.g. clay, so water cannot infiltrate
• steep-sided valley, so water runs down the slope rather than infiltrating into the soil
• flat flood plain, making the water hard to drain
• small river basin so the tributaries join the main river quickly, which increases discharge
• deforestation, so less leaves for interception, and less roots to absorb water
• urbanisation, so the land is covered in impermeable concrete and tarmac, increasing surface run-off