Flooding

What is meant by a river flood?

A river floods when the level of the water

exceeds bankfull and flows onto the

floodplain.

Floodplain

Rivers regularly flood. When they do so they

deposit some of their load. This material is

known as silt.

The land on either side of a river that floods

is known as its floodplain.

Floodplain

Higher land

Floodplain

Notice how brown the water is. It is this colour because of the silt it is carrying..

Our school is on the floodplain of the River Thames but is protected by flood defences

Edge of floodplain

The Thames

Dagenham Park

Dagenham Heathway

Floodplain worksheet

Complete the floodplain worksheet

Flood hydrographs

These are used to show the relationship

between rainfall and the discharge of a river

Discharge is the volume of water flowing

past a point in a river during a period of time.

It is usually measured in cubic metres per

second i.e. cumecs

TIME

DIS

CH

AR

GE

(cu

mec

s)

FLOOD HYDROGRAPH

TIME days or hours

DIS

CH

AR

GE

(cu

mec

s)

FLOOD HYDROGRAPH

Bankfull level

Lag time Peak discharge

Peak rainfall

Key terms

• Peak rainfall

• Peak discharge

• Lag time

• Bankfull level

Write out simple definitions of these terms.

• A hydrograph shows us the difference between the peak rainfall and peak discharge. This difference is called lag time. The greater the lag time, the less chance there is of a flood; while a short lag time shows that water has reached the river channel quickly.

• A rise in discharge on the graph is called the rising limb; the falling limb shows a decrease in discharge.

• The flood hydrograph below shows:• rainfall peaking at six hours • discharge peaking at 11 hours • lag time - the time between peak rainfall and peak

discharge - is five hours.

There is a lag time because _______________________________________________________________________________

Extension for the clever onesThere are two peaks to my hydrographbecause ________________________________________________________________________________________________________________________________

Deforestation and flooding

Rock

Trees

River

Soil

When it rains water falls to the ground. In a forested

drainage basin the trees ___________________________

_______________________________________________

_______________________________________________

_______________________________________________

_______________________________________________

_______________________________________________

_______________________________________________

Less water reaches the river and the water that does arrive

takes a long time to reach it. This means that the river has

A chance to discharge the rainwater and so flooding is less

likely.

intercept transpiration roots

Rock

Concrete

Drains

Urbanisation and flooding

River

TIME

DIS

CH

AR

GE

(cu

mec

s)

FLOOD HYDROGRAPH

Bankfull level

Lag time Peak discharge

Peak rainfall

Explain why there a time lag between peak

rainfall and peak discharge?

Use your diagram of the drainage basin to

help you.