GCSE-ready- Home
- Lessons
- GCSE & IGCSE Geography A
- Rivers: flooding, hydrographs and defences
Rivers: flooding, hydrographs and defences
🎬 The doodle video for this lesson is coming soon. Subscribe on YouTube to see it first.
When a river cannot hold all its water, it floods. Both nature and people decide how quickly that happens.
Causes of flooding
Physical: heavy or prolonged rain, saturated soil, impermeable rock, steep slopes, snowmelt.
Human: urbanisation (tarmac and concrete are impermeable, and drains carry water quickly to the river), deforestation (less interception), and some farming practices that compact the soil.
All of these make water reach the river faster, so the river level rises quickly.
Human: urbanisation (tarmac and concrete are impermeable, and drains carry water quickly to the river), deforestation (less interception), and some farming practices that compact the soil.
All of these make water reach the river faster, so the river level rises quickly.
Storm hydrographs
A hydrograph shows river discharge after a storm.
Lag time = time of peak discharge − time of peak rainfall.
A short lag time and high peak mean a higher flood risk. Urban, steep and saturated basins give 'flashy' hydrographs.
Base flow is the normal level of the river, fed by groundwater.
Lag time = time of peak discharge − time of peak rainfall.
A short lag time and high peak mean a higher flood risk. Urban, steep and saturated basins give 'flashy' hydrographs.
Base flow is the normal level of the river, fed by groundwater.
Managing floods
Hard engineering: dams and reservoirs store water and release it slowly; channelisation straightens and deepens the river so water flows away faster (but may move flooding downstream); embankments raise the banks.
Soft engineering: flood-plain zoning keeps buildings off land most at risk; washlands are areas of floodplain deliberately allowed to flood to store water; planting trees increases interception.
Soft engineering: flood-plain zoning keeps buildings off land most at risk; washlands are areas of floodplain deliberately allowed to flood to store water; planting trees increases interception.
Peak rainfall was at 9:00 and peak discharge at 21:00 on the same day. Find the lag time.
- Lag time = 21 − 9
Answer: 12 hours
Floods are caused by heavy rain, saturated or impermeable ground, steep slopes, urbanisation and deforestation. Lag time = peak discharge time − peak rainfall time; shorter lag means more risk. Dams, channelisation and embankments are hard engineering; zoning and washlands are soft.
Check you have got it
Answer 6 quick questions with instant marking. If you get one wrong, GCSE-ready shows you why and gives you another go. It is free, and you do not need an account.