What this lesson teaches
I can explain what causes fluvial floods, using rainfall, discharge and basin conditions.
Syllabus 9173, Topic 2.1, 3. Floods in the Humid Tropics. What factors contribute to fluvial flooding in the humid tropics?
Make a guess
Two storms bring exactly the same rain to a basin. The second comes after a week of heavy rain. Which is more likely to flood?
- Neither, because floods depend on yearly totals.
- Both equally, because the rain is the same.
- The first, because dry ground floods more easily.
- The second, because the ground is already full.
Show the answer
The second, because the ground is already full.
Saturated soil and high groundwater leave little space to store more. More water runs quickly to the river, so discharge can exceed the channel.
A river flood happens when discharge is more than the channel can hold, so water spills onto normally dry land.
Bankfull discharge is the most water a channel can carry without spilling over. A fluvial flood happens when the river overtops its banks and covers normally dry land nearby. Flooding from overwhelmed street drains is a different type, though both can happen together.
In the humid tropics, monsoon rain and tropical cyclones can drop huge amounts of water over hours or days. Conditions before the storm matter. Saturated soil and high groundwater leave little space to store more. When peaks from several tributaries arrive together, downstream flow soars.
Impermeable ground, steep slopes and many streams deliver water fast. Sediment, debris or a narrow channel reduce how much the river can hold. High water downstream can also block drainage. A large yearly rainfall total alone does not predict when a flood will happen or how big it will be.
People change both delivery and capacity. Paving and drains speed runoff. Clearing forests and packing soil reduce infiltration. Building on floodplains removes storage and puts more in harm's way. Dams can lower peaks if they have space, but badly timed releases add to floods. So explain the specific process, rather than treating all development as the same.
- Bankfull discharge
- The most flow a channel can hold.
- Fluvial flood
- River overtops its banks onto dry land.
- Antecedent conditions
- How wet the basin was before the storm.
Step by step
More water in
Rain and wet ground cause fast runoff.
Check capacity
Narrow channels, sediment or blockages.
Compare the two
Flooding starts when flow exceeds capacity.
Real examples to use
Thailand, 2011
Heavy monsoon rain and tropical storms caused months of flooding in the Chao Phraya basin. Over 800 people died.
Worked example: A capacity test
River A can carry 90 cubic metres per second within its banks. A storm peak of 110 floods it. River B can carry 150, so the same peak of 110 stays inside. Flooding depends on each channel's capacity.
Watch out for this
A big population causes the rain that makes a flood.
People change runoff and exposure. The weather makes the rain.
Check your understanding
Two storms bring the same rain, but the second follows a week of heavy rain. Why might its flood be bigger?
- Water moves faster under gravity on wet ground.
- Saturated ground and full stores send more water quickly.
- The second storm's rain falls more heavily.
Show the answer
Saturated ground and full stores send more water quickly.
Yes. There is less space to store the new rain.