What this lesson teaches
I can explain natural and human influences on the pathways water takes through a drainage basin.
Syllabus 9173, Topic 2.1, 2. Drainage Basin Hydrology. How do pathways in the drainage basin system vary in the tropics?
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
A town paves over fields and adds storm drains. How will its river respond to the next big storm?
- It will peak higher and sooner.
- The river will stop flowing during storms.
- It will not change, because the rain is the same.
- It will peak lower and later.
Show the answer
It will peak higher and sooner.
Paving stops water soaking in, and drains rush it to the river. More water arrives faster, so lag time shortens and the peak rises.
A storm hydrograph shows river flow over time, and a short lag with a high peak suggests water reached the river fast.
A storm hydrograph plots discharge, the river's flow rate, against time. The rising limb shows flow increasing as storm water arrives. Peak discharge is the highest flow. The recession limb shows flow falling as stored water drains away more slowly.
Compare it with rainfall from the same storm. Peak rainfall is the heaviest rain shown. Lag time is the gap between peak rainfall and peak discharge.
Put these in order
Follow a heavy storm through a river's hydrograph. Put the steps in order.
- Fast overland flow reaches the river first.
- Flow falls slowly on the recession limb as stored water drains away.
- The river reaches peak discharge after a lag time.
- Discharge rises steeply on the rising limb.
- Rain falls on the basin and reaches its peak intensity.
Show the answer
- Rain falls on the basin and reaches its peak intensity. Some water runs straight over the ground and through drains, so it reaches the channel quickly.
- Fast overland flow reaches the river first. Discharge starts to climb as this water arrives, forming the rising limb.
- Discharge rises steeply on the rising limb. Water from farther away and from throughflow keeps arriving until the most water is passing at once.
- The river reaches peak discharge after a lag time. Once the fast water has passed, slower water from soil and groundwater keeps draining in.
- Flow falls slowly on the recession limb as stored water drains away.
Heavy or long rain, saturated soil, impermeable ground and good drainage give a steep rising limb and a high, early peak. Urban drains shorten travel time. Wetlands, permeable ground and empty reservoirs slow water down and lower the peak.
Basin size and shape matter too. Water from a round, compact basin tends to arrive together. Water from a long, thin basin arrives more spread out. But where the storm falls and how wet the ground was can change this.
- Rising limb
- Flow increasing after rain.
- Lag time
- Time from peak rainfall to peak discharge.
- Recession limb
- Flow falling as stored water drains.
Step by step
Find both peaks
Read peak rainfall and peak discharge separately.
Measure the lag
Peak discharge time minus peak rainfall time.
Explain the shape
Link it to pathways, storage and the storm.
Worked example: A four-hour lag
In the example hydrograph, the heaviest hourly rain falls at hour 2, and discharge peaks at hour 6. The lag time is 4 hours. The graph alone cannot tell us whether paving, wet soil or something else caused it.
Watch out for this
Peak rainfall and peak discharge use the same units.
Rain is usually mm per interval; discharge is cubic metres per second. Compare their timing, not their numbers.
Check your understanding
Rainfall peaks at 14:00 and discharge peaks at 19:00. What is the lag time?
- 33 hours.
- 19 hours.
- 5 hours.
Show the answer
5 hours.
Yes. 19:00 minus 14:00 is 5 hours.
Check your understanding
Which change would shorten lag time and raise peak discharge?
- Planting forest across the basin's upper slopes.
- Restoring wetlands on the floodplain near the river.
- Paving much of the basin with impermeable surfaces.
Show the answer
Paving much of the basin with impermeable surfaces.
Water runs off quickly instead of soaking in.