What this lesson teaches
I can apply fieldwork to fluvial flood risk and mitigation strategies.
Syllabus 9173, Fieldwork. Students should devise geographical questions or hypotheses and follow through the fieldwork in the following areas: Fluvial flood risk and strategies to mitigate it.
I can draw a supported conclusion and evaluate the data and the methods used to collect and present it.
Syllabus 9173, Fieldwork, 3. Post-fieldwork stage. Students should be able to: draw conclusions in relation to the question/hypothesis posed; Evaluate the validity of the fieldwork in terms of data collected and methods used to collect and present data.
Make a guess
Discharge rises from upstream to downstream along a canal, and so does built-up land. Does that prove the buildings cause the extra flow?
- Yes, because built-up land floods.
- No, because discharge stays the same downstream.
- Yes, because both rise together.
- No. Tributaries also add flow downstream.
Show the answer
No. Tributaries also add flow downstream.
Discharge naturally grows as more water joins the channel. A pattern linking two things does not prove one causes the other.
Flood risk needs evidence of the hazard, such as discharge and freeboard, and of exposure, such as buildings close to the bank, collected over more than one day.
This lesson gives you a complete practice resource pack for the H2 fieldwork question on fluvial flood risk. Sungei Pelangi and all the data are invented. Read the map, the field sketches, the method and the results before you answer.
Five students tested this hypothesis: "Flood risk along Sungei Pelangi increases downstream as the land around the canal becomes more built-up." They chose four sites, A to D, from the park upstream to the town centre downstream. Around each site, they used satellite images to classify a 100 m by 100 m grid of 25 squares as built-up, green or water.
At each site, working only from bridges, they measured channel width with a tape and depth at three points with a weighted rope. They timed a float over 10 m three times to estimate surface velocity, and calculated discharge as width times mean depth times velocity. They measured freeboard, the height from the water surface to the top of the bank. All readings were taken on one dry weekday morning in February.
Most results fit the hypothesis. Built-up land rises from 28% at Site A to 88% at Site D. Discharge rises from 0.36 to 1.89 cubic metres per second, and freeboard falls from 2.1 m to 1.2 m. Buildings stand 45 m from the bank at A but only 8 m at D, so more people and property are exposed. Site B is an exception: its freeboard is 2.3 m, higher than at A, because the bank was raised when a new road was built.
The limits are serious. A dry-day visit shows normal flow, not storm flow, so it cannot show how close the canal comes to overflowing. A surface float moves faster than the water near the bed and walls, so velocity and discharge are overestimated. Discharge also rises downstream simply because more tributaries and drains join, not only because of land use. Records of past floods and the water-level sensor at Site D would add evidence of real high-water events.
Sort these
Tag each part of the Sungei Pelangi method: what kind of weakness is it, or is it a strength?
Groups: Timing flaw, Measurement flaw, Evidence missed, Strength.
- Depth was measured at only three points across the channel.
- All readings were taken on one dry weekday in the drier phase of the Northeast Monsoon.
- Past floods came only from news articles.
- The water-level sensor at Site D was available but not used.
- Each float was timed three times at each site and averaged.
- Velocity came from a float timed on the surface over 10 m.
- Freeboard was measured with a pole held from the bridge.
Show the answer
Timing flaw
- All readings were taken on one dry weekday in the drier phase of the Northeast Monsoon. The data shows normal low flow, not the storm flows that cause floods, so it cannot measure flood risk directly.
Measurement flaw
- Depth was measured at only three points across the channel. Three points may miss the deepest part of the channel, so the mean depth and the discharge may be inaccurate.
- Velocity came from a float timed on the surface over 10 m. Surface water moves faster than the average across the channel, so velocity and discharge are probably overestimated.
- Freeboard was measured with a pole held from the bridge. A bridge may not be typical of the banks, and a pole held from a height is hard to keep upright, which lowers accuracy.
Evidence missed
- Past floods came only from news articles. News reports cover mainly large or dramatic floods, so smaller floods may be missing from the record.
- The water-level sensor at Site D was available but not used. Sensor records would show how high the water rises in storms, the evidence the hypothesis most needs.
Strength
- Each float was timed three times at each site and averaged. Repeating a reading reduces the effect of one odd timing, which makes the velocity more reliable.
- Freeboard
- Height from the water surface up to the top of the bank.
- Discharge
- Width x mean depth x velocity, in cubic metres per second.
- Flood risk evidence
- Hazard (flow, freeboard) plus exposure (people and buildings nearby).
Step by step
Describe the trend
Quote figures for A to D, and name the exception.
Check what was measured
Normal flow on one dry day, not storm flow.
Weigh other causes
Tributaries and drains also add flow downstream.
Judge and improve
How far is the hypothesis supported, and what evidence is missing?
Worked example: Use the exception
Site B breaks the pattern: built-up land is higher than at A (52% against 28%), yet freeboard is higher too (2.3 m against 2.1 m). The map shows a new road built in 2024 where the bank was raised. So land use is not the only control on flood risk. Engineering can change it, which a strong evaluation should point out.
Watch out for this
Discharge rises downstream, so built-up land must be causing it.
Discharge rises downstream in most rivers because tributaries and drains join. The data show an association with land use, not proof that land use is the cause.
Check your understanding
Why can the dry-day readings not show the true flood risk at Site D?
- They show normal flow, not how high water rises in storms.
- Site D has more built-up land around it than Site A.
- The students measured the channel width with a tape measure.
Show the answer
They show normal flow, not how high water rises in storms.
Yes. Flood risk depends on high flows, which a dry morning cannot capture.