Does velocity increase downstream?
Compare river velocity at three safe, accessible sites.
The method
Time a float over 5 metres three times at each site. Use the mean travel time to estimate surface velocity.
| Site | Time 1 (s) | Time 2 (s) | Time 3 (s) |
|---|---|---|---|
| Upper | 12 | 14 | 13 |
| Middle | 10 | 9 | 11 |
| Lower | 7 | 8 | 6 |
Choose a presentation
Plot site order or distance downstream against estimated velocity. Do not plot the travel times and label them as velocity.
Follow a limitation through the enquiry
- Limitation
- A float measures surface movement and can catch on obstacles.
- Effect on the evidence
- A delayed float gives a longer travel time and underestimates velocity. This could distort the downstream pattern.
- A useful improvement
- Repeat measurements across safe parts of the channel and record obstructions. A suitable calibrated meter could measure velocity at specified depths, with trained supervision.
Try it yourself
Explain why the proposed improvement would help this enquiry. Then identify something it would still not establish.
One possible answer
Repeat measurements across safe parts of the channel and record obstructions. A suitable calibrated meter could measure velocity at specified depths, with trained supervision. This strengthens the evidence for the stated enquiry, but a small study still cannot represent every place or date.
Check your answer: did you address the question, use accurate evidence and explain the link?
Change the conditions
Imagine the same enquiry on another date or at another site. Which part of the method would you keep consistent? Explain why, using the enquiry aim.
What to look for
A useful answer identifies a specific measurement, sampling rule or time condition and explains how consistency makes the comparison fairer.