A river fieldwork investigation: discharge and an anomaly
A class tests the hypothesis that river velocity and discharge increase downstream on the Ballymore River. Three sites were sampled 1.2 km apart on one day. At each site the students measured width with a 30 m tape, depth at five points with a metre rule, and timed an orange float over 10 m three times.
| Site | Width (m) | Mean depth (m) | Float velocity (m/s) |
|---|---|---|---|
| 1 | 2.8 | 0.25 | 0.48 |
| 2 | 4.6 | 0.38 | 0.62 |
| 3 | 5.9 | 0.52 | 0.31 |
At Site 3 a tributary enters 40 m upstream, carrying grey water from a quarry settling pond. The float was timed near the left bank, where a fallen tree slowed the flow. At Site 2, depths were measured from a bridge deck 1.3 m above the bed because the water was too deep to wade. The class had one day, no flow meter, and three repeats per site. They calculated discharge as cross-sectional area multiplied by velocity, but did not measure velocity at different depths.
1. Calculate cross-sectional area and discharge for each site. Use A = width × mean depth and Q = A × velocity, and give answers to two decimal places.
Read the worked answer
Site 1: A = 2.8 × 0.25 = 0.70 m²; Q = 0.70 × 0.48 = 0.336 → 0.34 m³/s. Site 2: A = 4.6 × 0.38 = 1.748 → 1.75 m²; Q = 1.748 × 0.62 = 1.08376 → 1.08 m³/s. Site 3: A = 5.9 × 0.52 = 3.068 → 3.07 m²; Q = 3.068 × 0.31 = 0.95108 → 0.95 m³/s.
The results do not support a simple downstream increase. Discharge rises from Site 1 to Site 2 (0.34 to 1.08 m³/s), but falls at Site 3 to 0.95 m³/s even though width and depth are greatest there. Velocity rises from 0.48 to 0.62 m/s and then falls sharply to 0.31 m/s. Site 3 is the anomaly to explain.
2. Explain why Site 3's results do not fit the hypothesis, using the fieldwork details.
Read the worked answer
Discharge should normally increase downstream because tributaries add water. A tributary does enter 40 m upstream of Site 3, so the cross-sectional area (3.07 m²) is large. But Q = A × v, and the measured velocity is only 0.31 m/s. That low velocity is probably a measurement artefact, not a real fall in the river's discharge.
The float was timed near the left bank where a fallen tree slowed the flow. A float follows the fastest surface path only if placed in it; near a bank and behind an obstruction it can enter an eddy or slack water, so it underestimates the main channel velocity. The float also measures surface velocity, which is usually higher than mean velocity, but here the local effect dominates. A single float path cannot represent the whole cross-section; velocity should be measured at several points across the channel and at 0.6 of the depth.
The quarry settling pond may also affect the reading: grey water can indicate suspended sediment, and the tributary may create turbulence or a density difference, but this is secondary to the sampling position. The bridge at Site 2 made depth measurement safer, but at Site 3 the team may have avoided the deepest part, biasing mean depth. So the hypothesis is not disproved; the Site 3 velocity is unreliable.
3. Evaluate the reliability of the investigation and suggest two improvements.
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Reliability is limited. The study used one river, one day and three sites, so results cannot be generalised. Each site had only three float repeats, and the float method measures surface velocity, not mean velocity, so Q is likely overestimated if the float travels in the fastest thread. The team had no flow meter and did not measure velocity at different depths or across the channel. At Site 2, depths were taken from a bridge 1.3 m above the bed, which is safe but may miss the deepest part; at Site 3, sampling avoided the fastest water, creating the anomaly. There is no mention of repeating the whole survey or checking whether discharge changed during the day.
Improvements: (1) Use a current meter or a calibrated float method, measuring velocity at 0.6 depth at five points across each cross-section and then calculating a mean velocity. (2) Repeat the survey at least three times on different days and at the same time of day, and measure discharge at a safe waded section. Also record weather and antecedent conditions.
The investigation is useful as a pilot but not reliable enough to accept or reject the hypothesis. The Site 3 anomaly should be re-measured before conclusions are drawn.