River fieldwork: a conclusion under suspect conditions
A group of six students investigated downstream changes in the River Skell at three sites 2 km apart. At each site they measured channel width with a tape, depth at five points across a tape, and velocity with a float over 10 m, timed three times. They sampled 10 pebbles at each site and measured the long axis. Results: site 1 width 4.2 m, mean depth 0.35 m, mean velocity 0.42 m/s; site 2 width 7.8 m, mean depth 0.55 m, mean velocity 0.61 m/s; site 3 width 11.5 m, mean depth 0.72 m, mean velocity 0.78 m/s. At site 3 the measurements were taken 18 hours after 22 mm of rain. One group used a lighter float at site 2, and pebbles were collected only from the channel edge at all sites. The group concluded that discharge increases downstream and that channel efficiency increases because the channel becomes wider and deeper.
1. Calculate discharge at each site and assess whether the data support the conclusion that discharge increases downstream.
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Use Q = width × mean depth × mean velocity. Site 1: 4.2 × 0.35 × 0.42 = 0.62 m³/s. Site 2: 7.8 × 0.55 × 0.61 = 2.62 m³/s. Site 3: 11.5 × 0.72 × 0.78 = 6.46 m³/s. The calculated discharge rises downstream, so the broad conclusion is supported. However, the comparison is not robust. Site 3 was measured 18 hours after 22 mm of rain, so it was probably in recession flow and not equivalent to the other sites. The float measures surface velocity, which is faster than mean velocity, and a lighter float at site 2 would be affected more by wind. The depth was measured at only five points, and the pebbles were sampled only at the edge. The data therefore show a pattern but do not prove a clean downstream increase under comparable conditions.
2. Evaluate the reliability of the conclusion that channel efficiency increases downstream.
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Efficiency means the channel's ability to convey water for a given slope and roughness. The width and depth do increase downstream, which would raise the hydraulic radius and usually increase efficiency. Velocity also increases, but this is the weakest evidence because surface floats overestimate mean velocity and the float type changed at site 2. Site 3 was measured after rain, so its high velocity may reflect a higher discharge rather than a changed channel shape. Only three sites and one cross-section per site were used, and edge pebbles are not representative of the bed load. The conclusion is plausible but not reliable enough to accept without qualification. A repeat should control weather, use a flow meter at 0.6 depth, survey several cross-sections and sample pebbles randomly across the channel.
3. Suggest improvements for a repeat investigation.
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Measure all three sites on the same day in similar weather, and record rainfall and water level so that spate conditions can be excluded. Use a current meter or an impeller at 0.6 of the depth at several points across each cross-section, rather than a float. Survey at least three cross-sections per site and calculate cross-sectional area and discharge properly. Sample pebbles randomly across the channel, not only at the edge, and record at least 50 pebbles per site to characterise bed load. Add more sites to identify a trend rather than a three-point pattern. Repeat the measurements on at least two occasions and calculate means and ranges. These changes would improve precision, representativeness and comparability, allowing a stronger judgement about downstream change.