Stream-Table Sediment Sorting and Channel Response
A stream-table investigation uses two channels cut into the same sand-gravel mixture. Channel A has a slope of 0.020 m/m, width 0.40 m, average depth 0.05 m, and average velocity 0.90 m/s. Channel B has a slope of 0.005 m/m, width 0.80 m, average depth 0.10 m, and average velocity 0.225 m/s. The sediment mixture contains 2-mm gravel, 0.5-mm sand, and 0.02-mm silt. After 10 minutes, channel A's outlet has mostly gravel and coarse sand, while channel B's outlet has mostly silt and fine sand. A gravel delta forms where channel A enters a flat pool. Both channels start with the same sediment mixture and carry the same water volume.
1. Calculate discharge for each channel and use the results to explain why channel A transports coarser sediment even though the discharges are equal.
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Discharge is Q = width × average depth × average velocity. For channel A, Q = 0.40 m × 0.05 m × 0.90 m/s = 0.018 m³/s. For channel B, Q = 0.80 m × 0.10 m × 0.225 m/s = 0.018 m³/s. The discharges are equal, but channel A has a steeper slope and a much higher velocity. Higher velocity and slope increase shear stress on the bed and raise the stream's competence, so A can move gravel and coarse sand. Channel B has a larger cross-sectional area and lower velocity, so it can move only finer silt and sand. Discharge alone does not determine sediment size; velocity, slope, and channel geometry also control transport.
2. Explain why a gravel delta forms where channel A enters the flat pool, and predict what would happen to channel A's bed if its slope were reduced to 0.005 m/m while discharge stayed at 0.018 m³/s.
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When channel A enters the pool, the water slows abruptly. Velocity and shear stress drop, so the stream's competence decreases. Gravel is deposited first because it is the coarsest and heaviest material, followed by coarse sand; finer sediment can remain suspended and travel farther. That produces a delta of gravel and coarse sand at the outlet. If channel A's slope were reduced to 0.005 m/m at the same discharge, the channel would generally have lower velocity unless its width or depth changed. Lower velocity would reduce competence, so gravel would be deposited upstream and the bed would aggrade. Finer sediment would be carried farther into the pool. The channel might also begin to meander or braid as deposition steepened local gradients.
3. A manager proposes straightening channel A to reduce flooding. Predict the short-term and long-term effects on sediment transport and downstream deposition, and state one assumption.
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Straightening usually increases the channel slope and velocity in the short term. Faster flow increases erosion of the bed and banks and raises the sediment load. More sediment would be carried downstream and deposited where the channel enters a low-gradient reach or pool, potentially increasing downstream deposition and flood risk there. In the long term, the straightened channel may incise, lowering its bed, and then deposit sediment farther downstream. The local flood risk could shift rather than disappear. This prediction assumes that discharge remains about 0.018 m³/s and that the banks and bed are erodible sand and gravel. If the channel were lined with bedrock or concrete, the response would differ.