The Stream Table and the 40-House Slope
A class built two stream tables. Each tray held 3 kg of the same soil and was tilted at 10°. Tray A had bare soil. Tray B had soil covered with a layer of grass and roots. Students poured 4 L of water at the same rate into the top of each tray and collected runoff in a bucket. After the water settled, they dried and weighed the sediment. Tray A produced 86 g of sediment and a channel 2.5 cm deep. Tray B produced 19 g of sediment and no channel deeper than 0.4 cm. A nearby town plans to build 40 houses on a grassy slope beside a river. The builder proposes removing all grass, leveling the slope, and paving a road. Residents are worried about the river. The class must use their stream-table evidence to recommend a plan.
1. Use the stream-table data to explain how grass affects erosion.
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Tray A lost 86 g of sediment and cut a channel 2.5 cm deep. Tray B lost only 19 g and had no channel deeper than 0.4 cm. The main difference is the grass and roots. Roots bind soil particles together, making them harder to wash away. Grass also slows the runoff, so water has more time to soak into the soil instead of carrying sediment downhill. Because both trays used the same soil, slope, and water amount, the grass is the variable most likely responsible for the difference. The smaller sediment mass and shallower channel are evidence that grass reduces erosion.
2. Recommend a plan for the builder and predict its effect on the river. Include one trade-off.
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The builder should keep a wide strip of grass and trees along the river, limit paving, and direct runoff into a detention pond or rain garden. During construction, silt fences should be installed and checked after storms. These measures mimic Tray B by slowing water and holding soil in place. The prediction is that much less sediment will reach the river than if the slope is cleared and paved. A trade-off is that a buffer strip uses land that could be sold for houses, and detention ponds and fences cost money and need maintenance. The town may need smaller lots or shared green space, but the stream-table evidence supports keeping vegetation.
3. Identify one limitation of the stream-table model and describe how the class could improve it.
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The stream table is much smaller than a real hillside, and the soil is not compacted like real ground. Pouring 4 L quickly may not match local rainfall, and the tray has no river current or variety of plants. The model also does not show what happens over many years. The class could improve it by repeating trials, using local soil and a slope measured from the real site, testing several rain intensities, and adding a model river to see where sediment travels. Testing different buffer widths would help the town decide how much land to leave unpaved. These changes would make the evidence more reliable for the real slope.