Rain on the Paved Hillside
Builders cleared a grassy hillside above a bay on St. John to make a parking lot. The lot was graded and paved, with a curb and a storm drain that empties straight into the bay. Two weeks later, 6 cm of rain fell in two hours. Muddy brown water poured off the lot, across the road, and into the bay. A diver found a milky plume over the reef, and about 4 cm of sediment buried the seagrass bed near the drain. A grassy field a block away received the same 6 cm of rain, but no muddy water left it. Students ask why two places with the same rainfall behaved so differently.
1. Use the four Earth spheres to explain why the paved lot sent soil into the bay while the grassy field did not. Refer to the specific observations.
Read the worked answer
Setting up the spheres at each site
- Hydrosphere: the 6 cm of rain falling on both sites.
- Geosphere: the soil and slope underneath.
- Biosphere: the grass and shrubs on the field, and the reef and seagrass in the bay.
- Atmosphere: the storm that delivered the rain.
Why the field held its soil Grass roots and stems slow the water down and hold soil grains in place, so rain soaks in and moves slowly. Little or no water ran off, so nothing reached the drain.
Why the lot lost its soil Paving removed the plants and sealed the surface. Water could not soak in, so almost all 6 cm ran off fast, carrying loose soil from the cleared, graded areas. The curb and storm drain then piped that muddy water straight to the bay, where the geosphere (sediment) was dumped into the hydrosphere (bay water) on top of the biosphere (seagrass and reef).
Key point The rainfall was the same; the difference was whether living things and permeable ground were there to slow the water. One sphere (biosphere) was removed, and that changed everything downstream.
2. The owner proposes building a low wall around the lot to keep the mud in. Predict what happens in the next heavy rain and explain whether this protects the reef.
Read the worked answer
Prediction
- The wall traps water and mud inside the lot instead of stopping them from forming.
- In a 6 cm storm the lot fills with muddy water; it will spill over or push through the storm drain anyway, and if the wall holds, water stands on the pavement and seeps along the edges.
- Standing water on pavement can undercut the paving and wash around the wall's ends, so some mud still reaches the bay.
Does it protect the reef? Mostly no.
- A wall changes where the water goes, not how much water runs off the sealed surface. The lot still cannot absorb rain, so the same volume of muddy water must go somewhere.
- Because the drain still empties into the bay, any overflow carries the same sediment load to the reef and seagrass.
What would actually help Reducing runoff at the source - restoring vegetation or adding planted strips, using permeable paving, and directing water into a vegetated basin that soaks in and filters before it reaches the bay. The test of any plan is whether rain is absorbed or slowed on land, not whether the mud is hidden.
3. Propose two changes to the lot. For each, name the sphere interaction it changes and predict one measurable result the students could check after the next storm.
Read the worked answer
Change 1: Replace part of the pavement with permeable pavers and a planted border.
- Spheres changed: hydrosphere ↔ geosphere ↔ biosphere. Water can soak into the ground instead of running off, and plant roots hold soil.
- Measurable prediction: with the same 6 cm of rain, runoff leaving the lot should be less than before. Students could set out a marked bucket at the drain and compare volume and muddiness to the previous storm, or measure how long water keeps flowing after the rain stops.
Change 2: Dig a shallow vegetated basin between the lot and the drain.
- Spheres changed: hydrosphere ↔ geosphere (water slowed and filtered through soil) and biosphere (plants take up water).
- Measurable prediction: water reaching the bay should be clearer, and sediment on the seagrass bed should be thinner than 4 cm. Students could photograph the same seagrass quadrat and measure sediment depth before and after the next big storm.
Why these are fair tests Each change comes with a number to compare against the original storm (6 cm of rain, 4 cm of sediment, a visible plume). If a storm of similar size produces much less runoff and thinner sediment, the change helped; if not, the students should revise the plan rather than assume it worked.