Design an investigation

Permeable pavement and stormwater

Units 4, 5 and 8
10 points
About 23 minutes

You can use a calculator on this question, just like on exam day.

You read about a real-world environmental situation with a described figure or model, then pick out the parts of a scientific investigation (hypothesis, variables, controls), explain the science behind it, and propose solutions to the problems it raises. On the exam: Question 1 of 3 in the 70-minute free-response section (Section II is 40% of the exam score).

The question and its sources

A city's storm drains carry rainwater from streets and parking lots straight into Mill Creek, which floods after heavy storms. Before repaving its parking lots, the city's engineers tested whether permeable pavement would reduce stormwater runoff.

Figure 1: Diagram of one test pad

The diagram shows a side view of a rectangular parking-lot test pad, 10 m long and 5 m wide (50 m²), with a 2% slope. A sprinkler frame above the pad sprays artificial rain evenly over the surface.

Water that soaks into the pad drains down through a gravel layer and into the soil below. Water that does not soak in flows downhill across the surface into a gutter at the lower edge, which empties into a measuring tank.

The engineers built nine pads on the same soil: three of standard asphalt, three of permeable concrete pavers (blocks with gaps filled with small stones) and three of porous concrete (concrete full of tiny connected holes). Each pad received 25 mm of artificial rain in one hour, which is 1,250 liters of water per pad. They then measured the volume of water in each tank.

Source: Hypothetical study design

Table 1: Surface runoff from a 25 mm simulated storm

Surface	Mean runoff per pad (L)	Percent of applied water that ran off (%)
Standard asphalt	1,140	91
Permeable concrete pavers	210	17
Porous concrete	150	12

Propose one method, other than permeable pavement, that the city could use to reduce the amount of stormwater runoff reaching Mill Creek.

Your answer to part (j)

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