Permeable Pavement at Ovenden Cross Middle School

The Lot Behind the Gym

For years the back parking lot at Ovenden Cross Middle School, in the fictional town of Ovenden Cross, flooded in the same place. Every hard rain turned the low corner near the gym doors into a pond. Students stepped around it, and buses splashed through it. By the next afternoon the standing water was gone, and the lot looked ordinary again until the next storm.

The water had not soaked into the ground. It had run across the asphalt, picked up oil, brake dust, road salt, and grit, and slid down a storm drain at the curb. That drain does not lead to a treatment plant. It leads to Pelham Run, a creek that passes behind the soccer fields. After storms the creek ran gray, and the town's volunteer creek monitors kept finding a slick sheen near the outfall pipe.

Rain that falls on a hard surface has almost nowhere to go. Engineers who size storm drains use a rough number for how much of the rain a surface sheds. For pavement that number is about 0.9, meaning nearly all of it runs off. For a lawn the number is closer to 0.2, because grass and soil take most of the water in. A parking lot behaves like a small, fast river during a storm, and the faster water moves, the more it carries with it.

The district looked at three options. It could enlarge the storm drain, which would move the dirty water to the creek more quickly. It could add a basin at the edge of the lot to hold the water for a while. Or it could rebuild the worst section of the lot with permeable pavement, a surface built so water passes through rather than running across.

Permeable pavement looks a little coarse up close. The paving mix leaves small open spaces instead of filling them, and beneath the surface sits a bed of clean crushed stone, often more than a foot deep. Rain drops through the surface, spreads through the stone bed, and soaks slowly into the soil below. A perforated pipe called an underdrain runs through the stone to carry away whatever the soil cannot take during a long storm. The stone bed also holds sediment and some of the oil, so less of both reaches the creek.

The work took one summer. Crews removed about a third of the old lot, dug out the base, laid the stone bed, and paved the surface. The project cost the district about $186,000, and the town's stormwater fund covered part of it because the creek is a shared problem.

The results showed up in the first fall. The pond by the gym doors did not come back. Creek monitors reported that the sheen after storms was fainter, though they were careful to say that other work upstream had happened in the same year. The custodial staff learned a new job as well. Permeable pavement clogs if grit is allowed to fill its openings, so the lot now gets vacuumed twice a year with a machine borrowed from the town. A neglected permeable lot slowly becomes an ordinary one.

The district's facilities director tells visitors that the lot is not a cure for the creek. Buses still leak, and road salt still washes off in February. What changed is where the first inch of rain goes, because it now soaks into a bed of stone under the pavement instead of running straight to the drain.

1. What happened to the low corner of the parking lot after every hard rain?
2. According to the passage, where does the storm drain at the curb carry water?
3. The passage gives the number engineers use for how much of the rain a paved surface sheds. What is that number?
4. Why did the district reject the option of enlarging the storm drain? Support your answer with a detail from the passage.
5. The passage says a neglected permeable lot slowly becomes an ordinary one. Explain what that sentence means, using a detail from the paragraph about maintenance.
6. Creek monitors said the sheen after storms was fainter, but they added a caution. Why did they add that caution?
7. The facilities director says the lot is not a cure for the creek. Cite two details from the passage that support that statement.