Classroom activity Grade 7 Math
Sizing Brackenridge Rain Gardens: A Grade 7 Station Activity
A grade 7 station-rotation activity where groups compute roof runoff and size rain gardens for a fictional campus, using proportional reasoning and a runoff rule.
Objectives
- Students will compute roof runoff using the runoff rule of area times rainfall times 0.623.
- Students will size a rain garden proportionally from the runoff it must hold.
Vocabulary
- Rain garden
- A rain garden is a shallow planted dip in the ground that catches rain running off a roof or driveway and lets it soak slowly into the soil. Its deep-rooted plants help clean the water.
Also: runoff, proportional, impervious
Teacher overview
This is a station-rotation activity for groups of three or four. Each station card describes one roof on the fictional Brackenridge campus, and groups compute how much rain runs off that roof using a standard runoff rule. They then size a rain garden to soak up the runoff. The activity teaches the concept directly: because every roof uses the same runoff rule, groups see that runoff is proportional to roof area, and a bigger roof needs a bigger garden in the same ratio.
Activity Directions
- Before class, print four station cards, one for each roof: the Library (1,000 square feet), the Gym (2,000 square feet), the Art Room (1,500 square feet), and a synthesis card for a second Library storm. Place them at four spots around the room. Give each group a recording handout and a calculator.
- Explain the runoff rule to the whole class: gallons of runoff equal the roof area in square feet times the rainfall in inches times 0.623. The rule counts all the rain that falls on the roof, so it is the most that can run off; in a real storm somewhat less reaches the garden. Do one quick example on the board with a 100-square-foot roof and a 1-inch storm, giving about 62 gallons.
- Explain the rain-garden design rule the class will use: size the garden at 1 square foot of garden for every 15 gallons of runoff from a 1-inch storm.
- Send each group to a starting station. At each station, the group reads the roof size and rainfall, then completes the matching handout question. Groups rotate to the next station when you call time, about 5 minutes per station.
- Continue until every group has visited all four stations and answered all five handout questions.
- Gather the class. Ask groups to report their runoff for the Library and the Gym, and confirm that the Gym's runoff is exactly twice the Library's because its roof is twice as large.
- Close with two debrief questions: why does a roof twice as big produce twice the runoff, and why does a bigger roof need a bigger rain garden in the same ratio.
Student Handout
Use the runoff rule (area times rainfall times 0.623) and, in our scenario, the fictional Brackenridge design rule of 1 square foot of garden for every 15 gallons of runoff.
- Library station: the roof is 1,000 square feet and the storm drops 1 inch of rain. Find the runoff in gallons.
- Gym station: the roof is 2,000 square feet and the storm drops 1 inch of rain. Find the runoff, then tell whether it is twice the Library's runoff and why.
- Art Room station: the roof is 1,500 square feet and the storm drops 1 inch of rain. Find the runoff in gallons.
- Size a rain garden for the Art Room using the design rule of 1 square foot of garden for every 15 gallons of runoff.
- Synthesis station: a 2-inch storm falls on the Library roof of 1,000 square feet. Find the runoff and compare it with your answer from question 1.
From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.
Answer Key
- Answer: 623 gallons. 1,000 square feet × 1 inch × 0.623 = 623 gallons of runoff.
- Answer: 1,246 gallons, which is twice the Library's. 2,000 × 1 × 0.623 = 1,246 gallons. The Gym roof is twice the Library roof, and runoff is proportional to roof area, so the runoff is twice as much.
- Answer: 934.5 gallons. 1,500 square feet × 1 inch × 0.623 = 934.5 gallons of runoff.
- Answer: About 62 square feet. Divide runoff by the design rate: 934.5 ÷ 15 = 62.3, which rounds to about 62 square feet of garden.
- Answer: 1,246 gallons, twice question 1. 1,000 × 2 × 0.623 = 1,246 gallons. Doubling the rainfall doubles the runoff, because runoff is proportional to rainfall.
Differentiation
Support
Give groups a filled-in first line for question 1 so they see how the rule is set up before they compute the others. Provide the runoff rule and the design rule printed at the top of the handout and allow a calculator throughout.
Extension
Add a fifth station with a 2,500-square-foot roof and ask groups to size its rain garden for a 1-inch storm, then find how many times larger it is than the Library's garden and check that the ratio matches the roof ratio.
ELL supports
Post the runoff rule as a labeled picture: a roof, an arrow for rain, and a garden below. Pre-teach "runoff" and "rain garden" with the picture, and provide the sentence frame "The runoff is ___ gallons, so the garden is ___ square feet."