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Lesson plan Grade 8 Science

Larkspur Campus Rain Garden: A Stormwater Runoff Lesson

A 50-minute grade 8 science lesson where students calculate runoff from a fictional school's paved lot and model how a rain garden reduces stormwater.

Objectives

  1. Students will calculate stormwater runoff from a paved surface using area, rainfall, and a runoff coefficient.
  2. Students will compare runoff from an impervious surface and a rain garden to show the reduction.
  3. Students will explain how green infrastructure reduces the human impact of paving on a watershed.

Vocabulary

Impervious surface
An impervious surface is any hard surface, like a road, roof, or parking lot, that rain cannot soak through. The more impervious surface a town has, the more stormwater rushes off it.
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.
Green infrastructure
Green infrastructure uses soil and plants, like rain gardens, green roofs, and street trees, to soak up rainwater where it falls instead of piping it away. It copies the way nature handles rain.

Also: runoff, runoff coefficient

Teacher overview

When rain falls on a paved parking lot, almost all of it runs off into storm drains instead of soaking into the ground, which can overwhelm pipes and carry pollution to a nearby creek. This lesson uses the fictional Larkspur K-8 campus to have students calculate that runoff and then show how converting pavement to a rain garden cuts it sharply. Students use the relationship runoff (gallons) = area (sq ft) × rainfall (inches) × 0.623 × C, where C is a runoff coefficient near 0.9 for pavement and near 0.2 for a planted garden. A common misconception is that a rain garden mainly stores water; its larger effect is letting water soak into the ground so it never becomes runoff in the first place.

Procedure

Hook (5 min): Show a photo or quick sketch of a paved school parking lot during a storm, with water rushing toward a storm drain. Ask students to predict, in gallons, how much water a 1-inch storm sends off the fictional Larkspur lot. Record a few guesses on the board without comment.

Direct instruction (12 min): Introduce the terms impervious surface, runoff, and runoff coefficient. Present the relationship runoff (gallons) = area (sq ft) × rainfall (inches) × 0.623 × C. Explain that 0.623 converts one inch of rain over one square foot into gallons, and that C is the fraction of rain that runs off: near 0.9 for asphalt and near 0.2 for a rain garden. Model the Larkspur parking-lot calculation: 8,000 sq ft × 1 inch × 0.623 × 0.9 = 4,485.6, or about 4,486 gallons of runoff.

Guided practice (10 min): Work the same 8,000 sq ft area as a rain garden with C = 0.2 as a class: 8,000 × 1 × 0.623 × 0.2 = 996.8, about 997 gallons. Ask students what the difference between 4,486 and 997 gallons represents, and confirm it is the runoff the garden keeps out of the storm drain.

Independent practice (15 min): Students complete the handout questions on their own or in pairs, calculating runoff for the roof and for larger storms and finding the reduction from the rain garden.

Closure and exit ticket (8 min): Students answer handout question 6 on an index card, explaining how a rain garden reduces the human impact of paving. Collect the cards and confirm at least 80 percent name soaking water into the ground (infiltration) as the key mechanism before moving on.

Larkspur Campus Rain Garden: A Stormwater Runoff Lesson — Grade 8 Science
Name: ____________________________ Date: ______________

Student Handout

Use the following information about the fictional Larkspur K-8 campus. For every question, use runoff (gallons) = area (sq ft) × rainfall (inches) × 0.623 × C.

  1. Larkspur's parking lot is 8,000 sq ft of asphalt, with a runoff coefficient C = 0.9. For a 1-inch storm, calculate the runoff in gallons.
  2. Suppose that same 8,000 sq ft area became a rain garden with C = 0.2. Calculate the runoff for the same 1-inch storm.
  3. How many gallons of runoff does converting the lot to a rain garden keep out of the storm drain during that 1-inch storm?
  4. Larkspur's roof is 6,000 sq ft with C = 0.9. Calculate its runoff for a 1-inch storm.
  5. For a larger 2-inch storm, calculate the roof's runoff (still C = 0.9).
  6. Explain how a rain garden reduces the human impact of paving over ground, using the idea that water soaking into the soil never becomes runoff.

From Water for Schools (waterforschools.org). This material is in the public domain (CC0) and may be copied, edited, and shared freely.

Answer Key
  1. Answer: 4,486 gallons.
    8,000 × 1 × 0.623 × 0.9 = 4,485.6, which rounds to about 4,486 gallons of runoff.
  2. Answer: 997 gallons.
    8,000 × 1 × 0.623 × 0.2 = 996.8, which rounds to about 997 gallons.
  3. Answer: 3,489 gallons.
    4,486 − 997 = 3,489 gallons of runoff kept out of the storm drain by the rain garden (3,488.8 if you keep the unrounded values, which still rounds to 3,489).
  4. Answer: 3,364 gallons.
    6,000 × 1 × 0.623 × 0.9 = 3,364.2, which rounds to about 3,364 gallons.
  5. Answer: 6,728 gallons.
    6,000 × 2 × 0.623 × 0.9 = 6,728.4, which rounds to about 6,728 gallons; doubling the rainfall doubles the runoff.
  6. Answer: The garden lets rain soak into the soil instead of running off.
    A rain garden replaces a sealed surface with planted soil, so rain infiltrates into the ground rather than rushing to the storm drain; less runoff means less flooding downstream and less pollution carried to the creek.

Differentiation

Support (below grade level)

Provide the runoff relationship already set up for question 1, as 8,000 × 1 × 0.623 × 0.9, so students only carry out the multiplication. Let students use the class worked example as a template beside their own work.

Extension (above grade level)

Ask students to find what fraction of the parking-lot runoff the rain garden eliminates as a percent, then design a split where half the lot stays paved and half becomes garden and calculate the combined runoff.

ELL supports

Pre-teach "runoff," "impervious," and "soak in" with a demonstration: pour water on a tray and on a wet sponge, and compare how much runs off each. Provide a labeled version of the runoff relationship with each factor named.

Extensions

Have students measure a real paved area on their own campus, estimate its runoff for a typical local storm using the same relationship, and write a short proposal for where a rain garden could go and how much runoff it would keep out of the storm drain.