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Lesson plan Grade 10 Science

Thornwood Commons Rain Garden: Sizing Green Infrastructure

A 55-minute grade 10 lesson where students use the rational method to find a parking lot's peak runoff, then size a rain garden to hold the first inch of storm runoff.

Objectives

  1. Students will calculate peak stormwater runoff using the rational method Q = C x i x A.
  2. Students will size a rain garden footprint that holds a required storage volume.
  3. Students will explain how green infrastructure lowers peak runoff and protects water quality.

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.
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: rational method, runoff coefficient, rainfall intensity

Teacher overview

When rain falls on a paved parking lot, almost all of it runs off at once, overwhelming storm drains and carrying pollutants to the nearest stream. A rain garden is a shallow planted basin that catches and soaks in that runoff. This lesson uses the fictional Thornwood Commons parking lot to teach the rational method for peak runoff, Q = C x i x A, and then has students size a rain garden to store the first inch of storm runoff. A common student misconception is treating the runoff coefficient C as a unit rather than a fraction between 0 and 1; the procedure addresses this directly when the worked example is introduced.

Procedure

Hook (5 min): Ask students where the water goes when a heavy rain hits the school parking lot. Collect a few answers, then state the day's question: how big would a rain garden need to be to hold that water instead of sending it to the storm drain?

Direct instruction (12 min): Introduce the rational method, Q = C x i x A, where C is the runoff coefficient (about 0.9 for pavement and about 0.2 for lawn), i is rainfall intensity in inches per hour, and A is the drainage area in acres, giving Q in cubic feet per second. Emphasize that C is a fraction, not a unit. Work this example on the board: suppose a fictional paved courtyard is 0.1 acre in a storm with intensity 2 in/hr. Then Q = 0.9 x 2 x 0.1 = 0.18 cfs. Show every substitution.

Guided practice (10 min): Hand out the student handout. Work problems 1 and 2 together as a class: find the peak runoff for the 0.25-acre Thornwood Commons lot, then convert its area to square feet. Have students check each substitution against a partner before moving on.

Independent practice (18 min): Students complete handout problems 3 through 6 on their own or in pairs, finding the runoff volume, converting it to cubic feet, and sizing the rain garden footprint at two ponding depths. Circulate and check that students are dividing volume by depth correctly.

Formative check (5 min): Ask every student to answer on an index card: "If a rain garden must hold 907 cubic feet and ponds water 1 foot deep, what footprint area does it need?" Scan for the correct method (dividing 907 by 1 to get about 907 square feet) before moving on; reteach if more than a quarter of the class divides incorrectly.

Closure (5 min): Return to the opening question. Have two or three students state, in one sentence each, how a rain garden changes where the parking lot's water goes and why capturing the first inch matters most for water quality.

Thornwood Commons Rain Garden: Sizing Green Infrastructure — Grade 10 Science
Name: ____________________________ Date: ______________

Student Handout

Suppose the fictional Thornwood Commons parking lot is 0.25 acre of pavement (runoff coefficient C = 0.9) and the design storm has a rainfall intensity of 2 in/hr. Use 1 acre = 43,560 sq ft, the runoff conversion volume (gallons) = area (sq ft) x rainfall (in) x 0.623, and 1 cu ft = 7.48 gal.

  1. Calculate the lot's peak runoff Q in cubic feet per second, using Q = C x i x A with C = 0.9, i = 2 in/hr, and A = 0.25 acre.
  2. Convert the lot's area of 0.25 acre to square feet, using 1 acre = 43,560 sq ft.
  3. Find the runoff volume from the first inch of rain, in gallons, using volume = area (sq ft) x rainfall (in) x 0.623 with the area from problem 2.
  4. Convert the runoff volume from problem 3 into cubic feet, using 1 cu ft = 7.48 gal. Round to the nearest whole cubic foot.
  5. The rain garden will pond water 0.5 ft deep. Using volume = area x depth, find the footprint area in square feet needed to hold the cubic feet from problem 4.
  6. To shrink the footprint, the designer increases the ponding depth to 0.75 ft. Find the new footprint area, and state in one sentence the trade-off of building the garden deeper.

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: 0.45 cfs. Q = C x i x A = 0.9 x 2 x 0.25 = 0.45 cubic feet per second.
  2. Answer: 10,890 sq ft. 0.25 acre x 43,560 sq ft per acre = 10,890 square feet.
  3. Answer: 6,784 gal. 10,890 sq ft x 1 in x 0.623 = 6,784.47, which rounds to about 6,784 gallons.
  4. Answer: 907 cu ft. 6,784 gal divided by 7.48 gal per cu ft = 906.95, which rounds to 907 cubic feet.
  5. Answer: 1,814 sq ft. Area = volume divided by depth = 907 cu ft divided by 0.5 ft = 1,814 square feet.
  6. Answer: 1,209 sq ft. Area = 907 divided by 0.75 = 1,209.3, about 1,209 square feet. The deeper garden needs less land, but the deeper water takes longer to soak into the soil, which can leave standing water long enough to stress the plants, and deeper ponding can call for gentle edges or fencing for safety.

Differentiation

Support

Provide the rational-method formula with each variable already labeled and the courtyard worked example printed at the top of the handout. Give students the acre-to-square-foot conversion done for problem 2 so they can concentrate on the runoff and sizing steps.

Extension

Have students design a rain garden for the whole lot that captures the first two inches of runoff instead of one, recompute the required storage volume and footprint, and explain why doubling the captured depth does not simply double the peak runoff Q.

ELL supports

Pre-teach "runoff," "intensity," "ponding depth," and "footprint" with a cross-section sketch of a rain garden filling with water. Provide a filled-in conversion reference card and sentence frames such as "The rain garden needs a footprint of ___ square feet because ___."