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Lesson plan Grade 9 Engineering & Design

Sizing a Parking Lot Bioretention Cell by the Rational Method

A 55-minute grade 9 engineering lesson where students use the rational method to find parking lot runoff, compare it with lawn, and size a bioretention cell.

Objectives

  1. Students will calculate peak stormwater runoff from a paved area using the rational method Q = C x i x A.
  2. Students will size a bioretention cell to a stated footprint rule and compare paved and lawn runoff.

Vocabulary

Key words: rational method, runoff coefficient, rainfall intensity, bioretention cell, impervious

Teacher overview

When rain falls on a parking lot, almost none of it soaks in, so it rushes off as runoff that can flood streets and carry pollutants to a stream. A bioretention cell is a shallow planted basin that catches and slows that runoff. This lesson uses the rational method, a peak-flow method stormwater engineers use, to size a cell for a fictional campus parking lot. A common student misconception is that a paved surface and a lawn of the same size shed about the same amount of water; the lesson corrects that directly by comparing the two runoff coefficients.

Procedure

Hook (5 min): Project a photo or sketch of a flooded parking lot and ask students where the rainwater goes when it cannot soak into pavement. Record a few ideas without comment, then name the day's goal: to size a basin that catches that runoff for a fictional college lot.

Direct instruction (12 min): Introduce the rational method, Q = C x i x A, where Q is peak runoff in cubic feet per second, C is a runoff coefficient (about 0.9 for pavement and 0.2 for a lawn), i is rainfall intensity in inches per hour, and A is drainage area in acres. Explain that C is the share of rain that runs off rather than soaking in. Work the example for Millbrook Community College's lot: A = 0.8 acres of pavement, i = 2.5 inches per hour, so Q = 0.9 x 2.5 x 0.8 = 1.8 cubic feet per second.

Guided practice (12 min): With students, compute the runoff if the same 0.8-acre lot were lawn instead of pavement: Q = 0.2 x 2.5 x 0.8 = 0.4 cubic feet per second. Find the difference (1.8 - 0.4 = 1.4 cubic feet per second) and discuss why paving the same area produces well over four times the peak runoff.

Bioretention sizing (14 min): Tell students the design team uses a rule for this project that the cell's surface area equals 6 percent of the paved area it treats. Convert 0.8 acres of pavement to a cell size: 6 percent of 0.8 acres is 0.048 acres, and 0.048 x 43,560 square feet per acre is about 2,091 square feet. Discuss how a cell of that footprint, planted and dug about a foot deep, gives runoff a place to pond and soak in.

Independent practice (7 min): Students compute the peak runoff for a second paved lot on the handout: A = 0.5 acres, i = 3.0 inches per hour. They should find Q = 0.9 x 3.0 x 0.5 = 1.35 cubic feet per second.

Closure and exit ticket (5 min): Each student answers one question on a card: find the peak runoff from a 0.6-acre paved lot during a storm of 2.0 inches per hour. Collect and scan for the correct setup (0.9 x 2.0 x 0.6) before dismissal.

Sizing a Parking Lot Bioretention Cell by the Rational Method — Grade 9 Engineering & Design
Name: ____________________________ Date: ______________

Student Handout

Use the rational method, Q = C x i x A, to answer questions 1, 2, 4, and 5. In this formula Q is peak runoff in cubic feet per second, C is the runoff coefficient, i is rainfall intensity in inches per hour, and A is drainage area in acres. Use C = 0.9 for pavement and C = 0.2 for a lawn, and show your setup for every answer.

  1. The paved parking lot at Millbrook Community College covers A = 0.8 acres. During the design storm the rainfall intensity is i = 2.5 inches per hour. Find the peak runoff Q from the paved lot.

  2. Suppose the same 0.8-acre lot were planted as a lawn instead of paved, with the storm intensity still i = 2.5 inches per hour. Find the peak runoff Q from the lawn.

  3. The design team sizes the bioretention cell so that its surface area equals 6 percent of the paved area it treats. For the 0.8-acre paved lot, find the cell's footprint in square feet, using 1 acre = 43,560 square feet.

  4. A second paved lot covers A = 0.5 acres and drains during a storm of i = 3.0 inches per hour. Find the peak runoff Q from this lot.

  5. Exit ticket: A paved lot of A = 0.6 acres drains during a storm of i = 2.0 inches per hour. Find the peak runoff Q from this lot.

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: 1.8 cfs.
    Millbrook lot peak runoff = 0.9 x 2.5 x 0.8 = 1.8 cubic feet per second.

  2. Answer: 0.4 cfs.
    Lawn runoff = 0.2 x 2.5 x 0.8 = 0.4 cubic feet per second. The paved lot sheds 1.4 cubic feet per second more than the lawn version of the same area.

  3. Answer: About 2,091 sq ft (accept 2,090 or 2,091).
    6 percent of 0.8 acres = 0.048 acres; 0.048 x 43,560 = 2,090.88, about 2,091 square feet of bioretention cell.

  4. Answer: 1.35 cfs.
    Second lot peak runoff = 0.9 x 3.0 x 0.5 = 1.35 cubic feet per second.

  5. Answer: 1.08 cfs.
    Exit-ticket runoff = 0.9 x 2.0 x 0.6 = 1.08 cubic feet per second.

Differentiation

Support

Provide a partly worked rational-method template with the formula and units labeled and the pavement coefficient (0.9) already filled in, so students focus on multiplying the three values. Allow a calculator and give the acre-to-square-feet conversion on a reference card.

Extension

Ask students to find what rainfall intensity would push the Millbrook lot's peak runoff above 2.5 cubic feet per second, and to explain how a bigger design storm changes the size of the cell the team would need to build within the same paved area.

ELL supports

Pre-teach "runoff," "impervious," "intensity," and "bioretention" with a labeled cross-section of a paved lot draining into a planted basin. Provide a sentence frame for the exit ticket, such as "The peak runoff is ___ cubic feet per second because ___."

Extensions

Have students walk the school grounds with a supervising adult, staying out of driving lanes, and estimate the area of the largest paved surface, then apply the rational method with a local design-storm intensity from the public works department to estimate its peak runoff and the bioretention footprint it would need.