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Lesson plan Grade 10 Engineering & Design

Tanager Ridge Parking Lot Runoff: A Bioretention Design Lesson

A 55-minute grade 10 engineering lesson where students calculate parking-lot runoff and size a bioretention cell to capture it within a fixed footprint.

Objectives

  1. Students will calculate stormwater runoff volume from a paved area for a design storm.
  2. Students will size a bioretention cell to capture a target runoff volume within a fixed footprint.
  3. Students will evaluate whether a design meets capture, footprint, and cost criteria.

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.

Also: runoff, bioretention cell, ponding depth, runoff coefficient

Teacher overview

When rain falls on a paved parking lot, almost none of it soaks in, so it runs off fast and can overwhelm a nearby storm drain and creek. A bioretention cell, a shallow planted basin that collects runoff and lets it soak slowly into the ground, is a common engineering fix. In this lesson, students design a bioretention cell for the fictional Tanager Ridge High School parking lot, first calculating how much runoff a design storm produces and then sizing a cell to capture it within a fixed patch of lawn. A common misconception this lesson addresses is that a cell is sized for a bigger storm simply by digging it deeper. Ponding depth is kept shallow for safety and plant health, so engineers fix the depth and solve for the surface area the runoff needs.

Procedure

Hook (5 min): Show students a photo or quick sketch of a parking lot draining into a single storm grate during a downpour, with water backing up. Ask them to estimate, with a show of hands, whether a 1-inch storm on a lot the size of their gym produces closer to 700 gallons or 7,000 gallons of runoff. Record the split without revealing the answer yet.

Direct instruction (12 min): Introduce the runoff conversion that roof or lot area (sq ft) × rainfall (inches) × 0.623 = gallons of runoff, and explain that paved surfaces are treated as fully impervious. Then introduce the peak-flow version engineers use for pipe sizing, the rational method Q = C × i × A, where C is a runoff coefficient (about 0.9 for pavement and 0.2 for lawn), i is rainfall intensity in inches per hour, and A is area in acres, giving Q in cubic feet per second. Note that the class will use the volume conversion to size the cell and the rational method only at the end.

Model a worked example (10 min): Using the Tanager Ridge lot (a fictional site) of 12,000 square feet and a 1-inch design storm, model the runoff volume: 12,000 × 1 × 0.623 = 7,476 gallons. Then model sizing a bioretention cell with a fixed ponding depth of 0.5 foot: required surface area = runoff volume ÷ (ponding depth × 7.48 gallons per cubic foot) = 7,476 ÷ (0.5 × 7.48) = 7,476 ÷ 3.74 = about 1,999 square feet, so roughly 2,000 square feet. Point out that the available lawn is 2,200 square feet, so the design fits.

Guided practice (13 min): In pairs, students work a second case from the handout: a 9,000-square-foot section of the lot and the same 1-inch storm, with 1,200 square feet of lawn available for the cell. They calculate the runoff volume, then size a cell at a ponding depth of 0.75 foot. Circulate and check that pairs divide by (0.75 × 7.48), not by 0.75 alone.

Independent practice (10 min): Each student decides whether their sized cell meets three criteria and records a one-line judgment for each: it captures the full design-storm runoff, it fits the 1,200-square-foot footprint, and it uses the smaller of the two ponding depths, 0.5 foot or 0.75 foot, when both fit (since a shallower, wider cell is safer around a school). To judge the third criterion, students also size the cell at 0.5 foot. This is the trade-off evaluation.

Closure and exit ticket (5 min): Students answer one rational-method question on an index card: for a 0.5-acre paved area, a rainfall intensity of 2 inches per hour, and a pavement runoff coefficient of 0.9, find the peak runoff Q in cubic feet per second. Collect and scan for the correct setup (0.9 × 2 × 0.5) before dismissal.

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: 5,607 gallons.
    Guided practice runoff: 9,000 sq ft × 1 inch × 0.623 = 5,607 gallons.

  2. Answer: about 1,000 square feet.
    Guided practice cell size at 0.75-foot ponding depth: 5,607 ÷ (0.75 × 7.48) = 5,607 ÷ 5.61 = 999.5, which rounds to about 1,000 square feet.

  3. Answer: about 1,499 square feet.
    Independent practice cell size at 0.5-foot ponding depth: 5,607 ÷ (0.5 × 7.48) = 5,607 ÷ 3.74 = 1,499.2, about 1,499 square feet, which does not fit the 1,200 square feet of lawn.

  4. Answer: 0.75 foot; the 0.75-foot cell meets all three criteria.
    Independent practice: the cell of about 1,000 square feet at 0.75 foot holds the full 5,607 gallons and fits the 1,200 square feet of lawn. The 0.5-foot cell would need about 1,499 square feet, which does not fit, so 0.75 foot is the shallowest depth that fits.

  5. Answer: 0.9 cfs.
    Exit-ticket rational method: Q = C × i × A = 0.9 × 2 × 0.5 = 0.9 cubic feet per second.

Differentiation

Support (below grade level)

Provide the runoff conversion and the cell-sizing formula already arranged with labeled blanks, so students only substitute the numbers, and give the modeled Tanager Ridge example on the handout for reference while they work the guided-practice case.

Extension (above grade level)

Ask students to find what ponding depth would let the guided-practice cell fit in exactly 750 square feet instead of 1,000, then explain the trade-off between a deeper cell that saves space and a shallower cell that is safer and easier to plant.

ELL supports

Pre-teach runoff, impervious, ponding depth, and bioretention with a labeled cross-section diagram showing rain hitting pavement, running to a planted basin, and soaking in. Provide sentence frames for the independent-practice judgments: "This design captures ___ gallons, which meets / does not meet the target because ___."