Lesson plan Grade 12 Science
Green Infrastructure for the Ellersby Creek Watershed
A grade 12 lesson using the rational method to compare pre- and post-development runoff and to size a rain garden for a fictional campus roof's first flush.
Objectives
- Students will use the rational method to compare peak runoff before and after development.
- Students will size a rain garden to capture the first inch of runoff from a roof.
- Students will evaluate how green infrastructure reduces a development's impact on a creek.
Vocabulary
- First flush
- First flush is the very first bit of runoff at the start of a rainstorm, which often carries the most pollution because it washes off everything that built up on streets and roofs during the dry period before the rain. Later rainfall in the same storm is usually cleaner.
- 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: rational method, runoff coefficient, bioretention
Teacher overview
When a school builds a new parking lot and wing, the paved area sheds far more stormwater than the grass it replaced, and the extra flow scours the creek downstream. This lesson uses the rational method to put numbers on that change for a fictional campus beside Ellersby Creek, then has students size a rain garden to catch the roof's first flush. A common misconception is that a rain garden must hold an entire storm; students should see that capturing the first inch of runoff, which carries most of the pollutants, is a realistic and effective design target rather than an impossible one.
Procedure
Hook (5 min): Show two sketches of the same campus, one mostly lawn and one with a new lot and building. Ask students to predict, as a percentage, how much the peak runoff rises after the pavement goes in. Record predictions for later.
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 the dimensionless runoff coefficient, i is rainfall intensity in inches per hour, and A is drainage area in acres. Explain that a lawn has a low coefficient near 0.2 while pavement runs near 0.9, so paving raises C sharply. Introduce the first-flush idea and the roof-capture rule: gallons = roof area (square feet) x rainfall (inches) x 0.623, which gives the rain that falls on the roof; a real system collects somewhat less.
Watershed worked example (13 min): Work the Ellersby Creek campus with the class. The drainage area is 3 acres and the design storm intensity is 2.5 inches per hour. Before development the coefficient is 0.35, so Q = 0.35 x 2.5 x 3 = 2.625 cubic feet per second. After development it rises to 0.75, so Q = 0.75 x 2.5 x 3 = 5.625 cubic feet per second, an increase of about 3.0 cubic feet per second. Then size the roof capture: the new roof is 8,000 square feet, so catching the first inch holds 8,000 x 1 x 0.623 = 4,984 gallons in the rain garden. That is the rain that falls on the roof; a real system collects somewhat less.
Green infrastructure design activity (15 min): In pairs, students complete the handout, computing runoff before and after for a second sub-basin and the capture volume for its roof. They then write one sentence recommending whether the rain garden alone offsets the runoff increase, citing their numbers. Circulate and confirm students keep the area in acres and the intensity in inches per hour.
Exit ticket (10 min): As a formative check, students answer: if the post-development coefficient could be lowered from 0.75 back to 0.55 by adding permeable pavement, what would the new peak runoff be for the 3-acre, 2.5 inch-per-hour storm? Require the calculation and a one-sentence interpretation. Scan for the correct 4.125 cubic feet per second before dismissal.
Student Handout
Suppose the town reviews a second sub-basin on the same campus. It drains 2 acres, the design storm intensity is 2.5 inches per hour, the pre-development coefficient is 0.30, and the post-development coefficient is 0.70. A new roof in this sub-basin covers 5,000 square feet.
Find the peak runoff before development, in cubic feet per second.
Find the peak runoff after development, in cubic feet per second, and state the increase.
Find the gallons captured if a rain garden holds the first inch of runoff from the 5,000-square-foot roof.
In one or two sentences, state whether capturing the roof's first inch is enough to erase the runoff increase you found in item 2, and explain your reasoning.
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: 1.5 cubic feet per second.
Q = 0.30 x 2.5 x 2 = 1.5 cubic feet per second.Answer: 3.5 cubic feet per second, an increase of 2.0.
Q = 0.70 x 2.5 x 2 = 3.5 cubic feet per second, so the increase is 3.5 - 1.5 = 2.0 cubic feet per second.Answer: 3,115 gallons.
5,000 x 1 x 0.623 = 3,115 gallons captured. This is the rain that falls on the roof in the first inch; a real system collects somewhat less.Answer: No, capturing the roof's first inch does not erase the increase.
The rain garden holds a fixed volume from one roof, while the runoff increase is a peak flow rate across the whole 2-acre sub-basin. Accept any answer that notes the capture helps with the first flush but does not by itself cancel the higher peak flow, which needs added measures across the paved area.
Exit ticket answer: 4.125 cubic feet per second. Q = 0.55 x 2.5 x 3 = 4.125 cubic feet per second, which is lower than the 5.625 value, so permeable pavement meaningfully cuts the peak.
Differentiation
Support
Provide the rational-method formula with each variable pre-labeled with its unit, and list each sub-basin's drainage area and storm intensity in a table so students can focus on choosing the right coefficient for each surface.
Extension
Ask students to find the runoff coefficient that would hold the post-development peak on the 3-acre campus to no more than the pre-development 2.625 cubic feet per second, and describe what mix of surfaces could achieve it.
ELL supports
Pre-teach "runoff," "coefficient," "impervious," and "first flush" with paired images of a lawn and a parking lot during rain. Provide a sentence frame for item 4 that compares "captures the first inch" with "lowers the peak flow."
Extensions
Have students map the impervious surfaces on their own campus using an aerial image, estimate a rough runoff coefficient for the whole site, and propose one green-infrastructure feature that would lower it, presenting the before-and-after coefficients.