Water for Schools
Menu

Lesson plan Grade 9 Science

Windrow Creek's Watershed: Modeling Runoff After Development

A grade 9 Earth science lesson where students use the rational method to compare peak stormwater runoff before and after development in a fictional watershed.

Objectives

  1. Students will calculate a weighted runoff coefficient for a watershed with mixed land cover.
  2. Students will use the rational method to calculate peak stormwater runoff before and after land-use change.
  3. Students will explain how increased impervious surface creates a feedback effect on a watershed's runoff and groundwater recharge.

Vocabulary

Watershed
A watershed is all the land where rain that falls will drain to the same stream, river, or lake. Everyone lives in a watershed, no matter how far from the water they live.
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.
Infiltration
Infiltration is when rain or melted snow soaks down into the soil instead of flowing away across the top of it. Water that infiltrates can feed plant roots or keep sinking to become groundwater.
Baseflow
Baseflow is the water that keeps a stream flowing even when it has not rained for a while. It comes from groundwater slowly seeping into the streambed.

Also: runoff coefficient, peak runoff

Teacher overview

The Windrow Creek watershed (a fictional watershed used for this lesson) covers 500 acres that were mostly farmland and forest thirty years ago. Since then, suburban development has replaced much of that open land with roads, roofs, and parking lots. This lesson has students use the rational method, a standard stormwater engineering formula, to calculate how much that land-cover change increased the watershed's peak runoff during a storm, then connects that number to the feedback effects development creates in a watershed.

A common misconception this lesson addresses is treating "more pavement" and "more runoff" as simply two separate facts rather than a connected system. When vegetated ground is paved over, rain that once soaked into the soil (infiltration) now runs off the surface instead, which does two things at once: it raises peak flows during storms, increasing flooding and erosion risk, and it reduces the water reaching the aquifer below, which can lower the steady flow (baseflow) the creek gets between storms. Students should leave this lesson able to describe that as one connected feedback, not two unrelated changes.

Procedure

Hook (5 min): Describe or sketch two versions of the same fictional 500-acre Windrow Creek watershed: one mostly open fields and trees, one with dense housing, roads, and parking lots covering most of the ground. Ask students to predict what happens to a heavy rainstorm's runoff in the second version compared to the first, before any calculation.

Direct instruction (15 min): Introduce the rational method formula: peak runoff Q (in cubic feet per second, cfs) = runoff coefficient C × rainfall intensity i (inches per hour) × drainage area A (acres). Explain that C is a number between 0 and 1 representing how much of the rain on a surface becomes runoff rather than soaking in: pavement runs off at about C = 0.9, while lawn or open ground runs off at about C = 0.2. When a watershed has a mix of surfaces, engineers use a weighted average C based on what fraction of the area each surface type covers. Introduce the feedback concept: more impervious surface means more runoff and less infiltration, which increases erosion and flooding during storms while lowering the groundwater recharge that keeps the creek flowing between storms.

Guided practice (10 min): As a class, calculate Windrow Creek's pre-development peak runoff. Thirty years ago the watershed was almost entirely open field and forest, so C ≈ 0.2 for the whole 500 acres. Using a design storm intensity of 2 inches per hour: Q = 0.2 × 2 × 500 = 200 cfs. Record this number on the board as the baseline.

Independent practice (15 min): Students work individually or in pairs on the numbered items below, using the same 500-acre area and 2 inch-per-hour storm intensity.

  1. Today, about 60 percent of the watershed is impervious surface (C = 0.9) and the remaining 40 percent is still open lawn or field (C = 0.2). Calculate the watershed's current weighted runoff coefficient: C = (0.60 × 0.9) + (0.40 × 0.2).
  2. Using your weighted C from item 1, calculate today's peak runoff with the rational method: Q = C × 2 in/hr × 500 acres.
  3. Calculate the percent increase in peak runoff from the pre-development baseline (200 cfs) to today's value (your answer to item 2). Use: percent increase = (new − old) ÷ old × 100.
  4. Explain in two or three sentences how this increase in peak runoff could also reduce the amount of water reaching Windrow Creek's underlying aquifer, connecting the runoff increase to the infiltration decrease described in direct instruction.

Closure / exit ticket (5 min): Students answer individually on an index card: "Name one change to Windrow Creek's developed land (such as a new type of surface or a design feature) that could lower the weighted runoff coefficient back toward the pre-development value, and explain why it would work." Collect the cards and scan for an answer that correctly connects the proposed change to reduced impervious surface or increased infiltration 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: 0.62.
    (0.60 × 0.9) + (0.40 × 0.2) = 0.54 + 0.08 = 0.62.

  2. Answer: 620 cfs.
    Q = 0.62 × 2 in/hr × 500 acres = 620 cfs.

  3. Answer: 210 percent.
    Percent increase = (620 − 200) ÷ 200 × 100 = 420 ÷ 200 × 100 = 210 percent.

  4. Answer: Sample response.
    When more of the watershed is covered by roads, roofs, and parking lots, rain that would have soaked into open ground now flows quickly across the impervious surface into the creek instead, so less water has the chance to infiltrate down to the aquifer; over time, that reduced infiltration can lower the aquifer's recharge and the steady baseflow the creek receives between storms.

Exit ticket answer: Answers will vary. A strong response names a specific change, such as permeable pavement, rain gardens, or preserved open space in new developments, and explains that it works by increasing infiltration (letting more water soak in) or reducing the effective impervious area, both of which lower the weighted C.

Differentiation

Support (below grade level)

Provide the weighted-average formula for item 1 with the percentages and C values already substituted, leaving only the arithmetic for students to complete. Give the rational method formula as a labeled reference card with each variable defined.

Extension (above grade level)

Ask students to calculate what percentage impervious surface (in place of the current 60 percent) would be needed to keep today's peak runoff at or below 400 cfs, working backward through the weighted-average and rational-method formulas.

ELL supports

Pre-teach "impervious," "infiltration," and "runoff coefficient" using a simple two-panel diagram: rain soaking into open ground on one side, rain running off pavement into a storm drain on the other, labeled in English and students' home languages where possible. Allow item 4 and the exit ticket to be answered with a labeled diagram plus one sentence instead of a full paragraph.