Green Infrastructure Design: Rain Garden Challenge
Stormwater infiltration testing, sized for Detroit's combined sewer system.
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Standards
HS-ESS3-4 — Evaluate or refine a technological solution that reduces impacts of human activities on natural systems.
Why Does This Matter?
Detroit's combined sewer system carries stormwater and wastewater in the same pipes in some areas. During heavy rain, this can overwhelm the system and release untreated water into the Detroit River. Rain gardens are one real, low-cost tool the city uses to soak up runoff before it ever reaches a sewer.
Objective
Test how different soil and plant setups affect stormwater infiltration rate, and design a rain garden sized to handle runoff from a model roof or parking lot.
Materials
- 3–4 clear plastic cups or bottles with drainage holes
- Different soil/media samples (plain soil, sand-amended soil, mulch-topped soil, potting mix with roots/plants)
- Water and a measuring cup
- Stopwatch
- Tray to catch drained water
Safety: Wipe up water spills immediately to prevent slipping. Wash hands after handling soil.
Procedure
- Fill each cup with a different soil/media sample to the same depth, and poke consistent drainage holes in the bottom of each.
- Pour the same measured amount of water into each cup at the same time.
- Time how long it takes for water to stop draining from each cup, and measure how much water collects in the tray below versus how much is absorbed.
- Calculate the percent of water absorbed vs. drained for each sample.
- Using the best-performing media, calculate the area of rain garden needed to absorb runoff from a model roof of a given size and rainfall amount (formula provided by teacher).
Data Table
| Soil/media type | Water added (mL) | Water drained (mL) | % absorbed | Drain time (s) |
|---|---|---|---|---|
Analysis Questions
- Which soil/media type absorbed the most water? What might explain why?
- How did adding plants/roots change infiltration compared to bare soil?
- Using your best-performing media's absorption rate, how large would a rain garden need to be to handle runoff from a 500-square-foot roof during a 1-inch rainstorm?
- Beyond reducing sewer overflow, list two other benefits a rain garden might provide to a Detroit neighborhood.