CSLB C-27 Landscaping ContractorSite Preparation and DrainageMedium
A landscape contractor is designing a low-impact development (LID) feature to manage stormwater runoff from a residential driveway. The design includes a bioretention area (rain garden) that needs to effectively filter pollutants and allow infiltration. Which characteristic is crucial for the soil media used in this bioretention area?
- ALow organic matter content to prevent clogging
- BHigh compaction resistance for stability
- CHigh clay content for nutrient retention
- DRapid infiltration rate (4-8 inches/hour)
Show answer & explanationAnswer & explanation
Correct answer: D. Rapid infiltration rate (4-8 inches/hour)
Bioretention areas rely on rapid infiltration to capture and treat stormwater. A key characteristic of the engineered soil media is a sufficiently high infiltration rate (typically 4-8 inches/hour) to prevent ponding and promote downward movement of water.
Why the other options are wrong
- A. Bioretention media typically requires moderate organic matter content (e.g., 2-5%) for biological activity and pollutant removal, not low content.
- B. While some stability is needed, excessively high compaction resistance is not the primary crucial characteristic; proper grading and material selection achieve stability.
- C. High clay content would impede infiltration, counteracting the primary function of a bioretention area.
Bioretention Soil Media Characteristic
Engineered soil media used in bioretention areas (rain gardens) must have a rapid infiltration rate (typically 4-8 inches/hour) to effectively manage stormwater runoff.
- Allows quick absorption of stormwater.
- Prevents prolonged surface ponding.
- Critical for pollutant removal through filtration and biological processes.
- Typically a mix of sand, compost, and topsoil.
Memory trick: Rain gardens need to soak it up FAST to filter and flow.