CSLB C-27 Landscaping ContractorSite Preparation and DrainageMedium
A landscape contractor is designing a surface drainage system for a large, flat commercial property. The plan includes a series of grass-lined swales to direct stormwater runoff. According to best practices, what is the minimum longitudinal slope recommended for these swales to ensure effective drainage without excessive erosion?
- A0.5%
- B2.0%
- C1.0%
- D3.0%
Show answer & explanationAnswer & explanation
Correct answer: C. 1.0%
A minimum longitudinal slope of 1.0% (or 1%) is generally recommended for grass-lined swales to ensure adequate water flow and prevent standing water, while also minimizing the risk of erosion. Slopes less than 1% may lead to ponding, and significantly steeper slopes can cause erosion.
Why the other options are wrong
- A. 0.5% is often too flat for effective drainage in a swale, leading to ponding and mosquito breeding.
- B. 2.0% might be acceptable in some situations but starts to increase the risk of erosion, especially on longer swales.
- D. 3.0% is generally too steep for a grass-lined swale, as it would likely cause significant erosion, especially during heavy rainfall events.
Minimum Swale Longitudinal Slope
The minimum longitudinal slope for grass-lined swales is typically 1.0% to ensure positive drainage and prevent ponding.
- Prevents standing water and associated issues.
- Balances effective flow with erosion control.
- Steeper slopes increase erosion risk.
- Flatter slopes cause ponding.
Memory trick: One percent for flow, or water won't go!