CSLB C-27 Landscaping ContractorSite Preparation and DrainageMedium
A C-27 contractor is installing a new surface drainage system for a large commercial parking lot. The design calls for concrete swales to direct runoff to a storm drain. What is the minimum longitudinal slope typically recommended for concrete swales to ensure effective drainage and prevent standing water?
- A1.0%
- B0.25%
- C2.0%
- D0.5%
Show answer & explanationAnswer & explanation
Correct answer: D. 0.5%
For concrete swales, a minimum longitudinal slope of 0.5% (or 1/2 inch per 10 feet) is generally recommended to ensure positive drainage and prevent standing water. This slope is sufficient to move water without excessive velocity.
Why the other options are wrong
- A. 1.0% is a good slope but not the *minimum* required; it would increase flow velocity more than necessary for basic drainage.
- B. 0.25% is often too flat for concrete swales and may lead to standing water or slow drainage, encouraging debris accumulation.
- C. 2.0% would create too high a velocity for a typical swale, potentially causing erosion at discharge points or requiring more extensive construction.
Minimum Concrete Swale Slope
The minimum longitudinal slope for concrete swales is 0.5% to ensure positive drainage and prevent standing water.
- Equivalent to 1/2 inch drop per 10 feet.
- Prevents debris accumulation.
- Ensures water flows consistently to discharge points.
Memory trick: Half a percent makes water flow, not halt.