CSLB C-27 Landscaping ContractorSite Preparation and DrainageHard
A landscape contractor is designing a surface drainage system for a large residential property. The design includes a series of swales to direct stormwater away from the house and towards a designated infiltration area. What is the minimum recommended longitudinal slope for a grass-lined swale to ensure effective drainage and prevent standing water?
- A0.5%
- B3.0%
- C2.0%
- D1.0%
Show answer & explanationAnswer & explanation
Correct answer: A. 0.5%
For effective drainage and to prevent standing water, a grass-lined swale typically requires a minimum longitudinal slope of 0.5% (or 0.005 feet per foot). While steeper slopes drain faster, 0.5% is generally considered the minimum for adequate flow without excessive erosion.
Why the other options are wrong
- B. 3.0% is a relatively steep slope for a grass-lined swale and might lead to increased erosion if not properly designed.
- C. 2.0% is a steeper slope often used for faster drainage or in areas with higher runoff, but not the minimum.
- D. 1.0% is a good general purpose slope, but not the minimum required.
Minimum Swale Longitudinal Slope
The smallest acceptable slope along the length of a swale, typically 0.5%, to ensure continuous water flow and prevent standing water.
- Ensures positive drainage.
- Prevents water stagnation and mosquito breeding.
- Balances drainage efficiency with erosion control.
Memory trick: Slope to Flow, Not to Grow.