OUTDOOR
How to Calculate a Drainage Swale
Contractor method for estimating a trapezoidal swale excavation volume, grade and fall.
A drainage swale is a shallow channel that moves water. Its excavation volume comes from a trapezoidal cross-section times the length.
THE SHORT ANSWER
How do you calculate a drainage swale?
Average the top and bottom width, multiply by depth and length for excavation volume. A 50 ft swale with 6 ft and 2 ft widths at 1.5 ft deep is 300 cu ft; a 1% grade adds 0.5 ft of fall.
Measure the cross-section
Record the top width, bottom width and depth. The average of the widths times the depth is the section area.
- Length
- Top width
- Bottom width
- Depth
Compute the section area
A trapezoid uses the average width times the depth. (6 + 2) ÷ 2 × 1.5 = 6 sq ft.
Multiply by the length
The volume is the section area times the swale length. 6 sq ft × 50 ft = 300 cu ft.
Check grade and fall
The calculator converts longitudinal grade into total fall. It estimates excavation geometry only; hydraulic capacity, erosion control and outlet design still require professional verification.