Swale Sizing Calculator
Calculate swale sizing accurately for your build. Get material quantities, waste allowances, and project cost breakdowns.
Reviewed by Abdullah, Technical Content Specialist
Formula
Q = (1.49/n) x A x R^(2/3) x S^(1/2)
Manning equation for open channel flow. Q is the flow rate in cfs, n is the roughness coefficient, A is the cross-sectional area in sq ft, R is the hydraulic radius (A/P) in ft, and S is the longitudinal slope in ft/ft. For a triangular swale with side slope z:1, the area is z*d^2 and the wetted perimeter is 2*d*sqrt(1+z^2).
Worked Examples
Example 1: Residential Swale Design
Problem:Size a triangular swale to carry 5 cfs with a 2% slope, Manning n = 0.035, and 3:1 side slopes.
Solution:Using Manning equation iteratively:\nDepth d = 0.84 ft (10.1 in)\nArea A = 3 x 0.84^2 = 2.12 sq ft\nTop width = 2 x 3 x 0.84 = 5.04 ft\nVelocity = 2.36 fps
Result:Swale depth = 0.84 ft, top width = 5.04 ft, velocity = 2.36 fps
Example 2: Parking Lot Perimeter Swale
Problem:Design a swale for 10 cfs with a 3% slope, n = 0.030, and 4:1 side slopes.
Solution:Using Manning equation iteratively:\nDepth d = 0.87 ft (10.4 in)\nArea A = 4 x 0.87^2 = 3.03 sq ft\nTop width = 2 x 4 x 0.87 = 6.96 ft\nVelocity = 3.30 fps
Result:Swale depth = 0.87 ft, top width = 6.96 ft, velocity = 3.30 fps
Frequently Asked Questions
What is a swale and when should it be used?
A swale is a shallow, vegetated open channel designed to convey, filter, and infiltrate stormwater runoff. Swales are commonly used in residential developments, parking lot perimeters, roadway medians, and agricultural settings as a low-cost alternative to underground storm sewers. They provide water quality benefits by filtering sediment and pollutants through vegetation and soil. Swales work best on gentle slopes between 1% and 6% and are typically designed as trapezoidal or triangular cross-sections.
How is Manning equation used for swale design?
Manning equation calculates the flow capacity of an open channel based on its cross-sectional geometry, roughness, and slope. The formula Q = (1.49/n) x A x R^(2/3) x S^(1/2) gives the flow rate in cubic feet per second, where n is the Manning roughness coefficient, A is the cross-sectional flow area, R is the hydraulic radius (area divided by wetted perimeter), and S is the longitudinal slope. Engineers use this equation iteratively to determine the required swale depth and width for a given design flow rate.
How do I determine if my swale design has adequate capacity?
A swale has adequate capacity when the calculated flow depth for the design storm is less than the maximum allowable depth, leaving freeboard for larger storms. Typical freeboard requirements range from 0.25 to 0.5 feet above the design water surface. Additionally, check that flow velocity stays below 4 to 5 feet per second to prevent erosion of the vegetation lining. If the required depth exceeds the maximum or velocity is too high, increase the swale width, flatten the side slopes, or reduce the slope by lengthening the channel path.
References
Reviewed by Abdullah, Technical Content Specialist · Editorial policy