Vegetated Swale Capacity Calculator

Calculate vegetated swale capacity, normal depth, velocity, freeboard, and Froude number with Manning's equation and vegetation-retardance roughness.

Calculate Vegetated Swale Hydraulics

For educational purposes only. Not a substitute for professional engineering judgment.

Input Parameters

Design Requirements

Design Flow

cfs

Peak flow rate the swale must convey

Swale Length

Total length for residence time calculation (optional)

Channel Geometry

Longitudinal Slope

Channel slope along flow direction (ft/ft or m/m)

Side Slope (H:V)

Horizontal to vertical ratio (e.g., 4 means 4:1)

Bottom Width

Width of flat bottom (0 for triangular section)

Channel Shape

Cross-sectional shape of the swale

Vegetation Properties

Vegetation Type

Type of grass/vegetation lining the swale

Max Permissible Velocity

ft/s

Override default velocity limit for erosion control

Vegetated Swale Design Overview

Vegetated swales convey stormwater while providing filtration and infiltration. Design uses Manning's equation with variable roughness coefficients that depend on flow depth, velocity, and grass retardance.

Normal Depth - Uniform flow depth for design flow
Velocity Check - Must be below permissible velocity for erosion control
Froude Number - Flow regime indicator (subcritical preferred)
Residence Time - Contact time for water quality treatment

Vegetation Retardance Classes

Vegetation types and their retardance classes
Vegetation Type	Class	Height Range
Bermuda Grass	C	6-12 inches
Buffalo Grass	C	3-6 inches
Kentucky Bluegrass	C	4-10 inches
Tall Fescue	B	8-18 inches
Native Grass Mix	B	12-24 inches
Unmowed/Dense Grass	A	> 24 inches
Short Mowed Grass	D	2-4 inches

Source: Chow (1959) Open-Channel Hydraulics, Table 5-6

Manning's n Values by Retardance Class

Manning roughness values by vegetation retardance class and velocity-radius product
Class	Low VR	Medium VR	High VR	Description
A	0.50	0.25	0.150	Very high retardance - dense, tall vegetation
B	0.35	0.15	0.080	High retardance - tall grass (18-24 in)
C	0.25	0.10	0.050	Moderate retardance - medium grass (6-12 in)
D	0.15	0.06	0.035	Low retardance - short grass (2-6 in)
E	0.08	0.04	0.025	Very low retardance - very short grass

VR = Velocity x Hydraulic Radius. Source: HEC-22 Table 7-6

Permissible Velocities (ft/s)

Permissible swale velocities in feet per second by retardance class and slope range
Retardance Class	Slope 0-5%	Slope 5-10%	Slope > 10%
Class A	8.0	7.0	6.0
Class B	7.0	6.0	5.0
Class C	6.0	5.0	4.0
Class D	5.0	4.0	3.0
Class E	4.0	3.0	2.5

For erosion-resistant soils. Source: HEC-22 Table 7-5