1. Norman W. Garrick
Design of Grass Swales

2. Swales are commonly GRASS-LINED
Overview
Swales are designed to convey water as part of an OPEN storm water system – they are not meant to be continuously wet
Typically the cross-section of swales are PARABOLIC in shape but other cross-sections are sometimes used
Swales are commonly GRASS-LINED
The type and height of grass affects the resistance to flow – an important factor for design

3. Overview Swale Design aims to
Limit erosion damage by making sure permissible velocity in not exceeded during peak flow
Ensure sufficient capacity to accommodate peak flow
The maximum or permissible velocity for a given swale depends on
The soil’s resistance to erosion
Type of grass
Height of grass
Slope of swale

4. Overview
The height of grass is an important design parameter but it changes with maintenance – the design should be based on the range of conditions expected
Low grass – results in maximum velocity so low grass should be used to design for stability (erosion damage)
High grass – results in minimum velocity so high grass should be used to design for capacity

5. Overview
The swale should not be designed to slow the water that is entering it unless sedimentation is desired
If sedimentation is desired, water can be slowed by
Decreasing the slope of the swale
Using wider swale
Using taller vegetation
As sedimentation accumulates, the character and capacity of the swale changes.
Maintenance is required to remove sediment and to repair erosion

6. Swale Design Formulas
Open channel flow, such as in a swale, is based on two formulas
Manning’s Equation
Continuity Equation

7. Swale Design Formulas Manning’s Equation V = (1.486 R2/3 S1/2) / n
Velocity of flow in an open channel is given by Manning’s Equation
V = (1.486 R2/3 S1/2) / n
V – flow velocity, ft/s
N – Manning’s roughness coefficient for open channels
R – hydraulic radius, ft
S – channel slope, ft/ft

8. Swale Design Formulas R - Hydraulic Radius
R = cross-section area of flow / wet perimeter
Water Surface
Wet Perimeter 5 4 1 20
Area = 20 sq, ft
WP = = 22 ft
R = 20/22 = 0.91 ft
Area = 20 sq, ft
WP = = 13 ft
R = 20/13 = 1.54 ft
Larger R = less resistance to flow

9. Swale Design Formulas Continuity Equation q = A V
Flow rate and velocity
q = A V
q – flow in cu. ft/s
A – cross-section area for flow, sq, ft
V – flow velocity, ft/s

10. Area and R of Parabolic Swales
Area A = 2/3 WD
Hydraulic Radius R = (W2D) / (1.5 W2 + 4 D2)
Width Ratio W2 = W1 (D2/D1)0.5