1. Design of Swales 2
Norman W. Garrick
CE 4410 Spring Lecture 17

2. Calculations for Trapezoidal Swale
Calculate the a) flow velocity , b) flow volume for the trapezoidal vegetative swale shown. The slope of the channel is 2% and the Manning’s roughness coefficient is 0.10.
2.438 m
1.219 m m
1.362 m

3. Calculations for Trapezoidal Swale
Calculate the flow velocity V = (R2/3 S1/2)/n >>>>>>> note… metric… do not include 1.48 R = Area/Wetted Perimeter A = ( )* = m2 WP = = m R = 5.162/ = m
2.438 m
1.219 m m
1.362 m

4. Calculations for Trapezoidal Swale
Calculate the flow velocity
V = (R2/3 S1/2)/n
R = m
S = 0.02
n = 0.10
V = * /0.10
Velocity = 0.81 m/s2
2.438 m
1.219 m m
1.362 m

5. Calculations for Trapezoidal Swale
Calculate the flow volume q = AV = * 0.81 Flow Volume = 1.81 cu. m./s
2.438 m
1.219 m m
1.362 m

6. Designing Parabolic Swales Using Charts

7. Retardance for Parabolic Swale
Each chart is based on a RETARDANCE value The retardance varies with the type of grass, how good the stand is, and the height of the grass Retardance A provides the most resistance to flow Retardance E the least resistance to flow

8. Retardance A
Weeping Lovegrass 30 in

9. Retardance B
Bermuda Grass 12 in

10. Retardance E
Bermuda Grass 1.5 in

11. Swale Design with Chart
Design a swale that will have a good stand of Kentucky bluegrass that will be mowed to stay between 2 and 10 inches tall. The soil is easily eroded and the grade of the swale is 5% This swale should be designed to carry 36 cubic feet per second of water.

12. Swale Design with Chart
We need to design so that
Permissible Velocity is not exceeded
We have enough capacity

13. Swale Design with Chart
What is the critical condition for PERMISSIBLE VELOCITY?
Good stand Kentucky Blue Grass, 2 inches high
Good stand Kentucky Blue Grass, 10 inches high
What is the critical condition for CAPACITY?

14. Swale Design with Chart
PERMISSIBLE VELOCITY Good stand Kentucky Blue Grass, 2 inches high Permissible velocity = 4 ft/s ….. Table 13.1 Retardance for 2 inch high Kentucky Blue Grass is Retardance D Need A, W, D for design

15. Swale Design with Chart
PERMISSIBLE VELOCITY Need A, W, D for design q = AV 36 = A*4 >>>>> A = 9 ft2 Get R from Chart Figure 13.9(d) R = 0.43

16. Swale Design with Chart
PERMISSIBLE VELOCITY R = 0.43 ft, A = 9 ft2 Try, D = 1.5 R D = 1.5*0.43 = 0.65 ft
Parabolic Swale
Hydraulic Radius = Cross-section Area/Wetted Perimeter
Hydraulic Radius: R = W2D/(1.5W2+4D2)
Area: A = 2/3 WD

17. Swale Design with Chart
PERMISSIBLE VELOCITY Substitute for D = 0.65 into A = 2/3 WD 9 = 2/3 W*0.65 W = 20.8 ft

18. Swale Design with Chart
When the grass is 2 inches high W = 20.8 ft and D = 0.65 ft is ok What happens when grass is 10 inch high? Flow velocity slows down Area needed for carrying the peak run-off increases
W1 = 20.8 ft
D1 = 0.65 ft

19. Swale Design with Chart
Do we have enough capacity for high grass situation? When grass is 10 inch high Retardance is C We need larger D Assume D = 0.75 ft
D2 = ?

20. Swale Design with Chart
Assume D = 0.75 ft Again take R = 1.5 D >>>>> D = R/1.5 = 0.5 ft From Figure 13.9c V = 3.4 ft2/s
D2 = ?

21. Swale Design with Chart
V = 3.4 ft2/s Is D = 0.75 adequate for q = 36 cubic feet/second q = AV
D2 = ?

22. Swale Design with Chart
To get A we need to calculate W2 W2 = W1 (D2/D1)0.5 = 20.8(0.75/0.65)0.5 = 22.3 ft A = 2/3 WD = ft2
W2 = ?
D2 = 0.75

23. Swale Design with Chart
q = AV = 11.15*3.4 = cubic feet/s Adequate capacity
W2 = 22.3 ft
D2 = 0.75