1. Surface Drainage: Slope and Velocity of Overland Flow.
Unit
Surface Drainage: Slope and Velocity of Overland Flow.

2. Objectives 1. Quantify the relationship between slope and velocity.
2. Describe the impact of roughness (friction) on velocity.
3. Make design decisions relative to slope and roughness to control water velocities in surface drains.

3. Terms
Non-linear relationship
Runoff
Slope
Velocity

4. How does slope affect velocity?
- Velocity, distance divided by time, increases at a ratio of v to the square root of the slope.

5. Velocity
- Slope is the amount of rise divided by the amount of run or distance traveled.

6. Manning’s Equation
-can help estimate the velocity of a fluid in a channel with many variables.

7. What affect does friction have on velocity?

8. Friction affects. - Friction affects the velocity
of water in a channel.
- The roughness coefficient determines the speed of the water.

9. - Winding, sluggish and weedy banks slow water down more than smooth straight channels.

10. Runoff -is water and soil that travel along the soil surface.
- The runoff of a land area is dependent on the infiltration and percolation rates of the soil as well as the precipitation levels.

11. Runoff
- The precipitation levels cannot be controlled. The soil characteristics will change over time, and as erosion increases the soil characteristics will differ.

12. How can changing slope and friction control water velocity in surface drains?

13. - The roughness, slope, and velocity of a channel all impact erosion and surface drainage.

14. -As the roughness of a channel increases the velocity of the water decreases. This forms an indirect relationship.

15. -Roughness is difficult to change in an existing waterway but can be affected in a newly created waterway situation.

16. -The velocity of water flowing overland in an open channel varies as the square root of the slope in ft/ft. The relationship is nonlinear.

17. -Slope is topographical and cannot easily be changed in a large channel but may be altered in smaller channels.

18. MANNING’S EQUATION

19. Roughness Coefficient n, for the Manning Formula Lined channels
Type and Description of Channel
n Value
Asphalt
0.015
Concrete –0.013
Concrete rubble –0.030
Smooth metal –0.015
Corrugated metal –0.026
Plastic –0.011
Wood –0.015

20. Vegetated Channels* Bermuda grass 0.04–0.20 Kudzu 0.07–0.23
Dense, uniform stands of green vegetation more than 10 in. long
Bermuda grass –0.20
Kudzu –0.23
Common Lespedeza –0.095
Dense, uniform stands of green vegetation cut to a length of less than 2.5 in.
Short Bermuda grass –0.11
Kudzu –0.16

21. Earth Channel and Natural Streams
Clean, straight bank, full stage
0.025–0.040
Winding, some pools and shoals, clean
0.035–0.055
Winding, some weeds and stones
0.033–0.045
Sluggish river reaches, weedy or with very deep pools
0.050–0.150

22. Earth Channel and Natural Streams
Clean, straight bank,
full stage
0.025–0.040
Winding, some weeds
and stones
0.033–0.045
Winding, some pools and
shoals, clean
0.035–0.055
Sluggish river reaches, weedy
or with very deep pools
0.050–0.150

23. Pipe 0.009 Asbestos cement Cast iron 0.011– 0.015
Clay or concrete (4–12 in.) – 0.020
Corrugated metal –
Corrugated plastic (2–4 in.)
Riveted and spiral steel –0.017
Vitrified sewer pipe –0.017
Wrought iron –0.017