1. Start Audio Lecture! FOR462: Watershed Science & Management 1 Streamflow Estimation Module 8.6

2. FOR462: Watershed Science & Management 2 How do we estimate flow where no gauge exists? Particularly pertinent question Where gauge existed, use correlation Relationship may not be valid in areas of extensive landcover modification Estimate based on hydrologic similarity Roughly the same volume of flow per unit area will be generated from hydrologically homogeneous areas Manning’s Equation !! Useful for design purposes

3. FOR462: Watershed Science & Management 3 Manning’s Equation For uniform flow in a channel: Where: v = average velocity (ft s -1 ) R = hydraulic radius of channel (ft) S = slope of channel (dimensionless) n = Manning’s roughness coefficient Note: in SI units, replace 1.5 by 1 !!

4. FOR462: Watershed Science & Management 4 Manning’s Equation Hydraulic Radius of Channel: R = A/P Where: A is the cross sectional area P is the wetted perimeter d w Estimate from geometrical relationships, or surveyed cross section Why use wetted perimeter and not total perimeter?

5. FOR462: Watershed Science & Management 5 Manning’s Equation Hydraulic Radius of Channel: R = A/P drop Estimate from topographic maps, or survey stream reach distance Slope of Channel: S = drop / distance

6. FOR462: Watershed Science & Management 6 Manning’s Equation Hydraulic Radius of Channel: R = A/P Slope of Channel: S = drop / distance Finally we must estimate Manning’s n range: 0.01 (concrete) – 0.1 (coarse bed, dense brush) n depends on: bed material, vegetation, channel configuration, sinuosity varies with stage Herein lies the true art of the method !!

7. FOR462: Watershed Science & Management 7 Palouse River at Colfax: n  0.01

8. FOR462: Watershed Science & Management 8 Columbia River at Vernita: n = 0.024

9. FOR462: Watershed Science & Management 9 Clearwater at Kamiah: n = 0.033

10. FOR462: Watershed Science & Management 10 Spokane River at Spokane: n = 0.038

11. FOR462: Watershed Science & Management 11 Grande Ronde at LaGrande: n = 0.043

12. FOR462: Watershed Science & Management 12 SF Clearwater at Grangeville: n = 0.05

13. FOR462: Watershed Science & Management 13 Boundary Creek at Porthill: n = 0.073

14. FOR462: Watershed Science & Management 14 Ötz near Ötz Village : n = 0.12?

15. FOR462: Watershed Science & Management 15 Red River near Elk City: n = ?

16. FOR462: Watershed Science & Management 16 Paradise Creek near CNR: n = ?

17. FOR462: Watershed Science & Management 17 Example 50 ft. 10 ft. Estimate bankfull flow…