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Streamflow Estimation Module 8.6: Methods for Flow Estimation without Gauges

Learn how to estimate streamflow where no gauge is present through Manning's Equation and other hydrologic principles, considering hydraulic radius, slope, and roughness coefficients.

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Streamflow Estimation Module 8.6: Methods for Flow Estimation without Gauges

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  1. Streamflow Estimation Module 8.6

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

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

  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. Palouse River at Colfax: n @ 0.01

  8. Columbia River at Vernita: n = 0.024

  9. Clearwater at Kamiah: n = 0.033

  10. Spokane River at Spokane: n = 0.038

  11. Grande Ronde at LaGrande: n = 0.043

  12. SF Clearwater at Grangeville: n = 0.05

  13. Boundary Creek at Porthill: n = 0.073

  14. Ötz near Ötz Village : n = 0.12?

  15. Red River near Elk City: n = ?

  16. Paradise Creek near CNR: n = ?

  17. Example 10 ft. 50 ft. Estimate bankfull flow…

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