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Estimating Streamflow in Unmonitored Areas: Techniques and Considerations

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This module explores methods for estimating streamflow where no gauging stations exist. It emphasizes the use of hydrologic similarity and correlation with nearby gauges, recognizing the limitations caused by land cover changes. It introduces Manning's Equation for calculating flow in channels, detailing the importance of hydraulic radius, channel slope, and roughness coefficients. The module provides insights into estimating these parameters from topographic maps and using various case studies to illustrate the range of roughness coefficients applicable in different river conditions.

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Estimating Streamflow in Unmonitored Areas: Techniques and Considerations

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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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