start audio lecture! for462: watershed science & management 1 streamflow estimation module 8.6
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FOR462: Watershed Science & Management FOR462: Watershed Science & Management
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Streamflow EstimationStreamflow Estimation
Module 8.6
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FOR462: Watershed Science & Management
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
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FOR462: Watershed Science & Management
Manning’s Equation
For uniform flow in a channel:
21325.1SR
nv =
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 !!
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FOR462: Watershed Science & Management
Manning’s Equation
21325.1SR
nv =
Hydraulic Radius of Channel:R = A/P
Where:A is the cross sectional areaP is the wetted perimeter
d
wEstimate from geometrical relationships, or surveyed
cross section
Why use wetted
perimeter and not
total perimeter?
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FOR462: Watershed Science & Management
Manning’s Equation
21325.1SR
nv =
Hydraulic Radius of Channel:R = A/P
drop
Estimate from topographic maps, or survey stream reach
distance
Slope of Channel:S = drop / distance
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FOR462: Watershed Science & Management
Manning’s Equation
21325.1SR
nv =
Hydraulic Radius of Channel:R = A/P
Slope of Channel:S = drop / distance
Finally we must estimate Manning’s nrange: 0.01 (concrete) – 0.1 (coarse bed, dense brush)n depends on:• bed material, vegetation, channel configuration, sinuosity• varies with stageHerein lies the true art of the method !!