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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 !!

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FOR462: Watershed Science & Management

Palouse River at Colfax: n 0.01

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FOR462: Watershed Science & Management

Columbia River at Vernita: n = 0.024

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FOR462: Watershed Science & Management

Clearwater at Kamiah: n = 0.033

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FOR462: Watershed Science & Management

Spokane River at Spokane: n = 0.038

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FOR462: Watershed Science & Management

Grande Ronde at LaGrande: n = 0.043

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FOR462: Watershed Science & Management

SF Clearwater at Grangeville: n = 0.05

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FOR462: Watershed Science & Management

Boundary Creek at Porthill: n = 0.073

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FOR462: Watershed Science & Management

Ötz near Ötz Village : n = 0.12?

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FOR462: Watershed Science & Management

Red River near Elk City: n = ?

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FOR462: Watershed Science & Management

Paradise Creek near CNR: n = ?

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FOR462: Watershed Science & Management

Example

21325.1SR

nv =

50 ft.

10 ft.

Estimate bankfull flow…