Conduit Outlet Expansion Examples

Message: Conduit Outlet Expansions are simple structures but the downstream channel should be armored.

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The conduit outlet expansion structure reduces eddies and turbulence that are typically associated with a sudden expansion and provides protection for the toe of the dam and the channel side-slopes in this transition zone.

Considerations

Tailwater Depth The effectiveness of the conduit outlet expansion can be greatly increased by ensuring adequate tailwater. The tailwater may be maintained with either a downstream weir or an appropriately configured downstream channel.

Downstream Channel Protection Typically, it is necessary to provide some form of channel protection downstream from the conduit outlet expansion.

Tailwater Depth The length necessary to prevent excessive scour without a downstream stilling basin is a function of the Froude number, tailwater depth, and conduit diameter.

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  Half isometric drawing of one half of a USDA-NRCS type PWD basin.

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  Outlet expansion into downstream channel. The discharge is 270 cfs with an approximate Froude number equal to 1.6.

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  The discharge is 400 cfs with an approximate Froude number equal to 2.0.

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  Minor scour exists downstream from the structure after sustained discharge of 270 cfs.

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  7.5-foot horseshoe conduit discharging into an outlet expansion with a depressed floor. Downstream channel armoring has also been provided.

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  Design Guidelines

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  Riprap armored channel.

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  Grouted riprap armored channel.

Revision ID: 1624
Revision Date: 06/27/2022