Applied Hydrology

Flood Routing & Detention Basin Design

Professor Ke-Sheng Cheng Dept. of Bioenvironmental Systems Engineering National Taiwan University

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The storage effect

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The storage effect of a water-retention structure or a channel reach can change the shape of the inflow hydrograph. Therefore two types of routing procedures are needed to calculate the outflow hydrograph.

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Level-pool routing

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Channel routing

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Level-Pool Routing (Reservoir Routing)

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For a reservoir the outflow is uniquely controlled by the water depth (or stage) of the reservoir pool. Also, the reservoir storage is a single-value function of the stage. Flow velocity in the reservoir is usually very low.

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Comparison of flow routing under invariable and variable stage-discharge (S~Q) relationships Invariable S~Q relationship Variable S~Q relationship

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Discharge under variable S~Q relationship

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Reservoir routing

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Modified puls method (Storage indication method)

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An example

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Given the following storage-outflow relationship and inflow hydrograph, calculate the outflow hydrograph. (Use 10 minute routing interval).

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Stage~storage~outflow relationship

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Reservoir routing as a computation of mass balance

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For a reservoir the outflow is uniquely controlled by the water depth (or stage) of the reservoir pool. Also, the reservoir storage is a single-value function of the stage.

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As a result, given a stage the reservoir storage and outflow can be uniquely determined, if the stage~storage and stage~outflow relationships are known.

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An intuitive trial-and-error approach for reservoir routing can be implemented.

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Alternative solution

Stage~storage Stage~discharge

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Spreadsheet calculation

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Inflow-outflow-(10*storage)

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Example of reservoir routing

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Channel routing

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