Flood Routing & Detention Basin Design Professor Ke-Sheng Cheng Dept. of Bioenvironmental Systems Engineering National Taiwan University

The storage effect 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. Level-pool routing Channel routing

Level-Pool Routing (Reservoir Routing) 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.

Invariable S~Q relationship

Reservoir routing Modified puls method (Storage indication method)

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

Stage~storage~outflow relationship

Reservoir routing as a computation of mass balance 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. 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. An intuitive trial-and-error approach for reservoir routing can be implemented.

Alternative solution Stage~storage Stage~discharge

Spreadsheet calculation

Inflow-outflow-(10*storage)

Example of reservoir routing