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

2. 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

3. 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.

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

5. Discharge under variable S~Q relationship

6. Reservoir routing
Modified puls method (Storage indication method)

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

10. Stage~storage~outflow relationship

12. 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.

13. Alternative solution
Stage~storage
Stage~discharge

14. Spreadsheet calculation

15. Inflow-outflow-(10*storage)

16. Channel routing