Flood Routing

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Flood Routing Detention Basin Design

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

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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

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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.
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Comparison of flow routing under invariable and
variable stage-discharge relationships
Invariable SQ relationship
Variable SQ relationship
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Discharge under variable SQ relationship
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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).

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Stagestorageoutflow relationship
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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
  stagestorage and stageoutflow relationships are
  known.
• An intuitive trial-and-error approach for
  reservoir routing can be implemented.

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Alternative solution
Stagestorage
Stagedischarge
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Spreadsheet calculation
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Inflow-outflow-(10storage)
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Channel routing