Reservoir Routing

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Reservoir Routing
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Reservoir Routing

• Similar to channel storage routing
• Outflow controlled by the principle spillway at a
  rate depending on the height of water above the
  inlet.
• Outflow is at a maximum where it crosses the
  inflow hydrograph

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Reservoir Routing

• Storage is at a maximum when outflow is at a
  maximum.
• Storage volume required in the reservoir is the
  area between the inflow and outflow hydrographs
  prior to the peak outflow.
• If the initial water level is the spillway crest
  than the volumes under the inflow and outflow
  hydrographs are equal.

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Graphical Routing Puls Method

• Graphically solve the continuity equation using
  storage characteristic curves.
• Stage-storage curve is developed from topographic
  information relative to the reservoir site.
• Stage-discharge curve is based on the hydraulics
  of the reservoir outlet.
• Routing interval should be 10 to 25 of the time
  to peak of the inflow hydrograph.

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Primary Uses

• Flood Peak Reduction
• Detention Storage
• Storm Water Management

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Flood Peak Reduction

• Based on
• Inflow Hydrograph
• Allowable downstream peak flow
• Storage requirements
• Outlet hydraulic characteristics

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Design Procedure for Flood Peak Reduction

• Determine inflow hydrograph for design event.
• Determine allowable peak outflow from the
  reservoir. Based on
• Downstream conditions
• Governing regulations
• Estimate the storage requirement by
• Plot the allowable peak outflow on the falling
  limb of the inflow hydrograph
• Approximate the outflow hydrograph up to the peak
  outflow as either a straight line or an estimated
  curved line.
• Estimate the storage required as the volume of
  flow between the inflow and outflow hydrographs.

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Design Procedure for Flood Peak Reduction

• Estimate the maximum stage from the stage-storage
  relationship.
• Determine the spillway hydraulic configuration
  required to produce the peak flow with the
  available storage.
• Route the inflow through the reservoir based on
  this spillway.
• Adjust the spillway characteristics and repeat
  previous step until a satisfactory design is
  achieved.

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Detention Storage

• Detention storage time is a measure of the
  average residence time of the water in a
  reservoir.
• Useful as measure of opportunity time for
  sediment to settle out.
• Actual and theoretical detention time may differ
  because of dead storage in the reservoir.
• Desirable to minimize dead storage when designing
  sediment basins.

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Theoretical Detention Time

• In a rectangular basin with a constant flow rate

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Detention time for variable flow

• Plug flow concept (first in-first out)
• Divide inflow hydrograph into equal volume plugs.
• Route each plug through the reservoir and
  determine detention time for each.
• Average the detention time for all of the plugs.

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Centroid Concept

• Theoretical detention time is defined as the time
  difference between centroids of the inflow and
  outflow hydrographs.

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Centroid Concept

• Tm can be calculated from
• Or approximated by

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Storm Water Management

• Used to reduce peak flows.
• Regulations often require that peak discharge
  from a design storm after a land-use change not
  exceed the peak discharge prior to the land-use
  change.
• Often requires detention storage be built in to
  the development plans.

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