Detention Design

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Detention Design

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Objectives

• Know what a detention basin is
• Compute a reservoir routing by hand
• Inflow Hydrograph
• Storage and Outflow versus WSE
• Routing Table

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Detention Basin-Purposes

• Store water temporarily during a storm and
  release the stored water slowly
• Attenuate the flow
• Store first-flush
• Design for infiltration

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Detention Basins

• On-Site
• Regional

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Detention Basins

• Inflow (ditch or pipe)
• Storage
• Outflow (orifice or weir)
• Emergency spillway

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Routing

• Method used to model the outflow hydrograph
• Based on continuity equation
• Water in varies
• Water out varies

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Information Needed to Route

• Inflow hydrograph
• Relation of storage volume to elevation in the
  proposed detention basin
• Relation of outflow to water level elevation
  (discharge rating)

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Computing Storage Volumes

• Elevation-Area
• Average end-area

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Discharge Rating

• Calculate outflows based on water elevation in
  the detention pond
• Orifice and weir equations are used

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Set up table with small time steps

• Water in Water Out Difference in Storage
• (water in is changing w/ time)
• (water out is a function of basin WSE)
• (basin WSE is a function of basin geometry)

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Reservoir Routing

• Avg Inflow (I) Avg Outflow (O) Change in
  storage per some increment of time
• If IO there is no change in water elevation
• If IgtO then water surface rises
• If IltO then water surface decreases

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Reservoir RoutingOver some time increment (1 to
2)

• I1 and I2 are inflows at beginning and end of
  time increment
• O1 and O2 are outflows
• S2 and S1 are actual storage volumes
• Delta t is the time from 1 to 2

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Equation

• (I1I2)/2 (O1O2)/2 (S2-S1)/t
• Rearrange to
• (I1I2) (2S1/t)-O12S2/t O2
• Left hand side knowns
• Right-hand side - unknowns

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Equation

• O2 and S2 -2 unknowns need 2 equations
• There is a relationship between outflow and
  storage volume

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Example 14-5Inflow Hydrograph
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Example 14-5 Storage Vol Discharge vs WSE
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Example 14-5Routing Table
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