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Detention Design

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Detention Design

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    1. 1 Detention Design

    2. 2 Objectives Know what a detention basin is Compute a reservoir routing by hand Inflow Hydrograph Storage and Outflow versus WSE Routing Table

    3. 3

    4. 4 Detention Basin-Purposes Store water temporarily during a storm and release the stored water slowly Attenuate the flow Store first-flush Design for infiltration

    5. 5 Detention Basins On-Site Regional

    6. 6 Detention Basins Inflow (ditch or pipe) Storage Outflow (orifice or weir) Emergency spillway

    7. 7 Routing Method used to model the outflow hydrograph Based on continuity equation Water in varies Water out varies

    8. 8 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)

    9. 9 Computing Storage Volumes Elevation-Area Average end-area

    10. 10 Discharge Rating Calculate outflows based on water elevation in the detention pond Orifice and weir equations are used

    11. 11 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)

    12. 12 Reservoir Routing Avg Inflow (I) – Avg Outflow (O) = Change in storage per some increment of time If I=O there is no change in water elevation If I>O then water surface rises If I<O then water surface decreases

    13. 13 Reservoir Routing Over 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

    14. 14 Equation (I1+I2)/2 – (O1+O2)/2 = (S2-S1)/t Rearrange to: (I1+I2) + [(2*S1/t)-O1]=2*S2/t + O2 Left hand side – knowns Right-hand side - unknowns

    15. 15 Equation O2 and S2 -2 unknowns; need 2 equations There is a relationship between outflow and storage volume

    16. 16 Example 14-5 Inflow Hydrograph

    17. 17 Example 14-5 Storage Vol & Discharge vs WSE

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    21. 21 Example 14-5 Routing Table

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