1 Detention Routing

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

3 Detention Basins Inflow (ditch or pipe) Storage Outflow (orifice or weir or combination) Berm Emergency spillway

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

5 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)

6 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)

7 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

8 Reservoir Routing Over some time increment (1 to 2) I 1 and I 2 are inflows at beginning and end of time increment O 1 and O 2 are outflows S 2 and S 1 are actual storage volumes Delta t is the time from 1 to 2 Note-variables in red are initially known

9 Equation (I 1 +I 2 )/2 – (O 1 +O 2 )/2 = (S 2 -S 1 )/t Rearrange to: (I 1 +I 2 ) + [(2*S 1 /t)-O 1 ]=2*S 2 /t + O 2 Left hand side – knowns Right-hand side - unknowns

10 Equation O 2 and S 2 -2 unknowns; need 2 equations There is a relationship between outflow and storage volume

11 Example 14-5 Inflow Hydrograph

12 Example 14-5 Storage Vol & Discharge vs WSE Cell G9=2*7500/(1440)-2.5=7.9 cfs Cell H9=2*7500/(1440)+2.5=12.9 cfs

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Example 14-5 Routing Table 16

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