Original Application Part 8 - Stormwater Details [As Published] · 132008.1787

                                                          Stormwater Hydrograph for OSD whole site
                    160
                    140
                                                                                                                        No Detention
Flow (Litres/sec)



                    120
                                                                                                                        With Detention
                    100
                     80
                     60
                     40
                     20
                      0
                          0                   2   4          6           8           10        12          14          16          18      20
                                                                          Time (mins)




                     Method:
                     Total inflow in litres =             total outflow + storage (over any given time period)
                     Total outflow           =           Amount over weir + amount absorbed
                     Total storage            =          Static storage below the overflow + dynamic storage above the overflow
                                                         (Any storage utilised in a gravel bed is adjusted for the void ratio.)
                     Storage above the weir, and flow over the weir, are dependant on the depth above the weir, or outlet pipe IL.
                     All variables in the above equation are known except the depth above the weir or o'flow device.
                     To avoid intergration of the above formulas, time increments are used to solve for depth.
                     The time increment used is 0.1mins. These increments are hidden on the spreadsheet below to save space.
                     Flow increments are progressively added to produce the result.
                     Any difference between the final output volume and the input vol is approximately the amount still held in storage.
                     This storage is gradually absorbed, or used for irrigation between rainfall events or run out, or slow leaked out.

                     If the tank outlet is a weir, the weir formula is used to calculate the depth of water in the tank
                     If the tank outlet is a pipe, a pipe 'part flow' formula is used for all tank depths up to the pipe obvert level.
                     As the water depth in the tank rises, the pipe is then checked for both an orifice and pipe flow (manning) formula.
                     The formula that gives the most restriction to the flow, is the one selected.
                     This is then used to calculate the depth of water in the tank, or other storage device.
                     Sometimes a slight wobble can be seen in the curve where the transitions between formulas occur.
                     The tank water level is also adjusted for exit losses equating to 1.5V^2/(2g)



                                                          Depth of water in tank above outlet IL


                                     1400.0
                                     1200.0
                                     1000.0
                          Depth mm




                                      800.0
                                                                                                                                depth
                                      600.0
                                      400.0
                                      200.0
                                        0.0
                                              0       5             10               15              20              25
                                                                         Time mins