RFI - Engineering - Stormwater

additional storage above this level will be                             0.075 m3/s. 7/85mm orifice
therefore be available.                                                 plate holes (or equivalent) are
In some instances, multiple small outlets                               therefore required in the
may be able to be used instead of an orifice                            middle third of the orifice
plate. The capacity of the outlet pipes (which                          plate.
is dependent on factors such as grade and                       iii.    In the top third of the plate,
materials) should be verified to ensure that                            the average depth of water
they do not become the hydraulic control.                               above the centroid is 0.3m,
The multiple pipes should drain to a single                             and it will be required to
pit before connecting to Council’s                                      convey 1/12 of 0.15m3/s, i.e.
underground drainage network.                                           0.0125 m3/s. 1/105mm orifice
                                                                        plate hole (or equivalent) is
Orifice Plate Sizing Example                                            therefore required in the top
An example of the orifice plate sizing is                               third of the orifice plate.
provided below for an example of a 0.5ha             Step 4. The orifice plate holes are able to
rural-residential block being developed into a               physically fit onto the 600mm
medium density development. The volume of                    diameter orifice plate, as shown in
storage required is 210m3. The pre-                          Figure 1, and so multiple outlet
development 10% AEP peak flow is                             pipes will not be required.
0.15m3/s (current day) and 0.18m3/s (at
2100). The depth of the proposed tank is
1.2m.
Step 1. The outlet pipe will be required to
        have a diameter of half of 1.2m, i.e.
        600mm.
Step 2. The target Qo is 0.15m3/s.
Step 3. The following orifice plate holes will
        be required:
          i.   The centroid of the bottom
               third is at 0.1m. The average                           Figure 1 Orifice Plate
               depth of water above the                                  Set Out Example
               centre of the holes at 2/3 of         Step 5. The 600mm RCP outlet at a
               the tank depth is therefore                   minimum grade of 0.2% will have a
               0.7m. The orifice plate holes                 capacity of 0.27m3/s, and so will
               are required to convey 5/12 of                have sufficient capacity to convey
               0.15m3/s, i.e. 0.0625 m3/s.                   the future climate 10% AEP peak
               4/95mm holes (or equivalent)                  flow of 0.18m3/s.
               are therefore required in the
                                                     Step 6. The minimum debris screen area is
               bottom third of the orifice
                                                             20 times the total orifice plate hole
               plate.
                                                             area, i.e 1.53m2.
         ii.   In the middle third of the
               plate, the average depth of
               water above the centroid is
               0.5m, and it will be required to
               convey 1/2 of 0.15m3/s, i.e.



                                                  3 of 3   Fact Sheet On-Site Detention Orifice Plate Sizing