RFI - Engineering - Stormwater

A removable debris screen, in accordance                         The orifice plate holes should be
with the Section BN5.5.2(p) of QUDM (2017)                       spaced so that the:
should be provided upstream of the orifice                        i.    bottom third of the plate can
plate. A sump pit, which houses the outlet                              convey 5/12 of Q0 when H is
orifice, will also be necessary. The access                             2/3 of the tank depth
chamber to the underground storage should
be adequately sized to enable the debris                          ii.   middle third of the plate can
screen to be removed from the tank.                                     convey 1/2 of Q0 when H is
                                                                        2/3 of the tank depth
The orifice plate should be made from a
corrosion resistant 5mm thick stainless steel.                   iii.   the top third of the plate can
The orifice plate will have numerous holes at                           convey 1/12 of Q0 when H is
specified levels and diameters cut out of it.                           2/3 of the tank depth
The minimum acceptable orifice plate hole             Step 4. Multiple holes will be required in
diameter is 25mm.                                             each section of the orifice plate. The
                                                              minimum acceptable orifice plate
Orifice Plate Sizing Process                                  hole diameter is 25mm. Check that
The orifice plate should be designed using                    the proposed number of holes can
the following steps:                                          fit within the available orifice plate
                                                              area over the outlet pipe. In order to
Step 1. Size the outlet pipe to which the                     accommodate the required number
        orifice plate will attach so that it has              of holes, multiple outlet pipes or a
        a diameter half the depth of the                      box culvert may be required. The
        underground tank. Multiple pipes                      procedure to size the orifice plate
        can be used to achieve the required                   holes will not be modified, the holes
        total depth, in order to allow smaller                will just be spread over a greater
        outlet pipes to be used.                              area.
Step 2. Determine the current climate pre-            Step 5. Check that the outlet pipe/s has
        development 10% AEP peak flow                         sufficient capacity (based on
        based on A Review of Simple Peak                      Colebrook-White equation) to
        Flow Estimation Methods for use on                    convey the 10% AEP (at 2100)
        the Sunshine Coast following the                      peak pre-development flows.
        release of ARR 2016 (SCC, 2017).
        This will be the target Qo.                   Step 6. Size the removable debris screen
                                                              so that the area (including blocked
Step 3. Size the required orifice plate holes                 area) is at least 20 times the total
        based on the following equation:                      orifice area. A grid mesh should be
         Qo = C A (2gH)1/2                                    used (e.g. Weldlok A40/203 or
         Where:                                               similar).

         C = 0.6 (unless otherwise justified)         An example of the orifice plate is attached.

         A = total orifice area (m2)                  The proposed outlet design should help to
                                                      ensure that post-development peak flows
         g = acceleration due to gravity              from the site are attenuated to pre-
         (m2/s)                                       development levels for all storm events, up
         H = depth of water above the                 to and including the 1 in 100 AEP (at 2100).
         centroid of orifice plate hole (m)           The depth of water (H) used to size the
                                                      orifice plate is limited to two thirds of the
                                                      depth of the tank. During major storms,


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