Meyer Judgment Sealed

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To achieve a non-worsening effect the proposed development must not discharge peak flows
larger than what was identified in the pre-development case.

Using an orifice equation, the infiltration trench (detention tank) outlet orifices can be sized.
The 10% AEP flow rate as shown in Table 1 is 37 L/s. However, consideration must also be
given to the infiltration flow rate of 2 L/s and the bypass flow of 7 L/s. Therefore, the effective
design flow rate for design purposes is 28 L/s (Q0 as described in the guideline).

An example of sizing the orifice for the bottom-third of the orifice plate is shown below. Using
an 86mm dia. orifice with a 2/3 tank height of 0.6m.

                                          𝑄 = 𝐶𝐴√2𝑔ℎ

                           𝑄 = 0.6 × 0.006√2 × 9.81 × 0.56 = 12 𝑙/𝑠


Where:
C = 0.6
A = pipe cross-sectional area (m2)
g = gravity (m/s2)
h = height of water above centre of orifice (m)

Therefore, an 86mm dia orifice will have a discharge of 12 l/s. In accordance with the guidelines
the maximum outflow allowable for the bottom-third of the orifice plate must be 5/12 of Q0 which
equates to 12 L/s. Therefore, the orifice size is acceptable.

The full calculations for the sizing of the remaining orifices are shown in Appendix B. The
calculations have demonstrated that the following orifice arrangement is required to match pre-
development peak discharges:

   •   Bottom-third: 1 x 86mm dia.
   •   Middle-third: 1 x 98mm dia.
   •   Top-third: 1 x 41mm dia.

Further the downstream outlet pipe will be a 225mm dia pipe at 1% grade with a flow capacity
of 45 L/s. The pipe is required to cater for the pre-development 10% AEP 2100 flow of 44 L/s.
Therefore, the outlet pipe size is adequate. In the event that the outlet system is blocked the
overland flow path has adequate capacity downstream to ensure the surrounding floor levels
will not be inundated.

The headwall for the outlet pipe has been provided with rip rap for scour protection. Based on
a 225mm outlet pipe with an outlet velocity of 1 m/s the required rip rap is determined to be
100mm rock over a length of 675mm minimum. Refer to Figure B1 for the determination of the
scour protection. As mentioned previously a level spread is also being provided to ensure the
stormwater discharges as sheet flow.

Further, the catchment upstream of the detention tank has been designed to convey the 1%
AEP flows to the detention tank. With the inclusion of the proposed stormwater management
strategy the development is considered to have a non-worsening effect to the surrounding
properties.




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