Development Application part 7 of 10 Stormwater Management Report - MCU13/0092

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3.      Site Hydrology

3.1 Method and Approach


The stormwater discharges from this site were calculated using the rational method. This method as described in the
Queensland Urban Drainage Manual (QUDM)is commonly used in municipal hydrology and is more than adequate
for the design requirements. The Rational Method is based around the Rational formula:



Q=-
      C/lyA
y
  360



Where:             Oy      =   Peak flow rate (m3/s)for the average recurrence interval (ARI)of y years

                   Cy      =   Coefficient of runoff for ARI of y years

                   A       =   Area of catchment (ha)
                   t
                       I   =
                               Average rainfall intensity (mmlhr)for design duration of t hours and
                       y

                               an ARI of y years.



When designing the stormwater drainage system the following points have been considered:


Stormwater treatment shall be designed for a 3 month storm event, calculated as Q1 x 0.5; and Stormwater
reticulation is to cater for Q3 month ,Q20 and Q100 storm events.


For the purposes of this stormwater quality assessment, the area of the proposed development requiring stormwater
quality treatment has been divided up into 4 catchments:

Table 1 details catchment names, areas and storm event flows for each of the 4 catchments based on the Rational
Formula. Refer to Appendix B for details of catchment calculations and Appendix A, drawing 13-024-C34, for
catchment areas.


                                               Table 1: Catchment and Flow


     Catchment             Catchment Area        Q100 Major Flow          Q20 Minor Flow     Q3 month Treatable Flow
     Catchment A               0.3494 ha              0.306 m3/s             0.221 m3/s               0.043 m3/s
     Catchment B               0.1023 ha              0.090 m3/s             0.065 m3/s               0.013 m3/s
     Catchment C               0.1893 ha              0.166 m3/s             0.119 m3/s               0.023 m3/s
     Catchment D               0.0094 ha              0.008 m3/s             0.006 m3/s               0.001 m3/s




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