17155 - Stormwater Quantity Assessment Letter - Rev 0 · OPW17/0612

                                                                       MORGAN CONSULTING
                                                                      E NGINEERS P TY LTD
                                                                                   16003/CG
                                                                                       Page 4




Table 3 summaries the post development un-mitigated peak discharges calculated by XP-
RAFTS for a range of design storms.

      Table 3: XP-RAFTS Post Development Unmitigated Peak Discharge
                                  XP-RAFTS Post Development (Unmitigated)
                       ARI
                                              Peak Discharge
                     (Years)
                                                   (m3/s)
                        1                 1.622 (1yr 25min storm)
                        2                 2.126 (2yr 25min storm)
                        5                 2.696 (5yr 25min storm)
                       10                 3.022 (10yr 25min storm)
                       20                 3.473 (20yr 25min storm)
                       50                 3.647 (50yr 25min storm)
                       100               4.050 (100yr 25min storm)

Refer to the attached post development unmitigated peak flow rate hydrographs from XP-
RAFTS.

As seen in Table 2 and Table 3 there will be a net increase in the 1 year and 2 year ARI
event peak flow from the development and reduction in peak flow for all other ARI events.
Therefore on site detention will be required to mitigate the increase in peak flows for the 1
year and 2 year ARIs.

The decrease in peak flow for the 5 to 100 year ARIs for the post development scenario can
be attributed to the considerable decrease in slope of the site due to the development. XP-
Rafts utilises the Laurenson Method to calculate peak flows which specifically accounts for
slope and other parameters while also allowing for storage within the catchment.

       2.3 Post Development Mitigated

Stormwater detention will need to be provided to ensure that the pre development peak
flows for the 1 and 2 year ARI’s listed in Table 2 are not exceeded for the proposed
development. It is proposed to provide detention storage at the following locations within
the development:

   1. 503m3 within the open space area adjacent to the pool within Precinct F;

   2. 67m3 within the stormwater reuse tank located in front of Building 4; and

   3. 48m3 storage tank under the basement slab of Buildings 23, 24 and 25 for total
      storage of 144m3.

Refer to the operational works drawings for the storage details.