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

                                                                     MORGAN CONSULTING
                                                                    E NGINEERS P TY LTD
                                                                                 16003/CG
                                                                                     Page 5




The storage nodes were included in the post development XP-RAFTS model and the model
input parameters summarised in Table 1 have been maintained in the XP-RAFTS post
development mitigated model for each of the nodes as described above. Refer to the
enclosed Post Development Catchment Plan for details.

Table 4 summaries the post development mitigated peak discharges calculated by XP-RAFTS
for a range of design storms.

      Table 4: XP-RAFTS Post Development Mitigated Peak Discharge
                                  XP-RAFTS Post Development (Mitigated)
                       ARI
                                              Peak Discharge
                     (Years)
                                                   (m3/s)
                       1                  1.452 (1yr 25min storm)
                       2                  1.832 (2yr 25min storm)
                       5                  2.446 (5yr 25min storm)
                      10                 2.718 (10yr 25min storm)
                      20                 3.190 (20yr 25min storm)
                      50                 3.370 (50yr 25min storm)
                      100               3.737 (100yr 25min storm)

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

A review of the XP-RAFTS results shown in Table 2 and Table 4 confirm the proposed
mitigation implemented within the development will maintain pre development flows (or
less) for all design storms except for the 1 year ARI.

The net increase in the 1 year ARI event flow from the development is 0.020m3/s which is
considered minor when compared to the total development flow. Although there is a minor
increase in flow, it should be noted that the development is 54% impervious which is
generally in keeping with the 50% imperviousness assumed in the master drainage plan
report. It should also be noted that the XP-RAFTS model has not taken into account the
storage provided in the bio-retention basins within the development, the long swales within
Catchments A and C or the extensive pipe network throughout the site.

The pipe drainage infrastructure in Resort Drive and the stormwater property connections to
the development have been designed for the minor (10 year ARI) storm event. The pipe
capacities and design flows have been included on the post development catchment plans
for reference. Based on this, there is sufficient capacity within the existing drainage
network to cater for the minor net increase in the 1 year ARI flow from the development.