Attachment C - Stormwater report

Figure 8 – Pre- vs Un-Mitigated Post-Development Hydrographs – 1% AEP Event.

The peak flow rate from the site for the pre-development case has been assessed and the design
hydrograph for the 1% AEP plotted as Figure 8. To assess the peak flow rate during the post-development,
unmitigated scenario, an additional 2.016 ha (i.e. 140 m x 144 m roof area) of the impervious area was
added to the model. A summary of the pre- and post-development peak discharge for the 1% and 63% AEP
events is included as Table 14.
Table 14 – Performance of Mitigation Measures – Peak Flow Rate at LPD

                                                                      3                                       3
                AEP                           Pre-Development (m /s)           Post-Development – mitigated (m /s)
                 1%                                       5.24                                5.24
                63%                                       1.09                                1.09


Results of this assessment demonstrate there is no increase in peak flow rate from the site due to the
construction of the greenhouse. We typically observe an increase in flow rate due to an increase in
impervious surfaces. However, due to the remainder of the site and the flat grades across the site, no
increase was observed in this scenario. It is noted from Figure 8 that the leading edge of the hydrograph
rises quicker in the post-development scenario. This can be attributed to the increase in impervious area.
Further assessment of the XP-SWMM modelling results reveals an increase in runoff volume does occur for
the post-development scenario, but does not translate to an increase in peak flow rate. A comparison of
the runoff volumes for the pre- and post-development scenarios is contained below in Figure 9, and
demonstrates an increase in runoff volume of approximately 540 m3 during the post development scenario.


Stormwater Management Plan – Lot 1 on SP108094, 305 Lake Macdonald Drive                                      Page 23

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