RFI - Attachment 3 - Stormwater Management Plan

                                          Figure 5.2 – DRAINS Post-development catchments



Modelling of the pre and post development conditions for peak flow conditions under the 1% A.E.P storm event has
found that the development will increase run-off rates as shown below in Table 5.1 Existing and proposed site peak
discharge rates.

Table 5.1 Existing and proposed site peak discharge rates

                                                                55 Carpenters Rd
             A.E.P                                                                                                   Qchange (%)
                                       Qexisting (m³/s)            Qproposed (m³/s)        Qchange (m³/s)
                           1EY                        0.411                   0.472                     0.061                       15%
                          50%                         0.481                     0.59                    0.109                       23%
                        0.2 EY                          0.75                    0.94                      0.19                      25%
                          10%                         0.936                     1.11                    0.174                       19%
                            5%                            1.1                   1.29                      0.19                      17%
                            2%                          1.28                    1.53                      0.25                      20%
                            1%                          1.42                    1.72                       0.3                      21%


The site will discharge these run-off volumes into the downstream drainage feature, which conveys an upstream
catchment of approximately 1.2km². The results of the flood modelling for this existing catchment under peak 1%
AEP existing flow conditions have been assessed to determine the impacts of the proposed increase in flow rates
from the development.

From the results, the total flow across the flooded width of approximately 55m downstream of the discharge point
from the site was determined as a baseline. Taking the product of the vector velocity and flow depth grid results
across a section through the channel finds a total flow rate of 22.4m³/s is carried. An increase of 0.3m³/s under the
1% 2100 AEP flood conditions represents a change of 1.3% to the existing flow rate, and an increase in flow depth
of approximately 5mm over the width of the channel.

Relative to the existing catchment and flows being carried, attenuating the proposed development site would offer
insignificant benefits relative to the total existing quantities, and the increased flows are unlikely to result in
worsening to downstream conditions that would post risk or hazard compared to the existing flow rates. It is therefore
proposed that attenuation is not required for the subject development.




Project number 21020                                                                                                Page 28 of 36
File RECOR Concept Stormwater Management Report - 21020.docx, 2022-09-05
Revision 0


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