Swmp

   Developed          62       78          96         110          126          147          163
     (L/s)

      Mitigated     36(+3)   48(+4)       60(-4)     70(-7)      87(-5)       109(0)       121(-2)
        (L/s)



As shown in Table 6.4 the proposed OSD facilities have demonstrated the proposed mitigated peak
discharge flow rates will not exceed the total pre-development flows for all design storms above 2 year
ARI, up to and including the 1% AEP Design Storm. We note that there is a marginal increase in the
mitigated developed peak flow rate for the Q1(1EY) and Q2 (2EY) storm events of 3 & 6L/s. However,
the increase in stormwater discharge is considered very minor and the total discharge rates are below
that of the existing site Q10 peak flows which is the design ARI for the Council’s stormwater drainage
system, it is also noted the proposed site was never intended as a greenfield site (0%) impervious and
as such imposing full detention requirements is excessive. Therefore, the effect on the downstream
network is considered to be insignificant and negligible.



Further, all stormwater discharging site in the developed case will discharge to the bio-retention basin
which contains an extended detention depth of 200mm. This equates to approximately 4kL of additional
detention storage which has not been considered in the calculation of peak stormwater discharge rates
for the developed site for the minor storm events. The peak stormwater discharge values are therefore
considered to be conservative and are expected to be less than the existing site. We consider the
developed site will not have a non-worsening effect on surrounding properties and downstream
network, with regard to stormwater discharge.



The proposed stormwater detention system is therefore considered to be adequate for the
development.



6.2       Proposed Drainage:

The proposed concept site drainage is detailed in drawings SCG 33094 – C5.1 (Appendix D) and is
discussed below.



As defined in the Noosa Plan 2020 stormwater drainage must be designed in accordance with QUDM
2017, therefore the following design storms were adopted: -



          1.      Minor Drainage System         -   10% AEP ARI (Q10 Storm)
          2.      Major Drainage System         -    1% AEP ARI (Q100 Storm)


6.3       Manage Allotment and Cross Allotment Flows:

The adjacent site described as lot 2 on RP 77499 (No. 21 Sidoni St) fall towards shared boundary with
the site. All existing cross allotment flows from the neighboring properties must also be considered,
managed and maintained. Clear unhindered drainage paths must be provided and maintained during

 SCG Consulting engineers                            15                     SCG Ref: 33094(SWM).Rev A