RFI - Appendix D- Stormwater Management Plan · MCU22/0140

                                                                                                                  ingeniir


6.3     Detention Results

The Opal Lane Units detention tanks are proposed to discharge to Opal Lane via two 100mm diameter kerb
adapters; with total peak discharge rates to Opal Lane estimated at 37L/s for a 1% AEP 10 min critical storm
which is an increase on the current discharge to Opal Lane of 21L/s, with the increase attributed to the
redirection of surface flows currently discharging across the western boundary.

As there is no piped infrastructure within Opal Lane it is impractical to detain the surface flows, and the
redirection of flows into a Council controlled discharge point is considered an improvement on the current
drainage scenario and is considered a preferable outcome. All flows (pre-developed and developed) enter the
same existing stormwater infrastructure to the west of the site, as the total site discharge is maintained there
can be no worsening to the performance of the existing infrastructure.

The Ruby Street Units detention tank is proposed to discharge to the field inlet in Ruby Street via a 150mm
diameter pipe; with total peak discharge rates to Ruby Street estimated at 37L/s for a 1% AEP 10 min critical
storm which is an improvement on the current discharge to Ruby Street of 59L/s.

                                 WBNM WHOLE OF SITE DETENTION SUMMARY
                                  Pre-Development     Post-Development
                    AEP                          Duration                         Duration          Performance
                                 Q (m3/s)                         Q (m3/s)
                                                  (min)                            (min)
                   63.20%            0.021                15          0.013                15             62%
                   39.35%            0.029                15          0.027                15             93%
                   18.14%            0.039                15          0.039                15            100%
                    10%              0.049                10          0.046                15             94%
                     5%               0.06                10          0.052                15             87%
                     2%              0.067                10          0.064                10             96%
                     1%              0.074                10          0.074                10            100%
                             Table 7: Comparison of pre and post development site discharge rates

As demonstrated in Table 7, the proposed detention system successfully mitigates stormwater discharge for
the proposed development to pre-development discharge rates.

The proposed development improves on existing stormwater discharge arrangement by redirecting surface
flows currently discharging across the western boundary into Council controlled infrastructure.

As the proposed development maintains current peak discharge rates, it is not considered to affect the
existing capacity of the downstream infrastructure.


7.    FLOODING
The site is in an elevated position locally and regionally and is not known to have any flood risk. A review of
the Noosa Shire Council’s Intramaps Interactive Mapping (accessed 23/04/2022) indicates that the site is not
affected by any known flood studies. Refer Plate 2 for an excerpt of the mapping system.




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