Minor Change Report MCU24 0142 01 & OPW24-0185.01 (As Published)

               •        Noosaville Local Plan Code;
               •        Dual Occupancy and Multiple Dwelling Code;
               •        Works Codes;
               •        Acid Sulfate Soils Overlay Code
               •        Flooding and Inundation Overlay Code.


               Existing Conditions 38 and 66 (Operational Works Approval)
               The applicant has requested that conditions 38 and 66 be amended to reference an amended
               Stormwater Management Report which proposes a revised detention system.
               Approved Stormwater Management System
               The approved system proposed to collect stormwater via a series of pits and pipes and attenuate
               flows in a 10m3 detention tank before discharging into Robert Street via the existing kerb
               adapter(s).
               The rationale for providing detention in the first instance is unusual. The stormwater detention
               calculations incorrectly assumed a lower fraction impervious (0.4) than what currently exists on
               site (~0.8). Additionally, the developed fraction impervious (1.0) was assumed higher than
               expected through the use (~0.9). In combination, these calculations overstate the increase in
               development flows to a point where detention could reasonably be avoided.
               The size of the low-flow orifice (30mm) is unusually small and limited to a maximum flow rate of
               ~1.7 L/s. For context, the SWMP reports that during very frequent, frequent, and infrequent events
               the development will produce peak flow rates of 27 L/s, 59 L/s and 103 L/s, respectively. The net
               result is likely that the detention tank will rapidly fill during the frequent and infrequent events and
               engage the high overflow weir, meaning in events other than very frequent event, peak storm flows
               will not be attenuated and coincide with peak storm flows external to the site, meaning the detention
               system will be ineffective. Further, it is likely the inefficiency of the outlet structure will result in
               system blockages noting now blockage screen as been provided in accordance with AS3500.
               Amended Stormwater Management System
               The amended system removes the 10m3 detention tank and instead relies on 142 metres of 300mm
               dia. uPVC to attenuate flows via the outlet structure and discharge into Robert Street. The pipes
               connect through a series of manholes and run two-to-three pipes in parallel.
               Assessment of Request
               A Request for Further Advice was issued seeking more information on some of the areas of non-
               compliance (fall, ability to empty between events, parallel pipes, roofwater connections,
               maintenance access, impacts on Robert Street, etc.) and advised the applicant the system was
               unnecessarily complex, unlikely to function and unlikely to be supported.
               The response and overall review of the system as referred to Council’s external stormwater
               consultant who provided the following comments:
                    •    None of the above-mentioned issues with the sizing and design methodology have been
                         addressed.
                    •    The calculations in the SWMP include a 177mm orifice being used in the calculations even
                         though a 30mm orifice is retained in the design.
                    •    There is only 400mm level difference between the overflow weir in the control pit and the
                         surface levels at the most upstream pits. No HGL calculations are provided, and it seems
                         reasonably likely that surcharge of these pits will occur in moderate rainfall events.
                    •    There are a number of pipe connections and changes of direction that seem to indicate
                         they would be done by gluing PVC pipe to couplings. Normal practice for surface
                         stormwater systems is that pits would be provided at these types of junctions (refer
                         AS3500.3).
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Document Set ID: 24562651
Version: 8, Version Date: 06/03/2026