Attachment C - SWMP Report by Bax Engineering_compressed

           STORMWATER QUANITY MANAGEMENT

         DRAINAGE AND EXISTING TOPOGRAPHY

             Lawful Point of Discharge

In general, the site is elevated at around 19.0m AHD in Bartlett Street and grades at approximately 2% towards
an existing low point at around RL 18.0m AHD in the south-west corner. There is an existing drainage pit in this
location which will be utilised as a lawful point of discharge for roofwater and carpark drainage. Due to the
proposed design of the building, overland flows from major events will be diverted to the Bartlett Road
frontage in the north. This will ensure any pipe blockages will not create flooding to the tenancies.



             External Catchment and Overland Flows

In general, the site experiences no overland flow from any upper catchments.



             Stormwater Infrastructure

The proposed stormwater system utilises a 225mm diameter pipes and will ultimately discharge to the existing
pipe system. Due to the calculated volume of the upstream catchment connected to this system (0.076m3/s)
it is proposed to upgrade a section of the existing network to a 300mm diameter. This should ensure that the
capacity of the system is not exceeded with the proposed connection of the new impervious area for the site
(0.033m3/s) is connected.
Of this volume 0.033 m3/s is a minor storm event and would be directed to the existing pipe system.
Overland flows from any major storm events will be directed to the Bartlett Street frontage. This calculated
volume is 0.054m3/s. Therefore the overland flow less the piped system would result in a volume of 0.021m3/s
discharging to the northern frontage.


In accordance with QUDM Table 7.3.5 and 7.3.6, road flows must be restricted for transverse flows to d.V
values of <0.3m2/s for minor flows and 0.6m2/s for major flows. These flows are also to be contained wholly
within the reserve
Flows were calculated at the outlet point using Izzard’s equation with the velocity calculated using the
Manning’s equation. The d.V value from the site based on post development conditions was less than 0.04
and therefore well under the allowable limit.




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   2261 david low way, peregian beach, qld 4573      acn: 626 671 092            [email protected]