2014-10-14 Planning & Organisation Committee Agenda - Pages 1 - 50

PLANNING & ORGANISATION COMMITTEE MEETING AGENDA                              14 OCTOBER 2014


Flood Impacts of the Revised Upgrade Design
As part of the Hydrology and Water report (2014) titled "Garnet Street Culvert Upgrade Hydraulic
Assessment” a series of Peak Flood Level Impact maps have been provided for the 2.Year ARI
(39% AEP), 10.Year ARI (10% AEP) and 100.Year ARI (1% AEP) rainfall events [see
Attachment 1]. These maps show the increase and decrease in levels between the most recent
design scenario and existing scenario.
Copies of these results are located in Appendix C of the Hydrology and Water report and a
summary discussion of these results is provided below.
   The upgrade provides a significant improvement to the flood immunity of the Cooroy Creek
     crossing. The current road crossing is not trafficable in events as frequent as the 2 year
     ARI, while the revised upgrade design will remain trafficable during events up to the 10
     year ARI. Road closure times during large flood events will also be significantly reduced.
   The proposed works are predicted to reduce 2 year ARI flood levels upstream of Garnet
     Street by approximately 200mm. Peak 2 year ARI flood levels immediately downstream of
     Garnet Street reduce by up to 90mm.
   The proposed works are predicted to reduce 10 year ARI flood levels upstream of Garnet
     Street by approximately 20 to 50mm. Peak 10 year ARI flood levels immediately
     downstream of the Garnet Street culverts reduce by up to 100mm. There is also a
     localised peak flood level impact of approximately 100mm located over the sporting fields
     immediately downstream of the proposed western culverts. The 10 year ARI impact map
     shows flood level increases over Garnet Street which is due to the proposed raising of this
     road. It is noted that while flood levels over the road have increased, peak flood depths
     over the road have actually reduced due to the elevated road level.
   There are some localised peak flood level impacts for the 100 year ARI event of up to
     20mm upstream of the Garnet Street road reserve. These impacts are contained over a
     vacant, low-lying bushland site which is already subject to peak 100 year ARI depths in the
     existing case of generally between 0.5 to 1m. There is also a localised peak flood level
     impact of approximately 180mm located over the sporting fields immediately downstream
     of the proposed western culverts.
   The upgraded crossing is not predicted to be trafficable for regular vehicles during a 100
     year ARI flood however, peak depths and velocities over the upgraded crossing during this
     event are such that it would be expected to remain trafficable for emergency vehicles.
   The sensitivity testing of the 50% culvert blockage factor predicts that peak 100 year ARI
     flood levels will increase by up to approximately 100mm. Generally speaking, this level of
     flood level impact is not considered to pose a large increase to the flood risk for the area
     however it is recommended that Noosa Council undertake on-going inspection of these
     culverts and periodic clearing of potential debris upstream to minimise the likelihood of this
     high level of blockage.


Assessment
Council officers have had protracted discussions with the applicant’s engineers regarding the
Garnet Street road upgrade requirements and generally agree with the points submitted above.
The applicant was asked to engage the hydraulic engineer that undertook Council’s recent flood
modelling for Cooroy, Rob Stewart of Hydraulic and Water Management Consulting Pty Ltd, and
he has prepared the referenced report (which can be provided to Councillors on request). The
Peak Flood Level Impact maps from the report are included as Attachment 1.
Given the complex flood modelling issues involved, Council had the report peer reviewed by
Steve Chamberlain of O2 Environment + Engineering. He advised:



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