132004.240018.A - Request to change an existing Approval

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        Bridge Option Analysis

        Consideration   has been given to a proposed       bridge across Cooroy Creek however, this option      has been
        dismissed given the flood mitigation measures that are provided by the culverts.

        The culverts in their proposed form will reguiate flows to ensure upstream and downstream conditions are
        "balanced" to a point where non.worsening of private properties is achieved. The construction of a bridge
        effectively removes this mitigation and has the potential to increase flooding downstream of Garnet Street.

        Furthermore, given the substantial floodway width across Garnet Street and the physicai constraints of the
        Ccoroy Creek alignment, construction of a bridge is not considered a viable option.

        Blockage Factors

        Queensland Urban Drainage Manual (QUDM - 2013) provides suggested a blockage factor of 0% for a clear
        opening greater than 3.0m. Table 10.4.1 - Suggested blockage factars far culverts highlights that a design
        blockage value of 20% should be assigned to culverts less than 3.0m in height or less than S.Omwidth.

        A 20% blockage factor is considered appropriate given the culvert sizes and site characteristics and has been
        adopted for modelling purposes by Hydrology and Water when undertaking the flood modelling analysis.

        In accordance with best practice, a sensitivity test has also been carried out using a blockage factor of 50% in
        order to gain an understanding of any additional flood risk associated with a higher blockage scenario. This
        sensitivity test is documented in the Hydrology and Water report (2014) titled "Garnet Street Culvert Upgrade
        Hydraulic Assessment".

        Flood Impacts of the Revised Upgrade Design

        As part of the Hydrology and Water report (2014) titled "Garnet Street Culvert Upgrade Hydraulic Assessmen('
        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) rainfali events. These maps show the increase and decrease in levels between
        the most recent design scenario and existing scenario.

        Caples of these results are located in Appendix C of the Hydrology and Water report (2014) and a summary
        discussion of these results is provided below.

        o   The upgrade provides a significant improvement to the flood immunity of the Cooroy Creek crossing. The
            current road crossing is un-trafficable in events as frequent as the 2 year ARI, while the revised upgrade
            design wili remain trafficable during events up to the 10 year ARI. Road closure times during large flood
            events will also be significantly   reduced.

        o   The proposed works are predicted to reduce 2 year ARI flood levels upstream of Garnet Street by
            approximately 200mm. Peak 2 year ARI flood leveis immediately downstream of Garnet Street reduce by
            up to 90mm.

        o   The proposed works are predicted to reduce 10 year ARI flood levels upstream of Garnet Street by
            approximately 20 to SOmm. Peak 10 year ARI flood levels immediately downstream of the Garnet Street
            culverts reduce by up to 1oomm. 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 fiood depths over
            the road have actually reduced due to the elevated road level.

        o   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 cuiverts.

        o   The upgraded crossing is not predicted to be trafficable for regular vehicles during a 100 year ARt 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.

        o   The sensitivity testing of the 50% culvert biockage 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




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