Response to information request REC13/0095

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         Your Reference:


         Our Reference:       2013-08
                                                                   Brisbane Stonnwater Management Pty Ltd
                                                                     ACN 094 878 734 ABN 75 094 878 734
                                                                        92 Alkoomie Street, Wynnum Nth, Qld
                                                                          PO Box 8119, Wynnum Nth, Q 4178
         Date:      6 Sept 2013                                           Ph 07 33963383      Fax 07 39010285
                                                                            Email:    [email protected]


        Callaghan & Toth
        POBox 2305,Noosa Heads, Q4567


        Attn:    Mr Lex Toth


        Dear Sir,


        Re:      Proposed 3 Lot Subdivision At 55 Lake MacDonald Drive, Cooroy

        In response to your emailed request of 16 August 2013,I offer
                                                                   my advice in relation to Items 3 and 5 of
        SeRC’s 24 July 2013 request for further infonnation about the above proposal; namely-


        3     Provide an amendedplan which accurately represents the regionalflood extent on site (Council
              flood extent mapping indicates thatpart ofLot 5 is within inundation extents).

        5     Provide additional information (or an updatedflood study) which demonstrates the roughness
              (manning’s)assumptions adopted in the originalflood study allow for revegetation ofthe main
              drainage line. Alternatively provide a sensitivity analysis ofincreased roughness assumptions and
              amendflood level extents and lot layout as necessary


        The 100 Year Flood Boundary For The Existing Floodplain Condition


        Lawson & Treloar Pty Ltd (L&T)undertook a flood study ofthe creeks in the Cooroy area for Council
        in July 2002.I understand that some parts ofthat study have recently been updated for Council
                                                                                                      by other
        consultants, but the updated parts do not include the tributary alongside the proposed subdivision.
        Accordingly, I have fonnulated my following advice on the basis that the 2002 L&T flood study is the
        latest available study of flood levels in the subject area.


        The schematic on the following page of this letter illustrates the floodway / subdivision scenario. One
        ofL&T’s model cross-sections abuts the upstream boundary of Lot 5, and that lot extends about 35
        percent of the floodway distance between that cross-section and the next downstream cross-section. I
        understand that L&T’s predictions of the 100 year flqod levels at those two cross-sections, for their
        assessment ofthe existing Manning’s n values, are respectively 101.79 m AHD and 100.62 m AHD,a<;

        marked in the last row of data in the schematic. I therefore infer that the 100
                                                                                          year flood level at the
        upstream boundary of Lot 5 for the existing floodway condition is 101.79 m AHD,and by arbitrarily
        proportioning 35 percent ofthe level differential between the upstream and downstream cross-sections,



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