Response to information request REC13/0095

 I infer that the corresponding 100 year flood level at the downstream boundary of Lot 5 is 101.38 m
 AHD,as also indicated in the last row of the schematic.

 I note that Callaghan & Toth have already redrawn the existing 100
                                                                            year flood boundary on their
drawing number 1225-19 to accord with the above advice, and for the sake of completeness I have
attached a copy ofthat revised drawing to this letter. The redrawn line is based on field survey of the
topography ofLot 5,rather than Council’s Lidar topography,and is transcribed to the schematic below.
The existing 100 year flood boundary is seen to be mostly outside the boundary of Lot 5, except for a
small area at the south-east corner ofthe lot. Please note that the flood boundary on the schematic does
not include the flood boundary alongside the local gully, which runs alongside the eastern boundary of
the lot. Please also note that the proposed filling of Lot 5 extends across the southern boundary ofthe
lot and intrudes slightly into the existing flood 100 year boundary the area offilled flood storage will    -




be 12 metre long, with a maximum width of 1.2 metres and a maximum depth of0.08 metre, for which
a prismoidal approximation suggests a displaced floodplain
                                                              storage volume ofjust 0.4 cubic metre. This
loss of storage should not adversely affect upstream and downstream interests.


                                                                                              Proposed Min.F    Levels: 102.99
                                                                                                                   102.68
                                                        Inferred 100 yr flood boundaries, Pre-fi!ling
                                                                                                                                ’\
                                                         For n = 0.15                   Post-Filling
                                                                   \..
                                                                    ’\
                                                                                           0 yr flood boundary
                                                                                         Based on L&T flood levels _
                                                                                         For existing n values
                                                                                                                     ~\.
                                                                                                                      \\
                                                                                                                                          LOT 5


                                                                                                                                     ---______---_
                                                                    v’,                                                    /1
                                                              -t:,?f) :                             Proposed toe of fill    :
                                                    <;)
                                                        f:>              1                                                  1

                                                                         :                    4f"~/1\I                      :
                                                                         1                                                  1
                                                                         1                                                  1
                                                                         1
                                                                         1
                                                                         1
                                                                         \
                                                                          \
                                                                             \
                                                                              \
                                                                                 \
                                                                                  \
                                                                                     \
  100 year Flood levels (mAHD)                                                        \
                                                                                         .,
                                                                                         1


                                                                                         1                                  ,                     1
  85M inference for n value of 0.15
                                                                              100.89                                  101.66                  102.07
  (50%more than 0.10 ~ Add 0.14 m)                                                       1                                  1                     1
                                                                                         1                                  1                     1



  BSM inference for n value of 0.10
                                                                                         ~                                  ~                     ~
                                                                              100.75                                  101.52                 101.93
 (25%more than 0.08 ~ Add 0.07 m )                                                       I                                  1                     1
                                                                                         I                                  1                     1

                                                                                         I                                  I                     1
                                                                                         1                                  I                     1
 L&T prediction for existing n values + 25 %                                             I                                  I                     1

  Adds 0.06   -
                  0.07 m to the existing flood levels                         100.68                                  101.45                 101.86
                                                                                         I                                  I                     I
 (BSM inference ~ n          =   1.25 x 0.064 = 0.080 )                                  I                                  I                     I
                                                                                         I                                  1                     1

                                                                                         I                                  1                     I
  L&T prediction for existing n values of:                                               I                                  I                     I
                                                                                         1                                  I                     1
  Creek bed 0.06                                                                         I                                  I                     I

                                                                                         !                               ~                        ~
  Cleared floodplain 0.03 to 0.05
                                                                              100.62                                  101.38                 101.79
  Uncleared floodplain 0.06 to 0.12                                                      I                                                        I
                                                                                         I                                                        I
  (BSM inference ~ average n 0.064 )                                                     I                                                        I
                                                                                         ,
                                                                                                                                                  I




 8SM interpolation of flood levels at 35% of the distance from the U/S XSect to the Dls XSect                              -




The 100 Year Flood Boundary For A Revegetated Floodplain


I have been advised by Council’s officers that they are seeking a sensitivity analysis for Manning’s n
values of 0.1 0 and 0.15.




BSM Doc File 55LkMacD RFI.Doc
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