REFERENCED DOCUMENT 100 Year Boundary for the Existing Floodplain Condition - REC13/0095

 I understand that L&T’s assessment of the existing Manning’s values ranged from 0.06 in the creek
 be~ to 0.03     -
                      0.06 in the cleared floodplain areas, to 0.06   -
                                                                          0.12 in the uncleared floodplain areas.
 Given that the overbank areas in the subject vicinity are mostly grassed paddocks, I infer that L&T’s
 model would have simulated an average n value across the floodway ofjust below 0.065 at these cross-
sections;   -
                or   nominally 0.064 as I have indicated in the last row of the preceding schematic.

I understand that L&T also undertook a sensitivity analysis with Manning’s n values 25 percent higher
than the existing values, resulting in the flood levels in the second-last row ofthe preceding schematic.
These flood levels are 0.07 meter higher than the corresponding levels for the existing floodplain
condition. On the basis ofmy nominal inference about average n values in the preceding paragraph, the
L&T model probably simulated an averaged Manning’s n value ofabout 0.064 + 25%                   =
                                                                                                     0.080 for this
sensitivity scenario, still short of Council’s requested values of 0.10 and 0.15.

I have used the abovementioned L&T sensitivity test as a guide to the impacts offurther increases in the

Manning’s n value. An increase in the Manning’s value to 0.10 would represent a 25 percent increase
on the abovementioned     value of 0.080, or the same % increase as in L&T’s sensitivity test. This
suggests that a Manning’s n of 0.1 0 would add about another 0.07 metre to L&T’s sensitivity flood
levels, thereby resulting in the flood levels in the third-last row of the preceding schematic. A further
increase,to a Manning’s n 0.15,would represent a 50 percent increase on the value of0.10, or double
the percentage increase in L&T’s sensitivity test, which suggests that this would add about twice 0.07
metre (i.e. 0.14 metre) to the flood levels for the 0.10 value        as indicated in the top row of the
                                                                           -




preceding  schematic.


Both ofthe abovementioned increases in Manning’s n value (to 0.10 and 0.15)would, in the absence of

any allotment filling, shift the 100 year flood boundary inside the Lot 5 boundary, as indicated for the
"Pre-Filling"scenario on Callaghan &Toth’s attached drawing and in the schematic on the preceding
page of this letter. However, Lot 5 is proposed to be filled to and beyond the lot boundary, thereby
raising the topography at the lot boundary enough to prevent any additional intrusion of the flood
boundary into the lot, as indicated by the realigned 100 year flood boundaries for the "Post-Filling /
n=0.15" scenario on the attached drawing and in the schematic. The house-pad is proposed to be filled
to a minimum level of 102.68 m AHD, or 0.61 metre higher than the 100 year flood level at the

upstream comer ofthe lot for a Manning’s n of0.15. Given that the 100 year flood depth in this area is
about 1.8 metre, that clearance of 0.61 metre represents a freeboard of about 34 percent of the flood
depth, above the "fully revegetated" flood level. The floor level on Lot 5 is proposed to be even higher,
at 102.99 m AHD,resulting in a freeboard of 0.92 metre (ormore than 50 percent ofthe flood depth)

above the "fully revegetated" flood level. I acknowledge that the abovementioned flood levels for the
revegetated floodway have been derived from some approximate inferences rather than detailed
modelling, but I suggest that the generosity ofthe abovementioned freeboards will be more than enough
to offset any shortcomings in those inferences.



Please do not hesitate to contact me if you require any clarification of the above advice. In the
meantime, I remain,

Yours faithfully,

      (?
~".~~
~"-~
Denis Ogle, RPEQ 2405, Director.




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