06/0210 - Response to request for information darted 14/03/06 [As Published] · 132006.210

4. HECRAS MODELLING

A HECRAS model has been compiled to translate the design flow rates to flood profiles along the
main floodway and along the flowpath of the kerb overflow.

The cross-sections alignments for the main floodway are shown in Figure 2 while those for the kerb
overflow are shown in Figure 3. Both of those figures also show the surveyed topographic data that
was used to define the cross-section shapes.

The model was started from a 1.25 percent slope-area condition at cross-section 1, about 80 metres
downstream of Cooroy Mountain Road.

Cross-section 18 represented the full width of the kerb overflow at the junction of Topaz and Kauri
streets (see Figure 3). The topography immediately downstream of the kerb has the effect of
deflecting part of the overflow into a formalised drainage channel to the left, while the remainder of
the overflow is deflected to the right and down alongside the subject property, through cross-
sections 17 to 12. The amount of the overflow which is deflected to the right was identified by
locating the left-hand channel marker of cross-section 18 at the dividing line for the left and right
deflections. The overflow to the right was then the sum of the "channel" flow and the "right
overbank" flow at that cross-section; - which equated to 0.30 cumecs or about half of the total 100
year overflow of 0.59 cumecs.:

The condition of the existing floodways is typified by the photographs in Figure 4. The main
floodway in the vicinity of the subject site (cross-sections 8 to 11) is covered in long grass with
some scattered trees; - these cross-sections were conservatively assigned a Manning’s n of 0.10 in
the HECRAS model. Downstream of cross-section 8, the main floodway is significantly overgrown,
for which a Manning’s n value of 0.15 was assigned to cross-sections 1 to 7. The course taken by
the kerb overflow is basically mown grass, for which a Manning’s n value of 0.05 was assigned to
cross-sections 12 to 18.

Printouts of the HECRAS results for the existing floodways are shown on pages 11 and 12 of the
Appendix. The predicted flood flows and levels alongside the subject property are summarised in
Table 1. The flood levels are also marked on Figure 5.

Table 1: Predicted 100 Year Flood Flows And Levels

 

 

 

 

 

 

 

 

 

 

 

Cross-Section Flow (m3/s) Level (m AHD)
17 0.3 107.45
16 0.3 107.09
15 0.3 106.76
14 0.3 106.36
13 0.3 106.01
12 0.3 105.72
11 7.6 105.33
10 7.6 105.32
Brisbane Stormwater Management P/L Doc File Topaz.Rpt - 30 Oct 2006 Page 3