BsmRevRpt2

7.0     RUNOFF RATES TO FLOODWAY MODEL


7.1     Catchment Areas And Subdivision

Flood profiles along floodways such as the subject tributaries are affected by the hydraulic capacity of the
adjoining downstream reaches and by any waterway structures in those downstream reaches.
Accordingly, the Extran model extends some distance downstream of the site to the convergence of the
two tributaries just downstream of Pearsons Road, as shown in Figure 3. The catchment modelling
therefore includes not just the contributing areas alongside and upstream of the site, but also the areas
downstream of the site to that termination point downstream of Pearsons Road. The extent of the 151.7
hectare overall catchment is shown in Figures 4 and 5. The site occupies 8.0 hectare, or about 5 percent,
of the overall catchment area.

The catchment has been subdivided into 23 subareas for modelling of the pre-development scenario (as in
Figure 3), and 41 subareas for the post-development scenario (as in Figure 4). The subdivision of the
areas external to the site is identical between the two scenarios, whereas the subdivision of those areas
which discharge to the floodways within the site are treated differently between the two scenarios, to
allow for accurate modelling of the increased runoff from the proposed urban development of 65 percent
of the site area. The modelling of the post-development scenario also accounts for the Q5 splits of the
inflows to the bio-basins, which necessitates doubling-up of the relevant six subareas, taking the total
sub-area count to 47 as marked on Figure 5. The impervious fractions applied to each of the modelled
subareas are summarised in Table 12, on the following page of this report.


7.2     Design Rainfall Data

The Bureau of Meteorology has recently updated the rainfall intensity-frequency-duration data that was
previously published in Volume 1 of Australian Rainfall and Runoff (1987). The previous data was based
on rainfalls recorded up to 1985, whereas the latest data is based on rainfalls recorded up to 2010 / 2011.
There has also been a change of nomenclature, whereby the previous description of severity in terms of
“average recurrence interval” has been updated to “annual exceedence probability” for storms from 2-
year to 100 year severity. Rainfall intensities for the 1 year severity are still quoted for an average
recurrence interval of once per year. The updating has increased the rainfall intensities at higher ARI’s
(or lesser exceedence probabilities) by about 4 percent, compared to the previous data. The updated
intensity-frequency-duration data for Cooroy is reproduced in Appendix A of this report.




      Brisbane Stormwater Management Pty Ltd Document File: ValkevSDF.Doc                          Page 12