BsmRevRpt2

1.0      SUMMARY

This report has been revised in response to Noosa Council’s 18 July 2014 information request. Responses
to the relevant items in that information request are stated in Section 3 of the report. The applicant has
also received a copy of a 3 September 2014 submission to Council from the owners of an adjoining
property (JR & F Saunders), and comments on that submission are stated in Section 4 of the report.

This analysis has found that the development’s stormwater detention requirement can be achieved by
deliberately “roughening” the floodways (by revegetating the riparian zones, as required by Council), by
excavating a side-storage alongside the western tributary, and by constructing a restrictive footbridge
across the eastern tributary to back-up the flood profiles in that floodway, to augment the small amount of
conventional detention storage that can be gained in two bio-basins on the site (as in Figures 9 to 12).

In fact, the nett gains in flood storage volumes generated by these works will substantially exceed the
detention volumes that would otherwise be required of a fully conventional detention system, as shown in
Table 1, below.

Table 1:  Comparison Of Pre-Development And Post-Development Floodplain Volumes (m3)
           To Required Detention Volumes
                       1Year     2 Year  5 Year    10 Year   20 Year      50 year    100 Year
  Pre-Development      5536       6311    8787      10202     11576        13125      14483
  Floodway Storage
  Post-Development     5909       6775    9607      11697     13389        15383      16849
  Floodway Storage
   Floodway Gain        373       464      820       1495      1813        2258        2366
   Plus Bio-Basins      162       183      252        252       252          252        252
 Less Park, Bund Fill    -0         -0      -0         -0        -5          -40        -72
      Nett Gain         535       647     1072       1747      2060        2470        2546
 Req’d Extra Volume
    (Conventional       395       525     770        910       1095        1370        1585
      Detention)

The abovementioned works, together with a minor regrading of the invert profile around node 5 (as also
shown on Figure 10) will result in the development having practically nil adverse impact on flood levels
on adjoining properties, as summarised in Tables 19 to 21 of this report.

The works will also result in practically nil adverse impact on downstream peak flow rates, apart from a
slight increase in flows along the western tributary through the immediately adjoining downstream
property (No2 Pearsons Road). That increase in creek flows across the adjoining property results from the
deliberate diversion of local runoff away from that adjoining property, and directly to the western
floodway upstream of that property. Downstream of the natural point of inflow to the floodway for that
runoff, the flows revert to their pre-development rates.

The consequent flood levels for design of the development are summarised in Tables 19 to 21 and in
Figure 13 of this report. A sensitivity test has identified the impact of uncontrolled floodplain regrowth
on the abovementioned design levels. The consequent 100 year flood levels are summarised in Table 24
and in Figure 13, and appropriate account should be taken of these levels when considering appropriate
design levels for the development.




      Brisbane Stormwater Management Pty Ltd Document File: ValkevSDF.Doc                           Page 3