BsmRevRpt2

The impact of the proposed development on peak flow rates downstream of the site is summarised in
Table 23, below. Flow rates leaving the site via the eastern tributary (nodes 15 – 16) will be slightly
reduced. Flow rates leaving the site via the western tributary (nodes 29.5 - 30) will increase, and this is a
result of the diversion of local stormwater runoff away from No 2 Pearsons Road and directly to the
western floodway. Further downstream along the western floodway, past the junction with the remaining
local runoff from that adjoining property (nodes 31-32), the post-development flow rates will be close to
the pre-development flow rates. Further downstream, at the downstream end of the model (nodes 38-39),
where the two tributaries have converged, the flow-rates will be unchanged or slightly reduced.

Table 23 :    Pre / Post-Development Peak Flow Rates (m3/sec) Through Downstream Properties
 Between        1 Year      2 Year      5 Year      10 Year       20 Year     50 Year     100 Year
  Nodes
   15 - 16     6.8 / 6.7     8.4 / 8.3    13.8 /13.4    17.4 / 17.3   20.7 / 19.9   23.8 / 22.6   27.0 / 25.0
 29.5 - 30     5.0 / 5.3     6.3 / 6.7    9.6 / 10.3    11.7 / 12.3   13.7 / 14.9   16.0 /18.1    17.8 / 20.5
   31 - 32     5.4 / 5.6     6.7 / 6.8    9.9 / 10.3    12.5 / 12.4   17.7 / 17.7   26.3 / 26.1   31.5 / 31.2
   38 - 39     12.7/12.7    15.8 / 15.8   24.2 / 24.1   30.1 / 29.6   37.9 / 37.6   47.9 / 47.0   55.1 / 53.7

The flood levels predicted by the post-development model have been compared to those for the pre-
development scenario in Table 19 to 21 on preceding pages of this report, for ARI’s from 1 to 100 years.
Yellow highlighting is used in those tables to identify nodes that are within the site boundaries. The flood
level affluxes on all external properties as a result of the proposed development are seen to be practically
zero (maximum 0.01 metre) or negative (beneficial), when quoted to the nearest centimetre.

The corresponding 100 year post-development flood profiles are shown on Figure 10, and the post-
development 100 year boundary is mapped on Figure 13.

9.3     Sensitivity Tests

The sensitivity of the development to uncontrolled regrowth along the floodways was examined by
applying a Manning’s n of value of 0.15 to the overbank riparian zones of all the cross-sections of the
model. The additional impact of the footbridge being blocked by debris was simulated by assuming that
the handrails could be completely blocked by debris and the under-bridge aperture could be 50 percent
blocked. The 100 year flood levels predicted by these tests are summarised in Table 24, on the following
page of this report, and in Figures 10 and 13. The design levels for the development platform should take
due account of these flood levels.




      Brisbane Stormwater Management Pty Ltd Document File: ValkevSDF.Doc                           Page 22