Glen - REC13/0146 Flood Study BSM 30 April 2014. Email 2 Attachment 2

The impact of the proposed development on peak flow rates downstream of the site is summarised in
Table 20, 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 29-30) 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 flows will revert to the
pre-development rates (within +/- 0.01 m3/sec). Further downstream, at the downstream end of the model
(nodes 38-39), where the two tributaries have converged, the flow-rates will be slightly reduced.

Table 20 :    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 / 13.5   17.3 / 17.2   20.4 / 20.4   22.9 / 22.8   26.0 / 25.1
 29 - 30      5.1 / 5.7     6.4 / 6.7    9.9 / 10.3    11.9 / 12.8   14.4 / 14.9   17.6 / 18.2   20.0 / 20.6
 31 - 32      5.6 / 5.6     6.8 / 6.9    10.2 / 10.3   12.6 / 12.5   18.1 / 18.1   26.5 / 26.6   31.9 / 31.4
 38 - 39      12.8/12.6    15.9 / 15.9   24.3 / 24.2   30.2 / 30.0   38.1 / 37.8   47.9 / 47.8   55.0 / 54.1

The flood levels predicted by the post-development model have been compared to those for the pre-
development scenario in Table 16 to 18 on preceding pages of this report, for ARI’s from 1 to 100 years.
Yellow highlighting is used in those two 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 generally seen
to be practically zero (maximum 0.01 metre), when quoted to the nearest centimetre.

The only exception to the abovementioned generality regarding affluxes is at node 23, just upstream of
the site boundary in the western tributary, where the affluxes will range from 0.02 metre in a 10 year
storm to 0.05 metre in a 100 year storm. The western channel between node 23 and the site boundary was
designed in 2006 to be deliberately revegetated to a Manning’s n of 0.15, as part of the adjoining Blue
Wren Place development. That revegetation was installed at the time of that development, but has since
been removed, thereby reducing the present (“pre-development”) Manning’s n in this area to about 0.04,
consequently decreasing the flood levels at node 23 to about 0.4 metre below the original design levels.
The post-development flood levels as a result of Valkev’s development, while being 0.02 to 0.05 metre
above the present levels, will nevertheless be at least 0.3 metre below the original design levels for this
area, and the quoted affluxes are therefore suggested to be of no consequence. This is illustrated by the
levels of the adjoining lot in Blue Wren Place, which are marked on the longitudinal floodway profiles in
Figure 10 of this report.

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.

7.3     Sensitivity Test

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 all overbank sectors of all of cross-sections of the model. The
100 year flood levels predicted by this test are summarised in Table 21, 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 18