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

1.0      SUMMARY

This analysis has found that the development’s stormwater detention requirement can be achieved by
enlarging the flood storage on the site, by the means shown on Figures 9 to 11 of this report, to augment
the small amount of conventional detention storage that can be gained in two bio-basins on the site (as in
Figure 12). The additional storage volumes generated by these works, after allowing for the small
amounts of flood storage that will be lost to filling of lot margins and active parkland, are compared to
the detention storages that would be required of a fully-conventional detention system, in Table S1:

Table S1:  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      5456       6184    8515       9927     11352       12986       14493
  Floodway Storage
  Post-Development     5781       6591    9349      11121     12976       14807       16240
  Floodway Storage
   Floodway Gain        325       407      834       1194      1624        1821       1747
   Plus Bio-Basins      162       183      252        252       252         252         252
 Less Active Park Fill   -0         -0      -0         -0        -0         -18         -45
      Nett Gain         487       590     1086       1446      1876        2055       1954
 Req’d Extra Volume
   (From Table 3)       395       525     770        910       1095        1370       1585

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 16 to 18 of this report. The only exception to this
generality 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 works will also result in 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 on the adjoining property results in 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, on the adjoining property, the
flows revert to their pre-development rates.

The consequent flood levels for design of the development are summarised in Tables 16 to 18 and in
Figures 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 21
and in Figures 13, and appropriate account should be taken of these possible flood levels when
considering appropriate design levels for the development.



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