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

7.0     FLOOD PROFILES, POST-DEVELOPMENT

7.1     Mitigation Concept For Increased Stormwater Runoff

The impact of the increased site runoff on flood profiles along the floodways is proposed to be mitigated
mostly by increasing the floodway storage on the site, and partly by conventional detention storage in
two of the proposed bio-basins. The increase in the floodway storage will be achieved by deliberately
roughening the floodways (by revegetating the riparian zones, as required by Council) and by
constructing a restrictive footbridge across the eastern tributary to back-up the flood profiles in that
tributary floodway.

A small additional amount of storage is also required to offset the minor intrusions of the development
platform into the pre-development 100 year flood boundary, and for some minor regrading of an artificial
hump in the existing low-flow channel around node 5 of the Extran model (as seen on Figure 10).

The full suite of the proposed modifications to the flood storage system is shown on Figure 9, with
additional detail on Figures 10 to 12.

The revegetation of the riparian zones, by planting of shrubs (typically, Lomandra sp) and trees at
densities of one item per 1.5 to 2 square metres, is presumed to increase the Manning’s n of the presently
grassed areas from 0.03/0.05 to 0.07.

The detention storages in the bio-basins are accounted as only the “live” storages, in the narrow range of
elevation between the riser overflow level and the weir level. The overflow level in the riser will be
controlled by an overflow slot in one side of the riser, with details as in Figure 12.

7.2     Model Simulation

An Extran model of the post-development scenario was compiled by adjusting a copy of the pre-
development model so as to include the reshaped and revegetated floodways and associated increases in
Manning’s n values, together with the inclusion of the proposed footbridge, the minor regrading of the
invert around Node 5, and hypothetical vertical walls at the boundaries of the proposed allotments.

The achievement of the required increase in floodplain volume, to offset the development’s stormwater
detention requirement, is summarised in Table 19, below:

Table 19:  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
         c.f.
 Req’d Extra Volume     395       525     770        910       1095        1370       1585
   (From Table 3)



      Brisbane Stormwater Management Pty Ltd Document File: ValkevSDF.Doc                        Page 17