BsmRevRpt2

9.0     FLOOD PROFILES, POST-DEVELOPMENT

9.1     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), 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 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.

9.2     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 (with zero blockage), 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 22, below:

Table 22:  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
   (From Table 6)       395       525     770        910       1095        1370       1585



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