BsmRevRpt2

constructed during this stage will easily offset the detention requirement for this stage. The riparian
vegetation for the eastern tributary will be planted during this stage and, even when only newly planted, it
should substantially offset the shortfall in hard storage at the 1 year ARI level. The subsequent growth of
that vegetation will confirm the over-provision of storage for Stage 5 and provide a surplus of storage for
the start of subsequent stages.

Table 3: Detention Requirement –vs- Hard Storage Provision, For Stage 5
 ARI (years)                           1          10           100
 Detention Requirement (m3)           75          190          350
 Hard Storage In Stage 5
 - Bio-Basin (m3)                     49          63            63
 - Footbridge (m3)                     4          635          690
 - Total (m3)                         53          698          753
 - % Of Requirement                  73%         367%         215%


2.      The report outlines 50mm of afflux upstream in a Q100 event relying on externalities to the site
        for justification. Any afflux will not be supported.

The above request relates to flood levels at node 23 of the flood model, in the western tributary, upstream
of the site. This revision of the report is based on a revised floodway design, in which the western
tributary channel between nodes 23 and 25 of the flood model is now proposed to be revegetated to a
similar density as the existing channel (estimated Manning’s n value 0.055), rather than roughened to a
Manning’s n value of 0.068 as in the previous design. Consequently, the affluxes at node 23 of the flood
model are now less than zero (i.e. beneficial).


3.      The 100mm impact reported at Node 29 results in localised peak flood level impacts at 2
        Pearson’s Rd. Agreement by the owner should be provided acknowledging the flood impact on
        their property.

The previously reported 100 mm impact was an anomalous prediction, given that the profile of the
floodway at this location was proposed to be lowered by about 200 mm. Testing of the model’s
sensitivities revealed some issues with respect to the manner in which the software simulated the effect of
a “hump” in invert of the existing channel, downstream of node 29, leading to an erroneous prediction of
the pre-development flood levels at node 29. The model has since been revised, with an additional node
(29.5) located on the crest of the existing “hump”, thereby correcting the anomalous prediction. The
model of the post-development scenario has also been revised in this area, with the addition of a 424
cubic metre side-storage to be excavated into the overbank area alongside the northern bank of the
channel at node 29.5. The nett result of these changes is that there is now predicted to be a – 140 mm
afflux, rather than the previous + 100 mm afflux, at node 29.


4.      The western tributary is a minor waterway for fish passage. Given the extent of works in the
        waterway and the proposed works are an integral part of the modelling, Council recommends the
        applicant gain written support from DAFF for the proposed works.

Refer to Callaghan & Toth response.




     Brisbane Stormwater Management Pty Ltd Document File: ValkevSDF.Doc                             Page 6