BsmRevRpt2

3.0     RESPONSE TO NOOSA COUNCIL INFORMATION REQUEST

The following comments are offered in response to Noosa Council’s information request of 18 July 2014.
The requests are reproduced in italics, with a response immediately following each request:

1.      The report details that in a Q100 event an additional 1,585m³ of storage is to be provided. Only
        252 m³ is proposed to be provided in detention basins and the balance has been created by
        floodplain roughness via the planting of vegetation. This equates to nearly 85% of the storage to
        rely on vegetation plantings. Council will not accept this form of detention.

In the previous report, the 100 year volume in the bio-basins was, as noted above, 252 cubic metres.
However, the 1747 cubic metre gain in 100 year floodway volume included about 200 cubic metres of
“hard” storage from the headwater afflux at the proposed footbridge, which was not separately identified
in that previous report. This took the total “hard” storage increase to about 450 cubic metres, leaving the
vegetation (”soft” storage) to account for at least the remaining 1135 cubic metres (or about 72 percent)
of the 1585 cubic metre requirement.

In this revised report, an additional 424 cubic metre side-storage is proposed to be excavated into the
overbank area alongside the northern bank of the western channel at node 29.5. Furthermore, the opening
area of the footbridge across the eastern channel is proposed to be significantly reduced, which will in
turn raise the 10 year headwater level and consequently the deck level by 0.13 metre, which will further
raise the upstream afflux storage. The nett effect of these changes will be a significantly increased
proportion of “hard’ storage, and conversely a significantly reduced requirement for “soft” storage to
make up the required overall volume. The revised make-up of the volumes, for ARI’s of 1, 10 and 100
years, is summarised in Table 2, below.

Table 2: Relative Proportions of “Hard” and “Soft” Flood Storage
 ARI (years)                                           1          10            100
 Required Volume (m3)                                 395        910            1585
 Proposed Hard Storage (m3)
 - Footbridge                                          4         635            690
 - Other (bio-basins, channel cut/fill, side-storage) 187        114            274
 - Total hard storage                                 191        749            964
 - Proportion of requirement                          48%        82%            61%
 Remainder, Soft Storage, Req’d (m3)                  204        161            621
 - Proportion of requirement                          52%        18%            39%
 Actual Soft Storage Proposed (m3)                    344        998            1582

Given that the hard storage is now proposed to make up about half or more of the overall storage
requirement, and that the soft storage will provide about twice or more of the remaining overall
requirement, this outcome is suggested to be an appropriate strategy in the longer-term context.

The appropriateness of the strategy in the shorter-term horizon, relative to the timeframe required for the
revegetation to become reasonably well-established, can be seen in the context of the planned progress of
the development platform vis-à-vis the planned progress of the floodplain storage works. The next stage
of the development is Stage 5, which comprises the remainder of the area labelled “Model D” on Figure 2
of this report. The equivalent detention requirements for the “Model D” area (from Section 5 of this
report) are compared to the hard storage that will be constructed during that stage, in Table 3, on the
following page of this report. Apart from a slight shortfall at the 1 year ARI level, the hard storage


      Brisbane Stormwater Management Pty Ltd Document File: ValkevSDF.Doc                           Page 5