Letter to Noosa Council - 20 Mitti Street

 “Adamson Town Planning, specialising in providing higher order town planning advice”




    •   the flow across the subject land is intermittent from the Noosa National Park with the site
        being dry most of the time and predominantly containing trickle flows;
    •   about 30% of the total flow from the upstream catchment passes through the site with the
        remaining 70% being within the Noosa National Park;
    •   adjacent to the upstream (southern) boundary the flow depth beside the existing extension in
        the Defined Flood Event (DFE - 1%AEP) is 400-500mm deep;
    •   the maximum flow depth across the driveway low point is about 390mm and the velocity of
        flow complies with the relevant standard being less than 0.45m/s (Table 7.4.5 from QUDM for
        “Vehicle safety: Transverse flow limits (no risk to life) e.g. road intersection”);
    •   the consulting engineers (Barlow Shelley) being structural and civil engineers have designed
        the dwelling extension to withstand the localised stormwater flows and to be substantially
        above the DFE; and
    •   the critical time of concentration for the DFE is 33mins so the 1%AEP event starts as ‘dry’ at
        0mins and peaks at 33mins, the maximum velocity and depth then subsides at approx. 60-
        65mins back to ‘dry’. Therefore, the peak (worst case) flows across the driveway and the site
        for approximately 10-15mins.

Barlow Shelley has provided additional advice which is provided as Attachment 3. This advice
addresses the issues raised in the email from Council to Martoo Consulting on 27 June 2018, and the
following comments are made:
    • the advice confirms approximately 30% of the flow passes through the site west of the new
        extension with the remaining 70% by passing the site within the Noosa National Park;
    • any local increases in flow depth due to debris build-up against the downstream tennis court
        fence will not be significant that would not affect the existing extension to the dwelling. This
        is because the floor level is about 800mm above the 1% AEP at the downstream face and
        580mm above at the upstream face. This provides an adequate factor of safety and complies
        with Council’s codes and policies;
    • Barlow Shelley being structural engineers have confirmed that the residence was designed to
        suit the conditions and accommodates the modelled flows as notated in the reports submitted
        to date; and
    • the modelling already addresses the full catchment and provides enough detail for the
        assessment, particularly considering that the extension is existing and has been designed,
        certified, approved and built to suit the site conditions.

This is unlike an area that is subject to river flooding under the Natural Hazards and Landslide Overlay
(OM6.3), which does not apply to the subject land. Land subject to river flooding in the Noosa Heads,
Noosaville and Tewantin localities can be developed where in accordance with specific outcome O2
of the Detached House Code. In these locations many properties are inundated by flooding for
extended periods and have restricted access, yet development is allowed, subject to meeting relevant
criteria. These same provisions apply, and the existing extension to the dwelling has been designed
and constructed to satisfy these provisions.

In this case having regard to the design of the existing extension, the associated access and the
engineering advice provided, in my opinion, the risk to people and property is minimal. This is because:

                                                                                             5|Page
18019 – 20 Mitti Street, Noosa Heads, August 2018