MCU14-0039 Flood report and flood man Plan rev August 2015

Como Glasshouse P/L C/‐ Martoo Consulting P/L
Flood Impact Assessment – 305 Lake McDonald Drive Cooroy

In relation to the greenhouse structure, modelling showed that flood impacts external to the site were
unacceptable when the greenhouse was modelled as a fully blocked structure. Given the structure
construction being plastic wall sheeting, it is unlikely that the structure would in fact operate in a
manner that would preclude flooding internally and it is likely that the plastic walls would become
compromised by flood waters. Indeed, information provided from the client noted that in the most
recent historical flood event the greenhouse walls were in fact compromised with flood waters flowing
through the structure.
For the purposes of this investigation, the greenhouse structure has been modelled to allow flood
waters to pass through the structure, with the Manning’s “n” roughness increased to reflect the nature
of the internal plantings and to allow for some blockage associated with the marginal reduction in
flood conveyance associated with structure columns. Given the type of structure (plastic sheeting)
and the contents of the greenhouse (tomato plants – refer Figure 2), a Manning’s “n” roughness of
0.07 was adopted, with a sensitivity analysis also undertaken using n=0.1. This approach is considered
to be very conservative given the internal plantings currently in existence as well as the fully vegetated
conditions at full plant growth conditions likely to be encountered within the greenhouse. We
additionally note that it is intended to provide the greenhouse with lay‐down wall sections to the
height of the 1% AEP flood depth to facilitate flood water flows through the structure during times of
significant flooding. Given these aspects, the modelling approach using a higher roughness condition
is considered to be conservatively appropriate for this investigation.




                                   Figure 2 – Greenhouse Internals


The existing case TUFLOW model was revised to include the increased roughness associated with the
greenhouse structure as well as the since constructed earthworks at the site (including bunds) to
represent the developed site conditions. The developed case model has been analysed for same
design events and used to quantify flood impacts associated with site development. Hydraulic results
are discussed in the following section of this report.

3917‐01 / R01 v02 ‐ 31/08/2015                                                                          6