B07056.W-01A Complete - Stormwater [As Published] · 132007.2398

Concept Stormwater Management Plan
RSL Care – Cooroy Retirement Village



4.0       FLOOD INVESTIGATION
A search of Noosa Council’s Planning and Development publications was undertaken. It was found that the site
falls within the Cooroy and Lake McDonald Locality Plan. The Natural Hazard: Landslide and Flooding Overlay
to this locality (Map OM3.3) shows that the site does not fall within the flood hazard area as nominated by
Council.

Using the peak flows presented in Section 3.4, existing peak flood levels for the 100 year ARI storm has been
determined using weir and Manning’s equations across the downstream extents of the northern and southern
dams.

A total of six cross sections have been used to determine the existing peak flood levels. Two sections were used
for weir equations across the northern dam, as flows are limited by two distinct control points in topography.

Four sections were used across the southern dam. As the second section was located across an existing
embankment the weir equation was utilised in this location. The Manning’s equation was used for all other
sections, assuming a roughness of 0.045. Channel slope was adopted from the survey provided.

The upstream and downstream water surface levels determined for the 100 flood levels at each location are
presented below in Table 4.1. A detailed summary of the weir and Manning’s equation calculations is presented
in Appendix C.
                                  Table 4.1: Calculated 100 year ARI Flood Levels
                                                            Flood Level (m. AHD)
                                Extent
                                                    Northern Dam          Southern Dam
                              Upstream                 100.60                 102.88
                             Downstream                 99.69                 101.15


Flood extents for the existing 100 year ARI flows have been determined and plotted. Refer to the Existing Flood
Extents plan (Drawing No. B07056-SK10) in Appendix A for details.

As discussed previously the increase in 100 year ARI peak flows resulting from the development will be reduced
to existing peak flows by the proposed detention basins. Refer to Section 5.0 for further details.

Therefore the extent of flooding will not worsen as a result of the development. Furthermore, detailed design of
roads and embankments at the downstream extents of the site will be undertaken at Operational-Works stage.
Details of flood extents for the future design can be provided if required at this stage.




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