2018-05-14 GC Agenda Attachment 1 to Item 6 Lake MacDonald Dr Community Reference Group Investigations Report

6. Ultimate Catchment Urbanisation
During the CRG consultation a concern was raised about the potential impact on flooding
conditions that may arise from continued development in the upstream contributing catchment.

A commitment was provided to the CRG to engage WMA to use the upgraded flood model to
simulate the potential changes to flood levels in the vicinity of Highland Drive due to increased
development upstream.

Development within Noosa Council is planned and regulated by the Noosa Plan. The Noosa Plan
is the planning scheme for the Noosa Shire and sets down the planning framework to guide its
future. The Noosa Plan reflects the intent of the SEQ Regional Plan which provides a regional
framework for growth management in SEQ. Through these documents areas for urban
development are identified and regulated.

Developments undertaken within Noosa Shire must be compatible with planned land use and
address provisions outlined in the Noosa Plan. These provisions may cover all manner of issues;
however, there are specific provisions related to the management of stormwater runoff. These
provisions require the applicant to demonstrate that a proposed development will not increase
runoff as a result of the development of a site. The anticipated runoff generated on a site is
increased by the amount of impervious (hardstand) areas, such as roofs, roads and pathways. To
compensate for the additional impervious area, onsite detention is often included.

The provisions of the Planning Scheme are designed to negate the impact of increased impervious
surfaces due to new development. However, for the purposes of this investigation the analysis has
not included any offset to additional runoff that would be required as a condition of a development
approval. In this case, we are able to determine a theoretically worse case outcome for additional
runoff from upstream urban development and test the maximum influence on peak flood levels
throughout the catchment.

To analyse this, existing impervious areas of modelled sub-catchments were determined from a
review of aerial images. For each sub-catchment an overall percentage of imperviousness was
determined. For the ultimate development scenario the increase in overall percentage of
imperviousness is calculated based upon the planned extent of urban development.

For this investigation area, future increases in urban development are limited to the Cooroy
township area. The full extent of the planned urban footprint of Cooroy township is only 7% of the
entire catchment area contributing runoff to Highland Drive. Within this urban footprint of Cooroy
many areas are already developed and hence the overall percentage increase due to ultimate
development levels is a proportion of the urban area (7%).

The estimated changes in percentage imperviousness of the Cooroy sub-catchments within the
ultimate urban footprint are shown in Figure 3.




Lake McDonald Flooding Community Reference Group Report                                       Page 11