Attachment D - Stormwater and Water Quality report

                                                                                Callaghan & Toth


5.0 FLOODING ASSESSMENT SUMMARY
The flood study for the site was undertaken by Hydrology & Water Management Consulting.
The results of the flood study and assessment are summarised below.

5.1      Existing Conditions

The site is bounded to the south and east by Cooroy Creek.

The site is subject to ponding of floodwaters from the adjacent watercourses and the Q100 year
ARI flood levels (i.e. 1% AEP event) (see Appendix A in Flood Study). The Lower Mill site is
affected by flood levels across a range of events. The proposed carparks and access roads
and pathways are below the Q100 year inundation line.

The flood assessment from Hydrology and Water (See Appendix 4) reports on the hydrologic
and hydraulic performance of the site including upstream and downstream catchments.

5.2      Proposed Drainage System

The flood study determined that the proposed level changes for the carpark (HWMC used
Council’s design levels) together with the proposed changes to the south of Cooroy Creek for
the new hinterland playground, that the flood depths over the modelled events were comparable
to the existing case; provided structures nominated in the flood study are removed from the
playground design. As stated in the report:

         Modelling has demonstrated that care is required to ensure that the design of the park
         will not increase flood risk on surrounding properties. Iterative modelling of various
         mitigation scenarios has been used to develop several mitigation measures that have
         successful reduced offsite flood impacts to an acceptable level.

Following discussions with Council staff, it is understood that mitigation scenarios include
lowering some levels of proposed mounds and to removing proposed viewing platforms into the
creek, to ensure that the flood levels of the proposed case were equivalent to the existing case.

As such the playground design is being amended to ensure that flood levels and velocities are
consistent with the modelling scenarios proposed by HWMC.

It should also be noted that management requirements for the contaminated land area on Lot 7
need to be considered when undertaking detailed design of the carpark levels in association
with flood modelling. Carpark levels may need to be amended i.e. raised to ensure that capping
is not damaged. However, this may affect the flood model, resulting in carpark levels needing
to encroach into the capping of contaminated material. The SMP in Appendix B details the
management requirements of this area.




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2001 Lower Mill Carpark Extension SWMP                                       February 2020