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

8. Lake MacDonald Dam outflows
During the CRG consultation some stakeholders reported their observations of changes over-time
in the way flood waters rose and receded from the northern end of the estate. These stakeholders
reported it was their experience that flood waters had previously not risen as much from the north
compared to more recent observations and had also previously receded much quicker than they
appeared to do now.

The interaction between flood flows from Six Mile Creek and the outflows from Lake MacDonald
Dam were investigated using the upgraded flood model to further understand these observations.
To explore this, outflows from Lake MacDonald were removed from the upgraded flood model for
moderate and major flood event simulations. The design flood model assumes Lake MacDonald is
full at the commencement of the rainfall event and hence flows from this catchment occur once
they pass through the dam. Removing the outflows from the dam is representative of a scenario
where the dam water levels are sufficiently below the dam’s spillway to allow the dam to store all
the runoff generated from the catchment without overtopping the spillway.

Figure 21 presents the flood model results near Hamilton Road. These results demonstrate the
backwater effect which can occur due to discharges from the Dam. The Dam spillway is not gated
and hence there is no control for allowing or preventing flows to discharge from the dam once dam
water levels have reached the spillway crest.

The results confirm that there is an interaction between the outflow from the Dam and flood flows
from the main branch of Six Mile Creek which results in a backwater effect which may elevate peak
flood levels and extend the duration of inundation.




          Increase in flood levels
          when Dam outflow is
          included




                                                                           Reference level




                                                   Increase in duration of inundation
                                                   when Dam outflow is included




Figure 21: No Dam outflow - moderate flood event




Lake McDonald Flooding Community Reference Group Report                                      Page 36