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

4. Historical Flooding and Flood Records
Flood records refer to rainfall and water level records for an historical flood event. Over long
periods of time all catchments are likely to eventually experience a significant rainfall and flooding
event. The historical frequencies of such events are typically identified from long-term rainfall and
water level records and local anecdotal reports.

Rainfall records describe the intensity and duration of rainfall recorded at a particular location for a
specific event. Real rainfall patterns are different for every storm and where the rainfall falls across
a catchment may not be consistent everywhere in the catchment. Rainfall records are typically
recorded by automatic rainfall stations; however, these may only be accurate in the vicinity of the
gauge. Daily rainfall totals are also often collected throughout a region but these devices do not
reveal the intensity of rainfall which may have fallen throughout the day of the record.

Water level records are recorded levels of how high the water has reached at a particular location.
These are generally recorded after a flood event by surveying flood marks and flood debris. In
some locations automatic gauges record the rise and fall of water levels over time.

When available historical flood records are used to calibrate flood models to confirm the models
reflect the natural conditions of a catchment. However, relatively few catchments possess detailed
records of historical rainfall and water levels.

Unfortunately, to date, there are no automatic water level gauges upstream of Highland Drive to
provide continuous records of the rise and fall of flood waters within the local waterways. The
nearest automatic water level gauge is located on Lake MacDonald Drive downstream of the dam
spillway.

Previous flood studies have utilised recorded peak flood levels obtained from field surveys
undertaken following major events.

Rainfall records for the upstream contributing catchment are limited to relatively recent events
recorded at the Cooroy, Black Mountain and Lake MacDonald rainfall stations. Over the past 25
years the contributing catchment has experienced several notable rainfall events. Such events
include:
   •   February 1992;

   •   February 1999;

   •   April 2009 (2 events); and

   •   February 2012.

Longer term historical rainfall records extending back to the late 1800’s are available from the
Eumundi rainfall station. Refer to Figure 2 below. These historical records are typically daily
records and do not reveal the local intensity of significant rainfall events which may occur within
periods of hours.




Lake McDonald Flooding Community Reference Group Report                                          Page 7