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

Section 2

© Noosa Council 2017

www.noosa.qld.gov.au

General Enquiries:
By telephone: (07) 5329 6500
By email: [email protected]
Fax: (07) 5329 6501
Street Address: 9 Pelican Street, TEWANTIN
Postal address: PO Box 141, TEWANTIN QLD 4565

Section 3

Table of Contents
1.    Executive Summary ............................................................................................................. 3
2.    Introduction .......................................................................................................................... 4
3.    Catchment Description ........................................................................................................ 5
      3.1         Catchment Area to Highland Drive ........................................................................... 5
      3.2         Contributing Sub-Catchments .................................................................................. 5
      3.3         Contributing Catchment Land Use............................................................................ 5
4.    Historical Flooding and Flood Records ............................................................................. 7
      4.1         February 2012 Flood Event ...................................................................................... 8
5.    Six Mile Creek Flood Study ................................................................................................. 9
      5.1         Previous studies ....................................................................................................... 9
      5.2         Six Mile Creek Flood Study Upgrade........................................................................ 9
      5.3         Design Flood Events ................................................................................................ 9
6.    Ultimate Catchment Urbanisation..................................................................................... 11
      6.1         Discussion of results .............................................................................................. 19
7.    Flood Mitigation Scenarios for Lake MacDonald Drive ................................................... 20
      7.1         Mitigation Options .................................................................................................. 20
      7.2         Council considerations for mitigation options .......................................................... 31
      7.3         Flood storage & blockages ..................................................................................... 32
      7.4         Sensitivity of water supply pipe crossing ................................................................ 33
8.    Lake MacDonald Dam outflows ........................................................................................ 36
9.    Six Mile Creek Flood Warning Stations ............................................................................ 37
      9.1         Flood Warning - during a rainfall/flood event .......................................................... 38
10.   Conclusions ....................................................................................................................... 39
11.   Where to from here? .......................................................................................................... 40
      11.1        Catchment flood risk assessment. .......................................................................... 40
      11.2        Develop a catchment wide flood mitigation strategy. .............................................. 40
      11.3        Update flood emergency planning maps. ............................................................... 40
      11.4        Update planning overlays for new Planning Scheme .............................................. 40
      11.5        Continue to monitor rainfall and runoff records in catchment. ................................. 40




Lake McDonald Flooding Community Reference Group Report                                                                              Page 1

Section 4

Table of Figures
Figure 1: Contributing catchment to Highland Drive ........................................................................ 6
Figure 2: Historical Rainfall Records – Eumundi ............................................................................. 8
Figure 3: Percentage imperviousness of the Cooroy sub-catchments ........................................... 12
Figure 4: Hydrograph reference locations ..................................................................................... 14
Figure 5: Model Result Hydrographs - Location 2 ......................................................................... 15
Figure 6: Model Result Hydrograph - Location 3 ........................................................................... 15
Figure 7: Model Result Hydrograph - Location 5 ........................................................................... 16
Figure 8: Model Results Hydrograph - Location 7 ......................................................................... 16
Figure 9: Unmitigated Change in Flood Levels due to Ultimate Urbanisation - 5%AEP Event ....... 17
Figure 10: Unmitigated Change in Flood Levels due to Ultimate Urbanisation - 1%AEP Event ..... 18
Figure 11: Low level earth bund flood mitigation option - Moderate Flood Event .......................... 22
Figure 12: Low level earth bund flood mitigation option - Major Flood Event ................................ 23
Figure 13: High level earth bund flood mitigation option - Moderate Flood Event .......................... 24
Figure 14: High level earth bund flood mitigation option – Major Flood Event ............................... 25
Figure 15: Channel works (A) – Moderate Flood Event ................................................................. 27
Figure 16: Channel works (A) – Major Flood Event ....................................................................... 28
Figure 17: Channel works (A+B) – Moderate Flood Event ............................................................ 29
Figure 18: Channel works (A+B) – Major Flood Event................................................................... 30
Figure 20: Flood model results for moderate flood event............................................................... 34
Figure 21: Flood model results for major flood event ..................................................................... 35
Figure 22: No Dam outflow - moderate flood event ....................................................................... 36

Table 1: Mitigation Option Assessment ......................................................................................... 31
Table 2: Hydraulic Blockage Factors ............................................................................................. 32




Lake McDonald Flooding Community Reference Group Report                                                                    Page 2

Section 5

1. Executive Summary
A flood study of the Six Mile Creek catchment was conducted in 2017 to combine past studies of
Cooroy, Six Mile Creek and Pomona catchments into one continuous flood study. By reviewing the
flood studies in the Six Mile Creek catchment we are able to apply the latest in modelling
techniques, data collection and computational ability to continually refine and improve our
knowledge of the flood plain. The Six Mile Creek flood study upgrade was conducted by WMA
Water Pty Ltd in conjunction with HydraLogic, acting as an independent review, as well as Noosa
Council.

This sub-report of the Six Mile Creek Flood Study is an investigation into concerns raised by the
Lake McDonald community with regard to flooding in their area. The investigation looks to address
concerns specifically raised by the community either through the multiple community reference
group forums Noosa Council has conducted, site visits conducted jointly by Noosa Council, WMA
Water and HydraLogic or by communication directly with the residents.

This report has also investigated several possible mitigation solutions that were jointly proposed
with the community reference group to mitigate the effects of flooding within the Lake McDonald
community. The investigations concluded that none of the possible mitigation solutions would be
viable as they offered little benefit to the community and had large costs either financially,
environmentally and/or had an adverse effect on other areas of the community.

The report does note positively that Noosa Council has invested strongly in its understanding of
this catchment through this flood study and also by installing five new flood monitoring gauges
within the catchment, three of which are upstream and useful for this community. These gauges
will allow the community to monitor rainfall as it occurs as well as rising water levels within the Six
Mile catchment before it impacts members of this community.

Members of the Lake McDonald community have been provided with instructions on how to access
this information through the Bureau of Meteorology website and have been shown where to find
other resources relating to flood awareness on the Noosa Council website.

This report concludes a major body of work in consulting with the community and understanding
the flooding within Six Mile Creek, but also specifically around the Lake McDonald community;
however, this does not represent the end of the this work. Noosa Council will now be able use this
investigation to inform future work including flood risk assessments, catchment wide mitigation
options, alert systems, disaster management mapping and disaster response.




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Section 6

2. Introduction
In mid-2016 Noosa Council invited residents in the vicinity of Lake MacDonald Drive Estate to
participate in a Community Reference Group (CRG) which was formed following concerns raised
by residents with regards to observed flooding conditions in the local area.

The first CRG meeting was held on 19 July 2016 and was attended by local residents, council
officers and consultants and chaired by Cr Frank Wilkie.

The meeting sought to inform the CRG members of Council’s current understanding of the
catchment with regards to rainfall and flooding and to gather details of flooding observations and
concerns from the local community.

Council’s understandings of the flooding characteristics of the catchment were informed by a series
of flood studies which had been undertaken throughout the previous 15years. The deficiencies of
these studies were noted and a commitment provided to begin to address these through an
upgrade of the relevant flood studies. This upgrade would provide improved confidence in
modelling flooding conditions and also will provide a useful tool to test changes in flooding
conditions due to changes in the catchment or landscape.

Observations and concerns raised by reference group members included:
   •   The severity of the February 2012 flood event;

   •   Observed changes in flooding characteristics over time with regards to the frequency and
       speed of flood waters;

   •   Observed changes in the duration of inundation from flood waters;

   •   The potential for additional runoff due to increased urbanisation of upstream catchments;

   •   Possible flood mitigation works; and

   •   Ability to receive warnings of rising flood waters.

Following the first CRG meeting Council proceeded with preparing a tender to upgrade the Six Mile
Creek Flood Study. This tender was awarded to engineering consultants WMA Pty Ltd in late 2016.

Following the awarding of the tender, site inspections were coordinated by Council to visit CRG
stakeholder’s properties and other key locations throughout the catchment to inform the flood study
upgrade.

Following extensive data collection and development of a new computer flood model of the entire
catchment was prepared by WMA.

The new flood model was used to reproduce the flood event of February 2012 and to test a variety
of flood mitigation options suggested by the stakeholders. The models were also used to
investigate potential increase in runoff due to continued upstream urbanisation.

A second CRG meeting was conducted on 19th September 2017 to present and discuss these
results.

This report summaries these findings.




Lake McDonald Flooding Community Reference Group Report                                       Page 4

Section 7

3. Catchment Description
A catchment is an area of land where all surface water resulting from rainfall falling within that area
will accumulate and flow to (or past) a point at the end of the defined catchment.

For the purposes of this report and the associated investigations the end of the catchment is
defined in the vicinity of Highland Drive. The entire Six Mile Creek catchment extends downstream
beyond the Noosa Council boundary to a confluence with the Mary River south of Gympie.


3.1      Catchment Area to Highland Drive
The total upstream contributing catchment area to Highland Drive is approximately 4,500 hectares
and extends approximately 11km upstream. Refer to Figure 1 for contributing catchment extent.


3.2      Contributing Sub-Catchments
The upstream contributing catchment includes the sub-catchments of:
   •   Cudgerie Creek

   •   Cooroy Creek (including Cooroy township)

   •   Six Mile Creek Left Branch and

   •   Two local drainage tributaries

3.3      Contributing Catchment Land Use
Land uses within the contributing catchment are predominantly rural and rural residential with
urban (commercial and residential) land uses within Cooroy township. The total urban area of
Cooroy township, as defined by the ultimate urban footprint of the SEQ Regional Plan, is
equivalent to approximately 7% of the contributing catchment area.

The contributing catchment and associated waterways and drainage lines also includes several
road crossings and the north coast rail line. The influence of each crossing on local and regional
flooding depends upon the size of the bridge or culverts at each crossing and the elevation of the
roadway and associated embankments. Significant road crossings in the contributing catchment
include: the Bruce Highway, Garnet Street, Elm Street and Mary River Road.

Depending upon the size of a flood event and hence the volume of flood waters, road crossings
often act as ‘control’ within a catchment where water can only pass through or over the structure at
a rate determined by the size of the crossing opening and elevation of the roadway and
embankments.




Lake McDonald Flooding Community Reference Group Report                                         Page 5

Section 8

Figure 1: Contributing catchment to Highland Drive




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Section 9

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

Section 10

Figure 2: Historical Rainfall Records – Eumundi



4.1      February 2012 Flood Event
The February 2012 flood event is the most significant flood for this catchment in recent records.
This event was characterised by a very intense and localised storm cell where 182 mm of rainfall
was recorded at the Cooroy rainfall station in 1 hour between 9:50 pm and 10:45pm on the 24th
February. Total rainfall volume for this relatively short event at the Cooroy rainfall station was
332mm.

Compared to other long term rainfall records the statistical likelihood of an event of this intensity
occurring is approximately 0.2% in any given year which is statistically equivalent to approximately
once every 500 years.

The new flood study model has been calibrated to reproduce the February 2012 event based upon
the recorded rainfall records and available peak water levels throughout the catchment.
The calibration process was also informed by observations and experiences provided by CRG
stakeholders and the model results generally reflect these observations. These observations
included the rapid rate of rise in flood water and the extent of the inundation at various locations in
the vicinity of Highland Drive.

The upgraded flood model has been deemed to have been sufficiently calibrated to peak flood
records throughout the catchment and observations of the CRG stakeholders.




Lake McDonald Flooding Community Reference Group Report                                         Page 8

Section 11

5. Six Mile Creek Flood Study
A flood study is an investigation into the flooding characteristics of a location. A modern flood study
will typically utilise a flood model which is a computer simulation of rainfall and runoff in a
catchment and its waterways. Flood models require information to represent the physical
characteristics of the contributing catchments and the waterways which convey the runoff.

Most flood modelling is a two-step process. The first step is to develop a model which simulates
the runoff generated from rainfall over a catchment area. This step is referred to as hydrology. The
second step is then to take this runoff and to simulate how it might travel down the main
waterways. This second step is referred to as hydraulics. Both hydrology and hydraulics relate to
the study of water.

Flood studies can be used for land use planning; infrastructure design, development assessment
and disaster management.


5.1      Previous studies
Over time several flood studies had previously been undertaken to inform planning and design
decisions for the floodplains of Cooroy Creek, Six Mile Creek and Cooroora Creek (Pomona).
Whilst each of these locations are connected within the catchment the models were prepared at
different times using different software and inconsistent parameters.

In terms of available flood studies, Highland Drive is awkwardly positioned between the lower
extent of previous flood models prepared for Cooroy Creek and the upper extent of flood models
prepared for Six Mile Creek. This created problems for confidently interpreting flooding conditions
at the overlap of these two areas as flood models are constrained by the nearby boundaries
conditions applied at the top and bottom of the models.


5.2      Six Mile Creek Flood Study Upgrade
The Six Mile Creek flood study upgrade has improved the continuity of modelling between the
Cooroy Creek system and main branch of Six Mile Creek. The upgrade process has delivered a
contemporary suite of connected flood models for the entire catchment (to downstream of Cooran).
The modelling platform is a two dimensional flood model (TUFLOW) and utilises detailed ground
terrain information obtained from aerial laser scanning (sometimes referred to a LiDAR).

The upgraded flood model has been calibrated to the February 2012 event. The calibration
process has referred to rainfall and peak water records and the observations and experiences
provided by CRG stakeholders.

To ensure planning and infrastructure designs are consistent throughout regions standardised
design rainfall patterns are applied to flood models to generate design flood results.


5.3      Design Flood Events
Design rainfall events are not real rainfall events. Design rainfall events can vary in magnitude to
reflect different sized flood events which may need to be considered for different forms of planning
and infrastructure design.

Design flood events are often described with reference to the statistical frequency that they are
expected to occur. This is sometimes referred to as the Average Recurrence Interval (ARI). An
example application of this is the minimum floor levels of residential homes are required to be


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Section 12

above the 1 in 100 year ARI design storm level. Another example may be a small road crossing
may be required to be designed to be drivable for a 1 in 10 year ARI design storm event.

More recently, the flooding industry has moved away from adopting this type of terminology to
describe flood events, as it was found to be potentially misinterpreted that once an event of a
particular size (i.e. 1 in 10 year event) had occurred it would not occur again for that period of time.
This is not correct. To address this, the industry now prefers to describe these events by the
percentage (%) chance of the event occurring in any given year. This is referred to as the Annual
Exceedance Probability (AEP). Therefore; in general terms a 1 in 100 year event may be described
as having a 1% AEP which is a 1% chance of the event occurring in any given year. Similarly a 1 in
10 year event has a 10% AEP i.e. a 10% chance of occurring in any given year and hence a 1 in 1
year event has a 100% AEP as it is expected to happen at least once a year.

The upgraded Six Mile Creek Flood Study will provide a new suite of design flood levels for the
main floodplains and waterways of Six Mile Creek, including Cooroy Creek and Cooroora Creek
(Pomona).




Lake McDonald Flooding Community Reference Group Report                                        Page 10

Section 13

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.




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Section 14

Figure 3: Percentage imperviousness of the Cooroy sub-catchments



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Section 15

The increases in impervious percentages were applied to the upgraded flood study’s hydrologic
model to determine the potential increase in runoff from each sub-catchment. This runoff was then
added to the hydraulic model to determine the impact on floodplain conditions downstream.
To assess the potential impact of increased impervious areas the model’s flood level results are
extracted and compared at selected locations. These locations are shown in Figure 4.

Hydrographs of water level results plotted against time have been prepared for each reference
location for two sample flood events for both existing and ultimate urbanisation conditions.

Selections of these hydrographs are presented in Figure 5 to Figure 8.

The extents of potential increases in flood levels due to unmitigated urbanisation are shown in
Figure 9 and Figure 10.




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Section 16

Figure 4: Hydrograph reference locations




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Section 17

Figure 5: Model Result Hydrographs - Location 2




Figure 6: Model Result Hydrograph - Location 3




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Section 18

Figure 7: Model Result Hydrograph - Location 5




Figure 8: Model Results Hydrograph - Location 7




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Section 19

Figure 9: Unmitigated Change in Flood Levels due to Ultimate Urbanisation - 5%AEP Event




Lake McDonald Flooding Community Reference Group Report                                   Page 17

Section 20

Figure 10: Unmitigated Change in Flood Levels due to Ultimate Urbanisation - 1%AEP Event




Lake McDonald Flooding Community Reference Group Report                                    Page 18

Section 21

6.1      Discussion of results
The results show there is a small increase in flood levels immediately downstream of some of the
urban areas. However, these small increases are attenuated as flood waters expand across the
floodplain and pass through the significant waterway crossing of the north coast rail line. The rail
line includes a high embankment which guides upstream flood waters towards the bridge opening
immediately upstream of Elm Street. The constriction of the bridge acts as a hydraulic control
allowing controlled rates of flood waters to pass through the bridge opening. This controlling affect
removes any evidence of the small increase in flood levels simulated upstream of the crossing.
This analysis demonstrates that if the planning scheme did not contain provisions to manage
additional stormwater runoff then a small increase in design flood levels could be observed in
areas upstream of the north coast rail line; however, these increases do not translate to areas
downstream of the rail line.

On the basis of the simulation results and with consideration of the provisions of the Planning
Scheme this analysis concludes that planned increases in urbanisation within the Cooroy township
urban footprint will not increase peak flood levels in the vicinity of Highland Drive, Lake
MacDonald.




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Section 22

7. Flood Mitigation Scenarios for Lake
   MacDonald Drive
Throughout the CRG consultation a selection of flood mitigation options were suggested by CRG
stakeholders with the view of reducing the frequency and severity of flooding in the vicinity of
Highland Drive.

In general terms these options included works to reduce the occurrence of flood waters entering
the overland flow path which leads to Highland Drive and also works to improve the passage of
floodwaters through the main channel of Six Mile Creek.

WMA were engaged to utilise the upgraded flood model to test the effectiveness of each flood
mitigation suggestion (including combinations of these).

For clarity, these options have been analysed only for the purposes of assessing their potential
effectiveness on reducing flood levels and frequency. These preliminary assessments have been
undertaken for the purposes of responding to CRG stakeholder suggestions and should not be
interpreted as a commitment or willingness from Council to undertake any of the works described
in these options without further detailed investigations and appropriate environmental approvals.

The mitigation options modelled and an assessment of the general effectiveness of each option is
described in the following sub-sections. Each option is assessed for its benefit to the Highland
Drive community and impact on other areas. Each option was modelled for a 5% AEP (moderate)
flood event and 1% AEP (major) flood event.


7.1      Mitigation Options

Option 1: Low level earth bund from Liane Drive
The flood plain which extends to the west from the end of Liane Drive is crossed by a number of
shallow depressions which convey flood waters to Highland Drive via the overland flow path as
water levels rise upstream. Refer to Image 5-1. These depressions are expected to be engaged
relatively frequently. The objective of this option is to construct an earth bund to fill across the entry
to these low level depressions and reduce the frequency of minor flood flows entering the overland
flow path. Major flows would overtop the low level bund.

The flood model results demonstrate reductions in flood levels in the vicinity of Highland Drive of
between 100 to 250mm for a moderate flood event; however, no reduction for a major flood event.

Localised upstream increases in flood levels of between 500mm and 1m were evident immediately
upstream of the bund. Downstream along the main branch of Six Mile Creek flood levels increased
by approximately 100 to 250mm for the moderate flood event with no change for the major flood
event.

Refer to Figure 11 and Figure 12.




Lake McDonald Flooding Community Reference Group Report                                           Page 20

Section 23

Image 5-1: Shallow floodplain channels near Liane Drive



Option 2A - High level earth bund from Liane Drive
As per Option 1; however, with the earth bund raised to an elevation above the major flood level.
This would require the earth bund to be approximately 2m above the natural ground surface.

The flood model results demonstrated similar reductions in flood levels in the vicinity of Highland
Drive as seen for Option 1.

Localised upstream increases in flood levels exceed 1m immediately upstream of the higher bund.
Downstream along the main branch of Six Mile Creek flood levels increased by approximately 100
to 250mm for both moderate and major flood events. Refer to Figure 13 and Figure 14.




Lake McDonald Flooding Community Reference Group Report                                       Page 21

Section 24

Figure 11: Low level earth bund flood mitigation option - Moderate Flood Event




Lake McDonald Flooding Community Reference Group Report                          Page 22

Section 25

Figure 12: Low level earth bund flood mitigation option - Major Flood Event




Lake McDonald Flooding Community Reference Group Report                       Page 23

Section 26

Figure 13: High level earth bund flood mitigation option - Moderate Flood Event




Lake McDonald Flooding Community Reference Group Report                           Page 24

Section 27

Figure 14: High level earth bund flood mitigation option – Major Flood Event




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Section 28

Option 2B – Extended high level earth bund


As per Option 2A; however, the high level earth bund is extended along the rear (western) property
boundaries of those properties where flood waters currently pass from the main channel of Six Mile
Creek to the overland flow path which leads to Hamilton Road.

Based upon the results obtained from modelling Option 2A the option to extend the bund and
further exacerbate the impact was deemed unfeasible. Localised increases in upstream flood
levels of up to and beyond 1m would be expected with additional increases above 250mm also
expected in the main branch of Six Mile Creek.

Option 3 - Channel works (a)
The main water channel of Six Mile Creek adjacent to the western end of Liane Drive includes a
sharp right angle bend with high downstream embankment. This suggested mitigation option was
to excavate a channel to link the bends of the main channel to effectively straighten the main
channel and improve the efficiency of flood flows.

The flood model results for this option show no reduction in flood levels in the vicinity of Highland
Drive and only small localised reductions for either the moderate or major flood events.

Refer to Figure 15 and Figure 16.


Option 4 - Channel Works (a + b)
As per Option 3; however, the proposed works are extended to include and straighten the next
significant upstream bend of the main channel.

The flood model results for this option also showed no reduction in flood levels in the vicinity of
Highland Drive and only relatively small localised reductions for either the moderate or major flood
events.

Refer to Figure 17 and Figure 18.




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Section 29

Figure 15: Channel works (A) – Moderate Flood Event



Lake McDonald Flooding Community Reference Group Report   Page 27

Section 30

Figure 16: Channel works (A) – Major Flood Event


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Section 31

Figure 17: Channel works (A+B) – Moderate Flood Event


Lake McDonald Flooding Community Reference Group Report   Page 29

Section 32

   Figure 18: Channel works (A+B) – Major Flood Event




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Section 33

7.2       Council considerations for mitigation options
The above mitigation options explored by Council to investigate possible relief of flood depth and
extent within the area surrounding Liane drive have provided results which need to be evaluated
within a consistent framework across the entire Shire.

When considering flood mitigation options within the Shire, the Council is required to evaluate each
option for its effectiveness for the local flooding area and its value within the context of the entire
Shire. In order to do this the Council must first perform a risk assessment of any affected land
parcels within the Shire. The framework for doing this work is set out on State and National levels
and would assess each land parcel for the consequence of a natural disaster, in this case flooding.
Typically these assessments evaluate each parcel of land by assessing:

    1. Loss of life. What is the potential loss of life for a given event in this area?
    2. Financial and Economic. What are the impacts to the local/state economy from a given
       event?
    3. Community & Social. What are the impacts to places of cultural significance and community
       cohesion?
    4. Public Administration. What is the ability of the governing bodies to cope with a disaster of
       this magnitude?
    5. Environmental. What is the damage to natural resources, ecosystems or species?

Once this risk assessment is conducted across the Shire, Council would consider potential
mitigation options within the context of the following questions to assess the effectiveness and
viability of each option:

    1.   What are the positive consequences to providing this mitigation?
    2.   Are there any negative consequences to providing this mitigation?
    3.   What are the cost implications for this mitigation option?
    4.   What are the environmental implications of this option?

With relation to the mitigation options provided in section 7.1, an assessment of each option is
provided in table 1 below:

Table 1: Mitigation Option Assessment

Option              Positive                Negative                 Estimated Environmental
                    Consequences:           Consequences:            Cost      Considerations
                    Flood level reduction   Flood level increases    ($AUD)
                    for Liane Drive         in adjacent areas.
                    (mm)                    (mm)
1: Low Earth        100-250                 500-1000                 250k         Low if no clearing
Bund                                                                              is required and
2: High Earth       100-250                 >1000                    630k         built on private
Bund                                                                              land or road
                                                                                  reserve
3: Single           0                       0                        1.5M         High due to
Channel                                                                           clearing and
4: Double           0                       0                        2.8M         disruption of
Channel                                                                           sensitive habitat
5: Combination      100-250                 >1000                    3.7M

Given the limited benefits, consequences to adjacent land parcels and large costs involved with
each of these options, Council is unlikely to proceed with any of these options.




Lake McDonald Flooding Community Reference Group Report                                        Page 31

Section 34

7.3      Flood storage & blockages
Within the Six Mile Creek catchment there are a number of hydraulic structures that affect the
performance of the flood plain and consequently the flooding extent experienced during flood
events. Hydraulic structures within the catchment can affect the flood waters by either presenting a
blockage or they can provide storage of flood waters such that they do not contribute to flooding at
its peak.

Several questions have been raised both within the CRG and subsequent to it about specific
structures, such as the SEQ water pipes that cross in the vicinity of the lake McDonald community,
and more generally about the consequence that these hydraulic structures have on the
performance of the flood plain.

Firstly, storage devices such as dams or retention ponds that are on private land either for water
storage or flood mitigation are assumed to be full when modelling catchment wide response to
flooding. Given that the volume currently within these devices for any given event is unknowable, a
conservative approach is taken by assuming these storage devices provide no attenuation to flood
waters.

This is in line with assuming that the ground soil condition is in a fully saturated state before any
design flood event takes place. By making these two assumptions, the flood model is assuming a
‘worst-case’ scenario for water storage when flooding events do occur.

While this assumption removes the benefit of flood storage within the model, new developments
within any catchment are required within the planning scheme to present non-worsening of water
run-off into the catchment. Typically, this is done by designing water retention basins that
effectively offsets natural water retention in the soil with man-made retention in basins.

Secondly, structures that are within the flow path of flood waters can block flood waters, which can
alter the peak flood levels both upstream and downstream of the structure. Six Mile Creek varies
between urbanised through Cooroy, Pomona and Cooran to moderately wooded in other areas. As
a consequence of this, vegetation and build up including fallen trees and sedimentation may block
structures within the flood water flow path.

The design blockage is the blockage condition that is most likely to occur during a given design
storm and needs to be an “average” of all potential blockage conditions to ensure that the
calculated design flood levels reflect the defined probability. This blockage condition is well defined
within Australian Runoff and Rainfall and the adopted blockage for this flood model are provided in
table 2 below.

Table 2: Hydraulic Blockage Factors

      Location         Width/Diameter      50% to 5% AEP       2% and 1% AEP            < 1% AEP
Cooroy Township              < 3m                25%                  50%                 100%
Cooroy Township              > 3m                0%                   10%                  20%
   External to               < 3m                50%                 100%                 100%
Cooroy Township
   External to               > 3m                10%                  20%                  20%
Cooroy Township




Lake McDonald Flooding Community Reference Group Report                                        Page 32

Section 35

7.4      Sensitivity of water supply pipe crossing
Extending from an easement at the end of Liane Drive are four water supply pipes which span the
main channel and project from the creek embankments approximately 1.5m above the creek bed.
The elevations and diameters of each pipe differ slightly with the largest pipe also including metal
frame presumably to prevent people walking across the pipe. Refer to Image 5-2 and Image 5-3.




Image 5-2 Water Supply Pipes across the main channel




Image 5-3 Metal barrier frame attached to pipe crossing




Lake McDonald Flooding Community Reference Group Report                                      Page 33

Section 36

CRG stakeholders raised concerns about the impact these pipes may be having on flood levels. To
investigate these concerns the upgraded flood model was run for a moderate and major flood
event with and without the pipes included in the model. For the simulation which included the
pipes, the model configuration assumed a complete blockage of flows in the main channel beneath
the pipes and across the pipe profiles.

The flood model results for both the moderate and major flood events showed no change to peak
flood levels at Highland Drive due to the presence of the water supply pipe. Refer to Figure 19
and Figure 20 for flood model results graphed as water level verses time.




             No increase in
             peak flood levels




          Small increase in
          flood levels as
          water rises over
          the pipes.




Figure 19: Flood model results for moderate flood event




Lake McDonald Flooding Community Reference Group Report                                  Page 34

Section 37

            No increase in
            peak flood levels




         Small increase in
         flood levels as
         water rises over
         the pipes.




Figure 20: Flood model results for major flood event




Lake McDonald Flooding Community Reference Group Report   Page 35

Section 38

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

Section 39

9. Six Mile Creek Flood Warning Stations
Until recently, there were 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. However, through
the course of this project Council has been successful in applying for a funding subsidy through the
Department of Infrastructure, Local Government and Planning (DILGP) for the supply and
installation of 6 new flood warning stations. Three of these new stations are located upstream of
Highland Drive.

The flood warning stations measure rainfall at the station and water levels in an adjacent
waterway. The records are transmitted via radio frequencies to data collection points at the Council
Chambers in Tewantin and to the Bureau of Meteorology (BoM) in Brisbane. After data is received
from a station it is translated into either a rainfall depth or water level. Data is recorded and sent
from the stations as it happens providing real time observations of rainfall and flooding conditions
at these locations.

The new flood warning stations positioned upstream of Highland Drive at located at:
    •   Garnet Street Cooroy

    •   Elm St, Cooroy – between the road and rail line; and

    •   Liane Drive, Lake MacDonald. (Refer to Image 7-1)

In addition to providing real-time observations of conditions the recorded data from these stations
provides a historical reference of rainfall and water level responses for the catchment which can be
later used to continue to improve the understanding of catchments and floodplains. Over time, after
more significant flood events have been recorded, Council may use these records to once again
review and improve the calibration of a flood model.




Image 7-4: Liane Drive Flood Warning Station

Lake McDonald Flooding Community Reference Group Report                                       Page 37

Section 40

9.1      Flood Warning - during a rainfall/flood event
Members of the public can access near real-time rainfall and water level readings from these
stations via the BoM website. Observing the readings for the Garnet St and Elm St stations can
provide residents in the vicinity of Highland Drive with some warning as to whether upstream flood
levels are continuing to rise or are receding. Readings from the Liane Drive station can also be
accessed however these will not provide sufficient warning to nearby residents.

Recorded rainfall and water levels for these gauges can be accessed in the following manner:
Go to www.bom.gov.au

From the top menu banner select QLD and then Rainfall & River Conditions
Users can then choose to display indicators via the map by selecting Zoom in to: menu on the
right side of screen and then selecting Mary River. This will display locations for rainfall and river
height stations. For water levels select River Conditions from the Display on Map box and click
on a station to display a chart of the readings.

Users can also choose to navigate directly to the station data by selecting Rainfall & River Data
from the Other Links: menu. Once selected, choose the Mooloolah, Maroochy, Noosa, Mary
data link from the Latest River Height Data selection. Stations within the Six Mile Creek
catchment will be displayed within the Mary River basin station sets.

Click Plot to display a graph of the water levels for each station.




Lake McDonald Flooding Community Reference Group Report                                         Page 38

Section 41

10. Conclusions
The CRG consultation process and associated investigations informed by the upgrade of the Six
Mile Creek flood model support the following conclusions.

The February 2012 flood event was a statistically exceptional event based upon the intensity and
duration of the rainfall which led to the event. The magnitude of this event is significantly greater
than the standard (1% AEP) design flood event typically adopted for land use planning and
infrastructure design.

Without the benefit of long-term water level records in the vicinity of Highland Drive or in the
upstream catchment it is not possible to explore changes in the frequency of runoff from rainfall
events in the catchment over periods of time. Long-term rainfall records do show seasonal and
decadal trends in rainfall volumes which may account for observations of increased runoff/flood
frequency over recent periods compared to periods of lower average rainfall volumes. Runoff from
the contributing catchment is generated from relatively short duration (i.e. hours) rainfall events.
The frequencies of these events are not readily discernible from the long-term data for Eumundi.

The potential effects on flooding conditions of increased development in the contributing catchment
has been investigated and determined that planned increases in urbanisation within the Cooroy
township urban footprint will not increase peak flood levels in the vicinity of Highland Drive, Lake
MacDonald.

A range of flood mitigation options have been tested to evaluate the potential effectiveness of each
option to reduce the frequency or severity of flooding in the vicinity of Highland Drive. Whilst some
options demonstrated modest reductions in localised peak flood levels no option was found to be
viable due to associated impacts on adjacent properties, cost benefit indicators and environmental
constraints.

An investigation found when outflow from Lake MacDonald Dam coincides with flood flows from
the main branch of Six Mile Creek a backwater effect can result which may elevate peak flood
levels and extend the duration of inundation in the northern end of Highland Drive.
Three new flood warning stations have recently been installed upstream of Highland Drive. Real-
time flood reading from these gauges can be accessed via BoM website and may provide warning
of impending flooding conditions. Records from these stations will also assist with improving
Council’s understanding of the rainfall and runoff relationship for this catchment over time.




Lake McDonald Flooding Community Reference Group Report                                        Page 39

Section 42

11. Where to from here?
11.1    Catchment flood risk assessment.
A catchment wide flood risk assessment will be undertaken following the completion of the Six Mile
Creek flood model upgrade. The flood risk assessment will seek to evaluate the vulnerability of all
land parcels impacted by flooding in terms of frequency and consequence. The risk assessment
will be used identify and prioritise the risk to properties throughout the floodplain. Where
appropriate, responses to reduce the risks will be considered. Such responses may include flood
mitigation options, planning scheme provisions, disaster management strategies and community
awareness.


11.2    Develop a catchment wide flood mitigation strategy.
Flood mitigation works identified as possible responses to reduce flood risk throughout the
catchment will assessed and prioritised based upon the overall flood risk, benefit to the community,
coast effectiveness and environmental impact.


11.3    Update flood emergency planning maps.
The Council’s flood emergency planning maps will be updated to include the results of the
upgraded Six Mile Creek flood model. The flood planning layers are available via Council’s online
interactive mapping portal. https://noosacouncil.wixsite.com/interactivemaps/home


11.4    Update planning overlays for new Planning Scheme
The catchment flood risk assessment and flood planning maps will be used to update the planning
overlays for the new Noosa Planning Scheme. Planning overlays act as mapping trigger which
may require applications within the overlay areas to respond to special provisions.


11.5    Continue to monitor rainfall and runoff records in catchment.
Rainfall and water levels heights will continue be monitored via the new flood warning stations
installed throughout the catchment. Over time the flood model calibration may be reviewed with
regards to other significant flood events recorded within the catchment.




Lake McDonald Flooding Community Reference Group Report                                     Page 40