CONDAMINE RIVER: FLOOD INTELLIGENCE REVIEW DRAFT FOR PUBLIC REVIEW
SEPTEMBER 2017
Toowoomba Regional Council
PROJECT DETAILS
Report Title Condamine River: Flood Intelligence Review Draft for Public Review Client Toowoomba Regional Council WMS Project Manager Blake Boulton Job Number Document Name 0022_Condamine_FI_Review_Draft.docx
DOCUMENT STATUS
Revision Doc Report Distributed Report Reviewer Number Type Author to Date 00 Draft TM BB TRC 13/09/2017 Draft for 01 Public TM BB Public 22/09/2017 Review
REVISION STATUS
Revision Number Description
00 Draft Report 01 Draft Report for Public Review
COPYRIGHT AND NON-DISCLOSURE NOTICE The contents and layout of this report are subject to copyright owned by Water Modelling Solutions Pty Ltd (WMS). The report may not be copied or used without our prior written consent for any purpose other than the purpose indicated in this report.
The sole purpose of this report and the associated services performed by WMS is to provide the information required in accordance with the scope of services set out in the contract between WMS and the Client. That scope of services was defined by the requests of the Client, by the time and budgetary constraints imposed by the Client, and by the availability of data and other relevant information.
In preparing this report, WMS has assumed that all data, reports and any other information provided to us by the Client, on behalf of the Client, or by third parties is complete and accurate, unless stated otherwise.
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Contents 1 Introduction ...... 1 1.1 Background ...... 1 1.1.1 Project Background and Objectives ...... 1 1.1.2 Flood History ...... 1 1.1.3 Modelling History ...... 1 1.2 Data Available ...... 2 1.2.1 Modelling Data ...... 2 1.2.2 GIS Data ...... 3 1.2.3 Gauge Data ...... 3 1.2.4 Community Supplied Data ...... 3 1.3 Study Area ...... 3 1.3.1 Floodplain Topography ...... 3 1.3.2 Floodplain Development ...... 3 1.3.3 Floodplain Community ...... 4 1.3.4 Catchment Area ...... 5 1.4 Study Methodology ...... 8 2 Site Visit and Landholder Meetings ...... 9 2.1 Site Visit ...... 9 2.2 Community Meetings ...... 9 2.2.1 Community Key Issues ...... 9 3 Stakeholder Consultation ...... 10 3.1 Agency Stakeholders ...... 10 3.1.1 BoM Data Provided ...... 10 4 Existing Gauge Analysis ...... 12 4.1 Existing Gauge Network ...... 12 4.1.1 Existing Rain Gauges ...... 13 4.2 Flood Timing and Levels ...... 14 4.2.1 Flood Timing ...... 14 4.2.2 Comparative Flood Levels ...... 14 4.3 BoM Forecasts and Timing ...... 16 4.4 Gauging Summary ...... 16 5 Hydraulic Modelling Analysis ...... 18 5.1 Modelled Events ...... 18 5.2 Flood Behaviour ...... 18
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5.2.1 Flood Depths ...... 19 5.2.2 Flood Velocities ...... 19 5.2.3 Flood Duration ...... 19 5.2.4 Flood Hazard ...... 21 5.3 Probability of Inundation ...... 21 5.4 Time to Inundation ...... 21 6 Flood Intelligence Products ...... 22 6.1 Flood Depth Relationships ...... 22 6.2 Flood Timing ...... 22 6.3 Flood Intelligence Cards ...... 22 7 Recommendations ...... 27 7.1 Communication of Flood Warnings ...... 27 7.1.1 Agency Communications ...... 27 7.1.2 Community Communication ...... 27 7.2 Expansion of Gauging Network ...... 27 7.2.1 Tributary Inflows ...... 27 7.2.2 Major Breakouts ...... 27 7.2.3 Summary ...... 28 7.3 Additional Modelling with Existing Tools ...... 30 7.3.1 Condamine River Model – Rising Hydrograph or Additional Smaller Events ...... 30 7.3.2 Rain on Grid Whole of LGA Modelling for Ungauged Tributaries ...... 30 7.4 Scope for New Condamine Hydrological Model ...... 31 8 References ...... 32 9 Appendix A. Summary of Issues Raised During Community Meetings ...... 33 10 Appendix B: Evacuation Route Mapping ...... 35
Table Index
Table 1 Key Community Statistics for the Study Area ...... 4 Table 2 Approximate Catchment Areas ...... 5 Table 3 Community Groups Contacted ...... 9 Table 4 Agency Stakeholders Contacted ...... 10 Table 5 Existing Flow Gauging Network ...... 12 Table 6 Estimated Flood Timing...... 14 Table 7 Flood Peak Relationships - Warwick to downstream Gauges ...... 15 Table 8 Flood Peak Relationships - Tummaville to downstream Gauges ...... 15
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Table 9 BoM Forecast Parameters for Gauges within the Study Area ...... 16 Table 10 Modelled Events vs BoM Categories ...... 18 Table 11 Tummaville Gauge Flood Intelligence Card (area between Talgai TW Gauge and Yarramalong Rd) ...... 23 Table 12 Centenary Bridge Gauge Flood Intelligence Card (area between Yarramalong Rd and Horrick Road) ...... 24 Table 13 North Condamine at Pampas Gauge Flood Intelligence Card (area along the North Condamine between Yarramalong Rd and Melrose Rd) ...... 24 Table 14 North Condamine at Lone Pine Gauge Flood Intelligence Card (area along the North Condamine between Melrose Rd and West Prairie Rd) ...... 25 Table 15 Cecil Weir Gauge Flood Intelligence Card (area between Horrick Road and Wanka Rd) .. 26
Figure Index
Figure 1 Yearly Maximum Water Levels at Tummaville with BoM Flood Categories ...... 2 Figure 2 Location Map of the Study Area ...... 6 Figure 3 Condamine Catchment Area ...... 7 Figure 4 BoM Travel Time and Gauge Correlations ...... 11 Figure 5 Hydrograph Comparison - Centenary Bridge to Tummaville for March - April 2017 flood ... 13 Figure 6 Flood Behaviour Description - Spatial Location ...... 20 Figure 7 Potential New Gauging Sites ...... 29 Figure 8 Example of Staged Rising Hydrograph Inflow ...... 30
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1 Introduction
1.1 BACKGROUND 1.1.1 Project Background and Objectives The Condamine River is the largest floodplain within Toowoomba Regional Council’s (TRC) local government area. The floodplain is crossed by regional roads, has several towns located within or around it and is a mostly comprised of productive agricultural land. Flooding has the potential to put lives in danger as well as damage both public and private infrastructure within the floodplain.
Council is required by law to manage flood hazard within their local government area and are responsible for ensuring communities are warned of the potential flood, necessary evacuations occur, roads are closed prior to becoming a flood risk and to assist recovering from flooding. As part of meeting this obligation, Council has commissioned Water Modelling Solutions to undertake a Flood Intelligence Review of the Condamine floodplain.
The key objectives of the review were to:
• Determine what flood intelligence is currently available to different stakeholders and how this is being applied. • Determine what data is available to improve the existing flood intelligence. • Apply the available data to prepare products to improve flood intelligence. • Determine what data should be acquired to further improve flood intelligence. 1.1.2 Flood History Figure 1 shows the highest yearly recorded level at the Tummaville gauge within the study area. The figure shows that the Condamine floodplain is relatively frequently flooded, there have been 13 years with floods that exceeded the “Major” flood level assigned by BoM since 1965, with three of these occurring in the past seven years. There have also been 17 years with floods that exceed the BoM “Moderate” level and 32 years with floods that exceeded the “Minor” level.
The flood on record occurred at the end of 2010 and is still well remembered by the local community with an additional independent major flood occurring in 2013 and another flood occurring in 2017. Note that the 2017 flood was more significant downstream of Tummaville due to tributary inflows.
Recent flooding in the Condamine has had a significant economic impact with the loss of crops and livestock (including poultry), damage to on farm infrastructure such as head ditches and supply channels and damage to residential properties. Flooding in the Condamine can also have a large regional effect on the transport network by closing the Gore Highway, which is the main road between Toowoomba and Goondiwindi. 1.1.3 Modelling History The Condamine River has been modelled since at least 2003, when an URBS hydrological model was developed by the Bureau of Meteorology (BoM) for use in flood forecasting and flood warnings. In 2012, SKM undertook a review and revision of the URBS model which was calibrated to the 2010/11 flood event. In 2013 the URBS model was then extended from Tummaville down to Cecil Plains and a combined one and two-dimensional hydraulic model was developed using the MIKE FLOOD software.
The MIKE FLOOD hydraulic model was developed to route flooding from the Upper Condamine down to Cecil Plains where a detailed flood study was undertaken. The modelled area outside of Cecil Plains relied on an elevation product known as SRTM (Shuttle Radar Topography Mission) which has low
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accuracy and resolution (30 m pixels). Therefore, there was limited confidence with the hydraulic model.
The hydraulic model was then updated in 2015 by Water Modelling Solutions using newly acquired LiDAR (Light Detection and Ranging) elevation data which has much higher accuracy and resolution (1 m pixels) and covered the majority of the study area. Further model calibration and validation was undertaken to improve the confidence in the updated hydraulic model.
In 2016 Council also commissioned another modelling project using the “Rain on Grid” modelling approach to map flood extents in all the minor tributaries and flow paths within the Condamine catchment. This project took advantage of improved LiDAR coverage and recent software advances and provides information on flood conditions away from the main channels.
This study primarily relies on information gained from the 2015 update of the Condamine River Flood Study and is supplemented by the Rain on Grid model information.
Figure 1 Yearly Maximum Water Levels at Tummaville with BoM Flood Categories
1.2 DATA AVAILABLE 1.2.1 Modelling Data Water surface levels, depths, velocities and hydrographs were available from the updated Condamine River Flood Study for the 1 in 10, 1 in 20, 1 in 100, 1 in 500 Year ARI events and the calibrated 2010/11 flood event. These data were then used to produce additional products such as the “time to inundation” and “duration of inundation”.
Peak water depth for the 1 in 100 Year ARI event was also available from the TRC Rain on Grid model that covers the tributary inflows.
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1.2.2 GIS Data GIS data was available through the files that were supplied to Water Modelling Solutions as part of the Update of the Condamine River Flood Study project and Rain on Grid model project. This included shapefiles of the roads and rail alignments, critical infrastructure such as emergency services, gauge locations and land use data. 1.2.3 Gauge Data Historic gauge data was primarily sourced through the QLD Department of Natural Resources and Mines (DNRM) “Water Monitoring Information Portal” with some additional data provided by Council and BoM. Where possible, data was obtained on an hourly basis for the relevant period of record. 1.2.4 Community Supplied Data Council supplied relevant community data that was collected as part of the development of the Condamine River Flood Study. The community data included:
• Supplied maps and photographs showing historical flood levels. • Surveys of community flood experience, preparedness and preferences for flood mitigation options. A brief qualitative review of the community supplied data showed that generally additional flood gauges and warnings were strongly preferred management options for flooding with many respondents within the study area also preferring further community education. 1.3 STUDY AREA The study area extends from the Talgai TW gauge near Victoria Hill to downstream of Cecil Plains. Figure 2 shows the extent of the study area with relevant gauges, roads and key locations. The study area is essentially the extent of Condamine River flooding shown on Figure 2 and the surrounding areas that would be affected by the flood. 1.3.1 Floodplain Topography The headwaters of the Condamine River are located to the south east of the study area against the Great Dividing Range. The river flows through the town of Warwick in Southern Downs Regional Council and then enters the TRC LGA at around Talgai, where it is joined by Dalrymple Creek. Flow is generally to the north west throughout the study area and the river breaks its banks at several locations with breakout flow generally returning to the river at a point downstream.
The Condamine River floodplain is relatively flat with an average grade of around 0.1%, flow is generally confined to a corridor of around 10 – 20 km wide. Floodwaters are generally slow moving with relatively slow rates of rise with a high duration of flood waters remaining on the floodplain.
The main channels within the study area are; the Condamine River which runs through the entire study area; the North Condamine Branch which breaks from the main branch downstream of Ellangowan and re-joins the main branch downstream of Cecil Plains; Middle and Thanes Creek which run to the south of the Condamine River between Ellangowan, and Pampas and Grass Tree Creek which breaks on the south side of the main Condamine branch just downstream of Tummaville and re-joins the main branch downstream of Pampas. 1.3.2 Floodplain Development The floodplain has been primarily developed for agricultural purposes, with irrigated agriculture making up the majority of floodplain development. The remaining areas are generally used for grazing livestock as well as some small towns (e.g. Pampas, Ellangowan, Cecil Plains) and farm households.
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As part of the irrigated agriculture development, there is on farm storages, above ground supply channels and other works that can locally impact the passage of flood waters. Anecdotal evidence also suggests the time of year and condition of crops can significantly impact flow distribution.
There is also substantial public infrastructure within the floodplain, including the Gore Highway, Toowoomba-Kararra Rd, Toowoomba Cecil Plains Road and the dis-used Pampas Railway Line. When these roads are cut, access from Tooowoomba to a number of towns is also cut, including; Millmeran, Cecil Plains and Leyburn. 1.3.3 Floodplain Community The local farming community is generally flood prepared, resilient and has a strong community network where members assist each other with flood intelligence as well as provide physical assistance to their neighbours, including rescuing stock and resupply during floods. However, this network is limited to long term residents and other more transient members would be less flood ready.
Table 1 shows key statistics for the community taken from the 2016 Census for the Millmeran SA2 area. Around 1,500 of the population are located in the rural areas, around 250 in Cecil Plains and the remainder (around 1,500) within Millmeran. Some rural areas outside of the floodplain are also included in the Millmeran SA2 and the rural community within the floodplain has been estimated at around 500 from a house count using aerial imagery.
The data shows that generally the community is relatively similar to the rest of Queensland, however there are some key differences with some aspects of flood resilience, particularly:
• There is a higher proportion of elderly people that may require additional assistance than the average QLD community. • There is a lower median household income which may impact on the ability to recover from flood damages. • There is a higher rate of car ownership, indicating a greater ability to evacuate, only around 30 residences indicated that they did not own a car and these are more likely to be within Millmeran rather than on the Condamine floodplain. • There is a lower rate of non-english speakers who may struggle to understand flood warnings. The data shows that around 30% of the dwellings within the study area (2016 Census: Darling Downs – East SA3) are rental accommodation and this could be used as an estimate for the proportion of the population that would have limited flood experience and would require the most assistance.
Table 1 Key Community Statistics for the Study Area
Study Area Key Statistic All of QLD (Millmeran SA2) Population 3,310 N/A Average People per Dwelling 2.51 2.54 Percentage Elderly Population ( > 65 years of age) 20.2% 15.8% Percentage Very Young Population ( < 5 Years of age) 5.3% 6.3% Percentage Young Population (5 – 14 Years of age) 12.4% 13.1% Percentage Rental Properties 29.7% 30.9% Percentage non-English Speaking Households 6.4% 13.5% Median Household Income ($/week) 1,117 1,402 Number of Households with no Vehicles 32 N/A Number of Vehicles per Household 1.96 1.71
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1.3.4 Catchment Area The catchment area of the Condamine River as far down as Cecil Plains has been estimated using approximate methods and broken down into broad categories as shown in Figure 3. These catchment areas show the different potential water sources, however they cannot be used to pro-rata the proportion of flood waters. For example, the catchment upstream of Warwick is only around 20% of the Tummaville catchment, however in 2016 around 60% of the flow at Tummaville was sourced from upstream of Warwick. This is due to the difference in rainfall patterns and catchment characteristics. Nonetheless it can be seen that the catchment area of tributaries within the study area make up a significant proportion of the total catchment and may contribute heavily to flooding.
Table 2 Approximate Catchment Areas
Approximate Catchment Area Catchment Description (km2) Condamine River Upstream of Warwick 1,400 Condamine River Warwick to Talgai TW 3,100 Dalrymple Creek 1,350 Main Branch Tributaries Talgai – Centenary Bridge 1,550 Main Branch Tributaries Centenary Bridge – Cecil Plains 500 North Condamine Tributaries 1,140
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Figure 2 Location Map of the Study Area
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Figure 3 Condamine Catchment Area
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1.4 STUDY METHODOLOGY The project was undertaken in three main stages. The first stage was to gather and review existing data to determine what flood intelligence is currently available and identify the key issues within the floodplain (Sections 1 to 3). This was primarily done through site inspection, stakeholder consultation and review of the existing flood studies.
The second stage was to analyse the existing data, particularly the gauge, community supplied and modelling data to produce a range of flood intelligence products that will assist in emergency management (Sections 4 to 6).
The third stage was to identify gaps in the flood intelligence and make recommendations on how to improve them (Section 7). These gaps were typically raised as part of stages one and two.
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2 Site Visit and Landholder Meetings
2.1 SITE VISIT A drive-by inspection of the main evacuation routes within the study area was undertaken on the 15th and 16th of August. Main roads and some local roads on both sides of the river were covered between Toowoomba and Millmeran and then from Millmeran to Cecil Plains and from Talgai to Pampas. The primary goal of the site visit was to gain a familiarity with the local area and topography.
Road conditions on public roads were generally reasonably good and floodway signs were observed in many locations along main roads. Local roads were generally unsigned with respect to flooding. 2.2 COMMUNITY MEETINGS Contact was made with several community groups to organise meetings to obtain community data on road closures and flood impacts. Contact was made with these groups either via phone call and an email was sent if there was no answer or available phone number. The landholder groups that were contacted are shown in Table 3.
Community meetings were held at three locations on the 15th of August, the first meeting was held from 9 am to 11 am near Yandilla with representatives from Millmeran Landcare and five local landholders. The second meeting was held with a local landholder near Cecil Plains and the third was held in the Pampas Community Hall with approximately 10 landholders.
At each of the landholder meetings, the project background and objectives were discussed and the meetings were then held as an open forum for attendees to raise issues, point out key locations and provide other flood intelligence.
Table 3 Community Groups Contacted
Community Group Response Millmeran Landcare Organised two meetings with local landholders Michael Haggerty (Pampas Landholders) Organised meeting with local landholders Condamine Alliance No substantive information/assistance provided Condamine-Balonne Water Committee No response Condamine Headwaters Landcare Group Provided general information, raised ARTC issue Condamine Catchment Management No substantive information/assistance provided Association 2.2.1 Community Key Issues Detailed notes from the community meetings are provided in Appendix A and specific flood intelligence has been incorporated into the products developed as part of this project. Some of the key issues raised during the meetings are:
• A lack of communication from Council (or other agencies) with respect to flood warnings. Generally, those in attendance would warn each other during floods but this only extends to residents with strong community links. • A lack of communication from Council with respect to other flood related issues within the floodplain (E.g. Inland rail alignment, on farm storage impacts, maintenance of culverts). • There is a desire to see additional gauging and warning, particularly for the ungauged tributaries • There was an issue with the Centenary Bridge gauge during the most recent flooding and some confusion about what rainfall gauges in local tributaries are available.
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3 Stakeholder Consultation
3.1 AGENCY STAKEHOLDERS A number of key agency stakeholders were contacted and asked to provide both agency input as well as any local knowledge, the stakeholders contacted and their responses are shown in Table 4. Council stakeholders provided general goals and key issues with the current emergency management system while BoM provided a range of data and information used in their flood forecasting system. Local SunWater representatives provided limited information while QLD DNRM and Southern Downs Regional Council did not respond to enquiries.
BoM expressed a desire to work more collaboratively with Council and indicated that during a flood they generally attempt to contact Council and provide specific flood warnings, while Council indicated that they generally receive no warning from BoM.
This disconnect between Council and BoM is likely an issue that can be easily resolved by introducing the correct agency officers to be contacted during emergencies.
Table 4 Agency Stakeholders Contacted
Agency Response Council Internal Stakeholders Provided over-arching goals and key issues Bureau of Meteorology Provided data (see below) SunWater No substantive information/assistance provided QLD SES No contact provided QLD Department of Natural Resources No response and Mines Southern Downs Regional Council No response
3.1.1 BoM Data Provided BoM provided a range of data, including:
• Travel time and correlations between gauges, shown in Figure 4. • Flood impacts in relation to gauge height for selected gauges, which have been included in the Flood Intelligence Cards. • Peak flood heights for various gauges, which have formed part of the gauge analysis. The travel time and correlations show that while there is a relatively strong correlation between the gauge heights, there is a significant variation in the travel time between gauges. The travel time between Tummaville and Centenary Bridge varies between 4 hours and 20 hours with an average of 10 hours while between Centenary Bridge the travel time varies between 12 hours and around 70 hours with an average of 30 hours. There does not appear to be a link between flood severity and the travel time.
It is unknown how the BoM has estimated these travel times and the high variability could be due to using an automated procedure where the peak values are used. Condamine River floods tend to have very broad flat peaks, so while the “peak” might take a long time to reach the downstream area, the initial flooding may occur much quicker.
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Figure 4 BoM Travel Time and Gauge Correlations
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4 Existing Gauge Analysis
4.1 EXISTING GAUGE NETWORK The Condamine River is served by several gauges within the study area and upstream. These gauges are described in Table 5 and shown graphically on Figure 2. However, there are currently no gauges on major tributaries within the study area nor on upstream tributaries, with the exception of Dalrymple Ck.
The lack of tributary gauges creates issues, such as in the March-April 2017 event where the Centenary Bridge gauge rose approximately 10 hours prior to the gauge upstream at Tummaville, this was due to intense local rainfall in the tributaries that feed into the Condamine between the two gauges. Figure 5 shows the rising limb of the event for these gauges and the downstream gauge (Centenary Bridge) leads the upstream gauge by a significant margin throughout the event.
There are additional gauges within the network that are reported on by the BoM, including; Pratten, and Ellangowan, however data for these gauges was not available and anecdotally these gauges are not used by the community for flood warning.
Table 5 Existing Flow Gauging Network
Gauge Gauge Name Location (Description) Number Condamine River at Condamine River well upstream of the study area, 422310 Warwick appropriate for early warning. Condamine River at Talgai Condamine River near Victoria Hill, appropriate for 422355 Tailwater Ellangowan. Condamine River midway between Talgai and Condamine River at 422323 Pampas, appropriate for Tummaville, local roads as Tummaville well as the major Grass Tree Ck breakout. North Condamine River at North Condamine Branch at Pampas, appropriate Pampas 422347 for Pampas, Gore Hwy and surrounding area on the right floodplain. Condamine River at the Gore Hwy, appropriate for Condamine River at 541158 Pampas, Millmeran, Gore Hwy, Leyburn Rd and Centenary Bridge other local roads. North Condamine Branch approximately halfway North Condamine River at 422345 between the Pampas and Cecil Weir gauges, Line Pine appropriate for local roads. Just upstream of Cecil Plains, downstream of Condamine River at Cecil confluence with North Condamine River, 422316 Weir appropriate for Cecil Plains and surrounding area and Lake Broadwater floodway.
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Figure 5 Hydrograph Comparison - Centenary Bridge to Tummaville for March - April 2017 flood 4.1.1 Existing Rain Gauges Where stream gauges are unavailable, it is often possible to roughly predict flooding based on rainfall. Based on a manual reading of a rain gauge the intelligence that can be gained is broadly limited to whether or not a flood is likely, with some uncertainty in-between. To make precise predictions requires an automated system that can account for antecedent conditions and utilises a complex set of models.
The rainfall gauges that are utilised by BoM within the ungauged tributaries are shown on the Catchment Map in Figure 3. In the upstream area south of Leyburn there are a number of gauges that could be used for predicting tributary flooding (e.g. St Aubyn, Strathyre, Warahgai). Further downstream in the Grass Tree Ck catchments to the south of Millmeran there is only one location (Grays Gate). Downstream of Pampas there is also only one gauge available (Turallin) and in the North Condamine tributaries there are two available (Cambooya and Felton).
Anecdotally it is the Grass Tree Ck tributaries that are causing the most significant issues as they are downstream of Tummaville, and therefore no warning is available for the area around Pampas. They also appear to contribute enough flow to cause flooding in the Condamine, for example in the March 2017 event.
Data from the Grays Gate TM (041550) gauge was not available for the March 2017 flood, however daily read data from the nearby Carramar Gauge (041318) showed 55 mm fell on the 30th and 51 mm on the 31st of March 2017. This suggests that if greater than 50 mm falls in a day, there may be some flooding that occurs downstream in the Condamine around Pampas and Yandilla. A brief review of the past five years of daily rainfall suggests that there have been seven events with 40 mm or greater in a single day and three events with around 100 mm falling within two days. Complete gauge data was not available for Centenary Bridge to check whether these rainfall events produced flooding independent of the Condamine.
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4.2 FLOOD TIMING AND LEVELS 4.2.1 Flood Timing Flood Timing can vary considerably between floods and the use of peak flow timing can be significantly affected by the shape of the hydrograph. This is particularly problematic in floodplains such as the Condamine where flood hydrographs are typically flat and have no distinct peak.
Interestingly, the North Condamine River tends to peak at gauges that are downstream of the breakout than the gauges upstream on the main branch (e.g. North Condamine at Pampas vs. Tummaville). This is likely due to the Grass Tree Creek floodway. Essentially, once the Grass Tree Creek overland flow path is activated, nearly all additional flow goes down either the main Condamine River or Grass Tree Ck, rather than down the North Condamine branch. This is commonly referred to as a relief valve effect. In many floods, the Nth Condamine branches will rise and then flatten off while the main branch gauges will continue to rise. This makes predicting the flood travel time difficult using observed data.
Travel time estimates have been provided in Table 6, these have been based on a combination of; estimates provided by BoM, gauge Analysis (between 1996 and 2016) and model results. For the main river, gauge analysis was primarily used and estimates are based on an average of a range of floods while for the north Condamine locations estimates are based on modelled travel times as the gauges here tend to flatten off as previously discussed.
These travel times are useful for estimating the amount of time available for the flood to peak at different locations, however the travel time could vary significantly (as shown in Figure 4) or become confounded by local tributary run off as discussed in Section 4.1 and shown in Figure 5. The reported travel times are generally conservative (i.e. quicker), however it is possible for flooding to occur faster than those reported.
Table 6 Estimated Flood Timing
Cumulative Approximate Travel Upstream Gauge Downstream Gauge Time from Time (hours) Warwick Warwick Talgai TW 10 10 Talgai TW Tummaville 22 32 Tummaville Centenary Bridge 7 39 Centenary Bridge Cecil Weir 17 56 Pampas (Nth Tummaville 2 24 Condamine Pampas (Nth Lone Pine (Nth 5 29 Condamine) Condamine)
4.2.2 Comparative Flood Levels Correlation between gauge peaks was undertaken using the yearly maximum from 1996 – 2016. Overall it was found that there was a good correlation between the Warwick gauge and the gauges downstream, particularly at Talgai TW, Tummaville and Cecil Weir. This suggests that the relationship could be relied upon to provide a relatively accurate prediction of flood height at these other locations based on a prediction made at Warwick.
These predictions can be made on the assumption that the flood is primarily fed by rainfall upstream of Warwick, the strong relationships between gauges suggest that in most cases this is a reasonable assumption.
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Table 7 shows the relationships between the Warwick and the downstream gauges and the strength of the relationship. Table 8 shows the relationships between Tummaville and the downstream gauges.
The “Level” variable in the relationship is the predicted level at that gauge relative to the level at Warwick or Tummaville (Z). For example, if the BoM issues a warning at Warwick of 8 m, then the estimated flood height at Tummaville is 8 m multiplied by 1.145 plus 0.57, which equals 9.73.
The reported relationship strength (r2) is made up of values between zero and one, where a r2 of zero would indicate that the two gauges are not correlated at all and an r2 of one would indicate that the relationship would perfectly predict the flood height. Typically, values above around 0.8 are considered a good relationship.
For the North Condamine gauges (at Pampas and Lone Pine) the flood heights tend to be fairly flat regardless of the flood height in the main river. For example, a river height at Tummaville of around 4 m will produce a height in the North Condamine at Pampas of around 2.5 – 3 m, if the river height at Tummaville increases to 12 m, an 8 m rise, then the North Condamine at Pampas will only increase to around 3 – 3.5 m, a 0.5 m rise. This is likely due to the relief valve effect of the Grass Tree Ck overland flow path. Essentially, once this overland flow path is activated, nearly all additional flow goes down either the main Condamine River or Grass Tree Ck, rather than down the North Condamine branch.
The relationships in Table 7 and Table 8 are a useful tool for predicting potential flood heights for the study area with sufficient warning time to take action, however care needs to be taken as the relationships are not perfect and will only provide an approximately flood level. The eventual flood height will likely be influenced by local topographic conditions (e.g. crops, debris) as well as hydrological conditions such as contributions from tributaries.
Table 7 Flood Peak Relationships - Warwick to downstream Gauges
Relationship (Z indicates Gauge Relationship Strength (r2) height at Warwick) Tummaville Level = 1.145 * Z + 0.57 0.94 Talgai TW Level = 0.952 * Z + 9.37 0.86 Centenary Bridge Level = 0.428 * Z + 4.94 0.74 Cecil Weir Level = 0.678 * Z + 3.66 0.80 Nth Condamine Pampas Level = 0.173 * Z + 2.14 0.62
Table 8 Flood Peak Relationships - Tummaville to downstream Gauges
Relationship (Z indicates Gauge Relationship Strength (r2) height at Tummaville) Centenary Bridge Level = 0.311 * Z + 4.58 0.73 Cecil Weir Level = 0.466 * Z + 3.41 0.85 Nth Condamine Pampas Level = 0.123 * Z + 2.06 0.78
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4.3 BOM FORECASTS AND TIMING The Bureau of Meteorology (BoM) provides quantified flood warnings for two locations within the study area; Tummaville and Cecil Plains, as well as upstream at Warwick, as per the Service Level Specification for Flood Forecasting and Warning Services (BoM, 2013). BoM will provide a forecast flood level at these two locations as per the parameters outlined in Table 9.
A quantified warning is issued for both gauges within the study area when “Moderate” flooding is reached. “Moderate” flooding is approximately 2.2 m below the smallest modelled event (10% AEP) level at both locations. BoM aims to provide the warning at Tummaville at least 18 hours prior the peak and 24 hours prior to the peak at Cecil Plains. At both locations BoM aims to have at least 70% of its forecasts within 0.3 m of the eventual peak.
The BoM warnings are contingent on flooding occurring down the main river from Warwick, if a flood is generated by tributary inflows downstream (e.g. below Tummaville) then it is possible that flooding can occur at Cecil Plains without BoM issuing a flood warning. This is shown in Figure 5.
Similarly, BoM cannot provide quantified flood warnings for flooding occurring on tributary flows but may provide a generalised “Flood Watch” which will indicate a broad area and won’t provide an estimated level.
Table 9 BoM Forecast Parameters for Gauges within the Study Area
Tummaville Cecil Plains Warwick Parameter (041342) (041410) (041534) Minor 5 6 5 Flood Levels (m) Moderate 8 7 6 Major 9 8 7 Target Warning Lead Time 18 24 6 (hours) Trigger Height (m) > 8 > 7 >5 70% of Forecast Accuracy (m) +/- 0.3 +/- 0.3 +/-0.3 Gauge Zero (m AHD) 380.926 347.507 443.63
4.4 GAUGING SUMMARY The main Condamine river has several gauges along its length that are useful for the development of flood intelligence cards, the gauges are also relatively well correlated and warnings from Warwick can be used to estimate flood heights and impacts downstream. Given the long travel times from Warwick this provides useful time for warnings to be issued, evacuations and road closures to be enacted.
The gauge analysis for the North Condamine branch suggests that the North Condamine is relatively unresponsive to the flood magnitude and it will often rise to a relatively stable level (i.e. the hydrograph flattens off) well before the peak flow is reached in the main Condamine River. This suggests that action may need to be taken earlier on in areas affected by North Condamine River flooding.
The BoM service level specification for QLD suggests that they provide quantified warnings at Warwick, Tummaville and Cecil Plains when flooding reaches “moderate” levels. If warnings from Warwick can be relied upon then this provides at least 16 hours for flood peaks to reach Talgai (warning time plus travel time) and 28 hours for flood peaks to reach Tummaville. Interestingly, BoM will only provide 18 hours warning time for levels directly at Tummaville.
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Should a BoM warning not be available, then the travel times and relationships can be used in conjunction with current levels at Warwick to estimate flood heights. For example, if a flood appears to be peaking at Warwick at around 7 m, then it can estimated then the flood will peak around 8 m at Centenary Bridge in around 56 hours. These estimates are subject to a degree of uncertainty in both the magnitude and timing, so early action is preferable.
The above analysis is largely contingent on the flood water coming from the Condamine upstream, there are several tributaries within the study area that can cause flooding to occur well before flood waters from upstream reach the study area. The March-April 2017 flood provides a perfect example of where the existing gauge network is inadequate as the flooding occurred around Pampas prior to the water rising upstream at Tummaville. This event highlights the need for more data (both observed and modelled) on these tributaries and it is recommended that emergency managers keep an eye on rainfall that is occurring in these tributary catchments.
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5 Hydraulic Modelling Analysis
5.1 MODELLED EVENTS The Condamine River Flood Study includes modelled events with flood magnitudes ranging from the 1 in 10 Year ARI through to the 1 in 500 Year ARI, while the Rain on Grid model only includes the 1 in 100 Year ARI event.
The modelled events relative to the gauge heights are shown in Table 10. The model events have been scaled based on a comparison between model and gauge data for the 2010/11, for example at Tummaville the modelled 100 year ARI flood level is 0.24 m higher than the modelled 2010/11 flood at the gauge location. The observed flood height was 11.13 m for the 2010/11 flood therefore the 100 Year ARI is 11.37 m (11.13 + 0.24 m).
For the locations that have a BoM flood category, it can be seen that the smallest modelled event (1 in 10 Year ARI) is significantly greater than the “Major Category”. This is problematic for the development of flood intelligence cards as there is no data for minor, moderate or major floods up to the level of the 1 in 10 Year ARI. Any impacts shown for this flood may occur in floods that are smaller. For example, the Grass Tree Ck floodway is active during the 1 in 10 Year ARI event, but it is also likely to be active in floods smaller than this.
Table 10 Modelled Events vs BoM Categories
Nth Nth Talgai Centenary Cecil Event ARI Tummaville Condamine Condamine TW Bridge Weir Pampas Lone Pine 10 15.18 10.76 7.97 8.82 3.52 7.40 20 15.34 10.95 8.15 9.1 3.55 7.51 100 15.65 11.37 8.52 9.78 3.67 7.84 500 16.05 11.74 8.92 10.36 3.81 8.01 2010/11 15.66 11.13 8.3 9.22 3.59 6.37 Historic BoM N/A 5 N/A 6 N/A 3.5 Minor BoM N/A 8 N/A 7 N/A 4 Moderate BoM N/A 9 N/A 8 N/A 5 Major
5.2 FLOOD BEHAVIOUR A detailed description of flood behaviour is available within the flood study report (TRC, 2014) and this section is intended to provide a quick overview for emergency management.
The numbering used in this section relates to the numbers used in Figure 6.
1. At the upstream end of the Study Area, flow breaks primarily from the left bank between Talgai and Ellangowan through Middle and Thanes Creek. 2. Between Ellangowan and Tummaville there is a minor breakout to the north upstream of Hodson Creek, however the river flow is largely confined to the river and the North Condamine branch and the Middle/Thanes Creek flow paths to the south.
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3. At Tummaville the Middle/Thanes Ck overland flows on the south side of the river split between returning to the river and continuing on into the Grass Tree Creek Floodway which re-joins the main river flow further downstream at the Gore Highway. 4. Overbank flow is widespread between the main and northern Condamine branches (around 10 km) between Tummaville and the Gore Highway at Pampas. 5. Downstream of the Gore Highway the flow is relatively contained to the main branch floodway (around 5 km wide) and a narrow floodway along the North Condamine Branch. 6. Downstream of the Lone Pine Gauge the North Condamine branch breaks on both the left and right banks, with the left bank flow crossing to the Main branch around Cecil Plains. This forms a wide (9 km) flowpath between the main and northern branch up to the railway line at Cecil Plains. 7. Flow from the right bank of the Northern branch continues to the north through the railway line at Nangwee before joining overland flow from Linthorpe Creek and flowing into Ashall Creek and re-joining the main branch downstream of the study area. 8. The main branch of the Condamine continues downstream of Cecil Plains in a relatively confined (around 2.5 km) floodway. 9. Downstream of Cecil Plains, flow can break into an overland flow path to the west of the Condamine River that feeds Broadwater Lake (through Grassdale). 10. Ungauged tributary flows can produce widespread flooding on the left bank of the main branch and Grass Tree Ck through relatively wide overland flow paths. 5.2.1 Flood Depths In relatively frequent floods such as the 1 in 10 Year ARI, the floodplain depths are generally between 0.5 and 1 m in the main flow paths along the Condamine River branches and Grass Tree Ck. In less frequent floods (such as the 1 in 100 Year ARI) the average depth increases to around 1 – 2 m between the Gore Highway and Cecil Plains along the main branch. Upstream of the Gore Highway flood depths generally remain around 0.5 – 1 m with some areas greater than 1 m just upstream of the Gore Highway. In extreme events, such as the 1 in 500 Year ARI, the majority of the floodplain exceeds 1 m in depth. 5.2.2 Flood Velocities Given the flat nature of the floodplain, the flow velocities are relatively low. During a 1 in 10 Year ARI event, the average floodplain velocities is around 0.5 to 1 m/s with higher velocities in the river and other channels. The flood velocity only increases marginally for floods greater than this and even in an extreme event such as the 1 in 500 Year ARI the floodplain velocities are mostly between 0.5 and 1 m/s. 5.2.3 Flood Duration The modelled flood events do not include the full recession of the flood and it is difficult to estimate the peak duration of inundation, this is common practice as models are typically developed to estimate the peak flood conditions and running the model past this point is time consuming and expensive.
During the 2010/11 flood, the river level exceeded the “Minor” flood level at Tummaville for approximately 18 days, while in January and February of 2013 it exceeded “Minor” for approximately 8 days. This suggests that flooding can last for significant lengths of time and potentially isolate people for much longer as it takes time for ponded water on the floodplain to recede or infiltrate and for unsealed roads to become trafficable.
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Figure 6 Flood Behaviour Description - Spatial Location
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5.2.4 Flood Hazard Flood Hazard has been defined using the Queensland Reconstruction Authorities categories (QLDRA, 2012). Hazard is considered low where the combination of depth is less than 0.4 m and velocity is less than 1.5 m/s. This broadly conforms to other research (Smith, 2015) where combinations of depth and velocities greater than this are considered unsafe for vehicles, children and the elderly. As the flood velocities in the study area are typically under 1 m/s for even extreme events, the flood hazard is largely controlled by exceeding the 0.4 m depth criteria. In the 1 in 10 Year event the majority of the floodplain is between 0.5 and 1 m depth and therefore the hazard is considered significant or greater.
As the flood magnitude increases, the area of significant flood hazard increases and in extreme events the majority of the floodplain would be categorised as “Significant” or “High” which is considered unsafe for all people and vehicles. 5.3 PROBABILITY OF INUNDATION Given that the smallest modelled event is the 1 in 10 Year ARI and it inundates the majority of the floodplain, it can be inferred that significant floods occur at least as frequently as this and likely more frequently. Figure 1 shows that in the past 10 years there have been 6 floods that exceeded the minor level and 3 floods that exceeded the major level at Tummaville. While this may be a “wetter” period than average, it suggests that flooding is a common occurrence and flood preparation and resilience is crucial for those within the floodplain.
The probability (and gauge height) of roads within the floodplain being cut is shown in Appendix B where roads are considered “cut” when inundation reaches “Significant” Flood Hazard. It can be seen throughout the entire floodplain most roads within the network are cut in floods smaller than the 1 in 10 Year ARI event. 5.4 TIME TO INUNDATION Time to inundation has been calculated for the road network in the 1 in 500 Year ARI flood. This flood was selected as it is the fastest rising modelled flood and therefore the estimated time to roads becoming cut is generally conservative.
Estimates of the time until roads become inundated are shown in Appendix B. These time estimates are based on the river beginning to rise at the Talgai TW gauge. Low lying roads such as the Tummaville Rd as it crosses the Condamine River are cut in around 9 hours from the flooding to begin while roads up on the floodplain generally take between 18 – 30 hours to be cut between Talgai TW and Tummaville and 21 – 40 hours around Pampas and 21 – 60 hours downstream to Cecil Plains.
The mapping in Appendix B shows each road within the study area and the time it takes to become cut off. In most floods, these values are likely to be conservative, however they will provide a relative assessment of when roads will be cut i.e. road segments with smaller values will be cut before segments with higher values. The exception to this case is where roads are flooded by ungauged tributary inflows.
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6 Flood Intelligence Products
Several flood intelligence products have been developed as part of this study. It should be noted that these products largely rely on modelling as well as relationships between gauge data that have some variability and error. While conservative estimates in calculating these data have generally been made, it is recommended that these methods are used conservatively (i.e. assume a higher level of impact that will occur quicker than calculated). 6.1 FLOOD DEPTH RELATIONSHIPS Flood depth relationships are useful for predicting flood heights at locations where downstream of gauges or where there are no flood warnings. As described in Section 4.2.2 and presented in Table 7 and Table 8, flood depth relationships have been produced for the gauges within the system.
It is anticipated that these relationships can be used alongside the Flood Intelligence Cards to predict flooding impacts during an event. For example, if the Warwick gauge is peaking at 9 m, then the relationship in Table 7 could be used to calculate that the Tummaville gauge will reach approximately 10.9 m and the Tummaville Flood Intelligence Card (Table 11) would determine the likely flood impacts. 6.2 FLOOD TIMING Similarly to the flood depth relationships, average flood timing has been calculated and is presented in Table 6 and in the mapping in Appendix B.
These data can be used to estimate the time available to prepare for flooding. For example, if a flood is peaking at Warwick then Table 6 could be used to estimate that there is approximately 22 hours till the peak is reached at the Talgai TW gauge. Assuming the flood is large enough to cut roads, the maps in Appendix B could then be used to add on the number of hours till they become cut. For example, Tummaville Rd as it crosses the Condamine River will typically get cut in around 9 hours after flooding begins at Talgai, therefore there is around 31 hours to ensure that the road is closed to prevent people from driving over the bridge. 6.3 FLOOD INTELLIGENCE CARDS Flood Intelligence Cards have been produced for the main gauges along the river. As discussed in Section 5.1, the smallest modelled event is the 1 in 10 Year ARI which is above the “Major” BoM flood level at the relevant gauges. This severely limits the data that can be provided by the modelling in the Flood Intelligence Cards as most roads are cut below this point. Additional smaller modelled events would greatly improve the data available in the Flood Intelligence Cards.
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Table 11 Tummaville Gauge Flood Intelligence Card (area between Talgai TW Gauge and Yarramalong Rd)
Water Depth Impacts Data Source 4.2 • The Tummaville Rd at the Bridge is Cut BoM 8 • Local crops start to become affected BoM 10.4 • Some rural houses are affected BoM 10.76 NOTE THESE IMPACTS MAY OCCUR AT A LOWER LEVEL Model (1 in 10 Year ARI Flood) • Thanes and Middle Creek floodways are active between Talgai and Tummaville with depths exceeding 1 m • Grass Tree Creek Floodway is operating with flood depths around 0.5 – 0.8 m • Toowoomba-Karara Rd is cut • Clifton-Leyburn Rd is cut • Ryeford-Pratten Rd is cut • Millmeran-Leyburn Rd is cut upstream and downstream of intersection with Tummaville Rd • Grass Tree Rd cut by Grass Tree Ck and floodway • All local roads between Holmes Rd and Yarramalong Rd cut on both sides of the floodplain • North Condamine River upstream of Pampas is generally confined to banks 11.13 • Floodplain depths generally exceeding 0.5 m with large areas exceeding 1 m Model (Modelled 2010/11 Event) 11.37 • The right bank of the North Condamine floodplain begins to activate Model (1 in 100 Year ARI Flood) • Tummaville Rd is cut to the North of the North Condamine River • The area between Grass Tree Ck and Condamine River begins to be mostly flooded (between Grass Tree Rd and Reichle Rd) flood depths generally less than 0.3 m 11.74 • The majority of the floodplain between Grass Tree Ck and North Condamine from Talgai to Gore Model (1 in 500 Year ARI Flood) Highway is inundated with flow depths greater than 1 m • All local roads are cut
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Table 12 Centenary Bridge Gauge Flood Intelligence Card (area between Yarramalong Rd and Horrick Road)
Water Depth Impacts Data Source 6.7 • Water begins to cover Gore Highway Community 6.83 • One lane of Gore Highway is cut Community 7.97 NOTE THESE IMPACTS MAY OCCUR AT A LOWER LEVEL Model (1 in 10 Year ARI Flood) • Gore Highway and most local roads between Gore Highway and Bostock Rd are cut, isolating a number of houses • Bostock Rd is cut • Hall Rd is cut • Reichle Rd is cut • Grass Tree Rd is cut • Millmeran – Leyburn Rd is cut • Millmeran – Cecil Plains Rd is Cut • Flood Depths exceed 1 m in many locations, water has overtopped the railway line • Huff Rd is cut • Ladner Rd is cut 8.3 • Reservoir Road is Cut Model (Modelled 2010/11 Event) 8.52 • Most local roads between Bostock Road and Horrick Road are cut Model (1 in 100 Year ARI Flood)
Table 13 North Condamine at Pampas Gauge Flood Intelligence Card (area along the North Condamine between Yarramalong Rd and Melrose Rd)
Water Depth Impacts Data Source 3.52 NOTE THESE IMPACTS MAY OCCUR AT A LOWER LEVEL Model (1 in 10 Year ARI Flood) • The Gore Highway is cut at the North Branch • Hanlon Rd is cut to the west of the Nth Condamine Branch • Bostock Rd is cut • Yarramalong Rd is cut at the North Branch 3.55 • Melrose Rd is Cut Model (1 in 20 Year ARI Flood) 3.59 • Hanlon Road Model (Modelled 2010/11 Event) 3.67 • Hanlon Rd is cut to the east of the Nth Condamine Branch Model (1 in 100 Year ARI Flood)
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Table 14 North Condamine at Lone Pine Gauge Flood Intelligence Card (area along the North Condamine between Melrose Rd and West Prairie Rd)
Water Depth Impacts Data Source 6.37 NOTE THESE IMPACTS MAY OCCUR AT A LOWER LEVEL Model (Modelled 2010/11 Event) • Branchview Rd is cut • Merson Rd is cut • Snake Gully Rd is cut • Toowoomba – Cecil Plains Rd is cut • Coggan Rd is cut • West Prairie Rd is cut • Bazley Rd is cut • Tipton – Horraine Rd is cut to the north of Watson Rd intersection 7.85 • Properties to the west on Nangwee Rd and north of Branchview Rd are isolated Model (1 in 100 Year ARI Flood) • McKeever Rd is cut • Olsen Rd west of McPherson Rd is cut • Rowland Rd East is cut • Nangwee Rd is cut between Branchview and Merson Rd is cut • Jeffries Rd is cut 8.09 • Olsen Rd east of McPherson Rd is cut Model (1 in 500 Year ARI Flood)
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Table 15 Cecil Weir Gauge Flood Intelligence Card (area between Horrick Road and Wanka Rd)
Water Depth Impacts Data Source 6 • Condamine River is running at bank full BoM 6.9 • The Condamine River Bridge (Toowoomba Cecil Plains Road) is affected BoM 7 • Local crops are affected BoM 7.1 • Toowoomba - Cecil Plains Road is cut BoM 7.8 • Cecil Plains – Dalby Road is cut approximately 5 km north of town BoM 8 • Water Reaches Cecil Plains Homestead BoM 8.82 • NOTE THESE IMPACTS MAY OCCUR AT A LOWER LEVEL Model (1 in 10 Year ARI Flood) • Properties between the Condamine and North Condamine Branches are isolated south of Toowoomba-Cecil Plains Rd • Millmeran – Cecil Plains Rd is cut • Toowoomba – Cecil Plains Rd is cut • Pampas Horraine Rd is cut • Storey Lane is cut • Cook Rd is cut • River Rd is cut • McGrath Rd is cut • Coggan Rd is cut • Routley Rd is cut • Wanka Rd is cut 9.1 • All local roads are cut and properties within the floodplain isolated Model (1 in 100 Year ARI Flood) • Ladner Rd is cut • Percy Jurgs is cut • Grassdale is cut • Hutt Rd is cut • Boundary Rd is cut
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7 Recommendations
7.1 COMMUNICATION OF FLOOD WARNINGS 7.1.1 Agency Communications During stakeholder consultation it was raised that there are often difficulties with communication between Council and BoM with both parties expressing a desire to improve communication.
It is recommended that a meeting is held between BoM Flood Forecasting team and Council emergency managers to discuss lines of communication, what data can be available and how it can be provided during flood events. It is recommended that this meeting is repeated annually to prevent miscommunication due to staff turnover. 7.1.2 Community Communication As discussed previously, community members are currently obtaining flood warnings from within the community rather than from Council. There was a desire within the community for improved flood warning that was raised in both the Flood Study Consultation and during community meetings held for this project.
This improved flood warning could take the shape of a publicly available website with predicted flood levels and road closures or utilise existing social media (Council’s Facebook or Twitter accounts) or an SMS alert service to provide a warning.
The community also expressed concerns about flood related development in the local area (Inland Rail, updated planning scheme and culvert maintenance) and a lack of communication from Council in this regard. It is recommended that Council officers hold meetings or have discussions with the community to address some of these issues where possible and articulate the Council’s position on them. 7.2 EXPANSION OF GAUGING NETWORK 7.2.1 Tributary Inflows The existing gauging network covers flow from the Condamine River relatively well. However, as the March 2017 event showed, the ungauged tributaries, particularly those that flow into Grass Tree Ck can cause flooding without any form of warning and time to prepare.
There are several potential locations along Grass Tree Ck (such as at Reichle Rd) that could serve as locations for additional gauges, however these would provide little warning time. Analysis of the Rain on Grid model shows that the flow from these tributaries is widespread and not contained by their channels. However, some of the tributaries such as Canal Ck, Speers Ck, Sandy Ck or Back Ck could have gauges located on them and these may be representative of all the local tributaries. Further modelling and analysis would be required to confirm which tributary was the most representative. Also, a gauge along Back Ck at Millmeran may also have additional benefits for flood intelligence for local Millmeran flooding.
If no stream gauge can be installed that would provide sufficient warning time, it is suggested that the existing rain gauge network be improved and some analysis is undertaken to determine the rainfall threshold for flooding to occur (e.g. 50 mm in one day). Due to the highly variable nature of rainfall, it is suggested that a number of these gauges be installed to ensure that all storms are captured. 7.2.2 Major Breakouts The Grass Tree Ck breakout forms a significant flow path of water and potentially isolates several properties. It is currently unknown at what river height this breakout occurs (relative to the Tummaville
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gauge) and the exact location, an additional gauge at the upstream end of the Grass Tree Ck breakout along Millmeran-Leyburn Road would provide information on when the flow path is activated and when it recedes.
An additional gauge may be useful for the Broadwater Lake breakout downstream of Cecil Plains, however this was not raised as a significant issue by the community. 7.2.3 Summary A small focused study examining the potential sites for the Grass Tree Ck gauges (upstream for breakout and downstream for tributary flows) and tributary stream or rain gauges would need to be undertaken to determine the most appropriate location from both hydraulic control and warning time perspectives. The rough location for the suggested gauges is shown in Figure 7.
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Figure 7 Potential New Gauging Sites
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7.3 ADDITIONAL MODELLING WITH EXISTING TOOLS 7.3.1 Condamine River Model – Rising Hydrograph or Additional Smaller Events The smallest modelled flood is the 1 in 10 Year ARI, which is largely higher than the “Major” BoM flood level and when it occurs, most of the road network is cut throughout the study area. Therefore, there is very limited information of the sequencing of the road network getting cut by floodwaters and very little information about the impacts of a “minor” or “moderate” flood and the events in between these and the 1 in 10 Year ARI event.
The Flood Intelligence Cards could be greatly improved by obtaining flooding data for smaller more frequent events.
It is recommended that the Condamine River Flood Study model is run for either a series of smaller events to quantify the impacts at these lower levels, or run using a staged and slowly rising hydrograph. Results can then be analysed at key timesteps and relationships between the gauge level and flood impacts calculated at these timesteps. This approach has been used in similar studies in similar floodplains and an example of the inflow hydrograph is shown in Figure 8.
These flood events would not need an associated Annual Exceedance Probability and so the hydrological model would not need to be run.
Figure 8 Example of Staged Rising Hydrograph Inflow 7.3.2 Rain on Grid Whole of LGA Modelling for Ungauged Tributaries Currently there is very little information regarding the flooding that can be caused by the ungauged tributaries. The new Rain on Grid “Whole of LGA” modelling could be used to significantly improve the knowledge and provide data to produce a Flood Intelligence Card for either a rain gauge or for a new stream gauge on the currently ungauged tributaries.
If emergency planning occurs assuming only flooding from the Condamine River then in many locations there is the potential for roads to be cut and properties to become isolated well before anticipated. Therefore, it is recommended that the Rain on Grid modelling be expanded to provide flood intelligence for the Condamine River study area.
This would require a series of model runs using different rainfall depths over different time periods and the flood impacts of those combinations could be examined and a sequence of road closures could be developed based on tributary rainfall.
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7.4 SCOPE FOR NEW CONDAMINE HYDROLOGICAL MODEL It is understood that the existing URBS hydrological model relies on old date, dated software and modelling approaches and has a number of issues both with the results and in running the model.
Given the importance of the Condamine River it is recommended that a new hydrological model be developed that can provide more accurate results for use in hydraulic modelling, particularly around tributary inflows where there is no observed data to calibrate against.
The Condamine River spans several local government areas, with the majority of it’s catchment located in Southern Downs Regional Council and flood impacts felt in a number of Councils downstream of Toowoomba. Therefore, a new hydrological model would be a valuable tool for a number of Councils in addition to Toowoomba and it is recommended that discussions be held with these Councils on a joint effort to improve modelling in the Condamine.
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8 References
Bureau of Meteorology (BoM), 2013, Service Level Specification for Flood Forecasting and Warning Services for Queensland v2.0
Bureau of Meteorology (BoM), 2017, QLD Flood Warning Station Details (http://www.bom.gov.au/qld/flood/networks/index.shtml)
Department of Natural Resources and Mines, 2017, Water Monitoring Information Portal (https://water-monitoring.information.qld.gov.au/)
QLDRA (2012). Planning for stronger, more resilient floodplains. Part 2 – Measures to support floodplain management in future planning schemes. Queensland Reconstruction Authority.
Toowoomba Regional Council, 2014, 2D Flood Study for areas within the Upper Condamine River Floodplain Broad Scale Model Study Area including Cecil Plains.
Smith G, McLuckie D (2015) Delineating Hazardous Flood Conditions to People and Property, Australian National Floodplain Management Conference 2015.
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9 Appendix A. Summary of Issues Raised During Community Meetings
General:
• The proposed Inland Rail will affect the hydraulics within the study area and if it is constructed the data from this project would need to be revised to reflect the new flood behaviour. Note: However, having an accurate and robust flood model will allow any Inland rail impacts to be better understood by Council and its community. • There is general concern that community input to Council projects are not being utilised and no feedback is provided to the community on the use of their data. Note: Council assures the community that ALL previously supplied data is definitely being utilised. • There is a well-established local community with a long flood memory (39 floods or around 80 years). • There is some concern surround the proposed planning scheme amendments. Council is currently revising information that was previously provided and will present new information early 2018. Note: The larger TRC Planning Study is due for the further round of statutory 6 months community consultation
Warning:
• Local residents currently rely on either their reading of gauges, primarily Tummaville and upstream as well as headwaters rain gauges, or through contacting upstream neighbours to estimate the potential flood size. • The only warning available from institutions (BoM, Council, SES etc) is the public BoM warning and generalised warnings on 747 AM. No specific warning (such as SMS or phone calls) are received by locals. • The BoM generally warning times are around 18 – 24 hours for floods coming from the river upstream, no warning for flooding that occurs due to tributaries. • There is a transient population in rental properties that do not have flood experience and/or not connected to the community and therefore receive no specific warnings. An anecdotal estimate is that this population turns over every 3 or so years. • There has been a strong reliance on individual knowledge over the years (e.g. Lyndon Pfeffer) and some of this knowledge is being lost over time.
Evacuation:
• Evacuation is generally undertaken early or otherwise residents collect supplies and stay home. • Properties can be isolated upwards of 7 days. • Some residents are placing and removing road closed signs on low lying bridges. • There is a general perception not to rely on the SES. • 24 hours is ideal for flood preparation and reduce potential damage (e.g. lift equipment). • There is concern regarding the amount of time taken to assess bridge conditions after flooding and re-open roads.
Tributaries:
• The tributaries that flow into the Grass Tree Creek flow path can produce enough runoff to cause floods within the Condamine that can cut evacuation routes. Flooding occurs much quicker from these tributaries than from main river flooding. • These tributaries are currently not gauged and no warning is available for flooding due to tributary inflows.
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Flood Behaviour:
• The Powerhouse mine is affecting flood behaviour upstream of Millmerran. • The existing railway has a large influence on flooding behaviour and its culverts are generally not maintained (i.e. become blocked by debris). • The existing railway has suffered significant damage in recent flooding (Gilgai Lane blowout). • The existing railway and Gore Highway are exacerbating flooding issues and require additional culverts. • Significant community input was gathered by Council on the “Big Map” which should still be relevant. Note: this will be used as input into the report. • There is a perception that floods are arriving more rapidly due to conversion of land upstream from grazing to cropping and also that crops within the floodplain have a major effect on the flow distribution. • The flow distribution between the North and main branch of the river is variable depending on the flood. E.g. In 2011 there was significant local rainfall in the North Branch which led to much greater flooding). • Existing farm storages are funnelling flow onto neighbouring properties in some cases.
Gauging System:
• The Centenary Bridge gauge in the most recent flood was only recording levels approximately every 3 hours. • There are no gauges on the Grass Tree Creek flow path or in tributary inflows. • There was some confusion about the availability of rain gauge information in the Grass Tree Ck tributary catchments (i.e. Millmeran gauge now that the post office is not being read?). • There was a desire that if new gauges were installed that the data be made publicly available. An “app” would be particularly useful • There has been a 0.15 m change in gauge datum from when the automatic gauge was installed Centenary Bridge upstream (manual gauge was located downstream)
Key Locations Identified:
• Gore Highway first cut to the west of the Centenary Bridge (water on road at 6.7 m, covering one lane at 6.825 m, reading at Centenary Bridge) • Gore Highway can be cut by local runoff just to the west of the “S Bend” near Millmeran (3ft deep in Dec 2010) • The Grass Tree Creek breakout from the main river (waist height at power pole in 2013), this flow path is difficult to gauge as it is largely overland with no defined channel although a number of potential locations were discussed • Grass Tree Creek tributaries were identified as causing significant flooding independent of mainstream flooding (e.g. in 2017, 5.5 m at Centenary Bridge while Tummaville is only at 1.5 m) • Crawlers Creek local rainfall can cut the Cecil Weir Road to the south • “The Long Swamp” downstream of Cecil Plains (feeding Broadwater Lake) is not gauged • Bridges on Cecil Plains Road identified as a potential gauge site (only overtopped in 2011 flood) • Other low-lying bridges on local roads were identified.
0022_Condamine_FI_Review_Draft.docx | 22 September 2017
10 Appendix B: Evacuation Route Mapping
0022_Condamine_FI_Review_Draft.docx | 22 September 2017
Figure B1 Tummaville Area - Road Closures during a 1 in 10 Year ARI Event (10.76 m at Tummaville)
Figure B2 Tummaville Area - Road Closures during a 1 in 100 Year ARI Event (10.37 m at Tummaville)
Figure B3 Tummaville Area - Road Closures during a 1 in 500 Year ARI Event (11.74 m at Tummaville)
Figure B4 Tummaville Area - Timing of Road Closures during a 1 in 500 Year ARI Event
Figure B5 Pampas Area - Road Closures during a 1 in 10 Year ARI Event (7.97 m at Centenary Bridge, 3.52 m North Condamine (Pampas))
Figure B6 Pampas Area - Road Closures during a 1 in 100 Year ARI Event (8.52 m at Centenary Bridge, 3.67 m North Condamine (Pampas))
Figure B7 Pampas Area - Road Closures during a 1 in 500 Year ARI Event (8.92 m at Centenary Bridge, 3.81 m North Condamine (Pampas))
Figure B8 Pampas Area - Timing of Road Closures during a 1 in 500 Year ARI Event
Figure B9 Cecil Plains Area - Road Closures during a 1 in 10 Year ARI Event (8.82 m at Cecil Weir, 7.4 m North Condamine (Lone Pine))
Figure B10 Cecil Plains Area - Road Closures during a 1 in 10 Year ARI Event (9.78 m at Cecil Weir, 7.85 m North Condamine (Lone Pine))
Figure B11 Cecil Plains Area - Road Closures during a 1 in 500 Year ARI Event (10.36 m at Cecil Weir, 8.01 m North Condamine (Lone Pine))
Figure B12 Cecil Plains Area - Timing of Road Closures during a 1 in 500 Year ARI Event