The Big Flood Final Report

 

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Final report of The Big Flood ARC project

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THE BIG FLOOD: WILL IT HAPPEN AGAIN? www.thebigflood.com.au UNDERSTANDING PREDICTING MANAGING Final report

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This project was funded by the Australian Research Council (ARC) Linkage Project (ARC LP 120200093 2013-2016). Disclaimer The material contained in this report is produced for general information only. It is not intended as professional advice on specific applications. It is the responsibility of the user to determine the suitability and appropriateness of the material contained in this publication to specific applications. No person should act or fail to act on the basis of any material contained in this publication without first obtaining specific independent professional advice. The Big Flood Project team expressly disclaim any and all liability to any person in respect of anything done by any such person in reliance, whether in whole or in part, on this publication. The Big Flood Project team give no warranty in relation to the data (including without limitation, accuracy, reliability, completeness or fitness for a particular purpose). To the maximum extent permitted by applicable law, in no event shall the Big Flood Project team be liable for any special, incidental, indirect, or consequential damages whatsoever (including, but not limited to, damages for loss of profits or confidential or other information, for business interruption, for personal injury, for loss of privacy, for failure to meet any duty including of good faith or of reasonable care, for negligence, and for any other pecuniary or other loss whatsoever including, without limitation, legal costs on a solicitor own client basis) arising out of, or in any way related to, the use of or inability to use the data. Acknowledgements A great many people assisted us on this project and we are very grateful for their input and expertise. Thanks to; James Grove (Uni. Melb), Robert Denham (DSITI), Annegret Larsen (UQ), Giri Kinhal (DSITI), Peter Todd (DNRM), Paul Lawrence (DSITI), Ken Brook (DSITI), Fiona Watson (DSITI), Dan Tindall (DSITI), Cate Dent (DSITI), Rob Dehayr (DSITI), Morag Stewart (Seqwater) Sonya Monk (DSITI), Kate Dolan (DSITI), Michael O’Loughlin (DSITI), Loraine Smith (DSITI), Dan Brough (DSITI), Jeremy Manders (DSITI), Bernie Powell (DSITI), Ian Hall (DSITI), Don Malcolm (DSITI), Taka Furichi (DSITI), Joanne Burton (DSITI), Justine Kemp (GU), Jerry Maroulis (WUR); Arnaud Temme (KU); Richard Collins (LVRC), Belinda Whelband (LVRC), Quinten Underwood (LVRC), Kate Hughes (UQ), Rebecca Bartley (CSIRO) and the Sippel Families and property owners in the Lockyer Valley.

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Contents Introduction 2 Approach 3 The Lockyer Valley 8 What happened? The Flood Sediment movement Where did the sediment go? What was the impact on Moreton Bay? 11 13 17 19 Why did it happen? Valley evolution Channel and floodplain characteristics Historical channel adjustment Flood energy and stream power 20 22 24 27 Will it happen again? Predicting flood frequency Extending the flood record Integrating paleoflood data 28 30 32 Managing future floods Key findings Flood hazard Soil on the paddock Downstream impacts Future trajectories Integrated catchment action plans 33 34 36 37 39 40 References 41 Final report THE BIG FLOOD: WILL IT HAPPEN AGAIN? 1

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Introduction The urgency to understand and predict the magnitude and timing of floods in Eastern Australia reached a critical point following widespread flooding across large parts of Queensland, NSW and Victoria in January 2011. Twenty-two lives were lost in the Lockyer Creek floods in southeast Queensland (SEQ) in the summer of 2011. The total damage to public infrastructure as a result of this flood was estimated at about $2 billion. Whilst the hydrological characteristics of the Lockyer Creek 2011 event have now been evaluated through Coronial Enquiries, there remains concern about the timing of the next ‘big-event’ and how other populated settlements in similar settings may be affected. River discharge records are too short to determine the likely recurrence intervals of these extreme flood events with any certainty. Climate change predictions for Australia also indicate increased incidences of extreme flood events with some areas being at greater risk than others. Understanding the frequency and causes of extreme flood events is crucial for social and economic planning and environmental protection. SEQ has one of the fastest growing populations in Australia; currently around 2.8 million people, and expected to increase to ~4.4 million by 2031. The associated expenditure on infrastructure is expected to exceed $100 billion. This overall goal of this project was to contribute to the improved understanding, prediction and management of extreme flood events in the Lockyer Valley and broader SEQ region. Major aims of the project • Reconstruct a time series of major flood events for Lockyer Creek extending back more than 1000 years. • Predict river channel and floodplain geomorphic susceptibility to floods in the Lockyer Valley and locate areas of high risk. • Incorporate research findings into climate change predictions, water quality protection and river management in Australia. STATE LIBRARY OF QUEENSLAND 2 THE BIG FLOOD: WILL IT HAPPEN AGAIN? Final report

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Approach A range of innovative methods and approaches were used to answer the project questions. Collectively these tools allowed us to develop a source-to-sink framework for assessing the impacts of floods on our rivers and receiving waters. Geomorphic change detection Geomorphic change detection (GCD) using high resolution LiDAR digital elevation models from different time periods (pre- postflood) was used to determine the extent and magnitude of change following a flood (Figure 1). The application used in the Lockyer was the largest scale this approach has been applied and gave very accurate estimates of erosion, deposition and sediment redistribution 3. Figure 1. Pre-flood, post-flood and DEM of difference Final report THE BIG FLOOD: WILL IT HAPPEN AGAIN? 3

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Approach Reconstructing the long-term evolution of the valley? To put the present river channel of Lockyer Creek into perspective, we investigated the long-term evolution of the valley over several time periods; Pleistocene (~ 250, 000 years ago); Holocene (last 10,000 years) and Historic (~ 200 years ago). Old river features such as river terraces were mapped in the Lockyer valley. We used DSITI’s Geoprobe drill rig (Figure 2) to obtain samples of the deep alluvium stored in the Lockyer’s floodplain. We sampled sediments from 30m deep in the floodplain. A total of 6944 bore records for the Lockyer catchment were extracted from the Queensland groundwater database with 2330 records having a record of depth to bedrock. These were used to construct the bedrock palaeovalley 20. Reconstructing the post-European settlement river adjustment story reconstructed? Time was spent at the State Library of Queensland and the State Archives at Runcorn searching for the following types of information: • Explorers’ journals often have descriptions of the landscape, vegetation and rivers they encountered. • Old on-ground photographs of the landscape, rivers, old bridges (Figure 4) to establish what the landscape was like at the time of the photographs being taken. • Parish maps that contain information on channel planform and sometimes floodplain vegetation descriptions. Figure 2. Drill rig • Bridge surveys to determine any changes in channel capacity. • Old photographs at bridges that can be rephotographed in the field (Figure 3). • One of the most useful forms of information is the historical air photograph record (Figure 5). All the air photos are orthorectified in ArcGIS and analysed for recognisable geomorphic adjustments between timeslices. Changes in channel position, width, and a range of other geomorphic adjustments can be detected. In the Lockyer Valley the first set of air photographs were flown in 1933. The most complete set of parish maps is from the 1890s. Google Earth can also be used to detect more recent changes 9. Figure 3. Helidon (Drover’s Crossing), 1890s and 2014. STATE LIBRARY OF QUEENSLAND K. FRYIRS 4 THE BIG FLOOD: WILL IT HAPPEN AGAIN? Final report

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STATE LIBRARY OF QUEENSLAND Approach Figure 4. Gatton O’Connors Bridge ca 1924 Figure 5. Historical air photograph record Reconstructing flood frequency Floods often leave a layer of overbank sediment which can build up over time to provide a record of past flood events. These were located in the bank profiles exposed during the flood. They are horizontal and can range in thickness from 10cm to nearly 1m (Figure 6). We sampled 41 sites down the length of the main Lockyer Creek. Sediments were taken in the field and taken back to the laboratory for dating using Optically Stimulated Luminescence (OSL) dating. OSL can measure the last time each grain of sand was exposed to sunlight, allowing us to determine a burial age for each flood unit. OSL is one of the most accurate ways to date the age of river sediment. A data base of over ~ 180 OSL ages have been compiled through this project 19. Figure 6. Flood unit sampling Final report THE BIG FLOOD: WILL IT HAPPEN AGAIN? 5

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Approach Reconstructing flood magnitude Slack-water deposits (Figure 7 and Figure 8) are flood-sediments deposited in slow flow or backwater zones generally on the margins of bedrock or laterally stable channels and are protected from subsequent erosion. The timing of the flood is determined by OSL dating and the magnitude of the flood is reconstructed based on the minimum stage height of flow required to inundate the slack-water deposit. A calibrated hydraulic model is built based on topographic surveys and/or LiDAR DEMs and is used to estimate paleoflood magnitude. Paleoflood reconstructions based on slack-water deposits have been reconstructed from six sites across Southeast Queensland and the Wide BayBurnett 24. Figure 7. Slack water deposits Reconstructing past rainfall patterns Records of past rainfall in the region are relatively short- only ~ 100 or so and patchy in coverage throughout SEQ. One way of reconstructing past rainfall patterns is dendrochronology- the study of tree ring growth (Figure 9). Several species of trees were sampled in the region to reconstruct their growth rate 11. H. HAINES ARI Figure 8. Slack water deposits 6 THE BIG FLOOD: WILL IT HAPPEN AGAIN? Final report Figure 9. Tree rings can be assessed to reconstructed past rainfall patterns

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Approach Modelling landscape response to climate and landuse change To evaluate potential future channel response to climate and land use change, a river evolution model (REM) is developed to examine movement of sediment through the system. The interaction of two factors (1) changing hydrological regime and (2) riparian vegetation are examined based on scenario modelling. A cellular automata model called CAESAR-Lisflood is used to develop a Lockyer Valley REM (Figure 10) which is calibrated using floodplain deposition rates derived from OSL dating and geomorphic change measured after the 2011 and 2013 flood events. The Lockyer Valley REM identifies possible trajectories of channel response over the next 100 to 500 years. How floods impact on downstream water quality and ecosystem health? To evaluate the impact of the 2011 flood on delivering muddy sediments to Moreton Bay four cores were collected from the bay for particle size analysis (PSA) and dating with Optically Stimulated Luminescence (OSL) for determining deposition rates over time and estimating the volume of sediment delivered to Moreton Bay by the flood. To evaluate the impact of the 2011 flood on delivering metal contaminants (Lead, Zinc and Copper) to Moreton Bay, 22 sediment samples were collected for geochemical analysis with inductively coupled plasma optical emission spectrometry (ICPOES) 8. Figure 10. Lockyer Valley river evolution model Murphys Creek Helidon Grantham THE CONVERSATION Final report THE BIG FLOOD: WILL IT HAPPEN AGAIN? 7

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The Lockyer Valley Located in South East Queensland The Great Dividing Range forms the western boundary of the Lockyer Creek catchment (~3000 km2). The Creek flows east with numerous tributaries joining (including Murphy’s Creek, Ma Ma Creek, Tenthill Creek and Laidley Creek) before it’s confluence with the Mid Brisbane River just downstream of Lake Wivenhoe (Figure 11). Figure 11. Lockyer Valley Catchment MURPHY’S CREEK LAIDLEY CREEK MA MA CREEK TENTHILL CREEK Climate The climate of the Lockyer Valley is sub-humid, subtropical and strongly seasonal, with 65-70% of total rainfall occurring between October and March, in part due to higher precipitation intensities associated with summer storms generated by sub-tropical lows (Figure 12). The area experiences highly variable multi-year rainfall regimes and decadal trends of above- and below-average rainfall. Hydrologically, this manifests in high streamflow variability. Figure 12. Average monthly rainfall for Helidon (1870-2015) 8 THE BIG FLOOD: WILL IT HAPPEN AGAIN? Final report

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The Lockyer Valley Geology The catchment geology comprises Main Range Volcanics (Olivine basalt) on the divide. The headwaters have incised down to, and flow across, the Marburg subgroup (Jurassic sandstones, siltstones, shale). Quaternary terrace and floodplain alluvium deposits commence near Helidon along the main channel down to the mid-Brisbane River confluence (Figure 13). Soils The soils in the Lockyer Valley are some of the most productive in Australia and support an important agricultural industry in the region. European settlement European settlement and exploration of the Lockyer Valley began in 1823. The explorer’s notes of Allan Cunningham during his 1829 excursion depict the Lockyer Creek basin as having mixed forests with variable density along with abundant grassland plains and pastures in close proximity to Lockyer Creek (Steele, 1972). Through the early and mid-1800’s the Lockyer Valley region was used by squatters to manage sheep. Widespread vegetation clearance from floodplains occurred when established farming began in the late 1800’s, producing corn, alfalfa, potatoes, pumpkins, citrus fruits and dairy products. Walloon subgroup Marbug subgroup Main Range volcanics MG-Tv Cressbrook Creek Group MG-PTRg Helidon Sandstone Water body MG-CPm Quaternary Alluvium Figure 13. Geology of the Lockyer Catchment Based on or contains data provided by the State of Queensland (Department of Natural Resources and Mines) 2012 STATE LIBRARY OF QUEENSLAND Final report THE BIG FLOOD: WILL IT HAPPEN AGAIN? 9

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The Lockyer Valley Today’s landuse Since European settlement, two-thirds of native vegetation has been cleared for agricultural purposes (Apan et al., 2002). Riparian vegetation is highly variable through the 20th century, but there is a noticeable increase in within-macrochannel vegetation density since 1974. Widespread irrigation of farmland developed through the early and mid-1900’s. The latter part of the 20th century saw the reduction of dairy production to accommodate expanding crop farming, along with the establishment of large-scale beef production. Today, land use in the region is dominated by pasture (47%), followed by woody vegetation (41%) and crops (11%) (Figure 14). Figure 14. Today’s land use Based on or contains data provided by the State of Queensland (Department of Natural Resources and Mines) 2012 Intensive uses Production from dryland agriculture and plantations Conservation and natural environments Production from irrigated agriculture and plantations Water Production from relatively natural environments Other 10 THE BIG FLOOD: WILL IT HAPPEN AGAIN? Final report STATE LIBRARY OF QUEENSLAND

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WHAT HAPPENED? The Flood Wettest year on record 2010-2011 was the wettest year on record for the state of Queensland, and the wettest year since 1974 for SEQ. The second half of 2010 and early 2011 was characterized by one of the four strongest La Niña events since 1900. Strong La Niña events are often associated with extreme rainfall and widespread flooding in eastern Australia. The extremely heavy rain in early January 2011 (Figure 15) fell on the near-saturated catchments of the Brisbane River causing it to overtop its banks, resulting in an area of inundation equivalent to the total land area of France and Germany combined (Figure 16). Figure 15. December 2010-January 2011 rainfall at Helidon Low pressure system moves in In 1974 the heaviest rains in south east Queensland occurred close to the coast, whereas in 2011 the heaviest rainfalls spread further inland, particularly on the western fringe of the Brisbane River catchment and on the Great Dividing Range. On 10 January, a low-pressure system moved inland over the catchment (Figure 17), colliding with upper level and monsoon troughs and intensified in the north and west of the Lockyer Valley. Rainfall intensities on 10 January ranged from 58 mm in 1 hour at Toowoomba on the catchment divide, 90 mm in 1 hour on the escarpment near Spring Bluff to an estimated 150 mm in 2 hours over Fifteen Mile and Alice Creek subcatchments. Helidon and Gatton received ≤ 11 mm. On the 11 January rainfall persisted for 12 hours over the central and southern catchments resulting in higher rainfall totals than the 10 January event but at lower rainfall intensities. Figure 17. Rain radar from January 10, 2011 Figure 16. Flood inundation extent in Brisbane VEN DEN HONERT, R.D. AND MCANENEY, J. Final report THE BIG FLOOD: WILL IT HAPPEN AGAIN? 11

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SKM, 2012 The Flood Flooding varied across the catchment In spite of the significant magnitude of the January 2011 flood event, flood inundation varied significantly downstream (Figure 18). Flooding occupied the whole valley in Murphy’s Creek which has a steep, narrow channel close to the valley sides. The valley widens as the channel meanders past the town of Murphy’s Creek. The flood waters throughout the bedrock channel of lower Murphys Creek- upper Lockyer Creek were fully contained in the channel and were rapidly conveyed towards Helidon. A relatively large channel conveyed flood waters at high speed past Helidon towards Figure 18. Flood inundation extent Grantham (Figure 19). We refer to this as a macrochannel, a channel which is capable of containing large discharges and contains a number of inset surfaces. A reduction in the size of the channel past Grantham resulted in floodwaters spilling out across the entire floodplain at high velocities. At Gatton the channel increased in size around the large bend which could convey the majority of the flood waters limiting floodplain inundation. Below Gatton the channel size decreases and natural levees are present along the channel. Floodwaters breached the natural levees at low points and flowed out across the floodplain generally following the path of older channels. Figure 19. Flood inundation extent at Heldion and Grantham The natural levee ceases over the lower 16 km of Lockyer Creek leading to the confluence of the Brisbane River, and the channel size increases. As flood waters approached the confluence, they were backed up by flow in the Brisbane River causing valley wide inundation for many days 2. SKM, 2012 12 THE BIG FLOOD: WILL IT HAPPEN AGAIN? Final report

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Sediment movement Once rainfall and overland flow discharges exceed the resistance offered by the soil and vegetation, erosion occurs. Once detached, sediments can be transported throughout the catchment. The 2011 flood caused large amounts of sediment to be moved from both the hillslopes and the channel (Figure 20). Some sources of sediment connected to the channel, while others remained disconnected. Figure 20. Large amounts of sediment were moved from hillslopes and channels in the catchment Final report THE BIG FLOOD: WILL IT HAPPEN AGAIN? 13

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