A statistical analysis of flood hydrology and bankfull discharge for the Mitchell River catchment, Queensland, Australia Paul Rustomji January 2010 Water for a Healthy Country Flagship Report series ISSN: 1835-095X Australia is founding its future on science and innovation. Its national science agency, CSIRO, is a powerhouse of ideas, technologies and skills. CSIRO initiated the National Research Flagships to address Australia’s major research challenges and opportunities. They apply large scale, long term, multidisciplinary science and aim for widespread adoption of solutions. The Flagship Collaboration Fund supports the best and brightest researchers to address these complex challenges through partnerships between CSIRO, universities, research agencies and industry. The Water for a Healthy Country Flagship aims to achieve a tenfold increase in the economic, social and environmental benefits from water by 2025. The work contained in this report is collaboration between CSIRO and the Tropical Rivers and Coastal Knowledge (TRaCK) research program. For more information about Water for a Healthy Country Flagship or the National Research Flagship Initiative visit www.csiro.au/org/HealthyCountry.html TRaCK brings together leading tropical river researchers and managers from Charles Darwin University, Griffith University, University of Western Australia, CSIRO, James Cook University, Australian National University, Geoscience Australia, Environ- mental Research Institute of the Supervising Scientist, Australian Institute of Marine Science, North Australia Indigenous Land and Sea Management Alliance, and the Governments of Queensland, Northern Territory and Western Australia. TRaCK receives major funding for its research through the Australian Government’s Commonwealth Environment Research Facilities initiative; the Australian Government’s Raising National Water Standards Program; Land and Water Australia; the Fisheries Research and Development Corporation and the Queensland Government’s Smart State Innovation Fund Citation: Rustomji, P., (2010) A statistical analysis of flood hydrology and bankfull discharge for the Mitchell River catchment, Queensland, Australia. CSIRO: Water for a Healthy Country National Research Flagship [01/2010] Copyright and Disclaimer ⃝c 2010 CSIRO To the extent permitted by law, all rights are reserved and no part of this publication covered by copyright may be reproduced or copied in any form or by any means except with the written permission of CSIRO. Important Disclaimer: CSIRO advises that the information contained in this publication comprises general statements based on scientific research. The reader is advised and needs to be aware that such information may be incomplete or unable to be used in any specific situation. No reliance or actions must therefore be made on that information without seeking prior expert professional, scientific and technical advice. To the extent permitted by law, CSIRO (including its employees and consultants) excludes all liability to any person for any consequences, including but not limited to all losses, damages, costs, expenses and any other compensa- tion, arising directly or indirectly from using this publication (in part or in whole) and any information or material contained in it. Cover Photograph: Meris satellite image of south western Gulf of Carpentaria on 15 February 2009. The Mitchell River is the major river flowing to the gulf in the upper right corner of the image. ⃝c 2009 European Space Agency. Image origin: http://mrrs.eo.esa.int/mrrs/images/2009/02/15/MER_FR__0PNPDE20090215_003544_000001862076_ 00274_36400_1443.N1_49976CD4_image_0260.jpg For further information about this publication: Paul Rustomji, CSIRO Land and Water [email protected] To find out more about TRaCK Visit: http://www.track.gov.au/ email: [email protected] phone: 08 8946 7444 Contents Acknowledgements xv Executive Summary xvi 1 Introduction 1 2 Study Site 1 3 Methods 4 3.1 Flood frequency analysis . 4 3.2 Plotting positions . 8 3.3 Probability density function selection . 8 3.4 Flood quantile estimation . 9 3.5 Bankfull discharge analysis . 11 4 Results 11 4.1 Threshold selection for identification of flood events . 11 4.2 Identification of flood peaks . 11 4.3 Probability distribution selection using L-moment ratio diagrams . 12 4.4 Flood quantile estimation . 12 4.5 Regional flood quantile estimation . 16 4.6 Bankfull discharge and its recurrence interval . 20 5 Conclusions 23 References 24 Appendices 27 A 919001C Mary Creek at Mary Farms 29 B 919002A Lynd River at Lyndbrook 33 C 919003A Mitchell River at O.K. Br 37 D 919005A Rifle Ck at Fonthill 41 iii E 919006A Lynd River at Torwood 45 F 919007A Hodgkinson River at Piggy Hut 49 G 919008A Tate River at Torwood 53 H 919009A Mitchell River at Koolatah 57 I 919011A Mitchell River at Gamboola 61 J 919012A Galvin Ck at Reid Ck Junction 65 K 919013A McLeod River at Mulligan HWY 69 L 919014A Mitchell River at Cooktown Crossing 73 M 919201A Palmer River at Goldfields 77 N 919204A Palmer River at Palmer River at Drumduff 81 O 919205A North Palmer River at 4.8 Km 85 P 919305B Walsh River at Nullinga 89 Q 919309A Walsh River at Trimbles Crossing 93 R 919310A Walsh River at Rookwood 97 S 919311A Walsh River at Flatrock 101 T 919312A Elizabeth Ck at Greenmantle 105 iv List of Figures 1 Map of the Mitchell River catchment showing gauging stations, main drainage lines, elevation and mean annual rainfall isohyets. 3 2 L-moment ratio diagrams for flood peak data from the Mitchell River catchment. 12 3 Fitted flood frequency curves (solid line) and 95% confidence intervals (dashed line) for the Mitchell River catchment. The observed flood peaks are shown with open triangle symbols. 14 4 Fitted flood frequency curves (solid line) and 95% confidence intervals (dashed line) for the Mitchell River catchment. The observed flood peaks are shown with open triangle symbols. 15 5 Downstream trends in fitted flood quantiles (Q2 denotes 1:2 year recurrence interval flood) and mean annual flow (MAF) along the main stem of the Mitchell River. 17 6 Observed versus predicted plots of selected flood quantiles for the Mitchell River catchment using upstream catchment area (km2) and mean annual up- stream rainfall (mm) as predictive variables. The dashed line indicates the line of perfect agreement. Note gauge 919009A (Mitchell River at Koolatah) has been omitted from model formulation for events with >5 year recurrence interval and is shown with an open circle plotting symbol. 19 7 Channel cross sections, streamflow gaugings and rating curves for gauging stations in the Mitchell River catchment. The dashed horizontal line shows the maximum observed stage at the gauge. 21 8 Channel cross sections, streamflow gaugings and rating curves for gauging stations in the Mitchell River catchment. The dashed horizontal line shows the maximum observed stage at the gauge. 22 9 Threshold selection steps. 30 10 Linear-scale hydrograph showing peaks (shown by } symbols) identified in the peaks over threshold analysis. 30 11 Log-scaled hydrograph showing peaks (shown by } symbols) identified in the peaks over threshold analysis. 31 v 12 Fitted flood frequency curve for station 919001C. Dashed lines indicate a 95% confidence interval for the prediction. Note curve is only fitted to events with an average recurrence interval ≥ 1 year. 31 13 Threshold selection steps. 34 14 Linear-scale hydrograph showing peaks (shown by } symbols) identified in the peaks over threshold analysis. 34 15 Log-scaled hydrograph showing peaks (shown by } symbols) identified in the peaks over threshold analysis. 35 16 Fitted flood frequency curve for station 919002A. Dashed lines indicate a 95% confidence interval for the prediction. Note curve is only fitted to events with an average recurrence interval ≥ 1 year. 35 17 Threshold selection steps. 38 18 Linear-scale hydrograph showing peaks (shown by } symbols) identified in the peaks over threshold analysis. 38 19 Log-scaled hydrograph showing peaks (shown by } symbols) identified in the peaks over threshold analysis. 39 20 Fitted flood frequency curve for station 919003A. Dashed lines indicate a 95% confidence interval for the prediction. Note curve is only fitted to events with an average recurrence interval ≥ 1 year. 39 21 Threshold selection steps. 42 22 Linear-scale hydrograph showing peaks (shown by } symbols) identified in the peaks over threshold analysis. 42 23 Log-scaled hydrograph showing peaks (shown by } symbols) identified in the peaks over threshold analysis. 43 24 Fitted flood frequency curve for station 919005A. Dashed lines indicate a 95% confidence interval for the prediction. Note curve is only fitted to events with an average recurrence interval ≥ 1 year. 43 25 Threshold selection steps. 46 26 Linear-scale hydrograph showing peaks (shown by } symbols) identified in the peaks over threshold analysis. 46 vi 27 Log-scaled hydrograph showing peaks (shown by } symbols) identified in the peaks over threshold analysis. 47 28 Fitted flood frequency curve for station 919006A. Dashed lines indicate a 95% confidence interval for the prediction. Note curve is only fitted to events with an average recurrence interval ≥ 1 year. 47 29 Threshold selection steps. 50 30 Linear-scale hydrograph showing peaks (shown by } symbols) identified in the peaks over threshold analysis. 50 31 Log-scaled hydrograph showing peaks (shown by } symbols) identified in the peaks over threshold analysis. 51 32 Fitted flood frequency curve for station 919007A. Dashed lines indicate a 95% confidence interval for the prediction. Note curve is only fitted to events with an average recurrence interval ≥ 1 year. 51 33 Threshold selection steps. 54 34 Linear-scale hydrograph showing peaks (shown by } symbols) identified in the peaks over threshold analysis. 54 35 Log-scaled hydrograph showing peaks (shown by } symbols) identified in the peaks over threshold analysis. 55 36 Fitted flood frequency curve for station 919008A. Dashed lines indicate a 95% confidence interval for the prediction. Note curve is only fitted to events with an average recurrence interval ≥ 1 year. 55 37 Threshold selection steps. 58 38 Linear-scale hydrograph showing peaks (shown by } symbols) identified in the peaks over threshold analysis.