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
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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]
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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
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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...... 58
39 Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis...... 59
40 Fitted flood frequency curve for station 919009A. Dashed lines indicate a 95% confidence interval for the prediction. Note curve is only fitted to events with an average recurrence interval ≥ 1 year...... 59
41 Threshold selection steps...... 62
vii 42 Linear-scale hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis...... 62
43 Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis...... 63
44 Fitted flood frequency curve for station 919011A. Dashed lines indicate a 95% confidence interval for the prediction. Note curve is only fitted to events with an average recurrence interval ≥ 1 year...... 63
45 Threshold selection steps...... 66
46 Linear-scale hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis...... 66
47 Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis...... 67
48 Fitted flood frequency curve for station 919012A. Dashed lines indicate a 95% confidence interval for the prediction. Note curve is only fitted to events with an average recurrence interval ≥ 1 year...... 67
49 Threshold selection steps...... 70
50 Linear-scale hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis...... 70
51 Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis...... 71
52 Fitted flood frequency curve for station 919013A. Dashed lines indicate a 95% confidence interval for the prediction. Note curve is only fitted to events with an average recurrence interval ≥ 1 year...... 71
53 Threshold selection steps...... 74
54 Linear-scale hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis...... 74
55 Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis...... 75
56 Fitted flood frequency curve for station 919014A. Dashed lines indicate a 95% confidence interval for the prediction. Note curve is only fitted to events with an average recurrence interval ≥ 1 year...... 75
viii 57 Threshold selection steps...... 78
58 Linear-scale hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis...... 78
59 Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis...... 79
60 Fitted flood frequency curve for station 919201A. Dashed lines indicate a 95% confidence interval for the prediction. Note curve is only fitted to events with an average recurrence interval ≥ 1 year...... 79
61 Threshold selection steps...... 82
62 Linear-scale hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis...... 82
63 Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis...... 83
64 Fitted flood frequency curve for station 919204A. Dashed lines indicate a 95% confidence interval for the prediction. Note curve is only fitted to events with an average recurrence interval ≥ 1 year...... 83
65 Threshold selection steps...... 86
66 Linear-scale hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis...... 86
67 Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis...... 87
68 Fitted flood frequency curve for station 919205A. Dashed lines indicate a 95% confidence interval for the prediction. Note curve is only fitted to events with an average recurrence interval ≥ 1 year...... 87
69 Threshold selection steps...... 90
70 Linear-scale hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis...... 90
71 Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis...... 91
ix 72 Fitted flood frequency curve for station 919305B. Dashed lines indicate a 95% confidence interval for the prediction. Note curve is only fitted to events with an average recurrence interval ≥ 1 year...... 91
73 Threshold selection steps...... 94
74 Linear-scale hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis...... 94
75 Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis...... 95
76 Fitted flood frequency curve for station 919309A. Dashed lines indicate a 95% confidence interval for the prediction. Note curve is only fitted to events with an average recurrence interval ≥ 1 year...... 95
77 Threshold selection steps...... 98
78 Linear-scale hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis...... 98
79 Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis...... 99
80 Fitted flood frequency curve for station 919310A. Dashed lines indicate a 95% confidence interval for the prediction. Note curve is only fitted to events with an average recurrence interval ≥ 1 year...... 99
81 Threshold selection steps...... 102
82 Linear-scale hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis...... 102
83 Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis...... 103
84 Fitted flood frequency curve for station 919311A. Dashed lines indicate a 95% confidence interval for the prediction. Note curve is only fitted to events with an average recurrence interval ≥ 1 year...... 103
85 Threshold selection steps...... 106
86 Linear-scale hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis...... 106
x 87 Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis...... 107 88 Fitted flood frequency curve for station 919312A. Dashed lines indicate a 95% confidence interval for the prediction. Note curve is only fitted to events with an average recurrence interval ≥ 1 year...... 107
xi List of Tables
1 Gauging stations in the Mitchell River catchment with data used to determine whether or not a gauge’s data was suitable for hydrologic regionalisation (indi- cated by the “include” column). 1 MGS denotes maximum gauge stage. . . . . 6
2 Flow threshold and inter-flood gap details for analysis stations...... 7
3 Fitted parameters for the Generalised Pareto distribution...... 13
4 Comparison of peak instantaneous flood magnitude (as represented by re- gional flood quantile relationships) between the Daly and Mitchell Rivers. Peak instantaneous flood magnitude on the Mitchell River is 1.6 to 2.1 times larger that on the Daly River for catchments with comparable catchment area and mean annual rainfall...... 18
5 Flood peaks identified by the peaks over threshold analysis for station 919001C. 29
6 Fitted flood quantiles for station 919001C. Values have been reported to four significant digits...... 29
7 Flood peaks identified by the peaks over threshold analysis for station 919002A. 33
8 Fitted flood quantiles for station 919002A. Values have been reported to four significant digits...... 33
9 Flood peaks identified by the peaks over threshold analysis for station 919003A. 37
10 Fitted flood quantiles for station 919003A. Values have been reported to four significant digits...... 37
11 Flood peaks identified by the peaks over threshold analysis for station 919005A. 41
12 Fitted flood quantiles for station 919005A. Values have been reported to four significant digits...... 41
13 Flood peaks identified by the peaks over threshold analysis for station 919006A. 45
14 Fitted flood quantiles for station 919006A. Values have been reported to four significant digits...... 45
15 Flood peaks identified by the peaks over threshold analysis for station 919007A. 49
16 Fitted flood quantiles for station 919007A. Values have been reported to four significant digits...... 49
17 Flood peaks identified by the peaks over threshold analysis for station 919008A. 53
xii 18 Fitted flood quantiles for station 919008A. Values have been reported to four significant digits...... 53
19 Flood peaks identified by the peaks over threshold analysis for station 919009A. 57
20 Fitted flood quantiles for station 919009A. Values have been reported to four significant digits...... 57
21 Flood peaks identified by the peaks over threshold analysis for station 919011A. 61
22 Fitted flood quantiles for station 919011A. Values have been reported to four significant digits...... 61
23 Flood peaks identified by the peaks over threshold analysis for station 919012A. 65
24 Fitted flood quantiles for station 919012A. Values have been reported to four significant digits...... 65
25 Flood peaks identified by the peaks over threshold analysis for station 919013A. 69
26 Fitted flood quantiles for station 919013A. Values have been reported to four significant digits...... 69
27 Flood peaks identified by the peaks over threshold analysis for station 919014A. 73
28 Fitted flood quantiles for station 919014A. Values have been reported to four significant digits...... 73
29 Flood peaks identified by the peaks over threshold analysis for station 919201A. 77
30 Fitted flood quantiles for station 919201A. Values have been reported to four significant digits...... 77
31 Flood peaks identified by the peaks over threshold analysis for station 919204A. 81
32 Fitted flood quantiles for station 919204A. Values have been reported to four significant digits...... 81
33 Flood peaks identified by the peaks over threshold analysis for station 919205A. 85
34 Fitted flood quantiles for station 919205A. Values have been reported to four significant digits...... 85
35 Flood peaks identified by the peaks over threshold analysis for station 919305B. 89
36 Fitted flood quantiles for station 919305B. Values have been reported to four significant digits...... 89
37 Flood peaks identified by the peaks over threshold analysis for station 919309A. 93
xiii 38 Fitted flood quantiles for station 919309A. Values have been reported to four significant digits...... 93 39 Flood peaks identified by the peaks over threshold analysis for station 919310A. 97 40 Fitted flood quantiles for station 919310A. Values have been reported to four significant digits...... 97 41 Flood peaks identified by the peaks over threshold analysis for station 919311A.101 42 Fitted flood quantiles for station 919311A. Values have been reported to four significant digits...... 101 43 Flood peaks identified by the peaks over threshold analysis for station 919312A.105 44 Fitted flood quantiles for station 919312A. Values have been reported to four significant digits...... 105
xiv Acknowledgements
This research was funded as part of the Tropical Rivers and Coastal Knowledge (TRaCK) Research Program. TRaCK is funded jointly by:
• the Australian Government Department of the Environment, Water, Heritage and the Arts
• the National Water Commission’s Raising National Water Standards Programme
• Land & Water Australia’s Tropical Rivers Programme
• the Queensland Government’s Smart State Strategy
• the Fisheries Research and Development Corporation
• and CSIRO’s Water for a Healthy Country Flagship.
The Queensland Government’s Department of Environment and Resource Management collected and provided the hydrologic data. Andrew Brooks is thanked for useful discussions about catchment geomorphology and hydrology. Cuan Petheram and Gary Caitcheon are thanked for reviewing a draft of this manuscript.
xv Executive Summary
This report presents a flood frequency analysis for twenty gauging stations within the Mitchell River catchment. A flood frequency analysis allows the estimation of the magnitude of se- lected flood quantiles, such as a 1 in 20 year flood, at particular gauging stations. A series of statistical relationships were developed to allow flood quantile estimation at ungauged lo- cations. Gauging station cross sections were examined to identify bankfull discharge and its corresponding recurrence interval. However, this could not be achieved because the majority of gauging stations appear to be incised into either older alluvium (terraces) or bedrock val- leys and consequently did not have ‘self-formed channels’. An analysis of the downstream trends in fitted flood quantiles along the main stem of the Mitchell River indicates that floods with a recurrence interval of 1 in 2 years are generally contained within the channel (or at least the losses to floodplains and distributaries are proportionally constant downstream). However, for events with recurrence intervals of 5 years or more, losses of flood flows to the floodplain and distributary channels within the Mitchell River mega-fan region are notable. Peak flood flows at the downstream-most gauge (Mitchell River at Koolatah, 919009A) have an effective upper bound of ∼ 6600m3s−1 for events with a recurrence interval greater than 1 in 20 years; any discharges generated by the upstream catchment in excess of this appear to be diverted onto the floodplain and distributary channel system within the mega-fan region.
xvi 1 Introduction
Project 4.2 (Regional scale sediment and nutrient budgets) of the Tropical Rivers and Coastal Knowledge (TRaCK) research hub is concerned with the identification of erosion processes and sediment sources in the Mitchell River catchment, Queensland. One component of this research involves application of the SedNet model (Prosser et al. 2001) to model catch- ment sediment and nutrient budgets. A suite of hydrologic parameters, some of which relate to flood flows, are required to run the SedNet model (Wilkinson et al. 2006). This report presents a statistical analysis of flood hydrology in the Mitchell River catchment firstly as contribution to understanding the hydrology of a relatively large tropical river system (by Aus- tralian standards) and secondly to derive some of the required hydrologic parameters for use in the modelling of catchment scale sediment budgets in the Mitchell River catchment.
2 Study Site
The Mitchell River catchment (71,000 km2) shown in Figure 1 drains the western flank of Cape York Peninsula, flowing to the Gulf of Carpentaria. Galloway et al. (1970) conducted a landscape suitability assessment of the Mitchell River catchment and surrounding areas and the following catchment characteristics are summarised from this report (unless otherwise noted):
• Relief: The eastern third of the catchment comprises a bedrock dominated landscape of varying dissection of granitic, volcanic and sedimentary lithology (the ‘Eastern High- lands’ and ‘Central Uplands’ regions). A series of alluvial plains, aged from Tertiary to modern, dominate the landscape westwards of these uplands (Grimes and Doutch 1978) through to a narrow coastal plain 3-25 km in width fringing the western extent of Cape York. The Mitchell River has incised into these plains (referred to as a ‘mega- fan’ by Brooks et al., 2009), with maximum incision occurring approximately 400 km upstream of the coast and decreasing coastwards (Brooks et al. 2009). The morpho- logical apex of the mega-fan is near the junction of the Mitchell and Lynd Rivers (see Figure 1), though the current hydrologic/delta apex is located below the confluence of the Mitchell and Palmer Rivers. Below this apex, flood flows spread extensively
1 across a large number of distributary channels before reaching the coastal plains and ultimately the sea.
• Climate: The area has a sub-humid to humid tropical climate with marked wet and dry seasons. Practically all rains falls in the months from November to April inclusive. Catchment rainfall is moderate (∼ 1200 mm/yr in the vicinity of the Gulf of Carpentaria and decreases inland to below 800 mm/yr in the southern and western regions. Small zones of high rainfall (> 2000 mm/yr) occur in the catchments in the north-eastern and eastern headwaters. Historic maximum daily observed rainfall values at Kowanyama Airport are ∼300-350 mm, with values of ∼ 300mm per day being recorded at other lo- cations in the catchment (http://www.bom.gov.au/climate/averages/). Tem- peratures are fairly high throughout the year, varying between 17 ◦C and 23 ◦C in the dry season and 32 ◦C and 37 ◦C in the wet season (Crowley and Garnett 2000).
• Vegetation: Eucalypt and paperbark woodlands are common throughout the study area though grasslands predominate on the alluvial plains flanking the main river chan- nels (Neldner et al. 1997).
• Land Use: Grazing of beef cattle on native pastures has been the predominant landuse in the catchment for approximately the last 120 years, prior to which the landscape was managed by its indigenous inhabitants. There has been minimal clearance of native vegetation though some evidence exists in the region for Melaleuca encroachment into grassland environments due to altered burning regimes (Crowley and Garnett 1998).
2 GULF OF
CARPENTARIA Alice Kowanyama Mitchell River CORAL SEA River
1200 mm
Nassau River 919009A Palmer 919205A 919204A River 919201A 1200 mm 1000 mm 2000 mm
919012A 919003A 919013A 919011A 919309A Mitchell River919014A 919001C 919312A 919005A 919007A
800 mm 919310A 1000 mm Walsh River 919311A 919305B 120° E 140° E 800 mm Dimbulah 919008A -10° S Mitchell River Tate River catchment 919006A 1200 mm Australia 1000 mm Lynd River -30° S 919002A 800 mm
Figure 1: Map of the Mitchell River catchment showing gauging stations, main drainage lines, elevation and mean annual rainfall isohyets.
3 3 Methods
3.1 Flood frequency analysis
Daily maximum streamflow observations were obtained for the 25 stations listed in Table 1 from the Queensland Government’s hydrographic agency. These stations were examined for completeness of record and adequacy of gaugings used to derived the stage-discharge rating curve. Stations 919004A, 919202A and 919203A were rejected on the basis of having a very large (39–64%) percentage of their total flow volume occurring at stage heights greater than the maximum gauged stage, implying the discharge estimates for these stations at high flows are likely to be quite uncertain. Station 919001A had a very short record and was also rejected, whilst the data for station 919001B was merged with station 919001C. Of the remaining stations, 919305A and 919312A also had moderately high proportions of flow greater than the maximum gauged stage. However these were small catchments and the maximum gauged stage was moderately close in absolute terms to the maximum observed stage and it was considered that the high flows for these gauges could be sufficiently reliably estimated as to be useable for the purposes of this analysis. Figure 1 shows the locations of the selected gauges.
A peaks-over-threshold analysis has been used to identify statistically independent flood peaks. This approach requires a threshold discharge to be selected to differentiate flood from non-flood conditions. As a single flood (or a single wet season) may have multiple peaks, the second step in a peaks over threshold approach is to specify a minimum time period for which discharge must be below the threshold value for a sequence of floods to be considered independent. We follow the recommendation of Lang et al. (1999) that a range of threshold values be explored and have, for each station conducted a peaks over threshold analysis using a stepped sequence of thresholds. Lang et al. (1999) recommend that the threshold be chosen such that the distribution of the mean exceedence of flood peaks above the threshold range is a linear function of threshold magnitude and secondly, the selection of the largest threshold within this range that gives a mean number of floods per year greater than two. For the Mitchell River catchment, more emphasis was placed on identifying the peak in mean number of floods per year as the mean exceedence criteria was deemed to be of lesser use. Different interflood periods were also selected for the various gauges in
4 the Mitchell catchment as a uniform interflood period produced produced some undesirably low or high values for the mean number of floods per year for a number of stations (as a rule of thumb this value should be between 1 and 2.5). This is not surprising given the large variation in catchment sizes and hence hydrologic conditions at the selected gauging stations. Note that the inter-flood period pertains to the period between the time of the falling limb of the previous flood crossing the threshold and the time when the rising limb of the next flood crosses the threshold, not the time between flood peaks. Table 2 lists the threshold discharge and interflood gap used to derive the flood peaks for each station. The peaks over threshold analysis was conducted within R (R Development Core Team 2005) using the “pot” (Peaks Over Threshold) and “decluster” algorithms in the Extreme Values in R package (McNeil 2007).
5 area start end percent maximum gauged maximum observed flow above MGS1 flow above MGS station station name (km2) date date days < fair data stage (m) stage (m) (% days) (% volume) include notes
919001B Mary Creek at Mary Farms 89 25/2/1970 25/7/1986 11.7 3.91 5.25 0 0 in combine with 919001C 919001C Mary Creek at Mary Farms 88 24/5/1985 20/12/1988 31.09 2.81 3.23 0.4 1.2 in 919002A Lynd River at Lyndbrook 1215 15/1/1968 17/5/1992 38.94 3.78 6.25 1.6 10.6 in 919003A Mitchell River at O.K. Br 7535 16/10/1967 15/8/2008 18.34 12.66 13.06 0 0.4 in 919005A Rifle Ck at Fonthill 365 2/9/1968 3/7/2008 11.24 6.21 8.96 0.3 1 in 919006A Lynd River at Torwood 4325 7/3/1969 22/10/1988 22.51 7.47 9.95 0.4 9.3 in 919007A Hodgkinson River at Piggy Hut 1720 19/12/1968 3/1/1990 34.84 8.64 10.33 0.1 5.6 in 919008A Tate River at Torwood 4350 17/12/1972 13/8/1988 36.35 3.16 9.14 11.4 15.8 in 919009A Mitchell River at Koolatah 46050 9/7/1972 12/3/2008 30.8 11.89 11.95 0 0.7 in 919011A Mitchell River at Gamboola 20460 16/12/1971 2/8/2008 20.2 17.95 17.93 0 0 in 919012A Galvin Ck at Reid Ck Junction 163 15/12/1971 13/8/1982 55.91 2.74 5.29 1.9 8.3 in 919013A McLeod River at Mulligan HWY 530 18/1/1973 27/6/2008 20.25 8.72 8.29 0 0 in 919014A Mitchell River at Cooktown Crossing 2574 19/8/1999 12/5/2008 8.58 7.17 9.66 0.3 2.7 in 6 919201A Palmer River at Goldfields 530 12/12/1967 30/6/2008 26.61 5.06 6.81 0.1 0.8 in 919204A Palmer River at Drumduff 7750 8/7/1972 10/8/2008 37.21 12.38 12.81 0 0 in 919205A North Palmer River at 4.8 Km 430 13/11/1973 5/6/1988 32.76 2.5 4.66 2 19.7 in 919305B Walsh River at Nullinga 325 2/1/1956 24/9/2000 22.53 3.35 4.88 0.3 29.4 in 919309A Walsh River at Trimbles Crossing 9040 10/9/1967 20/6/2008 32.75 10.52 15.12 0.2 1.9 in 919310A Walsh River at Rookwood 5025 14/10/1967 7/8/2008 18.43 7.99 11.93 0.3 3.4 in 919311A Walsh River at Flatrock 2770 11/10/1968 17/10/2008 21.15 8.59 10.67 0.1 0 in 919312A Elizabeth Ck at Greenmantle 620 18/12/1969 6/7/1988 35 2.62 4.51 2.5 21.5 in 919001A Mary Ck at Brooklyn 90 02/09/1960 29/09/1960 16.82 0.95 4.32 17.3 NA out 919004A Tate River at Ootann 1630 1/9/1967 12/7/1988 27.22 2.59 5.73 8.4 56.8 out 919202A Palmer River at Maytown 2210 17/12/1968 27/7/1988 21.85 4.79 10.42 4 39.2 out 919203A Palmer River at Strathleven 7070 30/10/1969 20/10/1988 25.9 2.35 11.65 13.3 64 out
Table 1: Gauging stations in the Mitchell River catchment with data used to determine whether or not a gauge’s data was suitable for hydrologic regionalisation (indicated by the “include” column). 1 MGS denotes maximum gauge stage. threshold Inter-flood gap station station name (m3s−1) (days)
919001C Mary Creek at Mary Farms 25 15 919002A Lynd River at Lyndbrook 50 15 919003A Mitchell River at O.K. Br 150 30 919005A Rifle Ck at Fonthill 20 30 919006A Lynd River at Torwood 70 30 919007A Hodgkinson River at Piggy Hut 30 30 919008A Tate River at Torwood 50 30 919009A Mitchell River at Koolatah 200 30 919011A Mitchell River at Gamboola 200 30 919012A Galvin Ck at Reid Ck Junction 23 15 919013A McLeod River at Mulligan HWY 50 30 919014A Mitchell River at Cooktown Crossing 50 15 919201A Palmer River at Goldfields 40 30 919204A Palmer River at Drumduff 150 30 919205A North Palmer River at 4.8 Km 20 15 919305B Walsh River at Nullinga 20 20 919309A Walsh River at Trimbles Crossing 171 30 919310A Walsh River at Rookwood 200 30 919311A Walsh River at Flatrock 40 30 919312A Elizabeth Ck at Greenmantle 25 30
Table 2: Flow threshold and inter-flood gap details for analysis stations.
7 3.2 Plotting positions
Plotting positions for the observed flood series were calculated according to Cunnane (1978) using the formula:
n + 0.2 t = (1) r − 0.4
where n is the number of years of record and r is the sample rank, and the flood with a recurrence interval of t years is denoted Qt.
3.3 Probability density function selection
A critical issue in flood frequency analyses is the selection of an appropriate probability den- sity function to represent the observed flood series. Both in Australia and north America, the Pearson Type-III distribution fitted to the log-transformed flood series (referred to as the log Pearson-III distribution) has traditionally been recommended for flood frequency modelling (see for example Pilgrim and Doran 1987). However, Vogel et al. (1993) and Rustomji et al. (2009) observed that other statistical distributions may potentially be more appropriate for Australian data. Here, L-moment ratio diagrams (Hosking 1990; Vogel and Fennessey 1993; Hosking and Wallis 1997) have been used to select a suitable probability density function.
A sample of flood peaks can be characterised by four statistical moments: the first and second moments are the mean value and standard deviation respectively, which essentially indicate the magnitude and variability of the distribution, yet provide no discrimination about which theoretical distribution is closest to the characteristics of the data. The third and fourth moments, being measures of skewness and kurtosis, allow for discrimination between the shapes of different probability density functions. Hence, they can be used to select a proba- bility density function that most closely resembles the shape of the data. L-moments, being linear combinations of the sample data (as opposed to the exponentiated combinations of traditional moments) have also been argued to be more robust estimators of a distribution’s shape as they are less sensitive to extreme events (Vogel and Fennessey 1993). L-moment ratio diagrams are plots of L-skewness versus L-kurtosis onto which the L-skewness and L-kurtosis values for each dataset (ie. selection of flood peaks) are plotted. Then, theoretical L-skewness and L-kurtosis values (as given in Hosking and Wallis 1997) for the contender
8 probability density functions to be evaluated are also plotted (they may be shown as curves or points depending on the nature of the theoretical distribution). The theoretical distribution to which the observed values are closest can then be evaluated, either numerically or visu- ally. In this case, a visual examination of the L-moment ratio diagram was used to select the distribution.
3.4 Flood quantile estimation
L-moments have also been used to estimate the parameters of the selected flood frequency distribution. As is shown in Figure 2, the Generalised Pareto (abbreviated as GPA) distribu- tion appears to be a fair representation of the shape of the flood frequency distribution for gauging stations in the Mitchell River catchment. The GPA distribution has three parameters: ξ (location), α (scale) and κ (shape). The quantile function for the GPA distribution is: α − − κ ̸ ξ + κ (1 (1 F) ), κ = 0 x(F) = (2) ξ − αlog(1 − F), κ = 0
where x(F) is the quantile for non-exceedance probability F. All parameters have been estimated from the sample L-moments (as per Hosking 1990, 1996; Hosking and Wallis 1997) using the “lmomco” package (Asquith 2007) in R (R Development Core Team 2005) and are listed for each station in Table 3. Confidence intervals for the flood quantiles with return periods greater than 2 years have also been calculated using Monte Carlo simulation and an assumed normal error distribution around the fitted flood frequency curve, using the method described by Asquith (2007):
1. For nsim simulation runs (ideally a very large number, in this case nsim = 1000), sam- ples of size n are drawn from Q(F,θ) using the randomly selected F values drawn from a uniform distribution with range 0 to 1 and θ is the parameter set estimated from the original data.
2. The L-moments of the simulated sample are computed and a GEV distribution is fitted to these simulated L-moments resulting in a slightly different parameter set θ∗ from that determined from the original data.
3. The F-quantile of the synthetic distribution is computed and placed into a vector.
9 4. The process of simulating the sample, computing the L-moments, computing the distri- bution parameters, and solving for the F-quantile is repeated for the specified number of simulation runs.
5. This process is repeated for a sufficient number of non-exceedence probabilities F to draw smooth confidence limits around the main curve
The parameters of a normal distribution are estimated for each quantile F using L-moments and the 2.5th and 97.5th quantiles of this normal error distribution are used to provide a 95% confidence interval for the model fit.
10 3.5 Bankfull discharge analysis
Bankfull discharge is the discharge at which flow overtops the river banks and spills from the channel onto the floodplain. Understanding its occurrence within the catchment is criti- cal for understanding hydrologic linkages between the channel and the floodplain. Bankfull discharge can potentially be estimated through examination of the shape of a gauging sta- tion’s rating curve with its cross section. Bankfull stage may be evident from the surveyed cross section and from an inflection in the rating curve for a given station. Consequently, rat- ing curves (the relationship between stage height and discharge) and channel cross section data was obtained from the Queensland Government.
4 Results
4.1 Threshold selection for identification of flood events
For each gauging station, the results of the threshold identification algorithm are shown in the Appendices (see for example Figure 25). Well defined peaks were identified in the number of flood events/year statistic for approximately half the gauges and the threshold value associ- ated with this peak was used to guide the threshold selection. In other cases either multiple peaks were evident in the form of a peak at a relatively low threshold value and another at more intermediate discharges. Generally, the low thresholds resulted in > 2 flood peaks per year and experience has suggested selection of an alternate peak that produces between 1 and 2 flood events per year produces acceptable results. The specific thresholds identified for each gauging station (based on the peak in the “mean number of floods/year” curve) are listed in Table 2, with values ranging from 20 to 200 m3s−1. Interflood gap periods also listed in Table 2 range from 15 to 30 days.
4.2 Identification of flood peaks
Each station’s flood peaks, identified by the peaks over threshold analysis and derived using the thresholds and inter-flood gaps listed in Table 2 are listed in the Appendix, along with their calculated plotting positions. Both linear and log-scaled hydrographs for each station are also given in the Appendix with the flood peaks identified by the peaks over threshold
11 analysis shown by open diamond symbols.
4.3 Probability distribution selection using L-moment ratio diagrams
The L-moment ratio diagram for the peaks over threshold flood series’ from the Mitchell River catchment is shown in Figure 2. As mentioned above, the Generalised Pareto (GPA) distribution appears a suitable distribution for modelling the distribution of flood peaks in the Mitchell River catchment as the curve for this distribution appears to most closely bisect the distribution of L-moment ratios calculated from the peaks over threshold flood series.
0.7 Generalised Logarithmic Generalised Extreme Value Generalised Pareto 0.6 Log Normal Pearson Type III Peaks Over Threshold Series 0.5
0.4
0.3
0.2 L−Kurtosis
0.1
0.0
−0.1
−0.2 −0.4 −0.2 0.0 0.2 0.4 0.6 0.8 L−Skewness
Figure 2: L-moment ratio diagrams for flood peak data from the Mitchell River catchment.
4.4 Flood quantile estimation
The three parameters of the GPA distribution calculated from the sample L-moments (based on floods with an estimated return period > 1 year) are listed in Table 3. Figures 3 and 4 show the fitted flood frequency curves for all stations. Equivalent, larger plots are shown for
12 each station in the appendix along with estimates of 7 selected flood quantiles. The fitted flood frequency curves generally fit the observed data well.
Station ξ α κ
919001C 89.22 47.88 -0.4719 919002A 128.4 366.2 -0.06052 919003A 249.8 1628 -0.09405 919005A 44.34 299.3 0.2708 919006A 122.8 1558 0.1388 919007A 92.72 645.6 -0.1967 919008A 168.3 655.6 0.1881 919009A -374.6 9324 1.332 919011A 527 3001 0.04775 919012A 160.4 106.1 -0.213 919013A 30.38 511.3 -0.2054 919014A -105.4 1840 0.7057 919201A 36.12 563.6 0.2846 919204A 54.67 1271 -0.01497 919205A 26.72 267.6 0.4108 919305B 26.14 176.5 -0.2479 919309A 324.4 868.7 -0.08766 919310A 322.3 1092 -0.09791 919311A 10.06 1153 0.05897 919312A 116.3 498.2 0.4557
Table 3: Fitted parameters for the Generalised Pareto distribution.
One of the more distinctive flood frequency curves is that of station 919009A (Mitchell River at Koolatah). This is the station with the largest catchment area, yet the discharge data show a distinctive upper limit - the eight largest events have flood peaks within the range 6011–6358 m3s−1, all of which are well below the peaks from upstream gauges such as 919011A and 919003A. This is not due to the exclusion from the analysis of flows above this rate as flows > 6000m3s−1 have been assigned a ‘normal reading’ quality code. Nor is it considered likely to be due to attenuation of the flood peak as it travels downstream. Upstream gauges show substantial increases in discharge for their largest few events. This characteristic is consistent with losses of flood flow from the channel to distributaries up- stream of station 919009A when the discharge would otherwise exceed 6000m3s−1. Indeed, this characteristic can be seen in an (approximate) downstream profile of fitted flood quan- tiles and mean annual flow for stations 919001C, 919005A, 919014A, 919003A, 919011A
13 104 2 919001C Mary Creek at Mary Farms 10 88 km2 ) 2 − ) 1 − km s 1 3 3 1 −
10 s
m 10 3 (
m ( Q
Q
102 1.1 1.2 1.5 2 3 4 5 10 20 50 100 Average Return Interval (Years)
Figure 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.
14 104 104 919013A McLeod River at Mulligan HWY 919014A Mitchell River at Cooktown Crossing 2 2 530 km 101 2574 km ) )
2 0 2 − −
) ) 10 1 1 − − km km s s 1 1 3 3
3 − 3 −
10 s 10 s m m 3 3 ( (
0 10 m m ( ( Q Q −1 Q 10 Q
102 102 1.1 1.2 1.5 2 3 4 5 10 20 50 100 1.1 1.2 1.5 2 3 4 5 10 20 50 100 Average Return Interval (Years) Average Return Interval (Years) 104 104 919201A Palmer River at Goldfields 919204A Palmer River at Drumduff 0 2 2 10 530 km 101 7750 km ) ) 2 2 − − ) ) 1 1 − − km km s s 1 1 3 3
3 − 3 −
10 s 10 −1 s m m 3 3
( ( 10
0 10 m m ( ( Q Q
Q Q
102 102 1.1 1.2 1.5 2 3 4 5 10 20 50 100 1.1 1.2 1.5 2 3 4 5 10 20 50 100 Average Return Interval (Years) Average Return Interval (Years) 103 104 919205A North Palmer River at 4.8 Km 919305B Walsh River at Nullinga 2 2 430 km 325 km 1 100 10 ) ) 2 2 − −
) ) 3
1 1 10 − − km km s s 1 1 3 3 2 − 100 −
10 s s m m 3 3 ( (
−1 m 2 m ( 10 ( Q 10 Q
Q 10−1 Q
101 101 1.1 1.2 1.5 2 3 4 5 10 20 50 100 1.1 1.2 1.5 2 3 4 5 10 20 50 100 Average Return Interval (Years) Average Return Interval (Years) 104 0 104 919309A Walsh River at Trimbles Crossing 10 919310A Walsh River at Rookwood 2 2 9040 km 5025 km 100 ) ) 2 2 − − ) ) 1 1 − − km km s s 1 1 3 3 3 −1 − 3 −
10 s 10 s
m 10 m 3 3 ( (
−1 m m
( 10 ( Q Q
Q Q
102 102 1.1 1.2 1.5 2 3 4 5 10 20 50 100 1.1 1.2 1.5 2 3 4 5 10 20 50 100 Average Return Interval (Years) Average Return Interval (Years) 104 104 919311A Walsh River at Flatrock 919312A Elizabeth Ck at Greenmantle 1 2770 km2 620 km2 10 ) )
0 2 2
10 − − ) ) 1 1 − − km km s s 1 1 3 3
3 − 3 −
10 s 10 s m m 3 3
( ( 0
10 m m ( ( Q Q −1
10 Q Q
102 102 1.1 1.2 1.5 2 3 4 5 10 20 50 100 1.1 1.2 1.5 2 3 4 5 10 20 50 100 Average Return Interval (Years) Average Return Interval (Years)
Figure 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 and 919009A as shown in Figure 5. For the 1:2 and 1:5 year events as well as for mean annual flow, discharge increases downstream for all of these gauges suggesting flow is gen- erally contained within the channel and is accumulative downstream. For the 1:10 year event, the discharge at station 919011A exceeds that of downstream station 919009A despite the latter having approximately twice the catchment area and this pattern is accentuated for rarer events. This suggests that for events with recurrence intervals greater than approximately 5 years, substantial amounts of flow are lost from the channel prior to reaching the Koolatah gauge. These losses are likely to flow into rivers such as the Nassau River to the south of the Mitchell (but within the same AWRC basin) and also into the Staaten River which is in the next AWRC basin southwards.
4.5 Regional flood quantile estimation
The capacity to predict flood quantiles at ungauged locations is valuable for a range of issues including modelling of floodplain inundation. Using a selection of flood quantiles derived from the flood frequency analysis described above, a series of regional regression relationships have been developed. Note that due to the reasons discussed above about significant chan- nel losses occurring along the Mitchell River upstream of the Koolatah gauge for events with recurrence intervals > 5 years, this station has been omitted from the model formulation for such events though the predictions for this station are shown for reference only. The following model provided a good fit to the observed flood quantiles:
√ Qx = b × area × rain (3)
where Qx is the flood quantile with an average return period of x years, b is a parameter estimated by least squares regression and area and rain are upstream catchment area (km2) and mean annual upstream rainfall (mm) respectively. The gridded mean annual rainfall sur- face derived by Jeffrey et al. (2001) has been used in this case. Note that this is an empirical, statistically based relationship, not one based entirely on physical hydrology. Figure 6 shows the observed versus predicted plots of the model fits along with the fitted values of b. The 95% confidence intervals of the GPA flood frequency curves are shown along with the 95% prediction interval of the regression relationship for each return period. The fitted relation- ships and b values were all highly statistically significant and the models explain a very large
16 4000 6000
5000 3000
) ) 4000 /s /s 3 3
m 2000 m 3000 ( (
2 5
Q Q 2000 1000 1000
0 0
919001C 919005A 919014A 919003A 919011A 919009A 919001C 919005A 919014A 919003A 919011A 919009A 8000 10000
8000 6000 ) ) /s /s
3 3 6000
m 4000 m ( (
10 20 4000 Q Q 2000 2000
0 0
919001C 919005A 919014A 919003A 919011A 919009A 919001C 919005A 919014A 919003A 919011A 919009A 12000 14000
10000 12000
) 10000 ) 8000 /s /s 3 3 8000 m m
6000 ( (
6000 50 100
Q 4000 Q 4000 2000 2000 0 0
919001C 919005A 919014A 919003A 919011A 919009A 919001C 919005A 919014A 919003A 919011A 919009A 8
) 6 ML
6 10
4 x (
2 MAF
0
919001C 919005A 919014A 919003A 919011A 919009A
Figure 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 portion of the observed variance (adjusted R2 ≥ 0.9 in all cases) and there is congruence between observed and predicted values for almost all data points when their uncertainties are considered. As expected, observed discharges at the Koolatah gauge are substantially less than those that would be predicted from the hydrologic regionalisations (which assum- ing gaining systems), and for events such as the 1:25 year flood, approximately 50% of the discharge that would be expected to be observed at the Koolatah gauge appears to have been lost to upstream distributary flows. By way of comparison, a similar flood frequency analysis to this was undertaken for the Daly River catchment in the Northern Territory by Rustomji (2009). An identical function for flood quantile regionalisation was adopted and comparison of the coefficients for these equations (bMitchell versus bDaly as per Equation 3) allows for an assessment of the relative sizes of peak flood magnitudes for a given catchment area and mean upstream rainfall, as shown in Table 4. Peak instantaneous flood magnitude on the Mitchell River is 1.6 to 2.1 times greater than for an event of similar return period, upstream catchment area and mean annual rainfall in the Daly River catchment. This difference could potentially be attributed to two factors: (1) steeper headwaters in the Mitchell River’s catchment generating ‘peakier’ floods (though not necessarily greater total flow volumes), and (2) a higher ratio between the magnitude of flood generating rainfall events and mean annual rainfall (which is used as a predictive variable in Equation 3) for the Mitchell River catchment relative to the Daly.
bMitchell Recurrence interval (yrs) bDaly bMitchell bDaly
2 0.011 0.018 1.6 5 0.018 0.032 1.7 10 0.023 0.045 1.7 20 0.028 0.058 2.1 25 0.030 0.062 2.1 50 0.035 0.074 2.1 100 0.041 0.088 2.1
Table 4: Comparison of peak instantaneous flood magnitude (as represented by regional flood quantile relation- ships) between the Daly and Mitchell Rivers. Peak instantaneous flood magnitude on the Mitchell River is 1.6 to 2.1 times larger that on the Daly River for catchments with comparable catchment area and mean annual rainfall.
18 5000 8000 ) ) 1 Q2 = 0.018 area * rain 1 Q5 = 0.032 area * rain − −
s 2 s 2
3 3 4000 Adj. R = 0.95 Adj. R = 0.95 m m
( ( 6000
3000 4000 quantile quantile
2000 2000 1000
Predicted 0 Predicted 0 0 1000 2000 3000 4000 5000 0 1000 2000 3000 4000 5000 6000 7000 − − Observed quantile (m3s 1) Observed quantile (m3s 1)
12000 14000 ) ) 1 Q10 = 0.045 area * rain 1 Q20 = 0.058 area * rain − −
s 2 s 2 3 10000 Adj. R = 0.93 3 12000 Adj. R = 0.92 919009A
m 919009A m ( (
10000 8000 8000 6000
quantile quantile 6000
4000 4000 2000 2000
Predicted 0 Predicted 0 0 2000 4000 6000 8000 10000 0 2000 4000 6000 8000 10000 12000 − − Observed quantile (m3s 1) Observed quantile (m3s 1)
20000 ) ) 1 15000 Q25 = 0.062 area * rain 1 Q50 = 0.074 area * rain − −
s 2 s 2 3 Adj. R = 0.92 3 Adj. R = 0.91 919009A m m 919009A ( ( 15000
10000 10000 quantile quantile
5000 5000
Predicted 0 Predicted 0 0 2000 4000 6000 8000 10000 12000 0 2000 4000 6000 8000 12000 − − Observed quantile (m3s 1) Observed quantile (m3s 1)
25000 ) 1 Q100 = 0.088 area * rain −
s 2
3 20000 Adj. R = 0.9 m ( 919009A
15000 quantile
10000
5000
Predicted 0 0 5000 10000 15000 20000 − Observed quantile (m3s 1)
Figure 6: Observed versus predicted plots of selected flood quantiles for the Mitchell River catchment using upstream catchment area (km2) and mean annual upstream 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 4.6 Bankfull discharge and its recurrence interval
Figures 7 and 8 show channel cross sections, streamflow gaugings and rating curves for gauging stations in the Mitchell River catchment. Note that multiple surveyed sections often exist for a single gauge and every effort has been made to show as many as possible in these figures. Unfortunately it is difficult to ascertain what a “natural” bank top is from these cross sections. In many cases, the maximum observed stage is below what could potentially be termed a bank top based on the cross sectional morphology alone (e.g. for station 919201A, 919012A). For such stations, this indicates the gauging station has been sited within the al- luvium of older terraces as it would be expected that the bank top of a natural, self formed channel was lower in elevation than the maximum observed stage at a gauging station. In other cases, such as 919310A or 919312A, the station appears to be located in a bedrock valley. Both of these attributes are desirable from the perspective of siting a gauging station but render these cross sections unsuitable for ascertaining what channel depth might be for a self-formed river channel. Galloway et al. (1970) and Brooks et al. (2009) document ex- tensive incision of the Mitchell River into older fan surfaces and for the Mitchell River itself, it is probably only the lower 150 km of channel that sits within the contemporary Holocene fan (Fan M5 of Grimes and Doutch, 1978) that has what could be referred to as a self formed channel. No gauging station cross sections are located within this reach of river. However, what is known based on the preceding analysis of downstream flow patterns is that floods with a recurrence interval of approximately 1:2 years appear to be mostly con- tained within the channel and losses to distributaries are minimal (or at the very least losses are proportionally constant along the length of main channel). For events with a recurrence interval of 1:5 years, disproportionate losses to distributaries downstream of gauging station 919011A are detectable (see Figure 5). This implies that something approximating bank full flow (ie. the flow where significant volume of flow leaves the channel onto floodplains or other distributary channels) likely falls within the range of 2 to 5 years, at least along the length of channel between gauges 919011A and 919009A.
20 − − Q (m3s 1) Q (m3s 1) 0 200 400 600 800 0 500 1000 1500 2000 2500 919001C Mary Creek at Mary Farms 919002A Lynd River at Lyndbrook 2 2 8 Area = 88 km Area = 1215 km 15
6
10
4 Height (m) Height (m) 5 2
0 0 0 50 100 150 200 250 300 0 50 100 150 200 250 300 350 Chainage (m) Chainage (m)
− − Q (m3s 1) Q (m3s 1) 0 2000 4000 6000 8000 10000 12000 0 200 400 600 800 1000 25 919003A Mitchell River at O.K. Br 919005A Rifle Ck at Fonthill 15 Area = 7535 km2 Area = 365 km2 20
15 10
10 Height (m) Height (m) 5
5
0 0 0 100 200 300 400 500 0 100 200 300 400 500 600 Chainage (m) Chainage (m)
− − Q (m3s 1) Q (m3s 1) 0 1000 2000 3000 4000 5000 0 1000 2000 3000 4000 5000 50 919006A Lynd River at Torwood 919007A Hodgkinson River at Piggy Hut 15 Area = 4325 km2 Area = 1720 km2 40
30 10
20 Height (m) Height (m) 5
10
0 0 0 100 200 300 400 0 100 200 300 400 Chainage (m) Chainage (m)
− − Q (m3s 1) Q (m3s 1) 0 500 1000 1500 2000 0 2000 4000 6000 8000 25 919008A Tate River at Torwood 919009A Mitchell River at Koolatah 15 Area = 4350 km2 Area = 46050 km2 20
15 10
10 Height (m) Height (m) 5
5
0 0 0 50 100 150 200 250 300 0 200 400 600 Chainage (m) Chainage (m)
− − Q (m3s 1) Q (m3s 1) 0 2000 4000 6000 8000 10000 12000 0 200 400 600 800 25 12 919011A Mitchell River at Gamboola 919012A Galvin Ck at Reid Ck Junction Area = 20460 km2 Area = 163 km2 10 20
8 15 6 10 Height (m) Height (m) 4
5 2
0 0 0 100 200 300 400 500 0 20 40 60 80 100 120 140 Chainage (m) Chainage (m)
Figure 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 − − Q (m3s 1) Q (m3s 1) 0 500 1000 1500 2000 2500 3000 3500 0 500 1000 1500 2000 2500 3000 919013A McLeod River at Mulligan HWY 919014A Mitchell River at Cooktown Crossing 15 15 Area = 530 km2 Area = 2574 km2
10 10 Height (m) Height (m) 5 5
0 0 0 100 200 300 400 500 600 −200 0 200 400 600 800 1000 1200 Chainage (m) Chainage (m)
− − Q (m3s 1) Q (m3s 1) 0 500 1000 1500 2000 0 1000 2000 3000 4000 5000 30 20 919201A Palmer River at Goldfields 919204A Palmer River at Drumduff Area = 530 km2 Area = 7750 km2 25 15 20
15 10
Height (m) 10 Height (m) 5 5
0 0 0 50 100 150 200 250 0 100 200 300 400 500 Chainage (m) Chainage (m)
− − Q (m3s 1) Q (m3s 1) 0 200 400 600 800 1000 1200 1400 0 500 1000 1500 20 12 919205A North Palmer River at 4.8 Km 919305B Walsh River at Nullinga Area = 430 km2 Area = 325 km2 10 15 8
10 6
Height (m) Height (m) 4 5 2
0 0 0 50 100 150 200 0 50 100 150 200 250 Chainage (m) Chainage (m)
− − Q (m3s 1) Q (m3s 1) 0 1000 2000 3000 4000 5000 0 1000 2000 3000 4000 5000 6000 7000 25 30 919309A Walsh River at Trimbles Crossing 919310A Walsh River at Rookwood Area = 9040 km2 Area = 5025 km2 25 20
20 15 15 10 Height (m) Height (m) 10
5 5
0 0 0 100 200 300 400 500 0 100 200 300 400 Chainage (m) Chainage (m)
− − Q (m3s 1) Q (m3s 1) 0 1000 2000 3000 4000 0 200 400 600 800 1000 1200 25 25 919311A Walsh River at Flatrock 919312A Elizabeth Ck at Greenmantle Area = 2770 km2 Area = 620 km2 20 20
15 15
10 10 Height (m) Height (m)
5 5
0 0 0 50 100 150 200 250 300 0 50 100 150 200 250 300 350 Chainage (m) Chainage (m)
Figure 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 5 Conclusions
This report presented a flood frequency analysis for 20 stations in the Mitchell River catch- ment. Flood peaks were identified using a peaks-over-threshold approach and the flood frequency distributions was modelled using the Generalised Pareto distribution. Fitted flood frequency quantiles were presented for a selected number of quantiles. Regional regression relationships were also developed allowing for the prediction of selected flood quantiles at ungauged locations using catchment area and mean annual upstream rainfall as predictive variables. However, these relationships are not suitable for prediction of flood quantiles with recurrence intervals > 5 years downstream of gauging station 919011A due to the loss of flood flows to distributary channels downstream of this gauge. Finally, an analysis of gauging station cross sections failed to identify bankfull discharge rates. This was largely due to the location of many gauging stations within either river terraces or bedrock valleys, implying that the channel margin sediments were generally not those deposited by the current flow regime, at least upstream of the Mitchell River’s junction with the Palmer River. However, along the Mitchell River below its confluence with the Lynd River, floods with a recurrence interval be- tween 2 and 4 years begin to spill out of the channel into distributary channels. Consequently, downstream of gauge 919011A, flow within the main channel of the Mitchell River does not increase downstream with increasing catchment area for events with a recurrence interval of ∼ 5 years or greater.
23 References
Asquith, W. H. (2007). lmomco: L-moments, Trimmed L-moments, L-comoments, and Many Distributions. R package version 0.84.
Brooks, A. P., Shellberg, J. G., Knight, J. and Spencer, J. (2009). Alluvial gully erosion: an example from the Mitchell fluvial megafan, Queensland, Earth Surface Processes and Landforms 34: 1951–1969.
Crowley, G. M. and Garnett, S. T. (1998). Vegetation change in the grasslands and grassy woodlands of east-central Cape York Peninsula, Australia, Pacific Conservation Biology 4: 132–148.
Crowley, G. M. and Garnett, S. T. (2000). Changing fire management in the pastoral lands of Cape York Peninsula or northeast Australia, 1623-1996, Australian Geographical Studies 38: 10–26.
Cunnane, C. (1978). Unbiased plotting positions - A review, Journal of Hydrology 37: 205– 222.
Galloway, R. W., Gunn, R. H. and Story, R. (1970). Lands of the Mitchell-Normanby Area, Queensland, Land Research Series 26, Commonwealth Scientific and Industrial Research Organisation.
Grimes, K. G. and Doutch, H. F. (1978). The late Cainozoic evolution of the Carpentaria Plains, North Queensland, BMR Journal of Australian Geology and Geophysics 3: 101– 112.
Hosking, J. R. M. (1990). L-moments: analysis and estimation of distributions using linear combinations of order statistics, Journal of the Royal Statistical Society (B) 52: 105–124.
Hosking, J. R. M. (1996). FORTRAN routines for use with the method of L-moments: Version 3, IBM Research Report RC20525, T.J. Watson Research Center,, Yorktown Heights, New York.
Hosking, J. R. M. and Wallis, J. R. (1997). Regional frequency analysis: an approach based on L-moments, Cambridge University Press.
24 Jeffrey, S., Carter, J., Moodie, K. and Beswick, A. (2001). Using spatial interpolation to construct a comprehensive archive of Australian climate data, Environmental Modelling and Software 16: 309–330.
Lang, M., Ouarda, T. B. M. J. and Bobee,´ B. (1999). Towards operational guidelines for over-threshold modeling, Journal of Hydrology 225: 103–117.
McNeil, A. (2007). Extreme Values in R. R port by Alec Stephenson. R package version 1.5. URL: http://www.maths.lancs.ac.uk/ stephena/
Neldner, V. J., Fensham, R. J., Clarkson, J. R. and Stanton, J. P. (1997). The natural grass- lands of Cape York Peninsula, Australia: Description, distribution and conservation status, Biological Conservation 81: 121–136.
Pilgrim, D. H. and Doran, D. G. (1987). Flood frequency analysis, in D. H. Pilgrim (ed.), Aus- tralian rainfall and runoff: a guide to flood estimation, Academic Press, Sydney, chapter 10, pp. 197–236.
Prosser, I. P.,Rustomji, P.,Young, W. J., Moran, C. J. and Hughes, A. O. (2001). Constructing River Basin Sediment Budgets for the National Land and Water Resources Audit, Technical Report 15/01, CSIRO Land and Water.
R Development Core Team (2005). R: A Language and Environment for Statistical Comput- ing, R Foundation for Statistical Computing, Vienna, Austria. ISBN 3-900051-07-0. URL: http://www.R-project.org
Rustomji, P. (2009). A statistical analysis of flood hydrology and bankfull discharge for the Daly River catchment, Northern Territory, Australia., Water for a Healthy Country Report September 2009, CSIRO.
Rustomji, P., Bennett, N. and Chiew, F. (2009). Flood variability east of Australia’s Great Dividing Range, Journal of Hydrology 374: 196–208.
Vogel, R. M. and Fennessey, N. M. (1993). L moment diagrams should replace product moment diagrams, Water Resources Research 29: 1745–1752.
Vogel, R. M., McMahon, T. A. and Chiew, F. H. S. (1993). Floodflow frequency model selec- tion in Australia, Journal of Hydrology 146: 421–449.
25 Wilkinson, S., Young, W. and DeRose, R. (2006). Regionalizing mean annual flow and daily flow variability for basin-scale sediment and nutrient modelling, Hydrological Processes 20: 2769–2786.
26 Appendices
27 28 A 919001C Mary Creek at Mary Farms
Return Period Q Water (years) (m3s−1) Year Year 0.55 25.9 1986 1985/1986 0.57 27.5 1982 1981/1982 0.58 30.2 1972 1971/1972 0.60 30.9 1984 1983/1984 0.62 31.6 1978 1977/1978 0.64 34.6 1979 1978/1979 0.66 36.2 1973 1973/1974 Return Lower Estimated Upper 0.68 36.2 1977 1976/1977 0.71 37.0 1978 1977/1978 Period C.I. Quantile C.I. 0.73 37.0 1981 1981/1982 0.76 45.6 1986 1985/1986 (years) (m3s−1) (m3s−1) (m3s−1) 0.79 46.7 1975 1974/1975 0.82 47.7 1981 1980/1981 2 118 128 141 0.86 58.3 1985 1984/1985 0.89 70.7 1975 1975/1976 5 171 205 239 0.94 71.7 1978 1977/1978 0.98 80.1 1973 1972/1973 10 221 288 345 1.03 80.1 1985 1984/1985 1.09 83.4 1983 1982/1983 20 273 405 513 1.15 103.8 1973 1972/1973 1.22 109.1 1976 1975/1976 25 285 451 588 1.29 110.2 1987 1986/1987 50 332 630 875 1.38 113.1 1976 1975/1976 1.49 116.9 1982 1981/1982 100 340 879 1366 1.60 125.4 1972 1971/1972 1.74 125.4 1980 1979/1980 1.91 127.5 1980 1979/1980 Table 6: Fitted flood quantiles for station 2.10 129.7 1979 1978/1979 2.35 133.0 1974 1973/1974 919001C. Values have been reported to four 2.66 155.0 1985 1984/1985 3.06 157.4 1981 1980/1981 significant digits. 3.61 164.8 1970 1969/1970 4.39 171.1 1977 1976/1977 5.61 250.0 1975 1974/1975 7.77 302.7 1981 1980/1981 12.62 330.9 1971 1970/1971 33.67 708.0 1979 1978/1979
Table 5: Flood peaks identified by the peaks over threshold analysis for station 919001C.
29 919001C 5 250
4 200 ) 1 − s
3 m ( 3 150
2 100 exceedance
mean
mean number of floods/year mean number 1 50
0 0 0 20 40 60 80 100 140 180 − threshold (m3 s 1)
Figure 9: Threshold selection steps.
919001C Mary Creek at Mary Farms (88 km2) − Threshold = 25 m3 s 1 Interflood period = 15 days 800
700 )
1 600 − s
3 m ( 500
400 streamflow
300 maximum
Daily 200
100
0 1970 1975 1980 1985 Year
Figure 10: Linear-scale hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
30 919001C Mary Creek at Mary Farms (88 km2) 3 −1 103 Threshold = 25 m s Interflood period = 15 days ) 1 − s
3 102 m (
streamflow
101 maximum
Daily
100 1970 1975 1980 1985 Year
Figure 11: Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
4 10 2 919001C Mary Creek at Mary Farms 88 km2 10 ) 2 − ) 1 − km s 1
3 3 −
10 1 s m 3
( 10
m ( Q
Q
102 1.1 1.2 1.5 2 3 4 5 10 20 50 100 Average Return Interval (Years)
Figure 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 32 B 919002A Lynd River at Lyndbrook
Return Period Q Water (years) (m3s−1) Year Year 0.59 59.6 1969 1968/1969 0.61 59.6 1987 1986/1987 0.62 61.7 1976 1975/1976 0.64 68.1 1975 1975/1976 0.65 69.3 1988 1988/1989 0.67 71.6 1983 1982/1983 0.69 78.7 1991 1991/1992 0.71 79.9 1985 1985/1986 0.73 81.2 1974 1974/1975 0.75 82.4 1969 1969/1970 Return Lower Estimated Upper 0.77 86.3 1968 1967/1968 0.80 95.8 1985 1984/1985 Period C.I. Quantile C.I. 0.82 116.7 1968 1967/1968 0.85 131.6 1975 1974/1975 (years) (m3s−1) (m3s−1) (m3s−1) 0.88 136.8 1970 1970/1971 0.91 153.3 1970 1969/1970 2 323 388 452 0.95 161.1 1990 1989/1990 0.98 165.0 1981 1981/1982 5 612 748 875 1.02 167.8 1977 1976/1977 1.07 187.9 1986 1985/1986 10 837 1033 1216 1.12 190.9 1973 1972/1973 1.17 190.9 1989 1989/1990 20 1043 1331 1617 1.22 195.4 1989 1988/1989 1.29 207.7 1976 1975/1976 25 1106 1430 1754 1.35 209.2 1975 1974/1975 50 1247 1745 2249 1.43 236.6 1976 1975/1976 1.52 271.1 1990 1989/1990 100 1360 2073 2791 1.62 294.7 1988 1987/1988 1.73 353.0 1977 1976/1977 1.85 353.0 1979 1978/1979 Table 8: Fitted flood quantiles for station 2.00 369.5 1971 1970/1971 2.17 388.4 1968 1967/1968 919002A. Values have been reported to four 2.38 403.5 1977 1976/1977 2.62 618.5 1975 1974/1975 significant digits. 2.93 642.0 1974 1973/1974 3.32 674.7 1989 1989/1990 3.82 681.3 1980 1979/1980 4.50 741.9 1986 1985/1986 5.48 797.4 1979 1978/1979 7.00 801.8 1992 1991/1992 9.69 813.8 1981 1980/1981 15.75 1502.4 1991 1990/1991 42.00 1662.7 1984 1983/1984
Table 7: Flood peaks identified by the peaks over threshold analysis for station 919002A.
33 919002A 3.0 500
2.5 400 ) 1 − s
2.0 3 m ( 300
1.5
200 exceedance
1.0 mean
mean number of floods/year mean number 100 0.5
0.0 0 0 50 100 200 300 400 500 − threshold (m3 s 1)
Figure 13: Threshold selection steps.
919002A Lynd River at Lyndbrook (1215 km2) − Threshold = 50 m3 s 1 Interflood period = 15 days 1800
1600
1400 ) 1 − s
3
m 1200 (
1000 streamflow
800
maximum 600
Daily 400
200
0 1968 1973 1978 1983 1988 1993 Year
Figure 14: Linear-scale hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
34 919002A Lynd River at Lyndbrook (1215 km2) 3 −1 104 Threshold = 50 m s Interflood period = 15 days
3
) 10 1 − s
3 m (
102 streamflow
maximum
101 Daily
100 1968 1973 1978 1983 1988 1993 Year
Figure 15: Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
104 919002A Lynd River at Lyndbrook 1215 km2 ) 2 − ) 1
− 0 km s
10 1
3 3 −
10 s m 3 (
m ( Q
Q
10−1 102 1.1 1.2 1.5 2 3 4 5 10 20 50 100 Average Return Interval (Years)
Figure 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 36 C 919003A Mitchell River at O.K. Br
Return Period Q Water (years) (m3s−1) Year Year 0.70 153 1976 1976/1977 0.71 164 1968 1967/1968 0.72 171 1984 1984/1985 0.73 173 1982 1982/1983 0.75 173 1995 1995/1996 0.76 179 1986 1986/1987 0.77 195 1993 1993/1994 0.79 203 2003 2002/2003 0.80 217 2005 2004/2005 0.82 245 1982 1981/1982 0.83 262 1969 1969/1970 0.85 272 1973 1973/1974 0.87 289 1994 1993/1994 0.89 307 1982 1981/1982 0.91 340 2000 2000/2001 0.93 407 1981 1981/1982 0.95 413 2002 2001/2002 0.97 415 1970 1969/1970 0.99 443 1970 1969/1970 Return Lower Estimated Upper 1.01 489 1989 1989/1990 1.04 506 2006 2005/2006 Period C.I. Quantile C.I. 1.07 506 2004 2004/2005 1.09 518 1982 1981/1982 (years) (m3s−1) (m3s−1) (m3s−1) 1.12 530 1978 1977/1978 1.15 546 1990 1989/1990 2 1131 1416 1706 1.19 552 1969 1968/1969 1.22 555 1988 1987/1988 5 2442 3079 3729 1.26 577 1983 1982/1983 1.29 577 1993 1992/1993 10 3471 4436 5373 1.34 667 1975 1974/1975 1.38 680 1992 1991/1992 20 4425 5885 7333 1.43 756 1983 1982/1983 1.48 770 1998 1997/1998 25 4718 6371 7956 1.53 779 1987 1986/1987 50 5479 7950 10499 1.59 806 1987 1987/1988 1.65 861 1986 1985/1986 100 6133 10000 12997 1.72 880 2003 2002/2003 1.79 1123 1988 1988/1989 1.87 1294 1976 1975/1976 Table 10: Fitted flood quantiles for station 1.95 1447 1969 1968/1969 2.05 1455 1984 1983/1984 919003A. Values have been reported to four 2.15 1644 2006 2005/2006 2.27 1701 1968 1967/1968 significant digits. 2.40 1745 1973 1972/1973 2.54 1793 2006 2005/2006 2.71 1870 1995 1994/1995 2.89 2030 2004 2003/2004 3.10 2054 1980 1979/1980 3.35 2308 1981 1980/1981 3.64 2609 1974 1973/1974 3.98 3427 2001 2000/2001 4.40 3452 1996 1995/1996 4.91 3496 1971 1970/1971 5.55 3907 1977 1976/1977 6.39 3919 2007 2006/2007 7.54 4012 2008 2007/2008 9.17 4468 2009 2008/2009 11.72 4504 1972 1971/1972 16.23 5658 2000 1999/2000 26.38 6349 1999 1998/1999 70.33 8165 1979 1978/1979
Table 9: Flood peaks identified by the peaks over threshold analysis for station 919003A.
37 919003A 2.5 2500
2.0 2000 ) 1 − s
3 m ( 1.5 1500
1.0 1000 exceedance
mean
mean number of floods/year mean number 0.5 500
0.0 0 0 200 600 1000 1400 1800 − threshold (m3 s 1)
Figure 17: Threshold selection steps.
919003A Mitchell River at O.K. Br (7535 km2) − Threshold = 150 m3 s 1 Interflood period = 30 days 9000
8000
7000 ) 1 − s
3
m 6000 (
5000 streamflow
4000
maximum 3000
Daily 2000
1000
0 1967 1972 1977 1982 1987 1992 1997 2002 2007 Year
Figure 18: Linear-scale hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
38 919003A Mitchell River at O.K. Br (7535 km2) 3 −1 104 Threshold = 150 m s Interflood period = 30 days
3
) 10 1 − s
3 m (
102 streamflow
maximum
101 Daily
100 1967 1972 1977 1982 1987 1992 1997 2002 2007 Year
Figure 19: Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
105 919003A Mitchell River at O.K. Br 7535 km2 101
104 0 ) 10 2 − ) 1 − km s 1 3 − s m 3 (
m ( Q
103 Q 10−1
102 1.1 1.2 1.5 2 3 4 5 10 20 50 100 Average Return Interval (Years)
Figure 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 40 D 919005A Rifle Ck at Fonthill
Return Period Q Water (years) (m3s−1) Year Year 0.77 22.3 1983 1982/1983 0.78 25.1 1970 1970/1971 0.80 27.8 1992 1991/1992 0.81 28.4 1981 1980/1981 0.83 30.3 1975 1974/1975 0.85 38.8 1988 1987/1988 0.87 40.8 2006 2005/2006 0.88 42.2 1996 1996/1997 0.90 46.4 1969 1968/1969 0.92 50.9 2002 2001/2002 0.94 58.7 1976 1976/1977 0.97 59.3 1990 1989/1990 0.99 61.1 1970 1969/1970 1.01 61.6 1993 1992/1993 1.04 71.7 2000 2000/2001 1.07 77.0 1984 1983/1984 Return Lower Estimated Upper 1.10 78.7 1972 1971/1972 1.13 80.7 2003 2002/2003 Period C.I. Quantile C.I. 1.16 88.4 2005 2004/2005 1.19 95.1 1986 1985/1986 (years) (m3s−1) (m3s−1) (m3s−1) 1.23 101.2 2008 2007/2008 1.26 107.9 1980 1979/1980 2 193 233 273 1.30 110.6 1978 1977/1978 1.35 115.2 2005 2004/2005 5 371 435 499 1.39 121.0 1970 1969/1970 1.44 130.4 1969 1968/1969 10 478 557 636 1.49 153.2 2009 2008/2009 1.55 165.2 2007 2006/2007 20 567 658 745 1.61 189.2 1975 1974/1975 25 585 687 792 1.67 189.7 1994 1993/1994 1.75 213.7 1987 1986/1987 50 640 766 904 1.82 217.9 1982 1981/1982 1.91 234.3 1997 1996/1997 100 669 832 1003 2.00 247.2 1989 1988/1989 2.10 255.8 1991 1990/1991 2.22 273.6 1985 1984/1985 Table 12: Fitted flood quantiles for station 2.34 281.8 2001 2000/2001 2.48 293.9 1972 1971/1972 919005A. Values have been reported to four 2.64 294.9 1990 1989/1990 2.82 302.6 2008 2007/2008 significant digits. 3.03 319.1 1977 1976/1977 3.27 341.2 2000 1999/2000 3.55 370.4 1983 1982/1983 3.89 431.3 1976 1975/1976 4.29 434.6 1971 1970/1971 4.79 437.6 2006 2005/2006 5.42 439.0 1998 1997/1998 6.24 473.3 1974 1973/1974 7.36 517.7 1981 1980/1981 8.96 519.6 1973 1972/1973 11.44 521.5 2004 2003/2004 15.85 618.7 1995 1994/1995 25.75 620.7 1999 1998/1999 68.67 876.9 1979 1978/1979
Table 11: Flood peaks identified by the peaks over threshold analysis for station 919005A.
41 919005A 3.0 250
2.5 200 ) 1 − s
2.0 3 m ( 150
1.5
100 exceedance
1.0 mean
mean number of floods/year mean number 50 0.5
0.0 0 0 20 40 60 80 100 140 180 − threshold (m3 s 1)
Figure 21: Threshold selection steps.
919005A Rifle Ck at Fonthill (365 km2) − Threshold = 20 m3 s 1 Interflood period = 30 days 900
800
700 ) 1 − s
3
m 600 (
500 streamflow
400
maximum 300
Daily 200
100
0 1968 1973 1978 1983 1988 1993 1998 2003 2008 Year
Figure 22: Linear-scale hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
42 919005A Rifle Ck at Fonthill (365 km2) 3 −1 103 Threshold = 20 m s Interflood period = 30 days ) 1 − s
3 102 m (
streamflow
101 maximum
Daily
100 1968 1973 1978 1983 1988 1993 1998 2003 2008 Year
Figure 23: Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
104 919005A Rifle Ck at Fonthill 365 km2
101
103 ) 2 − ) 1 − km
s 0 1 3
10 − s m 3 (
m ( Q
102 Q
10−1
101 1.1 1.2 1.5 2 3 4 5 10 20 50 100 Average Return Interval (Years)
Figure 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 44 E 919006A Lynd River at Torwood
Return Period Q Water (years) (m3s−1) Year Year 0.65 73.9 1978 1978/1979 0.67 75.8 1976 1976/1977 0.69 81.6 1982 1981/1982 0.72 94.1 1983 1983/1984 0.74 118.5 1969 1968/1969 0.77 143.7 1981 1981/1982 Return Lower Estimated Upper 0.80 153.7 1969 1968/1969 0.83 191.1 1985 1984/1985 Period C.I. Quantile C.I. 0.86 192.8 1985 1984/1985 − − − (years) (m3s 1) (m3s 1) (m3s 1) 0.90 201.3 1987 1987/1988 0.94 217.1 1983 1982/1983 0.98 229.9 1978 1977/1978 2 908 1152 1399 1.03 237.4 1982 1981/1982 1.08 341.9 1984 1984/1985 5 1954 2370 2778 1.14 358.8 1970 1969/1970 10 2654 3193 3713 1.20 404.3 1987 1986/1987 1.28 473.2 1977 1976/1977 20 3252 3941 4600 1.36 514.7 1983 1982/1983 1.45 854.3 1977 1977/1978 25 3416 4166 4920 1.56 879.4 1978 1977/1978 1.68 883.6 1971 1970/1971 50 3723 4825 5896 1.83 900.6 1977 1976/1977 2.00 913.3 1976 1975/1976 100 4097 5423 6811 2.21 1194.2 1980 1979/1980 2.47 1475.4 1986 1985/1986 Table 14: Fitted flood quantiles for station 2.79 2036.0 1979 1978/1979 3.21 2064.1 1988 1987/1988 3.79 2171.6 1984 1983/1984 919006A. Values have been reported to four 4.61 2327.7 1972 1971/1972 5.89 2482.7 1973 1972/1973 significant digits. 8.15 2635.4 1981 1980/1981 13.25 3752.4 1975 1974/1975 35.33 4402.4 1974 1973/1974
Table 13: Flood peaks identified by the peaks over threshold analysis for station 919006A.
45 919006A 2.5 1400
1200 2.0 ) 1 − s
1000 3 m ( 1.5 800
600 1.0 exceedance
400 mean
mean number of floods/year mean number 0.5 200
0.0 0 0 50 100 200 300 400 500 − threshold (m3 s 1)
Figure 25: Threshold selection steps.
919006A Lynd River at Torwood (4325 km2) − Threshold = 70 m3 s 1 Interflood period = 30 days 4500
4000
3500 ) 1 − s
3
m 3000 (
2500 streamflow
2000
maximum 1500
Daily 1000
500
0 1968 1973 1978 1983 1988 Year
Figure 26: Linear-scale hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
46 919006A Lynd River at Torwood (4325 km2) 3 −1 104 Threshold = 70 m s Interflood period = 30 days
3
) 10 1 − s
3 m (
102 streamflow
maximum
101 Daily
100 1968 1973 1978 1983 1988 Year
Figure 27: Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
104 919006A Lynd River at Torwood 4325 km2
100 ) 2 − ) 1 − km s 1
3 3 −
10 s m 3 (
m ( Q
Q 10−1
102 1.1 1.2 1.5 2 3 4 5 10 20 50 100 Average Return Interval (Years)
Figure 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 48 F 919007A Hodgkinson River at Piggy Hut
Return Period Q Water (years) (m3s−1) Year Year 0.69 30.1 1969 1968/1969 0.72 30.6 1979 1979/1980 0.74 36.0 1970 1970/1971 0.77 41.0 1969 1968/1969 0.80 51.0 1983 1982/1983 Return Lower Estimated Upper 0.83 51.6 1983 1982/1983 0.86 56.1 1988 1987/1988 Period C.I. Quantile C.I. 0.90 65.1 1982 1981/1982 − − − (years) (m3s 1) (m3s 1) (m3s 1) 0.94 72.3 1984 1984/1985 0.98 92.8 1983 1982/1983 1.03 143.1 1985 1984/1985 2 450 572 699 1.08 171.3 1970 1969/1970 1.14 198.6 1981 1981/1982 5 1022 1315 1607 1.20 198.6 1982 1981/1982 10 1498 1973 2426 1.28 262.1 1970 1969/1970 1.36 385.7 1975 1974/1975 20 1953 2727 3420 1.45 385.7 1978 1977/1978 1.56 419.1 1990 1989/1990 25 2076 2993 3833 1.68 444.3 1987 1987/1988 1.83 484.6 1972 1971/1972 50 2409 3896 5277 2.00 515.1 1976 1975/1976 2.21 579.4 1986 1985/1986 100 2682 4931 7195 2.47 690.3 1984 1983/1984 2.79 722.1 1980 1979/1980 Table 16: Fitted flood quantiles for station 3.21 1030.1 1981 1980/1981 3.79 1161.8 1973 1972/1973 4.61 1286.7 1971 1970/1971 919007A. Values have been reported to four 5.89 1435.5 1974 1973/1974 8.15 2375.7 1977 1976/1977 significant digits. 13.25 2955.2 1972 1971/1972 35.33 2978.9 1979 1978/1979
Table 15: Flood peaks identified by the peaks over threshold analysis for station 919007A.
49 919007A 2.0 1000
800 ) 1
1.5 − s
3 m ( 600
1.0
400 exceedance
0.5 mean
mean number of floods/year mean number 200
0.0 0 0 50 100 200 300 400 500 − threshold (m3 s 1)
Figure 29: Threshold selection steps.
919007A Hodgkinson River at Piggy Hut (1720 km2) − Threshold = 30 m3 s 1 Interflood period = 30 days 3000
2500 ) 1 − s
3
m 2000 (
1500 streamflow
maximum 1000
Daily
500
0 1968 1973 1978 1983 1988 Year
Figure 30: Linear-scale hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
50 919007A Hodgkinson River at Piggy Hut (1720 km2) 3 −1 104 Threshold = 30 m s Interflood period = 30 days
3
) 10 1 − s
3 m (
102 streamflow
maximum
101 Daily
100 1968 1973 1978 1983 1988 Year
Figure 31: Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
104 919007A Hodgkinson River at Piggy Hut 1720 km2
0 ) 2
10 − ) 1 − km s 1
3 3 −
10 s m 3 (
m ( Q
Q
10−1
102 1.1 1.2 1.5 2 3 4 5 10 20 50 100 Average Return Interval (Years)
Figure 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 52 G 919008A Tate River at Torwood
Return Period Q Water (years) (m3s−1) Year Year 0.58 55.7 1985 1984/1985 0.60 56.7 1982 1982/1983 0.62 57.0 1987 1987/1988 0.65 67.0 1983 1982/1983 Return Lower Estimated Upper 0.67 69.3 1977 1977/1978 0.70 79.7 1978 1977/1978 Period C.I. Quantile C.I. 0.73 90.1 1977 1976/1977 0.76 114.7 1983 1983/1984 (years) (m3s−1) (m3s−1) (m3s−1) 0.80 122.8 1973 1973/1974 0.83 151.1 1985 1984/1985 2 504 594 689 0.88 152.8 1975 1974/1975 0.92 157.7 1981 1981/1982 5 918 1079 1236 0.98 178.0 1982 1981/1982 1.04 190.7 1984 1984/1985 10 1191 1393 1593 1.10 293.4 1983 1982/1983 1.18 310.4 1973 1972/1973 20 1403 1670 1925 1.26 316.2 1978 1977/1978 25 1452 1751 2039 1.37 347.0 1975 1975/1976 1.48 434.6 1987 1986/1987 50 1618 1984 2349 1.62 435.5 1980 1979/1980 1.79 475.7 1982 1981/1982 100 1730 2188 2692 2.00 670.2 1981 1980/1981 2.26 750.4 1984 1983/1984 2.61 801.8 1972 1971/1972 Table 18: Fitted flood quantiles for station 3.07 815.0 1986 1985/1986 3.74 851.0 1979 1978/1979 919008A. Values have been reported to four 4.78 961.6 1977 1976/1977 6.62 1434.3 1975 1974/1975 significant digits. 10.75 1483.6 1988 1987/1988 28.67 1669.0 1974 1973/1974
Table 17: Flood peaks identified by the peaks over threshold analysis for station 919008A.
53 919008A 2.5 700
600 2.0 ) 1 − s
500 3 m ( 1.5 400
300 1.0 exceedance
200 mean
mean number of floods/year mean number 0.5 100
0.0 0 0 50 100 200 300 400 500 − threshold (m3 s 1)
Figure 33: Threshold selection steps.
919008A Tate River at Torwood (4350 km2) − Threshold = 50 m3 s 1 Interflood period = 30 days 1800
1600
1400 ) 1 − s
3
m 1200 (
1000 streamflow
800
maximum 600
Daily 400
200
0 1972 1977 1982 1987 Year
Figure 34: Linear-scale hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
54 919008A Tate River at Torwood (4350 km2) 3 −1 104 Threshold = 50 m s Interflood period = 30 days
3
) 10 1 − s
3 m (
102 streamflow
maximum
101 Daily
100 1972 1977 1982 1987 Year
Figure 35: Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
104 919008A Tate River at Torwood 4350 km2
100 ) 2 − ) 1 − km s 1
3 3 −
10 s m 3 (
m ( Q
10−1 Q
102 1.1 1.2 1.5 2 3 4 5 10 20 50 100 Average Return Interval (Years)
Figure 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 56 H 919009A Mitchell River at Koolatah
Return Period Q Water (years) (m3s−1) Year Year 0.99 272 2003 2002/2003 1.02 349 1985 1984/1985 1.05 377 1993 1992/1993 1.08 446 1984 1984/1985 1.11 739 1982 1981/1982 1.15 889 1988 1987/1988 1.18 903 1983 1982/1983 1.22 1061 1983 1982/1983 Return Lower Estimated Upper 1.27 1076 1981 1981/1982 1.31 1487 1987 1986/1987 Period C.I. Quantile C.I. 1.36 1646 2003 2002/2003 − − − (years) (m3s 1) (m3s 1) (m3s 1) 1.41 1668 1978 1977/1978 1.47 2666 2004 2003/2004 1.53 2701 1986 1985/1986 2 3222 3845 4497 1.60 2731 2005 2004/2005 1.68 3297 2009 2008/2009 5 5428 5806 6192 1.76 3414 1988 1987/1988 10 6041 6300 6552 1.85 3738 1980 1979/1980 1.95 3951 1997 1996/1997 20 6245 6497 6763 2.06 4168 1984 1983/1984 2.18 4306 1975 1974/1975 25 6272 6530 6814 2.32 4518 1973 1972/1973 2.48 4590 1976 1975/1976 50 6284 6588 6927 2.66 4666 1995 1994/1995 2.87 4722 2002 2001/2002 100 6290 6611 6985 3.12 4828 1996 1995/1996 3.42 5001 2006 2005/2006 Table 20: Fitted flood quantiles for station 3.77 5293 1998 1997/1998 4.21 5859 1981 1980/1981 4.76 6011 2007 2006/2007 919009A. Values have been reported to four 5.48 6074 2001 2000/2001 6.46 6081 1999 1998/1999 significant digits. 7.87 6140 1977 1976/1977 10.06 6183 1979 1978/1979 13.92 6255 2008 2007/2008 22.62 6270 1974 1973/1974 60.33 6358 2000 1999/2000
Table 19: Flood peaks identified by the peaks over threshold analysis for station 919009A.
57 919009A 2.0 4000 ) 1
1.5 3000 − s
3 m (
1.0 2000 exceedance
0.5 1000 mean mean number of floods/year mean number
0.0 0 0 200 600 1000 1400 1800 − threshold (m3 s 1)
Figure 37: Threshold selection steps.
919009A Mitchell River at Koolatah (46050 km2) − Threshold = 200 m3 s 1 Interflood period = 30 days 7000
6000 ) 1 − s
5000 3 m (
4000 streamflow
3000 maximum
2000 Daily
1000
0 1972 1977 1982 1987 1992 1997 2002 2007 Year
Figure 38: Linear-scale hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
58 919009A Mitchell River at Koolatah (46050 km2) 3 −1 104 Threshold = 200 m s Interflood period = 30 days
3
) 10 1 − s
3 m (
102 streamflow
maximum
101 Daily
100 1972 1977 1982 1987 1992 1997 2002 2007 Year
Figure 39: Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
104 919009A Mitchell River at Koolatah 46050 km2
10−1 ) 2 − ) 1 − km s 1
3 3 −
10 s m 3 (
m ( Q
10−2 Q
102 1.1 1.2 1.5 2 3 4 5 10 20 50 100 Average Return Interval (Years)
Figure 40: Fitted flood frequency curve for station 919009A. Dashed lines indicate a 95% confidence interval for the prediction. Note curve is only fitted to events with an average recurrence interval ≥ 1 year.
59 60 I 919011A Mitchell River at Gamboola
Return Period Q Water (years) (m3s−1) Year Year 0.79 224 1976 1976/1977 0.80 245 1988 1988/1989 0.82 287 1988 1987/1988 0.84 366 1990 1989/1990 0.86 380 1984 1984/1985 0.88 411 1985 1984/1985 0.90 434 1983 1982/1983 0.92 639 1977 1976/1977 0.94 686 1982 1981/1982 0.96 739 1983 1982/1983 0.99 771 1978 1977/1978 1.02 877 2005 2004/2005 1.04 904 1982 1981/1982 1.07 957 1994 1993/1994 Return Lower Estimated Upper 1.10 999 1981 1981/1982 1.14 1040 2003 2002/2003 Period C.I. Quantile C.I. 1.17 1078 1987 1986/1987 − − − (years) (m3s 1) (m3s 1) (m3s 1) 1.21 1115 1987 1987/1988 1.25 1238 1990 1989/1990 1.29 1277 1989 1989/1990 2 2084 2573 3066 1.34 1343 2002 2001/2002 1.38 1382 1992 1991/1992 5 4235 5176 6101 1.44 1433 1993 1992/1993 10 5839 7071 8241 1.49 1558 1975 1974/1975 1.55 1622 1984 1983/1984 20 7156 8904 10611 1.62 1770 2006 2005/2006 1.69 1902 2004 2003/2004 25 7486 9482 11399 1.77 1961 1988 1987/1988 1.85 2144 1988 1988/1989 50 8502 11236 13982 1.95 2188 1980 1979/1980 2.05 2466 1973 1972/1973 100 9214 12934 16996 2.17 2895 2006 2005/2006 2.30 2901 1981 1980/1981 Table 22: Fitted flood quantiles for station 2.45 2950 1995 1994/1995 2.62 3065 1986 1985/1986 2.81 3150 1976 1975/1976 919011A. Values have been reported to four 3.03 3888 1998 1997/1998 3.29 4246 1997 1996/1997 significant digits. 3.60 4740 1996 1995/1996 3.98 4770 2001 2000/2001 4.44 5624 2008 2007/2008 5.03 5816 1974 1973/1974 5.79 6602 2007 2006/2007 6.82 7047 2009 2008/2009 8.30 7499 1977 1976/1977 10.61 8233 2000 1999/2000 14.69 8287 1979 1978/1979 23.88 8882 1972 1971/1972 63.67 9023 1999 1998/1999
Table 21: Flood peaks identified by the peaks over threshold analysis for station 919011A.
61 919011A 2.0 4000 ) 1
1.5 3000 − s
3 m (
1.0 2000 exceedance
0.5 1000 mean mean number of floods/year mean number
0.0 0 0 200 600 1000 1400 1800 − threshold (m3 s 1)
Figure 41: Threshold selection steps.
919011A Mitchell River at Gamboola (20460 km2) − Threshold = 200 m3 s 1 Interflood period = 30 days 10000
9000
8000 ) 1 − s
3 7000 m (
6000
5000 streamflow
4000 maximum 3000 Daily 2000
1000
0 1971 1976 1981 1986 1991 1996 2001 2006 Year
Figure 42: Linear-scale hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
62 919011A Mitchell River at Gamboola (20460 km2) 3 −1 104 Threshold = 200 m s Interflood period = 30 days
3
) 10 1 − s
3 m (
102 streamflow
maximum
101 Daily
100 1971 1976 1981 1986 1991 1996 2001 2006 Year
Figure 43: Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
105 919011A Mitchell River at Gamboola 20460 km2
100
104 ) 2 − ) 1 − km s 1 3 − s m 3 ( −1
10 m ( Q
103 Q
10−2
102 1.1 1.2 1.5 2 3 4 5 10 20 50 100 Average Return Interval (Years)
Figure 44: Fitted flood frequency curve for station 919011A. Dashed lines indicate a 95% confidence interval for the prediction. Note curve is only fitted to events with an average recurrence interval ≥ 1 year.
63 64 J 919012A Galvin Ck at Reid Ck Junction
Return Period Q Water (years) (m3s−1) Year Year
0.47 23.8 1973 1972/1973 Return Lower Estimated Upper 0.50 24.0 1980 1979/1980 0.52 26.3 1975 1975/1976 Period C.I. Quantile C.I. 0.54 27.4 1975 1974/1975 0.57 28.6 1974 1974/1975 (years) (m3s−1) (m3s−1) (m3s−1) 0.60 31.4 1980 1980/1981 0.64 32.0 1978 1977/1978 2 220 240 260 0.67 35.2 1973 1972/1973 0.72 37.2 1982 1981/1982 5 315 364 413 0.77 43.4 1975 1974/1975 0.82 43.7 1975 1974/1975 10 399 476 551 0.89 60.1 1979 1979/1980 0.97 89.5 1977 1976/1977 20 465 605 744 1.06 182.5 1973 1972/1973 25 490 651 807 1.17 186.1 1979 1978/1979 1.30 192.3 1976 1975/1976 50 549 809 1060 1.47 197.4 1972 1971/1972 1.70 201.3 1976 1975/1976 100 584 1000 1397 2.00 224.1 1980 1979/1980 2.43 279.6 1972 1971/1972 3.11 332.4 1981 1980/1981 Table 24: Fitted flood quantiles for station 4.31 370.5 1979 1978/1979 7.00 483.3 1974 1973/1974 919012A. Values have been reported to four 18.67 598.5 1977 1976/1977 significant digits. Table 23: Flood peaks identified by the peaks over threshold analysis for station 919012A.
65 919012A 2.5 250
2.0 200 ) 1 − s
3 m ( 1.5 150
1.0 100 exceedance
mean
mean number of floods/year mean number 0.5 50
0.0 0 0 20 40 60 80 100 140 180 − threshold (m3 s 1)
Figure 45: Threshold selection steps.
919012A Galvin Ck at Reid Ck Junction (163 km2) − Threshold = 23 m3 s 1 Interflood period = 15 days 600
550
500 )
1 450 − s
3
m 400 (
350
300 streamflow
250
maximum 200
Daily 150
100
50
0 1971 1976 1981 Year
Figure 46: Linear-scale hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
66 919012A Galvin Ck at Reid Ck Junction (163 km2) 3 −1 103 Threshold = 23 m s Interflood period = 15 days ) 1 − s
3 102 m (
streamflow
101 maximum
Daily
100 1971 1976 1981 Year
Figure 47: Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
104 919012A Galvin Ck at Reid Ck Junction 163 km2 )
1 2 −
) 10 1 − km s 1
3 3 −
10 s m 3 (
m ( Q
Q
100
102 1.1 1.2 1.5 2 3 4 5 10 20 50 100 Average Return Interval (Years)
Figure 48: Fitted flood frequency curve for station 919012A. Dashed lines indicate a 95% confidence interval for the prediction. Note curve is only fitted to events with an average recurrence interval ≥ 1 year.
67 68 K 919013A McLeod River at Mulligan HWY
Return Period Q Water (years) (m3s−1) Year Year 0.82 52.1 1983 1983/1984 0.84 52.6 2007 2006/2007 0.86 58.1 2006 2005/2006 0.89 73.4 1985 1984/1985 0.91 78.6 1987 1987/1988 0.93 84.9 1988 1988/1989 0.96 85.2 1989 1989/1990 0.99 85.5 1990 1989/1990 1.02 87.1 1984 1983/1984 1.05 89.5 1999 1998/1999 Return Lower Estimated Upper 1.08 93.4 1986 1985/1986 1.12 100.8 1985 1984/1985 Period C.I. Quantile C.I. 1.16 120.6 2005 2004/2005 1.20 140.8 2000 1999/2000 (years) (m3s−1) (m3s−1) (m3s−1) 1.24 182.9 1991 1990/1991 1.29 187.5 1990 1989/1990 2 314 411 512 1.34 215.1 1980 1979/1980 1.39 233.0 2009 2008/2009 5 775 1006 1221 1.45 240.3 1976 1975/1976 1.51 257.2 1983 1982/1983 10 1153 1536 1884 1.58 267.9 2000 2000/2001 1.66 274.8 1977 1976/1977 20 1504 2147 2755 1.74 275.8 2005 2004/2005 25 1622 2363 3084 1.84 278.7 1987 1986/1987 1.94 291.3 2007 2006/2007 50 1953 3101 4280 2.06 301.2 1998 1997/1998 2.19 344.8 2006 2005/2006 100 2064 3952 5850 2.34 421.3 1974 1973/1974 2.51 435.8 1978 1977/1978 2.71 448.5 1986 1985/1986 Table 26: Fitted flood quantiles for station 2.95 591.3 1975 1974/1975 3.23 840.7 1980 1979/1980 919013A. Values have been reported to four 3.56 994.0 2006 2005/2006 3.98 1039.0 1981 1980/1981 significant digits. 4.50 1246.6 2004 2003/2004 5.18 1315.1 1989 1988/1989 6.11 1342.7 1973 1972/1973 7.43 1526.0 2001 2000/2001 9.50 1599.0 2008 2007/2008 13.15 1804.6 2000 1999/2000 21.38 2524.1 1999 1998/1999 57.00 2799.0 1979 1978/1979
Table 25: Flood peaks identified by the peaks over threshold analysis for station 919013A.
69 919013A 4 1000
800 ) 1
3 − s
3 m ( 600
2
400 exceedance
1 mean
mean number of floods/year mean number 200
0 0 0 50 100 200 300 400 500 − threshold (m3 s 1)
Figure 49: Threshold selection steps.
919013A McLeod River at Mulligan HWY (530 km2) − Threshold = 50 m3 s 1 Interflood period = 30 days 3000
2500 ) 1 − s
3
m 2000 (
1500 streamflow
maximum 1000
Daily
500
0 1973 1978 1983 1988 1993 1998 2003 2008 Year
Figure 50: Linear-scale hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
70 919013A McLeod River at Mulligan HWY (530 km2) 3 −1 104 Threshold = 50 m s Interflood period = 30 days
3
) 10 1 − s
3 m (
102 streamflow
maximum
101 Daily
100 1973 1978 1983 1988 1993 1998 2003 2008 Year
Figure 51: Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
104 919013A McLeod River at Mulligan HWY 530 km2
101 ) 2 − ) 1 − km s 1
3 3 −
10 s m 3 (
m ( Q 0
10 Q
102 1.1 1.2 1.5 2 3 4 5 10 20 50 100 Average Return Interval (Years)
Figure 52: Fitted flood frequency curve for station 919013A. Dashed lines indicate a 95% confidence interval for the prediction. Note curve is only fitted to events with an average recurrence interval ≥ 1 year.
71 72 L 919014A Mitchell River at Cooktown Crossing
Return Period Q Water (years) (m3s−1) Year Year Return Lower Estimated Upper 0.56 54.7 2001 2001/2002 0.59 64.4 2004 2004/2005 Period C.I. Quantile C.I. 0.62 69.0 2008 2008/2009 0.66 69.5 2002 2001/2002 (years) (m3s−1) (m3s−1) (m3s−1) 0.69 72.7 2002 2002/2003 0.73 92.7 2002 2001/2002 2 719 903 1091 0.78 93.2 2004 2003/2004 0.84 175.4 1999 1999/2000 5 1465 1665 1853 0.90 181.3 2003 2003/2004 0.97 211.3 2006 2005/2006 10 1808 1989 2158 1.05 211.8 2003 2002/2003 1.15 217.9 2005 2004/2005 20 2001 2187 2365 1.27 244.8 2000 2000/2001 25 2031 2233 2432 1.42 328.2 2005 2004/2005 1.61 401.3 2008 2007/2008 50 2120 2337 2567 1.85 856.2 2007 2006/2007 2.18 1094.3 2006 2005/2006 100 2148 2401 2695 2.65 1355.5 2004 2003/2004 3.39 1528.2 2009 2008/2009 4.69 1668.9 2001 2000/2001 Table 28: Fitted flood quantiles for station 7.62 1830.6 2008 2007/2008 20.33 1943.5 2000 1999/2000 919014A. Values have been reported to four significant digits. Table 27: Flood peaks identified by the peaks over threshold analysis for station 919014A.
73 919014A 2.0 1200
1000 ) 1
1.5 − s
800 3 m (
1.0 600 exceedance
400
0.5 mean mean number of floods/year mean number 200
0.0 0 0 20 40 60 80 100 140 180 − threshold (m3 s 1)
Figure 53: Threshold selection steps.
919014A Mitchell River at Cooktown Crossing (2574 km2) − Threshold = 50 m3 s 1 Interflood period = 15 days 2000
1800
1600 ) 1 − s
3 1400 m (
1200
1000 streamflow
800 maximum 600 Daily 400
200
0 1999 2004 2009 Year
Figure 54: Linear-scale hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
74 919014A Mitchell River at Cooktown Crossing (2574 km2) 3 −1 104 Threshold = 50 m s Interflood period = 15 days
3
) 10 1 − s
3 m (
102 streamflow
maximum
101 Daily
100 1999 2004 2009 Year
Figure 55: Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
104 919014A Mitchell River at Cooktown Crossing 2574 km2
100 ) 2 − ) 1 − km s 1
3 3 −
10 s m 3 (
m ( Q
Q
10−1
102 1.1 1.2 1.5 2 3 4 5 10 20 50 100 Average Return Interval (Years)
Figure 56: Fitted flood frequency curve for station 919014A. Dashed lines indicate a 95% confidence interval for the prediction. Note curve is only fitted to events with an average recurrence interval ≥ 1 year.
75 76 M 919201A Palmer River at Goldfields
Return Period Q Water (years) (m3s−1) Year Year 0.78 40.5 1990 1989/1990 0.79 41.3 2005 2004/2005 0.81 45.3 1993 1993/1994 0.82 46.5 1985 1984/1985 0.84 51.0 2000 2000/2001 0.85 54.2 1983 1982/1983 0.87 66.2 2004 2004/2005 0.89 71.3 1969 1969/1970 0.91 73.7 1985 1984/1985 0.93 78.2 2005 2004/2005 0.95 81.0 1982 1981/1982 0.97 81.3 1994 1993/1994 0.99 81.7 2008 2007/2008 1.01 87.6 1969 1968/1969 1.04 89.0 1986 1986/1987 1.06 96.3 1987 1987/1988 1.09 100.7 1984 1984/1985 Return Lower Estimated Upper 1.12 101.6 1988 1988/1989 1.15 102.4 2002 2001/2002 Period C.I. Quantile C.I. 1.18 133.0 1970 1969/1970 1.21 134.8 1987 1986/1987 (years) (m3s−1) (m3s−1) (m3s−1) 1.25 137.4 1986 1985/1986 1.29 141.3 1986 1985/1986 2 312 391 468 1.33 162.5 1970 1969/1970 1.37 196.5 1996 1995/1996 5 646 764 884 1.41 201.6 1998 1997/1998 1.46 210.1 2003 2002/2003 10 852 1000 1117 1.51 217.4 1992 1991/1992 1.57 234.8 1975 1974/1975 20 1003 1172 1333 1.62 303.3 1995 1994/1995 25 1046 1224 1407 1.69 307.3 1984 1983/1984 1.76 322.8 2006 2005/2006 50 1132 1366 1610 1.83 397.5 1968 1967/1968 1.91 413.1 1978 1977/1978 100 1184 1482 1785 2.00 423.1 1973 1972/1973 2.10 432.0 2006 2005/2006 2.20 474.4 1976 1975/1976 Table 30: Fitted flood quantiles for station 2.32 489.0 1989 1988/1989 2.45 506.4 1981 1980/1981 919201A. Values have been reported to four 2.60 534.0 2006 2005/2006 2.77 586.6 1972 1971/1972 significant digits. 2.96 607.3 2008 2007/2008 3.18 627.5 1977 1976/1977 3.43 628.5 1991 1990/1991 3.72 665.5 1974 1973/1974 4.08 668.8 2007 2006/2007 4.50 671.6 2004 2003/2004 5.02 690.5 2001 2000/2001 5.68 744.9 1997 1996/1997 6.55 755.8 2009 2008/2009 7.71 966.8 2000 1999/2000 9.39 1029.5 1980 1979/1980 12.00 1068.0 1971 1970/1971 16.62 1103.5 1999 1998/1999 27.00 1194.2 1979 1978/1979 72.00 1459.7 1996 1995/1996
Table 29: Flood peaks identified by the peaks over threshold analysis for station 919201A.
77 919201A 3.0 400
2.5 ) 1
300 − s
2.0 3 m (
1.5 200 exceedance
1.0
100 mean mean number of floods/year mean number 0.5
0.0 0 0 50 100 200 300 400 500 − threshold (m3 s 1)
Figure 57: Threshold selection steps.
919201A Palmer River at Goldfields (530 km2) − Threshold = 40 m3 s 1 Interflood period = 30 days 1600
1400 )
1 1200 − s
3 m ( 1000
800 streamflow
600 maximum
Daily 400
200
0 1967 1972 1977 1982 1987 1992 1997 2002 2007 Year
Figure 58: Linear-scale hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
78 919201A Palmer River at Goldfields (530 km2) 3 −1 104 Threshold = 40 m s Interflood period = 30 days
3
) 10 1 − s
3 m (
102 streamflow
maximum
101 Daily
100 1967 1972 1977 1982 1987 1992 1997 2002 2007 Year
Figure 59: Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
104 919201A Palmer River at Goldfields 530 km2
101 ) 2 − ) 1 − km s 1
3 3 −
10 s m 3 (
m ( Q 0
10 Q
102 1.1 1.2 1.5 2 3 4 5 10 20 50 100 Average Return Interval (Years)
Figure 60: Fitted flood frequency curve for station 919201A. Dashed lines indicate a 95% confidence interval for the prediction. Note curve is only fitted to events with an average recurrence interval ≥ 1 year.
79 80 N 919204A Palmer River at Palmer River at Drumduff
Return Period Q Water (years) (m3s−1) Year Year 0.93 168 1988 1987/1988 0.96 216 1987 1986/1987 0.99 230 1982 1981/1982 1.02 238 1981 1981/1982 1.06 249 2005 2004/2005 1.09 269 1987 1987/1988 Return Lower Estimated Upper 1.14 317 1985 1984/1985 1.18 324 1982 1981/1982 Period C.I. Quantile C.I. 1.23 330 2005 2004/2005 − − − (years) (m3s 1) (m3s 1) (m3s 1) 1.28 343 2001 2000/2001 1.34 432 1983 1982/1983 1.40 433 2003 2002/2003 2 737 940 1157 1.47 440 1984 1984/1985 1.54 502 1978 1977/1978 5 1684 2126 2560 1.62 502 1983 1982/1983 10 2405 3033 3613 1.72 530 1986 1985/1986 1.82 597 2006 2005/2006 20 3045 3950 4851 1.94 851 1973 1972/1973 2.07 869 1975 1974/1975 25 3211 4247 5230 2.22 1072 1984 1983/1984 2.40 1178 2002 2001/2002 50 3621 5177 6598 2.60 1214 1980 1979/1980 2.85 1440 1981 1980/1981 100 4063 6116 8059 3.15 1454 1976 1975/1976 3.51 1941 2004 2003/2004 Table 32: Fitted flood quantiles for station 3.97 2230 2008 2007/2008 4.58 2376 1974 1973/1974 5.39 2472 1999 1998/1999 919204A. Values have been reported to four 6.57 3085 1977 1976/1977 8.39 3258 2001 2000/2001 significant digits. 11.62 3535 2000 1999/2000 18.88 3781 2007 2006/2007 50.33 4099 1979 1978/1979
Table 31: Flood peaks identified by the peaks over threshold analysis for station 919204A.
81 919204A 2.0 2000 ) 1
1.5 1500 − s
3 m (
1.0 1000 exceedance
0.5 500 mean mean number of floods/year mean number
0.0 0 0 200 600 1000 1400 1800 − threshold (m3 s 1)
Figure 61: Threshold selection steps.
919204A Palmer River at Drumduff (7750 km2) − Threshold = 150 m3 s 1 Interflood period = 30 days 4500
4000
3500 ) 1 − s
3
m 3000 (
2500 streamflow
2000
maximum 1500
Daily 1000
500
0 1972 1977 1982 1987 1992 1997 2002 2007 Year
Figure 62: Linear-scale hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
82 919204A Palmer River at Drumduff (7750 km2) 3 −1 104 Threshold = 150 m s Interflood period = 30 days
3
) 10 1 − s
3 m (
102 streamflow
maximum
101 Daily
100 1972 1977 1982 1987 1992 1997 2002 2007 Year
Figure 63: Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
104 2 919204A Palmer River at Drumduff 7750 km 100 ) 2 − ) 1 − km s 1
3 3 −
10 s m 3
( −1
10 m ( Q
Q
102 1.1 1.2 1.5 2 3 4 5 10 20 50 100 Average Return Interval (Years)
Figure 64: Fitted flood frequency curve for station 919204A. Dashed lines indicate a 95% confidence interval for the prediction. Note curve is only fitted to events with an average recurrence interval ≥ 1 year.
83 84 O 919205A North Palmer River at 4.8 Km
Return Period Q Water (years) (m3s−1) Year Year 0.53 20.0 1982 1982/1983 0.55 20.4 1983 1982/1983 0.57 20.4 1988 1987/1988 0.59 20.8 1980 1979/1980 Return Lower Estimated Upper 0.62 22.4 1987 1986/1987 0.64 27.3 1985 1984/1985 Period C.I. Quantile C.I. 0.67 28.1 1988 1987/1988 − − − (years) (m3s 1) (m3s 1) (m3s 1) 0.70 31.3 1977 1977/1978 0.74 31.4 1987 1986/1987 0.78 46.3 1978 1977/1978 2 155 188 221 0.82 49.2 1976 1976/1977 0.86 56.8 1983 1982/1983 5 295 342 388 0.92 57.3 1975 1974/1975 10 375 425 473 0.97 64.8 1981 1981/1982 1.04 68.2 1984 1984/1985 20 430 488 541 1.12 72.8 1982 1981/1982 1.21 79.5 1975 1974/1975 25 444 505 567 1.31 97.1 1986 1985/1986 1.43 110.0 1983 1983/1984 50 475 548 621 1.58 115.2 1975 1974/1975 1.77 124.6 1986 1985/1986 100 489 580 675 2.00 139.4 1986 1985/1986 2.30 231.5 1984 1983/1984 Table 34: Fitted flood quantiles for station 2.71 312.1 1977 1976/1977 3.30 312.1 1981 1980/1981 4.22 336.0 1980 1979/1980 919205A. Values have been reported to four 5.85 402.9 1974 1973/1974 9.50 418.0 1979 1978/1979 significant digits. 25.33 426.9 1976 1975/1976
Table 33: Flood peaks identified by the peaks over threshold analysis for station 919205A.
85 919205A 2.5 250
2.0 200 ) 1 − s
3 m ( 1.5 150
1.0 100 exceedance
mean
mean number of floods/year mean number 0.5 50
0.0 0 0 20 40 60 80 100 140 180 − threshold (m3 s 1)
Figure 65: Threshold selection steps.
919205A North Palmer River at 4.8 Km (430 km2) − Threshold = 20 m3 s 1 Interflood period = 15 days 450
400
350 ) 1 − s
3
m 300 (
250 streamflow
200
maximum 150
Daily 100
50
0 1973 1978 1983 1988 Year
Figure 66: Linear-scale hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
86 919205A North Palmer River at 4.8 Km (430 km2) 3 −1 103 Threshold = 20 m s Interflood period = 15 days ) 1 − s
3 102 m (
streamflow
101 maximum
Daily
100 1973 1978 1983 1988 Year
Figure 67: Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
103 919205A North Palmer River at 4.8 Km 430 km2
100 ) 2 − ) 1 − km s 1
3 2 −
10 s m 3 (
m ( Q
Q 10−1
101 1.1 1.2 1.5 2 3 4 5 10 20 50 100 Average Return Interval (Years)
Figure 68: Fitted flood frequency curve for station 919205A. Dashed lines indicate a 95% confidence interval for the prediction. Note curve is only fitted to events with an average recurrence interval ≥ 1 year.
87 88 P 919305B Walsh River at Nullinga
Return Period Q Water (years) (m3s−1) Year Year 0.77 20.0 1982 1981/1982 0.79 20.3 1960 1959/1960 0.80 20.3 1961 1961/1962 0.82 23.7 1973 1972/1973 0.84 26.4 1971 1970/1971 0.86 26.4 1975 1974/1975 0.88 26.7 1959 1958/1959 0.90 29.6 1963 1962/1963 0.92 30.0 1959 1958/1959 0.94 30.5 1982 1981/1982 0.96 31.0 1964 1964/1965 0.99 37.6 1976 1975/1976 1.02 38.4 1956 1955/1956 1.04 38.6 1987 1986/1987 Return Lower Estimated Upper 1.07 38.7 1959 1959/1960 1.10 41.3 1964 1963/1964 Period C.I. Quantile C.I. 1.14 58.1 1960 1959/1960 1.17 59.5 1975 1974/1975 (years) (m3s−1) (m3s−1) (m3s−1) 1.21 61.5 1972 1971/1972 1.25 66.0 1981 1980/1981 2 126 160 197 1.29 72.5 1987 1987/1988 1.34 86.0 1980 1979/1980 5 290 375 462 1.38 86.1 1976 1975/1976 1.44 89.3 1989 1988/1989 10 427 574 718 1.49 93.6 1961 1960/1961 1.55 96.0 1992 1991/1992 20 569 811 1041 1.62 101.5 1966 1965/1966 25 603 896 1183 1.69 105.9 1989 1989/1990 1.77 141.7 1963 1962/1963 50 708 1192 1647 1.85 171.6 1981 1980/1981 1.95 174.7 1971 1970/1971 100 777 1544 2257 2.05 175.1 1984 1983/1984 2.17 182.7 1956 1955/1956 2.30 194.2 1977 1976/1977 Table 36: Fitted flood quantiles for station 2.45 195.9 1964 1963/1964 2.62 230.1 1988 1987/1988 919305B. Values have been reported to four 2.81 247.9 1979 1978/1979 3.03 262.4 1990 1989/1990 significant digits. 3.29 293.9 1973 1972/1973 3.60 310.9 1962 1961/1962 3.98 318.5 1991 1990/1991 4.44 367.2 1968 1967/1968 5.03 388.7 1974 1973/1974 5.79 451.9 1958 1957/1958 6.82 492.3 1979 1978/1979 8.30 495.1 1972 1971/1972 10.61 500.6 1957 1956/1957 14.69 525.7 1986 1985/1986 23.88 1267.4 1977 1976/1977 63.67 1391.8 1967 1966/1967
Table 35: Flood peaks identified by the peaks over threshold analysis for station 919305B.
89 919305B 2.5 350
300 2.0 ) 1 − s
250 3 m ( 1.5 200
150 1.0 exceedance
100 mean
mean number of floods/year mean number 0.5 50
0.0 0 0 20 40 60 80 100 140 180 − threshold (m3 s 1)
Figure 69: Threshold selection steps.
919305B Walsh River at Nullinga (325 km2) − Threshold = 20 m3 s 1 Interflood period = 20 days 1400
1200 ) 1 − s
1000 3 m (
800 streamflow
600 maximum
400 Daily
200
0 1956 1961 1966 1971 1976 1981 1986 1991 Year
Figure 70: Linear-scale hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
90 919305B Walsh River at Nullinga (325 km2) 3 −1 104 Threshold = 20 m s Interflood period = 20 days
3
) 10 1 − s
3 m (
102 streamflow
maximum
101 Daily
100 1956 1961 1966 1971 1976 1981 1986 1991 Year
Figure 71: Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
104 919305B Walsh River at Nullinga 325 km2
101
103 ) 2 − ) 1 − km s 1
3 0 10 − s m 3 (
m ( Q
102 Q
10−1
101 1.1 1.2 1.5 2 3 4 5 10 20 50 100 Average Return Interval (Years)
Figure 72: Fitted flood frequency curve for station 919305B. Dashed lines indicate a 95% confidence interval for the prediction. Note curve is only fitted to events with an average recurrence interval ≥ 1 year.
91 92 Q 919309A Walsh River at Trimbles Crossing
Return Period Q Water (years) (m3s−1) Year Year 0.80 193 1984 1984/1985 0.82 194 1990 1989/1990 0.83 205 1969 1968/1969 0.85 208 1983 1982/1983 0.87 215 1982 1981/1982 0.89 217 1983 1982/1983 0.91 241 1977 1977/1978 0.93 248 1970 1969/1970 0.95 265 1975 1974/1975 0.97 275 1985 1984/1985 0.99 312 1970 1970/1971 1.01 364 1981 1981/1982 1.04 416 1978 1977/1978 1.07 429 1994 1993/1994 1.09 443 1987 1986/1987 1.12 456 1973 1973/1974 Return Lower Estimated Upper 1.15 462 1969 1968/1969 1.19 499 1988 1987/1988 Period C.I. Quantile C.I. 1.22 505 1993 1992/1993 − − − (years) (m3s 1) (m3s 1) (m3s 1) 1.26 510 2003 2002/2003 1.29 529 1992 1991/1992 1.34 568 1990 1989/1990 2 794 945 1102 1.38 575 1997 1996/1997 1.43 673 2005 2004/2005 5 1494 1826 2138 1.48 705 1976 1975/1976 10 2015 2541 3039 1.53 730 2006 2005/2006 1.59 746 1982 1981/1982 20 2521 3301 4036 1.65 776 2002 2001/2002 1.72 816 1968 1967/1968 25 2692 3555 4397 1.79 816 1989 1989/1990 1.87 862 1973 1972/1973 50 3040 4378 5634 1.95 875 1988 1988/1989 2.05 897 1975 1974/1975 100 3340 5253 7176 2.15 898 2006 2005/2006 2.27 904 1980 1979/1980 Table 38: Fitted flood quantiles for station 2.40 966 1984 1983/1984 2.54 1223 1981 1980/1981 2.71 1249 1988 1987/1988 919309A. Values have been reported to four 2.89 1250 2004 2003/2004 3.10 1269 1986 1985/1986 significant digits. 3.35 1376 1998 1997/1998 3.64 1525 2001 2000/2001 3.98 1670 1996 1995/1996 4.40 1687 1971 1970/1971 4.91 1954 1979 1978/1979 5.55 2085 1974 1973/1974 6.39 2677 2008 2007/2008 7.54 2681 1977 1976/1977 9.17 2814 2007 2006/2007 11.72 2905 2009 2008/2009 16.23 3400 2000 1999/2000 26.38 3472 1972 1971/1972 70.33 3962 1999 1998/1999
Table 37: Flood peaks identified by the peaks over threshold analysis for station 919309A.
93 919309A 2.0 1200
1000 ) 1
1.5 − s
800 3 m (
1.0 600 exceedance
400
0.5 mean mean number of floods/year mean number 200
0.0 0 0 200 600 1000 1400 1800 − threshold (m3 s 1)
Figure 73: Threshold selection steps.
919309A Walsh River at Trimbles Crossing (9040 km2) − Threshold = 171 m3 s 1 Interflood period = 30 days 4000
3500 )
1 3000 − s
3 m ( 2500
2000 streamflow
1500 maximum
Daily 1000
500
0 1967 1972 1977 1982 1987 1992 1997 2002 2007 Year
Figure 74: Linear-scale hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
94 919309A Walsh River at Trimbles Crossing (9040 km2) 3 −1 104 Threshold = 171 m s Interflood period = 30 days
3
) 10 1 − s
3 m (
102 streamflow
maximum
101 Daily
100 1967 1972 1977 1982 1987 1992 1997 2002 2007 Year
Figure 75: Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
4 10 0 919309A Walsh River at Trimbles Crossing 9040 km2 10 ) 2 − ) 1 − km s 1
3 3 −1 −
10 s m
10 3 (
m ( Q
Q
102 1.1 1.2 1.5 2 3 4 5 10 20 50 100 Average Return Interval (Years)
Figure 76: Fitted flood frequency curve for station 919309A. Dashed lines indicate a 95% confidence interval for the prediction. Note curve is only fitted to events with an average recurrence interval ≥ 1 year.
95 96 R 919310A Walsh River at Rookwood
Return Period Q Water (years) (m3s−1) Year Year 0.83 209 1993 1992/1993 0.85 217 1987 1986/1987 0.87 237 1969 1968/1969 0.89 261 1985 1985/1986 0.91 271 1989 1988/1989 0.93 291 1970 1970/1971 0.95 336 2002 2001/2002 0.97 336 1975 1974/1975 0.99 349 2002 2001/2002 1.01 371 2004 2004/2005 1.04 386 1978 1977/1978 1.07 436 1987 1986/1987 1.09 444 2006 2005/2006 1.12 461 1992 1991/1992 1.15 484 1980 1979/1980 Return Lower Estimated Upper 1.19 514 2004 2003/2004 1.22 531 2003 2002/2003 Period C.I. Quantile C.I. 1.26 580 1977 1977/1978 − − − (years) (m3s 1) (m3s 1) (m3s 1) 1.29 591 2004 2003/2004 1.34 620 1988 1988/1989 1.38 648 1995 1994/1995 2 917 1105 1305 1.43 784 2005 2004/2005 1.48 824 1996 1995/1996 5 1809 2225 2637 1.53 854 1987 1987/1988 10 2491 3142 3746 1.59 877 1990 1989/1990 1.65 937 1972 1971/1972 20 3162 4122 5065 1.72 970 1984 1983/1984 1.79 985 1976 1975/1976 25 3379 4453 5526 1.87 1035 1973 1972/1973 1.95 1074 1982 1981/1982 50 3859 5525 7243 2.05 1183 1968 1967/1968 2.15 1183 1981 1980/1981 100 4203 6674 9224 2.27 1206 1997 1997/1998 2.40 1225 2006 2005/2006 Table 40: Fitted flood quantiles for station 2.54 1309 1989 1989/1990 2.71 1324 1975 1974/1975 2.89 1367 2007 2006/2007 919310A. Values have been reported to four 3.10 1409 1971 1970/1971 3.35 1438 2001 2000/2001 significant digits. 3.64 1698 1988 1987/1988 3.98 2190 1986 1985/1986 4.40 2304 1991 1990/1991 4.91 2652 2008 2007/2008 5.55 2757 1997 1996/1997 6.39 2832 1974 1973/1974 7.54 2852 2009 2008/2009 9.17 2972 1979 1978/1979 11.72 3793 2000 1999/2000 16.23 4206 1977 1976/1977 26.38 4524 1999 1998/1999 70.33 5526 1972 1971/1972
Table 39: Flood peaks identified by the peaks over threshold analysis for station 919310A.
97 919310A 3.0 1500
2.5 ) 1 − s
2.0 1000 3 m (
1.5 exceedance
1.0 500 mean mean number of floods/year mean number 0.5
0.0 0 0 200 600 1000 1400 1800 − threshold (m3 s 1)
Figure 77: Threshold selection steps.
919310A Walsh River at Rookwood (5025 km2) − Threshold = 200 m3 s 1 Interflood period = 30 days 6000
5500
5000 )
1 4500 − s
3
m 4000 (
3500
3000 streamflow
2500
maximum 2000
Daily 1500
1000
500
0 1967 1972 1977 1982 1987 1992 1997 2002 2007 Year
Figure 78: Linear-scale hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
98 919310A Walsh River at Rookwood (5025 km2) 3 −1 104 Threshold = 200 m s Interflood period = 30 days
3
) 10 1 − s
3 m (
102 streamflow
maximum
101 Daily
100 1967 1972 1977 1982 1987 1992 1997 2002 2007 Year
Figure 79: Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
104 919310A Walsh River at Rookwood 5025 km2
100 ) 2 − ) 1 − km s 1
3 3 −
10 s m 3 (
m ( Q −1 10 Q
102 1.1 1.2 1.5 2 3 4 5 10 20 50 100 Average Return Interval (Years)
Figure 80: Fitted flood frequency curve for station 919310A. Dashed lines indicate a 95% confidence interval for the prediction. Note curve is only fitted to events with an average recurrence interval ≥ 1 year.
99 100 S 919311A Walsh River at Flatrock
Return Period Q Water (years) (m3s−1) Year Year 0.83 44.3 2008 2008/2009 0.85 48.3 1990 1989/1990 0.87 51.3 1968 1968/1969 0.88 52.7 1996 1995/1996 0.90 54.1 1977 1976/1977 0.92 60.2 1995 1995/1996 0.94 84.7 1985 1984/1985 0.97 108.2 1985 1984/1985 0.99 109.6 1993 1992/1993 1.01 117.4 1981 1981/1982 1.04 135.9 1982 1981/1982 1.07 141.2 1985 1985/1986 1.10 144.9 1983 1982/1983 1.13 180.2 1982 1981/1982 Return Lower Estimated Upper 1.16 187.7 2000 2000/2001 1.19 189.4 1985 1984/1985 Period C.I. Quantile C.I. 1.23 193.9 1969 1968/1969 1.26 270.3 1970 1969/1970 (years) (m3s−1) (m3s−1) (m3s−1) 1.30 277.3 1978 1977/1978 1.35 315.4 1980 1979/1980 2 615 793 983 1.39 354.9 2002 2001/2002 1.44 419.3 2006 2005/2006 5 1430 1780 2121 1.49 460.9 1996 1995/1996 1.55 533.5 1992 1991/1992 10 2028 2493 2952 1.61 544.1 1984 1983/1984 1.67 571.9 2003 2002/2003 20 2529 3176 3807 1.75 606.1 2004 2003/2004 25 2656 3390 4083 1.82 772.3 1998 1997/1998 1.91 781.5 1976 1975/1976 50 2998 4038 5097 2.00 811.7 1973 1972/1973 2.10 825.8 2005 2004/2005 100 3212 4660 6093 2.22 918.4 1990 1989/1990 2.34 934.6 1987 1987/1988 2.48 994.4 1971 1970/1971 Table 42: Fitted flood quantiles for station 2.64 1038.7 1975 1974/1975 2.82 1137.6 2001 2000/2001 919311A. Values have been reported to four 3.03 1144.7 1981 1980/1981 3.27 1156.0 2007 2006/2007 significant digits. 3.55 1248.8 2006 2005/2006 3.89 1323.4 1989 1989/1990 4.29 1808.8 1988 1987/1988 4.79 2002.1 1986 1985/1986 5.42 2372.8 1974 1973/1974 6.24 2503.2 1972 1971/1972 7.36 2503.2 1979 1978/1979 8.96 2571.9 1997 1996/1997 11.44 2649.2 2008 2007/2008 15.85 2664.5 1977 1976/1977 25.75 3598.4 1999 1998/1999 68.67 3645.7 2000 1999/2000
Table 41: Flood peaks identified by the peaks over threshold analysis for station 919311A.
101 919311A 2.5 1000
2.0 800 ) 1 − s
3 m ( 1.5 600
1.0 400 exceedance
mean
mean number of floods/year mean number 0.5 200
0.0 0 0 50 100 200 300 400 500 − threshold (m3 s 1)
Figure 81: Threshold selection steps.
919311A Walsh River at Flatrock (2770 km2) − Threshold = 40 m3 s 1 Interflood period = 30 days 4000
3500 )
1 3000 − s
3 m ( 2500
2000 streamflow
1500 maximum
Daily 1000
500
0 1968 1973 1978 1983 1988 1993 1998 2003 2008 Year
Figure 82: Linear-scale hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
102 919311A Walsh River at Flatrock (2770 km2) 3 −1 104 Threshold = 40 m s Interflood period = 30 days
3
) 10 1 − s
3 m (
102 streamflow
maximum
101 Daily
100 1968 1973 1978 1983 1988 1993 1998 2003 2008 Year
Figure 83: Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
104 919311A Walsh River at Flatrock 2770 km2
100 ) 2 − ) 1 − km s 1
3 3 −
10 s m 3 (
m ( Q
Q
10−1
102 1.1 1.2 1.5 2 3 4 5 10 20 50 100 Average Return Interval (Years)
Figure 84: Fitted flood frequency curve for station 919311A. Dashed lines indicate a 95% confidence interval for the prediction. Note curve is only fitted to events with an average recurrence interval ≥ 1 year.
103 104 T 919312A Elizabeth Ck at Greenmantle
Return Period Q Water (years) (m3s−1) Year Year 0.70 41.3 1975 1974/1975 0.72 54.1 1970 1969/1970 0.75 54.1 1973 1973/1974 Return Lower Estimated Upper 0.78 71.8 1970 1970/1971 0.81 72.8 1969 1969/1970 Period C.I. Quantile C.I. 0.85 74.7 1976 1976/1977 0.89 90.3 1981 1981/1982 (years) (m3s−1) (m3s−1) (m3s−1) 0.93 101.6 1984 1984/1985 0.98 122.5 1977 1977/1978 2 353 412 471 1.03 137.2 1987 1986/1987 1.09 152.7 1974 1974/1975 5 603 684 764 1.16 157.1 1983 1982/1983 1.23 218.9 1982 1981/1982 10 740 827 910 1.32 271.6 1985 1984/1985 1.41 306.8 1986 1985/1986 20 833 930 1028 1.52 325.8 1982 1981/1982 25 857 1000 1057 1.66 386.0 1984 1983/1984 1.81 433.4 1988 1987/1988 50 901 1026 1151 2.00 449.0 1987 1987/1988 2.23 453.5 1971 1970/1971 100 926 1075 1229 2.53 471.8 1976 1975/1976 2.91 483.3 1981 1980/1981 3.43 565.7 1973 1972/1973 Table 44: Fitted flood quantiles for station 4.17 570.8 1980 1979/1980 5.33 687.2 1974 1973/1974 919312A. Values have been reported to four 7.38 737.8 1977 1976/1977 12.00 912.8 1972 1971/1972 significant digits. 32.00 990.5 1979 1978/1979
Table 43: Flood peaks identified by the peaks over threshold analysis for station 919312A.
105 919312A 2.0 400 ) 1
1.5 300 − s
3 m (
1.0 200 exceedance
0.5 100 mean mean number of floods/year mean number
0.0 0 0 50 100 200 300 400 500 − threshold (m3 s 1)
Figure 85: Threshold selection steps.
919312A Elizabeth Ck at Greenmantle (620 km2) − Threshold = 25 m3 s 1 Interflood period = 30 days 1000
900
800 ) 1 − s
3 700 m (
600
500 streamflow
400 maximum 300 Daily 200
100
0 1969 1974 1979 1984 1989 Year
Figure 86: Linear-scale hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
106 919312A Elizabeth Ck at Greenmantle (620 km2) 3 −1 103 Threshold = 25 m s Interflood period = 30 days ) 1 − s
3 102 m (
streamflow
101 maximum
Daily
100 1969 1974 1979 1984 1989 Year
Figure 87: Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold analysis.
104 919312A Elizabeth Ck at Greenmantle 620 km2 101 ) 2 − ) 1 − km s 1
3 3 −
10 s m 3 (
m
0 ( Q
10 Q
102 1.1 1.2 1.5 2 3 4 5 10 20 50 100 Average Return Interval (Years)
Figure 88: Fitted flood frequency curve for station 919312A. Dashed lines indicate a 95% confidence interval for the prediction. Note curve is only fitted to events with an average recurrence interval ≥ 1 year.
107 108