a statistical analysis of flood hydrology and bankfull discharge for … · 2010-12-16 · a...

Paul Rustomji

A statistical analysis of flood hydrology and bankfull

discharge for the Mitchell River catchment, Queensland,

Australia

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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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






16 Fitted flood frequency curve for station 919002A. Dashed lines indicate a 95%






















vi































vii
































viii






























ix

72 Fitted flood frequency curve for station 919305B. Dashed lines indicate a 95%






























x






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















xii



























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



xiii






41 Flood peaks identified by the peaks over threshold analysis for station 919311A.101



43 Flood peaks identified by the peaks over threshold analysis for station 919312A.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

http://www.bom.gov.au/climate/averages/

919309A

919204A

919012A

919006A

919014A919011A

919312A

919008A

919002A

919305B919311A

919013A

919007A919005A

919001C

919201A

919205A

919009A

919310A

919003A

Kowanyama

140° E120° E

-10° S

-30° S

Palmer River

Lynd River

Tate River

Mitchell

Walsh River

River

Mitchell R

iver

Alice

River

Nassau River

GU

LF

OF

C

AR

PE

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RIA

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SEA

Mitchell River

catchment

Australia

1200 mm

1000 mm

800 mm

800 mm

800 mm

1200 mm

2000 mm

1000 mm

1000 mm

1200 mm

Dimbulah

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).

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umga

uge

stag

e.

6

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:

t =n + 0.2r − 0.4

(1)

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:

x(F) =

ξ + α

κ (1 − (1 − F)κ), κ = 0

ξ − αlog(1 − F), κ = 0(2)

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.

−0.2

−0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

L−K

urto

sis

−0.4 −0.2 0.0 0.2 0.4 0.6 0.8

L−Skewness

Generalised LogarithmicGeneralised Extreme ValueGeneralised ParetoLog NormalPearson Type IIIPeaks Over Threshold Series

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

102

103

104

Q (

m3 s−1

)

1.1 1.2 1.5 2 3 4 5 10 20 50 100Average Return Interval (Years)

919001C Mary Creek at Mary Farms88 km2

101

102

Q (

m3 s−1

km−2

)

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

102

103

104

Q (

m3 s−1

)


919013A McLeod River at Mulligan HWY530 km2

100

101

Q (

m3 s−1

km−2

)

102

103

104

Q (

m3 s−1

)


919014A Mitchell River at Cooktown Crossing2574 km2

10−1

100

Q (

m3 s−1

km−2

)

102

103

104

Q (

m3 s−1

)


919201A Palmer River at Goldfields530 km2

100

101

Q (

m3 s−1

km−2

)102

103

104

Q (

m3 s−1

)1.1 1.2 1.5 2 3 4 5 10 20 50 100

Average Return Interval (Years)

919204A Palmer River at Drumduff7750 km2

10−1

100

Q (

m3 s−1

km−2

)

101

102

103

Q (

m3 s−1

)


919205A North Palmer River at 4.8 Km430 km2

10−1

100

Q (

m3 s−1

km−2

)

101

102

103

104

Q (

m3 s−1

)


919305B Walsh River at Nullinga325 km2

10−1

100

101

Q (

m3 s−1

km−2

)102

103

104

Q (

m3 s−1

)


919309A Walsh River at Trimbles Crossing9040 km2

10−1

100

Q (

m3 s−1

km−2

)

102

103

104

Q (

m3 s−1

)


919310A Walsh River at Rookwood5025 km2

10−1

100

Q (

m3 s−1

km−2

)

102

103

104

Q (

m3 s−1

)


919311A Walsh River at Flatrock2770 km2

10−1

100

Q (

m3 s−1

km−2

)

102

103

104

Q (

m3 s−1

)


919312A Elizabeth Ck at Greenmantle620 km2

100

101

Q (

m3 s−1

km−2

)

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

9190

01C

9190

05A

9190

14A

9190

03A

9190

11A

9190

09A

0

1000

2000

3000

4000

Q2 (

m3 /s

)

9190

01C

9190

05A

9190

14A

9190

03A

9190

11A

9190

09A

0

1000

2000

3000

4000

5000

6000

Q5 (

m3 /s

)

9190

01C

9190

05A

9190

14A

9190

03A

9190

11A

9190

09A

0

2000

4000

6000

8000

Q10

(m3 /s

)

9190

01C

9190

05A

9190

14A

9190

03A

9190

11A

9190

09A

0

2000

4000

6000

8000

10000

Q20

(m3 /s

)

9190

01C

9190

05A

9190

14A

9190

03A

9190

11A

9190

09A

0

2000

4000

6000

8000

10000

12000

Q50

(m3 /s

)

9190

01C

9190

05A

9190

14A

9190

03A

9190

11A

9190

09A

0

2000

4000

6000

8000

10000

12000

14000

Q10

0 (m

3 /s)

9190

01C

9190

05A

9190

14A

9190

03A

9190

11A

9190

09A

0

2

4

6

8

MA

F (x

106 M

L)

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.

Recurrence interval (yrs) bDaly bMitchellbMitchell

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

0 1000 2000 3000 4000 5000

Observed quantile (m3s−1)

0

1000

2000

3000

4000

5000

Pre

dic

ted

qu

anti

le (

m3 s−1

) Q2 = 0.018 area * rain

Adj. R2 = 0.95

0 1000 2000 3000 4000 5000 6000 7000


0

2000

4000

6000

8000

Pre

dic

ted

qu

anti

le (

m3 s−1

) Q5 = 0.032 area * rain

Adj. R2 = 0.95

0 2000 4000 6000 8000 10000


0

2000

4000

6000

8000

10000

12000

Pre

dic

ted

qu

anti

le (

m3 s−1

)

919009A

Q10 = 0.045 area * rain

Adj. R2 = 0.93

0 2000 4000 6000 8000 10000 12000


0

2000

4000

6000

8000

10000

12000

14000

Pre

dic

ted

qu

anti

le (

m3 s−1

)

919009A

Q20 = 0.058 area * rain

Adj. R2 = 0.92

0 2000 4000 6000 8000 10000 12000


0

5000

10000

15000

Pre

dic

ted

qu

anti

le (

m3 s−1

)

919009A

Q25 = 0.062 area * rain

Adj. R2 = 0.92

0 2000 4000 6000 8000 12000


0

5000

10000

15000

20000

Pre

dic

ted

qu

anti

le (

m3 s−1

)

919009A

Q50 = 0.074 area * rain

Adj. R2 = 0.91

0 5000 10000 15000 20000


0

5000

10000

15000

20000

25000

Pre

dic

ted

qu

anti

le (

m3 s−1

)

919009A

Q100 = 0.088 area * rain

Adj. R2 = 0.9

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

0 50 100 150 200 250 300Chainage (m)

0

2

4

6

8

Hei

ght (

m)

919001C Mary Creek at Mary FarmsArea = 88 km2

0 200 400 600 800

Q (m3s−1)

0 50 100 150 200 250 300 350Chainage (m)

0

5

10

15

Hei

ght (

m)

919002A Lynd River at LyndbrookArea = 1215 km2

0 500 1000 1500 2000 2500

Q (m3s−1)

0 100 200 300 400 500Chainage (m)

0

5

10

15

20

25

Hei

ght (

m)

919003A Mitchell River at O.K. BrArea = 7535 km2

0 2000 4000 6000 8000 10000 12000

Q (m3s−1)

0 100 200 300 400 500 600Chainage (m)

0

5

10

15

Hei

ght (

m)

919005A Rifle Ck at FonthillArea = 365 km2

0 200 400 600 800 1000

Q (m3s−1)

0 100 200 300 400Chainage (m)

0

10

20

30

40

50

Hei

ght (

m)

919006A Lynd River at TorwoodArea = 4325 km2

0 1000 2000 3000 4000 5000

Q (m3s−1)

0 100 200 300 400Chainage (m)

0

5

10

15

Hei

ght (

m)

919007A Hodgkinson River at Piggy HutArea = 1720 km2

0 1000 2000 3000 4000 5000

Q (m3s−1)

0 50 100 150 200 250 300Chainage (m)

0

5

10

15

20

25

Hei

ght (

m)

919008A Tate River at TorwoodArea = 4350 km2

0 500 1000 1500 2000

Q (m3s−1)

0 200 400 600Chainage (m)

0

5

10

15

Hei

ght (

m)

919009A Mitchell River at KoolatahArea = 46050 km2

0 2000 4000 6000 8000

Q (m3s−1)

0 100 200 300 400 500Chainage (m)

0

5

10

15

20

25

Hei

ght (

m)

919011A Mitchell River at GamboolaArea = 20460 km2

0 2000 4000 6000 8000 10000 12000

Q (m3s−1)

0 20 40 60 80 100 120 140Chainage (m)

0

2

4

6

8

10

12

Hei

ght (

m)

919012A Galvin Ck at Reid Ck JunctionArea = 163 km2

0 200 400 600 800

Q (m3s−1)

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

0 100 200 300 400 500 600Chainage (m)

0

5

10

15

Hei

ght (

m)

919013A McLeod River at Mulligan HWYArea = 530 km2

0 500 1000 1500 2000 2500 3000 3500

Q (m3s−1)

−200 0 200 400 600 800 1000 1200Chainage (m)

0

5

10

15

Hei

ght (

m)

919014A Mitchell River at Cooktown CrossingArea = 2574 km2

0 500 1000 1500 2000 2500 3000

Q (m3s−1)

0 50 100 150 200 250Chainage (m)

0

5

10

15

20

25

30

Hei

ght (

m)

919201A Palmer River at GoldfieldsArea = 530 km2

0 500 1000 1500 2000

Q (m3s−1)

0 100 200 300 400 500Chainage (m)

0

5

10

15

20

Hei

ght (

m)

919204A Palmer River at DrumduffArea = 7750 km2

0 1000 2000 3000 4000 5000

Q (m3s−1)

0 50 100 150 200Chainage (m)

0

5

10

15

20

Hei

ght (

m)

919205A North Palmer River at 4.8 KmArea = 430 km2

0 200 400 600 800 1000 1200 1400

Q (m3s−1)

0 50 100 150 200 250Chainage (m)

0

2

4

6

8

10

12

Hei

ght (

m)

919305B Walsh River at NullingaArea = 325 km2

0 500 1000 1500

Q (m3s−1)

0 100 200 300 400 500Chainage (m)

0

5

10

15

20

25

Hei

ght (

m)

919309A Walsh River at Trimbles CrossingArea = 9040 km2

0 1000 2000 3000 4000 5000

Q (m3s−1)

0 100 200 300 400Chainage (m)

0

5

10

15

20

25

30

Hei

ght (

m)

919310A Walsh River at RookwoodArea = 5025 km2

0 1000 2000 3000 4000 5000 6000 7000

Q (m3s−1)

0 50 100 150 200 250 300Chainage (m)

0

5

10

15

20

25

Hei

ght (

m)

919311A Walsh River at FlatrockArea = 2770 km2

0 1000 2000 3000 4000

Q (m3s−1)

0 50 100 150 200 250 300 350Chainage (m)

0

5

10

15

20

25

Hei

ght (

m)

919312A Elizabeth Ck at GreenmantleArea = 620 km2

0 200 400 600 800 1000 1200

Q (m3s−1)

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.

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an example from the Mitchell fluvial megafan, Queensland, Earth Surface Processes and

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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

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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

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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

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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

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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

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over-threshold modeling, Journal of Hydrology 225: 103–117.

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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

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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

A 919001C Mary Creek at Mary Farms

ReturnPeriod Q Water(years) (m3s−1) Year Year

0.55 25.9 1986 1985/19860.57 27.5 1982 1981/19820.58 30.2 1972 1971/19720.60 30.9 1984 1983/19840.62 31.6 1978 1977/19780.64 34.6 1979 1978/19790.66 36.2 1973 1973/19740.68 36.2 1977 1976/19770.71 37.0 1978 1977/19780.73 37.0 1981 1981/19820.76 45.6 1986 1985/19860.79 46.7 1975 1974/19750.82 47.7 1981 1980/19810.86 58.3 1985 1984/19850.89 70.7 1975 1975/19760.94 71.7 1978 1977/19780.98 80.1 1973 1972/19731.03 80.1 1985 1984/19851.09 83.4 1983 1982/19831.15 103.8 1973 1972/19731.22 109.1 1976 1975/19761.29 110.2 1987 1986/19871.38 113.1 1976 1975/19761.49 116.9 1982 1981/19821.60 125.4 1972 1971/19721.74 125.4 1980 1979/19801.91 127.5 1980 1979/19802.10 129.7 1979 1978/19792.35 133.0 1974 1973/19742.66 155.0 1985 1984/19853.06 157.4 1981 1980/19813.61 164.8 1970 1969/19704.39 171.1 1977 1976/19775.61 250.0 1975 1974/19757.77 302.7 1981 1980/1981

12.62 330.9 1971 1970/197133.67 708.0 1979 1978/1979

Table 5: Flood peaks identified by the peaksover threshold analysis for station 919001C.

Return Lower Estimated Upper

Period C.I. Quantile C.I.

(years) (m3s−1) (m3s−1) (m3s−1)

2 118 128 141

5 171 205 239

10 221 288 345

20 273 405 513

25 285 451 588

50 332 630 875

100 340 879 1366

Table 6: Fitted flood quantiles for station

919001C. Values have been reported to four

significant digits.

29

0

1

2

3

4

5

mea

n nu

mbe

r of

floo

ds/y

ear

0 20 40 60 80 100 140 180

threshold (m3 s−1)

0

50

100

150

200

250

mea

n ex

ceed

ance

(m3 s

−1)

919001C

Figure 9: Threshold selection steps.

1970 1975 1980 1985

Year

0

100

200

300

400

500

600

700

800

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919001C Mary Creek at Mary Farms (88 km2)Threshold = 25 m3 s−1 Interflood period = 15 days

Figure 10: Linear-scale hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold

analysis.

30

1970 1975 1980 1985

Year

100

101

102

103

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919001C Mary Creek at Mary Farms (88 km2)Threshold = 25 m3 s−1 Interflood period = 15 days

Figure 11: Log-scaled hydrograph showing peaks (shown by ♢ symbols) identified in the peaks over threshold

analysis.

102

103

104

Q (

m3 s−1

)


919001C Mary Creek at Mary Farms 88 km2

101

102

Q (

m3 s−1

km−2

)

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

B 919002A Lynd River at Lyndbrook


0.59 59.6 1969 1968/19690.61 59.6 1987 1986/19870.62 61.7 1976 1975/19760.64 68.1 1975 1975/19760.65 69.3 1988 1988/19890.67 71.6 1983 1982/19830.69 78.7 1991 1991/19920.71 79.9 1985 1985/19860.73 81.2 1974 1974/19750.75 82.4 1969 1969/19700.77 86.3 1968 1967/19680.80 95.8 1985 1984/19850.82 116.7 1968 1967/19680.85 131.6 1975 1974/19750.88 136.8 1970 1970/19710.91 153.3 1970 1969/19700.95 161.1 1990 1989/19900.98 165.0 1981 1981/19821.02 167.8 1977 1976/19771.07 187.9 1986 1985/19861.12 190.9 1973 1972/19731.17 190.9 1989 1989/19901.22 195.4 1989 1988/19891.29 207.7 1976 1975/19761.35 209.2 1975 1974/19751.43 236.6 1976 1975/19761.52 271.1 1990 1989/19901.62 294.7 1988 1987/19881.73 353.0 1977 1976/19771.85 353.0 1979 1978/19792.00 369.5 1971 1970/19712.17 388.4 1968 1967/19682.38 403.5 1977 1976/19772.62 618.5 1975 1974/19752.93 642.0 1974 1973/19743.32 674.7 1989 1989/19903.82 681.3 1980 1979/19804.50 741.9 1986 1985/19865.48 797.4 1979 1978/19797.00 801.8 1992 1991/19929.69 813.8 1981 1980/1981

15.75 1502.4 1991 1990/199142.00 1662.7 1984 1983/1984

Table 7: Flood peaks identified by the peaksover threshold analysis for station 919002A.



(years) (m3s−1) (m3s−1) (m3s−1)

2 323 388 452

5 612 748 875

10 837 1033 1216

20 1043 1331 1617

25 1106 1430 1754

50 1247 1745 2249

100 1360 2073 2791


919002A. Values have been reported to four

significant digits.

33

0.0

0.5

1.0

1.5

2.0

2.5

3.0

mea

n nu

mbe

r of

floo

ds/y

ear

0 50 100 200 300 400 500


0

100

200

300

400

500

mea

n ex

ceed

ance

(m3 s

−1)

919002A


1968 1973 1978 1983 1988 1993

Year

0

200

400

600

800

1000

1200

1400

1600

1800

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919002A Lynd River at Lyndbrook (1215 km2)Threshold = 50 m3 s−1 Interflood period = 15 days


analysis.

34

1968 1973 1978 1983 1988 1993

Year

100

101

102

103

104

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919002A Lynd River at Lyndbrook (1215 km2)Threshold = 50 m3 s−1 Interflood period = 15 days


analysis.

102

103

104

Q (

m3 s−1

)


919002A Lynd River at Lyndbrook 1215 km2

10−1

100Q

(m

3 s−1km

−2)

Figure 16: Fitted flood frequency curve for station 919002A. Dashed lines indicate a 95% confidence interval for


35

C 919003A Mitchell River at O.K. Br


0.70 153 1976 1976/19770.71 164 1968 1967/19680.72 171 1984 1984/19850.73 173 1982 1982/19830.75 173 1995 1995/19960.76 179 1986 1986/19870.77 195 1993 1993/19940.79 203 2003 2002/20030.80 217 2005 2004/20050.82 245 1982 1981/19820.83 262 1969 1969/19700.85 272 1973 1973/19740.87 289 1994 1993/19940.89 307 1982 1981/19820.91 340 2000 2000/20010.93 407 1981 1981/19820.95 413 2002 2001/20020.97 415 1970 1969/19700.99 443 1970 1969/19701.01 489 1989 1989/19901.04 506 2006 2005/20061.07 506 2004 2004/20051.09 518 1982 1981/19821.12 530 1978 1977/19781.15 546 1990 1989/19901.19 552 1969 1968/19691.22 555 1988 1987/19881.26 577 1983 1982/19831.29 577 1993 1992/19931.34 667 1975 1974/19751.38 680 1992 1991/19921.43 756 1983 1982/19831.48 770 1998 1997/19981.53 779 1987 1986/19871.59 806 1987 1987/19881.65 861 1986 1985/19861.72 880 2003 2002/20031.79 1123 1988 1988/19891.87 1294 1976 1975/19761.95 1447 1969 1968/19692.05 1455 1984 1983/19842.15 1644 2006 2005/20062.27 1701 1968 1967/19682.40 1745 1973 1972/19732.54 1793 2006 2005/20062.71 1870 1995 1994/19952.89 2030 2004 2003/20043.10 2054 1980 1979/19803.35 2308 1981 1980/19813.64 2609 1974 1973/19743.98 3427 2001 2000/20014.40 3452 1996 1995/19964.91 3496 1971 1970/19715.55 3907 1977 1976/19776.39 3919 2007 2006/20077.54 4012 2008 2007/20089.17 4468 2009 2008/2009

11.72 4504 1972 1971/197216.23 5658 2000 1999/200026.38 6349 1999 1998/199970.33 8165 1979 1978/1979

Table 9: Flood peaks identified by the peaksover threshold analysis for station 919003A.



(years) (m3s−1) (m3s−1) (m3s−1)

2 1131 1416 1706

5 2442 3079 3729

10 3471 4436 5373

20 4425 5885 7333

25 4718 6371 7956

50 5479 7950 10499

100 6133 10000 12997



significant digits.

37

0.0

0.5

1.0

1.5

2.0

2.5

mea

n nu

mbe

r of

floo

ds/y

ear

0 200 600 1000 1400 1800


0

500

1000

1500

2000

2500

mea

n ex

ceed

ance

(m3 s

−1)

919003A


1967 1972 1977 1982 1987 1992 1997 2002 2007

Year

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919003A Mitchell River at O.K. Br (7535 km2)Threshold = 150 m3 s−1 Interflood period = 30 days


analysis.

38

1967 1972 1977 1982 1987 1992 1997 2002 2007

Year

100

101

102

103

104

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919003A Mitchell River at O.K. Br (7535 km2)Threshold = 150 m3 s−1 Interflood period = 30 days


analysis.

102

103

104

105

Q (

m3 s−1

)


919003A Mitchell River at O.K. Br 7535 km2

10−1

100

101

Q (

m3 s−1

km−2

)



39

D 919005A Rifle Ck at Fonthill


0.77 22.3 1983 1982/19830.78 25.1 1970 1970/19710.80 27.8 1992 1991/19920.81 28.4 1981 1980/19810.83 30.3 1975 1974/19750.85 38.8 1988 1987/19880.87 40.8 2006 2005/20060.88 42.2 1996 1996/19970.90 46.4 1969 1968/19690.92 50.9 2002 2001/20020.94 58.7 1976 1976/19770.97 59.3 1990 1989/19900.99 61.1 1970 1969/19701.01 61.6 1993 1992/19931.04 71.7 2000 2000/20011.07 77.0 1984 1983/19841.10 78.7 1972 1971/19721.13 80.7 2003 2002/20031.16 88.4 2005 2004/20051.19 95.1 1986 1985/19861.23 101.2 2008 2007/20081.26 107.9 1980 1979/19801.30 110.6 1978 1977/19781.35 115.2 2005 2004/20051.39 121.0 1970 1969/19701.44 130.4 1969 1968/19691.49 153.2 2009 2008/20091.55 165.2 2007 2006/20071.61 189.2 1975 1974/19751.67 189.7 1994 1993/19941.75 213.7 1987 1986/19871.82 217.9 1982 1981/19821.91 234.3 1997 1996/19972.00 247.2 1989 1988/19892.10 255.8 1991 1990/19912.22 273.6 1985 1984/19852.34 281.8 2001 2000/20012.48 293.9 1972 1971/19722.64 294.9 1990 1989/19902.82 302.6 2008 2007/20083.03 319.1 1977 1976/19773.27 341.2 2000 1999/20003.55 370.4 1983 1982/19833.89 431.3 1976 1975/19764.29 434.6 1971 1970/19714.79 437.6 2006 2005/20065.42 439.0 1998 1997/19986.24 473.3 1974 1973/19747.36 517.7 1981 1980/19818.96 519.6 1973 1972/1973

11.44 521.5 2004 2003/200415.85 618.7 1995 1994/199525.75 620.7 1999 1998/199968.67 876.9 1979 1978/1979

Table 11: Flood peaks identified by thepeaks over threshold analysis for station919005A.



(years) (m3s−1) (m3s−1) (m3s−1)

2 193 233 273

5 371 435 499

10 478 557 636

20 567 658 745

25 585 687 792

50 640 766 904

100 669 832 1003



significant digits.

41

0.0

0.5

1.0

1.5

2.0

2.5

3.0

mea

n nu

mbe

r of

floo

ds/y

ear

0 20 40 60 80 100 140 180


0

50

100

150

200

250

mea

n ex

ceed

ance

(m3 s

−1)

919005A


1968 1973 1978 1983 1988 1993 1998 2003 2008

Year

0

100

200

300

400

500

600

700

800

900

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919005A Rifle Ck at Fonthill (365 km2)Threshold = 20 m3 s−1 Interflood period = 30 days


analysis.

42

1968 1973 1978 1983 1988 1993 1998 2003 2008

Year

100

101

102

103

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919005A Rifle Ck at Fonthill (365 km2)Threshold = 20 m3 s−1 Interflood period = 30 days


analysis.

101

102

103

104

Q (

m3 s−1

)


919005A Rifle Ck at Fonthill 365 km2

10−1

100

101

Q (

m3 s−1

km−2

)



43

E 919006A Lynd River at Torwood


0.65 73.9 1978 1978/19790.67 75.8 1976 1976/19770.69 81.6 1982 1981/19820.72 94.1 1983 1983/19840.74 118.5 1969 1968/19690.77 143.7 1981 1981/19820.80 153.7 1969 1968/19690.83 191.1 1985 1984/19850.86 192.8 1985 1984/19850.90 201.3 1987 1987/19880.94 217.1 1983 1982/19830.98 229.9 1978 1977/19781.03 237.4 1982 1981/19821.08 341.9 1984 1984/19851.14 358.8 1970 1969/19701.20 404.3 1987 1986/19871.28 473.2 1977 1976/19771.36 514.7 1983 1982/19831.45 854.3 1977 1977/19781.56 879.4 1978 1977/19781.68 883.6 1971 1970/19711.83 900.6 1977 1976/19772.00 913.3 1976 1975/19762.21 1194.2 1980 1979/19802.47 1475.4 1986 1985/19862.79 2036.0 1979 1978/19793.21 2064.1 1988 1987/19883.79 2171.6 1984 1983/19844.61 2327.7 1972 1971/19725.89 2482.7 1973 1972/19738.15 2635.4 1981 1980/1981

13.25 3752.4 1975 1974/197535.33 4402.4 1974 1973/1974




(years) (m3s−1) (m3s−1) (m3s−1)

2 908 1152 1399

5 1954 2370 2778

10 2654 3193 3713

20 3252 3941 4600

25 3416 4166 4920

50 3723 4825 5896

100 4097 5423 6811



significant digits.

45

0.0

0.5

1.0

1.5

2.0

2.5

mea

n nu

mbe

r of

floo

ds/y

ear

0 50 100 200 300 400 500


0

200

400

600

800

1000

1200

1400

mea

n ex

ceed

ance

(m3 s

−1)

919006A


1968 1973 1978 1983 1988

Year

0

500

1000

1500

2000

2500

3000

3500

4000

4500

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919006A Lynd River at Torwood (4325 km2)Threshold = 70 m3 s−1 Interflood period = 30 days


analysis.

46

1968 1973 1978 1983 1988

Year

100

101

102

103

104

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919006A Lynd River at Torwood (4325 km2)Threshold = 70 m3 s−1 Interflood period = 30 days


analysis.

102

103

104

Q (

m3 s−1

)


919006A Lynd River at Torwood 4325 km2

10−1

100

Q (

m3 s−1

km−2

)



47

F 919007A Hodgkinson River at Piggy Hut


0.69 30.1 1969 1968/19690.72 30.6 1979 1979/19800.74 36.0 1970 1970/19710.77 41.0 1969 1968/19690.80 51.0 1983 1982/19830.83 51.6 1983 1982/19830.86 56.1 1988 1987/19880.90 65.1 1982 1981/19820.94 72.3 1984 1984/19850.98 92.8 1983 1982/19831.03 143.1 1985 1984/19851.08 171.3 1970 1969/19701.14 198.6 1981 1981/19821.20 198.6 1982 1981/19821.28 262.1 1970 1969/19701.36 385.7 1975 1974/19751.45 385.7 1978 1977/19781.56 419.1 1990 1989/19901.68 444.3 1987 1987/19881.83 484.6 1972 1971/19722.00 515.1 1976 1975/19762.21 579.4 1986 1985/19862.47 690.3 1984 1983/19842.79 722.1 1980 1979/19803.21 1030.1 1981 1980/19813.79 1161.8 1973 1972/19734.61 1286.7 1971 1970/19715.89 1435.5 1974 1973/19748.15 2375.7 1977 1976/1977

13.25 2955.2 1972 1971/197235.33 2978.9 1979 1978/1979




(years) (m3s−1) (m3s−1) (m3s−1)

2 450 572 699

5 1022 1315 1607

10 1498 1973 2426

20 1953 2727 3420

25 2076 2993 3833

50 2409 3896 5277

100 2682 4931 7195



significant digits.

49

0.0

0.5

1.0

1.5

2.0

mea

n nu

mbe

r of

floo

ds/y

ear

0 50 100 200 300 400 500


0

200

400

600

800

1000

mea

n ex

ceed

ance

(m3 s

−1)

919007A


1968 1973 1978 1983 1988

Year

0

500

1000

1500

2000

2500

3000

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919007A Hodgkinson River at Piggy Hut (1720 km2)Threshold = 30 m3 s−1 Interflood period = 30 days


analysis.

50

1968 1973 1978 1983 1988

Year

100

101

102

103

104

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919007A Hodgkinson River at Piggy Hut (1720 km2)Threshold = 30 m3 s−1 Interflood period = 30 days


analysis.

102

103

104

Q (

m3 s−1

)


919007A Hodgkinson River at Piggy Hut 1720 km2

10−1

100

Q (

m3 s−1

km−2

)



51

G 919008A Tate River at Torwood


0.58 55.7 1985 1984/19850.60 56.7 1982 1982/19830.62 57.0 1987 1987/19880.65 67.0 1983 1982/19830.67 69.3 1977 1977/19780.70 79.7 1978 1977/19780.73 90.1 1977 1976/19770.76 114.7 1983 1983/19840.80 122.8 1973 1973/19740.83 151.1 1985 1984/19850.88 152.8 1975 1974/19750.92 157.7 1981 1981/19820.98 178.0 1982 1981/19821.04 190.7 1984 1984/19851.10 293.4 1983 1982/19831.18 310.4 1973 1972/19731.26 316.2 1978 1977/19781.37 347.0 1975 1975/19761.48 434.6 1987 1986/19871.62 435.5 1980 1979/19801.79 475.7 1982 1981/19822.00 670.2 1981 1980/19812.26 750.4 1984 1983/19842.61 801.8 1972 1971/19723.07 815.0 1986 1985/19863.74 851.0 1979 1978/19794.78 961.6 1977 1976/19776.62 1434.3 1975 1974/1975

10.75 1483.6 1988 1987/198828.67 1669.0 1974 1973/1974




(years) (m3s−1) (m3s−1) (m3s−1)

2 504 594 689

5 918 1079 1236

10 1191 1393 1593

20 1403 1670 1925

25 1452 1751 2039

50 1618 1984 2349

100 1730 2188 2692



significant digits.

53

0.0

0.5

1.0

1.5

2.0

2.5

mea

n nu

mbe

r of

floo

ds/y

ear

0 50 100 200 300 400 500


0

100

200

300

400

500

600

700

mea

n ex

ceed

ance

(m3 s

−1)

919008A


1972 1977 1982 1987

Year

0

200

400

600

800

1000

1200

1400

1600

1800

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919008A Tate River at Torwood (4350 km2)Threshold = 50 m3 s−1 Interflood period = 30 days


analysis.

54

1972 1977 1982 1987

Year

100

101

102

103

104

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919008A Tate River at Torwood (4350 km2)Threshold = 50 m3 s−1 Interflood period = 30 days


analysis.

102

103

104

Q (

m3 s−1

)


919008A Tate River at Torwood 4350 km2

10−1

100

Q (

m3 s−1

km−2

)



55

H 919009A Mitchell River at Koolatah


0.99 272 2003 2002/20031.02 349 1985 1984/19851.05 377 1993 1992/19931.08 446 1984 1984/19851.11 739 1982 1981/19821.15 889 1988 1987/19881.18 903 1983 1982/19831.22 1061 1983 1982/19831.27 1076 1981 1981/19821.31 1487 1987 1986/19871.36 1646 2003 2002/20031.41 1668 1978 1977/19781.47 2666 2004 2003/20041.53 2701 1986 1985/19861.60 2731 2005 2004/20051.68 3297 2009 2008/20091.76 3414 1988 1987/19881.85 3738 1980 1979/19801.95 3951 1997 1996/19972.06 4168 1984 1983/19842.18 4306 1975 1974/19752.32 4518 1973 1972/19732.48 4590 1976 1975/19762.66 4666 1995 1994/19952.87 4722 2002 2001/20023.12 4828 1996 1995/19963.42 5001 2006 2005/20063.77 5293 1998 1997/19984.21 5859 1981 1980/19814.76 6011 2007 2006/20075.48 6074 2001 2000/20016.46 6081 1999 1998/19997.87 6140 1977 1976/1977

10.06 6183 1979 1978/197913.92 6255 2008 2007/200822.62 6270 1974 1973/197460.33 6358 2000 1999/2000




(years) (m3s−1) (m3s−1) (m3s−1)

2 3222 3845 4497

5 5428 5806 6192

10 6041 6300 6552

20 6245 6497 6763

25 6272 6530 6814

50 6284 6588 6927

100 6290 6611 6985



significant digits.

57

0.0

0.5

1.0

1.5

2.0

mea

n nu

mbe

r of

floo

ds/y

ear

0 200 600 1000 1400 1800


0

1000

2000

3000

4000

mea

n ex

ceed

ance

(m3 s

−1)

919009A


1972 1977 1982 1987 1992 1997 2002 2007

Year

0

1000

2000

3000

4000

5000

6000

7000

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919009A Mitchell River at Koolatah (46050 km2)Threshold = 200 m3 s−1 Interflood period = 30 days


analysis.

58

1972 1977 1982 1987 1992 1997 2002 2007

Year

100

101

102

103

104

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919009A Mitchell River at Koolatah (46050 km2)Threshold = 200 m3 s−1 Interflood period = 30 days


analysis.

102

103

104

Q (

m3 s−1

)


919009A Mitchell River at Koolatah 46050 km2

10−2

10−1

Q (

m3 s−1

km−2

)



59

I 919011A Mitchell River at Gamboola


0.79 224 1976 1976/19770.80 245 1988 1988/19890.82 287 1988 1987/19880.84 366 1990 1989/19900.86 380 1984 1984/19850.88 411 1985 1984/19850.90 434 1983 1982/19830.92 639 1977 1976/19770.94 686 1982 1981/19820.96 739 1983 1982/19830.99 771 1978 1977/19781.02 877 2005 2004/20051.04 904 1982 1981/19821.07 957 1994 1993/19941.10 999 1981 1981/19821.14 1040 2003 2002/20031.17 1078 1987 1986/19871.21 1115 1987 1987/19881.25 1238 1990 1989/19901.29 1277 1989 1989/19901.34 1343 2002 2001/20021.38 1382 1992 1991/19921.44 1433 1993 1992/19931.49 1558 1975 1974/19751.55 1622 1984 1983/19841.62 1770 2006 2005/20061.69 1902 2004 2003/20041.77 1961 1988 1987/19881.85 2144 1988 1988/19891.95 2188 1980 1979/19802.05 2466 1973 1972/19732.17 2895 2006 2005/20062.30 2901 1981 1980/19812.45 2950 1995 1994/19952.62 3065 1986 1985/19862.81 3150 1976 1975/19763.03 3888 1998 1997/19983.29 4246 1997 1996/19973.60 4740 1996 1995/19963.98 4770 2001 2000/20014.44 5624 2008 2007/20085.03 5816 1974 1973/19745.79 6602 2007 2006/20076.82 7047 2009 2008/20098.30 7499 1977 1976/1977

10.61 8233 2000 1999/200014.69 8287 1979 1978/197923.88 8882 1972 1971/197263.67 9023 1999 1998/1999




(years) (m3s−1) (m3s−1) (m3s−1)

2 2084 2573 3066

5 4235 5176 6101

10 5839 7071 8241

20 7156 8904 10611

25 7486 9482 11399

50 8502 11236 13982

100 9214 12934 16996



significant digits.

61

0.0

0.5

1.0

1.5

2.0

mea

n nu

mbe

r of

floo

ds/y

ear

0 200 600 1000 1400 1800


0

1000

2000

3000

4000

mea

n ex

ceed

ance

(m3 s

−1)

919011A


1971 1976 1981 1986 1991 1996 2001 2006

Year

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919011A Mitchell River at Gamboola (20460 km2)Threshold = 200 m3 s−1 Interflood period = 30 days


analysis.

62

1971 1976 1981 1986 1991 1996 2001 2006

Year

100

101

102

103

104

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919011A Mitchell River at Gamboola (20460 km2)Threshold = 200 m3 s−1 Interflood period = 30 days


analysis.

102

103

104

105

Q (

m3 s−1

)


919011A Mitchell River at Gamboola 20460 km2

10−2

10−1

100

Q (

m3 s−1

km−2

)



63

J 919012A Galvin Ck at Reid Ck Junction


0.47 23.8 1973 1972/19730.50 24.0 1980 1979/19800.52 26.3 1975 1975/19760.54 27.4 1975 1974/19750.57 28.6 1974 1974/19750.60 31.4 1980 1980/19810.64 32.0 1978 1977/19780.67 35.2 1973 1972/19730.72 37.2 1982 1981/19820.77 43.4 1975 1974/19750.82 43.7 1975 1974/19750.89 60.1 1979 1979/19800.97 89.5 1977 1976/19771.06 182.5 1973 1972/19731.17 186.1 1979 1978/19791.30 192.3 1976 1975/19761.47 197.4 1972 1971/19721.70 201.3 1976 1975/19762.00 224.1 1980 1979/19802.43 279.6 1972 1971/19723.11 332.4 1981 1980/19814.31 370.5 1979 1978/19797.00 483.3 1974 1973/1974

18.67 598.5 1977 1976/1977




(years) (m3s−1) (m3s−1) (m3s−1)

2 220 240 260

5 315 364 413

10 399 476 551

20 465 605 744

25 490 651 807

50 549 809 1060

100 584 1000 1397



significant digits.

65

0.0

0.5

1.0

1.5

2.0

2.5

mea

n nu

mbe

r of

floo

ds/y

ear

0 20 40 60 80 100 140 180


0

50

100

150

200

250

mea

n ex

ceed

ance

(m3 s

−1)

919012A


1971 1976 1981

Year

0

50

100

150

200

250

300

350

400

450

500

550

600

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919012A Galvin Ck at Reid Ck Junction (163 km2)Threshold = 23 m3 s−1 Interflood period = 15 days


analysis.

66

1971 1976 1981

Year

100

101

102

103

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919012A Galvin Ck at Reid Ck Junction (163 km2)Threshold = 23 m3 s−1 Interflood period = 15 days


analysis.

102

103

104

Q (

m3 s−1

)


919012A Galvin Ck at Reid Ck Junction 163 km2

100

101

Q (

m3 s−1

km−2

)



67

K 919013A McLeod River at Mulligan HWY


0.82 52.1 1983 1983/19840.84 52.6 2007 2006/20070.86 58.1 2006 2005/20060.89 73.4 1985 1984/19850.91 78.6 1987 1987/19880.93 84.9 1988 1988/19890.96 85.2 1989 1989/19900.99 85.5 1990 1989/19901.02 87.1 1984 1983/19841.05 89.5 1999 1998/19991.08 93.4 1986 1985/19861.12 100.8 1985 1984/19851.16 120.6 2005 2004/20051.20 140.8 2000 1999/20001.24 182.9 1991 1990/19911.29 187.5 1990 1989/19901.34 215.1 1980 1979/19801.39 233.0 2009 2008/20091.45 240.3 1976 1975/19761.51 257.2 1983 1982/19831.58 267.9 2000 2000/20011.66 274.8 1977 1976/19771.74 275.8 2005 2004/20051.84 278.7 1987 1986/19871.94 291.3 2007 2006/20072.06 301.2 1998 1997/19982.19 344.8 2006 2005/20062.34 421.3 1974 1973/19742.51 435.8 1978 1977/19782.71 448.5 1986 1985/19862.95 591.3 1975 1974/19753.23 840.7 1980 1979/19803.56 994.0 2006 2005/20063.98 1039.0 1981 1980/19814.50 1246.6 2004 2003/20045.18 1315.1 1989 1988/19896.11 1342.7 1973 1972/19737.43 1526.0 2001 2000/20019.50 1599.0 2008 2007/2008

13.15 1804.6 2000 1999/200021.38 2524.1 1999 1998/199957.00 2799.0 1979 1978/1979




(years) (m3s−1) (m3s−1) (m3s−1)

2 314 411 512

5 775 1006 1221

10 1153 1536 1884

20 1504 2147 2755

25 1622 2363 3084

50 1953 3101 4280

100 2064 3952 5850



significant digits.

69

0

1

2

3

4

mea

n nu

mbe

r of

floo

ds/y

ear

0 50 100 200 300 400 500


0

200

400

600

800

1000

mea

n ex

ceed

ance

(m3 s

−1)

919013A


1973 1978 1983 1988 1993 1998 2003 2008

Year

0

500

1000

1500

2000

2500

3000

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919013A McLeod River at Mulligan HWY (530 km2)Threshold = 50 m3 s−1 Interflood period = 30 days


analysis.

70

1973 1978 1983 1988 1993 1998 2003 2008

Year

100

101

102

103

104

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919013A McLeod River at Mulligan HWY (530 km2)Threshold = 50 m3 s−1 Interflood period = 30 days


analysis.

102

103

104

Q (

m3 s−1

)


919013A McLeod River at Mulligan HWY 530 km2

100

101

Q (

m3 s−1

km−2

)



71

L 919014A Mitchell River at Cooktown Crossing


0.56 54.7 2001 2001/20020.59 64.4 2004 2004/20050.62 69.0 2008 2008/20090.66 69.5 2002 2001/20020.69 72.7 2002 2002/20030.73 92.7 2002 2001/20020.78 93.2 2004 2003/20040.84 175.4 1999 1999/20000.90 181.3 2003 2003/20040.97 211.3 2006 2005/20061.05 211.8 2003 2002/20031.15 217.9 2005 2004/20051.27 244.8 2000 2000/20011.42 328.2 2005 2004/20051.61 401.3 2008 2007/20081.85 856.2 2007 2006/20072.18 1094.3 2006 2005/20062.65 1355.5 2004 2003/20043.39 1528.2 2009 2008/20094.69 1668.9 2001 2000/20017.62 1830.6 2008 2007/2008

20.33 1943.5 2000 1999/2000




(years) (m3s−1) (m3s−1) (m3s−1)

2 719 903 1091

5 1465 1665 1853

10 1808 1989 2158

20 2001 2187 2365

25 2031 2233 2432

50 2120 2337 2567

100 2148 2401 2695



significant digits.

73

0.0

0.5

1.0

1.5

2.0

mea

n nu

mbe

r of

floo

ds/y

ear

0 20 40 60 80 100 140 180


0

200

400

600

800

1000

1200

mea

n ex

ceed

ance

(m3 s

−1)

919014A


1999 2004 2009

Year

0

200

400

600

800

1000

1200

1400

1600

1800

2000

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919014A Mitchell River at Cooktown Crossing (2574 km2)Threshold = 50 m3 s−1 Interflood period = 15 days


analysis.

74

1999 2004 2009

Year

100

101

102

103

104

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919014A Mitchell River at Cooktown Crossing (2574 km2)Threshold = 50 m3 s−1 Interflood period = 15 days


analysis.

102

103

104

Q (

m3 s−1

)


919014A Mitchell River at Cooktown Crossing 2574 km2

10−1

100

Q (

m3 s−1

km−2

)



75

M 919201A Palmer River at Goldfields


0.78 40.5 1990 1989/19900.79 41.3 2005 2004/20050.81 45.3 1993 1993/19940.82 46.5 1985 1984/19850.84 51.0 2000 2000/20010.85 54.2 1983 1982/19830.87 66.2 2004 2004/20050.89 71.3 1969 1969/19700.91 73.7 1985 1984/19850.93 78.2 2005 2004/20050.95 81.0 1982 1981/19820.97 81.3 1994 1993/19940.99 81.7 2008 2007/20081.01 87.6 1969 1968/19691.04 89.0 1986 1986/19871.06 96.3 1987 1987/19881.09 100.7 1984 1984/19851.12 101.6 1988 1988/19891.15 102.4 2002 2001/20021.18 133.0 1970 1969/19701.21 134.8 1987 1986/19871.25 137.4 1986 1985/19861.29 141.3 1986 1985/19861.33 162.5 1970 1969/19701.37 196.5 1996 1995/19961.41 201.6 1998 1997/19981.46 210.1 2003 2002/20031.51 217.4 1992 1991/19921.57 234.8 1975 1974/19751.62 303.3 1995 1994/19951.69 307.3 1984 1983/19841.76 322.8 2006 2005/20061.83 397.5 1968 1967/19681.91 413.1 1978 1977/19782.00 423.1 1973 1972/19732.10 432.0 2006 2005/20062.20 474.4 1976 1975/19762.32 489.0 1989 1988/19892.45 506.4 1981 1980/19812.60 534.0 2006 2005/20062.77 586.6 1972 1971/19722.96 607.3 2008 2007/20083.18 627.5 1977 1976/19773.43 628.5 1991 1990/19913.72 665.5 1974 1973/19744.08 668.8 2007 2006/20074.50 671.6 2004 2003/20045.02 690.5 2001 2000/20015.68 744.9 1997 1996/19976.55 755.8 2009 2008/20097.71 966.8 2000 1999/20009.39 1029.5 1980 1979/1980

12.00 1068.0 1971 1970/197116.62 1103.5 1999 1998/199927.00 1194.2 1979 1978/197972.00 1459.7 1996 1995/1996




(years) (m3s−1) (m3s−1) (m3s−1)

2 312 391 468

5 646 764 884

10 852 1000 1117

20 1003 1172 1333

25 1046 1224 1407

50 1132 1366 1610

100 1184 1482 1785



significant digits.

77

0.0

0.5

1.0

1.5

2.0

2.5

3.0

mea

n nu

mbe

r of

floo

ds/y

ear

0 50 100 200 300 400 500


0

100

200

300

400

mea

n ex

ceed

ance

(m3 s

−1)

919201A


1967 1972 1977 1982 1987 1992 1997 2002 2007

Year

0

200

400

600

800

1000

1200

1400

1600

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919201A Palmer River at Goldfields (530 km2)Threshold = 40 m3 s−1 Interflood period = 30 days


analysis.

78

1967 1972 1977 1982 1987 1992 1997 2002 2007

Year

100

101

102

103

104

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919201A Palmer River at Goldfields (530 km2)Threshold = 40 m3 s−1 Interflood period = 30 days


analysis.

102

103

104

Q (

m3 s−1

)


919201A Palmer River at Goldfields 530 km2

100

101

Q (

m3 s−1

km−2

)



79

N 919204A Palmer River at Palmer River at Drumduff


0.93 168 1988 1987/19880.96 216 1987 1986/19870.99 230 1982 1981/19821.02 238 1981 1981/19821.06 249 2005 2004/20051.09 269 1987 1987/19881.14 317 1985 1984/19851.18 324 1982 1981/19821.23 330 2005 2004/20051.28 343 2001 2000/20011.34 432 1983 1982/19831.40 433 2003 2002/20031.47 440 1984 1984/19851.54 502 1978 1977/19781.62 502 1983 1982/19831.72 530 1986 1985/19861.82 597 2006 2005/20061.94 851 1973 1972/19732.07 869 1975 1974/19752.22 1072 1984 1983/19842.40 1178 2002 2001/20022.60 1214 1980 1979/19802.85 1440 1981 1980/19813.15 1454 1976 1975/19763.51 1941 2004 2003/20043.97 2230 2008 2007/20084.58 2376 1974 1973/19745.39 2472 1999 1998/19996.57 3085 1977 1976/19778.39 3258 2001 2000/2001

11.62 3535 2000 1999/200018.88 3781 2007 2006/200750.33 4099 1979 1978/1979




(years) (m3s−1) (m3s−1) (m3s−1)

2 737 940 1157

5 1684 2126 2560

10 2405 3033 3613

20 3045 3950 4851

25 3211 4247 5230

50 3621 5177 6598

100 4063 6116 8059



significant digits.

81

0.0

0.5

1.0

1.5

2.0

mea

n nu

mbe

r of

floo

ds/y

ear

0 200 600 1000 1400 1800


0

500

1000

1500

2000

mea

n ex

ceed

ance

(m3 s

−1)

919204A


1972 1977 1982 1987 1992 1997 2002 2007

Year

0

500

1000

1500

2000

2500

3000

3500

4000

4500

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919204A Palmer River at Drumduff (7750 km2)Threshold = 150 m3 s−1 Interflood period = 30 days


analysis.

82

1972 1977 1982 1987 1992 1997 2002 2007

Year

100

101

102

103

104

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919204A Palmer River at Drumduff (7750 km2)Threshold = 150 m3 s−1 Interflood period = 30 days


analysis.

102

103

104

Q (

m3 s−1

)


919204A Palmer River at Drumduff 7750 km2

10−1

100

Q (

m3 s−1

km−2

)



83

O 919205A North Palmer River at 4.8 Km


0.53 20.0 1982 1982/19830.55 20.4 1983 1982/19830.57 20.4 1988 1987/19880.59 20.8 1980 1979/19800.62 22.4 1987 1986/19870.64 27.3 1985 1984/19850.67 28.1 1988 1987/19880.70 31.3 1977 1977/19780.74 31.4 1987 1986/19870.78 46.3 1978 1977/19780.82 49.2 1976 1976/19770.86 56.8 1983 1982/19830.92 57.3 1975 1974/19750.97 64.8 1981 1981/19821.04 68.2 1984 1984/19851.12 72.8 1982 1981/19821.21 79.5 1975 1974/19751.31 97.1 1986 1985/19861.43 110.0 1983 1983/19841.58 115.2 1975 1974/19751.77 124.6 1986 1985/19862.00 139.4 1986 1985/19862.30 231.5 1984 1983/19842.71 312.1 1977 1976/19773.30 312.1 1981 1980/19814.22 336.0 1980 1979/19805.85 402.9 1974 1973/19749.50 418.0 1979 1978/1979

25.33 426.9 1976 1975/1976




(years) (m3s−1) (m3s−1) (m3s−1)

2 155 188 221

5 295 342 388

10 375 425 473

20 430 488 541

25 444 505 567

50 475 548 621

100 489 580 675



significant digits.

85

0.0

0.5

1.0

1.5

2.0

2.5

mea

n nu

mbe

r of

floo

ds/y

ear

0 20 40 60 80 100 140 180


0

50

100

150

200

250

mea

n ex

ceed

ance

(m3 s

−1)

919205A


1973 1978 1983 1988

Year

0

50

100

150

200

250

300

350

400

450

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919205A North Palmer River at 4.8 Km (430 km2)Threshold = 20 m3 s−1 Interflood period = 15 days


analysis.

86

1973 1978 1983 1988

Year

100

101

102

103

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919205A North Palmer River at 4.8 Km (430 km2)Threshold = 20 m3 s−1 Interflood period = 15 days


analysis.

101

102

103

Q (

m3 s−1

)


919205A North Palmer River at 4.8 Km 430 km2

10−1

100

Q (

m3 s−1

km−2

)



87

P 919305B Walsh River at Nullinga


0.77 20.0 1982 1981/19820.79 20.3 1960 1959/19600.80 20.3 1961 1961/19620.82 23.7 1973 1972/19730.84 26.4 1971 1970/19710.86 26.4 1975 1974/19750.88 26.7 1959 1958/19590.90 29.6 1963 1962/19630.92 30.0 1959 1958/19590.94 30.5 1982 1981/19820.96 31.0 1964 1964/19650.99 37.6 1976 1975/19761.02 38.4 1956 1955/19561.04 38.6 1987 1986/19871.07 38.7 1959 1959/19601.10 41.3 1964 1963/19641.14 58.1 1960 1959/19601.17 59.5 1975 1974/19751.21 61.5 1972 1971/19721.25 66.0 1981 1980/19811.29 72.5 1987 1987/19881.34 86.0 1980 1979/19801.38 86.1 1976 1975/19761.44 89.3 1989 1988/19891.49 93.6 1961 1960/19611.55 96.0 1992 1991/19921.62 101.5 1966 1965/19661.69 105.9 1989 1989/19901.77 141.7 1963 1962/19631.85 171.6 1981 1980/19811.95 174.7 1971 1970/19712.05 175.1 1984 1983/19842.17 182.7 1956 1955/19562.30 194.2 1977 1976/19772.45 195.9 1964 1963/19642.62 230.1 1988 1987/19882.81 247.9 1979 1978/19793.03 262.4 1990 1989/19903.29 293.9 1973 1972/19733.60 310.9 1962 1961/19623.98 318.5 1991 1990/19914.44 367.2 1968 1967/19685.03 388.7 1974 1973/19745.79 451.9 1958 1957/19586.82 492.3 1979 1978/19798.30 495.1 1972 1971/1972

10.61 500.6 1957 1956/195714.69 525.7 1986 1985/198623.88 1267.4 1977 1976/197763.67 1391.8 1967 1966/1967

Table 35: Flood peaks identified by thepeaks over threshold analysis for station919305B.



(years) (m3s−1) (m3s−1) (m3s−1)

2 126 160 197

5 290 375 462

10 427 574 718

20 569 811 1041

25 603 896 1183

50 708 1192 1647

100 777 1544 2257


919305B. Values have been reported to four

significant digits.

89

0.0

0.5

1.0

1.5

2.0

2.5

mea

n nu

mbe

r of

floo

ds/y

ear

0 20 40 60 80 100 140 180


0

50

100

150

200

250

300

350

mea

n ex

ceed

ance

(m3 s

−1)

919305B


1956 1961 1966 1971 1976 1981 1986 1991

Year

0

200

400

600

800

1000

1200

1400

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919305B Walsh River at Nullinga (325 km2)Threshold = 20 m3 s−1 Interflood period = 20 days


analysis.

90

1956 1961 1966 1971 1976 1981 1986 1991

Year

100

101

102

103

104

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919305B Walsh River at Nullinga (325 km2)Threshold = 20 m3 s−1 Interflood period = 20 days


analysis.

101

102

103

104

Q (

m3 s−1

)


919305B Walsh River at Nullinga 325 km2

10−1

100

101

Q (

m3 s−1

km−2

)

Figure 72: Fitted flood frequency curve for station 919305B. Dashed lines indicate a 95% confidence interval for


91

Q 919309A Walsh River at Trimbles Crossing


0.80 193 1984 1984/19850.82 194 1990 1989/19900.83 205 1969 1968/19690.85 208 1983 1982/19830.87 215 1982 1981/19820.89 217 1983 1982/19830.91 241 1977 1977/19780.93 248 1970 1969/19700.95 265 1975 1974/19750.97 275 1985 1984/19850.99 312 1970 1970/19711.01 364 1981 1981/19821.04 416 1978 1977/19781.07 429 1994 1993/19941.09 443 1987 1986/19871.12 456 1973 1973/19741.15 462 1969 1968/19691.19 499 1988 1987/19881.22 505 1993 1992/19931.26 510 2003 2002/20031.29 529 1992 1991/19921.34 568 1990 1989/19901.38 575 1997 1996/19971.43 673 2005 2004/20051.48 705 1976 1975/19761.53 730 2006 2005/20061.59 746 1982 1981/19821.65 776 2002 2001/20021.72 816 1968 1967/19681.79 816 1989 1989/19901.87 862 1973 1972/19731.95 875 1988 1988/19892.05 897 1975 1974/19752.15 898 2006 2005/20062.27 904 1980 1979/19802.40 966 1984 1983/19842.54 1223 1981 1980/19812.71 1249 1988 1987/19882.89 1250 2004 2003/20043.10 1269 1986 1985/19863.35 1376 1998 1997/19983.64 1525 2001 2000/20013.98 1670 1996 1995/19964.40 1687 1971 1970/19714.91 1954 1979 1978/19795.55 2085 1974 1973/19746.39 2677 2008 2007/20087.54 2681 1977 1976/19779.17 2814 2007 2006/2007

11.72 2905 2009 2008/200916.23 3400 2000 1999/200026.38 3472 1972 1971/197270.33 3962 1999 1998/1999




(years) (m3s−1) (m3s−1) (m3s−1)

2 794 945 1102

5 1494 1826 2138

10 2015 2541 3039

20 2521 3301 4036

25 2692 3555 4397

50 3040 4378 5634

100 3340 5253 7176



significant digits.

93

0.0

0.5

1.0

1.5

2.0

mea

n nu

mbe

r of

floo

ds/y

ear

0 200 600 1000 1400 1800


0

200

400

600

800

1000

1200

mea

n ex

ceed

ance

(m3 s

−1)

919309A


1967 1972 1977 1982 1987 1992 1997 2002 2007

Year

0

500

1000

1500

2000

2500

3000

3500

4000

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919309A Walsh River at Trimbles Crossing (9040 km2)Threshold = 171 m3 s−1 Interflood period = 30 days


analysis.

94

1967 1972 1977 1982 1987 1992 1997 2002 2007

Year

100

101

102

103

104

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919309A Walsh River at Trimbles Crossing (9040 km2)Threshold = 171 m3 s−1 Interflood period = 30 days


analysis.

102

103

104

Q (

m3 s−1

)


919309A Walsh River at Trimbles Crossing 9040 km2

10−1

100

Q (

m3 s−1

km−2

)



95

R 919310A Walsh River at Rookwood


0.83 209 1993 1992/19930.85 217 1987 1986/19870.87 237 1969 1968/19690.89 261 1985 1985/19860.91 271 1989 1988/19890.93 291 1970 1970/19710.95 336 2002 2001/20020.97 336 1975 1974/19750.99 349 2002 2001/20021.01 371 2004 2004/20051.04 386 1978 1977/19781.07 436 1987 1986/19871.09 444 2006 2005/20061.12 461 1992 1991/19921.15 484 1980 1979/19801.19 514 2004 2003/20041.22 531 2003 2002/20031.26 580 1977 1977/19781.29 591 2004 2003/20041.34 620 1988 1988/19891.38 648 1995 1994/19951.43 784 2005 2004/20051.48 824 1996 1995/19961.53 854 1987 1987/19881.59 877 1990 1989/19901.65 937 1972 1971/19721.72 970 1984 1983/19841.79 985 1976 1975/19761.87 1035 1973 1972/19731.95 1074 1982 1981/19822.05 1183 1968 1967/19682.15 1183 1981 1980/19812.27 1206 1997 1997/19982.40 1225 2006 2005/20062.54 1309 1989 1989/19902.71 1324 1975 1974/19752.89 1367 2007 2006/20073.10 1409 1971 1970/19713.35 1438 2001 2000/20013.64 1698 1988 1987/19883.98 2190 1986 1985/19864.40 2304 1991 1990/19914.91 2652 2008 2007/20085.55 2757 1997 1996/19976.39 2832 1974 1973/19747.54 2852 2009 2008/20099.17 2972 1979 1978/1979

11.72 3793 2000 1999/200016.23 4206 1977 1976/197726.38 4524 1999 1998/199970.33 5526 1972 1971/1972




(years) (m3s−1) (m3s−1) (m3s−1)

2 917 1105 1305

5 1809 2225 2637

10 2491 3142 3746

20 3162 4122 5065

25 3379 4453 5526

50 3859 5525 7243

100 4203 6674 9224



significant digits.

97

0.0

0.5

1.0

1.5

2.0

2.5

3.0

mea

n nu

mbe

r of

floo

ds/y

ear

0 200 600 1000 1400 1800


0

500

1000

1500

mea

n ex

ceed

ance

(m3 s

−1)

919310A


1967 1972 1977 1982 1987 1992 1997 2002 2007

Year

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

5500

6000

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919310A Walsh River at Rookwood (5025 km2)Threshold = 200 m3 s−1 Interflood period = 30 days


analysis.

98

1967 1972 1977 1982 1987 1992 1997 2002 2007

Year

100

101

102

103

104

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919310A Walsh River at Rookwood (5025 km2)Threshold = 200 m3 s−1 Interflood period = 30 days


analysis.

102

103

104

Q (

m3 s−1

)


919310A Walsh River at Rookwood 5025 km2

10−1

100

Q (

m3 s−1

km−2

)



99

S 919311A Walsh River at Flatrock


0.83 44.3 2008 2008/20090.85 48.3 1990 1989/19900.87 51.3 1968 1968/19690.88 52.7 1996 1995/19960.90 54.1 1977 1976/19770.92 60.2 1995 1995/19960.94 84.7 1985 1984/19850.97 108.2 1985 1984/19850.99 109.6 1993 1992/19931.01 117.4 1981 1981/19821.04 135.9 1982 1981/19821.07 141.2 1985 1985/19861.10 144.9 1983 1982/19831.13 180.2 1982 1981/19821.16 187.7 2000 2000/20011.19 189.4 1985 1984/19851.23 193.9 1969 1968/19691.26 270.3 1970 1969/19701.30 277.3 1978 1977/19781.35 315.4 1980 1979/19801.39 354.9 2002 2001/20021.44 419.3 2006 2005/20061.49 460.9 1996 1995/19961.55 533.5 1992 1991/19921.61 544.1 1984 1983/19841.67 571.9 2003 2002/20031.75 606.1 2004 2003/20041.82 772.3 1998 1997/19981.91 781.5 1976 1975/19762.00 811.7 1973 1972/19732.10 825.8 2005 2004/20052.22 918.4 1990 1989/19902.34 934.6 1987 1987/19882.48 994.4 1971 1970/19712.64 1038.7 1975 1974/19752.82 1137.6 2001 2000/20013.03 1144.7 1981 1980/19813.27 1156.0 2007 2006/20073.55 1248.8 2006 2005/20063.89 1323.4 1989 1989/19904.29 1808.8 1988 1987/19884.79 2002.1 1986 1985/19865.42 2372.8 1974 1973/19746.24 2503.2 1972 1971/19727.36 2503.2 1979 1978/19798.96 2571.9 1997 1996/1997

11.44 2649.2 2008 2007/200815.85 2664.5 1977 1976/197725.75 3598.4 1999 1998/199968.67 3645.7 2000 1999/2000




(years) (m3s−1) (m3s−1) (m3s−1)

2 615 793 983

5 1430 1780 2121

10 2028 2493 2952

20 2529 3176 3807

25 2656 3390 4083

50 2998 4038 5097

100 3212 4660 6093



significant digits.

101

0.0

0.5

1.0

1.5

2.0

2.5

mea

n nu

mbe

r of

floo

ds/y

ear

0 50 100 200 300 400 500


0

200

400

600

800

1000

mea

n ex

ceed

ance

(m3 s

−1)

919311A


1968 1973 1978 1983 1988 1993 1998 2003 2008

Year

0

500

1000

1500

2000

2500

3000

3500

4000

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919311A Walsh River at Flatrock (2770 km2)Threshold = 40 m3 s−1 Interflood period = 30 days


analysis.

102

1968 1973 1978 1983 1988 1993 1998 2003 2008

Year

100

101

102

103

104

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919311A Walsh River at Flatrock (2770 km2)Threshold = 40 m3 s−1 Interflood period = 30 days


analysis.

102

103

104

Q (

m3 s−1

)


919311A Walsh River at Flatrock 2770 km2

10−1

100

Q (

m3 s−1

km−2

)



103

T 919312A Elizabeth Ck at Greenmantle


0.70 41.3 1975 1974/19750.72 54.1 1970 1969/19700.75 54.1 1973 1973/19740.78 71.8 1970 1970/19710.81 72.8 1969 1969/19700.85 74.7 1976 1976/19770.89 90.3 1981 1981/19820.93 101.6 1984 1984/19850.98 122.5 1977 1977/19781.03 137.2 1987 1986/19871.09 152.7 1974 1974/19751.16 157.1 1983 1982/19831.23 218.9 1982 1981/19821.32 271.6 1985 1984/19851.41 306.8 1986 1985/19861.52 325.8 1982 1981/19821.66 386.0 1984 1983/19841.81 433.4 1988 1987/19882.00 449.0 1987 1987/19882.23 453.5 1971 1970/19712.53 471.8 1976 1975/19762.91 483.3 1981 1980/19813.43 565.7 1973 1972/19734.17 570.8 1980 1979/19805.33 687.2 1974 1973/19747.38 737.8 1977 1976/1977

12.00 912.8 1972 1971/197232.00 990.5 1979 1978/1979




(years) (m3s−1) (m3s−1) (m3s−1)

2 353 412 471

5 603 684 764

10 740 827 910

20 833 930 1028

25 857 1000 1057

50 901 1026 1151

100 926 1075 1229



significant digits.

105

0.0

0.5

1.0

1.5

2.0

mea

n nu

mbe

r of

floo

ds/y

ear

0 50 100 200 300 400 500


0

100

200

300

400

mea

n ex

ceed

ance

(m3 s

−1)

919312A


1969 1974 1979 1984 1989

Year

0

100

200

300

400

500

600

700

800

900

1000

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919312A Elizabeth Ck at Greenmantle (620 km2)Threshold = 25 m3 s−1 Interflood period = 30 days


analysis.

106

1969 1974 1979 1984 1989

Year

100

101

102

103

Dai

ly m

axim

um s

trea

mflo

w (m

3 s−1

)

919312A Elizabeth Ck at Greenmantle (620 km2)Threshold = 25 m3 s−1 Interflood period = 30 days


analysis.

102

103

104

Q (

m3 s−1

)


919312A Elizabeth Ck at Greenmantle 620 km2

100

101

Q (

m3 s−1

km−2

)



107

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