Intensity of Tropical Cyclones from Sedimentary and Erosional Evidence

Apart from determining the frequency of tropical cyclone occurrence, the ridge and terrace records of these events in Queensland can also be used to reconstruct the intensity of the events responsible for emplacement of individual ridges. The technique involves an assumption that the height of these landforms represents the minimum height of the storm inundation during the event responsible. The elevation of the ridges and terraces is accurately surveyed to datum, and samples of coral and/or shell radiocarbon dated or sands dated using OSL to determine the minimum height and times of inundation, respectively. The height of this inundation is then related to the intensity of the paleocyclone which is determined through

Table 2 Calculated central pressure of tropical cyclones responsible for emplacing ridges and eroding terraces

Location

Inun.

Hs

Set-up

Run-up

Surge

hPa mean

±1a

±2a

Wallaby Is.

5.08

3.9

0.39

1.2

3.43

861

9

30

4.1

3.6

0.36

1.1

2.58

896

8

29

3.6

3.4

0.38

1.0

2.24

910

8

29

Pormpuraaw

5

3.3

0.2

0.9

3.9

942

7

15

4

3

0.15

0.7

2.9

955

7

15

Princess Charlotte Bay

3.1

2.32

0.23

0.7

2.17

924

9

30

2.9

2.26

0.22

0.67

1.99

931

9

29

Red Cliff Pt.

6.1

5.2

0.52

1.56

3.99

900

12

25

4.9

4.7

0.47

1.4

3.0

926

12

24

3.8

4.13

0.41

1.24

2.11

949

12

23

2.12

3.1

0.31

0.92

0.86

982

10

17

Double Is

1.9

2.0

0.2

0.6

0.8

980

15

27

3.5

3.8

0.38

1.0

2.2

941

15

27

Fitzroy Is

4.5

7.4

0.74

2.2

1.5

894

20

39

3.9

6.7

0.67

2.0

1.2

912

19

36

Normanby Is

4.71

7.9

0.79

2.4

1.4

893

13

31

3.8

6.7

0.67

2.0

1.0

924

12

29

Cowley Bch

5

9.5

0.9

1.2

3.7

910

12

24

4

8.5

0.8

1.0

2.8

931

12

24

Curacoa Is

5.5

6.3

0.63

1.89

2.95

893

9

29

4.8

5.8

0.58

1.73

2.47

912

8

28

Lady Elliot Is

5.1

11.2

1.12

3.4

0.52

896

15

35

4.9

10.6

1.06

3.2

0.47

906

14

32

4.5

9.9

0.99

3.0

0.43

916

14

30

Inun. = inundation or height of ridge or terrace, Hs = significant wave height. Mean central pressure in hPa represents cyclone intensity if storm crossed at mean tide level which occurs at majority of time over full nodal (~19 yrs) tidal cycle. Central pressures are regarded as minimum intensity values.

Inun. = inundation or height of ridge or terrace, Hs = significant wave height. Mean central pressure in hPa represents cyclone intensity if storm crossed at mean tide level which occurs at majority of time over full nodal (~19 yrs) tidal cycle. Central pressures are regarded as minimum intensity values.

the use of numerical storm surge and shallow water wave models (Nott, 2003). The models are used to determine the relationship between surge height and central pressure for each location containing evidence of paleocyclones. Also, the relationship between surge height and translational velocity of the cyclone, the radius of maximum winds and the track angle of the cyclone as it approaches and crosses the coast are determined. Model results are compared to measured surge heights from recent or historical cyclone events near the study sites. The central pressure of the cyclone responsible for formation of the ridge or terrace is determined by modeling the magnitude of the surge plus wave set-up, and run-up and tide required to inundate the ridge or terrace.

The tide height at the time of the prehistoric event is unknown but can be estimated (at the 95% confidence level) to have occurred within the 2a probability tidal range of the frequency distribution nodal tide curve for each site (Table 2). Likewise, the tide height at the time can be estimated at the 1a probability tidal range (68% confidence level). These tidal ranges effectively form the uncertainty margins associated with the mean central pressure of the cyclone responsible for producing an inundation equal in height to the elevation of the ridge or terrace (Nott, 2003).

Nott and Hayne (2001) and Nott (2003) applied this technique to seven sites along the Queensland coast. More recently, analyses for the ridge sequences at Pormpuraaw and Cowley Beach have also been undertaken. The results are presented in Table 2. They show that the ridges at virtually all of the locations studied were emplaced during category 5 cyclones (category 5 being the most intense and category 1 being the least intense), which probably had central pressures of less than 920 hPa (Table 2). Even at the 95% uncertainty margin, these cyclones were still category 5 or at least severe category 4 events. The storms responsible for construction of the ridges at Princes Charlotte Bay (PCB) appear to have been less intense (929 ±31 hPa) than elsewhere; however, the lower elevation of these ridges may be due to limited sediment supply rather than less intense cyclones (Nott and Hayne, 2001).

In the immediate Cairns region the surge and wave numerical modeling revealed that a range of cyclones of varying intensities occurred between AD1800 and 1870. Table 2 presents the mean intensity and the 1 a and 2a range of intensities for individual cyclone events at each site. The highest terrace at Red Cliff Point was eroded by a cyclone with a mean central pressure of 900 ± 25 hPa at the 2a uncertainty margin. This suggests that there is only a 5% probability that this cyclone had a central pressure higher (weaker) than 925 hPa (Table 2). The mean central pressure of this storm (900 hPa) is close to the thermodynamic limit for tropical cyclones in this region [Holland, 1997]. The mean central pressures for the storms responsible for eroding each of the successively lower terraces were 926, 949 and 982 hPa, respectively. The latter figure accords closely with the central pressures (980, 975 hPa) of the two tropical cyclones to cross near Red Cliff Point in recent years (TC Justin and TC Steve) and which caused inundations close to the crest of Terrace 1. The mean central pressures for the storms responsible for depositing the shingle ridges at Fitzroy Island were 894 and 912 hPa (Table 2). The ridges at Double Island are lower in elevation compared to Fitzroy Island and were likely deposited by less intense cyclones. The numerical modeling and chronologies from the coral shingle ridges and eroded gravel terraces near Cairns suggest that one and possibly two intense (severe category 4 or category 5) tropical cyclones occurred here between AD 1815 and 1870.

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