The world record for a storm surge is 13 m, occurring during landfall of Tropical Cyclone (TC) Mahina in northeast Australia, March 1899. The central pressure of this cyclone has been reported as 914 hPa as measured by a ship's barometers during passage of the eye. The accuracy of these measurements is poor, as the only source for this information has been thirdhand accounts in an anonymously authored report. Modeling of this event, at 914 hPa, also suggests that the highest storm surge that could have been generated at the site of landfall of TC Mahina is approximately 5 m, well short of the reported 13 m. This study reexamined the evidence for this event and based on archived firsthand accounts, it seems more likely that the central pressure of this event was 880 hPa, which when modeled can generate a storm surge over 9 m and a total inundation (surge plus tide and wave action) of approximately 13 m. The central pressure of TC Mahina may also be the lowest recorded for a Southern Hemisphere tropical cyclone.
New evidence suggests that Tropical Cyclone Mahina had a central pressure of 880 hPa and could have produced a maximum storm surge of approximately 9 m and a total inundation of roughly 13 m.
According to the World Meteorological Organization (WMO), the largest ever-recorded storm surge was 13 m (WMO 2006). This occurred during Tropical Cyclone (TC) Mahina on 5 March 1899 near Bathurst Bay, northeast Australia. The robustness of this record is questionable because it is based on a thirdhand account in an anonymously authored report, known as the Outridge Booklet (Anonymous 1899g). The central pressure of TC Mahina is stated in this report as being recorded by a ship's barometer at 27 in. of mercury (914 hPa), but this too is a thirdhand account. Along with the lack of veracity associated with these reports, there is a further problem—using this central pressure, the Brisbane Tropical Cyclone Warning Centre using the Australian real-time system for forecasting tropical storm surges (Hubbert et al. 1991) could only generate a surge considerably lower than that reported. There is little doubt that this tropical cyclone occurred and that it was severe, for it resulted in the deaths of nearly 300 people and sank 54 vessels. However, the evidence for this world record storm surge has not been scrutinized. The reported central pressure of the cyclone does not support a storm surge of such height. Either the height of the storm surge or the central pressure of the cyclone—or both—has been misreported.
An initial attempt to resolve this issue by Nott and Hayne (2000) found no physical field evidence of a storm surge of this height throughout the area impacted by Mahina. However, observations of the impacts of recent severe tropical cyclone inundations throughout this broader region suggest there can be two types of physical debris left after these events— sand and carbonates (shell and corals), which remain well preserved for millennia, and vegetative matter, which decomposes within years. The latter type is often, but not always, deposited at a higher elevation within the coastal landscape, so it may be possible that the highest debris left by the Mahina inundation is no longer present. Hence, Nott and Hayne's (2000) conclusion may have been premature. Furthermore, it is also likely that other accounts, including eyewitness accounts of the storm surge and the cyclone's central pressure, may exist within historical archives. To this end this study aimed to reexamine the evidence for this world record storm surge by taking a three-pronged approach: Reexamine the physical evidence in the field, investigate any archived historical records of the event, and remodel the tropical cyclone and associated storm surge in light of any new evidence uncovered by the first two approaches.
THE EVENT AND SUBSEQUENT ANALYSIS.
Tropical Cyclone Mahina crossed the Queensland coast at Bathurst Bay on 5 March 1899. A booklet, The Pearling Disaster, 1899: A Memorial (Anonymous 1899g), was published only months after the cyclone by a Brisbane-based firm the Outridge Publishing Company. This report, now known as the Outridge Booklet (Anonymous 1899g), was produced to assist the grieving families of the “white men” who died during the storm. In contrast most of the dead were South Pacific Islanders working on the pearling luggers. Most of the accounts in the Outridge Booklet are taken from the Brisbane Courier, which in turn were reports from the search and rescue vessels Duke of Norfolk, White Star, and Warrego.
One of the most interesting elements of the report's contents is an account of the storm surge experienced by a Constable John Kenny. Constable Kenny of the 8-mi police station near Cooktown, along with four indigenous troopers and 10 horses, were heading toward Barrow Point (Fig. 1) to investigate an alleged spearing attack. The passage in the Outridge Booklet states that “Kenny reached Barrow Point on Saturday, 4th March, and camped about 6.p.m., on a ridge fully 40ft. above sea level, and about half a mile from the beach” (Anonymous 1899g, p.18–19).
The passage tells of Kenny's memory of the wind and rain steadily increasing to “hurricane force,” and the noticeable shift in wind direction from the southeast to the northeast. It then states that “shortly after the wind shifted to the north-east an immense tidal wave swept in shore, and reached waist deep on the ridge with the camp on it, completing the misery of the constable and troopers, also spoiling Kenny's watch” (Anonymous 1899g, p.18–19).
The report suggests that the inundation, which Kenny and the troopers endured, was approximately 42 ft, or 13 m above sea level.
The Outridge Booklet also recounts the story of Captain William Porter, whose schooner The Crest of the Wave was one of the few to survive the event. He, along with several fleets of pearling luggers, sheltered in Bathurst Bay (Fig. 1). Porter read the mercury barometer on his vessel during passage of the eye. As stated in the Outridge Booklet, “the glass continued to fall until 4:30am on Sunday, the 5th March, when it reached 27 inches” (Anonymous 1899g, p. 26)
However, these were not Porter's own words; like Kenny's account, they were written in the third person.
Clement Wragge, a Queensland government meteorologist at the time, provided details on the development and apparent track of Mahina from available meteorological data (Anonymous 1899g). Later, Whittingham (1958) investigated and attempted to reconstruct the details of Mahina. He produced a synoptic chart of the event using a pressure reading of 29.10 in. (~985.4 hPa) taken on the schooner Olive anchored north of Burkitt Island (almost 70 km northwest of Bathurst Bay) at about midnight along with Porter's central pressure reading during landfall and other meteorological data gathered by Wragge. Whittingham's (1958) chart suggests Mahina approached the coast from the northeast. Nott and Hayne (2000) searched the area of the surge impact suggested in the Outridge Booklet for debris that may have been deposited during the event. The highest debris (sand, shell, and coral) was found at approximately 5 m above mean sea level or Australian height datum (AHD). As a consequence they suggested that a storm surge or inundation of 13 m was unlikely.
Three approaches to try to uncover a more realistic estimate of either the central pressure of Mahina or the magnitude of the inundation were attempted. The first approach was to research the area of reported inundation for any physical evidence, such as elevated sand deposits, coral and shell fragments, or other possible debris, such as pumice, that may have been preserved at an elevation higher than previously discovered in the coastal landscape. This included searching the area around Bowen Bay, which is approximately 2 km southeast of Ninian Bay (Fig. 1), being the location where Nott and Hayne (2000) focused their search for physical debris. Bowen Bay faces east and it was thought that Constable Kenny may have been camped nearer to this area rather than the north-facing Ninian Bay. The east-facing aspect of Bowen Bay may also have allowed it to receive a larger inundation than Ninian Bay.
The second approach was to search the historical archives, such as newspaper reports of the event at the time, and any other documents, such as police or the coroner's reports. This evidence was predominantly obtained through Trove on the National Library of Australia's website (www.nla.gov.au/), as well as searching the Queensland archives and the John Oxley Library of the State Library of Queensland. The principal aim was to ascertain the accuracy of the reported TC central air pressure and storm surge height reported in the Outridge Booklet as well as to obtain any other information on wind direction, strength, and impacts to vegetation throughout the area.
The third approach was to model the storm surge using the two-dimensional hydrodynamic model GCOM2D (Hubbert and McInnes 1999). This numerical model solves a set of mathematical equations over an equally spaced grid to determine water depth, currents, topography, and bathymetry, and it incorporates wind stresses and atmospheric pressure gradients acting on the ocean surface and friction on the ocean floor. The model was run in a nested grid (coarse grid—1000-m resolution, and fine grid—100-m resolution) for locations within Ninian Bay, Bowen Bay, and Bathurst Bay (Fig. 1) to examine the variation in storm surge heights under a variety of meteorological scenarios.
GCOM2D has been tested against actual tropical cyclone surges and has generated realistic simulations of these events. Hubbert and McInnes (1999) simulated the storm surge generated by a severe tropical cyclone that struck Port Hedland, Western Australia, in 1939 and compared the results against known surge flooding levels within the area. The model reproduced the peak flood levels and areas of inundation to a high degree of accuracy. Nott and Hubbert (2005) also simulated the storm surges generated by severe Tropical Cyclones Vance (1999) and Chris (2002), and compared these to the heights of debris left in the coastal landscape by these inundations. In both cases GCOM2D accurately simulated the actual storm surges.
Survey of highest debris.
The sand dunes behind Bowen Bay and the granite headland at the northern end of the bay were examined for any evidence of high-elevation marine inundations. The highest debris (scattered marine shells) was found on the granite headland at 1.79 m AHD and in the dunes behind Bowen Bay at 1.62 m AHD.
A beach ridge containing coarse-grained pre-dominantly quartzose sands and scattered marine shells occurs on the landward side of the beach at Ninian Bay. This type of beach ridge has been shown by Nott (2006) and Nott et al. (2009) to be deposited by waves. These ridges are abundant within many embayments along the northeast Queensland coast and also around the Gulf of Carpentaria. They accumulate progressively over time and are a function of multiple marine inundations—each inundation leaving a sedimentary unit ranging between several centimeters and more than a meter thick. The lower units in a ridge can be deposited by relatively weak tropical cyclones, whereas the uppermost units of a high ridge, generally above 4 m AHD, are thought to be deposited by inundations associated with intense tropical cyclones (Nott 2006; Nott et al. 2009). The crest of the ridge backing Ninian Bay is 6.6 m AHD, which is one of the highest wave-constructed ridges surveyed to date along the northeast Queensland coast (Nott 2006; Nott et al. 2009). The crest of this ridge has the highest debris deposited by a marine inundation found both within Ninian Bay and the surrounding area.
Structure and movement of the hurricane
The Honorable John Douglas, Companion of the Order of St. Michael and St. George (CMG), a government resident at Thursday Island, visited the impacted area on the White Star. His report was published in the Brisbane Courier on 11 April 1899 (Douglas 1899). Douglas (1899) suggested that Bewick Island (Fig. 2) appeared to be the southern limit of the cyclone. At Newton Island, south of Bewick and approximately 60 km southeast of Cape Melville, there appeared no indication of the storm—the vegetation being untouched—whereas between Bewick and Hannah Island (northwest of Bathurst Bay) (Fig. 2), the trees and grass had been swept clean. This suggests that the zone of strongest winds was at least 50 km wide.
It is possible that hurricane-force winds extended 100 km either side of Mahina's eye. Ship reports (Crest of the Wave, Meg Merrilees, and Olive) suggest that the wind at both Cape Melville and Hannah Island became violent around midnight. The wind at this time was southeasterly at Cape Melville and southwesterly at Hannah Island.
Hannah Island is 90 km west-northwest (292° bearing) and Bewick Island is 47 km southeast (133° bearing) from the western tip of Cape Melville (Fig. 2). Normally on the east coast of Queensland, gales extend farther south than north when a severe tropical cyclone makes landfall. In this instance there was a strong coastal ridge to the south, which would have resulted in the zone of very destructive winds being larger in the southeast quadrant. It is possible therefore that Mahina passed close to Hannah Island as it approached landfall.
At Stapleton Island (close to Bewick Island; Fig. 2), a report from the cutter Spray stated that the wind started to blow from the southeast, and about midnight the wind changed, to the east. A report from the cutter Jamaica anchored at Lizard Island (115 km southeast of Cape Melville) stated that wind blew with hurricane force from the east on Saturday, 4 March, and on Sunday morning it blew with terrific force, shifting to the northeast, and finally died away from the northwest. These reports suggest that Mahina had an extensive band of hurricane-force winds to the southeast. The turning of the winds at the Jamaica and the Spray were consistent with a cyclone passing to the north of these locations and therefore a track from the northeast. This northeast track took Mahina directly over Musgrave Telegraph Station (Fig. 2), where all buildings were blown down. Whittingham (1958) also suggested that Mahina approached from the northeast.
The central pressure of Mahina is attributed to William Field Porter, an experienced master mariner and captain of the 112-t schooner Crest of the Wave, and the manager of a fleet owned by a consortium. This schooner was the only vessel to have experienced the eye of the cyclone and stay afloat. Porter, his wife, and their small child survived the event. Porter took readings from his vessel's barometer before the event and during passage of the eye. The Outridge Booklet states that Porter recorded a central pressure of 27 in. of mercury during the eye. However, this appears to be a thirdhand account.
However, there is primary evidence that Porter reported a reading of 26 in. of mercury. An article in the Brisbane Courier newspaper (15 March 1899) (Anonymous 1899d) reports a Japanese steamship Kasuga Maru stopped beside the stricken schooner Crest of the Wave at 0700 UTC 8 March, three days after the cyclone. A pilot aboard the steamship, Captain Craig, reported that Porter told him that the barometer fell to 26 in. Craig expressed doubt about this reading because he said he had experienced some of the worst typhoons ever in the “China seas” but had never known the barometer to fall lower than 27.30 in. An article in the Sydney Morning Herald (14 March 1899) (Anonymous 1899h) and an article in the Brisbane Courier (14 March 1899) (Anonymous 1899c) both state that Porter made a reading of 27 in. at 11 p.m. This was approximately five hours before the eye passed over Bathurst Bay, where Porter's vessel Crest of the Wave was anchored.
In a letter to his parents in Auckland, New Zealand, dated 6 March 1899, and sent from Crest of the Wave, Cape Melville, Porter states that he recorded a pressure of 26 in. (Porter 1899): “The wind was hard by this time we could not face it or see any distance in the driving rain. This continued till about 4.30 a.m. when the barometer was down to 26. Then came the lull, and what I feared would happen, the wind came around to N.W., that is right on the shore.”
In that letter, he describes the schooner breaching and his orders to cut away the masts. This letter was put aboard a passing steamer and was subsequently published in the New Zealand Herald on 1 April 1899 (Porter 1899).
A third report stating that Porter recorded this pressure is detailed in an article in the Brisbane Courier dated 18 March 1899 (Anonymous 1899a). The article reprints an extract of a report written by Captain Jenkins of the steamer Duke of Norfolk. In this report Jenkins states that Porter told him the barometer dropped to 26 in.
The marine inundation
As mentioned, the main source for the height of the storm surge during Mahina has been the Outridge Booklet. Constable Kenny, a policeman working for the Protector of Aborigines, was camped near Barrow Point during the cyclone. His account in the Outridge Booklet is third hand and it is difficult to assess its validity. To date no firsthand accounts of the event by Constable Kenny have been found.
Kenny's report in the Outridge Booklet is a copy of a report first published in the Brisbane Courier, Tuesday, 18 April 1899 (Anonymous 1899f). The report appears to be an account of a meeting between Kenny and the anonymous author. Without knowing who wrote the report, it is difficult to form an opinion as to the accuracy of its contents. However, the same account was published in a newspaper called the Week on 21 April 1899 (Anonymous 1899b), and that article describes it as an extract from a private letter sent by A. R. (Arthur) Vidgen to a friend in Brisbane. A. R. Vidgen was the brother of one of the survivors of the cyclone. Vidgen went to the scene on 10 March, five days after the cyclone, on the steamer Warrego. The letter appears to be a report of an interview with Kenny, either at the scene or in Cooktown.
A month after Mahina, Dr. Walter Roth, the northern protector of Aborigines, was commissioned by the Queensland government to visit the region to distribute goods to surviving coastal Aborigines. Roth submitted a detailed report of his observations of the impacted area to the Department of the Home Secretary (Roth 1899). Roth met Kenny and traversed the coastal country behind Bowen Bay and then to Ninian Bay and Bathurst Bay. Roth made two key observations relevant to this study. First, he noted and drew a map of the direction of tree fall throughout the area (Fig. 1); and second, he observed fragments of Aboriginal canoes high in trees deposited by the marine inundation. Roth's map of the tree falls shows that the majority of trees immediately inland of Bowen Bay and Ninian Bay fell toward the west to the west-northwest (Fig. 1), suggesting that the strongest winds here, at least during landfall or approach of Mahina, were from the east to the east-southeast. Farther north and northwest near Cape Melville and Bathurst Bay, the majority of trees fell toward the northwest, suggesting the strongest winds at TC landfall were from the southeast. As Roth and Kenny descended the high sand dune behind Ninian Bay, Roth was shown Aboriginal canoe fragments deposited in the trees that were, in his words, “fully 70 and 80 feet above high water level” (Roth 1899). This height is considerably higher than the height of the storm marine inundation attributed to Kenny in the Outridge Booklet (42 ft).
Newspaper articles at the time also made reference to indicators of the magnitude of the marine inundation. The Torres Strait Pilot newspaper published a report from Herbert Bowden (Bowden 1899), owner of the stranded schooner Meg Merrilees, who had inspected the islands of Princess Charlotte Bay and Bathurst Bay. In his report dated 11 March 1899 (Bowden 1899), Bowden says, “The shore appearances of the effects of the gale shows that the hurricane was terrific. All the trees have been swept bare of branches, leaves and bark as if they had been fired; and the islands are bared of grass. A large porpoise was found 20 feet up on a hillside.”
There were other reports that porpoises (dolphins) were washed 50 ft above sea level onto Flinders Island in Bathurst Bay (Anonymous 1899d), and other articles reported that the “porpoises” were 15 ft above sea level (Anonymous 1899i). Some of the newspaper articles also reported that stones were embedded in trees to a depth of 6 in., and that rocks weighing tons were thrown up (Anonymous 1899d).
A month after the storm, Constable Kenny was sent back to the scene to help find and bury bodies at Bathurst Bay. Sub Inspector Cooper of the Cooktown Police Station telegraphed the commissioner of police in Brisbane to report that Constable Kenny had found 37 bodies, several being 12 ft above the ground in trees (Anonymous 1899e).
The GCMO2D numerical storm surge and wind model was run using a number of different meteorological parameters, including track direction, central pressure, and radius of maximum winds. The height of the storm surge was determined for Bathurst Bay, Ninian Bay, and Bowen Bay. Track runs from the northeast, north-northeast, north, and northwest were run for TCs with central pressures between 914 and 880 hPa. The results can be seen in Figs. 3–6. The model runs using a 914-hPa (27 in.) central pressure only generated a storm surge between 2 and 4.5 m AHD at any of the three locations examined. The model runs using an 880-hPa (26 in.) central pressure produced much larger surges. The north and northwest track runs at 880-hPa central pressure produced surges between 3 and 5.3 m AHD (Fig. 3). The largest surges were generated by the northeast track runs with a 9.1-m AHD surge at Ninian Bay and an 8.3-m AHD surge at Bowen Bay (Fig. 3).
Sensitivity tests were also undertaken to ascertain surge levels under different radius of maximum winds (Rm) and translational velocities of the cyclone. These results are presented in Figs. 5 and 6. Figure 5 shows the surge height at various values versus translational velocity for Ninian Bay. Here, the cyclone was simulated to cross the coast so that the maximum winds, in all runs, occurred at Ninian Bay when the Rm was 60 km. The results show that surge heights vary little with changes in translational velocity. Figure 6 shows surge heights versus various values of Rm with the northeast cyclone track adjusted (moved east or west) for each run in order to generate the maximum winds at Ninian Bay. There were only very minor variations in surge height irrespective of the Rm. Indeed the surge height at Ninian Bay was almost identical between Rms of 30 and 60 km. All runs, both in the main model simulations and in the sensitivity tests, suggest that the largest surge achievable with an 880-hPa central pressure is a little over 9 m.
These numerical simulations were undertaken without incorporating the tide at the time of the event. The model runs showed that maximum surge occurred generally around 0430 UTC 5 March 1899, which is in agreement with the reports of the passage of the eye and maximum marine inundation by Porter's and Kenny's accounts, respectively, in the Outridge Booklet. According to Maritime Safety Queensland, the tide at Legget Island, being the nearest tidal port, at 0430 UTC 5 March 1899, was 2.32 m lowest astronomical tide (LAT). High tide was 2.34 m LAT at 0510 UTC. Maritime Safety Queensland also advised that these tide heights are overestimated by about 0.15 m because of sea level rise during the twentieth century. While the tide level cannot be directly added to the surge estimates, because of the nonliner relationship between the two, it is clear that if the barometric pressure of 26 in. (880 hPa) reported by Porter is correct, then the time of maximum surge was close to high tide and therefore the maximum storm tide inundation at Ninian Bay would have been higher than 9.1 m and potentially close to 9.5 m AHD (when the 2.34-m LAT tide height is converted to AHD).
The Outridge Booklet has to date been the sole source of information regarding the intensity of Mahina and the height of the associated marine inundation. This has formed the basis for the inundation becoming the world record of 13 m. However, this report is anonymously authored and the accounts of the central pressure and the inundation are third hand. The firsthand account of the central pressure by William Porter in a letter to his father states that he read 26 in. of mercury, not 27 in., as stated in the Outridge Booklet. Two newspaper articles also verify Porter's account in his letter. The captains of the vessels that transported Porter first to Townsville and then to Brisbane both state that Porter told them that the central pressure he recorded was 26 in. These primary accounts of Porter's measurement are at odds with the secondhand or thirdhand report of 27 in. stated in the Outridge Booklet. The reason that this figure was stated as 27 in. may have been because of the comments of the marine pilot Captain Craig, who said he doubted that the barometer could fall as low as 26 in., as he had never encountered a pressure lower than 27.30 in.
The three primary reports suggest that Porter did read 26 in. of mercury during passage of the eye, but he may not have been believed at the time. No other vessel that experienced the eye of Mahina and had a barometer aboard survived the storm. The other vessels in the area that did record air pressure were well away from the eye.
It was the practice in the late 1800s to document barometric readings to one decimal point in inches. The reading of 26 in. suggests that Porter may have rounded his reading to the nearest inch, and this is understandable given the circumstances and the likelihood there were reasonably rapid changes in air pressure at the time. So, it is possible that the air pressure was between 880.5 and 897.0 hPa (26.01–26.48 in.). No further detailed information on Porter's pressure reading has been found, so the reading of 26 in. is the most reliable measurement uncovered to date.
The height of the marine inundation generated during Mahina as stated in the Outridge Booklet is corroborated by Walter Roth's report. Roth, visiting the area with Constable Kenny approximately one month after the event, was shown fragments of Aboriginal canoes in trees as they descended a high sand ridge behind Ninian Bay. Roth states that these fragments were 70 ft above the high-water mark (Roth 1899). Roth studied biology at Oxford University and completed his medical training in London, United Kingdom; as a scientist his estimate carries some credence. The height of these fragments suggests that there was physical evidence in the coastal landscape that backed Kenny's account of the magnitude and extent of the inundation.
The height of the porpoise washed up on an island near Bathurst Bay also suggests that there was a substantial inundation and/or wave action on other parts of the coast during the event. The original account of the height of this marine animal was reported in the Torres Straits Pilot newspaper by Herbert Bowden, owner of the stranded schooner Meg Merrilees. He reported a large porpoise found 20 ft up on a hillside on one of the islands north of Bathurst Bay (Bowden 1899).
The numerical storm surge modeling suggests that a storm surge of 9.1 m AHD could occur with a TC approaching from the northeast and with a central pressure of 880 hPa or 26 in. (Fig. 3). The highest storm surge for a TC with a central pressure of 914 hPa or 27 in. is 4.6 m AHD at Bowen Bay and 3.8 m AHD at Ninian Bay (Fig. 3). If Kenny's account of a 13-m inundation during Mahina is correct, then it would appear that the central pressure of Mahina needed to be 880 hPa and that the TC needed to follow a track from the northeast. The difference of 4 m between the modeled storm surge at this central pressure and Kenny's account could be explained by the additional components of the inundation, such as wave action, wave setup, and wave runup plus the high tide at the time. These additional inundation components have amounted to approximately 25% of the total inundation during severe TCs in north Queensland over the past six years. For example, the total inundation generated by TC Yasi (with a recorded central pressure of 929 hPa) in March 2011 at Cardwell was 6.5 m AHD (surveyed by J. Nott to the highest vegetative debris lines) and the tide gauge–measured storm tide was 4.6 m AHD. Nott et al. (2009) reported a similar relationship for the inundation generated by TC Larry in 2006. In both of these cases, the total inundation was approximately 25%–30% higher than the storm tide. An inundation, but not storm surge per se, of 13 m at Ninian Bay could have been achieved if the high tide and the additional inundation components are combined with the modeled storm surge of 9.1 m AHD. Hence, it appears that if Porter's measurement of 26 in. (880 hPa) is correct, and this appears to be more likely than the 27 in. as stated in the Outridge Booklet, then an inundation of 13 m AHD during TC Mahina was possible at Ninian Bay. However, this is not a storm surge of 13 m, which is currently regarded as the world record (WMO 2006).
The remaining issue is the reliability of the Outridge Booklet report of an inundation level of 13 m at Ninian Bay. There is some modern observational evidence that supports this claim. The height of the highest-wave-deposited sediments in the area also occurs at Ninian Bay at an elevation of 6.6 m AHD. As discussed by Nott et al. (2009), and as observed at Cardwell following landfall of TC Yasi, sediments are often deposited at a considerably lower elevation than the highest debris deposited during the event. The highest debris is usually composed of materials that float, such as vegetative matter and marine fauna. Heavier materials, such as sand and shells, are typically transported landward during the event as either traction or saltation load, where they roll along the surface at the base of the flow or they roll and occasionally become suspended for short periods, respectively. These types of sediment require higher velocity flows to remain in transport than suspended sediments and are therefore deposited earlier as the flow diminishes in velocity. The sand sheet deposited at Cardwell during the TC Yasi inundation occurred at 3.6 m AHD (surveyed by J. Nott), being considerably lower than the suspended and floating load material (vegetative debris, in this case) deposited at 6.5 m AHD. Hence, it is entirely possible for the unit of sand at 6.6 m AHD at Ninian Bay to have been deposited during an inundation that reached 13 m AHD in the coastal landscape. The highest debris, which was most likely dominated by vegetation (both marine and terrestrial), that would have been deposited at approximately 13 m AHD during Mahina would have decomposed over the subsequent century and is therefore no longer evident.
There is substantial discrepancy between the reported 13-m-high marine inundation generated by TC Mahina in northeast Australia, March 1899, and the numerically modeled height of an inundation using the reported central pressure of 27 in. of mercury or 914 hPa. Further investigation into the physical evidence for this magnitude inundation in the coastal landscape showed that the highest-wave-emplaced sediments here lie at 6.6 m AHD, which is at a sufficient elevation to have been deposited by a 13-m-high inundation. Historical archives of firsthand accounts of this event suggest that the central pressure of Mahina was 26 in. or 880 hPa. Numerical modeling using this central pressure shows that it is possible for a 9.1-m AHD storm surge to occur at Ninian Bay, close to where Constable Kenny is thought to have camped and been inundated by the reported 42-ft or 13-m inundation. Combining the 9.1-m surge with the high tide at the time and the additional inundation components of wave action, wave setup, and wave run-up suggests that this reported 13-m inundation was possible.
The most important new information in this study is the central pressure of Mahina, measured by a ship's barometer at 26 in., and reported shortly after by Captain William Field Porter in his own words, in a letter to his father. Porter also stated this barometric reading to the pilot captain of the vessel that rendered him assistance three days after the event, and also to the captain of another passing vessel. In the first instance, this account appears in the captain's own words within the newspaper article. These accounts have a substantially greater degree of credibility than the thirdhand account of 27 in., as stated in the Outridge Booklet. If the barometer on Porter's schooner the Crest of the Wave was properly calibrated at the time, then it would appear that a marine inundation of 13 m might have been possible. However, this is not a storm surge of 13 m, for the numerical modeling suggests that the highest surge possible here is a little over 9 m with a central pressure of 880 hPa. If Porter's barometric reading is correct, then Mahina's central pressure of 880 hPa could also be the lowest central pressure recorded historically in the Southern Hemisphere.