The Role of Hurricanes in the Development of Reef ...

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Apr 23, 2007 - KEY WORDS:Solomon Islands, Ontong Java atoll, reef islands, hurricanes ...... advice of David Stoddart, Richard Chorley and Roger McLean.
The Role of Hurricanes in the Development of Reef Islands, Ontong Java Atoll, Solomon Islands T. P. Bayliss-Smith The Geographical Journal, Vol. 154, No. 3. (Nov., 1988), pp. 377-391. Stable URL: http://links.jstor.org/sici?sici=0016-7398%28198811%29154%3A3%3C377%3ATROHIT%3E2.0.CO%3B2-P The Geographical Journal is currently published by The Royal Geographical Society (with the Institute of British Geographers).

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The Geographical Journal, Vol. 154, No. 3, November 1988, pp. 377-391

THE ROLE OF HURRICANES IN THE DEVELOPMENT OF REEF

ISLANDS, ONTONG JAVA ATOLL, SOLOMON ISLANDS

T . P. BAYLISS-SMITH This paper describes recent geomorphic change on reef islands of Ontong Java atoll, Solomon Islands and discusses the nature of the 'steady state' equilibrium that has been proposed for cays and motus. The effects of a hurricane in 1967 are analysed through detailed field mapping in 1971. 1972 and 1986, and with reference to evidence from earlier charts and air photographs. It is concluded that the hurricane represents a high-magnitude low-frequency event necessary for the long-term replenishment of sediment on shorelines, but that in the short term such storms will seem to have mainly destructive effects. Since 1967 rubble ramparts thrown up by the hurricane have been largely reworked into stable landforms, mainly through the agency of more frequent, lower-magnitude storms. In parts of the atoll where coarse sediment was not deposited in 1967 there is a widespread retreat of seaward beaches. Such coastal erosion does not necessarily imply that islands have been destabilized by vegetation removal, climatic change or a rising sea level. It is only by considering the long-term sediment budget and the relaxation time of landforms in relation to major hurricanes that decisions can be reached about whether particular reef islands are in equilibrium with process. KEY

WORDS: Solomon Islands, Ontong Java atoll, reef islands, hurricanes

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E E F ISLANDS A R E OF FIVE main types: unvegetated cays, either sand or shingle; vegetated cays; motus, which are sand cays with shingle ridges, usually vegetated and with beachrock development; mangrove cays, either simple or the composite 'low wooded islands' with peripheral sand or shingle ridges; and finally emerged limestone islands (Steers, 1937; Stoddart and Steers, 1977; Stoddart et al., 1982; Hopley, 1982). Islands of the last category have been modified by relative sea-level change, but the other four types are generally considered to be equilibrium landforms continually adjusting to the controlling forces: Certain types of islands are restricted to certain reef areas, defined in terms of exposure to wave action. . . . While reef islands may thus be arranged together in an apparent series, into which all islands may at some point be fitted, the series is one of location and not of stage: it represents a series of equilibrium positions, not an evolutionary succession. (Stoddart and Steers, 1977: 97-8.)

The type of equilibrium envisaged for reef islands is a 'steady state' equilibrium between process and form (Chorley et a/., 1984). Fluctuations in process, through alterations in external factors such as sediment supply, wave direction, wave height and vegetation growth, will elicit a temporary response in island size, shape or location, but no long-term change is observable. For motus on atolls this idea was first made explicit by Wood-Jones (1910). Whereas earlier writers, for example Guppy (1889). had argued for an evolutionary model of landform development on atolls, Wood-Jones saw evidence on Cocos-Keeling for a balance between form and process. H e suggested that at equilibrium the beaches of atoll islands could not grow seawards any faster than the slow rate at which the seaward reefs themselves grew outwards. This idea is supported by recent research into the negative feedback processes that restrict sediment supply. It is argued that with the spread of sedimentary accumulations on reef tops the area available for active carbonate production is reduced. Ultimately a steady state situation will be reached where a given amount of sediment formed during a high-production period is re-sorted and relocated on the reef, and where any additional sediment supply is balanced by sediment export (Stoddart et al., 1 9 7 8 ~ ) . -+ Dr Tim Bayliss-Smith is a lecturer at the Department of Geography, Downing Place, Cambridge CB2 3EN. The paper was accepted for publication in October 1987.

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01988 The Royal Geographical Soclefy

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HURRICANES AND REEF ISLANDS, ONTONG JAVA

Fig. I . Location of Ontong Java atoll, showing the track of Cyclone Annie

There are, however, some problems with the concept of a steady state. Even if we accept that sufficient time has elapsed since sea level reached its present elevation for reef landforms to achieve this supposed equilibrium, it is unclear exactly what processes are involved, and over what time-scale. Normal waves, tides, winds and currents may be effective in reworking existing accumulations of reef sediment into an equilibrium form, but where does the material come from? How can storms be accommodated in this model, if they are liable to destroy at a stroke what years of slow sediment accumulation has produced? T o throw light on these questions, a number of studies have focused on the effects of low-frequency, high-magnitude events, i.e., tropical cyclones or hurricanes (Moorhouse, 1936; Blumenstock, 1958, 1961; McKee, 1959; Stoddart, 1963, 1971, 1972; Perkins and Enos, 1968; Maragos et al., 1973; Baines et al., 1974; Hopley, 1974; Hernandez-Avila et al., 1977; Woodroffe, 1983; Laboute, 1985; Harmelin-Vivien and Laboute, 1986; Fitchett, 1987). It would appear that catastrophic storms, as well as being destructive, are also the only mechanism capable of transporting large amounts of coarse sediment from reefs on to reef flats. It is likely that features such as shingle ramparts and rubble zones are only initiated in an episodic way during hurricanes, to be modified by subsequent normal wave action. It is also possible that reef islands are in some kind of equilibrium with occasional major storms, but in between these events appear to be in a state of disequilibrium (Stoddart, 1974; Hopley, 1982). The coral reef ecosystem might itself depend upon disturbance for the maintenance of its productivity and species diversity (Connell, 1978; Stoddart, 1985). At the same time there is a growing interest in secular change in the various factors influencing reef islands. The productivity of the reefs and the storminess of the nearshore zone may have reached a maximum at the time when post-glacial sea levels first reached their present elevation (the 'Holocene energy window'), and have declined since (McLean et al., 1978; Stoddart et al., 197%). Secondly, changes in climate and sea level have been identified as influencing the size and shape of reef islands (Verstappen, 1954; Tracey and Ladd, 1974; Pirazzoli et al., 1985; Flood, 1986).

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Thirdly, coastal protection is enhanced by strand vegetation, whose clearance by Man may adversely affect coastal stability (Stoddart, 1964, 1971; Stoddart et al., 1982; McLean, 1980). These factors may mean that the reef islands we observe are not in perfect adjustment to present-day processes of erosion, deposition and stabilization. If we find evidence for change in landforms it could reflect some alteration in one or more of these external controls, rather than a fluctuation around the steady state. Methods Most studies of geomorphic change on reef islands, like this one, have been based on the inference of process from form, in particular from island location, size and shape. Examples include the long-term studies of islands on the Great Barrier Reef (Stoddart et al., 1978a, b; Flood, 1986), and medium-term studies of the Belize Barrier Reef and Tonga (Stoddart, 1969, 1974; Stoddart et al., 1982; Woodroffe, 1983). Landform adjustments in the three years following a hurricane have been studied at Jaluit atoll in the Marshall Islands (Blumenstock et al., 1961) and at Funafuti atoll in Tuvalu (Baines and McLean, 1976). This present study examines geomorphic change on Ontong Java atoll, Solomon Islands, over the two decades since a major hurricane in 1967. This atoll demonstrates the geomorphic role of catastrophic storms in a region where the frequency of hurricanes is very low. Fieldwork took place in June 197CMay 1971, July-August 1972, and May-June 1986. Mapping was done through prismatic compass traverses, with all distances measured with a tape. In addition aerial photographs are available for 1943 (US Army, from Bishop Museum, Honolulu) and 1963 (RAF, from Directorate of Overseas Surveys, Southampton). These photographs, British Admiralty charts dating mainly from 1908, and the 1981 1:50000 maps of the Royal Australian Survey Corps, enable a longer-term assessment to be made of geomorphic changes in the last 80 years. Ontong Java atoll The atoll now known as Ontong Java is the largest of several atolls that lie north of the Solomon Islands between latitudes 5" and 6" S (Fig. 1). The atoll contains 120 vegetated islands, mostly shingle and sand accumulations of the elongated motu type. Mean tidal range is about 1.5 metres. Levelling profiles on Pelau, the second largest island, indicate that the seaward reef flat is about 1.0 metre below mean sea level (msl), with intertidal beaches between - 1.0 and +1.4 metres. Most of the island lies at 1.7-2.1 metres above msl, but artificial mounds formed by excavation of taro swamps rise to a maximum elevation of 7.6 metres. The taro swamps themselves are at -0.3 metres, utilizing fresh groundwater supplied by a high average rainfall, which totalled 3.4 metres per annum in 1970-1. The main crops are giant swamp taro (Cyrtosperma chamissonis) and taro (Colocasia esculenta), cultivated by a population which currently numbers 1500 people (Bayliss-Smith, 1974b, 1986). The larger islands are formed from linear ridges of coral shingle on their seaward sides, and on the lagoon side from finer gravel and sand deposits. The smaller ones, including those formed on patch reefs within the lagoon, are simple shingle or sand cays, with o r without beachrock. As in Kiribati to the east (Marshall and Jacobson, 1985), there is no convincing evidence for Holocene sea levels higher than at present. All land lies at elevations that are reached by present storm waves, and all the beachrock observed could have formed from the lithification of beach ridges similar in elevation to those thrown up by the 1967 cyclone. Some elevated micro-atolls were seen on the reef flat near Angii, but the eroded centres of these corals rise only 0.15-0.23 metres above the present level of coral growth, and their formation through the ponding of intertidal water levels cannot be ruled out. It seems reasonable, therefore, to assume that island landforms have for a long period developed in relation to geomorphic processes operating at present sea level. The 1967 hurricane The climate is dominated by South East Trades that blow from May to October, and by the more variable Northwesterlies in the period December to April. Tropical cyclones can occur during these latter months, although very seldom do they produce

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HURRICANES AND REEF ISLANDS, ONTONG JAVA

Fig. 2. A Hurricane damage to coconut palms

B Direction of windthrow

hurricane-force winds (Coleman, 1971). Sarfert recorded in 1910 only two oral accounts relating to storms big enough to destroy many trees (Sarfert and Damm, 1929). Genealogical data collected by Hogbin (1934) provide an approximate chronology which suggests that one of these hurricanes occurred about 1820 and the second about 1850. No major storms occurred between 1910 and 1967, by which time all memory of the nineteenth century hurricanes had disappeared. Cyclone Annie, in November 1967, was regarded by the islanders as so unprecedented that a new word (sakaloni) was adopted to describe it. No further hurricanes have occurred since then, suggesting an approximate mean recurrence interval since the early nineteenth century of 60 years. However, it should be noted that the atoll is extensive, with a maximum length of 70 kilometres and a width ranging from 11 to 26 kilometres, so that half or more of the islands will escape damage from any given storm. Cyclone Annie originated from an incipient low which persisted for several days to the east of the Solomon Islands at latitude 6" South (see Fig. 1). No accurate data on wind speeds along the storm track are available, but wind damage was heavy and locally severe in the Western ~ o l o m o n s A . t Sasamungga on Choiseul 13-20 ft (4-6 metre) waves and a heavy swell were reported (Bureau of Meteorology, 1968). The hurricane passed the southern shores of Ontong Java, but wind speed, wave height and storm surge can only be estimated. Damage to coconut palms is perhaps the most objective

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measure. When fieldwork commenced in June 1970 those palms uprooted almost three years previously were dead or dying, but still easy to identify despite the growth of new palms and saplings. Damage was assessed within 503 randomly-located 20 metres X 20 metres quadrats, in which all mature, immature and fallen coconut palms were counted and floristic and soils data were collected (Bayliss-Smith, 1974a). O n very small islands a complete enumeration of coconut palms was carried out. Compass bearings were taken on over 1500 fallen palms to establish direction of windthrow. These directions were remarkably consistent at a site, but between islands varied from SSW to WNW (Fig. 2). The distribution of coconut destruction suggests that severe wind damage (over 50 per cent destroyed) was restricted to islands in a 27-kilometre zone along the southern rim of the atoll. The rate of damage declined to 3C-50 per cent in a 10-kilometre zone further north, and rapidly diminished to only slight damage (1C-20 per cent) in the 10-kilometre zone beyond. About half of the atoll escaped significant wind damage. If peak wind velocity is correlated with the modal direction of windthrow, then in the zone of greatest wind damage WSW was the strongest wind direction. The destruction of palms is not just the result of high wind speeds. Waves may undercut roots along shorelines, and overwash can cause surface scouring and the saturation of sandy soils which reduces their shear strength (Kferfve and Dinnel, 1983). On Ontong Java maximum palm destruction occurred in southern parts of the atoll most exposed to overwash, and informants reported a continuous heavy surf ('like smoke') passing right across these islands at the peak of the storm. A combination of overwash and winds in excess of 215 kmlhour seems necessary to account for the observed pattern of catastrophic and very widespread damage. During Hurricane Hattie in British Honduras (Belize) in 1961, sustained wind speeds of up to 150 mph (240 kmlh) were associated with total coconut destruction on islands such as Goff's Cay (Stoddart, 1963, 1971). Apart from a few tiny cays which lost all their vegetation, on Ontong Java the worst damage was on Alunga Island with 83 per cent coconut destruction, suggesting less extreme winds than in Hurricane Hattie. Taro swamps were flooded with sea water at Kepae and southern Luangiua, but those at Kemalu and further west escaped damage. The geomorphic changes on Ontong Java also suggest that extreme conditions were experienced during Cyclone Annie. Even three years later seaward reefs along the southern rim of the atoll were virtually devoid of coral growth. Between Ngikolo and Lopaha, a distance of 35 kilometres, the storm produced an almost continuous rampart of rubble about 20 metres wide and 1-3 metres above mean sea level. This ridge is considerably longer than the 19-kilometre rampart reported from Funafuti atoll following Hurricane Bebe in 1972 (Maragos et al., 1973; Baines et a l . , 1974). By 1970 the Ontong Java ridge was reported by the islanders to be lower in many places than it had been immediately after the storm, a similar change to that noted on Funafuti (Baines and McLean, 1976).

Post-hurricane changes Seaward beaches of motu islands Even by 1970 the islanders were pointing to evidence that the 1967 storm ridge was not stable. Overtopped by swash, the ridge, in places, had already moved towards island shores, while on some reef flats between islands it had been reworked through wave attack from both sides into a low sheet of rubble. Detailed mapping of this landform on the islands of Kemalu and Nguakala provides evidence of these and subsequent changes. Both islands are outside the zone of maximum wind damage. Kemalu lost 31 per cent of coconuts and Nguakala 41 per cent, from an inferred wind direction of WSW (Fig. 3). Air photography from 1942 shows coconut woodland largely obscuring a narrow beach on both islands, apart from small spits facing the passage. Little change is evident from 1963 photographs, and the positions of both the 1942 and the 1963 shorelines are similar to the pre-hurricane shoreline which was reconstructed during field mapping in 1971.

HURRICANES AND REEF ISLANDS, ONTONG JAVA

Fig. 3.

A Nguakala and Kemalu Islands in 1942 and 1967 B Kemalu in 1971, 1972 and I986

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Three and a half years after the hurricane, in February 1971, the rubble ramparts which had formed around the seaward shores o f both islands were still in position. At Nguakala (Fig. 4) the rubble rampart had a width o f 4-17 metres above high water mark, and apart from the active seaward beach was clearly a stable feature, partially vegetated with ephemeral seedlings. It was separated from the old shoreline by a protected zone 10-20 metres wide and partly mantled by rubble, but mainly a scoured surface o f beachrock and reef flat, with pools o f tidal water in which fine sand and silt were being deposited. Larger amounts o f sand had been deposited as a spit behind the eastern end o f the barrier through wave refraction and, above high tide level, wind action. On this spit a pioneer vegetation dominated by Scaevola taccada was becoming established. The sand was mostly derived from newly-eroded material transported by ebb currents from the old shoreline near the village. Along the western shore o f Kemalu Island the hurricane debris itself formed a spit protecting the old shoreline (Fig. 3). Along this shoreline, as at Nguakala, active sand deposition was occurring, and a littoral hedge over 1 metre tall had become established along the pre-1967 beach. The ground cover was principally Cassytha filiformis and Lepturus repens grass, while the main shrubs were coconutsi Terminalia samoensis, Calophyllunz inophyllum and Cordia suhcordata. Eighteen months later, in July 1972, both islands were remapped (see Figs. 3 and 4). Almost all the geomorphic changes that had occurred were attributed by the inhabitants to south-westerly gales associated with Cyclone Ida, a small tropical cyclone in May 1972 which caused damage on Choiseul and Santa Isabel (British Solomon Islands, 1972). Ontong Java was 250 kilometres north-east o f the storm track, but the waves generated were sufficient to overtop the 1967 rubble rampart in most places, pushing it back up to 15 metres. As a result, on both Nguakala and Kemalu, the ridge had become joined to the old shoreline o f the island along most o f its length, covering previously-exposed beachrock. Both ends o f the Nguakala barrier were widened and lowered, while on Kemalu most o f the spit was flattened into an intertidal rubble sheet. All drift seedlings on the shingle were destroyed or severely damaged, but on the sandy deposits in the lee o f the shingle the pioneer strand vegetation mostly survived. In places new sand was burying former vegetation, but whether deposited by waves or by wind action was not clear. Fourteen years later, in May 1986, a return visit provided an opportunity to remap these features. The process o f consolidation o f the 1967shingle deposits has continued. At Kemalu (Fig. 3) a smooth new shoreline now extends all along the western end o f the island, with no trace o f the former shingle spit. New land has been added along at least 230 metres o f coast, with up to 25 metres o f new sand and shingle accretion. Longshore drift towards the lagoon may be continuing, but both the elevation o f the new beach and its vegetation cover suggest that the island has now attained a relatively stable configuration. Some o f the new land has been converted to coconut woodland. Elsewhere strand vegetation has persisted, dominated on shingle beaches by abundant Terminalia and Scaevola, frequent Guettarda speciosa, Pandanus tectorius and Calophyllurn, and occasional coconuts. The ground cover in shingle areas was patchy, with occasional Vigna marina, Lepturus and Triumfetta procumhens. Sandy areas nearer to the lagoon had a more disturbed vegetation, with more coconuts, no Terminalia, and few other trees and shrubs apart from abundant Scaevola. The ground cover was Cassytha, Stenotaphrum micranthum and Lepturus. Similar changes were recorded from Nguakala (Fig. 4). but here the eastern shingle spit persists. The other end o f the rubble rampart has been shifted by wave action towards the north-west, where it now forms a tombolo linking Nguakala with the neighbouring island, Pukukae. Two weeks prior to the fieldwork this coast had been battered by gales associated with Cyclone Namu, which had overtopped the tombolo in two places and severely damaged the pioneer strand vegetation dominated by Terminalia. The stability o f this feature may depend on sand deposition occurring on the leeward side, but in May 1986 there was little evidence for this happening. Some o f the material forming this tombolo was being transported as a rubble sheet towards the lagoon, which for very coarse material will act as a sediment sink, reducing the amount available for island replenishment. On the other hand some seasonal reconstruction o f

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