The Hydrology-Geomorpliology Interface: Rainfall, Floods, Sedimentation, Land Use (Proceedings of the Jerusalem Conference, May 1999). IAHS Publ. no. 261, 2000. 215 Hurricane floods as extreme geomorphic events AVIJIT GUPTA School of Geography, University of Leeds, Leeds LS2 9JT, UK e-mail: [email protected] Abstract Tropical cyclones (also known as hurricanes or typhoons) generate extreme floods in parts of the tropics and subtropics between about 10° and 30° of latitude. Accounts of the resulting high shear stress and unit stream power, enhanced stream competence, sediment transport and storage, and channel forms are now available for a limited number of streams in Australia, South Asia, and the Caribbean. Not only are these very large floods but they also tend to occur at an interval of decades, rather than as 50 year or 100 year floods. This paper discusses the hydrology of these extreme floods, areas of their occurrence, their effect on channel size and form, and the flushing of sediment downchannel. The recovery of the channel from its response to such floods is indirectly dependent on: (a) the size difference between the hurricane floods and floods that occur at smaller intervals such as 1-2 years, and (b) the texture of the sediment in the channel. Streams draining basins where tracks of tropical cyclones cross tectonically active terrains tend to be strongly controlled by extreme events. Key words Caribbean; channel form; flood hydrograph; floods; Holocene; Narmada River; sediment transport; tropical cyclones; Yallahs River INTRODUCTION Tropical cyclones (also known as hurricanes or typhoons) generate extreme floods in parts of the tropics and subtropics between about 10° and 30° of latitude. Accounts of the resulting high shear stress and unit stream power, enhanced stream competence, sediment transport and storage, and channel forms, are now available for a limited number of streams in Australia, South Asia, and the Caribbean. Not only are these very large floods, they also tend to occur with an interval of decades rather than as 50 year or 100 year floods. These large floods, which occur on the decadal scale, determine the size and coarse sediment of many river channels in the hurricane-affected areas. Other high flows, which are relatively smaller and occur between the big floods, build inset features in such channels. The final form therefore is a function of two different sizes and frequencies of floods. So is the sediment. Channels of this type have been reported from many areas: northeastern Australia (Wohl, 1992), India (Gupta, 1995), and the Greater Antilles group of islands in the Caribbean (Ahmad et al, 1993). Bourke & Pickup (1999) mention in-filling of channels widened by large floods in arid central Australia. The role of even bigger floods with a recurrence interval in thousands of years as discussed by Bourke & Pickup (1999) and Patton et al. (1993), however, has yet to be properly examined in the more humid seasonal tropics. The most impressive high-magnitude precipitation event in the tropics, tropical cyclones, require specific environmental conditions such as a sea level temperature above 27°C, which restricts their formation to the local summer. As the Coriolis force is another requirement, tropical cyclones do not strongly develop within about 10° of 216 Avijit Gupta the Equator. Tropical cyclones also tend not to develop beyond the 30°N and S parallels, although they may eventually move into temperate latitudes. They may travel for a considerable distance. For example, the tropical cyclones that affect the Caribbean and the Atlantic and Gulf Coasts of North America may begin as tropical depressions in the Atlantic, off the coast of West Africa. Tropical cyclones therefore have a geographical distribution (Simpson & Riehl, 1981; Reynolds, 1985). Certain estimates of the expected amount of rainfall are available for these areas. In general, a 24 h rainfall event in which hundreds of millimetres of rain falls is not unexpected in a decade, and the rain may be expected to continue for 2-3 days. Such rain translates into high-magnitude floods with very high shear stress and stream power. Tropical storms of less than hurricane strength also contribute to flooding in these streams. Unfortunately, a geomorphologist rarely sees these floods in the field or records their effects immediately after their passage. Stream gauges are rare, but even where they exist, high-magnitude floods like these have frequently turned out to be too large to be instrumented automatically. The information that we have is to a large extent derived from hydrological and geomorphological reconstruction from past flood evidence. This is indirect evidence, often anecdotal, and speculation from descriptive studies therefore becomes unavoidable. In justification, it can be said that the rivers are fascinating. This paper is a progress report on attempts to accumulate the available material. It is hoped that in future it will be possible to report more directly on the role of these high-magnitude floods on channel form and behaviour. A number of rivers in the seasonal tropics and subtropics display a channel-in-channel physiography. An inner channel is located within a much larger channel with steep banks. The origin and maintenance of the large channel have been attributed to high-magnitude floods that occur approximately at an interval of decades, whereas the smaller inner channel is used to convey water and sediment of the high flows during the annual wet seasons. This type of nested appearance of channels has been described from Australia (Pickup, 1991; Bourke & Pickup, 1999), South Asia (Gupta & Dutt, 1989; Rajaguru et al, 1995; Gupta et al, 1999), and the Caribbean (Gupta, 1975; Ahmad et al, 1993). The morphology and behaviour of these rivers are controlled by three variables: (a) high-magnitude floods which occur at decadal intervals, (b) the seasonality in annual discharge, and (c) the volume and texture of the sediment in motion (Gupta, 1995). HYDROLOGY OF EXTREME FLOODS Ideally, we should have a series of measurements of extreme floods with assigned probabilities for a number of rivers. In reality this does not happen. Not many stream gauges are. located on the rivers of the humid tropics, and those that do are often destroyed in these floods. There are certain estimates to compensate for this lack of information, but long records of good river discharge measurements are difficult to find. The role of extreme floods is also governed by a scale factor. The valleys of small tributary streams of first or second order often carry debris flows or hyperconcentrated flows rather than sediment-laden river discharges. At the other end of the scale, floods on larger rivers in the lowland are often attenuated compared to the sharper peaks that occur upstream. Such attenuation may tend towards the curve type A of unit stream Hurricane floods as extreme geomorphic events 217 power of Costa & O'Connor (1995). The flood may be one of long duration but with very low peak stream power that limits the effectiveness in changing channel form and floodplain features. Apart from the need for high peak power to operate over a moderate to high duration (curve B of Costa & O'Connor, 1995), effectiveness is also related to the texture of stream sediment and bed and bank material. Flood effects are best preserved when the sediment is coarse and the relief is high (Gupta, 1988) because in such environments the flood effects are difficult to undo. In view of problems in reliable data acquisition, surrogate or synthetic information is frequently used. Such surrogates may involve statistical analysis on rainfall data, using synthetic valuation of discharge such as calculating with an assumed Manning's n, and using regional figures for approximation. Such manipulations provide impressionistic estimations but reliability may remain low. An examination of the heavy 1 day rainfall figures from the tropics indicates that although figures in 103 mm (as have happened in Réunion, Taiwan, Philippines and Jamaica) are rare, figures in 102 mm are quite common in the humid tropics (Gupta, 1988, Table 3, which is a compilation from various sources). The high rainfall is usually from tropical disturbances from depressions to hurricanes. When the total rainfall for such tropical storms are cumulated (over several days), figures of several hundred millimetres become quite common. Rainfall in the storms, however, varies widely and therefore the occurrence of a hurricane does not necessarily guarantee a large flood. For example, rainfall from eight hurricanes between 1899 and 1999 over Puerto Rico varied between 135 and 725 mm (Scatena & Larsen, 1991; M. C. Larsen, personal communication). At the extreme end, in the four days between 5 and 8 November 1909, Silver Hills in the Blue Mountains of Jamaica received 2451 mm of rain. On the other hand, the 2789 mm of rain between 22 and 25 January 1960 over Bowden Pen, Jamaica, was frontal in character (Vickers, 1967). High rainfall events are of course more frequent when all sources are considered. Both Lirios (1969) and Evans (1972) estimated several hundred millimetres of rain to fall in 24 h in the Blue Mountains of Jamaica with a recurrence interval of as low as 5 years. Their estimates for the 25 year recurrence interval rainfall range between 350 and 735 mm approx­ imately. There are some disparities between their estimates but it is common to record such high rainfall with a recurrence interval of decades. Such figures are found in most of the tropical cyclone affected humid tropics. The geographical locations where tropical cyclone driven floods could be expected to affect river systems are: - north and northeastern Australia; - parts of southeast Asia which are affected by tropical cyclones as in Myanmar, Vietnam, and the Philippines; - parts of the Indian subcontinent; - Madagascar and neighbouring coastal areas of East Africa; - the Caribbean islands, the coastal areas of the Gulf of Mexico, and tropical and subtropical North America.