The Hydrology-Geomorpliology Interface: Rainfall, Floods, Sedimentation, Land Use (Proceedings o f 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: aviiit@foxhill.demon.co.uk

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 approx­imately. 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. These cover a large area despite the restrictions of high relief and coarse sediment. However, the currently available case studies do not cover all these areas.

Table 1 shows discharge figures for the streams of eastern Puerto Rico which has an efficient system of streamgauges. The following conclusions can be reached:

218 Avijit Gupta

Table 1 A selection of peak flood discharges, eastern Puer to Rico.

River and Area Peak discharge at recurrence Hurr icane flood peaks: Recorded station ( k m 2 ) interval of: Hugo Hortense Georges peak ( k m 2 )

2 year 5 year 10 year (1989) (1996) (1998) max.

Espiritu 22.3 200.5 340.0 433.2 427 370.9 382.2 566 Santo, at (8.9) (15.2) (19.4) (19.1) (16.6) (17.1) (25.4) Rio Grande Fajardo, at 38.6 229.6 381.1 483.6 666 182.0 240.1 Fajardo (5.9) (9.9) (12.5) (17.3) (4.7) (6.2) Mameyes , at 17.8 291.6 411.4 489.5 580 362.4 356.7 Sabana (16.4) (23.1) (27.5) (32.6) (20.4) (20.0)

Icacos, at 3.3 35.0 49.6 59.5 70 53.8 61.7 81 N a g u a b o (10.6) (15.0) (18.0) (21.2) (16.3) (18.6) (24.5) Quebrada 2.6 32.3 49.8 61.4 63.2 Sonadora , at (12.4) (19.1) (23.6) (24.1) El Verde

Note : All discharge figures are in m 3 s"1; figures within parenthesis are unit discharges in m J s"1 k m - 2 . Source: Scatena & Larsen (1991), U S Geological Survey (1997, 1999), Ramos-Gines (2000) .

- In this region, hurricane floods can have a recurrence interval of less than 10 years. In fact, some researchers suspect a clustering of hurricane and non-hurricane events (Gray, 1990). In any case, high-magnitude floods seem to recur with an

, -interval of decades. --""The hurricane floods and the 10 year recurrence interval flood have reasonably

high unit peak discharge values, although nowhere near the world limit. - The 2 year flood carries approximately half the discharge of a hurricane flood or a

flood with a 10 year recurrence interval. - This implies that the 2 year flood is able to remove and reorganize a significant

amount of relatively finer sediment. This general pattern is seen in many parts of the humid tropics where tropical cyclones occur at short time intervals. However, the dataset is not always available, as for the streams of Puerto Rico. But where datasets are available the pattern is similar. The gauging station at Mortakka on the Narmada River of India, for example, records wet monsoon peak discharges between 10 000 and 30 000m 3 s"' , but peak discharges ranging up to 55 000 m 3 s"1 have been recorded in extreme floods (Gupta et al., 1999). The gauging station at Jamtara, on the same river but further upstream, has values of 7115 m 3 s"1 and 18 391 m 3 s"1 for peak discharges of 2 and 50 year recurrence interval respectively. These figures describe river systems which have high floods in the wet season on a short-interval basis, and a set of bigger floods when tropical storms occur, at longer recurrence intervals. Such intervals, however, are of decadal length.

Figures 1 and 2 are two hydrographs for the Yallahs River at Llandewey, Jamaica. These are not hurricane floods but flood hydrographs which happen to be available. Table 2 puts these flood events into perspective. The hydrographs illustrate two non-hurricane floods, which as the estimate for the Gilbert flood shows, have recurrence intervals well below 25 years. In fact, data collected by the Underground Water Authority, Jamaica (ODP & WMO, 1988) show that peaks (Figs 1 and 2) were exceeded three times in the 15 years between 1972 and 1986. Both figures show multiple peaks in discharge rising steeply from lower values, especially during the

Hurricane floods as extreme geomorphic events 219

300

YALLAHS at Llandewey 30 OCT.- 1 NOV 1987 k -

- ! ^ 500mm boulder movement (Wi!liams,1983)

UNIT STREAM POWER \ y I \ \

! ^ i "**"•

1 : / J

J | \ vL'DISCHARGE x

j 15mm pebble movement (Williams. 1983)

J | \ vL'DISCHARGE x

j 15mm pebble movement (Williams. 1983)

E 5

1 0 0 0 =

1200 1800 31 OCT,

600 1200 1 NOV,

Fig . 1 Yal lahs River at Llandewey, Jamaica: flood hydrograph of 30 October-1 N o v e m b e r 1987 and the ability of the s t ream to t ransport coarse material .

_ 300

1= UJ

YALLAHS at L landewey 2 1 - 2 3 MAY 1991

1800 21 MAY

0600

1000 p

0 1200 1800 2400 22 MAY 23 MAY

Fig . 2 Yal lahs River at Llandewey, Jamaica: flood of 2 1 - 2 3 M a y 1991 and variation in unit s t ream power .

220 Avijit Gupta

T a b l e 2 Peak flood discharges on the Yallahs River at Llandewey, Jamaica.

F lood of 3 0 - 3 1 M a y 1986 453 .00 (gauged) F lood of 30 O c t o b e r - 1 November 1987 (Fig. 1) 438.84 (gauged) F lood of 2 1 - 2 3 M a y 1991 (Fig. 2) 450.16 (gauged) Hurr icane Gilbert flood, September 1988 523.5 (est imated), 25 year RI F lood of 1973 729.2 100 year RI 707.0 (H.E.C. 1 est imate)

Note : All flood measurements are in m 3 s"1

Source: The flood discharges of 1986, 1987 and 1991 are by courtesy of the Water Resources Authori ty, Jamaica. The rest are from the Office of Disaster Preparedness (Jamaica) , and the Wor ld Meteorological Organizat ion (ODP & W M O , 1988).

1991 event. It seems likely that a bigger flood, like that caused by Hurricane Gilbert, would have a similar pattern of multiple peaks, with pulses of water arriving from various sources at different times, but the peaks will be larger.

HURRICANE FLOODS AS A GEOMORPHIC EVENT

The changing unit stream power for the Yallahs at Llandewey has also been plotted for these two floods (Figs 1 and 2). It should be noted that the calculations have been carried out with certain approximations: the channel slope has been used instead of the energy slope; an average reach gradient has been used; the mean channel velocity has been determined by accepting a Manning's n of 0.055 (Barnes, 1967). The conclusions reached therefore should be seen as semi-quantitative trends rather than as carefully determined measurements. Even then, certain conclusions can be reached, particularly from Fig. 1. - The flood hydrograph shows more than one sharply rising peak and falling stage,

and the high flow continues for about half-a-day. - A significant difference exists between (a) the pre-flood and (b) the peak and near-

peak-stage unit stream power. - The rise in stream power is extremely steep. - According to the relationships established by Williams (1983), material as large as

500 mm boulders can be moved for nearly 12 h. The river has the capability to move cobbles, pebbles and sand for the duration of the entire flood.

- The very low pre-flood flows do not have the requisite unit power to move sediment in the channel.

- The Yallahs, during floods of this magnitude, is capable of moving considerable amounts of sediment resting in the channel, and also of eroding the coarse alluvium of bars, channel banks, flood plains and low terraces. Such activities probably happen in spurts.

- The long duration of the high discharge suggests that a high sediment concentration can only be maintained if sediment supply to the channel continues from upstream, either via tributaries or from steep slopes failing directly into the river.

Similar conclusions can be reached by examining the time-distribution of shear stress and mean velocity in floods. Hurricane floods or floods with a recurrence interval of, say, 20 years, will have even greater unit power and may last for several days.

Hurricane floods as extreme geomorphic events 221

No record of sediment moving down the Yallahs in a hurricane flood exists. High volume of sediment transfer is illustrated by the post-flood deposition on the Yallahs fan, on paved road crossings at low levels, and the breaking of the submarine cable off the mouth of the Yallahs during the Hurricane Flora flood of 1963 (Burke, 1967). There are also records of bridge destruction on the Yallahs indicating transfer of coarse sediment. Suspended sediment was measured for the much smaller Hope River at Gordon Town, Jamaica, as part of the environmental management of the Hope River Watershed Project. The Hope upstream of Gordon Town has only a 41 km 2 drainage basin. In the flood discharge associated with Hurricane Gilbert, 35 727 t were moved on 13 September 1988. The Hope during the dry season carries less than 20 t of suspended sediment daily. The total monthly sediment carried by the Hope in September 1988, 1989 and 1990 was 92 430 t, 567 t and 5137 t respectively (Thomas, 1991). This indicates the importance of hurricane floods (and floods in general) in the geomorphology of the seasonal tropics. The peak velocities of the Gilbert flood ranged between 1.2 and 7 m s"1 for the Jamaican rivers (ODP & WMO, 1988). Such velocities can move coarse sediment, and in large quantities.

There are several sources of the sediment. A very large amount of material is derived from slope failures either as shallow soil slips, deeper slides or debris flows along small tributaries. Landslides in eastern Jamaica have been studied over the last ten years by Rafi Ahmad and his co-workers at the University of the West Indies and Office of Disaster Preparedness, Jamaica. Several instances of field mapping exist. Manning et al. (1992) mapped 478 slope failures in 108 km of distance along roads in the area northwest of Kingston. These were mostly debris flows but also included rock falls, translational slides, and complex rotational slides. Maharaj (1993) mapped 866 slope failures in 15 Ion2 north of Kingston which gives an average of 58 slides per Ion2. After the intense rainfall of May 1991, the mapping was repeated. The second total was 950 with 540 re-activated old slides. Almost all the failures were debris flows and slides. Ahmad (1994) concluded that about 200-300 mm of rain in 24 hours would initiate shallow slides on slopes of more than 25°. In the hills of eastern Jamaica the recurrence interval of such rain varies between 2 and 10 years, depending on the station (Evans, 1972; Gupta, 1983).

These slides produce the large amount of sediment that moves in the high-magnitude floods. A sequence of events has been proposed in an earlier study (Ahmad et al, 1993). This involves: (a) damming of channels by coarse boulders, (b) boulders removed in subsequent large floods and randomly distributed in the channel, (c) building of small bars with large boulders downstream over time, and (d) building of large bars with almost no boulders further downstream. This of course depends on the types of material falling into the channel. Robinson et al. (1996) and Robinson & Laughton (1997) have discussed events leading to natural dams and their failures in Jamaica.

Other sources of sediment for the flood-prone Jamaican rivers are the bed and the bank material, and tributary contributions. However, a large part of the tributary contributed sediment begins as some kind of slope failure. Simon et al. (1990) have summarized the mechanism and causes of slope failures in the Luquillo Mountains of eastern Puerto Rico. There shallow soil slips and debris flows are released by short-duration high intensity rainfall. Larger events, related to tropical storms (including

222 Avijit Gupta

hurricanes), add larger and deeper slides to the list. More than 400 landslides were mapped in parts of the Luquillo Mountains after the passage of Hurricane Hugo in September 1989. The fact that landslides affected 0.11% of the surface area on the forested slopes indicates that large sediment supply in storms is an integral part of geomorphic processes and landform development in eastern Puerto Rico. Hurricane Hugo eroded 11 767 m 3 of material from this area (Larsen & Torres Sanchez, 1992, 1998). Most of it came down to flood-swollen streams with discharges as listed in Table 1.

A subsequent hurricane to visit Puerto Rico (Georges, 21-22 September 1998) caused about one slide per km 2 in the Luquillo Mountains area; the slides are mostly shallow soil slips and debris flow. A small channel draining a 24 km 2 basin in central Puerto Rico (which was affected more strongly by the hurricane) moved 2 m boulders and built large bars in boulders and cobbles for several kilometres along its course (M. C. Larsen, personal communication). This pattern has been reported from the stream channels of the mountainous islands in the Great Antilles group of islands in the Caribbean (Ahmad et al., 1993).

This collection of field descriptions and Fig. 1, taken together, suggest that an enormous volume of sediment moves out of the steep valleys of the Caribbean in large rainfall events, particularly in hurricanes (Fig. 3). Figure 1 shows that boulders have moved for more than 12 h in a flood which is probably repeated in less than 10 years. Even after the boulder-size material stopped moving, the Yallahs was still capable of moving smaller boulders, cobbles, pebbles and sand for a considerable length of time. Material falling into the streams therefore gets flushed out for a considerable distance.

z&!&~*, „ . * S . » J Ô » . . ~.tir?t,r.~.. ~

—

Fig. 3 Sediment in the channel of the Buff Bay River, Blue Mountains, Jamaica; the largest boulder seen has an intermediate diameter of about 5 m.

Hurricane floods as extreme geomorphic events 223

In larger floods (hurricane-driven?) material may even be flushed out on to the coastal fans leaving the channels relatively clear for a period until the next small flood event starts building bars as insets in a widened channel. Perched boulders and cobbles, possible evidence of macroturbulence in a flood with low sediment concentration (Cenderelli & Cluer, 1998), are seen scattered along all the regional rivers except where the channel gradient is less steep. It is likely that the earlier pulses of discharge (Fig. 1) remove a considerable amount of sediment, and the later pulses are responsible for the macroturbulence effect. Such forms may also happen in a random fashion between two slugs of moving sediment. Once deposition of coarse material starts, some of these locations tend to form cores for future bar building. Bars are also built in other locations, for example, as dictated by channel geometry.

In India, the flood-affected Narmada derives its sediment load from (a) erosion of bedrock over steeper sections and in gorges, (b) from an old Pleistocene river alluvium which forms its high banks, and (c) to a lesser extent, from the tributaries. It is a river whose channel is also formed and maintained by the passage of large-magnitude floods at intervals (Gupta et al, 1999).

These rivers which are affected by very large floods may have bedrock channels, or bedrock channels thinly veneered with coarse alluvium, for at least part of their courses. Case studies of such rivers include the Burdekin in northeastern Australia (Wohl, 1992), the Yallahs and Buff Bay in Jamaica (Gupta, 1975), a number of small streams in eastern Puerto Rico (Ahmad et al, 1993), the Narmada (Gupta et al, 1999) and Tapi (Kale et al, 1994) in India. Baker & Kale (1998) have recently summarized the effect of high-magnitude floods in shaping bedrock channels. The bedrock channels can have a wide multi-channelled reach or a gorge section where an inner gorge is flanked by scablands (Fig. 4). The erosion of the bedrock to produce a river channel, however, requires a high shear stress and stream power. Bed shear stresses of 103 Nm" 2 and unit stream powers of 104 Wnf 2 have been calculated (Baker et al, 1988; Wohl, 1992; Kale et al, 1994; Gupta, 1995; Baker & Kale, 1998).

In sum, where storms up to hurricane level occur in the seasonal tropics, the resulting high-magnitude floods erode very large channels that are then modified inside the channels by deposition in smaller peak flows. An understanding of the rivers of the seasonal tropics therefore requires investigation into the effect of these floods. However, the final appearance of the channels is also dependent on other factors such as local rock types and presence or absence of neotectonics (Gupta, 1995).

This discussion leads to two further questions. How old are the channel forms described, and is the evidence of such flood-affected channels preserved in the sedimentary records?

THE AGE OF THE CHANNEL FORMS

The available historical information varies among different parts of the humid tropics and thus it is not feasible to construct an overall temporal account. A summary account of the last ~18 000 years for South Asia is presented here and it is suggested that similar general trends might have occurred elsewhere. Most of the data drawn on come from three sources: offshore sedimentary evidence, research in the Thar Desert and its margins, and alluvial sequences in a few river valleys of Central India.

224 Avijit Gupta

Fig. 4 The Narmada River, India: inner gorge, and scabland at the shoulder of the gorge below a riverbank in Pleistocene alluvium.

Duplessy (1982) commented on the changing nature of the South Asian monsoon from the last glacial maximum at approximately 18 000 years BP. The difference between the 1 8 0 / 1 6 0 ratios of planktonic foraminifera deposited on the floor of the Arabian Sea, northern Indian Ocean and the Bay of Bengal at that time, indicates a weak southwestern monsoon and a stronger northeastern one. The discharges of a number of rivers including the large ones draining into the Bay of Bengal (the Ganga, the Brahmaputra, the Irrawaddy, the Salween), were reduced in a weak southwest monsoon system. The continental aridity of South Asia contrasted sharply with heavy oceanic precipitation limited to the south of 10°N latitude. The pattern seems to have changed at the onset of the Holocene leading up to the present stronger southwestern monsoon.

A series of investigations has been carried out in the general area of the Thar Desert, northwestern India, to determine climatic and environmental changes. For example, Wasson et al. (1984) provide a reconstruction of the moisture conditions based on study of the Didwana Lake. The lake changed from a hypersaline condition to fluct­uating saline and then to deep freshwater conditions at about 13 000 years BP. Around 6000 years BP, the lake had deep freshwater which shallowed a little over time, the lake turning ephemeral before 4000 years BP. The pattern of change of moisture cond­itions is similar to that described more recently for the Lunkaransar dry lake (Enzel et al, 1999). A general picture of wet conditions in the Early Holocene (-8000 years BP) followed by a desiccating phase prior to 4000 years BP and ultimately leading to present

Hurricane floods as extreme geomorphic events 225

conditions, seems to emerge from a number of studies. Two basic conclusions from the Thar Desert studies seem to be (a) a change to wetter conditions in the Early Holocene, followed by (b) drier conditions and fluctuating moisture availability.

Research in the river valleys of India has also suggested changes in moisture availability and river discharge patterns. Williams & Clarke (1984) described the change in the headwaters of the Son River, from sparse vegetation and high sediment yield during the last glacial maximum, to dense vegetation and low sediment yield during the warmer and wetter Early Holocene. They also mention a conspicuous Early Holocene channel enlargement caused by erosion of the Pleistocene valleyfill, that left the bed of the Son about 30 m below the old riverine plain. A change in channel pattern, size and shape in the Holocene has also been suggested for the Narmada River (Gupta et al., 1999). Large floods maintain the channel of the present Narmada, and such floods probably started to appear in the Holocene (Kale et al., 1993, 1994; Baker et al., 1995; Gupta et al., 1999). The history of large floods in central India so far has been extended to 5000 years BP (Kale et al., 1993) and a pattern of clustering of floods and low flows is beginning to emerge (Ely et al, 1996; Kale et al, 1997; Kale et al, 2000).

Large-scale climatic changes between the last glacial and Early Holocene have also been reported from Australia. Increased rainfall and rapid spread of rainforest in northeastern Queensland is reported by Kershaw (1978). No detailed investigations on the Quaternary flood history of the Caribbean have been reported. In Jamaica, the local geology indicates that the flood controlled rivers of the Blue Mountains, at least their present channels, are post-Pleistocene. If so, the rivers of the Blue Mountains, like the rivers of Central India, reached their present form and size only in the Holocene. The sea surface temperature dropped below 27°C for the month of August in the Bay of Bengal during the last glacial (Ruddiman, 1984) which negates the formation of tropical cyclones. A cooler ocean implies fewer storms of smaller size. A drier Pleistocene therefore should also be a time when floods were rare. Currently a year of high annual rainfall also implies a stormy year in the seasonal tropics.

It seems likely that at some time in the Holocene, with higher rainfall and more frequent and bigger storms, the streams excavated bigger channels to accommodate much enlarged flood peaks. The high flows of the wet season then deposited sediment inside the channel as insets to create the channel-in-channel physiography described for several rivers (Gupta, 1995). The present flood-controlled channels and their forms therefore have existed for only several thousand years. This conclusion, however, needs to be tested with proper dating of both alluvium and bedrock samples from such rivers. Erosion and sedimentation in these channels should be seen more as episodic events and not as happening at an average rate. The eroding of bedrock to form river gorges and scablands is more of a step-function than a continuous one.

FLOOD CHANNELS AND THE GEOLOGICAL RECORD

In a review paper dealing with fluvial forms, Fielding (1999) opined that fluvial environments representative of extreme climatic areas have not been adequately studied. He identified certain channel characteristics, such as a channel-in-channel physiography, as representative of areas where rainfall is strongly seasonal giving rise to pronounced variability in discharge. Facies models for this type of river are

226 Avijit Gupta

currently lacking in the geological literature but future work on their deposits and internal architecture should rectify this omission. There is a general lack of information on an important fluvial environment: "the group of large variable discharge or extreme discharge tropical rivers fed by cyclonic rain" (Fielding, 1999 p.502). Fielding has listed two rivers of this type from northeastern Australia (the Burdekin and the Pioneer), which have been studied for their fluvial sedimentology. The fan-delta of the Yallahs River of Jamaica can be added to the list (Wescott & Etheridge, 1980). Fluvial sedimentology of several Himalayan rivers, e.g. the Kosi (Singh et al., 1993), has recently been described. However, these accounts mostly deal with fan-type deposition and very little is available regarding the fluvial sedimentology of the extreme discharge tropical rivers.

CONCLUSION

The importance of high-magnitude floods, caused by rainfall from tropical cyclones, on channel form and stream behaviour has been discussed in a number of recent publications. This paper explores the hydrological background for such forms and processes with data from Jamaica, Puerto Rico and India. It is suggested that extreme discharge governed tropical rivers began to appear at the onset of the Holocene with the associated climate change.

Acknowledgements I would like to thank M. C. Larsen for the information on the flood discharges of the rivers of eastern Puerto Rico. Rafi Ahmad (University of the West Indies) and Francis Murphy and Michael Samuel (Water Resources Authority, Jamaica) very kindly provided me with hydrological information on the Yallahs River at short notice. This paper could not have been written without such acts of kindness and help. My familiarity with the extreme discharge rivers evolved gradually over the years and with help from a number of geomorphologists. I should acknowledge M. G. Wolman, Bud Frank, Rafi Ahmad, Fred Scatena, Matt Larsen, Vic Baker, S. N. Rajaguru and Vishwas Kale. A discussion with Ashok Singhvi regarding the climate change in the Thar Desert area since the last glacial maximum was extremely helpful. The paper is a much improved version after the careful review of Olav Slaymaker.

REFERENCES

Ahmad, R. (1994) Landslides in Jamaica: extent, significance and geological zonation. In: Environment and Development in the Caribbean: Geographical Perspectives (ed. by D. Barker & D. J. F. McGregor), 147-169. University of the West Indies Press, Barbados.

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