Influences on River Responses

High-magnitude rainfalls delivered by tropical cyclones normally produce extraordinarily big discharges in Pacific island rivers (Figs. 9.4 and 9.5). The nature of a river's response depends primarily on three groups of influences, which are discussed below. The first group are the meteorological characteristics of an individual tropical cyclone, such as the organisation of its cloud bands, speed of movement and the corresponding patterns in precipitation. The second set of factors are those concerning the physical geography of the landscape, particularly the types of geology, soils, topography and vegetation. The third group are parameters related to the geometric configuration of the river drainage basin, especially its size, shape and orientation.

During storm events, it is the combination of exceptional rainfall amounts and intensities that promotes swift transmission of moisture into stream channels, by saturating the soil and generating excess runoff (Bonell and Gilmour 1978, Walsh 1980). Contrary to popular belief, this can occur under a mature cover of tropical rainforest (Herwitz 1986). Storm duration is also important because rainfall interception by the vegetation canopy decreases in proportion to storm size (Jackson 1971). This means that the longer a storm lasts, the more the canopy interception decreases and throughfall increases as a percentage of total precipitation input. Windy conditions in tropical

Pictures Fiji During Cyclone Season
Fig. 9.4. View of the old main bridge over the Sigatoka River at Sigatoka town, southwest Viti Levu island in Fiji, during the flood produced by Tropical Cyclone Kina in December 1992. The water level is 10 m or more below the bridge at normal flow. Courtesy of the Fiji Times.

cyclones also aid moisture transfer to soils by further reducing the possibilities for vegetation interception (Herwitz 1985). If two or more cyclones strike an island during the same wet season, then forest damage such as foliage stripping, crown breakage and tree uprooting caused by the initial storm reduces the interception-storage capacities of the canopy during any later cyclone events (Walsh 1982), encouraging very rapid water loss from affected catchments.

The windward side of an island in relation to the direction of cyclone approach, which we might call the 'cyclone side', will benefit from orographic effects and generally receive much more rainfall than the sheltered side behind volcanic highlands. Consequently, rivers on the cyclone side of islands are more likely to experience severe floods. For Vanuatu, Samoa and Fiji, rivers on the north and west sides of the big islands are therefore more prone to cyclone-induced floods. This is because in addition to draining tall mountains that promote strong orographic effects, they also face the direction from which tropical cyclones normally arrive.

The speed and the proximity of a cyclone track in relation to the position and orientation of a particular river basin are also important influences on the flood magnitude. This is because a storm which travels slowly, close and roughly parallel to the long axis of a river basin, is likely to deliver big rainfalls to most of the river's main tributaries and thus produces a large flood. Flooding is less likely where a cyclone moves relatively quickly along a track that runs perpendicular to the orientation of the river system (Kostaschuk

Rewa River Island
Fig. 9.5. Peakflows produced by tropical cyclones from 1992 to 1999 in the Tontouta River on Grande Terre island, New Caledonia.1 Bankfull discharge at the hydrometric station is approximately 600 m3 s-1. Source: Observatoire de la Ressource en Eau, New Caledonia.

et al. 2001). This is illustrated by Table 9.3, which lists the chronology of tropical cyclones that have caused overbank floods at three hydrometric stations in the major Rewa River system in Fiji (Fig. 9.6) between 1970 and 1997. Tropical cyclones that had the greatest flood impacts were those that passed near the axis of the watershed. The centres of TCs Wally, Bebe and Kina, for example, all tracked within 50 km of the centre of the Rewa basin, and more importantly followed paths that ran more or less in the same direction as the basin orientation.

Since many volcanic island rivers have upper catchments dominated by rugged topography, this promotes a high degree of hydrological short-circuiting. This means that during a torrential cyclonic downpour, the precipitation is transferred quickly into the river channels, leading to a fast hydrological response. Rivers therefore display flashy behaviour, giving little time lag

1 Grande Terre island in New Caledonia is not a volcanic island like the large islands of neighbouring Solomon Islands, Vanuatu and Fiji. It is a piece of the ancient continent of Gondwanaland and comprises ultramafic rock types. Grande Terre has steep terrain and good flow records exist for the Tontouta River. So the Tontouta is a useful river for illustrating the effects of tropical cyclones on river hydrology in the South Pacific, in spite of the unusual (non-volcanic) basement geology.

Table 9.3. Chronology of tropical system of eastern Viti Levu island cyclones from 1970 to 1997 that caused overbank floods at three hydrometric stations in the Rewa River in Fiji.

Table 9.3. Chronology of tropical system of eastern Viti Levu island

Nabukaluka

Nairukuruku

Navolau

Storm duration

Flood duration

Flood duration

Flood duration

Tropical cyclone

(dd/mm/yy)

(days)

2p(m3 s-1)

(days)

Qp (m3 s-1)

(days)

Qv (m3 s"1)

Priscilla

14/12/70-18/12/70

<

<

-

-

1

2.003

Bebe

19/10/72-06/11/72

1

-

-

-

3

6.711

Lottie

05/12/73-12/12/73

<

<

-

-

2

2.304

Tina

24/04/74-28/04/74

<

<

-

-

1

3.100

Val

29/01/75-05/02/75

<

<

-

-

2

2.806

Wally

01/04/80-06/04/80

2

798

2

1,853

2

4.218

Hettie

27/01/82-30/01/82

<

<

2

1,609

2

3.311

Oscar

28/02/83-02/03/83

<

<

2

2,759

3

4.533

Nigel

19/01/85-20/01/85

<

<

2

1,368

1

2.734

Gavin

04/03/85-07/03/85

<

<

2

2,140

2

3,960

Sina

24/11/90-30/11/90

<

<

<

<

1

2,117

Joni

06/12/92-13/12/92

<

<

1

2,089

2

3.581

Kina

26/12/92-05/01/93

2

602

4

7.334

2

6.923

Gavin

04/03/97-11/03/97

<

<

3

6,384

1

3.927

June

03/05/97-05/05/97

<

<

-

-

3

4.015

Flood characteristics Number of overbank floods

Decadal frequency of overbank floods i J

Flood characteristics Number of overbank floods

Decadal frequency of overbank floods

Qp sample mean (m3 s 1) Qp standard deviation (m3 s-1)

700 1.1

Storm duration is the time that the tropical cyclone occupied Fijian waters, gp is the peak daily discharge caused by the cyclone, and flood duration is the period that the flow exceeded bankfull stage; < indicates the peakflow remained below bankfull discharge (i.e. no overbank flood), - indicates no data. Source: Hydrology Division of the Fiji Public Works Department.

Fiji Flooding Viti Levu
Fig. 9.6. The Rewa River on Viti Levu island in Fiji, showing its major tributaries and the sites of three long-term hydrometric stations operated by the Hydrology Division of the Fiji Public Works Department.

between the onset of intense rainfall and the rise of the rivers. On Grande Terre island in New Caledonia for instance, river discharge may increase 100-fold in less than an hour (ORE 2002).

The natural vegetation of many high islands across the South Pacific was originally forest, but extensive areas were cleaned and burned by early farmers. On the western drier sides of bigger islands like Viti Levu in Fiji this assisted the spread of savannah-like grasslands in the mid-Holocene. Further vegetation clearance and burning has been associated with the introduction of grazing animals, commercial cropping or the timber industry after the arrival of European colonists in the nineteenth century. These types of land-management practices and land-use changes exacerbate hydrological short-circuiting and enhance the size of cyclone-generated floods as a result.

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