| || || Ratiram, Anupama|
| || || Investigating the braided system of the Jourdain river, Espiritu Santo, Vanuatu |
Institution: University of the South Pacific.
Subject: Braided rivers -- Vanuatu -- Espiritu Santo
Call No.: pac QH 541 .5 .S7 R382 2013
Copyright:Under 10% of this thesis may be copied without the authors written permission
Abstract: Braided rivers are large rivers that rejoin and subdivide around islands and bars. Braided rivers are characterized by unstable behavior as they tend to be mobile. The Jourdain River is such a river that is found in the humid tropics. Most large braided rivers are found in proglacial to semi arid areas where the land area is large. The Jourdain River is a braided river that drains on the northern coast of Espiritu Santo, the largest island of Vanuatu, although the island’s area is comparatively small at approximately 2500 sq. kilometers. The Jourdain is the largest fluvial system draining through one of Vanuatu’s most important conservation parks, the Vatthe National Park. Most humid tropical islands have meandering rivers rather than braided rivers and therefore the Jourdain River and other braided rivers of Santo are unusual and this thesis presents the first detailed study of such a river system on a small tropical island. Braided rivers are characterized by unstable inerali as their channels tend to be mobile. The purpose of this thesis is to determine the cause(s) of braiding of the rivers on Santo. This is carried out through three aims: the first aim is to describe the main features and characteristics of the braided system of the Jourdain River; the second aim is to explain the reason for braiding of the Jourdain River and its dynamism; the third aim is to compare the characteristics and features of the Jourdain River to braided rivers in proglacial and semi arid areas to see what similarities and differences exist between them. Several variables affect the pattern of a river – climate and geology are known to be the main controls for braiding to occur, other environmental factors include variable stream discharge, steep valley slopes, abundant bed load and erodible banks. The thesis analysed sediment type and relative abundance, valley floor slope and assessed the tectonics in the area. Characteristics of the river were also investigated through studies of channel migration over several decades and calculation of braiding index (BI). For these variables to be tested field work was carried out in consecutive years and archival data such as aerial photographs were used to make measurements of channels characteristics. The characterist ics of the Jourdain River together with its stream variables were compared with rivers in semi arid and proglacial environments in other parts of the world; and comparison was made to other limited studies of braided rivers found in the tropics. The results showed that volcanic rocks were the most abundant and they normally occurred as gravel. Analysis of the valley floor slope of the tributaries and the river found that the braided reach has the lowest value of valley floor slope (braiding occurred with a valley floor slope of <0.3), likely due to a reduced gradient near to the coast and thus lower stream power and subsequent deposition of sediments. The BI of the river from 16 sections was 4.29 which is high and comparable to those in Arctic and semi arid areas; the river was characterized by high mobility of channels over the years (from 1962 to 2003) and even over a short term time scale. The volcanic rocks were mostly contributed from tributaries bringing in large amounts of sediments and also fine sediments from large landslides that occur close to the tributaries, and from the abrasion of rocks. Gravel within the river is also known to be reworked as was observed during the fieldwork; channel mobility was owed to sediment supply and transportation of these sediments through aggradation and degradation processes. In addition, the mobility was also related to natural disasters that caused a lot of changes within the river. Faulting and uplift of the island are producing channel migration mostly in a westward direction. Human activities in the catchment such as agriculture may also mobilize sediments although the contribution is likely to be minimal given the low population density in the area. The comparison of the characteristics of the Jourdain River with rivers from other climatic settings indicates that the Jourdain River is very similar to some other rivers; the most noticeable difference being that the rivers in proglacial and semi-arid areas on continents have a much wider braid belt than the Jourdain due to the larger land areas on continents but also due to the sparser vegetation cover that occurs in these environments. Another difference is that the Jourdain River has larger bar sizes than those described from rivers in proglacial and semi-arid areas, as is typical for tropical rivers, due to the vegetation cover that stabilizes sediments. Climate, discharge, geology/tectonics, sediment abundance, valley floor slope and probably also human activities are all working in synergy for braiding to occur in the Jourdain River and likely also for other rivers on Espiritu Santo. In terms of natural disasters, tropical cyclones tend to occur interannually, although tremors can occur on a daily basis in Espiritu Santo and the braiding is thus likely to be mostly due to sediment abundance triggered by tremors.