An Examination of Vertical Accretion of Floodplain Sediments in the Labasa River Sugarcane Belt of Northern Fiji: Rates, Influences and Contributing Processes

To purchase or authenticate to the full-text of this article, please visit this link: http://dx.doi.org/10.1111/j.1745-5871.2008.00533.x Byline: J.P. TERRY (1), R. LAL (2), S. GARIMELLA (3) Keywords: floodplains; accretion rates; Cs-137 method; Labasa River; Fiji Abstract: Abstract The 137Cs (caesium-137) method was used to investigate medium-term rates of sediment deposition on the floodplain of the Labasa River, on Vanua Levu island in northern Fiji. The Labasa basin is commercially important for sugarcane farming, which provides much revenue and sustains the economy in the greater Labasa area. Alluvium was sampled at three riverbank sites in vertical increments of 30 mm. Measured net vertical accretion rates, based on analysis of depth-profiles of 137Cs activity, ranged from 10 mm yr.sup.-1 at a low-lying site near a tributary confluence with the main river, to 60 mm yr.sup.-1 on a levee that was elevated slightly above the adjacent floodplain. These rates of accretion are high, but in general agreement with rates recorded using similar methods in other tropical Pacific island river systems. Soil erosion under sugarcane on hilly parts of the lower Labasa basin is an important factor in rapid floodplain development. Observations made during Cyclone Ami, which traversed Vanua Levu island in early 2003, illustrate the major contribution of severe storm events to floodplain sediment supply, by triggering numerous landslides on catchment slopes and in-channel debris floods, and by generating large-magnitude overbank floods. Author Affiliation: (1)Department of Geography, National University of Singapore, 1 Arts Link, Singapore 117570. (2)Division of Physics, The University of the South Pacific, Private Mail Bag, Suva, Fiji Islands. (3)School of Engineering and Physics, The University of the South Pacific, Private Mail Bag, Suva, Fiji Islands. Article History: Received 14 September 2007; Revised 9 November 2007; Accepted 29 November 2007 Article note: (*) Corresponding author. Email: geojpt@nus.edu.sg