Upscaling suspended-sediment flows in disturbed rainforest terrain: role of localised new sources

Modelling the impacts of complex distributions of sediment producing landforms on catchment-scale sediment behaviour requires an understanding of the role of (a) scale, and (b) new sediment-sources. This paper first examines the variations in sediment flux from 11 land-units ranging in size from the 0.1 ha 'landform-scale' to the 44 ha 'experimental-catchment'. Second, the work demonstrates the role of new sediment-sources with high sediment yields (sometimes triggered by extreme hydro-meteorological conditions) on behaviour at the landform to catchment scale. The analysis is based on 5-minute datalogged waterflow and suspended-sediment (calibrated turbidity) data from 11 weir and tipping-bucket gauging stations monitored over the period 1 July 1995 to 30 June 1996. Detailed work on sub-grid scale denudation and erodibility patterns was also undertaken. The study catchment is located within an area of tropical Borneo, where selective timber harvesting in 1988-89 left a mosaic of active erosional landforms including road gullies, eroding skid trails, mass movements and disturbed channel-heads.

Analysis of the time-series data indicated that the impact of localised road-culvert collapses and rotational landslides propagated through to the catchment-scale. Mass movements at new locations appeared part way through the time-series, giving rise to marked changes in the pattern of sediment flux within the catchment (compared to areas where gully or sheet-wash erosion dominated). This localised behaviour is likely to give rise to non-stationarity in sediment behaviour at the lumped catchment scale. Furthermore, not all of the mass movement events observed were related to extreme hydro-meteorological conditions and each had locally complex stability/erodibility patterns.

These conclusions lead the authors to investigate the value of transfer-functions (not requiring the definition of geomorphic parameters, such as stability/erodibility and topography) in identifying spatial and temporal patterns in suspended-sediment flux.