CONTROLS OF CONCENTRATION AND SIZE DISTRIBUTION OF SUSPENDED SEDIMENT IN LARGE LOWLAND RIVERS: DETAILED FIELD MEASUREMENTS FROM THE FLY RIVER, PAPUA NEW GUINEA

Detailed three dimensional flow velocity measurements and suspended sediment samples collected from the Fly River, Papua New Guinea provide a unique data set for the evaluation of the controls of the concentration and size distribution of suspended sediment in a large, low-gradient, sand-bedded river. At seven cross-sections along the middle and lower reaches of the Fly River, we collected vertical and cross-stream profiles of flow velocity and turbulence using an Acoustic Doppler Velocimeter (ADV). Concurrent with the velocity measurements, we collected depth specific suspended sediment samples to determine both sediment concentrations and grain size distributions. This dataset will be used to test and refine theoretical models for the prediction of suspended sediment concentrations in order to assist in modeling of sediment routing and floodplain sedimentation by overbank flow. Further, we hope that this data will provide insights on the interaction of silt with the channel bed. This interaction is of practical and theoretical importance to questions regarding controls on bed sorting and river aggradation. Preliminary analysis of this data indicates substantial deviations from linear stress profiles occur across large sections of the river channel. While the short duration of our measurements may have some bias due transient effects from dune migration and wake shedding, the data suggest a strong influence of channel meander bends and bed topography on both stress distributions and suspended sediment concentrations. High velocity cores occur along the outside of meander bends and below the water surface, similar to that observed in smaller meandering rivers. Suspended sediment data show strong vertical concentration gradients across a grain size range of 2 to 10 phi. Elevated sediment concentrations locally occur in association with the high velocity cores.