Sediment transport load in the Onyx River, Wright Valley

The sediment transport load in the Onyx River was investigated. The bedload transport rates were used in association with the 15 year flow record, to estimate sandur sedimentation rates. 38 cross sections were established on six neighbouring branches in a braided reach of the river. The branches were chosen to give a full range of channel sizes and discharges (up to 4m3/s). Five to six sections were placed on each branch. Across each section, measurements of water depth and velocity were made at 10-15 points. Within the zone in which sediment was visibly moving along the bed, 4-6 measurements of transport rate were made by placing an open mouthed baset sampler on the bed for periods of 5-30 seconds. Surveys were made to determine channel slope and the relative locations of the cross sections on each occasion, and a full topographic survey of the whole reach was also carried out. Repeated measurements were made at each site to establish relationships between sediment movement and hydraulic conditions, threshhold for movement, the impact of other factors such as bank erosion and implications for channel evolution. By making measurements on selected subsets of the cross sections, the following can be determined; i) the variations in transport rate along the channel, ii) the influence of channel size and discharge on transport rate, iii) the effect of diurnal variations in discharge and supply of sediment by bank caving on transport rate, iv) the total transport rate by the river during the period, v) the relationship between hydraulic conditions and transport rate at a point and vi) the characteristics of sediment moved under varying hydraulic conditions.