The combined use of wash load and bed material load rating curves for the calculation of total load: An example from the Burdekin River, Australia

In hydraulic response terms, the total clastic load of a river is comprised of wash load and bed material load components. Bed material load includes sediment moving as actual bed load and as suspended material originating from the bed. Classical suspended sediment rating curves do not differentiate between wash load and suspended bed material and their indiscriminant use can therefore lead to ambiguous results. Wash load must be delineated from the total suspension when using a rating curve. This is particularly important for rivers draining tropical catchments as these transport a large proportion of fine sediment. For rivers in which a relationship exists between wash load concentration and discharge, an estimate of fine sediment delivery may be obtained from a wash load rating curve. The remaining bed material load may then be calculated from physical principles. This technique is applied to the Burdekin River of northeast Australia.The Burdekin River drains a tropical semi-arid hinterland and transports a predominantly silt and clay wash load. A wash load rating curve is established by applying correction factors for suspended bed material under different flow rates to a classical suspended sediment rating curve. The resulting estimate of mean annual wash load is 3.0 million tonnes of silt and clay. The use of a theoretical bed load equation (ACKERS & WHITE 1973) together with hydraulic parameters on the coastal plain yields an estimate of mean annual bed material load of 0.45 million tonnes of sand.