Bedload transport efficiency of forested sand-bed streams in the seasonally wet tropics of northern Australia

The purpose of this research was to apply the bedload transport efficiency approach of Bagnold (1973) to a new bedload data set for two sand-bed streams in the seasonally wet tropics of northern Australia. Hand-held, pressure difference, Helley-Smith bedload samplers were used to measure bedload fluxes for the 1998/1999, 1999/2000, 2000/2001 and 2001/2002 wet seasons at the East Tributary and Swift Creek gauging stations in the Ngarradj Creek catchment at Jabiluka, NT, Australia. The East Tributary gauge is characterized by slightly higher stream powers than Swift Creek gauge and hydraulic geometry relations show that both stations respond to increasing discharge differently. Bedload ratings were defined as those that were not only statistically significant (ρ ≤ 0.05) but also explained at least (Adjusted R2 ≥) 0.60 of the variance in bedload flux. They were established between adjusted submersed bedload weight per unit width and time, and both unit and excess unit stream power for raw and log10-transformed data. Bagnold (1973) defined the capacity of a river to transport bedload at various percentage efficiencies. Most stream kinetic energy is expended overcoming internal resistance to flow within the fluid and only a very small proportion is expended in moving bedload. For East Tributary, bedload transport efficiency increased with increasing excess unit stream power but never exceeded 0.1%. For Swift Creek, bedload transport efficiency approximately followed a linear trend at a constant slope at about 0.3 % efficiency. This indicates that bedload transport at Swift Creek is at least three times more efficient than at East Tributary, most likely because of the wider cross section and less dense loading of large wood. This would permit a greater proportion of excess unit stream power to be expended on the bed. Furthermore, at-a-station hydraulic geometry equations differ between the stations, supporting the differences in bedload efficiency. Bagnold‟s (1973) approach seems to apply to the two gauges.