The continuous measurement of bedload discharge in a large alpine gravel bed river

The objective of this paper is to analyze the efficiency of a bedload-measuring trap for continuous sediment measurements. The trap is implemented into the river-bed of the Drau, located in Southern Austria. At the trap site the mean annual discharge is 64 m(3) s(-1), the slope is about 0.002 and the mean subsurface grain diameter is about 28 mm. Two criteria, hydraulic efficiency and sampling efficiency, are used to evaluate the performance of the trap. The hydraulic efficiency as well as the sampling efficiency are shown to be high. The advantage of the trap is that continuous and automatic bedload transport measurements can be obtained even in wide, alpine gravel bed rivers. We demonstrate that continuous bedload data are useful with respect to identifying initiation of motion. The comparison with formulas shows significant deviations between measurements and calculations. In such a supply limited river-bed, where bedload often moves over an armour layer, it is apparent that local hydraulic conditions explain only a fraction, about one third of the variation in the weak bedload flux.