The effects of antecedent flows on sediment entrainment in a mountain stream

Bedload transport in mountain streams is notoriously difficult to measure, and substantial efforts are currently devoted to develop and test reliable surrogate techniques for quantifying bedload transport rates and size. Tracers, and in particular Passive Integrated Transponders (PITs), represent a powerful method to assess particle dynamics. PITs are usually searched after floods using a portable antenna, and grain size of tracers are typically related to the peak of the events. However, antennas fixed on the channel bed have the potential to identify the actual discharge at the time of transport.

This work focuses on incipient motion of tracers measured with a stationary antenna in the upper part of a mountain basin (Saldur River, drainage area 18.6 km², Italian Alps), where a glacier (2.3 km²) determines significant daily discharge fluctuations in summer. During the study period (2011 to 2013) flow discharge ranged from 1 to 10 m³s^-1. Almost 600 clasts - ranging in diameter from 40 mm to about 0.5 m - were equipped with PITs and laid in a confined reach (6% slope) of the main channel featuring a bed morphology transitional from plane-bed to step-pool. PITs-clasts were gently placed on the bed surface few meters upstream of an antenna fixed on the channel bed, where flow stage is recorded every 10 min.

Preliminary results indicate that discharge at the time of passage above the antenna is only slightly related to the size of transported tracers, providing little evidence of size-selectivity conditions in this stream. The influence of antecedent flows on incipient motion was then investigated dividing the maximum discharge recorded between each PIT placement and its subsequent entrainment by the actual critical discharge at the time of movement (ratio Qmax/Qc). It results that only 45% of tracers moved at Qmax/Qc ~ 1, and 70% of tracers moved at Qmax/Qc < 1.5. Therefore, about 30% of tracers had to previously experience a discharge substantially higher than the one which actually mobilized them. Interestingly, coarser particles moved at higher Qmax/Qc ratios. These preliminary results shed some light on the importance of antecedent flows on incipient motion of coarse particles in a mountain stream, revealing that the magnitude of previous flows seems quite relevant for their entrainment.