BEDLOAD ENTRAINMENT, TRAVEL LENGTHS, STEP LENGTHS, REST PERIODS - STUDIED WITH PASSIVE (IRON, MAGNETIC) AND ACTIVE (RADIO) TRACER TECHNIQUES

This paper reports results from bedload transport investigations with active (radio) and passive (iron, magnetic) tracers in the Lainbach, a step-pool mountain river, in Bavaria, Southern Germany. The spatial distributions of the iron tracers after flood events can be best described by exponential or Gamma distributions. There is some indication of a tendency of size-selective transport of the iron tracers, but there is also a considerable amount of scatter in the correlations between weight (size) and travel length owing to the masking influence of other variables, such as the shape of the particles and different positions in the river bed. The experiments with artificial magnetic tracers showed that elongated pebbles (rods) had the longest mean transport distance, platy ones (discs) remained relatively close to the starting points. The particles from the pool showed the greatest transport lengths and a 100 per cent chance of being eroded. The Pebble Transmitter System (PETSY) consists of transmitters implanted into individual pebbles, a computerized receiver, a stationary antenna system with an antenna switchboard, and a data logging system. The particles do not move continuously but in a series of transport steps and non-movement intervals. A single value for a given size-class is not adequate to describe the critical conditions of entrainment under natural circumstances. A probability approach is much more suitable. The critical unit discharges (total discharge divided by active channel width) along the measuring reach are dependent on river bed morphology. In the steps bedload needs higher unit discharges to be entrained. Once entrained, the transport of bedload is stochastic in nature and the single particle transport is controlled by the step lengths and the duration of rest periods. The distributions of both parameters can be approximated by exponential functions. Applying the stochastic concept proposed by Einstein the mean values of the random variables 'step length' and 'duration of rest period' measured with the PETSY technique were used for the simulation of spatial distributions of bedload particles from point sources. More field and laboratory data are needed to include varying flow and roughness conditions with tracers representing different particle characteristics.