How many tagged stones are needed to estimate the mean travel distance of sediment from tracer experiments in gravel‐bed rivers?

Abstract

Tagged particles (painted, with inserted magnets, RFID labelled) used as tracers have often been deployed in field experiments focused on tracking the displacement of river gravels. In this way, fluvial geomorphologists have explored the relationships between mean travel distances and flow strength, among many other topics. However, these kinds of tracer experiments have typically been conducted without clear guidelines or rules on what constitutes an adequate sample size of tracers to minimise bias and uncertainty in displacement metrics. To fill this gap, we propose a probabilistic model that we applied to simulate and analyse the influence of the size of the tracer sample on the precision and accuracy of mean travel distances. We also used this model to explore how different causes of tracer loss (loss of frontrunners, buried tracers, due to signal collision and random) influence sample representativeness. Model results suggest that understanding the influence of sample size in particle tracking experiments in gravel‐bed rivers requires an explicit consideration of the different sources of tracer loss. According to the model, the loss of frontrunners and the consequent truncation of sampled travel distances is the most important factor conditioning the accuracy in the estimation of mean travel distances. Its impact is more significant than in the case of losses of buried tracers, losses due to signal collision and random losses resulting from low tracer densities or resulting from a poor survey. Based on our analysis, we concluded that 500–1000 tagged stones is an optimal tracer sample size for estimating mean travel distances. We hope that the outcomes of the proposed model could be used as a supporting tool to assist in the design and programming of future particle tracking experiments in gravel‐bed rivers.