Are we measuring morphological changes in gravel bed rivers with the appropriate frequency?

Repeated topographic surveys are commonly used to estimate bedload flux in large gravel bed rivers. This work investigates the role of the survey frequency in the provision of reliable quantifications of morphological changes. We carried out a set of flume experiments to measure the bias in volume estimations for different formative discharges and related bed morphology. Results show a strongly non linear relationship between morphological changes and the time between surveys, with a significant and increasing underestimation of the morphological changes for increasing time between surveys. This allowed us to estimate a temporal scale, quantified in a range of 30-150 min, at laboratory scale, equivalent to 3-20 hours of competent flow at the field scale. This means that field surveys separated by longer timesteps are likely to significantly underestimate the occurred morphological changes, with effects on our ability to estimate the bedload flux.