The Impact of Intermittency on Bed Load Sediment Transport

Sediment transport by wind or water near the threshold of grain motion is dominated by rare transport events. This intermittency makes it difficult to calibrate sediment transport laws, or to define an unambiguous threshold for grain entrainment, both of which are crucial for predicting sediment transport rates. We present a model that captures this intermittency and shows that the noisy statistics of sediment transport contain useful information about the sediment entrainment threshold and the variations in driving fluid stress. Using a combination of laboratory experiments and analytical results, we measure the threshold for grain entrainment in a novel way and introduce a new property, the “shear stress variability”, which predicts conditions under which transport will be intermittent. Our work suggests strategies for improving measurements and predictions of sediment flux and hints that the sediment transport law may change close to the threshold of motion. Plain Language Summary Sediment transport by wind or water is intermittent―Displaying long periods of low transport followed by sudden large transport events. Intermittency is commonly seen as unwanted noise that makes it difficult to predict sediment flux. We introduce a mathematical model for sediment transport near the threshold of motion which provides a possible explanation for why and when intermittency occurs, and show that the noisy statistics of intermittent sediment transport encode information about the threshold of sediment motion and the variations in driving fluid forces. This knowledge can help improve the accuracy of sediment transport predictions, which are central to many engineering and geological applications. The ingredients necessary for intermittency―Noise near a transition―apply to many systems beyond sediment transport. Key Points Sediment transport near the threshold of motion is intermittent, making it hard to measure average flux, which is crucial for predictions We use bifurcation theory in the presence of multiplicative noise to understand and describe the intermittency Applying this to flume experiments, we find a new way of measuring the threshold of motion and predicting when intermittency will be present