a) Application of the segmentation mask to the horizontal velocity on the τ₁-iso-surface for a single frame. b) Scatter plot of granular diameter vs. mean horizontal velocity of segmented granules for the entire time series, shown in red and the maximum horizontal velocity in blue. The trends for granules across the whole range of diameters are illustrated by linear fits as solid black lines and for granules with diameters larger than 750 km by a linear fit as a dashed dark-gray line (only shown for the mean horizontal velocity).