Boulder transport under mixed-lubrication friction

Abstract

Boulder transport by wave action may exploit thin gaps between the boulder and bedrock platform whereby a lubrication zone reduces the friction coefficient during sliding. We derive an effective mixed-lubrication friction coefficient, depending on boulder speed and the geometry of the lubricated region, and analyse its effect on boulder transport. Mixed lubrication enhances boulder dynamics when moving at a faster speed, the lubricated region is increased, or with fewer separated lubricated regions and smaller boulder aspect ratios. We compare our model against static friction and an empirically-fitted model.