Vegetation in open channels has a significant influence on the characteristics of bed load movement. To date, studies of incipient motion of sediment in open channels with submerged vegetation have been restricted by limitations in statistical approaches and measuring equipment. Researchers have also neglected incipient motion of sediment in the presence of vegetation when deriving semi-empirical equations for the rates of bed load transport, and have generally considered it the same as that without vegetation, which is clearly inconsistent with, and does not represent, the actual conditions. In this study, we therefore studied the vegetation and sandy bed in a natural river to determine the characteristics of incipient motion of sediment within rigid vegetation on a movable bed. From the spatially averaged velocity, we then described the incipient motion of sediment under the water-vegetation interface. We found that there were three phases of incipient motion of sediment, namely static, partially in motion, and entirely in motion. We defined the incipient motion of sediment from the conditions for the third process. We derived a semi-empirical equation for the sediment incipient velocity in the presence of submerged vegetation from our experimental results, and found that, of the factors explored, the vegetation factor Fv
had most influence on the incipient motion velocity of sediment with vegetation. For a given vegetation density, the average depth of the scour holes around the vegetation elements were linearly related with the spatially averaged velocity of the flow under the water-vegetation interface. These results demonstrate that our method gave reasonable estimates of the spatially averaged velocity for incipient motion of sediment.