We have theoretically and experimentally investigated the effects of vegetation on the flow resistanceand the bed-load transport. The results are summarized as follows; It is assumed that the flow resistance is composed of the drag force acting on vegetation and the shear stress interacting between the flow and the bed surface of mobile grains. Furthermore, adopting Eq. (2) for the vegetal retardance and Eq. (9) for the law of resistance in mobile beds respectively, the flow resistance in vegetated channels is given by Eq. (11). For the sake of simplicity, we introduce the coefficient Cdε which contains several influences of the drag coefficient Cd, the coefficient β, the vegetal flexibility, etc., then Eq. (11) is expressed by Eq. (15) . And Eq. (16) corresponds to the resistance law of channels filled with tufts of plants. On the other hand, we obtain the bed-load transport expression (Eq. (37) ) by means of the brief tractive force model (Eq. (22)-(31) ), and its applicability is experimentally confirmed. In this expression, Eq. (33) for the effective tractive force τ*e, Eq. (34) for the critical tractive force τ*c respectively should be adopted.
On the early dawn of the 23rd of May 1987, mud flows attacked the Osawa alluvial fan.I.T.V.camera, which is established at the top of the fan, successfully catched the phenomen a of flowing mudflows. It is said that the triggering mechanism of the mud flows occurred at Osawa valley is closelyrelated to the existence of freezing mud layer. This phenomenon was clearly proved by this time's dataand field surveys. This paper shows the outline of mud flows observations and the function of thescreen dam.