Following a heavy rainfall, a channelled debris flow, which was bigger than 100 000 m 3 , caused extensive damage to infrastructures on the alluvial fan of the Bez torrent. The observations which were made during the event as well as the geomorphological survey of the catchment enable us to understand better the triggering of this debris flow. At the beginning of the event, there was a mud flow mainly supplied with the gullies located upstream from the catchment. The debris flow which followed was linked to the glacial deposits eroded on the left river bank, downstream from the catchment. In the days following the flood, the channel was destabilized and the risk of new debris flow was worsened. Therefore, important earthworks and enrockments were quickly undertaken along this section of the channel.A la suite d'un violent orage, une lave torrentielle d'un volume superieur a 100 000 m 3 s'est repandue sur le cone de dejection du torrent du Bez, provoquant des degats considerables aux infrastructures. Les observations realisees durant l'evenement et les investigations menees dans le bassin versant nous permettent de mieux comprendre le declenchement de cette lave. Le flot boueux marquant le debut de l'evenement a ete en grande partie alimente par les ravinements affectant le haut du bassin versant. La lave a blocs qui a suivi s'est nourrie quant a elle des depots morainiques recouvrant la rive gauche du torrent, dans la partie inferieure du bassin versant. Au lendemain de la crue, on pouvait observer deux grandes entailles d'une profondeur maximale de 15 m. L'instabilite des berges qui en resultait a alors ete rapidement enrayee par d'importants travaux de terrassements et d'enrochements.