Abstract: The shape of longitudinal profile of a debris flow valley contains the information about the evolution of the basin and the development of the debris flow, and is a synthetical topographic factor which indicates the develoment of the debris flow. Taking the Liziyida valley along the Chendu-Kunming railway as an example, the paper forecasts the change of the forming index N of the longitudinal profile of the valley by using the unequal intervaI GM (1,1),and improves the accuracy of the forecast method. Finally relevant problems in forecasting valley evolution are discussed.

Key words: Debris flow, Longitudinal profile of valley, Gray forecast