The study of stone trails with painted tracers on unstable slopes in the Southern French Alps: preliminary results

Abstract

An experiment has been carrying out in the Baronnies Mountains at the altitude of 850 m since April 1988, with 6 lines of 100 painted tracers each, on a steep debris slope (21 to 32°) and on medium slope (12 to 20°). Stone tr ails are present among tufts of grass and small trees which represent the anthropic degradation of the original extensive oak forest. They more or less follow lines of maximum gradient. Some of the stone trails lead down into downcutting guilles in marls. Small steps are created by the tufts of grass which are covered with debris moving down the stone trails. The stone tralis show landmark of runoff but back analysis on the painted stones over the past 2 years shows that runoff is not chiefly responsible for the movement of the coarse debris. Sometimes the correlation coefficients reflect positive correlations with the lenght of runoff and the sine of slope angle but there is not a significant correlation coefficient with the reverse of particle size. The processes belonging to what has been called rock creep by SCHUMM (1967) is largely responsible for the distance moved. Among the possible processes of rock creep responsible for mov-ing the debris along the stone trails in the mountainous climate of the Baronnies, needle ice transport has been eliminated because it has not been noted in the field study. But numerous freeze-thaw cycles a year have involved frost creep as it has been confirmed with the observation of frost-heaving figures. For different reasons one can justify the relative reliability of the back analysis results: the time elapsed after the beginning of the experiment is not sufficient to allow each variable to be on site expressed clearly in the distance moved; the lenght of runoff measured was not realistic because there has been no heavy rainstorms: other variables such as the roughness of the ground surface or the shape of the debris influence the distance moved; lastly animals, which consist in a random variable, move the stones too.