Multi-temporal analysis of morphologic changes applying geomatic techniques. 70 years of torrential activity in the Rebaixader catchment (Central pyrenees)

The monitoring of morphologic changes in high-mountain environments is an important, but complex task. Terrestrial as well as airborne laser scanning (TLS and ALS) and digital photogrammetry (DP) using Unnamed Aerial Vehicle (UAV) can determine recent variations. In contrast, changes that occurred 20 or more years ago, need generally the application of DP and historic aerial photograph. We compared these four geomatic techniques and the resulting digital elevation models (DEM) that ranged over a 70-year time span (1946-2016). The accuracy of the different models and also the effect of different cell size were analysed. The accuracy analysis of the resulting DEMs shows important drawbacks, when historic aerial photographs are used. In these DEMs, the error in elevation can be important and a detailed analysis of morphologic changes is limited. Nowadays, the combination of DP and UAV is the technique with best cost-benefit ratio, although TLS could reveal similar precision. However, the application of TLS is restrained by the complex morphology and the presence of vegetation, which produce many shadows. Finally, the erosion rate was determined in our catchment. The resulting values range between 0.06 and 0.16 m 3 /m 2 /y, which coincides rather well with data observed in other studies.