Rapid lake infill following major rockfall (bergsturz) events revealed by ground-penetrating radar (GPR) measurements, Reintal, German Alps

The capability of GPR (ground-penetrating radar) measurements to decipher the sedimentary history of young alluvial plains is investigated. The sediment basins result from the infill of two lakes, which were dammed by high-magnitude rockfalls 400-600 and 200 years ago in an Alpine trough valley. The thickness and structure of the dammed sediments was derived from a series of longitudinal and cross profiles. In order to take advantage of both high penetration and high resolution measurements, we applied 25, 50, 100 and 200 MHz antennas. The radargrams allow for a detailed analysis of the history of the alluvial fill. The pre-existing valley bottom is clearly visible under one of the basins. The profiles display the former clayey lake bottom and illustrate several stages of lake infill. Small-scale sedimentary structures, such as topset beds or foreset beds, are discernible and provide proxy information on water level, sediment supply and phases of infill activity. At the second basin, discordant structures in the adjacent sediment cones indicate the abrupt rise of the base level resulting from the rockfall event. Radar-derived information and historical records of lake retreat were used to reconstruct the sediment budget of the 200 year lifespan of the younger alluvial plane. Therefore, it can be concluded that the sedimentation rates of the twentieth century (489 ± 87 m ³/yr) are several times larger than those of the nineteenth century (103 ± 7 m³/yr). A 130-yr rainstorm record suggests that the overall increase in sediment supply in the twentieth century corresponds to increased rainstorm frequencies during warm periods.