Ground penetrating radar for assessing sediment structures in the hyporheic zone of a prealpine river

Ground penetrating radar (GPR) was used to investigate the structure of the sediments in a floodprone gravel-bed river. Measurements were made on 2 gravel bars and in the adjacent channel. We buried a metallic object to calibrate depth and to visualize changes in GPR-profiles taken before and after digging. In addition, along 2 GPR-profiles the structure of the alluvial sediment of a gravel bar was visually verified by digging a trench down to the groundwater table. The GPR method enables,ne to distinguish sedimentary units of different texture with a vertical resolution of about 0.25 m. The investigation showed the depth of the alluvial valley fill, and it revealed geomorphological features such as lateral point bar accretion, fills of former pools, or open framework textures. These structures form a complex 3-dimensional mosaic of sediment patches. The GPR method also allowed us to document the changes resulting from bed-moving floods, such as in the formation of homogeneous gravel sheets during high magnitude floods. At some sites we observed intercalations of coarse organic matter between adjacent sedimentary units. Based on GPR data and the analysis of geomorphological structures near the study site, we developed a diagrammatic model of the sediments of this site in the lower Necker Valley.