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 o f the sediments in a floodprone gravel-bed river. Measurements were ma de on 2 gravel bars and in the adjacent channel. We buried a metallic object to calibrate depth and to visualize changes in GPR-profiles tak en before and after digging. In addition, along 2 GPR-profiles the str ucture of the alluvial sediment of a gravel bar was visually verified by digging a trench down to the groundwater table. The GPR method enab les,ne to distinguish sedimentary units of different texture with a ve rtical 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 f ramework textures. These structures form a complex 3-dimensional mosai c 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 s edimentary units. Based on GPR data and the analysis of geomorphologic al structures near the study site, we developed a diagrammatic model o f the sediments of this site in the lower Necker Valley.