| The geological setting is essential for occurrence of slope failure and landslide so that the geology may control the suspended sediment yield with different magnitude. In the Saru River watershed of central Hokkaido、 northern Japan、 the typhoon Etau in August 2003 brought heavy rainfall、 causing the slope failure and landslide and the highest sediment yield since 1960s and prolonged runoff of suspended sediment since then. To clarify the suspended sediment sources with watershed、 hydrological monitoring of discharge and turbidity、 fingerprinting technique using natural radionuclide、 and GIS analysis for slope failure and landslide areas were conducted in Nukabira River watershed、 a tributary of Saru River. Gamma ray spectrometry was conducted to determine the activity of environmental radionuclides、 including U-series、 Th-series、 cesium-137 and potassium-40. Statistical analysis showed the best composite fingerprints of Pb-212、 Ac-228 and K-40 to classify the suspended sediment sources into six geological units、 which are sedimentary rock、 Cretaceous accretionary complex consist of sedimentary rock、 basalt block and volcanic rock、 plutonic rock and metamorphic rock. The contribution of source group to suspended sediment was calculated according to the assumption that the inverse of the Mahalanobis distance in three tracer properties between each source and suspended sediment can represent the relative contribution of the source to suspended sediment. During the rainfall event on August 11、 2011、 dominant source of suspended sediment was found to be the areas consist of metamorphic rock (31%)、 sedimentary rock (30%) and accretionary sedimentary rock (24%). GIS analysis showed the spatial distribution of slope failure and landslide within the watershed and the high density in accretionary basalt block and metamorphic rock in slope failure and in metamorphic rock in landslide. The sediment yield of source group、 calculated from the amount of suspended sediment and contribution of source group、 was found to be the highest in metamorphic rock (4.7x103 t/km2)、 in consistence with the dense distribution of slope failure and landslide. Active landslide can provide the significant amount of sediment from hillslope to stream channel、 resulting in the source of fine sediment. In the area of accretionary basalt、 there was still inconsistency between the sediment yield and the density of slope failure and landslide. The low suspended sediment yield may be attributable to the lack of fine sediment on the bare slope along the stream channel and riverbed、 due to the large particle size and hardness of sediment. These results indicate that the natural radionuclides can be available to elucidate the suspended sediment sources and sediment yield of various geological units within a watershed. |
