Abstract The distribution and characteristics of marine surface sediments are a basic marine geological information. Large river floods are a frequent natural hazard that transport substantial terrigenous sediments into the marine environment. In August 2003, TY ETAU (0310) caused heavy rainfall in the southern coast of Hokkaido, north Japan, where some mountainous rivers in the Hidaka region flooded. Two deposition modes for the 2003 flood sediments can be identified by comparing the pre- and post-flood surface sediment distribution. Shore-normal shallow depressions off the mouth of the Saru and Atsubetsu rivers served as channels for the discharged floodwater preventing dispersion and maintaining the necessary water density to transport the materials as density bottom currents. This action also promoted long-distance transport of flood materials across the continental shelf. Absence of depression on the inner shelf off the mouth of the Niikappu and Shizunai rivers may have dispersed floodwaters near the river mouth and deposited the flood materials close to the shore. Marine geological mapping suggests that the differences in submarine topography (the presence or absence of shallow depressions) are closely related to the regional geological structure. Thus, submarine geology is a controlling factor of the seafloor environments influenced by the river flood.

Abstract Piping and erosion phenomena are serious problems affecting the integrity of buffer materials, which are an element of engineered barrier systems in the geological disposal of high-level radioactive waste. In this study, the outflow behaviour and the condition of buffer materials are investigated using a test pit drilled into host rock at the Horonobe Underground Research Laboratory to consider countermeasures to contain the outflow of the buffer material. The results are as follows: (1) Piping and erosion phenomena occur irrespective of the injection flow rate. However, when the rate is small, the buffer material is considered to be self-repairing and the outflow of the buffer material can be suppressed. (2) When the injection water contains large amounts of electrolytes, the surface of the buffer material peels off and precipitates, probably decreasing the waterproof performance. (3) Bentonite pellets are likely to be an effective countermeasure against piping and erosion.

Abstract Colloid concentration is an important parameter in models of colloid-facilitated transport. The purpose of the present study is to characterize colloid concentrations and colloid stability in natural groundwater from the Horonobe Underground Research Laboratory (URL) in Hokkaido, Japan. The particle sizes of colloids in groundwaters from the Horonobe URL range from several nanometres to c . 500 nm, with a mode particle size of c . 120 nm. Evaluation of colloid stability by DLVO theory suggests that larger colloids (i.e. >100 nm in diameter) would be more stable than smaller colloids in some groundwaters. The estimated colloid particle concentrations when considering the results of DLVO calculations ranged from 2.33 × 10 6 to 1.12 × 10 8 particles/ml, and mass concentrations were estimated to range from 45 to 1540 µg l −1 for diameters greater than 100 nm. Colloids in Horonobe groundwaters appear to be less stable, with a moderate potential for transport, than colloids investigated in similar international studies. This reduced stability may be due to relatively higher ionic strengths and moderate dissolved organic concentrations in Horonobe groundwaters compared to their international counterparts.