Existing simulation methods of long-term fluvial processes, represented as conventional dif ference scheme of dif fusion equation, cannot control about the expression of sediment transportation satisfactorily, particularly around confluences in middle and lower reach of the river. To address this, we newly developed a numeral model and simulation program of fluvial processes, in which sediment outflux is distributed to multiple grids with the formula that enlarges the fluvial transport efficiency downstream as a function of grain-size and drainage area (named “transport distance increment curve”). This formula can control the concavity of longitudinal profile. With this model, analyses about expressions of geomorphic development in longitudinal river profile were carried out. As a result, it was confirmed that, with accurate settings of parameters, this model can yield features of river profiles, considering sediment transportation around confluences. It is believed from the above that this model enables the expression of fluvial topography as seen in nature, by translating the concept of long-term fluvial process into parameters.

In a slender water body with a large aspect ratio, the angle of wind waves relative to the direction normal to the shoreline may exceed 45˚, resulting in the emergence of cuspate forelands and the subdivision of a lake, because the fetch distance along the principal axis becomes large. The BG model (a three-dimensional model for predicting beach changes based on Bagnold’s concept) was used to predict the segmentation of a rectangular lake by wind waves when the probability of occurrence of wind direction is given by a circular or elliptic distribution. The change in wave field and sand transport flux over time were calculated in detail to investigate the effect of the wave-sheltering effect of the cuspate forelands. It was concluded that a rectangular lake is subdivided into circular or elliptic lakes, when the probability of occurrence of wind direction is given by a circular or elliptic distribution, respectively. The impact of the construction of offshore breakwaters to the segmentation of a slender, rectangular water body was also investigated under the conditions with/without the construction of offshore breakwaters that reduce the fetch distance. The construction of the offshore breakwater caused great impact to the segmentation of a slender water body.

In the Shimanto accretionary complex in the southwest Japan, most of the recent deep-seated landslides (DSLs) were induced by heavy rainfall over 1000 mm in a few days. In contrast, in the areas of volcanic rocks or Neogene sediments in the northeast Japan, recent DSLs were induced by strong seismic motion larger than six on the Japanese earthquake damage scale. These facts suggest that the regional characteristics exist in the trigger factors that induces DSLs (i.e., rainfall or earthquake), and that these factors are related strongly to the geological features of DSL sites. We therefore studied the regional characteristics of rainfall and earthquake on the basis of the distribution maps of trigger factors and the processes of DSL. The processes of DSL induced by two factors and the geological influences on them are summarized as follows. In the Shimanto accretionary complex in the southwest Japan, DSL induced by heavy rainfall is more likely compared with that by earthquake. This is because of the higher precipitation in the relevant area and the convergence of seepage water into the cracks of potential slip surfaces formed by rock creep. Earthquake-induced DSL is rare because active faults in this region are few and the mountainous basement is relatively hard. On the other hands, in the areas of volcanic rocks or Neogene sediments in the northeast Japan, earthquake-induced DSL is likely because earthquake stimulates greater ground motion in these areas and the mountainous basement is relatively soft.

Lake Mochirippu is a brackish lagoon extending for a few kilometers from the narrow entrance on the Pacific coast, eastern Hokkaido, where a series of Tsunami have attacked. The previous studies presumed that the lake sediments contained more than 17 times of Tsunami deposits (Sawai et al., 2009). In this paper, we reported the details of diatom assemblages from the Tsunami layers and would discuss their sedimentary environment and transportation processes of Tsunami deposits using diatom valves. The drilling core (3.8 m) was taken at the lake side peat land east of the lake, and be divided into five sedimentary zones by their lithofacies and diatom characteristics. The second and fourths layers were Tsunami deposits, dated 17th Century and 2500-5500 years BP respectably. Although the tsunami deposits of the two layers were composed mainly by marine sands, the 80% of the diatom fossils were consist of freshwater species, and very few of oceanic species. They presumed that the marine sands were secondary shifted during and just after the tsunami event, and contained a lot of fresh water diatoms in them. The detail observation of tsunami layer, sliced 1-2 cm each, made clear diatom assemblages shifted systematically according to the small sedimentary units in the Tsunami.