Usually, pipes in soil layer are formed by concentration of ground water and activity of animals, decayed roots, living roots, cobbles and cracks caused by earthquake. These make soil layer structure heterogeneous. In addition, these heterogeneities give large influence to not only hydrologic processes in soil layer but also slope failures. Studies on pipes which are a portion in primary factor to make soil layer structure heterogeneous, were conducted much so far now. However, study on heterogeneities in soil layer has not been almost carried out, except studies regarding a pipe flow, which flows inside the macro-pores to be parallel to the surface soil layer.
We defined these factors that make soil layer structure heterogeneous with “Structural heterogeneity in soil layer”.
In this study, laboratory experiment was carried out to investigate influence of cobble in soil layer on hydrological process inside a slope and stabilization of a slope.
The following matters were confirmed by this laboratory experiment using a ball.
1) A cobble (ball) in soil layer decreases slope stability. In addition, slope stability decreases in inverse proportion to an increase of sectional area of the cobble.
2) Velocity of saturated throughflow becomes fast in the surroundings of the cobble.

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A wide devastated area has remained for more than 1300 years in the Tanakami Mountains located in the central Japan, and the sediment discharge has been a serious problem in the area. Various hillslope managements have been conducted since the mid-19th century. Consequently, the sediment discharge decreased into a range from 10 to the power of -3 to 10 to the power of -4 after the hillside works had been conducted. This study clarified the changes in vegetation cover over a 51-year period through the interpretation of aerial photographs taken in 1947, 1963, 1968, 1982 and 1998. Digital terrain models were also applied for the evaluation of the relationships between the vegetation cover and the topographical properties. The results showed that the area ratio of the bare and sparse forested slopes to the experimental area was remarkably reduced from 41.7% (in 1947) to 6.0% (in 1998). In the area where both hillside works and tending works were undertaken, the area ratio was especially reduced to only about 1.0% in 1998 regardless of the slope gradients. In contrast, in the area where only hillside works were undertaken, the area ratio was still 25.6% in 1998. It was not clear how much the area of sparse forested slopes was decreased by only hillside works especially in the slopes with a gradient exceeding 40 degrees. The results of this study suggested that both hillside and tending works were necessary for rapid vegetation recovery of both the bare slopes and sparse forested slopes.

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Flood-deposition of debris flows on coastal alluvial fans is unusual in Japan, except the one caused during the Great Hanshin Flood in 1938, and there are a lot of unknown factors with respect to the flood-deposition characteristics of debris flows. In addition, quantitative data on the runoff, flood, and deposition characteristics of debris flows in Venezuela are not sufficient in Japan, since the climate, and topographical and geological conditions are different between the two countries.
A field survey was conducted on the debris flows, which occurred between 14 and 16 December 1999 in the Province of Vargas, Republic of Venezuela, to verify the relationship between the volume of runoff sediment produced from debris flows and the topographical characteristics of the river basin and the relationship between the topographical characteristics of coastal alluvial fans and the flood-deposition characteristics of debris flows for the 13 major mountain streams in the province. As a result, the following findings were obtained :
(1) The area of an alluvial fan and the flood-deposition area of debris flow have a positive correlation.
(2) The larger the catchment area, the smaller the average alluvial fan gradient and the gradient, at which deposition starts.
(3) The gentler the alluvial fan gradient is (the further downstream the location is), the larger the flood distance (the maximum distance from the channel (stream bank) in the flood-deposition area on the alluvial fan) becomes.
(4) The catchment area and the runoff sediment volume have a positive correlation. The average runoff sediment volume per 1 km² of a catchment area is about 58 × 10³ m³/km², which is almost the same as the maximum value previously recorded in Japan.
(5) The calculation method for the movable sediment volume and the transportable sediment volume based on the “Guideline for Measures against Debris Flows (proposal) ” used in Japan is, to some extent, applicable for the calculation of debris flows in Venezuela.

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Shikoku island has several major tectonic lines, such as the Median tectonic line, the Mikabu tectonic line and the Butsuzo tectonic line. Influenced by the tectonic lines, the bedrocks in the Shikoku mountainous region are often weak. This causes frequent fracture-zone landslides and collapses in the area.
On the other hand, as the notion that the sediment issue in a watershed should be dealt collectively from the headwater area to the coastal drift sand area is proposed recently, we studied the actual condition of the sediment yield in the Shikoku mountainous region by the sedimentation in the reservoir of the dams and the erosion control dams to discuss about the new Sabo project plans.
Consequently, the average specific annual sediment yield in Shikoku mountainous region is estimated as 0.59 × 10³ (m³/km²/year), and the value of the produced unstable sediment at the disaster time is sometimes accumulated ten times as much as usual time.
And we pointed out one of the most serious sedimentation problems in the reservoir is the consumption of its effective storage capacity.

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In Okinawa, erosion of red sediment called Kunigami Maaji and its sedimentation in reef area cause serious environmental problem. We have made hydrological observations of red sediment discharge in 4 basins in Taira River, and clarified the followings.
1) In basin with broad pineapple fields, red sediment discharge at each rain event amount to some hundreds kilograms per ha and SS of river will come up to some thousands milligrams per little.
2) In the basin with broad pineapple fields, SS of river increases as rain intensity increase.
3) The relation between landuse in basins and SS or sediment discharge shows that pineapple fields are the main source of red sediment outflow.

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The phreatomagmatic eruption of Mt.Usu began on March 31, 2000. Previous volcanic activity had taken place in 1977-1978. The 2000 eruptions occurred at the western flank of Mt. Usu, close to the hot-spring town of Abuta, resulting in diastrophism, the formation of a number of small craters and eruption-induced mudflows.
The volume of sediment produced by the volcanic activity was estimated at 1.0 × 10⁵m³ in the Nishiyama River watershed. The mudflow sediment that overflowed into a residential area on April 10 had a similar grain size distribu-tion (d₅₀=0.41 mm) to that of the mudflow (d₅₀=0.38 mm) that occurred during the four days from May 29. The grain size of airborne volcanic ash deposited on the mudflow sediment was very fine (d₅₀=0.16 mm), almost the same size as the volcanic ash in 1978.
Since July 10, rainfall-induced mudflows and slope collapses have occurred on steep hill slopes on which the underlying vegetation has been covered with volcanic ash. Also, the crater walls have been weakened by rainfall and the flow of groundwater and may breakdown in the event of heavy rain. Thus, the risk of mudflows is high in the Nishiyama River watershed.
Abuta town urgently requires plans for prevention of sediment disasters and the establishment of new evacuation zones, which should be linked with any new urban planning of the town near Mt. Usu.

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The Tarija Basin, located in southern Bolivia, is subject to intense soil erosion and in recent decades rainstorm-induced soil loss and associated flooding have posed a serious hazard to local inhabitants. Intense soil erosion in the basin was initiated by deforestation associated with mining industries in the 17th century and thereafter it has been accelerated by intensive grazing and land reclamation. Poorly consolidated Quaternary sediments which comprise the center of the basin and mainly consist of silt and clay promote the erosion. The extremely low infiltration capacities of non-vegetated surfaces lead to rapidly generate surface runoff during rainstorms in the rainy season. This has led to extensive, well developed piping, rills and gullies. Consequently, in 1978 the Bolivian government established the Tarija Prefecture Reclamation Office (PERTT) in order to run programs aimed at soil conservation. In 1998 a project examing erosion control and reforestation in the Tarija Basin was established in collaboration with the Japan International Cooperation Agency (JICA). Three model sites were setup in order to improve and develop sustainable methods to mitigate soil erosion by participation of local inhabitants. The five-year project aims to construct fences against animal invasion and build earth dams, rock dams, infiltration conduits and terraces as well as planting native and exotic trees.

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