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The internal structure and geotechnical properties of landslide dams have rarely been examined in detail, although they largely control the longevity and potential failure mechanisms of the dams. During the 2004 Mid-Niigata Prefecture earthquake, thousands of landslides were triggered and many landslide dams were formed in the Imo River basin. Among these landslides dams, the biggest one that occurred in the Higashi-Takezawa district threatened both the upstream and downstream areas. Emergency and eventual permanent countermeasures were successfully performed to mitigate the hazards presented by the dam. The preserved landslide dam provides a good field study area for improving understanding of the formation and stability of similar features. To characterize its internal structure and to provide more reliable information for stability analysis, we used a multiple-channel surface-wave technique and microtremor array measurements to survey the dam's S-wave velocity (V-s) profile. Our results show that the integrated use of these methods is an effective approach for determining the S-wave velocity structure of landslide dams, irrespective of the mountainous environment in which they typically occur. The V-s profile suggests that the displaced landslide materials had been little disturbed during movement, although movement was rapid and over a long distance. We also discuss herein the landsliding mechanism and assess the breaching risk of the landslide dam due to overtopping. (C) 2016 Elsevier B.V. All rights reserved.

We propose a simulation model using viscous damping to predict the moving velocity (v) of a landslide before it reaches a strain limit. We call this model “Lumped mass damper model”, and it is based on a motion equation using a mass system model composed of a damper. The damper introduces a damping force in the opposite direction to the downward force (F) of the landslide, according to the landslide velocity. For slope-stability analysis, a simple model such as the Fellenius method is used. In the analysis, the resistance force (R) and driving force (D) are calculated in each individual slice and summed up for all slices. the safety factor (Fs) is indicated as Fs = R/D. Then, the equation of motion is shown as \( {\text{m}}\upalpha = {\text{F}} - {\text{A}} \cdot {\text{Cd}} \cdot {\text{v}} \). In the equation m is mass and α is acceleration of the landslide. After solving this motion equation, the formula ‘\( {\text{v}}{ \approx }{\text{F}}/{\text{A}} \cdot {\text{Cd}} \)’ is obtained. ‘A’ is the area of the slip plane of the landslide. ‘Cd’ is the viscosity coefficient of the damper, which exerts its effect on the slip surface of the landslide. Analytical results using this technique on the Kostanjek landslide in Croatia show that this Lumped mass damper model was able to reproduce the variations in landslide motion in response to the variation of groundwater level. Also, the Takino landslide of 4th August 1986 which was induced by embankment construction for a Japanese highway was successfully analyzed using this technique.

地震時の地すべり・土砂災害の被害を抑制するためには危険箇所の緊急点検等の対応を迅速・的確に行う必要がある。地すべり等を引き起こす斜面安定度の低下は水文状態と地震動とに影響される。水文状態については, 北陸地方で適用可能な広域水文指標が提案されている。地震動については, 全国の市町村単位で震度が計測されている。これにより, 市町村単位で水文状態と震度とを監視することが可能である。北陸地方で最近3ヵ年に発生した比較的大きな3つの地震災害については, 危険箇所緊急点検が行われ, 市町村単位で危険度判定が実施されてきた。そこで, 本研究では危険度判定により対策が必要とされた事例を水文指標と震度を用いて分析した。これにより, 地震発生時に, 震度が一定以上の区域内を対象とした危険箇所の崩壊危険性を, 水文状態と震度とから推定する手法を構築した。地震時の危険箇所の崩壊危険性は, 広域の水文状態を考慮することで, より精密に推定できることが示された。いくつかの広域水文指標について, 一定の閾値以上の震度の領域を点検対象とした場合に危険性の高い箇所を見出す的中率と捕捉率との比較を実施した。

In order to study the physical and chemical weathering and their impact on the shear strength of the soil mass, soil samples were collected from different depths ranging from the ground surface to a depth of 15m at two different drainage wells in a Mukohidehara landslide area. These specimens represent the residual soil near the surface, specimen at a depth range of ground water level fluctuation, less weathered specimen from the depth below the ground water level fluctuation, the remolded soil from sliding zone and the specimen below sliding zone. Measurement of Atterberg's limits, particle size analysis, X-ray diffraction analysis, slake durability test and X-ray fluorescence tests were carried out. Drained simple shear tests, and drained ring shear tests were also conducted on those specimens to measure their strength characteristics.Test results showed that fully softened shear strength of the less weathered specimen was very close to the peak shear strength of the specimen from the sliding zone. Test results showed no significant variations of index properties (liquid limit, plasticity index and clay fraction) , mineral compositions and chemical compositions with depth. Residual shear strengths of the specimen also did not vary much with depth. In order to compare these findings with rocks from the other types of formation, specimens were collected from the ground surface, ground water fluctuation zone, less weathered zone and sliding zone of three other landslides sites. The test results on the index properties, shear strength properties, mineralogical analysis, and bulk chemistry, of those landslides were consistent with the findings in Mukohidehara landslide.The test results of this study suggested that physical weathering plays a major role in the reduction of shear strength of mudstone, in comparison to the chemical weathering in the form of mineralogical and chemical alteration. Test results also suggested that physical weathering by slaking and disintegration is significant at ground water fluctuation zone in those mudstone formations.

Debris flows triggered by earthquake show different occurrence mechanism, movement characteristics and failure modes with the common debris flows under gravity, because of the influence of seismic force, snowpack, different topography, lithology etc. In this paper, the Tatsunokuchi debris flow induced by the 2011 Northern Nagano Prefecture Earthquake and Dozou-sawa debris flow caused by the 2008 Iwate-Miyagi Nairiku Earthquake are analyzed. The two debris flows can be considered as composite actions of seismic force and snow. This paper compares the occurrence conditions, movement processes and geomorphic characteristics of the two debris flows and simulates the runout distance and inundated area using a GIS-Based depth-averaged two-dimensional numerical model. The simulations reproduce the propagation and deposition across the actual three-dimensional topography and the results are in good agreement with the investigation by comparing with the photographs. The entrained-snow along the flow path alters the speed and runout distance of the debris flows. The lubricating effect of snow in the movement process is discussed.

Most of the slope failure disasters in a humid area such as that of Japan are caused by heavy rain. However, even for the case of heavy rainfall that occurs once in every 10 years, total area of slope failures seldom exceeds 10% of a watershed. From this background, we focused on the vein-like groundwater flows that increase pore-water pressure, and clarified the relationship between distributions of slope failures and groundwater veins. In this study, a 1-m-depth ground temperature survey and water-chemistry analyses at springs and boreholes were conducted in Zentoku area of Shikoku Island, southwestern Japan, to grasp the distribution of groundwater veins and their sources. Subsequently, slope-stability was analyzed to investigate the relationship between groundwater veins and slope failures at study sites. These results lead to the following conclusions: The slope failures appear to concentrate around shallow groundwater veins and groundwater veins rising from deep layers. This means that slope failures caused by these groundwater veins in addition to rainfall. Two types of groundwater originate in the deep layers: one has short storage time as indicated by the fact that dissolved substances are low; the other is stored for a lengthy period as noted by a high concentration of dissolved substances. By combining the results of stability analyses and distribution of groundwater veins, it is suggested that prediction of zones with high potential for slope failure can be more accurate.

Slope stabilization procedures for seven landslides in the Niigata Prefecture of Japan had provided an excellent and rare opportunity to examine the composition of ground water, and residual shear strength of the sliding surface soil, as well as unsheared mudstone. Ring shear tests were conducted on the soil samples collected from sliding surface soil, revealed during the installation of 3 m diameter drainage galleries. Shear strength and index properties of the soil were measured by mixing the soil with distilled water and sea water, and after leaching the NaCl from the pore water. The residual shear strength of the soil sample with distilled water was 3–5° lower than that with the sea water. The residual shear strength of the intact rock powder after leaching the dissolved salt was close to that of the soil from sliding surface, and the residual shear strength of the soil from sliding surface after mixing with sea water was close to that of the intact rock powder. The increase in residual shear strength...

A sufficient knowledge on the kinematics and development of landslides helps to adopt proper measures that can be used to protect slopes and the environment in general. This can be achieved by adequate monitoring programs. This paper presents the findings of intensive monitoring activities carried out on Shiidomari and Katanoo landslides found in Sado Island of Japan. More than one year of observation of the two landslides allowed defining some peculiar futures of their kinematics and style of development. The problem of slope instability in the two areas is generally accredited to various factors. But, both landslides were triggered by heavy rainfalls and snowmelt. Because of the outline of the area and the presence of relict topographic features, the Shiidomari landslide is considered to be a large-scale reactivation of old slope failures. The Katanoo landslide is, however, a first-time case. Geophysical investigations and drilling activities in Shiidomari indicated the presence of two slip planes. The deepest (80–100 m) of these is controlled by existing lineaments. Monitoring data suggests that the body of the landslide has subsided as much as 1.16 m just below the main scarp, but a centimeter in the central region. The toe sector also experienced a significant amount of subsidence, but this was counter-balanced by an uplift on the opposite side of the landslide. Hence, the landslide seems not any more active along the deepest slip surface, although it may extend upward and define a series of shallow shear planes around the crown. In the case of Katanoo, the landform characteristics, differential weathering, the road cut and groundwater fluctuations appeared to contribute much to determine the exact location of the landslide. Extensional cracks that preceded the landslide can be related to heavy rainfalls and the cold and warm cycles thereafter. Subsurface investigations and monitoring works indicated that the landslide has two slide blocks with different slip planes. During the observation period, the upper part of the landslide responded more effectively to rainfall and snowmelt than the middle and lower sections. The corresponding movements, however, appeared to settle about three months after failure. There were also little strain transmissions in boreholes and no significant change in the characteristics of the landslide. The kinematics of deformation of many of the slopes in Sado Island resembles that of Shiidomari landslide. But mass movements along highways and mountain roads are usually similar to Katanoo. Landslides of the type like Shiidomari may not show sudden and drastic failures, but are usually long lasting and can reactivate repeatedly along new, shallow shear planes. Monitoring works and long-term supervisions in these types of landslides are useful to identify impending failures and take the right measures before they brought about large-scale destruction to the environment.

Slope instability research and susceptibility mapping is a fundamental component of hazard management and an important basis for provision of measures aimed at decreasing the risk of living with landslides. On this basis, this paper presents the result of a comprehensive study on slope stability analyses and landslide susceptibility mapping carried out in part of Sado Island of Japan. Various types of landslides occurred in the island throughout history. Little is known about the triggering factors and severity of old landslides, but for many of the recent slope failures, the slope characteristics and stratigraphy are such that ground surfaces retain water perennially and landslides occur when additional moisture is induced during rainfall and snowmelt. A range of methods are available in literature for preparation of landslide susceptibility maps. In this study we used two methods namely, the analytical hierarchy process (AHP) and logistic regression, to produce and later compare two susceptibility maps. AHP is a semi-qualitative method, which involves a matrix-based pair-wise comparison of the contribution of different factors for landsliding. Logistic regression on the other hand promotes a multivariate statistical analysis with an objective to find the best-fitting model that describes the relationship between the presence or absence of landslides (dependent variable) and a set of causal factors (independent parameters). Elevation, lithology and slope gradient were casual factors in this study. The determinations of factor weights by AHP and logistic regression were preceded by the calculation of class weights (landslide densities) based on bivariate statistical analyses (BSA). The differences between the AHP derived susceptibility map and the logistic regression counterpart are relatively minor when broad-based classifications are considered. However, with an increase in the number of susceptibility classes, the logistic regression map gave more details but the one derived by AHP failed to do so. The reason is that the majority of pixels in the AHP map have high values, and an increase in the number of classes gives little change in the spatial distribution of susceptibility zones in the middle. To verify the practicality of the two susceptibility maps, both of them were compared with a landslide activity map containing 18 active landslide zones. The outcome was that the active landslide zones do not completely fit into the very high susceptibility class of both maps for various reasons. But 70% of these landslide zones fall into the high and very high susceptibility zones of the AHP map while this is 63% in the case of logistic regression. This indicates that despite the skewed distribution of susceptibility indices, the AHP map was better to capture the reality on the ground than the logistic regression equivalent.

Slope stabilization procedures for six landslides in the Niigata prefecture in Japan afforded an excellent opportunity to examine the residual drained shear strength of weathered mudstones. Installation of 3 m diameter drainage galleries allowed undisturbed block samples to be collected which contained the slickenside surface of the failure plane. Ring shear tests were conducted on undisturbed samples and remolded samples of the same material. In addition to the laboratory testing, back analysis was performed to calculate the residual shear strength parameters of the slopes. Good agreement was found between the residual friction angles determined from back analysis, tests on undisturbed samples, and tests on remolded specimens. Many of the landslide materials exhibited a small cohesion at low values of effective normal stress, but the secant friction angle achieved a constant value at higher normal stresses.

Mineralogical composition, measured with x-ray diffraction for soil specimens collected from more than 80 natural disaster areas, show smectite, kaolinite, mica, feldspar, and quartz as major constituent minerals. Based on the results of more than 35 mixtures of smectite, kaolinite, and quartz in different proportions, a new correlation method was proposed for estimation of the residual friction angle ( ϕr′ ) with the proportion of the minerals. The mineralogical compositions of the natural samples were used to estimate the ϕr′ based on the proposed triangular model. The estimation method resulted in values as close as 90% to the measured ϕr′ , and as close as 80% to the case of 53 samples tested by other researchers. The paper incorporates the correlation of ϕr′ with the liquid limit, the plasticity index, the clay fraction, the specific surface area, and the proportion of smectite. Various ranges are proposed, according to the influence of the constituent clay minerals. In addition, the applicable range...

NaCl solution has significant effect on the residual shear strength of soil from mudstone areas. Soil samples were collected from the intact rock (unsheared zone) and sliding surface (sheared zone) of five mudstone landslide areas. Mixtures of distilled water and the intact rock possessed higher electric conductivity than those of distilled water and the sliding surface soil. When these samples were leached with distilled water for five times, they showed a substantial decrease in the electric conductivity. The residual shear strength varies according to the NaCl concentration in the pore water. Ring shear tests were also carried out in the commercial standard clays as bentonite and kaoline to understand the shear strength characteristics of the mudstone soils. When bentonite was treated with NaCl solution as pore water, it showed about three times increase in residual shear strength and decrease to one quarter in liquid limit. The effect of NaCl solution as pore water in kaoline was comparatively low. The residual shear strength of soil from mudstone area also increased when it was treated with NaCl solution as pore water. The result of the X-ray diffraction analysis showed that the proportion of smectite is higher than that of kaolinite in the residual soil from mudstone. These smectite minerals were found to be highly sensitive to NaCl solution. Because of the sensitive changing character of smectite minerals to NaCl solution, soil samples with NaCl rich water showed lower value in consistency limits and higher value in the residual shear strength compared to soil samples with distilled water. The findings suggest that the residual shear strength and index properties should be considered as variable properties as a result of the change in the pore water. Such properties are useful to understand frequent re-activation of the landslide occurring in the mudstone areas.

本論文では, 東北地方の第三紀層地すべり地における水質調査結果に基づき, 大規模地すべり地における地下水流動機構に関する総合的考察を行った。水質調査結果を地下水位調査結果, 地下水追跡調査, 更には地質構造と対比し, 総合的な地下水解析を行うことが有効であることを示した。滝坂地すべり地を対象とした解析では, 地下水位の変動タイプを地質・地すべりブロック別に考慮し, 地下水水質並びに地下水追跡調査結果を組み合わせることで地下水の流動状況を把握した。更に, 地すべり地内における地下水のCl-イオン濃度分布の調査結果に基づいて, 融雪期の地下水位の上昇に花崗岩体を通過してくる深層地下水が関与している可能性が高いことを示した。平根地すべり地を対象とした解析では, 新第三紀層中から流入する地下水と, 地下深部から上昇してくる地下水の混合状況から, 各地すべりブロックにおける地下水流動状況を把握した。水質解析結果から推定された地下水流動状況は地下水追跡試験結果と合致している。以上の考察結果を総合して, 大規模地すべり地における総合的な地下水流動特性の解析手法を提案した。

新潟県東頸城地域の地すべり地では高濃度Na-Cl型地下水がしばしば見出される. 本研究では, 宇津俣地すべり地等で電気伝導度計を用いた詳細な地下水 (孔内水) 検層, 地下水の化学分析, ならびに酸素同位体比測定を実施した. その結果, 滑動が活発な地すべり土塊における地下水断面では, 化学躍層的な電気伝導度の急激な増加があり, 不活発な地すべり土塊では電気伝導度の有意な変化は認められない. 地すべり土塊内における地下水の水質は, 表層部より深部へ向かって, (1) Ca-HCO3, Na. Ca-HCO3型, (2) Na-SO4型, (3) Na-HCO3型, および (4) Na-Cl型に分類される. 酸素同位体比と塩化物イオン濃度の関係より, Na-Cl型地下水は西山層相当の続成作用を被った化石海水と天水起源の地下水との混合によって形成された. Na-Cl型地下水は異常高圧熱水としての深部化石海水が断層に沿って上昇し, 地すべり土塊に供給されていることを示唆する.

During the heavy rain on June 29, 1999, four slope failures occurred around the Taishi-do area in Zentoku landslide, Tokushima, Japan. Based on field investigation after these failures, and the monitored records of the three-dimensional shear displacement me-ters before the failures, it was made clear that 1) the failures were limited comparing to the rainfall ; 2) One of the failures showed precursory displacement of creeping failure. To clarify the failure mechanism, field investigations including one-meter depth of soil temperature survey, temperature measurements of spring water and groundwater in borehole were conducted, and pore water pressures generated at failure were estimated through back-analysis. Thereafter, the relationship between the location of failed slopes and flowing groundwater veins, and the relationship between groundwater vein and precursory displacement of failures were analyzed. It is found that: 1) Slope failure occurred above or near groundwater vein ; 2) There were two types of groundwater veins, one was surficial vein type with high water temperature, and the other was deep vein type originating from a depth between 49 and 70m with low water temperature ; 3) The precursory displacement of the creeping failure was associated with the groundwater vein.

This paper presents comprehensive study of the process and mechanism of the 7th March 2012 Kokkawa landslide in Japan. The landslide has the following dimensions; average width of 150 m, maximum length of about 500 m. The depth of the sliding surfce is about 20 m. The volume of the sliding soil mass is estimated to be 750,000 m3. Although no one was killed or injured, several house were completely destroyed by the sliding soil mass. In addition, a lot of inhabitants of neighbouring houses had to be evacuated. The Kokkawa landslide occurred in Joetsu City, which is located in the southwestern part of Niigata Prefecture. Many deep-seated landslide events have been recorded in the past in Joetsu City area. The area is also characterized by long and heavy bouts of hazardous snow falls. A lot of snow accumulation was observed in the vicinity of the landslide area during winter season. Clearly the landslide was triggered by snowmelt. The soil layer above the geological formation border probably contained high amount of water due to antecedent snowmelt before the occurrence of the landslide. It was remarkably observed that the sliding soil mass has long travel distance and high velocity at the early stage of the movement.

Two major movement modes of slope failures: landslides and debris-flows are simulated by two integrated models using GIS to represent the dynamic process across 3D terrains. Landslides occur when earth material moves downhill after failing along a shear zone. Debris flows can be differentiated from landslides by the pervasive, fluid-like deformation of the mobilized material. The formation of debris flows most often occurs as a result of a landslide partially or completely mobilizing into a debris flow. GIS-based revised Hovland’s 3D limit equilibrium model is used to simulate the movement and stoppage of a landslide. The 3D factor of safety will be calculated step by step during the sliding process simulation. Stoppage is defined by the factor of safety much greater than one and the velocity equal zero. GIS-based depth-averaged 2D numerical model is used to predict the runout distance and inundated area of a debris flow. The simulation displays the propagation and deposition and deposition of a debris flow across the complex topography. The two GIS-based integrated models are able to provide effective tools for hazard mapping and risk assessment.

The Grohovo Landslide, situated on the north-eastern slope in the central part of the Rječina River valley, is the largest active landslide along the Croatian part of the Adriatic Sea coast. The Grohovo Landslide is also a key pilot area for implementation of the monitoring activities of the Japanese-Croatian joint research project on “Risk identification and land-use planning for disaster mitigation of landslides and floods in Croatia”. This complex retrogressive landslide was reactivated in December 1996. It is considered that the basal surface of rupture is positioned at the contact between the slope deposits and the flysch bedrock. Based on the data from boreholes, geological mapping, geophysical surveys and the geological cross-section of the slope, the three dimensional shape and the position of the sliding surface were simulated using the inverse distance weighted interpolation. Using 3D extended Janbu’s simplified method, global stability of the Grohovo Landslide and stability of twelve separate landslide bodies were analyzed.

Landslides occur frequently in connection with other types of hazardous phenomena such as earthquakes and volcanic activities. Strong earthquakes often cause a large number of landslides, including large-scale landslides, in mountainous areas. Volcanic activities could trigger giant landslides or debris avalanches on mountain slopes. It is not uncommon that such large-scale landslides cause river blockage and form natural dams, which are vulnerable to collapse by overtopping and breaching. The sudden collapse of a landslide dam can cause a catastrophic flood in the downstream area. Submarine landslides are also common phenomena. Large-scale submarine landslides cause catastrophic tsunamis. In assessing those catastrophic cases as a whole, it is necessary to pay special attention to extremely high threats to vulnerable settlements in hazardous areas. The risk assessment of such complicated combined landslide disasters around the world, particularly in developing countries, is a significant step for identifying the appropriate mitigation strategy against catastrophic damage. In recent decades we have experienced remarkable disasters induced by landslides. For example, a huge number of landslides were induced by the Chi-Chi earthquake in Taiwan (1999), by the Mid-Niigata Prefecture earthquake in Japan (2004) and the Northern Pakistan earthquake (2005). Most recently the Wenchuan earthquake occurred with magnitude 8.0 in central part of China on May 12, 2008. This gigantic earthquake caused a tremendous number of landslides, as seen in the satellite picture in Fig. 23.1. Large-scale landslides occurred in volcanic areas, such as at Stromboli Volcano in Italy (2002) (Fig. 23.2) and on Leyte Island in Philippines (2006) (Fig. 23.3). Whole such events manifest the significance of a thematic session within the First World Landslide Forum focussing on the issue of risk mitigation targeting “landslides and multi-hazards”.

The Mid-Niigata earthquake occurred in 2004 and taught us a lesson about the possibility of landslide damming when the earthquake occurs after the softening of the ground by rainfall. Terano Landslide was considered stable before the earthquake. However, due to the earthquake, the landslide moved extensively and dammed the Imokawa River. A main landslide block was analyzed for a seismic loading condition using the observed peak ground acceleration. Soil samples were collected from numerous sliding surfaces, and fully softened and residual shear strengths were measured. Factor of safeties were calculated for the mobilization of fully softened and residual shear strengths during different peak ground accelerations (PGA) and different pore water pressure ratios. Quantitative risk to landslide damming was evaluated for different PGA and ground water level scenarios, using the Bishop's Simplified Stability Analysis Method modified for the dynamic loading. The study results showed that the landslide mass would fail even if the earthquake of PGA more than 0.4g occurred in a dry season.

It was a remarkable aspect that a tremendous number of landslides were triggered by the Mid-Niigata Prefecture Earthqauke. Because the epicenter was located at a depth of 13 km just in the landslide prone area of central part of the Niigata Prefecture, severe damage was caused by the earthqauke-induced landslides. Furtehermore many landslide dams were formed mainly in the watershed of the Imogawa-River by the displaced soil mass of the earthquake-induced landlslides. Some large landslide dams should pose a great threat of flood and debris flow in case of dam collapse to the settlemnt of the Ryuko-District in the downstream area of the watershed. It was urgently needed to arrange the emergency operations to avoid the destructive collapse of the major landslide dams. This contribution illustrates overview of landslide dams and emergency operations against dam collapse.

A 6.8M earthquake occurred in Niigata in October 2004 caused a huge damage that includes triggering of hundreds of landslides. Some of those landslides caused natural damming of the associated streams. Higashi Takezawa is one of the largest landslides that caused natural damming. Shear strengths of soil layers above and below the sliding surface of that landslide were remarkably different. Pseudo static analysis of the landslide showed that about 700 gal of peak ground acceleration (PGA) was enough to cause the slide moving at the time of earthquake. After the mobilization of residual shear strength, the stability was calculated and found to be reduced considerably, and about 350 gal of PGA was enough to keep the mass moving at that condition. However, if the earthquake has occurred in dry season or if the soil mass has not been softened by the excessive rainfall during the typhoon, the earthquake of that magnitude is not sufficient to cause failure. Dynamic safety factor of the landslide decreased exponentially with the increase in water pressure. This study is very useful to evaluate the stability of other potential landslides so that precaution can be taken against disaster during next earthquake events. BACKGROUND

The Hime River Basin is located in the northern part of Central Japan and is known as one of the areas where both erosional potential and sediment yield are extremely high in Japan. Landslides and debris flows triggered mainly by heavy rainfalls have frequently occurred in the basin. We have estimated the chemical weathering rates for nineteen watersheds in the Shirouma-Oike Volcano located in the western part of the basin. These rates have been simply estimated by the mass balance equation between solute fluxes of stream waters from each watershed and solute loss comparing fresh and weathered volcanic rocks and were calculated to be ranging from 0.15 to 3.24 mmyr-1. A watershed showing the highest rate of chemical weathering and solute flux corresponded to the area where the large-scale landslide occurred in 1911 and debris flows and landslides have continually occurred until now. Unstable sediments yielded by chemical weathering are thought to be an important factor of sediment disaster occurrences in the research area. Solute fluxes of each stream could be useful for susceptibility mappings of landslides and debris flows in each watershed.

1996年12月6日午前10時40分頃, 姫川支流の蒲原沢(第1図)で土石流が発生した. 土石流は下流の災害復旧工事現場を直撃し, 作業員14名が死亡あるいは行方不明となる惨事となった. 蒲原沢は急峻な地形(第2図)の荒廃した渓流である. 上空からの観察によって, 2か所(標高1350m付近と600m付近) の新規崩壊場所が確認された (写真2-4). 源頭部の崩壊は標高1350m付近にあり, 来馬層群と第四紀風吹火山噴出物との地層境界部(第3図)にあたる. この付近は地形的勾配変曲点であり, 侵食営力が集中しやすい. 標高600m付近の崩壊は, 渓岸斜面下部が侵食されることによって生じたとみられる. 今回の土石流は, 蒲原沢上流の標高1350m付近の源頭部で新たな斜面崩壊が発生し, 崩落した土砂が急勾配の沢を流下する過程で, 不安定な渓床堆積物の取り込みや渓岸侵食によって, 発達していったものと推定される. 蒲原沢では1995年7月11日の集中豪雨による姫川土砂災害の際にも国界橋より下流に新設した「新国界橋」を押し流す大規模な土石流が発生している.