In order to understand the relationship between plant communities and surface failures on mountain slopes, Hiroshima city and its surrounding area's hillsides slope of granite mountain ranges have been surveyed. The surveyed areas are the same in terms of climate, topography, geology, altitude, inclination of the slope and the condition of the soil, but plant communities are different. The hillside slope is covered with soil of weathered rock whose thickness is more than 50 centimeters and surface failures occurred at the upper sideslope that is generally known as the stable topography. In the surveyed regions, natural evergreen broad-leaved forest, substitution forest of summer-green tree, substitution forest of Japanese red pine, and an artificial hinoki forest are distributed throughout the area. The vegetation of the place where surface failures occurred on the slope consisted of Japanese red pine forest whose forest floor is covered by a fern dominantly and young hinoki forest. The dominance of the fern indicates that diseases or pests damaged the Japanese red pine. The common features of the forest where surface failures occurred on the slope are: dominance by the fern in forest floor, less than 70 percent vegetation cover in the canopy layer, more than 60 percent vegetation cover in field stratum, less than 30 percent mingling of vegetation with the evergreen broad-leaved species and a tree height of less than 10 meters. On the other hand, the common forest features on the slope where surface failures have not occurred are: more than 70 percent vegetation cover in the canopy layer, less than 30 percent vegetation cover in field stratum, more than 30 percent mingling with evergreen broad-leaved species and tree height of more than 14 meters. When the forest grows on the slope, although the slope basically becomes stable, the frequency of surface failures increases when the canopy layer becomes depressed. This is shown by the extreme increase of vegetation cover by the fern. The vegetation cover of the canopy layer is closely related with the hydrological balance of soil layer mainly caused by evapotranspiration. Surface failures occur due to the unstabilization of surface soil by water saturation of seepage water and movement of underground water. However, the forest that formed the perfect canopy layer has strong influence over evaporation : it can alleviate the seepage of rainfall and it regulates the amount of water contained in the soil. Because of the reasons above, it is inferred that the canopy layer functions to stabilize the soil layer. In contrast, on the slope that Japanese red pine forest whose forest floor is covered by the fern dominantly and young hinoki forests distributed, the function of hydrological balance to the soil by vegetation decreases and the instability of the soil layer occurs earlier, resulting in surface failures.

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To clarify the temporal changes in the Yue River caused through the processes of erosion and sediment discharge that followed the eruption of the Unzen volcano, field surveys, interpretation of aerial photographs, and runoff analysis of debris flows were carried out. It was found that the area contributing to the occurrence of debris flow declined with time as the area of gully erosion had increased. It was also found that the rate of sediment yield from gully erosion and sediment discharge by debris flows had peaked during October 1992 and August 1993, and then declined with time, though there was an increase during September 1996 and September 1997 after the occurrence of pyroclastic flows. Temporal changes in the processes involved in the debris flows following the eruption were recreated by constructing a runoff model for the Mizunashi river basin in the Unzen volcano. The results showed that this model is applicable to the Yue river basin. The frequency and the magnitude of debris flow occurrences decreased after October 1997 because of the recovery of the infiltration rate, the decrease in the frequency and the magnitude of the Hortonian overland flow, and the decrease in the area contributing to the Hortonian overland flow.

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Concerning the storage function method applied to runoff analysis at large, we tried to analyze by considering the effect of forest, on reference to the nine cases of dam-plan in Nagano Prefecture. As a result, we could propose the method of runoff analysis, which placed great importance on the initial constant of runoff model determined on the basis of the observed data, applied the value of the effective storage estimated by soil pore analysis to saturation rainfall, treated the coefficient of first runoff as the primary factor when identifying runoff model, and used properly the different model by raining antecedently or not. We calculated the flood discharge of Susuki-gawa river basin in Matsumoto city by this method, these results supported to use properly the different model and suggested to be able to estimate the discharge based on the limited rainfall period. Moreover, the lower flood discharge was estimated compared to the current method, therefore, we concluded that the effect of forest influenced this result.

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Large sediment disasters where dead persons reached to 200 persons occurred in December 2003 in Southern Leyte and Northern Mindanao of the Philippines due to heavy rainfall that hit these areas. Most of the casualties were due to the large landslide and the debris flow caused by the heavy rainfall that reached to three times as much as mean monthly precipitation. Especially, the landslide that occurred at San Francisco of Southern Leyte changed to debris flow and deposited huge volume of soil in that area, hitting the houses including the hall where many inhabitants sought refuge after an earlier collapse in a nearby slope. As a result, the number of casualties in this area reached to more than 120 persons. In addition to heavy rainfall, geological structure caused these disasters, because these landslides occurred along Philippines Fault that runs from north to south of Philippines and occurred on the andesite that was weathered and changed to clay. Therefore, there is possibility that sediment disasters occur in this area again. So, it is necessary to inform the people in the dangerous spots for sediment disasters and evacuate them as counter measures to mitigate such disasters.

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From the end of July to August 2, 2004, the 10_th Typhoon (called Namtheun) has damaged to the area around Kisawa Village and Kaminaka Town in Tokushima Prefecture, Japan. There were geologically sediment rocks and a lot of faults in these areas. Especially, on August 1, 2004, it brought extraordinarily heavy rainfall more than 1, 114mm of the daily rainfall-record in Japan onto these areas and a lot of slope failures and debris flows occurred. In the Shiraishi area, in Kaminaka Town, as many residents have paid attention to the occurrence of abnormal phenomena and most of them have opportunely evacuated, no lives were lost by debris flows. In the Azue area, in Kisawa-Village, the tendency of expansion of the landslide area was recognized. The areas, where large-scale debris flows and slope failures occurred, coincided with the areas, where both of a large quantity of cumulative rainfall and large rainfall intensity were brought. However, the critical values of rainfall, which had been related to the occurrence of sedimentrelated disaster, were two to four times greater than those in the cases of granite area in Hiroshima district in Japan.

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Several slope failures and debris flows occurred on September 29, 2004 in Miyagawa village, Mie prefecture. The geological background around Miyagawa village consists of metamorphic rocks of Sambagawa belt and sedimentary rocks of Chichibu belt, and the bedrocks near the surface were strongly weathered. The heavy rainfalls caused by the typhoon Mean (T 0421) triggered these sediment disasters. Cumulative rainfalls at several rain gauges in Miyagawa village were about 600-700mm, and hourly rainfalls were about 100-125mm/h around 8: 00 to 10: 00 a.m. Cumulative rainfall in Miyama town located at southern side of Miyagawa village exceeded 1, 000mm and the maximum hourly rainfall reached to 150mm/h (8: 00-9: 00 a.m.). Therefore, many slope failures and debris flows occurred from 8: 00 to 12: 00 a.m. As a result of these heavy rainfalls, six deaths and one missing person were recorded and 44 houses or sheds were completely broken or lost.

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Cemented sand and gravel (C.S.G) or Sabo soil cement is of a nature different from that of concrete. This paper describes a freezing-thawing test conducted using a poor quality construction material that was made of crusher-run and cement and had varying levels of strength. The objective of the test was to identify the freeze-thaw resistance of construction materials poorer in quality than concrete. The freezing-thawing test was conducted based on JIS A 1148 that is classified into two categories according to the method of freezing-thawing: test A (underwater freezing -thawing test) or test B (atmospheric freezing-thawing test) . And primary resonance frequency and mass were measured and relative dynamic modulus of elasticity and rate of reduction in mass were calculated. Durability factor (DF) was also calculated by these results. DF increased only slightly while compressive strength increased in test A. In test B, freezethaw resistance improved greatly with the increase of compressive strength. It was confirmed in the freezing-thawing test that the freeze-thaw resistance in test B was approximately ten times greater than that in test A as long as the same mix proportion was applied. The results of tests A and B, despite mutual difference between themselves, are almost in agreement with existing test results for IN-situ Stabilized Excavation Method (INSEM) materials with a zero slump.

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The author proposed gate sabo dams in 1988. The term ‘gates’ remind of ones used in flood control dams and/or ones operated frequently to release sediment. The gates of sabo dams are different from them and are usually open except during times when discharged sediment can cause trouble downstream due to significant aggradation or flooding. Such gate sabo dams are renamed ‘shutter sabo dams’. The concept of the shutter sabo dams is discussed in this paper. The shutter is usually made of steel pipes so that water is permeable. The shutter usually retracted and dropped into position when necessary. In order to operate shutter sabo dams properly, it is necessary to obtain a variety of technical information such as rainfall, short-term rainfall prediction data, water discharge rates, water level, sediment transport rates and riverbed elevation on a real-time basis. A real-time data transmission system for these items of data, therefore, needs to be developed. It is also expected that further research and development will be conducted on the mechanisms of the shutter operating information system, such as sensors for sediment transport rates and riverbed elevation changes, a high speed information analytical system, and so on.

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