The goal of this study was the development of a method of estimating a hydrograph of a mudflow passing through a slit check dam. Past research has shown that a slit check dam makes the hydrograph low. No study, however, has systematically clarified the influence of the sediment concentration and slit width on this effect. Therefore this research began with the clarification of the influence of these factors by flume experiments, and numerical simulations were performed to estimate the hydrograph.
The experiments were performed using a flume with a width of 10 cm, length of 300 cm, and height of 20 cm, set at gradients of 10° and 15° . The experiments were done using four dam models with slit widths of 1 cm, 2.3 cm, 5.3 cm, and 6.7 cm. Two mixtures of sand, Bentonite and water -sediment volumetric concentration of 40% and 20% were used. Each mixture was supplied from the upstream end of the channel at a constant rate with a tube pump. After the flow reached a steady state, one of the dam models was placed in the flume. The results show that the narrower the slit, the lower the discharge and the longer it is constrained. And the higher the sediment concentration, the lower the discharge.
The hydrographs were calculated using continue equation and momentum equation with MacCormack's method. It was assumed that maximum discharge with the specific energy in the upstream of dam model could flow out. The calculations accurately duplicated the experimental results.

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To describe long term changes in land cover conditions of hill slopes near villages in the northern part of Tanzawa mountains, Kanagawa Prefecture Japan, where the Kanto earthquake in 1922 and several heavy rain event during past fifty years caused many land slides, analysis has been made using aerial photography. Aerial photographs taken in 1940, 1947, 1960, 1971, 1973, 1980 and 1997 are overlaid to prepare an orthophotograph and a matching scale using graphic software together with topographical maps with land cover description issued in 1910, 1925, 1947 and 1973. The ridge part of this area was predominately a degraded grassland and foot slope area near villages was covered by forest before the earthquake in 1922 as shown in an old map. The degraded grassland of this area was a result of the utilization by local villagers in the nineteenth century. The bare land area in 1940 was not only caused by an earthquake but it was also caused by the previous degraded situation. From 1940 to 1960, clear cutting and preparatory treatment for the plantation of coniferous trees were detected over a considerable extent of the area by an aerial photograph taken in 1960. A heavy rainfall event in July of 1972 made several landslides as identified by a photograph taken in 1973. Planted coniferous trees and natural growing broad-leaved trees now lead to a crown closed forest where no bare land can be seen including the 1972 land slide area. These long-term changes of this area support the scenario that the general trend of the forest condition in Japan in the mountainous areas near villages without urbanization, shows a gradual change from a degraded vegetation and bare land caused by overuse to a crown closed forest with increasing biomass during the last hundred years.

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In order smoothly to carry out public works, an agreement formation of a citizen is necessary. Of course, for Sabo works also this isn't an exception of this case. Up to this time, Sabo works had been enforced for national land conservation as a main purpose. Recently, a social life of a citizen got rich, and social requests became various. Therefore, interests of a citizen extend to various influence effects outside national land conservation effects to be obtained by Sabo works, for example natural environment conservation effects. The value of people for a natural environment depends on a sense of a human being and an evaluation of the effect is different by every person. Therefore, evaluation methods for these natural environment conservation effects, especially to be obtained by Sabo works, had never been established. Recently, studies of evaluation of environment goods by means of sociological knowledge are put into practice. As evaluation methods of environment goods, various methods called TCM (Travel cost method), CVM (Contingent valuation method), Hedonic pricing method and Replacement cost method are proposed. Study area is Nishiiyayama village in Shikoku mountainous region. Yearly, people more than 400, 000 visit “ Kazurabashi” bridge in this village. The number of the visitor here shows an increase tendency. In this study, evaluation of natural environment conservation effects (Sightseeing resources conservation effects) brought about by Sabo works in Nishiiyayama village was carried out, by means of TCM. Following results were also obtained through this study. According to a dealing method of tour person who has the plural destinations and opportunity-cost, an appraised value changed greatly. However, an appraised value for sightseeing resources conservation effects in Nishiiyayama village region understood to be suited to 10% grade of Sabo works expenses, at least.

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The major sources of red sediment discharged from mountainous areas of Okinawa Prefecture are pineapple fields. The authors prepared hydrological observation fields representing different cultivation stages in the upstream of the Furushima river in Higashi-son. The following are the major findings.
1) The major period of red sediment outflow from pineapple fields ranges from 1 month to 1 year following the plowing-in performed to renew a field.
2) Excluding the times of large rainfalls in excess of 200 mm, the volume of sediment discharged per unit field surface where erosion is possible is almost identical at every cultivation stage.
3) Turbid water discharged from pineapple fields contains a large clay content that remains between 67% and 70% regardless of the cultivation stage.
4) The mean value of the discharge rate during a continuous rainfall of 30 mm or more -rainfall that increases the discharge of red sediment- is about 0.25. From 100 mm to 200 mm, it is about 0.2, but when the rainfall is higher, it is about 0.4.
5) When no rain falls for 200 hours or longer, the critical rainfall for red sediment outflow on a pineapple field is about 10 mm of cumulative rainfall. The critical rainfall intensity for red sediment discharge is between 0.3 and 0.6 mm/5 min.
6) The mean red sediment erosion depth caused by a continuous rainfall where the ground surface is exposed in a field is a maximum of several millimeters.

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When dealing with the Sabo type of dam, various methods have been implemented to reduce erosion by abrasive action from flowing water on the floor of the top part of the dam. Experiments have proven that use of granolithic concrete is cost efficient and effectively reduces erosion by abrasion of the top layer. However several problems still exist that prevent its' applicability in the field.
From experience, methods have been developed for designing and constructing the top part of the dam to prevent abrasion but there are many restrictions that still exist. In this sense there is a need to approach the whole problem from a performance related aspect.
In this thesis, performance design using granolithic concrete as the main construction material for the protection of the top of the Sabo dam, considers the workability of the concrete, the abrasive properties and the possibility of reducing cracks from arising.
The results obtained show that through the improvement of the mix ratios of the granolithic concrete, the difficulties sustained in the workability are reduced while the resistance to abrasive action is greatly improved. Actual use of this concrete and a combination of various experiments have shown that alterations in the mixture ratio effectively improve the performance of the concrete used for the Sabo dam floor.

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Unzen volcano had many pyroclastic flows from 1991 to 1995. The pyroclastic flow deposits widely and thickly covered the flank of the volcano and created a radical alteration of the hydrological and erosion regime of the basins. As a result of that, debris flows have frequently occurred at the rivers on the flank of the volcano.
In this paper, we analyzed the temporal variation of rainfall induced debris flows and the runoff characteristics of debris flows following the pyroclastic flows by using the dynamic records of debris flows observed from 1995 to 2000. Occurrence frequency and magnitude of the debris flow have decreased since the end of volcanic activity because of recovery of the infiltration rate and decrease of Horton's overland flow in frequency and magnitude.

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A conceptual process model is presented to analyse the whole process of devastation, denudation and recovery of forests and soils on low relief mountains. The model is composed of the four zones which represent a respective erosion type. The four zones are as follows : The mature forest zone where deep landslides ocurring as natural phenomena are predominant ; The preparatory zone of the active erosion front where shallow landslides are accerelated by forest deterioation and cutting ; The active erosion front zone where most sivere surface erosion and soil slides occur simultainiously ; The bareland zone where the surface soil is denudeted. The model clearif ed the spatial and the temporal process of devastation of forests and denudation of soils, and the importance of accerelation of soil slides by forest cuting in sediment disasters during the past half century.

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