We estimated the soil erosion rate in a small catchment of Hinoki (Chamaecyparis obtusa Sieb.) forest using Cs-137, and compared it with the sediment accumulation rate measured by direct analysis of core samples collected at the farm dam located on the bottom of the basin. Annual soil loss was calculated to be 3.1t/ha/yr using Cs-137 distribution on the hillslope. On the other hand, the soil loss was estimated to be 12.9t/ha/yr at the shallow points of the farm dam, and 1.0-1.4t/ha/yr at the deepest points. And it is expected that Cs-137 loss express the present conditions of surface soil at each sampling site. It is concluded that Cs-137 can be used as an indicator for estimation of surface soil erosion at Japanese Hinoki forest.

抄録全体を表示

Ten-minute mean windspeed, maximum windspeed (gust speed) and the corresponding directions above the top of a sand-control forest of Japanese black pine (Pinus thunbergii Parl.) were recorded continuously over a 20-month period. The maximum 10-minute windspeed recorded was 13.0ms^-1; prevailing wind direction ranged 145-180 degree north true. Gust factor changed with mean windspeed in linear function, and averaged at 1.92. The profile parameters of roughness length and displacement height over the sand-control forest canopy averaged at 0.52m and 5.6m. The cumulative distribution of windspeed was found to follow the Weibull distribution over the sand-control forest. The coefficients of the two-parameter Weibull model were k=1.36 (an exponent) and c=2.56ms^-1 (a constant). According to the method developed by building design purpose, the parameters of Gumbel distribution (Type I extreme value) for describing the conditional cumulative distribution of the annual extreme (10-minute mean windspeed) were estimated. The most likely extreme windspeed over the sand-control forest in a 50-year period was calculated to be 14.2-15.7ms^-1. Relationships between mean windspeed and gust factor showed that the data were normally distributed about the gust factor versus the regression line of mean windspeed. The relationships can be used to estimate the probability that a gust speed of given wind magnitude will be equaled or exceeded during a period when the mean windspeed is known. The results obtained in this study are very useful in understanding the general wind regime over the sand-control forest areas. It can be concluded that these observations are important for the establishment and management of the sand-control forests nearby the sea because there were few long-term measurements in the strong blown sand areas.

抄録全体を表示

This paper investigates the difference of topographical features between the mountain collapse of a shallow depth due to earthquake and that due to torrential rainfall by using neural network. Neural network learns through collapse data containing uncertainty obtained from the different pubulished papers. Results obtained by using neural network after learning are as follows
1) Collapse due to earthquake easily occurs at the mountain slope of the convex type in cross section and site on the summit.
2) Collapse due to rainfall easily occurs at the mountain foot and the slope with concave topographical feature.
3) Degree of certainty of the first conclusion is higher than the second.

抄録全体を表示

This paper presents an experimental study on the impact behavior of steel cell dam supported by truss frame. The repeated impact load with incremental collision velocity is applied to the steel cell dam supported by truss steel frame by using the horizontal weight collision test apparatus. The impact load and the dent deformation were examined for the various filled material conditions. The axial force in the diagonal member of the truss frame which supports the steel cell was also measured as the representative transmitted force of the steel cell dam. The impact force at the collision point was remarkably reduced by installing the steel cell filled with sand or gravel. Therefore, the axial force of the truss frame which supports the steel cell became significantly smaller than the impact force at the collision point.

抄録全体を表示

In most rivers, grains of sediment in transported downstream are smaller in size in the downstream direction. In the field of hydraulics for sediment transport, it is considered that the reduction in grain size in the downstream direction is due to selective transport. The abrasion of grains then is not taken into consideration.
However, in mountain streams with geologically fragile rock grains, grain abrasion might need to be considered.
The Yoshino River was chosen as the object of this study. First, the reduction in the grain size of the longitudinal bed material was investigated there. Secondly, we assumed that the grain size of the longitudinal bed material would be reduced by the collision of saltating bed-load grains with stationary bed grains. We executed free fall impact tests on three types of rocky boulders. Then, on the basis of the tests, the mechanism of the reduction in grain size was considered.
The summary of the test results suggests that: 1) in mountain streams with geologically fragile rock grains, individual grains possibly collided repeatedly with each other and were consequently reduced in grain size during their downstream transportation, and 2) as a result of the reduction in grain size, wash load was produced and possibly became the source of turbid water.

抄録全体を表示

Between August 1999 and January 2000, we conducted a survey on the changes in the 33 existing traditionally designed Sabo structures that were built using natural materials. They are located in the Inuyama District, which is in an Aichi Prefecture forest owned by the University of Tokyo.
These Sabo facilities were constructed between 1920 and 1950 in order to prevent the formation of sediment and its discharge from barren ground. Their foundation is comprised of earth dams, and their spillways are covered with either stones, steel wire gabions, or cncrete. Although approximately 50 to 70 years has passed since the Sabo facilities were constructed, the ones that had their spillways covered with stones or concrete have not been damaged.
On the other hand, all but one of the 19 Sabo facilities that had used steel wire gabions to cover their spillways had not been damaged. The damages to the steel wire gabions were broadly categorized into 2 conditions: one, corrosion of the steel wires which caused the stone filling to collapse; and two, deformation or destruction of the gabions, which were caused by debris flows and other external forces. No relationship was found between the cause of the damage and the degree of corrosion of the steel wire in the second condition. However, in the first condition, the corrosion of the gabions was found mostly in three places-areas close to the ground, in the joints between the gabions, and the areas that were at water level. It was assumed that all areas had constant exposure to high humidity.
The wire gabions, which were 4.2 milimeters in diameter and made from galvanized wire, had completely corroded over a period of 40 or 50 years, resulting in the gabions losing their ability to contain the stone filling. However, in some cases, the fine grain sediment between the individual stones has stayed securely in place so that the shape of the top of the spillway has not deformed even though the steel wire has corroded over time.
A study of the nearby vegetation revealed that a stratified structure had formed in the area surrounding the steel wire gabions some 70 years after the completion of the Sabo. The tree layers of the stratified structure included red pines and Quercus serrata.
The above findings have confirmed that the Sabo facilities that were constructed using steel wire gabions are not suitable for passively resisting the movement of sediment. However, they are capable of functioning as temporary structures preventing the motion of surface sediment for a certain period of time. In this manner, they help in the recovery of vegetation, and temporarily prevent sediment deposits from moving any further.

抄録全体を表示

A pyroclastic flow occurred on the south-west flank of Mt. Merapi in Indonesia on July 19, 1998. As a result, a huge amount of pyroclastic deposit filled the gorge of Sat River. In order to understand the process of gully erosion on the deposit, we conducted cross-section survey and took photographs at fixed points on the site, and calculated the sediment yield by gully erosion by using the data of cross-section survey. The results are summarized as follows: 1) Rainfall after the occurrence of the pyroclastic flow caused gully erosion on the doposit. Networks of gullies developed and the cross-sectional size of gullies rapidly increased during the first rainy season.
2) The total eroded sediment volume by gully erosion was estimated 680, 000m³ from the occurrence of the pyroclastic flow until February 2000. Most of the sediment yield by gully erosion occurred during the first half of the rainy season. Sediment yield by gully erosion did not occur during the dry season.
3) The annual specific sediment yield from the pyroclastic flow deposit of the first year was estimated 1.4×10⁶m³/km²/year. This value is almost similar to that of the pyroclastic flow deposit on Mt. Pinatubo, and is 1 to 2 orders of magnitude larger than that of several study examples on other volcanoes.

抄録全体を表示

The processes of debris flow has not been made clear. The repots of eyewitnesses help us understanding the processes of debris flow. The debris flow in Yokobayashi River, one of the debris flow torrents of ‘June 29, 1999 Hiroshima Disaster’, occurred at 4pm and it was seen by two people. The debris flow came two times from two tributaries. The situation reported by them was visualized with computer graphics.

抄録全体を表示