It could be considered that the sediment dynamics of a basin is characterized by its geomorphometric features, and knowing these features lead to appropriate performances in controlling and mitigating sediment disasters. Almost every year, during the monsoon, due to massive contribution of sediments from the Mahabharat and Siwalik Ranges of the Central Nepal, have been causing countless miserable events to the inhabitants and havoc in the regions. In order to understand the sediment dynamics of the regions, geomorphometric analysis on 16 sub-basins of the regions were conducted. Due to unavailability of sediment discharge and deposition data for the studied areas, floodplain coverage of the studied areas was used to evaluate the sediment dynamics of the regions. All the geomorphometric parameters were determined from 1/25, 000 topographical maps and DEMs. Regarding the floodplain coverage, topographical maps and Landsat-5 TM data acquired in 1988 were used. Apart from the aerial photographs interpretation, field surveys were also carried out to investigate and evaluate the sediment dynamics of the regions. It is suggested from the study that: 1) small basins of the Siwalik Hills have tendency to deposit sediment within the basin; thus, development of floodplains are predominant in the Siwalik Hills ; and 2) Mahabharat basins have capability to flush out most of the sediments yielded in the basins due to their high basin relief and stream drops. The study showed good response to the relationships between morphometric characteristics and sediment dynamics of the studied basins.
To resolve the problems resulting from sediment discharge, it is required to treat the entire area of sediment as the “sediment transport system”. To enable this, the actual state of sediment discharge must be grasped. In the case of mountainous rivers, it is desirable to trap sediment discharge at all depths, because bed load and suspended load and washload flow down being mixed. We developed the Total Load Trapping Equipment that can trap all sediment discharge passing through the overflow section at the unit width. We placed it at the Oshima check dam in Abe river and measured the sediment discharge. Based on the observations, the sediment discharge was quantified by sediment movement type and the following knowledge was gained; (1) In the case of floods of normal year scale, suspended load plus washload account for a far greater ratio than that of bed load in the total sediment discharge flowing down a mountainous river ; (2) The suspended load plus washload obtained at this river was greater by one order than those obtained at other rivers; (3) The bed load estimated by the Ashida, Takahashi, and Mizuyama's formula was greater by 3-5 orders than the measured amount in many cases, but the difference was within 1-2 orders in some cases obtained at peak flow rate. The bed load estimated by the Meyer Peter, Muller's formula was generally smaller than the measured amount, but the difference remained within 1-2 orders.
A volcanic mudflow chronology spanning the past 40, 000 years was developed for the Furano River, which is located on the foot of Mt. Tokachi, using stratigraphic analysis, radiocarbon dating, dendrochronology and historical archives. Mt. Tokachi is one of the most active volcanoes in Japan, and its volcanic eruptions have often caused large-scale mudflows like the 1926 event that caused 144 deaths along the Furano and Biei Rivers. To develop local programs for preventing volcanic disasters, knowledge of past disasters accompanied with volcanic mudflow chronologies are required. We have identified 14 total mudflows during the past 40, 000 years that occurred along the Furano River, including events in A.D. 1926, AD. 1857, AD. 1740, A.D. 1150-1690, A.D. 600-900, 1, 900 YBP, two in 2, 000-2, 300 YBP, 3, 200 YBP, 3, 500 YBP, 3, 700 YBP, 7, 500 YBP, 13, 500 YBP, and 39, 000 YBP. Some of them were also crosschecked with historical archives. Of the fourteen mudflows, a total of 11 occurred during the past 4, 000 years, and 8 occurred during the past 2, 000 years. These high-frequency mudflows aggraded the valley channel and riverbed to the elevation of Kami-Furano City's central part, which was not damaged by the 1926 mudflow. Therefore, the next mudflow will easily reach the city's center and increase overall damage. It is generally suggested that the frequency and magnitude of volcanic mudflows will work synergistically in increasing the risk of sediment disasters.
Application of free meteorological information on the Internet WWW to landslide disaster warnings, especially application of short term rain forecasts based on radar rain gauges, is a solution to appropriate warning and evacuation system. However, this information doesn't have enough accuracy in some cases. Hence, in this study, its reliability is examined with observed rain data, taking meteorological types into account. Eventually, following results are revealed. 1) The stronger the precipitation is, the less precise the information of short term forecast is. 2) Meteorological types have considerable effect on the accuracy of the information of short term forecast. The accuracy tends to reduce with the meteorological type related to approaching low pressure. Notwithstanding this tendency, it doesn't reduce in case of typhoon. 3) It is noteworthy that in strong rainfalls the short term rain forecast tends to give underestimated values.
This report presents the various test results of the Sabo-soil-cement in which the riverbed soils taken in basin of the Tenryu-River were used. First, the materials and the combination tests were carried out for the Sabo-soil-cement and the test results were examined to the applicability to the construction.Second, the wear resistance test was performed by the demand of the erosion control.Third, the freezing and thawing test was also performed to find the ratio of mass loss and the relative dynamic elastic modulus.As the test results, it was proven that the Sabo-soil-cement used herein was useful as the construction materials of the Sabo dam.
This paper proposes a new type of impact weight for simple penetration tests that is variable from 1- to 5-kgf in 1-kgf increments. Many simple penetration tests were conducted to estimate the effect of this weight in three actual forests. Experimental results provide a detailed description of the vertical distribution of penetration resistance values within the surface soil layer by using 2- and 3-kgf weights, which are lighter than conventional ones, i.e., 5-kgf. The ratio of penetration resistance values by using impact weights 5-kgf, 3-kgf, and 2-kgf, i.e., N5kgf : N3kgf : N2kgf, is nearly equal to 1.00:2.02:3.66. In addition, N3kgf nearly equals the resistance value obtained with Hasegawa's soil penetration test. These results show that variable impact weight is very useful for accurately estimating the physical characteristics of soil within forest slope where hillside collapse is liable to occur.
At first, the characteristics of two maximum acceleration estimation formulas by Cornell and Si-Midorikawa were studied. It was found that Cornell formula overestimates the maximum acceleration at the short distance from the earthquake fault than that by Si-Midorikawa. Secondarily, the collapsed land volume was compared with the maximum accelerations estimated from the both formulas. The correlation between collapsed land volume and maximum acceleration by Si-Midorikawa formula was better than that by Cornell formula, although the correlation was poor. Furthermore, It was confirmed that the Si-Midorikawa formula is more applicable than Cornell one, through a case study on Shikoku for the coming Tonankai-Nankai Earthquake. Eventually, Si-Midorikawa formula seems more advantageous than Cornell one for the rational explanation on the collapsed land volume with the maximum acceleration with distance attenuation.
During July 2006, a heavy rain in excess of 1, 000mm was recorded over a five-day period with about 400mm of maximum daily rainfall in the area where Kagoshima, Miyazaki and Kumamoto prefectures are close to each other. This is the largest rainstorm event in this area in the 31-year rainfall record. Due to this rain many small collapses, 101 to 102m3 in collapsed sediment, were widespread in the whole area ; and a small number of slope failures, 103 m3 in collapsed sediment, also occurred and three of them killed three people in northern Kagoshima. Besides, a deep-seated landslide that yielded ten to twenty thousand m3 of sediment occurred in Izumi. This is the largest of all massmovement events by this rain. Major sediment disasters that overtook this area and/or the vicinity during the last fifteen years demonstrate that landslides greater than the Izumi's slide by one to two orders of magnitude were often triggered by rainfall less than this rainstorm. Thus we conclude that magnitude of sediment movement caused by this rainstorm was considerably small in spite of the newly established record of rainfall.
A hydrophone is capable of determining the number of pulses when the sediment rate is high. This study varied the grain size, sediment rate, length of pipe, location where the grains of sand collided, and sensitivity of the microphone, and examined the relationships between these parameters and the number of pulses to determine characteristics using a hydrophone. The flume experiment used a rectangular 9-m-long flume with a changeable inclination and a rectangular flume 20m long and 1.0 m wide, with an inclination of 1/50. L- and U-type steel pipes with circular cross-sections were used. The outside diameter was 27.2mm and the inside diameter was 25.2mm. With the U-type pipe, a microphone was positioned at both ends, and a filter made of sponge was installed on one side. The following results were obtained: 1) the channel that best detected the pulse differed with the grain size; 2) when the sediment rate was high, there was a high probability that the number of pulses decreased unless the sensitivity of the microphone was adjusted using a filter; 3) the length over which the sediments collided influenced the change in the number of pulses more than did the length of the steel pipe; and 4) when the sensitivity was decreased by placing a filter over the microphone, pulses were detected even in the high sensitivity condition. Based on these characteristics of the hydrophone, it is thought that it is possible to correspond by shortening length over which the sand collides, and decreasing the microphone sensitivity under the condition the pulse's large the amount of the sediment rate decreasing.