A combined penetrometer-moisture probe (CPMP) was developed for simultaneous measurements of penetration resistance and soil water content distributions in a hsllslope. The probe uses the time domain reflectometry (TDR) technique to determine soil water content. The TDR moisture sensor consists of two parallel stainless wires, each 0.3 mm in diameter and 20cm long, coiled around a polyvinyl chloride (PVC) column 18mm in outer diameter. A calibration curve relating the dielectric constant measured by CPMP to water content was obtained in the laboratory for Toyoura standard sand, which was packed in a container together with the moisture sensor. The obtained data were described well by the mixing model approach. However, a different calibration curve was obtained for the same Toyoura standard sand, when the dielectric constant was measured during penetration tests. It was presumed that the difference could be attributed to inadequate contact between sand and moisture sensor. Subsequently, field experiments were conducted at a forested hillslope consisting of weathered granite soil to measure simultaneously the penetration resistance and water content along soil profiles. Results showed that penetration resistances measured with CPMP and commonly-used Hasegawa-type penetrometer were about the same. By using a specific calibration curve, the soil water content estimated by CPMP was similar to the water content obtained by the gravimetric method (R2=0.791). CPMP succeeded in measuring detailed water content distributions in the slope under various wetness conditions. The accuracy in the water content measurement was slightly increased when the soil-probe contact was evaluated by penetration resistance measurements.
Sediment management strategies are crucial to developing countries because of the limited available resources. Establishment of these strategies is hampered to some extent by the lack of reliable information on catchment sources. This paper reports the rainfall-sediment response of various types of land in the Shangshe catchment (528.8 ha) of the Dabie Mountains of Anhui Province, China. Field observation in the year 2000 showed 32 rainfall events happened, among which 18 events produced runoff. Comparison among SISSD hydrographs from the five monitored sites showed that the cultivated land produced the highest level of SISSD, which was 100 times higher than those of the tea garden, the Chinese fir forest, or the pine forest and 10 times higher than that of the river outlet of the Shangshe catchment. Integrating the field observation data of SISSD in representative kinds of land use within the catchment with Geographic Information Systems (GIS) and a hydro model, the GOIUH model was constructed to simulate the SISSD hydrograph of the river outlet. The resulting model showed that the calculated SISSD hydrographs compared quite well with the observed ones at the river outlet of the Shangshe catchment, as shown by the coefficient of correlation R2 being equal to 0.8. The semi-distributed sediment discharge model with a focus on the influence of land use in the Shangshe catchment helps to quantitatively understand the source of sediment discharge.
The Chiu-chiu-feng area is located about 20km southwest of Taichung city in the midwesten in part of Taiwan. This area consists of manny narrow ridges of a relatively large height. In the surrounding area of the Chiu-chiu-feng area, there was acceleration of 500-600 gal cased by the Chi-chi earthquake in September 1999. The ridge of the The Chiu-chiu-feng area were strongly accelerated by this earthquake, and many shallow slope failures occured around the ridge over the whole area. In December of that year, broken pieces of cobble were discovered by an investigation of the ridge. It was confirmed that the cobble had broken by a drop test of cobble on the ridge. Also, acceleration of about 1 -3 times that of gravitational acceleration might have arisen, as determined from the results of the earthquake response analysis of the narrow ridge of the survey site. It is liky that slope failures and sliding down of soil were triggerd by the strong acceleration which was in the manner of lifting and breaking cobbles on the rige.
Recently, comprehensive management of sediment in watershed has become increasingly important. The impacted area begins where the sediment is produced and extends to the continental coastline. One of the models of this comprehensive management is conversion of the closed-type sabo dams into the open-type. This involves a modification which creates a “slit portion”. Nowadays, this conversion has become popular in Japan. The structural stability of the dam body and strengthening methods of the converted dams was examined. Specifically, the decrease in structural strength due to “slit potion” was evaluated. The safety of sabo dam was evaluated using the threedimensional construction method instead of previous two-dimensional construction method. Finally, a method of strengthening the “slit potion” was recommended.
A gigantic landslide occurred on the caldera wall of Mt. Bawakaraeng (2830m) located in the uppermost reach of the Jeneberang River in South Sulawesi, Indonesia, on March 26, 2004. The huge mass of debris yielded from the gigantic landslide of Mt Bawakasraeng traveled about 7 km down the upper reach of the Jeneberang River with 500m to 800 m in width. Ten persons were killed and 22 others were unaccounted for in the accident. Twelve houses and one school were crushed or buried in the debris, and the damage was expected to run into around 2, 214 million Rp. The volume of the slide mass caused by gigantic landslide is estimated at about 240 million cubic meters with a head width of 1, 600m, a height to 700m to 800m, and a thick of approximately 200m. The other hand, the debris deposit has a totally volume of 272 million cubic meters on the upper reach of Jeneberang River, and 160 million cubic meters deposited within the caldera. The main cause of the landslide occurrence has been still unidentified It was the 782mm of cumulative rainfall during March 1 to 26 before the landsliding, and any earthquakes were not recorded around the day of occurrence, March 26. About three months have elapsed after the day of occurrence, and recorded a cumulative rainfall of 430mm. However, there has been generated the V -shaped or U-shaped valley deepening with the size of 50 m to 150 m in width, 30m to 80m in depth because of these materials easily being eroded. The eroded sediment volume up to now is estimated 14 million cubic meters by the site investigation. In next rainy season, there could be a great possibility of strong erosion and huge sediment transportation with debris flows. We recommend the implementation of urgent structural counter measures, such as excavation of riverbed, rising and construction of Sabo facilities, and non-structural counter measures, for example the early warning system, and establish of hazard map.
Heavy rainfalls occurred in Niigata and Fukushima Prefectures during July 12-13, 2004 (Total rainfall at Tochio City, Niigata was greater than 420mm) and in Fukui Prefecture during July 17-18 2004 (Total rainfall at Miyama Town in Fukui was greater than 280mm). Also, Typhoon Namtheun brought heavy rainfalls to the Kinki and Shikoku regions between July 30 and August 1, 2004. Total rainfall in the upper reaches of the Nakagawa River exceeded 1, 000mm. These rainfalls triggered a number of sediment related disasters. The NILIM and PWRI investigated these sediment related disasters shortly after they occurred. Here we report on (1) characteristics of sediment related disasters in Fukui and Tokushima, (2) spatial distributions of sediment related disasters and their relation to rainfall amount variability in Niigata and Fukui, and (3) effectiveness of sabo dams against debris flows.
A local heavy rainfall struck central area of Niigata Prefecture on 13th July 2004. It is reported that more than two thousands of slope movements occurred on the hill slopes and in the mountainous areas where the heavy rainfall concentrated. The geological background of the area consists of Tertiary mudstone and sandstone. Most of slope movements are shallow slope failure type in small dimension. However, not a few landslides and debris flows which displaced soil mass of more than several ten thousands cubic meters have occurred. Two people were killed by slope failures. Although the number of the victims is not high, tremendous volume of debris and sediment moved down and caused destruction of houses and blockage of roads. It should be noticed that evacuation maneuvers have been appropriately managed and it caused minimum victims comparing with a dimension of total slope movements. A large amount of unstable debris and sediment are still remaining in most of landslide areas and debris flow deposition areas. It is necessary to evaluate the volume and danger degree of such unstable materials in order to mitigate successional disasters in the near future.