Takaisoyama and Hose landslides occurred by heavy rainfall of typhoon in 25 July, 1892, in east Shikoku. These landslides blocked rivers and formed landslide dams. Many old documents illustrated outbursts and disasters of Takai-soyama and Hose Landslide dams. Takaisoyama landslide blocked Naka River and formed landslide dam of 71 m in height and 7.25×107 m3 in volume (Terado, 1970). The landslide dam broke down when the dam lake was filled with water after 1.87×105 seconds ( 52 hours), and the big flood rushed down the valley of Naka River, with the damage of more than 300 houses and the victims of 60 persons. Calculating by Manning formula, the peak flood discharge started by velocity of 10-12 m /second(average velocity of 5 m /second)and a volume of 3.0×104 m3/second, and rushed down by 3 hours at the distance of 49.3 km from the landslide dam. This flood reached by 7.5 hours at the distance of 72km in the estuary of Naka River. Hose Landslide blocked Kaifu River and formed landslide dam of 80 m in height and 3.3×107 m3 in volume. The landslide dam broke down after 1.08×105 seconds (30 hours) or 1.94 ×105 seconds (54 hours), and the big flood rushed down the valley of Kaifu River with the damage of more than 20 houses and the victims of 47 persons. This flood rushed down in 3-4 hours at the distance of 23 km in the estuary of Kaifu River.
Based on the results of field measurement of physical and mechanical properties of hillslope materials in granitic mountains, the angle of shearing resistance (φsat, degree), the cohesion (Csat, gf/cm2) and the hydraulic conductivity (K, cm/sec) of grus under saturated water content were deduced by two parameters of unit weight and grain size of soil as following regression equations : φsat=22.2γdCC0.0625+9.5(R2=0.71) Csat=133.6γd-55.7CC2.0-100.0(R2=0.75) log10K=-1.512γdCC2.0-0.211(R2=0.79) where γd is the dry unit weight of soils (gf/cm3) and CCD is the content for coarser grains than D mm. Since the values of CC0.0625 and CC2.0 can be easily measured by sieving method and γd is estimated using simplified dynamic cone penetrometer, these parameters can be estimated using the above equations at many points and depths of the soil layer on steep mountains.
In the past, several wire rope net dams were developed and installed, however eventually these dams broke. Based on this technology, various important improvements have been made and finally the development of a new ring net dam could be achieved. In the month of August 2002, the first ring net dam was completed at Mt. Yake Kamikamihorisawa. On July 18 th 2004, a stony debris flow hit this ring net dam. The dam was able to capture this heavy debris flow impact without major damages. The event of impact and debris capture was recorded by video. The biggest boulder diameters and the dam's deflection and transformation were analyzed in the survey. The actual mechanical behavior and function of the dam were compared with design values and measured values, e.g. by means of load cell data. Based on these experiences, it was found additionally that sediment removal can be executed safely and promptly by adopting the lancer stick. There is considerable validity to this dam's design method from these results, but of course, this new technology shall only be applied carefully. This paper reports about the above mentioned project and describes the improvement concept for future installations.
Since the slit sabo dams of clog type have been developed mainly using steel slits of vertical or lattice type, the effec-tiveness of the slit sabo dams using steel pipes installed in the lateral direction (lateral beams) for catching sediment in debris flow has not been examined much. Therefore, we experimentally examined how effectively a hybrid buttress type sabo dam using both vertical pillars (vertical slit) and lateral beams can catch mass-transport of sediment in debris flow, and further, how effectively another hybrid buttress sabo dam where subsidiary vertical materials are installed between the vertical slits can catch sparse-transport of sediment in traction. The results are summarized as follows : if the lateral beams of the hybrid buttress dam are installed at intervals of 1.0 ×d95, the same effect on catching sediment can be obtained as in the case where the intervals of vertical pillars are set to 1.5×d95. Moreover, it proves that, aside from the strength of the materials, even subsidiary vertical materials which are as slender as wire can catch sediment effectively if their installation intervals are narrow. This suggests that if it is necessary to install the vertical materials at narrow intervals, the use of a slender material like wire can be effective in securing the opening rate of the sabo dam.
Debris flows have been widely analyzed as multi-phase mixing flows in the discipline of continuous mechanics. With steep slope and little particle-liquid interaction, however, they can also be analyzed as a behavior of particle grains and fluids in discrete mechanics respectively. The applicability of Distinct Element Method (DEM), a class of method following the latter framework, was studied for a steep and rolling torrential ravine with the gradient of 22 degrees. The result of DEM simulation shows close similarity in its pattern with measured flow velocity, maximum flow depth, and flow winding at field observed points. Distinct Element Method can be applied to kinetically simulate flow velocity and depth affected by winding in torrential ravines.
When landslide dams are formed, it is demanded to predict flood discharge accurately. Therefore it is necessary to judge if landslide occurs inside the landslide dam during overtopping-erosion processes. The horizontal length of landslide dams is generally long when compared with the height, and the downstream slope is milder than the angles of repose, which denotes low possibility of landslide occurrence inside the landslide dam. Furthermore, when the sediment of landslide dam is coarse, as erosion by overtopping flow proceeds, the eroded sediment by overtopping is deposited on downstream slopes and the slope becomes more stable against the outburst. However when the sediment of landslide dam is fine or cohesive sand, or torrent gradient is steep enough to generate debris flow, the sediment will not be deposited on lower reach of the slope. In these cases, the possibility of landslides will increase according to the erosion proceeds. We investigated how the dimensions of landslide dams (the length, the height and the gradient of the downstream slopes) and the soil parameters of landslide dam's material (cohesion, the internal friction angle) influence the landslide dam's safety factor. Also, the conditions (the shape of the landslide dam and the soil parameters of the material) of slide occurrence are considered with calculation of safety factors using different calculation condition. As a result, we found out that downstream slope gradient and dam height have the strong influence on the safety factor of landslide occurrence. Moreover, it is indicated that outburst may occur even with low water level at the upstream area of the landslide dam when the sediment is fine.
Typhoon Nabi (No.14) which hit Japan in September, 2005 exposed her to danger for many hours. The maximum hourly rainfall was not so great, but continuous rainfall during 72 hours was over 500 mm in each area where suffered from severe damages. This typhoon caused incidences of 116 debris flows, 185 slope failures, 30 landslides. On account of that, 19 people were killed in Kagoshima, Miyazaki and Yamaguchi Prefecture, 3 people were missed in Oita Prefecture, 79 houses were destroyed completely and 21 houses were partially destroyed in Japan. Especially, severe damages were caused in Kyushu Region by greatly high continuous rainfall of this typhoon. Besides this region, 10 debris flows and a landslide occurred in Chugoku Region (509 mm of the maximum continuous rainfall, Yamaguchi Prefecture), and 29 slope failures, 7 debris flows and 3 landslides occurred in Shikoku Region (1203 mm of the maximum continuous rainfall, Tokushima Prefecture). To mention another feature of disasters by Typhoon Nabi, many large scales of landslides occurred in Miyazaki Prefecture. Especially a greatly large scale of landslide occurred on Mt. Wanituka in Tano Town. The total amount is estimated 5.61×106 m3 as collapsed soil, residual soil is estimated 3.52 ×106 m3, sediment deposition is estimated 1.08×106 m3.