For the non-structural measures of erosion control works, the improvement of accuracy in speculating slope failure generated in a concentrated manner has been advanced with improved technology in setting the critical line (CL) . However, speculating method for the slope failure generated within the safety zone of the set CL, known as the sediment-related disaster sporadically generated, still shows little improvement. Therefore, disability of warnings and evacuation was assumed to be the key in disaster prevention. In this study, we worked on the specifications of the slope failures sporadically generated, using the rough sets, at the southern part of Shimonoseki City where danger of slope failure is relatively high comparing with other regions in Japan. In addition, the setting of CL for region with more possible warnings and evacuation information was attempted not only on concentrated occurrence but also on the one sporadically generated in a specific region.

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In Japan, small wooden dams were built to prevent soil erosion and control sediment discharge. However, the wooden materials deteriorate under the actions of fungi that are controlled by temperature, oxygen and humidity, which cause the durability (service time) of wooden dam shorter than that of concrete and steel crib dam. Therefore, it is very important to evaluate the durability of dam in order to serve effectively for designing and maintenance works. The purpose of this study is to develop a new design method with considering the deterioration rate that allows evaluating the durability of crib dam. We defined a new term of the deterioration rate and have conducted the field measurement of actual deterioration rates of Cryptomeria japonica D.DoN and Chamaecyparis obtusa ENDL. at four wooden dams in Kyoto Prefecture. The actual deterioration rates of Cryptomeria japonica D.DoN and Chamaecyparis obtusa ENDL. were obtained. The diameter and cross sectional area of the sound part of deteriorated log were calculated using the deterioration rate and the time after completion of dam. Then the maximum bending, shear and tensile stresses acting on deteriorated log were calculated using the diameter and the cross sectional area of the sound part. We proposed three formulas for evaluating the durability considering the maximum stresses equal to allowable stresses of wood. The parameters in three formulas showed that the durability of small wooden crib dam depended upon the deterioration rate, log diameter, log length, dam height, and overflow depth. To increase the durability, the log diameter needs to be increased, and the dam height, overflow depth, as well as the log length need to be decreased. In an application of the new design method, a new diagram was also developed to show the durability of crib dam built with Cryptomeria japonica D.DoN.

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Examination and estimation of design flood discharges in upper river basins are critical for better river and erosion control planning. Annual maximum flood series were analyzed by 13 fitting methods in terms of SLSC goodness-of-fit criterion at 30 small-scale [mostly less than 100 km²] dam reservoirs in Hokuriku region where inflows are influenced by snow-melting but deemed unregulated. The inflow discharge data belong to the same hydrological regime and show mutual independency and proper randomness as a set of statistical sample. The analysis of fitting methods show that log Pearson Type III distribution is the best common probabilistic distribution for the sample annual maximum flood series in the region, which allowed us to apply regional flood frequency analysis method described in the U.S. Water Resource Council guideline Bulletin 17 B. 30 dam reservoirs are located within about one-hundred-mile radius and their flood series are pooled for the regional flood frequency analysis. Confidence intervals were estimated to become significantly smaller as regional information was incorporated than when estimated separately at each site. Concurrent omparative analysis by an index-flood method and a scaling method indicated that higher statistics such as skew coefficient at each observing station are indispensable in addition to mean, standard deviation, and coefficient of variation for further improvement of the estimation. Bulleting 17 B method, which adjusts site skew by regional information, seems promising to apply to small-scale non-regulated river basins in Hokuriku region. Further research is necessary to clarify how river basin characteristics are related to site skew in maximum annual flood series.

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A landslide dam can be formed by a landslide, a large-scale collapse, or a debris flow. A landslide dam can block a river, and during its collapse, the resulting debris flow and flood can cause disasters such as flooding in the downstream region. We therefore conducted this study within the context of crisis management. Sabo facilities were set up ahead of time in areas where landslide dams were likely to form, and after its formation, it was treated as an emergency ; i.e., in addition to the debris flow, the landslide dam itself was considered an object of the sabo plan. Thus, this study was able to confirm the possibility of using a model experiment to plan sabo facilities intended for a landslide dam. The model used in this study incorporated landslide dams with greater heights than in past experiments. We used a rectangular channel to conduct basic experiments on river dam collapses caused by overflow, and obtained the following results. We confirmed that landslide dam crown length and slope angle influenced peak discharge during collapse. We were able to reproduce the peak discharge when a landslide dam collapsed under identical conditions. Differences in sand size did not affect landslide dam peak discharge during collapse. The results indicate that the experiments could be used to verify the effects of sabo facilities on landslide dams.

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Flash floods occurred in Japan in 2006. The definition of “flash flood” is different in the world. Here in this report, it is defined as a flood which contain few sediment and with an abrupt rise of the water level. The authors made field surveys at the 2 sites where the flash floods occurred in 2006. As a result, we found different processes of the generation of the flash flood for each site. In one site, we found some evidences of the formation of small natural dams and some existing step pools in the upper stream. According to the eyewitness, flash floods occurred several times. It is considered that boulders and woody-debris transported by the water flow might have caused several dam-up-dam-break processes in the upper stream and caused flash floods observed several times in the downstream. On the other hand, in the other site, neither evidence of the formation of natural dams nor that of sediment transport have been found. The nature of the watersheds in the upper stream shows some properties favorable to generate flash floods, that is, the upper basin shows typical caldera morphology and rock surface appears distinctively. In addition, it is considered that there would have been localized strong rainstorm in the upper stream, which might have caused the occurrence of the flash floods.

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A spring storm attacked the central Japan on March 25, 2007 and brought heavy rainfall (total precipitation was excess 200mm/15 hour) to Mt. Fuji slopes with snowy and subarctic circumstances. Many slush avalanches, as well as surface avalanches, occurred on the western, southern and eastern slopes above timberline. Some of slush avalanches originatedfrom southern slope slid down 0.8-1.1km and damaged 14 traffic utilities and 2 houses. Fortunately road and houses wore closed and had no casualties. The avalanche chutes almost same as 1976 and 1988 events.

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In this report, we summarized the sediment related disasters triggered by the Noto-Hantou Earthquake on March 25, 2007. However Noto-Hantou area (peninsula) is remarked as landslide zone, many shallow slope failures were occurred rather than large scale slope failures like the case of Niigata Prefecture Chuetsu Earthquake. Three facilities of slope failure prevention works were damaged with crack and 0.6-1.0 m steps on the rigde of steep slope. Totally, the slope failures mainly consist of rock blocks along the coastal slope, and mainly consist of sediment and gravel at inland slope.

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On 18 March 2007 the summit Crater Lake of Mt. Ruapehu, New Zealand, breached a barrier of tephra emplaced by eruptions in 1995-96, resulting in the rapid release of 1.3 million m³ of water. The flood rapidly bulked by entraining snow, ice, rock debris and alluvium along the steep gorge of the upper Whangaehu River to form a debris flow that then transformed downstream into a hyper-concentrated and then sediment-laden stream flow during its passage to the Tasman Sea 155 km away. No lives were lost and infrastructural damage was minimal due to a comprehensive warning system developed in the decade before the lahar. A previous break-out lahar in 1953 caused a railway disaster at Tangiwai with the loss of 151 lives. The lahar flowed as a multi-peaked debris flow in the Whangaehu gorge, 7 km downstream of Crater Lake. Automatic lahar measuring equipment, including water level gauges, flow velocity sensors, conductivity samplers and seismographs were installed at key sites along the flow path, and were supplemented by sequential photographs captured by automatic monitoring cameras and observer teams. Flow velocity averaged 30 km/hr and water level rose by 8.2 m in 4 minutes in the gorge. The lahar caused riverbed aggradation in many places and environmental disturbance to the channel regime.

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