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Asaeda T.,Saitama University | Rajapakse L.,Public Works Research Institute PWRI | Kanoh M.,Saitama University
River Research and Applications | Year: 2010

To examine the effect of different growth forms of Sparganium erectum (Sparganiaceae) on its sediment trapping and retention characteristics, the plant phenology and morphological attributes based on stands along the edge of a river channel, river flow velocity distribution and sedimentation rates in- and outside the stands of a downstream site were examined over a 3-year period. A decomposition experiment was carried out to determine the respective fragmentation rates of S. erectum shoots and rhizomes. The preliminary monitoring revealed that S. erectum shoots attained distinct phonological stages; submerged in winter, subsequently emerging in late-spring, while the same cycle was followed by successive secondary cohorts. Our results highlighted that the growth form substantially affected the flow condition in- and outside the stands, thus affecting associated sedimentation rates. Although the sedimentation was high if the shoots were submerged in early-spring, the accumulated fine sediment layer was unleashed following shoot emergence, despite their large biomass and the resulting low flow velocity. The collapse of S. erectum shoots accelerated sedimentation again by increasing constriction to flow, producing a more preferable habitat for its soft roots and rhizomes. The collapse of emergent shoots, therefore, appeared to be a vital part of the inherent phonological cycle of S. erectum. Further, due to the high-decomposition rate, the collapsed shoots disappeared within 40-60 days, whereas the decomposed materials occupied a large fraction of the floating organic matter. The collapse of shoots increased flow resistance by -50%, though the stand area occupied a mere one-fifth of the channel. The seasonal observations provided further insight into the modification of flows due to the growth and changing growth form affecting associated fine sediment trapping and retention characteristics within the stands, deriving important management implications and highlighting the role of S. erectum as an ecosystem engineer in lowland streams. Copyright © 2009 John Wiley & Sons, Ltd. Copyright © 2009 John Wiley & Sons, Ltd. Source

Shrestha B.B.,Public Works Research Institute PWRI | Nakagawa H.,Kyoto University
Natural Hazards | Year: 2016

A massive landslide occurred at Jure village of Nepal on August 2, 2014. The landslide mass blocked the Sunkoshi River and created a landslide dam. Thirty-seven days after the formation, the Sunkoshi landslide dam breached on September 7, 2014. In this study, flood hazards due to the formation and failure of the Sunkoshi landslide dam were analyzed. Flood inundation due to impounded water in the upstream river valley of the dam was also analyzed. The outburst discharge from the dam breach on September 7, 2014 and flood characteristics along the downstream river valley were computed by using an erosion-based dam breach and flow model. Flood inundation maps were prepared by using the results obtained from the numerical simulations and geographical information system tools. The peak flow discharge of the Sunkoshi landslide dam outburst at the dam outlet was found to be 6436 m3/s, and the estimated time of the dam breach was found to be about 26 min. The estimated volume of water released from the impounded water in the upstream river valley of the dam was approximately 4.5 million m3. The calculated peak flow velocity and maximum flow depth along the river ranged mostly from 3 to 11 m/s and from 4 to 10 m, respectively. The dam breach had considerable impact on areas more than 30 km downstream from the dam. © 2016, Springer Science+Business Media Dordrecht. Source

Kubo K.,Public Works Research Institute PWRI
40th International Congress and Exposition on Noise Control Engineering 2011, INTER-NOISE 2011 | Year: 2011

Porous Elastic Rubber Surface (PERS) was developed in 1991 in Japan in order to reduce traffic noise. Around 1990, porous asphalt pavement had become popular as low noise pavement, while the effect did not last no more than 3 years. In order to improve this effect, PERS was developed by changing its aggregate from stones to rubber chips. While noise reduction effect was quite well, there were some troubles in its durability and skid resistance. PERS has been developed in its construction methods, aggregate type and so on. This paper shows its history of development and improvement. Copyright © (2011) by the Institute of Noise Control Engineering. Source

Shrestha B.B.,Public Works Research Institute PWRI | Nakagawa H.,Kyoto University | Kawaike K.,Kyoto University | Baba Y.,Kyoto University | Zhang H.,Kyoto University
Landslides | Year: 2013

In recent years, climate change and retreating glaciers constitute a major hazard in the Himalaya of South Asia. Glacial lakes are rapidly developing or increasing due to climate change. The rapid development of the lake may cause outburst of the lake. The outburst discharge from the glacial lake can cause catastrophic flooding and disaster in downstream area. Therefore, it is necessary to investigate the impact of climate change on glacial lakes and to understand the characteristics of the glacial lake outburst. In this study, the field assessment of Tsho Rolpa Glacial Lake in the Himalaya of Nepal has been presented and the impact of climate change on this glacial lake has been discussed. The Tsho Rolpa Glacial Lake is the largest and most potentially dangerous glacial lake in Nepal. In addition, a numerical model has been also developed for computing the characteristics of glacial lake outburst due to moraine dam failure by seepage and water overtopping. The numerical model is tested for the flume experimental cases. The simulated results of the outburst discharge, the dam surface erosion, and the temporal variation of the moisture movement in the dam are compared with those obtained from the hydraulic model experiments. The moisture profile calculated by numerical model was agreeable with the experimental moisture profile. The simulated failure surface of the dam due to seepage by considering the suction in slope stability analysis gave more agreeable results than the Janbu's simplified method. The results of the outburst discharge and dam surface erosion also agreed with the experimental results. © 2012 Springer-Verlag. Source

Osanai N.,Japan National Institute for Land and Infrastructure Management | Shimizu T.,Public Works Research Institute PWRI | Kuramoto K.,Chuden Engineering Consultants Co. | Kojima S.,Fujikawa Sabo Office | Noro T.,Snow Avalanche and Landslide Research Center
Landslides | Year: 2010

Early-warning systems for natural disasters are important tools for disaster risk reduction and for achieving sustainable development and livelihoods. In 2005, the Japanese government initiated a new nationwide early-warning system for landslides disasters. The main methodology of the system is to set a criterion for occurrences of debris flows and slope failures based on several rainfall indices (60-min cumulative rainfall and soil-water index) in each 5-km grid mesh covering all of Japan. Because many of the records of mass movements are lacking in scientific precision on timing and location, the system applies Radial Basis Function Network methods to set the criterion based primarily on rainfall data recorded as not triggering disasters. Since the end of March 2007, under torrential rainfall conditions, early-warning information has been disseminated as part of weather news using TV, radio, and the Internet. Because of the increasing worldwide recognition of the importance of early-warning systems for natural disaster reduction, the aim of this article is to introduce the new Japanese early-warning system to the international landslide community. In this article, the method, the system, and the result of its application to landslide disasters in 2009 are presented. © 2010 Springer-Verlag. Source

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