Channeling Destruction: The Osawa Failure Project

Although Mt. Fuji is treasured for its scenic beauty and cultural significance, the volcano is also a hotbed for natural disaster. The mountain’s steep topography combined with porous, unstable soil and volatile weather conditions contribute to frequent debris flows that come tearing down the mountainside. These flows tend to consist of snow melt, rain, silt, rocks, trees and whatever else the flow picks up on its mad dash down the mountain. Until recent decades, the towns surrounding Mt. Fuji were frequently ravaged by debris flows, causing major damage to infrastructure and resulting in many casualties. Since then, the Mt. Fuji Sabo Office has taken precaution by building extensive sediment control works around Mt. Fuji to contain these flows and protect surrounding communities.

 A dam and sediment pocket on the Osawa alluvial fan

The dams, concrete channels, and monitoring stations built below the Osawa Failure are a central part of Sabo’s sediment control project. The Osawa Failure is a steep and cavernous crevasse eroded into the western face of Mt. Fuji. During extended periods of heavy rain, The Osawa Failure is a frequent site of destructive debris flows that used to inundate the Osawa River alluvial fan. Now, such flows are channeled into a series of “sediment pockets” that contain the flowing sediment. After a debris flow, when these pockets become full, Sabo transports the sediment to Suruga Bay for use as beach nourishment soil. Since these public works have been completed, downstream communities have lived in safety from debris flows.

Key actors in this issue include a mix of geologic and anthropocentric forces. In the Osawa Failure concept map, actors fall under three categories: geologic features/forces of Mt. Fuji, the Fuji Sabo Office, local communities, and sediment destinations. Everything traces back to Mt. Fuji itself as the mountain sources debris flows and contains the Osawa Failure which funnels them onto the Osawa Alluvial Fan. The Mt. Fuji Sabo Office is another major actor as they are responsible for building and maintaining the sediment control works. Other actors that fall under the Sabo category are the dams and sediment pockets themselves because they bear the physical force of debris flows. The remaining key actors are the local communities being protected and the collected sediment’s final destinations which are the beaches of Suruga Bay.

Processes that connect these actors are links between Mt. Fuji, the Sabo Office, local communities, and the process by which the sediment gets to Suruga Bay. Local communities rely on the Mt. Fuji Sabo Office for maintenance of these projects and they rely on the dams for protection. Debris flows themselves are influenced by several processes including erosion, precipitation and steep topography. The sediment from these flows are linked to beach nourishment as they are used as coastal soil. All of these processes are ways that aspects of this problem are connected and how groups rely upon each other to contain debris flows.

All of these structures that the Mt. Fuji Sabo Office built below the Osawa Failure have contained several debris flows and saved countless lives. Residents of the Mt. Fuji area depend upon the Sabo Office for protection so they can live safely in an area so prone to disaster. With continued maintenance and monitoring, residents of the Mt. Fuji area will live in safety for years to come.

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