砂防学会誌

The Sabo (Erosion Control) Act, which dates back to 1897, is the oldest piece of legislation in effect with regard to policies for national territory conservation. The knowledge of the process of drafting the Sabo Bill, however, has been very much limited, largely due to the loss of relevant historical records as a consequence of a major earthquake which devastated the capital in 1923. This study sheds new light on this process drawing upon the findings from historical documents archived by some local authorities, including a petition by governors of prefectures calling for enactment in relation to Sabo works and the Draft Sabo Bil lwhich was issued by the Ministry of the Interior for consultation with prefectural governments. It is suggested that the processes of drafting the River Bill, which had been preceded by the Public River Bill, and the Sabo Bill were sequential and interconnected within the framework of the Ministry's fundamental policy for flood control which aimed to integrate actions to be taken in upper and lower reaches of rivers.

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The management of runoff and sediment discharge in a mountain watershed requires a watershed-management system (WMS) that predicts runoff and sediment discharge in any stream section under given rainfall conditions. As part of this system, a rainfall-runoff model was developed. As an example, the Sumiyoshi River on Mount Rokko was selected. Socalled zero-order torrents were treated as channels to evaluate erosion and debris flow. In the system, multilayer intermediate flow is introduced to explain the observed flow discharge to avoid eliminating the initial rainfall loss.

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A steel slit dam is one of Sabo dams with large opening formed by the combination of steel pipe, and its purpose is to capture debris flow and drift woods. There are three major advantages of it over concrete close dam : to catch drift woods, to keep designed entrapment volume, and to conserve mountain stream environment. So far, many steel slit dams have caught debris flow and drift woods, and those have been very effective to prevent disasters and minimize the damages. Comparing solid types with plane types, construction cost of solid types are more expensive than plane types because plane types need less steel. However, the plane types lack a sufficient redundancy. That means when a structural member was damaged, the potential to collapse whole structure would become higher. However the functional differences between plane types and solid types have been discussed a few. Therefore, the hydraulic experiments using two kinds of steel slit dam models were done. The results in all experiment cases show that the solid structure type has higher function than the plane structure type for the capture of debris flow.

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