愛知県・岐阜県における近自然河川工法の施工事例にみる問題点

Abstract

In Japan many attempts have been undertaken in the last decade to restore rivers to natural ones, which are called "Kinshizen-kasenkoho" (nature-oriented river construction, derived from the German word "Naturnaher Wasserbau"). We analyzed reports on the problems in the practices of such river restoration in Aichi and Gifu prefectures, Honshu, Japan. Some practices were evaluated in this study, and were added to the analysis. Many problems are found in the cover soil on concrete blocks, gabions, etc. The soil was sometimes washed out by flood. Many problems are also found in concrete blocks and gabions themselves. They are frequently inadequate to serve the substrata for plants, in case the river bank was revetted too massively, even if the blocks were hollowed out for the establishment of vegetation. Boulders for the habitat of fishes and other animals were sometimes washed away by flood. Contrary, hollows of concrete blocks for fish habitat were stuffed by sediment after flood in many cases. These cases suggest that wrong material was often adopted for the river restoration in this region. The hydraulic stress during flood had probably been underestimated in case the soil and boulders were washed out, or, on the other hand, overestimated when the revetment was inadequate for vegetation and the sediment deposited into the hollowed blocks. Material and other details of river restoration should be adopted after careful observation of geomorphology with appropriate estimate of hydraulic stress.