Flood control forest along the middle reach of the Kuji River, central Japan

Abstract

The flood control forest is a traditional river control measure that has been reevaluated as an effective and flood-tolerant method over the last decade. The flood control forest is important not only as a flood control measure or part of the local ecosystem but also as an educational site for disaster prevention awareness and environmental perception for the inhabitants of a frequently flooded region. The new River Law enacted in 1997 upholds the flood control forest, and its function of flood water control and since then this type of forest has been studied rather intensively. Ecological studies on riparian forests, on the other hand, focus on the relationship between forest dynamics and riverbed disturbance with sediment transport. To understand the dynamics of artificially formed flood control forests in local ecosystems, studies from both functional and ecological viewpoints are necessary. The author investigated the sediment control function of the flood control forest and examined its role in the development of riverbed landforms in the middle reaches of the Kuji River, Ibaraki Prefecture, central Japan, where patches of typical flood control forest are still well maintained. The study area of the flood control forest can be classified into two types based on aerial photograph interpretations and field observations : 1) flood control forest located in front of a terrace scarp or artificial embankment ; and 2) flood control forest located on natural levees. The first type has the function of protecting the artificial banks and terrace scarps from erosion by flood water. This function was confirmed in the literature and in interviews with local inhabitants. The grain size analysis of sediments of the natural levees with flood control forests, confirms that the sediment includes both the riverbed material and suspended sand. This indicates that the flood control forest screened the bed load by increasing the surface roughness of the riverbed, and that by letting the flood water remain in the forest for a while allows suspended sediments on the forest floor to be trapped. The structure of the natural levees shows the cumulative deposition of sediments of past floods. Flood control forests promote the development of natural levees by trapping the sediments on the forest floor through their screening function. The flood control forest can be appreciated as a "self-growing river control structure" that improves the flood control effect to harmonize with the geomorphic processes. The flood control forest also needs the social consciousness of the inhabitants and managers. The flood control forest should be evaluated in greater depth.