Abstract
For flood prevention projects, it is important to determine which stand structure variables of riparian forests have the greatest affect on flow resistance. Numerous studies have used hydraulic model experiments to investigate flow resistance, but stand structure dynamics are difficult to incorporate into such experiments. This study examines how river bed grain size reflects the degree of flow resistance by stand structure. We used three sizes of grain: D50 (grain size at 50% of grain size distribution), fine grain (< 0.075 mm) and very fine grain (< 0.005 mm). Two kinds of multiple regression analysis were used. The first analysis used forest floor elevation, stand structure (density, clear length, basal area) and floor vegetation (height, coverage ratio) as the independent variables and D50 and fine grain content as the dependent variables. The second analysis excluded floor vegetation from the independent variables used in the first analysis. In the first analysis, stand density and clear length were identified as the stand structure factors that most greatly affect flow resistance. In the second analysis, it was difficult to identify the degree to which stand structure factors affect flow resistance. These results indicate that consideration must be given to the effects of micro-topography and floor vegetation when the effect of stand structure on flow resistance is examined for river beds with the three grain sizes. The stand structure variables identified as influential, i.e., density and clear length, are important in studies on flow resistance. In studying riparian forests at different stages of succession, identifying stand structure factors that greatly affect flow resistance may enable examination of how spatiotemporal changes in stand structure influence the flow resistance.
