Journal of the Japan Society of Erosion Control Engineering,
Vol.56,No.4,p.4-11,2003
The relations between sediment discharge and forest cover in a weathered granite mountain area
Kazuhiro YABE
Abstract
In this study, the author applied the theory of sediment discharge to a mountain stream and examined its applicability for determining the sediment yield resulting from showers as well as the long]term applicability of the results for mountainous watersheds. Furthermore, using hydrological data obtained in the past 60years and data of sediment yields, the author analyzed the influence of the process of forest recovery on sediment yield. This study adopted one such formula, the Brown formula is considered applicable for bed material load. It became clear that sediment yield in Shirasaka watershed (the weathered granite zone) is proportional to runoff to the 1.40th power. However, coefficient values obtained by observation and by calculation are different. This fact suggests that other factors, for example washload, may have a significant impact on sediment yield. Next, using sediment data recorded in the past 60years for Shirasaka watershed and the result obtained from the sediment discharge theory, the author analyzed the relationship between sediment yield and the forest. Sediment yield has a tendency to decrease gradually. Here, variations in the Erosion Index (Ei) was studied thoroughly, where gEi" is the quotient when sediment yield is divided by runoff to the1.40th power. Changes in the gEi" align well with the changes in the bare]land ratio (La). Thus as a result of having analyzed relation of gEi" and gLa", it was expressed in an exponential function.
Key words:sediment discharge, forest cover, bedload, washload, erosion index