Journal of the Japan Society of Erosion Control Engineering, Vol.59,No.6,2007
The influence of the ratio of forested area on suspended sediment discharge
in the upper part of the Agatsuma River basin
NAMBA Yohe, HOTTA Norifumi, SUZUKI Masakazu and SHUIN Yasuhiro
Abstract
The relationship between the ratio of forested area (F) and suspended sediment
(SS) discharge was examined after eliminating the influence of geography as represented
by the average slope gradient in a basin (G). The research area comprised 75 basins
nested in the upper part of the Agatsuma River basin (707.9 km2). Using manual
sampling, 617 samples were collected between 1994 and 1998, and SS concentrations
were determined using the gravimetric method. Water discharge was measured at
the time of sampling to estimate SS discharge. After classifying all the data
into 3 and 4 groups according to F and G, respectively, sediment rating curves
for each group were obtained using the least squares method. By comparing the
sediment rating curves, the SS discharge in the basins was found to decrease with
higher F, even with scattered data, while no clear relationship was found between
SS discharge and G, which means that the forested area had a stronger influence
on SS discharge than the geographic characteristics of the research area. Furthermore,
we ascertained that the SS concentration converged on a lower value where F was
near 1, resulting in especially low SS discharge in wholly forested basins. From
these results, we conclude that it is difficult to evaluate SS discharge emphasizing
only F, although F is one of the important factors that strongly affects SS discharge.
The difficulty, we think, is caused by the existence of fields, which have a strong
negative correlation with F in these basins, and which generate SS according to
their local geographic characteristics. It is also possible that the relationship
between sediment yields and geographic characteristics, which has often been highlighted
in previous reports, can be applied only in basins where F is sufficiently high,
because most of the research areas in such studies have been limited to upper
mountain areas.
Key words:suspended sediment, the ratio of forested area, the average slope gradient,
the upper part of the Agatsuma river basin
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