Journal of the Japan Society of Erosion Control Engineering, Vol.59,No.5,2007
Sediment yield and transportation capacity in a forested watershed underlain
by weathered granite
Krishna Bahadur KARKI and Hirofumi SHIBANO
Abstract
Sediment transportation from a forested watershed (88.6ha) underlain by weathered
granite was observed in a reservoir at the outlet of the watershed from September
2001 to October 2004. Wash load concentration was observed at the reservoir exit
by manual sampling over 1 week, including a flood in October 2004. Bed load and
suspended load comprising total load, i.e. bed material load excluding wash load,
were calculated with hydrograph and sediment rating curve using the Einstein equation
based on the hydraulic parameters of a study channel. Applicability of the Ashida
& Michiue equation, and the Brown equation was also examined. Goodness of
fit for the Einstein equation was the most satisfactory of the methods we examined
because of the closeness of its total load estimation to that observed (3.67ton/ha/year)
over the study period as a whole. However, even the best]fit model showed an underestimation
during the earlier period. An overestimation of total load during the latter period
implied a supply limit in the watershed. The early underestimation occurred under
adequate sediment supply conditions following a large flood and the later overestimation
was attributed to a rapid stabilization of unstable sediment sources.
The proportion of bed load to total load was estimated by the model at 79% and
suspended load at 21%; wash load proportion to observed total load was 11%. These
results showed that the proportion of suspended load was at the same level as
the value quoted in other reports from Japan but wash load was not so small as
that. Grain size distribution was coarse (2.2 mm in D50) and uniform. The smaller
total load can be attributed partly to the uniformity of grain size from weathered
granite in comparison with bigger total load caused by the wider range grain size
distribution assumed in the model, and partly to the fact that the watershed was
well managed and covered by dense forest.
Key words:Bed load; Suspended load; Wash load; Hydrograph; Weathered granite
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