This paper aims to clear the movement about the sediments (boulder, cobble, gravel and sand), which fall down the downstream edge of the Sabo-dam, and so that, we can decide reasonablly to make the planning of its downstream slope.
In that case, we examined the trace marks impacted by falling sediments on such a place as already builted the Sabo-dams front apron and velocity of sediments on various conditions. The results show as follows:
1) The trace marks of the Sabo-dam's concrete front apron are grouped into three types on the plane figure (Fig. 3) and the fives on the vertical section (Fig. 5).
2) When the sediments fall down from the downstream edge of the Sabo-dam, those velocity change remarkablly, especially, the large stone (boulder, cobble, gravel) generally fall down inside of the rear nappe, but the more the amount of flow greater, the more it likely to show the sight of “within the nappe” or “along with the rear nappe”. The movement of the sediments, above mentioned, is quite similar to the one of singles.
3) The distribution forms on the falling sediments are quite similar to the one of the trace marks of the Sabo-dam's concrete front apron.
4) From the ovservations of the falling sediments trace caused by the flood, the sediments which flow down within the nappe or along with the rear nappe, are estimated about 60-70% of total amount.
5) In the case of the flood, when the streaming sediments fall down from the downstream edge of the Sabo-dam, the amount of sediments which are smaller than the average grain diameter are falling down within the nappe or along the rear nappe.
6) In the case of the flood, the bed-flow velocity which often show including the amount sediments in the stream is estimated by an extant 70-90% of the surface velocity.
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