Consideration on boulders & members' interval of open type steel sabo dam for capturing debris flow

Joji SHIMA, Keisuke YOSHIDA, Yoshihiro KAWAKAMI and Takahisa MIZUYAMA

Debris flow capturing function of open type steel sabo dam depends on the maximum boulder diameter and net interval of steel pipe members that composes the opening portion. This maximum boulder diameter (D95), deduced from the boulder diameter examination, is actually the 6th largest boulder, and members' interval is decided just by this boulder diameter. This means that disposition characteristic of boulder diameter at the actual site is not being put in consideration at all. So we examined the characteristics of boulders blocking the opening portion, based on debris flow capturing examples with boulders as their main capturing object. As a result, we found out that arch effect is being demonstrated by the 1 to 2 boulders captured between the members (primary blockage), and then these captured boulders capture small size boulders by arch effect (secondary blockage). This chain reaction was how the opening portion was being blocked. The area being occupied by boulders within the opening portion was approximately 70% for both primary and secondary blockage (blockage rate). Then we estimated how much the boulders sought out by the previous boulder diameter examination contributes to the blockage of opening portion, based on blockage rate of 70% and decreasing rate of debris flow's peak discharge, when setting the members' interval at 2 times the maximum boulder diameter. As a result, we found out that half of the subjected boulders contributed to capture debris flow when setting the members' interval at 2 times the maximum boulder diameter, and almost all the subjected boulders when setting the members' interval at 1 times the maximum boulder diameter. But there weren't many examples of opening portion being blocked just by boulders, and most debris flow capturing examples were those being blocked by driftwood. So we examined the boulder capturing function from the blockage condition of open type steel sabo dam capturing debris flow mixed with driftwood. As a result, we found out that the opening portion was blocked by arch effect, just like the boulder capturing result. But no chain reaction like primary blockage and secondary blockage could be observed with driftwood mixed, so it was made clear that the blockage of opening portion was done immediately by small size boulders.