Experimental Analysis of Woody Debris Trapping by Impermeable type Sabo Dam, filled with Sediment

Woody Debris carried by Debris Flow

Abstract

It has been reported that impermeable type Sabo dams, filled with sediment, trapped a considerable volume of woody debris, despite the fact that little woody debris could be trapped under the present design criteria. Authors presumed the dominant factor to determine the woody debris trapping rate was the ratio between the wood length in the woody debris and the spillway width. Therefore, “wood mixture rate, the wood longer than the spillway width which occupied in the total volume of the woody debris” (we simplify this term and write as “wood mixture rate”) strongly related the woody debris trapping rate. Following this idea, we conducted some flume experiments using various lengths of wood models. Consequently, woody debris movement in the sediment trapping zone were classified into 4 patterns (A to D), then it was confirmed that 2 patterns produced high woody debris trapping rates. One was trapped on the sediment before reached the spillway (pattern A) and this pattern could be seen when the discharge was small. The other one was trapped by clogging the spillway (pattern C) and this pattern could be seen when the discharge and the wood mixture rate were high, simultaneously. When the woody debris was trapped by pattern A or C, sediment was also trapped behind the woody debris. However, the trapping rate of pattern A was controlled by its discharge and flow time, while pattern C produced stable sediment trapping rate. Furthermore, it was confirmed that the “wood mixture rate” was an important factor to determine the woody debris moving patterns and the trapping rates.