Seventy percent of Japan is mountainous, and there are a number of residential areas near piedmonts and/or downstream alluvial flows. Debris flows, including driftwood, have occurred recently. Hence, it is necessary to take measures against such occurrences. Over the past 30 years, open Sabo dams with steel pipes that are a countermeasure against woody debris and debris flows have been constructed. However, it is difficult for designers to select the optimum spacing between pipes. Therefore, the concept of general entrapment tends to employ smaller spacing intervals between pipes to protect downstream areas, which have a high expectation of security, although open Sabo dams gradually lose their function of routine sediment transport to pass through. Furthermore, in current design methods, the maximum grain size (d95) is applied to determine the optimum spacing interval for entrapment, which does not take into account driftwood and roots. Thus, an effective method to evaluate the effects of woody debris is required to analyze the problems that arise due to trapping effects. We propose a novel method for evaluating the optimum spacing interval of open Sabo dams using a distinct element method. Moreover, the trapping mechanism of woody debris was investigated for open Sabo dams.