In planning for the arrangement of countermeasures in such a basin where has a large amount of sediment discharge, a series of grid check dam structures will be constructed. In this case, decision making of the space between steel pipes in consideration of change of characteristic of sediment rocks is substantial because characteristic of sediment rocks is used for design of grid check dam structures, and it change when it pass through the grid .This paper discusses an evaluation method of effect of grid shape on the probability of rocks capturing, and then an evaluation of effect of a series of grid check dam structures on the probability of rocks capturing. First, it is assumed that the capturing events are based on a probability process, i.e., a geometric distribution in a Bernoulli sequence. Second, the probability of what a rectangular space is stuffed by some rocks following log�]normal distribution diameter is estimated using Monte�]Carlo simulation. Third, in cases where same dam models were arranged at upstream and downstream, the number of rocks captured by the dam at downstream is less than 20% of number of rocks that pass through the dam at upstream. Fourth, in cases where the dam at downstream is designed to account for characteristic of rocks that pass through the dam at upstream, the number of rocks captured by the dam at downstream is about50%.

Key words�Fprobability of capture, grid check dam, consecutive arrangement, geometric distribution