This paper discusses an effect of the grid shape of open type check dam structures on the probability of rocks capturing. First, it is assumed that the capturing event is a probability process, i.e., a geometric distribution in a Bernoulli sequence. The probability associated with the event that a rectangular space is stuffed by some rocks which diameters follows the log-normal distribution is estimated by using Monte-Carlo simulation. In this estimation, the random effect is examined by two random variables, i.e., 95% probability of exceedence and a coefficient of variation of rock's diameters.
