This paper studies about inundation disasters via drainage channels induced by tsunami, which may occur even if high seawalls protect inside areas from tsunami. Since the inundation disasters may lead to losses of evacuation routes and be an obstacle to the safety measures, it is important to consider the countermeasures for industrial facilities from the view points of business continuity. An 1-D pipe network flow model, which is generally used for a surging analysis, was improved to evaluate the amount of overflows from pits of a channel network. It was found that the improved 1-D model could reproduce the almost same results from a 3-D model simulation. In addition, the characteristics of overflows induced by tsunami and the effects of countermeasures against that were examined by using the improved model.