This study is concerned with breach discharge from straight channels. Such breach characteristics as flow direction of breach flows, effective length of breach section were verified experimentally. It is found that breach characteristics of submerged outflows differ substantially from those of complete outflows. The analysis is directed toward the improvement of Honma’s overflow discharge formulas in terms of breach characteristics obtained in this study. It shows that the improved formulas can predict overflow discharge in both complete and submerged outflows with good accuracy.