STUDY ON WOODY DEBRIS BLOCKING AT SINGLE-SPAN BRIDGES IN MOUNTAINOUS STREAM

Abstract

 When bridges are blocked with accumulated woody debris, a high flow depth leads to overflow and causes damage. In mountainous streams, many single-span bridges exist in Japan due to narrow river width. In this study, we conducted laboratory experiments using bridge model without pier and considered factors for the blocking due to woody debris. When examining the blocking conditions, we applied the woody debris concentration at upstream of the bridge. From results, we found that the ratio of the woody debris length to the bridge clearance affect bridge blocking. Smaller ratio cases required larger concentration for blocking, and larger ratio cases required lower concentration for blocking. We also applied logistic regression analysis to experiment results and proposed an approximate expression for blocking possibility.