Management actions are required to extend the life-span of irrigation barrages for a long term cost reduction of an irrigation project. One of main problems for the extension is scouring damage of a barrage and its riprap by downstream riverbed excavation to protect downstream city areas from large floods. Therefore, we examined the mechanisms of downstream riprap destruction by progressing downstream riverbed fall to develop efficient repair methods for ripraps through hydraulic model tests on a condition of a typical irrigation barrage. Results show that destruction mechanisms are classified into two types; scouring type and slope type. In the scouring type, destruction progresses rapidly with scouring during a large flood. In the slope type, destruction progresses slowly with piping during small floods.
