Present situation and cause analysis of diversion dam failures in Japan

Diversion dams are settled in river channels to dam water. They obstruct flood flow and also affect sediment movement on riverbeds. Dam structures break due to local scouring of the riverbed caused by local flow around them. The causes of such failures should be studied by investigating how flow meanders over alternate bars during floods. Diversion dam failures are classified into three types according to cause. The first type of problem is caused by the difference between the riverbed height and the sill height. If the present riverbed is higher than the sill, the mobile gates do not close easily due to sand deposition on the sill. If the riverbed is lower than the sill, the riverbed in the downstream area of the weir is scoured, and the weir may be damaged. The second type is caused by the relation of the weir position to the alternate channel bars. The bar area is usually higher than the sill, and sand deposition on the sill causes problems for mobile gate operation. The riverbed below the weir at the pool section is scoured because the bed height is lower than the sill height. The third type is caused by the migration of alternate bars. A low water course changes due to migration of alternate bars after a flood. The sand deposition area from the upstream reach blocks the intake entrance. Before a diversion dam structure is designed, the situation and the probable changes in riverbed form should be investigated.