On conditions of sandbar clogging at confluences in the Upper Yellow River

Sandbar clogging occurred frequently at the confluence between hyper-concentrated flood flows from tributaries and the main stream in the Upper Yellow River, with consequence of severe flooding disasters. The properties of hyper-concentrated flood flow in tributaries and the characteristics of sandbar clogging at the confluences between tributary and main stream were analyzed based on field data for sandbar clogging events in the Upper Yellow River. Effects of key factors, such as discharge ratio (tributary discharge to that of main stream) and water amount and sediment load of hyper-concentrated floods, on the sandbar clogging were investigated. A general discriminant formulation for sandbar clogging at confluences was proposed based on the momentum principle. By taking account into the characteristics of flood and sediment transport in the main stream and tributaries, two criteria for sandbar clogging were further derived from the general discriminant formulation, one expressed by relationship between discharge ratio and water amount of tributary flood and another expressed by water amount and sediment load of tributary flood. Therefore, the occurrence of sandbar clogging can be predicted using discharge ratio, water amount and sediment load of tributary flood. This is of critical importance for preventing disasters caused by sandbar clogging at confluences that is associated with hyper-concentrated flood flows during rainstorm events in the Upper Yellow River.