A new method to evaluate floodwater for control/use in high-sediment rivers of Northwest China.

Evaluating the quantity of flood season floodwater that is difficult to control or use in rivers with high sediment concentration is an important part of water resource evaluation and floodwater resource utilisation. This study proposes a method coupling water quantity and quality to evaluate such floodwater. The method divides floodwater into floodwater that is difficult to control ('difficult-controllable') and floodwater that is controllable but difficult to use ('controllable but difficult-use'). A case study of the Bajiazui Reservoir in the Jing River in China's Loess Plateau is presented. The average annual quantity of difficult-controllable floodwater is 10.4 million m3 . The annual mean quantity of the difficult-controllable/-use floodwater is 38.1 million m3 . The majority of that amount (78.21%) comprised controllable but difficult-use floodwater. An analysis of 64 combinations of factors influencing the quantity of the difficult-controllable/-use floodwater shows that the sediment concentration of run-off is the primary factor influencing the difficult-controllable/-use floodwater. The reservoir's maximum flood release capacity and floodwater rejection coefficient are the primary factors affecting the difficult-controllable and controllable but difficult-use floodwater, respectively. The new evaluation method is superior to traditional methods and is suitable for evaluating difficult-controllable/-use floodwater in high-sediment rivers.