Optimization of water flushing in lowland urban river in Jiaxing, Zhejiang using dissolved oxygen as the indicator

River flushing has been considered as an effective tool for improving water quality in heavily polluted urban rivers. Dissolved oxygen (DO) as the evaluation index of water quality varied with time during and after water flushing. The time intervals between the flushing should be optimized such that the DO concentration remains above 2mg/L in the river. The duration between the flushing was found to be mainly correlated to the water temperature and the saturation level of dissolved oxygen. For the study river, when the temperature was below 15 ℃, there was an exponential relationship between the duration and saturation level of DO. For instance, the estimated duration was 157 hours or about 7 days for the subsequent flushing when the saturation level of DO was 90%. This duration was no more than 2 days when the temperature was above 15 ℃.