Analysis of the sediment remobilization phenomenon in a rain garden using CSTR theory

To investigate the impacts of inflow on overflow suspended solids (SS) concentration in rain gardens, field experiments were carried out in Guangming New District, Shenzhen, China. The pollutant degradation process was assumed to follow the first-order decay theory. Inflow rate and SS concentration data measured from a mild rainfall event were first used to calculate the decay constant based on the continuously stirred tank reactor (CSTR) theory, in which sediment remobilization was assumed to be negligible due to small inflow. Then, SS release rates of other rainfall events were calculated based on the known decay constant and the CSTR equations. Results show that maximum SS release rate has a strong correlation with the maximum inflow rate. Furthermore, sediment remobilization was observed in general, especially in cases of large inflow rate. Analyses show that the SS release rate is proportional to the inflow velocity. However, in the later stage of rainfall events, SS release rate drops more rapidly than the inflow velocity. This indicates the existence of a critical point, at which the inflow velocity does not have enough momentum to remobilize sediments. The study also demonstrates that small sediment particle sizes and uneven distribution of inflow could trigger sediment remobilization even with low inflow velocity.