Exploring urban runoff complexity: road-deposited sediment wash-off mechanisms and dynamics of constraints

To accurately figure out how much pollution comes from urban surface runoff and take steps to protect receiving water, we needed to fully understand how road-deposited sediments (RDS) wash off. Our research indicates that particles smaller than 100 μm imparted 59–73% of the wash-off load. Two instances of natural rainfall reduced the aggregate RDS mass by approximately 27–36%. On days without rain, the RDS particle shrank in size, but it became heavier after a downpour. The results showed that the source restricted the tiny particles washed off of RDS, while transport generally restricted the heavier particles washed off. We used 39 artificial rainfall events with different particle sizes to confirm our results on RDS wash-off. When compared to the heavier particles, tiny particles have a greater wash-off percentage, and when it comes to describing the wash-off mechanism, Fw values offer an inventive and insightful assessment. It has been assessed that tiny particles were source-restricted and this mechanism occurred during the initial stage, but heavier particles were transport-restricted and it occurred during the late stage. HIGHLIGHTS Natural rain reduces road-deposited sediment (RDS) by 27–36%, highlighting rainfall's cleansing effect on urban surfaces.; On rain-free days, RDS particles shrink, becoming heavier after downpours, revealing the dynamic interplay between rainfall and particle behavior in urban runoff.;