Urban drainage sediment deposition processes in pipes: an experimental study

Urban drainage pipes face increasing risk of sediment deposition, which reduces drainage capacity and increases blockage potential. This study experimentally investigated sediment deposition using a laboratory-scale drainage pipe system with a roughened pipe wall and an image-recognition-based monitoring method. 130 experiments were conducted by varying flow velocity, pipe slope, sediment concentration (loading conditions), and sediment type. Under the tested conditions, sand deposition development exhibited critical hydraulic thresholds, with critical pipe slopes of approximately 6 and 3‰ under low and high concentration conditions, respectively, and a critical flow velocity of 0.4–0.5 m/s. Higher sediment loading conditions promoted deposition, particularly at larger pipe slopes and flow velocities. Compared with sand, cohesive sediments produced longer and more spatially distributed deposits, while mixed sediments exhibited interaction effects rather than simple superposition. Sediment deposition was concentrated in the upstream pipe region, with the maximum deposition thickness occurring at approximately 2% of the pipe length, and generally followed a two-stage temporal evolution from rapid accumulation to quasi-equilibrium. These findings provide improved, empirically grounded understanding of sediment deposition processes, including the experimental identification of critical hydraulic thresholds and sediment-type interaction effects, offering practical guidance for drainage pipe design, targeted sediment removal, and proactive blockage management.