Sediment transport mechanisms through the sustainable vegetated flow networks

Understanding the pollution treatment efficiency of a sustainable urban drainage (SuDS) asset or network requiresthe influx, transport, detention and discharge of the pollutant within the system. To date event specific monitoringof sediment (primarily total suspended solids) concentrations in the inflow and discharge from SuDS have beenmonitored. Long term analysis of where the sediment is transported to and the residency time of this pollutantwithin the SuDS asset or network have not been unraveled due to the difficulty in monitoring specific sedimentparticulate movement. Using REO tracing methodology, sediment particulate movement has become possible. Intracing sediment movement from an urban surface the internal residency and transportation of this sediment hasillustrated SuDS asset differences in multi-event detention. Of key importance is the finding that sediment remainswithin the SuDS asset for extended periods of time, but that the location sediment detention changes. Thus, overmultiple rainfall-runoff events sediment is seen to move through the SuDS assets and network proving the assumptionthat detained sediment is permanent and stationary to be inaccurate. Furthermore, mass balance analysis ofSuDS sediment indicates that there is notable re-suspension and ongoing release of sediment from the SuDS overtime and cumulative rainfall-runoff events. Continued monitoring of sediment deposition and concentration in suspensionillustrates that sediment detention within SuDS decreases over time/multiple events, without stabilizingwithin a 12 month period. Repeated experiments show a consistent pattern of detention and release for the threeSuDS networks monitored in Scotland. Through consideration of both rainfall and flow factors the drivers of sedimenttransport within the monitored SuDS have been identified. Within the limitation of this field study the keydrivers to SuDS sediment detention efficiency (or transport of sediment through the system) have been identifiedas flow velocity, wetted surface area, stream power, rainfall occurrence and depth, Fr, Re and Rep.