Modelling the long-term suspended sedimentological effects on stormwater pond performance in an urban catchment

Abstract

The influence of long-term suspended sediment dynamics on stormwater pond performance is not negligible, but often neglected in pond design and performance evaluation. This paper provides systematic simulated quantification of long-term suspended sedimentological effects on stormwater pond performance. Integrated hydrological and two-dimensional hydro-morphodynamic modelling and simulations were carried out over a 32-year-period (1984-2015) covering 3896 rainfall events with a wide range of rainfall volumes, durations and intensities. Three event-based hypothetical rainfall scenarios; non-flood condition (5-year), sewer design condition (30-year), and river flood condition (100-year) rainfall events with 1-hr duration were also simulated for comparison between the traditional event-based approach and the novel approach presented in this study. Simulation results show that the flood peak attenuation and delay are more pronounced for small (< 5-year) and medium (< 30-year) flood events. The long-term continuous simulation results indicate that the pond provides positive annual trap efficiencies varying from 2% to 69% for 31 of 32 years, providing long term water quality benefits downstream. However, rainfall events in year 2012 flushed out the accumulated sedimentation as a shock load to the downstream river, leading to a negative trap efficiency of -11%. The spatially averaged sediment deposition rate, as predicted by the model, varies with a mean of 2 (1.34) cm/year over the study period, which resulted in a 24% loss in the pond’s volume over 32 years. The impacts of the loss in storage on pond flood attenuation capacity are explored at regular time intervals over the study period. The results indicate that reduction in the pond’s flood attenuation capacity is relatively more pronounced for medium (30-year) and extreme (100-year) flood events than the frequent small flood (5- year) events. The variation in annual sediment loading with rainfall quantities and patterns are also explored.