The optimisation of baffle arrangements for minimal sediment washout in standard stormwater sumps.

Abstract

Sediment in stormwater drainage systems is a matter of great concern due to its ability to collect within stormwater pipes to form blockages. Furthermore, sediments transport harmful pollutants, depositing them in stormwater discharge areas and disturbing the ecology of those areas. Standard stormwater sumps are a reliable and cost-effective solution to trap and retain sediment within stormwater drainage systems for manual removal. However, under high flowrate and fine sediment conditions, the effectiveness of standard sumps drastically decreases. A solution to this issue is the installation of baffles that can be retrofitted into the standard sump as a method of reducing flow velocities to allow sediments to settle to the sump bed. An aspect of this solution, as equally important as sediment trapping, is the prevention of sediment washout. During a storm event, sediment previously trapped in a stormwater sump can be washed out before the routine cleaning period. Therefore, it is paramount to ensure that the baffle arrangement effectively prevents the resuspension and washout of sediment already present in a standard sump. This dissertation investigates the use of baffle arrangements to minimise sediment washout in standard sumps and determine a baffle arrangement that will provide the best sediment retention results while remaining a feasible and practical solution in a real-world context. Three baffle arrangements, consisting of a combination of solid and semi-porous baffle plates of varying dimensions placed in different orientations, were chosen for this investigation based on their previous success with sediment trapping. The primary methodology of this research compared the effectiveness of these baffle arrangements in minimising sediment washout by comparing them to a control setup with no baffles, using a simple mass-balance process. Additional experimentation involved using an Acoustic Doppler Velocimeter (ADV) to develop flow velocity fields to motivate the observations made in the mass-balance tests. All baffle arrangements performed better than the control setup, with the least effective of the three designs improving sediment retention efficiency by 22% and the most effective by 71.5% under the worst-case condition. These results were then dimensionally analysed to determine the results of such testing under real-world conditions. The results of this analysis were also promising, with the best baffle arrangement achieving retained effluent concentrations of up to 0.4 kg/m3 when calibrated for typical standard sump dimensions and peak flowrate.