Longitudinal dispersion due to surcharged manholes
Greater environmental considerations and the desire to reduce pollution overflows to watercourses are requiring engineers to develop a better understanding of the processes involved in pollution transport through sewer networks. Furthermore, developments in modelling techniques and computer power are allowing urban drainage modellers to increase the complexity of their software and so demand additional data that can be incorporated. Presently, an important aspect is quantifying the retention time and dispersion of pollutants entering an urban drainage system. Manholes provide a means of sewer access for maintenance and inspection. Under storm flow conditions they are liable to surcharge above the level of the pipe soffit. This creates a storage volume that has an impact on the longitudinal dispersion and travel time of soluble pollutants in sewer systems. A laboratory investigation has been completed to quantify these effects for various manhole configurations. These include step heights between the inlet and outlet pipes, benching and extreme high surcharge conditions. In addition, re·analysis of previously acquired data has allowed variations in manhole diameter to be considered. Numerical modelling using computational fluid dynamics, combined with laser light sheet visualisation of the flow structures within manholes, has provided greater insight into the processes causing longitudinal dispersion. The coefficients required for two existing longitudinal dispersion models, the advection dispersion equation and the aggregated dead zone model, have been determined by means of an optimisation process. This has been undertaken with computer software specifically written for the purpose. The technique adopted for optimisation is fully detailed. Final conclusions regarding the longitudinal dispersion due to surcharged manholes are presented.