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.