Without any doubt, sewer systems are among the main parts of urban water systems. In traditional engineering, however, they have often been considered and designed as assets of static infrastructure – they actually represent a high capital and social value (which becomes apparent when considering the high costs for their replacement or when they are severely damaged). This special issue of Urban Water Journal therefore focuses on recent developments in the field of operation of these precious assets.
Obviously, there is a vast amount of work within this topic area being undertaken in various parts of the world. Therefore, this volume can only provide a small picture, a snapshot, representing a cross-section of some relevant work and results on this topic. The contributions to this volume, carefully selected by the guest editors of this issue, range from the discussion of specific details on sewer sediments (and how to avoid/remove them) over the definition of suitable performance indicators up to the attempt of a foresight into our future and the future of urban drainage systems at the end of this century.
This volume starts with a rather hot topic in today's drainage engineering – the issue of flushes on sediments: The paper by Campisano, Creaco, and Modica on Laboratory investigations on the effects of flushes on cohesive sediment beds describes a series of laboratory experiments, investigating the impacts of flushing gates on cohesive sediments. Not only additional data suitable also for the validation of mathematical models have been obtained, but also some conclusions have been drawn – the interested reader is invited to consult the paper.
In their paper Study of the shear stress in narrow channels: application to sewers, Bonakdari, Larrate, and Joannis, address the topic of shear stress in channels of a wide range of cross-sections by setting up different types of numerical models with various degrees of complexity (including detailed 3DNavier – Stokes equation modelling) and by using them for related analyses.
Since, despite all efforts, modelling of sediments involves a great deal of uncertainty, Freni, Mannina, and Viviani in their paper Uncertainty assessment of sewer sediment erosion modelling, present a detailed analysis of modelling approaches describing sediment erosion, including an assessment of the robustness of the models involving the Generalised Likelihood Uncertainty Efficiency method. Their analysis, based on data derived from a sampling campaign on an Italian catchment, observes a general trend of overparametrisation of some models.
The upscaling of results derived on a lab-scale to full-scale urban drainage structures constitutes the coretopic of the contribution Scaling solute transport characteristics of a surcharged manhole by Lau,Stovin, and Guymer. As their analysis based ontracer experiments suggests, the cumulative residence time distribution can be a useful tool for thedescription of travel time and dispersion characteristics.
After these contributions which focus on small particles within individual sewer pipes, the next paper attempts to describe the performance of an entire sewer system by suitable indicators: Engelhard, De Toffol and Rauch, in their contribution Sustainability of CSO performance indicators for compliance with ambient water quality targets, the authors analyse in a simulation study, followed by a correlation analysis, at the conclusion that none of the commonly used indicators corresponds well with acute impacts of urban discharges on receiving rivers, while mean annual rainfall volume and frequency of combined sewer overflows are linked to the extent of non-acute impacts.
Korving and van Noortwijk apply deep mathematical approaches to the assessment and evaluation of sewer degradation: Their contribution Bayesian updating of a prediction model for sewer degradation uses the judgement of individual experts for the estimation of a priori distribution functions, which are then used to derive predictions of sewer system degradation. A small example illustrates the mathematical framework derived.
The contribution of Muschalla entitled Optimisation of integrated urban wastewater systems using multi-objective evolution strategies goes even further beyond in scope: the contribution sets up a simulation model describing the urban wastewater system in a simplified way and applies it for an analysis and optimisation of various (static) design variants for urban wastewater management. The application of multi-objective optimisation methods allows a formal optimisation concomitantly with regard to hydraulic stress, maximum ammonia concentrations and maximum oxygen deficit in the receiving river.
With the aim of promoting the application of dynamic control of urban drainage systems, Schütze, Erbe, Haas, Scheer, and Weyand outline the fundamental principles of real-time control – which cannot be bought ‘off the shelf’. As a core element of their paper Sewer system real time control supported by the M180 guideline document, the authors report on the M180 guideline document compiled in Germany, which not only provides an evaluation procedure for RTC potential, derived from scientific findings combined with practical engineering expertise, but also give useful hints for embracing the various stakeholders' demands in setting up RTC systems.
This selection of contributions on the topic ‘Sewer Operation’ finally concludes with an outlook to the future: The question ‘How will sewer system operation look like at the end of this century?’ is addressed by Tait, Ashley, Cashman, Blanksby, and Saul in their paper Sewer system operation into the 21st century, study of selected responses from a UK perspective. A number of socio, political, economical and environmental factors, combined with the relevant institutional and regulatory frameworks is used – by means of expert workshops, studies and simulation exercises – to derive the most efficient responses to predicted changes in the short and long term.
Naturally, as the Guest Editor of this Special Issue, it is our hope that the reader will enjoy the multitude of contributions in this volume and will find them to be useful sources for his/her own work. However, my wishes for this issue are that the papers will stimulate some discussions, that readers contact the authors and/or present their own ideas in one of the forthcoming issues of this journal. May all the work on this wide topic area of sewer operation not be done for its own sake or solely for the sake of scientific merit, but actually to contribute to the improved utilisation of the infrastructure, significant part of which we have inherited from our forefathers and will have to hand them over to the generations to come.
We trust that the reader will enjoy browsing and studying this material. The editors hope that this volume will stimulate discussion among researchers and practitioners and contribute to better operation and maintenance of our sewer assets, resulting in a more sustainable global environment.