The hydraulic performance is the most important parameter for the analysis of the level service of the wastewater collection system. It depends on the dimension of the pipes, but also on temporal effect of structural deterioration, blockages, roots, sags, etc, which affect the capacity to transport wastewater and runoff, and to avoid local floods and excessive pollution discharges. Software for network flow capacity is commonly used, but these packages do not consider the capacity reduction, which takes place gradually over a long period of time.

As network deterioration continues, an urban area usually undergoes progressive urbanisation (i.e. increased impervious area, densification, increased population). All of these processes further deteriorate the hydraulic performance of the system. More intense rainfall due to climatic change might further deteriorate the situation.

The service reliability at the system level reflects the probability that parts of the wastewater network are out of function because of accidents (clogging, pipe burst) sediment formation or because of hydraulic overload. A service reliability analysis includes a hydraulic and a reliability model, the latter predicting the probability of network capacity failure.

The sewer system is connected to the wastewater treatment plant (WWTP) and the neighbouring natural compartments groundwater and surface waters. Therefore, the urban wastewater system must be perceived as an integrated, complex system, where changes in one of the subsystems affect the conditions in all the other subsystems. A damaged sewer system does not only fail to fulfil its task, that is complete transport of the sewage to the WWTP. Moreover, sewage is ex-filtrating into the ambient soil, thereby possibly polluting groundwater, or groundwater is infiltrating into the sewer and is thus partly depleted, the composition of combined sewer overflows (CSO) is changed and so is the resulting impact to the receiving water, the operation of the WWTP must be adapted to the altered conditions or its performance will suffer. Assessing the performance of the sewer system and evaluating the ecological and quantifying the economic consequences of rehabilitation strategies must include the effects on the WWTP operation and on the state of ground and surface waters.

The main objective of this work package, therefore, was to produce a model for hydraulic performance with temporal decline including the effect of this on the network interaction with the environment. Based on this, a model for estimation of the reliability for sewer and storm water network functionality was established.