Underground water infrastructure is essential for life in cities. The aging of these infrastructures requires maintenance strategies to maintain a minimum service level. Not all elements are equally important for the functioning of the infrastructure as a whole. Identifying the most critical elements in a network is crucial for formulating asset management strategies. The graph theory is presented as a means to identify the most critical elements in a network with respect to malfunctioning of the system as a whole. As opposed to conventional methods, the proposed method does not rely on iterative hydraulic calculations; instead, the structure of the network is taken as a starting point. In contrast to methods applied in practise, the results are independent on the chosen test-load. Because of the limited calculation effort, the method allows the analysis of large networks that are now, for practical reasons, beyond the scope of methods applied so-far.

9Gully pots are essential assets designed to relief the downstream system by trapping solids and 10 attached pollutants suspended in runoff. This study applied a methodology to develop a quantitative 11 gully pot sedimentation and blockage model. To this end, sediment bed level time series from 300 12 gully pots, spanning 15 months, were collected. A generalised linear mixed modelling (GLMM) 13 approach was applied to model and quantify the accumulation of solids in gully pots and to identify 14 relevant physical and catchment properties that influence the complex trapping processes. Results 15show that the retaining efficiency decreases as sediment bed levels increase. Two typical silting 16 evolutions were identified. Approximately 5% of all gully pots experienced progressive silting, 17 eventually resulting in a blockage. The other gully pots show stabilising sediment bed levels. The 18 depth of the sand trap, elapsed time since cleaning and the road type were identified to be the main 19properties discriminating progressive accumulation from stabilising sediment bed levels. 20

Background: Cooperatives delivering out of hours care in the Netherlands are hesitant about the use of expert systems during triage. Apart from the extra costs, cooperatives are not sure that quality of triage is sufficiently enhanced by these systems and believe that call duration will be prolonged drastically. No figures about the influence of the use of an expert system during triage on call duration and triage decisions in out of hours care in the Netherlands are available.

The accumulation of FOG (Fat, Oil and Grease) deposits in sewer pumping stations results in an increase in maintenance costs, malfunctioning of pumps and, a potential increase of wastewater spills in receiving open water bodies. It is thought that a variety of parameters (e.g. geometry of the pump sump, pump operation, socioeconomic parameters of the catchment) influences the built-up of FOG. Based on a database containing data of 126 pumping stations located in five Dutch municipalities a statistical model was built. It is shown that 3 parameters are most significant in explaining the occurrence of FOG deposits: mean income of the population in a catchment, the amount of energy (kinetic and potential) per m per day and the density of restaurants, bars and hotels in a catchment. Further it is shown that there are significant differences between municipalities that can be traced back to the local 'design paradigm'. For example, in Amsterdam, the design philosophy of discharging in the pump sump under the water surface (and hence maintaining a low level of turbulence in the pump sump) results in an increase of the probability of the formation of FOG.

The performance of lateral house connections can have a substantial impact on the overall level of service provided by sewer systems. However, knowledge on the failure probability of these components remains scarce. This paper analyses field data to determine failure rates. A distinction is made between different mechanisms that contribute to the overall failure probability of lateral house connections. Root causes that promote the occurrence of these failure mechanisms are derived from a literature review. Results of a trend analysis shows a time constant failure rate, which exceeds blockage rates reported for main sewers. Fat, oil, and grease deposits are the dominant failure mechanism for both a specific case study in Rotterdam and the Netherlands. Literature suggests misuse and the structural condition as main root causes for this failure mechanism. The sheer number of failures associated with lateral house connections suggests that these components should be taken into account in sewer asset management. ARTICLE HISTORY