Hidden Danger

Abstract

Continuous functioning of sewer systems and water distribution networks is crucial for liveability, public health and economic prosperity in urban areas. Progressive deterioration of these underground water infrastructures leads to an increased probability of failure. Maintenance is needed to ensure a desired level of functioning. For municipalities and drinking water companies as asset owners, the trend is to develop risk-based asset management. A risk-based asset management strategy links the likelihood of failure with the consequences. Hence, knowledge is needed on the condition of the underground water infrastructure and the consequences in case of failure. Within this strategy, a risk assessment should be performed to prioritise the maintenance and renewal of underground water infrastructure. Based on their criticality, resources can be applied appropriately. The more critical a conduit segment, the more important it is to perform maintenance. To contribute to risk-based asset management, this study investigates the consequences of failure in sewer systems and water distribution networks. In this research, a methodology is developed for mapping consequences of underground water infrastructure failure to guide asset management. Including the consequences of underground water infrastructure failure in the risk determination may lead to a different prioritisation of maintenance activities. Three failure mechanisms are considered: structural sewer failure, hydraulic sewer failure and water distribution network failure. Structural sewer failure leads to partial or complete loss of the load-carrying capacity, whereas hydraulic sewer failure occurs when a system does not meet serviceability requirements for system performance. For water distribution networks, it is assumed that hydraulic failure (caused by overpressure) and structural failure occur simultaneously. Hence, no distinction is made between failure mechanisms in water distribution networks. Using a screening method, the consequences of underground water infrastructure failure are mapped. The consequences of failure are expressed as the affected area and the characteristics of the built environment. The affected area consists of the sinkhole area and the flood zone. The characteristics of the built environment are displayed by means of consequence categories. Within this study, five different consequence categories are taken into account, using hydraulic modelling and (open) classified data. The consequence categories are: 1) Damage to buildings 2) Traffic obstruction 3) Impact on human health 4) Costs of conduit reconstruction 5) Drinking water supply outage. A flat, typical Dutch study area in Tuindorp (Utrecht) is used to test the methodology. For these five consequence categories, findings are illustrated and compared with the results of the hydraulic network functioning according to the Graph Theory method (GTM). This GTM determines the effects of failure of individual conduits on the functioning of a system as a whole, based on a simplified network structure using links and nodes. Results of the analyses show a positive relation between the individual consequences in case of hydraulic sewer failure. For example, critical conduits in the category `damage to buildings', are critical for `impact on human health' as well. Besides, there is a positive relation between the consequence categories and the hydraulic network functioning. Conduits with large diameters are stated as critical for both methods, yet dead-end segments are only critical for hydraulic network functioning. Contradictory, individual consequences are uncorrelated for structural sewer failure and water distribution network failure. The consequences are independent and can not be linked. Besides, there is no relation between the hydraulic network functioning and the individual consequences of structural sewer failure and water distribution network failure.