Showing papers by "Francois Clemens published in 2009"

Risk-based design of sewer system rehabilitation

Abstract: Risk and uncertainty are often not taken into account in decision-making on sewer rehabilitation. However, the assessments on which the decisions are based are considerably affected by uncertainties in external inputs, system behaviour and impacts. This is a problem of growing significance. Many sewer systems need expensive rehabilitation due to the deterioration in their performance brought about by changes in inputs, such as urbanization, urban renewal and climate, as well as decay of sewer infrastructure. Rehabilitation should be efficiently designed and implemented, and should also be effective with the objectives of minimizing costs and maintaining safety and reliability. In this article, a risk-based approach is presented, considering uncertainty in sewer system dimensions, natural variability in rainfall and uncertainty in the cost function describing environmental damage. In particular, the application of different shapes of cost functions is studied. The optimization method is illustrated with a ...

Uncertainty in risk analysis of urban pluvial flooding: a case study.

Abstract: The interest in urban flood risk is growing steadily over the last decades. Still, in the Netherlands no data is available to quantify urban flood risk. In this paper an estimation of urban flood frequencies is made in a detailed analysis of an urban catchment. Calculation results from a theoretical model are compared with data from a complaint register. The analysis in the case study shows that insufficient system maintenance condition is an important potential cause of urban flooding. The estimated flood frequency caused by severe rainfall is 4 events in 7 years or 0.6 per year, while the flood frequency caused by maintenance problems is 13 in 4 years or 3.3 per year. This includes 2 flood events that are caused by heavy rainfall and 11 events that are related to maintenance problems. The number of locations that suffer flooding caused by severe rainfall is more than 3 or 4 per event, while the number of locations that suffer flooding caused by maintenance problems is not more than 2 per event. It is expected that these numbers are representative for the rest of the Netherlands. Further research and data collection will very this assumption.

Evaluation of operational strategies for sewer flooding based on failure data

Abstract: Data from call centres at two municipalities were analysed in order to quantify flooding frequencies and associated flood risks for three main failure mechanisms causing urban flooding. The aim was to find out whether current operational strategies are efficient for flood prevention and if directions for improvement could be found. The results show that quantified flood risk for the two cases is well above the standard which is defined in sewer management plans. The analysis pointed out that gully pot blockages are the main cause of flooding and handling gully pot blockages should therefore be a priority for sewer operators. Reactive handling of calls, as is currently applied, is inefficient if all calls are reacted upon since a small portion of all calls report serious consequences like flooding in buildings or wastewater flooding. Preventive cleaning of sewer pipes proves to be an efficient strategy to reduce flooding due to sewer blockages as flood risk associated with sewer blockages is lower in case of higher cleaning sewer frequencies. Sewer blockages often have serious consequences, thus preventive handling is to be preferred to reactive cleaning. According to the results of this analysis, reduction of flooding sewer overloading is not of primary concern, because serious consequences for this failure mechanism are rare compared to other failure mechanisms.