Showing papers by "Francois Clemens published in 2008"

Fault tree analysis for urban flooding

Abstract: Traditional methods to evaluate flood risk mostly focus on storm events as the main cause of flooding. Fault tree analysis is a technique that is able to model all potential causes of flooding and to quantify both the overall probability of flooding and the contributions of all causes of flooding to the overall flood probability. This paper gives the results of a fault tree analysis for urban flooding for the case of Haarlem, a city of 105.000 inhabitants. Data from a complaint register, rainfall data and hydrodynamic model calculations are used to quantify the probabilities of the basic events in the fault tree. The flood probability that is calculated for Haarlem is 0.78/week. Gully pot blockages make the main contribution to flood probability: 79%, storm events contribute only 5%. This implies that in this case an increased efficiency of gully pot cleaning is a more effective strategy to reduce flood probability than to increase the drainage system capacity. Whether this is also the most cost-effective measure can only be decided if the risk calculation is completed with a quantification of the consequences of both types of events. To do this will be the next step in this study.

Microbial risk assessment for urban pluvial flooding

Abstract: Public health risks of urban pluvial flooding have so far received little attention in technical discussions. In this paper, the results of pathogen measurements in the sewer system of Utrecht and an urban flooding experiment are presented and used in an application of Quantitative Microbial Risk Assessment, an existing risk analysis method for the quantification of infection probabilities. This method uses ingested doses of pathogenic organisms for the calculation of infection probabilities. Ingested dose estimations are based on pathogen measurements. These samples have been analysed for concentrations of Campylobacter, Cryptosporidium and Giardia. Dose-response relations from literature are used to calculate infection probabilities for flood events. The results show that mean probabilities of obtaining a Campylobacter or Giardia infection as a result of contact with urban flood water are 2.8% and 0.6% per event respectively for adults and at least 5.7% and 1.0% per event for children, respectively. Infection probabilities for Cryptosporidium are about 1000 times lower than for Giardia. The infection probabilities found indicate that the health risk of urban flooding is higher than that of swimming in recreational freshwater environments, based on a comparison to the values for ‘acceptable risk’ as defined by the WHO for bathing water.

On gas transport in downward slopes of sewerage mains

Abstract: pressure mains are subject to gas pockets in declining sections. These gas pockets cause an additional head loss and an associated capacity reduction, which cannot be predicted with sufficient accuracy. The number of parameters, affecting downward gas transport in sewerage mains, is overwhelming. Therefore, the development of a numerical model is inevitable, starting from the knowledge base in the available literature. This paper includes a critical review of the literature on gas transport by flowing water in declining pipes. This review will show that the wide spread in the correlations for the clearing velocity, as reported by various investigators, is mainly caused by a subtle misinterpretation of the original data. Furthermore, new information from a few old references will be presented and integrated into a synthesis of the available literature. Finally, a new dimensionless velocity parameter is introduced, which is shown to be a more appropriate scaling parameter than the existing velocity scaling.