R7 PM Uncertainty Analysis in Risk Assessment: Influences on Decision-making
Thursday, 17 November 2005: 1:50 PM - 5:30 PM in 327-329

(ROS-1117-709605) Probabilistic risk assessment of urban wet-weather discharges.

Krejci, V.¹, Fankhauser, R.¹, Chèvre, N.¹, ¹ Swiss Federal Institute of Aquatic Science and Technology, Duebendorf, Switzerland

ABSTRACT- Urban wet-weather discharges, such as CSO (combined sewer overflows) and stormwater discharges could impair the receiving water in different ways: acute chemical pollution, contamination of sediments, etc. The main problems in assessing the risk of these discharges are linked with the random aspect of the cause (i.e. the rainfall) and the dynamic properties of pollutants during rain events. Furthermore, a lot of uncertainties are linked to the different pollution sources and transport processes in the sewer system and in the receiving waters. In this study, we propose a stochastic and probabilistic approach, taking the variability and the uncertainty of urban wet-weather discharges into account, to estimate the concentrations of pollutants resulting from urban discharges. These concentrations are compared with ecotoxicologically-based environmental quality criteria to avoid acute (ammonia toxicity, effects of turbidity and adsorbed compounds, dissolved oxygen) and chronic (accumulation of contaminated sediments from urban wet-weather discharges) risk for the aquatic ecosystem. This approach has been implemented in specific software, REBEKA II, in order to illustrate this new way of handling probabilities in risk assessment. The model parameters are specified through probability distributions. A Monte-Carlo procedure is used, with 10 years of rain data for each run. The results are expressed as a probability of fulfilling the different acute and chronic criteria. Discussions with authorities and engineers show a surprisingly good acceptance of this new approach. In fact, the first presentation of the concept generated heavy criticism from practitioners. But regular publication in specific journals, discussions, workshops and courses contributed to the acceptance of this method. In the near future, we expect that all new infrastructures for wet-weather control in Switzerland will be planned on the basis of this stochastic-probabilistic approach of risk assessment.

Key words: risk assessment, wet-weather discharges, uncertainty analysis, decision-making