Risk-based design of sewer system rehabilitation

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 case study on optimizing the storage capacity of a sewer system to balancing investment cost and damage due to combined sewer overflows.