The Aim of the paper is to develop an analytical procedure for calculating the dependability indicator of a specific tree-like transporting network, i.e., the water disposal system of a major city.Methods. It is suggested using the relative volume of sewage water released from a sewage system into the environment as the result of the failure of the former’s components as the measure of dependability. This paper presents a simple and convenient method for calculating such volume. The method is based on the representation of a tree-like sewage network as a combination of Y fragments referred to as structural elements. Every such element is formally replaced with its pseudo equivalent with a failure rate calculated assuming that the volumes of untreated waste water released by actual and fictitious elements are equal. A consistent application of this approach reduces the problem of estimation of the volume of released waste water to elementary subproblems that are solved using probability methods.Results. The problem of identifying the operational dependability indicator comes down to a recurrent stepwise procedure for equalization of Y fragments of the network, at each step of which the calculation data from the preceding step are used as the input data. Each such step, starting from the entry network, leads to another (virtual) network, for which the procedure repeats. The equalization process ends when the original network is represented by only one pseudo element, for which the dependability indicator is defined in an elementary way.Conclusion. The authors developed a convenient method for calculating the operational dependability indicator of a tree-like sewer network of a major city. Some practical problems are referred to that can be solved using the proposed method. Future lines of research in the domain were identified.