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In the water industry tunnels can be used to transfer water from a basin to other areas over varying distances. The construction of such tunnels is inherently risky and can result in unpredicted events and incidents. It is therefore necessary that thorough risk assessments are carried out as a priority of the owner, contractor and consultant organization. This is so that, through a systematic and logical plan, they can risk posed by these unforeseen events and incidents. In this paper, the risks and their main causes which are often encountered in such projects are identified and assessed. A fault tree method is applied in order to identify the main causes of events and incidents. By its nature a Risk assessment cannot be defined by absolute values and so fuzzy data must be used in order to calculate the probability of incidence and the severity of the risk. This is done on the four main criteria of time, cost, quality and safety. In order to estimate the significance of each criterion and to calculate the significance of the total influence of risk Analytic Hierarchy Process (AHP) is applied. In this paper the case study of Dasht-e Zahab water conveyance tunnel has been selected for discussion as it was subjected to severe and multiple hazards. The results obtained using the method were validated by conducting different interviews with the field experts. It was concluded that by applying the proposed methodology on the case study the risks of the project can be evaluated in a more methodical and accurate way than could be done without using the method. This approach is therefore recommended for similar types of projects where there are complicated risks that must be thoroughly investigated and understood.