The wastewater network is a critical infrastructure in a community and damages or disruption due to a hazard event implicate consequences in the economic security, public health and wellness of the community. Therefore, using an index to evaluate the vulnerability and the functionality of the system is essential for designers and utility managers for the design, operation and protection of wastewater network. In this paper, a functionality index for the wastewater network has been proposed. It is the product of three different indices: (i) the number of users still connected to the system, (ii) the quality of sewer discharge into the water body after the treatment, in term of two pollutants, biochemical oxygen demand and total suspended solids, and (iii) the presence of leaks into the network. Seaside, a small city in Oregon, in the West cost of USA has been selected as case of study using an earthquake scenario and a restoration plan. The results show the critical elements of the networks that under the observed operating conditions would not be able to present reliable performances. Using the proposed indices in a decision support tool for governmental agencies could give guidelines for the restoration of elements that have more weight in the functionality of the system.