In this study, optimal designs with minimum costs are obtained for various storm return periods. Then the risk analysis is used to determine the return period in which the design cost plus the damage risk cost is minimum. SWMM software was used to handle the simulation and the Network optimization was performed by using the genetic algorithm. The non-linear reservoir model to convert the rainfall into runoff and the dynamic wave model to perform the network hydraulic simulation in this software are utilized as a complicated simulation model. The results showed that the 10-year return-period storm in which the summation of the design and the damage risk costs are minimum is the one that should be selected. Also, the rational method of the software was applied as the simplest method of rainfall-runoff and the hydraulic calculations were performed using a Manning equation without considering the flow travel time. The results show that the return period of the risk analysis is the same as the first one whereas the total design costs are greater by 16.6%. Afterward, the classical rational method in which the flow travel time is considered was used to design the same network. The peak flows of the pipes were remarkably reduced, causing the design costs to be only 4.7% greater than the complicated precise method. It can be concluded that the simple classic rational method considering the flow travel time may be used in the design of storm sewer networks due to its acceptable accuracy and costs.

In this study, the risk analysis is used to determine the return period in which the design cost plus the damage risk cost is minimum. The damages include the roads and traffics, the lawn areas, and the residential and commercial buildings. The traffic damages are based on two factors, time of delay and social negative impacts. The non-linear reservoir model is used to convert the rainfall into runoff and the dynamic wave model is utilized to perform the network hydraulic simulation in SWMM software. This model is defined as a appropriate model. This model was applied in the risk analysis of a region in Tehran to obtain the optimal return period design. The results indicated that the optimal return period is 10 years. The rational method was also applied to the same region and same return period, but the total design cost of the rational method was greater by 5%. The damages due to the traffic include financial damages due to delays and loss of fuel resources in addition to the dissatisfaction of the people due to the psychological burden.