Composite open channel structures are an expensive infrastructure in terms of material, construction and maintenance, and therefore finding the optimum design becomes an important issue. This paper develops four different models for composite open channel and applies a new optimization algorithm to determine the optimal design of these models. The new algorithm called Charged System Search (CSS) is inspired by the governing laws from electrostatics physics and mechanics. CSS is a multi-agent approach and each agent is a Charged Particle (CP) which can affect others based on their fitness values and their separation distances. The results of the CSS are compared to those of ant colony optimization and the genetic algorithm method to highlight its superiority for determining the optimum design of composite open channel structures.