The increase in the population of cities and the formation of Great Metropolitan Areas has increased drastically the flow rates of waste water to the treatment plants. It means, in the coastal areas, an increase in diameter of the outfalls at the service of the treatment plants. Large diameter outfalls present design and construction problems of a different type and size to those of the smaller lines used until recently. The paper covers some of the problems encountered in the design and construction phases of so-called “big inch” outfalls.