The Relation Between the Rainfall and the Discharge of Sewers in Populous Districts

The most important question which arises in the construction of a sewerage system whose function is also the removal of the surface drainage, is with regard to the amount of storm water that will find its way into the sewers; and therefore a brief review of the modes of solution adopted in different places may be of interest. The proportion of rainfall contemplated to be admitted into the sewers varies within wide limits and depends largely upon the intensity and duration of the rain, the relative impermeability and slope of the surface, and the facility with which the storm waters can be diverted into suitable natural channels, either directly from the roofs and street gutters, or indirectly by means of storm-overflows from the sewers: the element of initial cost is also of vast significance in determining the limits, and, by being too narrowly considered, frequently leads to reductions of drainage capacity which must be supplemented a few years later by the construction of special relief conduits, whose expense is often much greater than the original amount saved, together with accrued interest.