TABLE OF CONTENTS Hydraulically, flows in sewers in a network interact, and the mutual flow interaction must be accounted for to achieve realistic results. In designing the sewers of a network, the constraints and assumptions on sizing sewers as discussed in Sec. 6.5.1 should be noted.
The rational method is the most commonly and traditionally used method for the design of sewer sizes. As described in Sec. 6.5, in the rational method each sewer is designed independently without direct, explicit consideration of the flow in other sewers. This can be done because to design a sewer, only the peak discharge, not the entire hydrograph, of the design-storm runoff is required. As previously explained in Sec. 6.5, each sewer has its own design storm. The information needed from upstream sewers is for the alignment and bury depth of the sewer in addition to the upstream flow time needed to estimate the time of concentration for the determination of the rainfall intensity i for the sewer to be designed.
Contrarily, in simulation of flow in an existing or predetermined sewer network for urban stormwater control and management, often the hydrograph, not merely the peak discharge, is sought, and a higher level of hydraulic analysis that considers the interaction of the sewer flows in the network is required. This network system analysis involves combining the hydraulics of individual sewers as described in Secs. 6.2 and 6.8, together with the hydraulics of junctions described in Sec. 6.7.
A sewer...
