TABLE OF CONTENTS One of the most useful pieces of information in solving urban drainage problems is knowing the flow carrying capacity of the channel or sewer. Knowing the sewer capacities allows a new approach in solving flood drainage problems by separating them into two parts: (1) the demand part of how much water needs to be drained, which is essentially a hydrology problem and (2) the supply part of how much can the sewer handle, that is, the capacity, which is a hydraulic problem. The flood drainage problem can be analyzed by comparing the two parts and then searching for a solution.
There are actually various kinds of sewer discharge capacities. There is the capacity for a single sewer. There is the capacity of the sewer network as a system, which usually is different from the capacity of individual sewers. For an open channel, often the maximum steady uniform flow that the channel can carry without spilling over bank is quoted as its capacity. For a sewer, the just-about-full gravity (open-channel) steady uniform flow is usually quoted.
In fact, for a subcritical free-surface flow, the discharge that the channel can carry depends on the downstream water level. For a sewer with a range of possible exit water levels, this requires repeated backwater profile computations. For a sewer network that has a number of connected channels, the number of backwater computations can easily become very large, making it nearly impossible, if not impractical, for a network capacity...
