TABLE OF CONTENTS Strictly speaking, as discussed in Sec. 6.2.2 storm sewer flows are generally unsteady and nonuniform. However, often the duration of the hydrograph of single rainstorm events is hours, the time of water flowing through individual sewers is minutes, whereas unsteady sewer flow computational time interval is often seconds. Therefore, within the time scale of the flow passing through the sewer or the computational time interval, the flow can be treated as steady without losing accuracy, that is, stepwise steady or quasi-steady. Extensive knowledge exists in the literature on the hydraulics of steady uniform and nonuniform free surface flow, for example, Chow (1959). To describe the water surface profiles of steady flows, two reference depths are often used. One is the normal depth and the corresponding normal flow, which will be described in the following subsection. The other is the critical depth at which the Froude number is equal to unity. The critical depth, which separates the supercritical flow from subcritical, is determined from
| (6.31) |  |
where the critical flow cross-sectional area A c and critical flow water surface width B c are each a function of either the critical depth h c or the central angle 
(in radians) subscribed by the free surface (Fig. 6.8). For a given discharge Q the right side of Eq. (6.31) is a constant, provided ? is constant. For circular sewer pipes Eq. (6.31) can be written as
| (6.32a) |  |
Straub suggested an approximation of...
