Sealing (Surcharging) Conditions

There may be conditions such that part of the section is flowing full, while part of the flow remains open. These conditions are called sealing conditions, and the sections are analyzed in separate parts. For sealing conditions, the portion of the section flowing full is analyzed as pressure flow, and the remaining portion is analyzed with gradually varied flow techniques.

Rapidly Varied Flow

Rapidly varied flow is turbulent flow resulting from the abrupt and pronounced curvature of flow streamlines into or out of a hydraulic control structure. Examples of rapidly varied flow include hydraulic jumps, bends, and bridge contractions. The hydraulic phenomenon that occurs when the flow passes rapidly from supercritical to subcritical flow is called a hydraulic jump. The most common occurrence of this within a gravity flow network occurs when there is a steep pipe discharging into a particularly high tailwater, as shown in the following figure.

There are significant losses associated with hydraulic jumps due to the amount of mixing and hydraulic turbulence that occurs. These forces are also highly erosive, so engineers typically try to prevent jumps from occurring in gravity flow systems, or at least try to predict the location of these jumps in order to provide adequate channel, pipe, or structure protection. The program does not perform any specific force analyses that seek to precisely locate the hydraulic jump, nor does it identify the occurrence of jumps that might happen as flows leave a steep pipe and enter a mild pipe. Rather it performs analyses sufficient to compute grades at structures.