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. . ~ ~ .. ~ . ~ l t ~ ~ . . t \ ~. . t \. k l t ~ I t . r ~ f ~ Flgnre I-lt--Culvert flowing full (no tail water at outlet end). tranquil. If F. = i.o, the flow is said to be critical. The three flow regimes are illustrat- ed in the depiction of a small dam in figure 1-12. Subcritical flow occurs upstream of the dam crest where the water is deep and the velocity is low. Super- critical flow occurs downstream of the dam crest where the water is shallow and the velocity is high. Critical flow occurs at the dam crest and represents the dividing point between the subcritical and supercritical flow regimes. To analyze free surface flow condi- tions, a point of known dePth and flow (control section) must first be identi- fied. A definable relationship exists between critical depth and critical flow at the dam crest, making it a convenient control section. f . l ~ t f , l ~ f r ~ ~ , ~ Identification of subcritical or super- critical flow is required to continue the analysis of free surface flow condi- tions. The example using the dam of figure 1-12 depicts both flow regimes. SubcrWcal flow characteristics, such as depth and velocity, can be affected by downstream disturbances or restric- tions. For example, if an obstruction is placed on the dam crest (control sec- tion), the water level upstream will rise. In the supercritical flow regime, flow characteristics are not affected by downstream disturbances. For example, an obstruction placed at the toe of the dam does not affect upstream water levels. The same type of flow illustrated by the small dam may occur in a steep culvert flowing partly full. (figure 1-13) In this situation, critical depth would occur at the culvert inlet, subcritical flow could exist in the upstream channel, and supercritical flow w("Jld cxist in the culvert barrel. A special type of free surface flow is called "just-full flow." This is a special condition where a pipe flows full with no pressure. The water surface just touches the crown of the pipe. The analysis of this type of flow is the same as for free surface flow. 2. Types of Flow Control. Inlet and outlet control are the two basic types of flow control defined in the research conducted by the NBS and the BPR. The basis for the classification system was the location of the control section. The characterization of pressure, subcri- tical, and supercritical flow regimes played an important role in determining the location of the control section and thus the type of control. The hydraulic capacity of a culvert depends upon a different combination of factors for each type of control. a. Inlet Control. Inlet control occurs when the culvert barrel is capable of conveying more flow than the inlet will accept. The control section of a culvert operating under inlet control is located just inside the entrance. Cri- tical depth occurs at or near this loca- tion, and the flow regime immediately downstream is supercritical. Figure 1-13 shows one typical inlet control flow condition, Hydraulic characteristics downstream of the inlet control section do not affect the culvert capacity. The upstream water surface elevation and the inlet geometry represent the major flow controls. The inlet geometry includes the barrel shape, cross-sectional area, and the inlet edge. (table I) 7