Hydraulic Design of Highway Culverts - DOT On-Line Publications

Figure I-13--Typical Inlet Control Flow Section
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, subcritical, 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. Critical depth occurs at or near this location, and the flow regime
immediately downstream is supercritical. Figure I-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 1).
b. Outlet Control. Outlet control flow occurs when the culvert barrel is not capable of conveying
as much flow as the inlet opening will accept. The control section for outlet control flow in a
culvert is located at the barrel exit or further downstream. Either subcritical or pressure flow
exists in the culvert barrel under these conditions. Figure I-14 shows two typical outlet control
flow conditions. All of the geometric and hydraulic characteristics of the culvert play a role in
determining its capacity. These characteristics include all of the factors governing inlet control,
the water surface elevation at the outlet, and the slope, length, and hydraulic roughness of the
culvert barrel (Table 1).
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