Full and partly full flow conditions in culverts

Articles > Full and partly full flow conditions in culverts

A culvert barrel may flow full over all of its length or partly full. Full flow in a culvert barrel is rare. Generally, at least part of the barrel flows partly full. A water surface profile calculation is the only way to accurately determine how much of the barrel flows full.


Full Flow

The hydraulic condition in a culvert flowing full is called pressure flow. If the crosssectional area of the culvert in pressure flow were increased, the flow area would expand. One condition which can create pressure flow in a culvert is the back pressure caused by a high downstream water surface elevation. A high upstream water surface elevation may also produce full flow (figure below). Regardless of the cause, the capacity of a culvert operating under pressure flow is affected by upstream and downstream conditions and by the hydraulic characteristics of the culvert.


Partly Full (Free Surface) Flow

Free surface flow or open channel flow may be categorized as subcritical, critical, or supercritical. A determination of the appropriate flow regime is accomplished by evaluating the dimensionless number, Fr, called the Froude number:
Fr = V/ (g yh)0.5
In this equation, V is the average velocity of flow, g is the gravitational acceleration, and yh is the hydraulic depth. The hydraulic depth is calculated by dividing the cross-sectional flow area by the width of the free water surface. When Fr > 1.0, the flow is supercritical and is characterized as swift. When Fr < 1.0, the flow is subcritical and characterized as smooth and tranquil. If Fr = 1.0, the flow is said to be critical.

The three flow regimes are illustrated in the depiction of a small dam in figure below. Subcritical flow occurs upstream of the dam crest where the water is deep and the velocity is low. Supercritical 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 conditions, a point of known depth and flow (control section) must first be identified. A definable relationship exists between critical depth and critical flow at the dam crest, making it a convenient control section.

Identification of subcritical or supercritical flow is required to continue the analysis of free surface flow conditions. The example using the dam of figure above depicts both flow regimes. Subcritical flow characteristics, such as depth and velocity, can be affected by downstream disturbances or restrictions. For example, if an obstruction is placed on the dam crest (control section), 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.

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