§ 7.4.1. Culvert Hydraulics

A.

Inlet Control Condition.

Inlet control for culverts may occur in two (2) ways.

1.

Unsubmerged: The headwater is not sufficient to submerge the top of the culvert opening and the culvert inlet slope is supercritical. The culvert inlet acts like a weir (Condition A, Figure 7-3 in Appendix D of this manual).

2.

Submerged: The headwater submerges the top of the culvert but the pipe does not flow full. The culvert inlet acts like an orifice (Condition B, Figure 7-3 in Appendix D of this manual).

The discharge capacity for several culvert materials, shapes, and inlet configurations under inlet control conditions are presented in the nomographs of the Federal Highway Administration. The nomographs are recommended for use in all inlet-control culvert calculations.

B.

Outlet Control Condition.

There are three (3) types of outlet control culvert flow conditions:

1.

The headwater submerges the culvert opening, and the culvert outlet is submerged by the tailwater. The culvert will flow full (Condition A, Figure 7-3 in Appendix D of this manual).

2.

The headwater submerges the culvert opening, the culvert outlet is not submerged by the tailwater (Condition B or C, Figure 7-3 in Appendix D of this manual).

3.

The headwater is insufficient to submerge the top of the culvert opening. The culvert slope is subcritical and the tailwater depth is lower than critical depth for the culvert (Condition D, Figure 7-3 in Appendix D of this manual).

The capacity of a culvert for outlet control is calculated using Bernoulli's Equation, which is based on the conservation of energy principle. In the application of this equation, an energy balance is determined between the headwater at the culvert inlet and the tailwater at the culvert outlet. This balance is a function of inlet losses, friction losses and velocity head (See Figure 7-4 in Appendix D of this manual).

Please refer to the publications of the Federal Highway Administration for design calculations.

C.

Depths of Tailwater and Headwater.

In culverts flowing with outlet control, tailwater is an important factor in computing both the headwater depth and the hydraulic capacity of a culvert. Thus, in many culvert designs, it becomes necessary to determine tailwater depth in the outlet channel.

Much engineering judgment and experience are needed to evaluate possible tailwater conditions during storms. A field inspection should be made to check on downstream controls and to determine water stages. Tailwater is often controlled by a downstream obstruction or by water stages in another stream.

An approximation of the depth of flow in a natural stream (outlet channel) can be made by using Manning's equation in the channel with normal flow condition (see Section 6.2.1, "Uniform Flow"). If the water surface in the outlet channel is established by downstream controls, a backwater analysis is required (see Section 6.2.2, "Gradually Varied Flow").

Please refer to the publications of the Federal Highway Administration for required designs.

A series of nomographs for various culvert materials and shapes have been developed by the Federal Highway Administration and the various pipe manufacturers. The nomographs include those for inlet control conditions, outlet control conditions and the critical depth for pipes of different shapes.