Culvert Design Considerations

The structural choice of a culvert and corresponding inlet is based on environmental considerations, risk to property, cost of construction and maintenance and also aesthetic considerations. The capacity of an existing culvert can be increased with an improved inlet.

Culvert Inlet Design

An improved inlet serves to funnel the flow into the culvert to remove the point of control from the face of the inlet to a throat located downstream from the face. The normal contraction of flow is included in the transition from the face to the throat. An improved inlet may be economical if the culvert is operating under inlet control, but not if the culvert is operating under outlet control.

An improved inlet may offer the advantage of increasing the capacity of an existing culvert that has become inadequate because of changes in the watershed which have increased the discharge to the culvert. However, improving a culvert inlet is not recommended in the following situations:

Available design procedures cannot accommodate an improved inlet when the face of the inlet is skewed to the flow entering the inlet.
Heavy debris loads could pass through the inlet entrance and become lodged further in the culvert due to the restriction at the throat.
The flow reduction at the throat may cause the culvert to flow as outlet control, which would negate any advantages of the improved inlet.
Improved inlets are usually costly to construct when compared with standard inlets.

Careful consideration should be given before selecting and using an improved inlet design. Guidelines for design can be found in FHWA publication, Hydraulic Design of Highway Culverts, HDS-5.

The recommended types of improved inlets are top-tapered inlets, side-tapered inlets, and slope-tapered inlets.

Top-Tapered Culvert Inlet

A simple transition of depth in a rectangular box culvert may improve the hydraulic efficiency. If the box culvert is operating under inlet control, the barrel of the culvert is more hydraulically efficient than the entrance geometry. The designer may reduce the barrel depth in transition from the original depth to a minimum of 1.0 feet (0.3 m) greater than the uniform depth of flow. The transition length should be a minimum of 20 feet (6 m) as shown in Figure 1, below. This method is arbitrary, and care should be exercised when the culvert is operating in inlet control.

In terms of design and construction, top-tapered transition inlet is effective, economical, and simple to construct. This type inlet improvement is desirable when designing a multiple barrel box culvert. Other inlet improvement types are not feasible for multiple barrel box culverts because of the need to taper or flare the sidewalls of the barrels.

Side-Tapered Culvert Inlet

Side-tapered inlets involve a widening of the face area of the culvert by tapering the sidewalls. Such inlets have two possible control sections as shown in : the face and the throat as shown in Figure 2. Maintain control at the throat for the design discharge in order to realize significant cost savings in the culvert barrel. This type of improvement is similar in operation to the flared inlet for pipes.

Slope-Tapered Culvert Inlet

The slope-tapered inlet incorporates the efficient flow characteristics of side-tapered inlets with a concentration of more of the total available culvert fall at the throat control section. Figure 3 shows a slope-tapered inlet. Generally, slope-tapered improvements are not practical for pipe culverts because of their complexity.

Some of the drawbacks of slope-tapered inlets are as follows:

Slope-tapered inlets have a tendency to allow sediment deposition; this can result in maintenance problems.
The degree of the slope taper is limited by how flat the remaining portion of pipe can be made without resulting in a mild slope.
The use of slope-tapered inlets can increase costs of structural excavation because of the lowering of the upstream end of the culvert.