Culvert location involves the horizontal and vertical alignment of the culvert with respect to both the stream and the highway. The culvert location affects hydraulic performance of the culvert, stream and embankment stability, construction and maintenance costs, and safety and integrity of the highway.

Ideally, you place a culvert in the natural channel (see Figure 8‑1). This location usually provides good alignment of the natural flow with the culvert entrance and outlet. It usually requires little structural excavation or channel work, which requires a USACE permit.

Establishing the culvert’s vertical orientation is usually a matter of placing the upstream flow line and downstream flow line elevations of the culvert at the same elevations as the existing streambed.

In some instances, the upstream flowline may need to be lowered or raised. Lowering the upstream flowline can provide an improved hydraulic operation but may create maintenance problems due to a higher potential for both sedimentation and scour. However, lowering the upstream flowline can also decrease the slope of the culvert, thereby slowing the velocity and decreasing the potential for downstream scour.

The placement of the downstream flowline of the culvert at a level higher than the roadway embankment toe of slope should be avoided. Such a configuration results in a waterfall that increases the potential for erosion.

Sometimes, extending a culvert to accommodate a widened roadway requires changing the flowline slope at one or both ends. Such a configuration is called a broken back culvert. In some cases, a broken back configuration can be designed to reduce the outlet velocity by introducing a hydraulic jump inside the culvert.