7.2 Effects of a Culvert

A culvert may cause an increase in upstream water surface elevations due to its restrictive cross section, similar to the effect of a bridge (discussed in Chapter 6). The opening of a culvert, however, is generally much smaller than the opening of a bridge and can therefore result in a greater increase in water surface elevations. Federal, state/provincial, and local laws often limit such increases.

Velocities through a culvert operating under open channel flow conditions are typically high unless they are reduced by a higher tailwater elevation caused by downstream effects. High-velocity flows exiting the culvert may cause scour and erosion immediately downstream, which may require an energy dissipater to control erosion.

If a culvert passes under a significant roadway embankment, the hydrologic and hydraulic effects can be similar to a dam with a conduit passing through the base. Upstream flood levels may be several feet (or meters) higher than they would be without the culvert and embankment. Higher flood levels can result in significant storage upstream of the embankment. High upstream ponding levels often require a hydrologic or hydraulic routing to properly analyze the effect of the culvert on the discharge hydrograph and to determine the correct peak discharge through the culvert. Section 7.9 discusses culvert routing.

As shown in Figure 7.3, the hydraulic design of a culvert should focus on the effects on upstream flood levels, controlling downstream scour potential, and including adequate freeboard at the roadway for the design flood. Applicable laws may also require that environmental considerations, such as fish passage, be taken into account. Additionally, the analysis of existing culverts should include an evaluation of upstream floodplain storage caused by the culvert embankment to best predict the discharge through the culvert.

Figure 7.3 Objectives for culvert placement.