The two basic culvert design criteria are allowable headwater and allowable velocity. Similar to the allowable headwater, the allowable outlet velocity is a design criterion that is unique to each culvert site. Allowable headwater usually governs the overall configuration of the culvert. However, the allowable outlet velocity only partially governs the overall culvert configuration but is the governing criterion in the selection and application of various downstream fixtures and appurtenances.

The velocity in the culvert is likely to be higher than that in the channel because the culvert usually constricts the available channel area. This increased velocity can cause streambed scour and bank erosion in the vicinity of the culvert outlet. There may also be eddies resulting from flow expansion. It is important to control the amount of scour at the culvert outlet because of the possibility of undermining of the headwall and loss of support of the culvert itself. Bank erosion may threaten nearby structures and may also disrupt the stability of the channel itself.

Scour prediction is somewhat subjective because the velocity at which erosion will occur is dependent upon many variables such as the characteristics of the bed and bank material, depth of flow in the channel and at the culvert outlet, velocity, velocity distribution, and the amount of sediment and other debris in the flow. Scour developed at the outlet of similar existing culverts in the vicinity is always a good guide in estimating potential scour at the outlet of proposed culverts.

Scour does not develop at all suspected locations because the susceptibility of the stream to scour is difficult to assess and the flow conditions that will cause scour do not occur at all flow rates. At locations where scour is expected to develop only during relatively rare flood events, the most economical solution may be to repair damage after it occurs.

At many locations, use of a simple outlet treatment (e.g., cutoff walls, aprons of concrete or riprap) will provide adequate protection against scour. At other locations, adjustment of the barrel slope may be sufficient to prevent damage from scour.

When the outlet velocity will greatly exceed the erosive velocity in the downstream channel, considerations should be given to energy dissipation devices (e.g., stilling basins, riprap basins). It should be recognized, however, that such structures are costly, many do not provide protection over a wide range of flow rates, some require a high tailwater to perform their intended function, and the outlet velocity of most culverts is not high enough to form a hydraulic jump that is efficient in dissipating energy. Therefore, selection and design of an energy dissipation device to meet the needs at a particular site requires a thorough study of expected outlet flow conditions and the performance of various devices. The cost of dissipation devices may dictate the design that provides outlet protection from low-frequency (high AEP) discharges and accepts the damage caused by larger floods. See Section 5 Velocity Protection and Control Devices.

Velocities of less than about 3 fps usually foster deposition of sediments. Therefore, 3 fps is recommended as a minimum for culvert design and operation.