A culvert, because of its hydraulic characteristics, generally increases the velocity of flow over that in a natural channel. High velocities are most critical just downstream from the culvert outlet and the erosion potential from the energy in the water must be considered in culvert design.
Culverts that produce velocities in the range of 3 to 10 feet per second (ft/s) tend to have fewer operational problems than culverts that produce velocities outside of that range. Varying the grade of the culvert generally has the most significant effect on changing the velocity, but because many culverts are placed at the natural grade of the existing channel, it is often difficult to alter this parameter. Other measures, such as changing the roughness characteristics of the barrel, increasing or decreasing the culvert size, or changing the culvert shape, must be investigated when it becomes necessary to modify the outlet velocity. Velocities less than 3 ft/s shall require a deviation from the State Hydraulics Office, thus needing approval from the RHE. Velocities more than 10 ft/s must be discussed with the RHE for potential solutions and final design exception approval by the RHE.
If velocities are less than about 3 ft/s, siltation in the culvert may become a problem. In those situations, it may be necessary to increase the velocity through the culvert or to provide oversized culverts. An oversized culvert will increase siltation in the culvert, but the larger size may prevent complete blocking and will facilitate cleaning. The PEO must consult with the RHE to determine the appropriate culvert size for this application.
If velocities exceed about 10 ft/s, abrasion due to bed load movement through the culvert and erosion downstream of the outlet can increase significantly. Abrasion is discussed in more detail in Chapter 8. Corrugated metal culverts may be designed with extra thickness to account for possible abrasion. Concrete box culverts and concrete arches may be designed with sacrificial steel inverts or extra slab thicknesses to resist abrasion. Thermoplastic pipe exhibits better abrasion characteristics than metal or concrete; see Chapter 8 for further guidance.
Adequate outlet channel or embankment protection must be designed to ensure that scour holes or culvert undermining will not occur. Energy dissipators can also be used to protect the culvert outlet and downstream property, as discussed in Section 3-4.7. Energy dissipators can significantly increase the cost of a culvert and should be considered only when required to prevent a large scour hole or as remedial construction.
Refer to HDS-5 for procedures used to calculate culvert velocities.
