3.3.2 Vertical Profile
Most culvert profiles should approximate the natural streambed. Other profiles may be chosen for either economic or hydraulic reasons. Slopes other than that of the natural stream, can be used to prevent stream degradation, minimise sedimentation, improve the hydraulic performance of the culvert, shorten the culvert, or reduce structural requirements. Culverts are typically smoother than the natural channel and hence an increase in flow velocity is expected. Careful design and detailing is required to control steam erosion and deposition. Slope alterations should, therefore, be given special attention to ensure that detrimental effects do not result from the change.
Culvert grading options are shown in Figure 3.3. The designer should check whether energy dissipation is required so that damage will not occur upstream or downstream (see Section 3.13 – Culvert Outlet Protection).
The culvert should be designed to:
- suit the outlet conditions even if inlet conditions have to be modified (e.g. a drop inlet to reduce potential scouring velocities through the culvert). High outlet velocities can cause erosion for a significant distance downstream of an outlet. Where high outlet velocities are expected, appropriate dissipation measures and/or protection measures will be required
- be free from sediment deposits, which tend to occur on the inside of stream bends, or where there is an abrupt change from the stream slope to a flatter grade in the culvert.
Source: Adapted from VicRoads (2003).