§ 7.3.0. CULVERT DISCHARGE VELOCITIES

Placement of a culvert crossing in a channel produces rapid changes in flow regime that can present erosion hazards both upstream and downstream of the culvert location. Design of the culvert should incorporate features that lessen these impacts to the receiving channel to the greatest extent possible. The following concepts should be considered in the design process. These erosion hazards are discussed in greater detail in the Federal Highway Administration (FHWA), Hydraulic Engineering Circular No. 14, "Hydraulic Design of Energy Dissipaters for Culverts and Channels":

• Whenever possible, the culvert axis should match the natural channel alignment upstream and downstream. Matching the channel alignment with the culvert axis prevents a bend in the channel that would be subject to erosion.

• Depressed entrances should be avoided. If capacity or depth of cover forces the use of a depressed entrance, the upstream apron should be designed to prevent progressive degradation of the upstream channel. Additionally, the potential for deposition should be considered.

• The local conditions within the channel reach should be evaluated to determine if there is degradation present. In a degrading channel, headcuts can migrate upstream and compromise the integrity of the culvert. If channel degradation is anticipated, the design should accommodate for future erosion.

• At a culvert outlet, exit velocities should be minimized to the greatest extent practical. Channel erodibility and local scour potential should be evaluated and taken into account. Due to the dynamic nature of alluvial channels, a flexible armoring system is preferred in streams subject to erosion. The City of Austin Standard 508S-20 (Stormdrain Outfall Protection, Culvert Under Roadway/Inline) may be used for protection of the receiving channel from erosive forces. Where degradation is expected, the volume of riprap should be increased to account for loss of base level downstream of the culvert outlet. The rock riprap used in this standard detail must be designed by the engineer for the specific hydraulic conditions present based on ECM 1.4.6(D). Where energy dissipation is needed at a culvert outlet, a rock riprap basin is preferred over rigid structures (see HEC-14 for design guidelines).

• If multiple boxes or culverts are necessary, different flowline elevations for each structure should be evaluated. The culverts in the center of the channel should be lower to match with the existing natural channel with the outlying culverts raised up to more closely coincide with the natural channel "terrace" elevation. This will help to minimize sedimentation in the outlying culverts, help preserve the integrity of the channel system, and reduce maintenance costs. A culvert may be depressed below the channel flowline, such that a natural channel bottom is maintained, as long as n-values and cross-sectional area are adjusted appropriately when modeling conveyance.

Riprap armoring is preferred; however, if site conditions necessitate a concrete apron, the minimum apron length which provides transition from a culvert outlet to an open channel shall be calculated from the following equation:

L = 0.2 VD (Eq. 7-2)

Where,

L = Apron length, feet

V = culvert discharge velocity, ft/sec

D = height of box culvert or diameter of pipe culvert, feet