The depth of water that exists in the channel downstream of a culvert is referred to as the tailwater and is shown in Figure 3-1 above. Tailwater is important because it can affect the depth of headwater necessary to pass a given design flow. This is especially true for culverts that are flowing in outlet control, as explained in HDS-5. Generally, one of three conditions will exist downstream of the culvert and the tailwater can be determined as described below:
• If the downstream channel is relatively undefined and depth of flow during the design event is considerably less than the culvert diameter, the tailwater can be ignored. An example of this might be a culvert discharging into a wide, flat area. In this case, the downstream channel will have little or no impact on the culvert discharge capacity or headwater.
• If the downstream channel is reasonably uniform in cross section, slope, and roughness, the tailwater may affect the culvert discharge capacity or headwater. In this case, the tailwater can be approximated by solving for the normal depth in the channel using Manning’s equation as described in Chapter 4.
• If the tailwater in the downstream channel is established by downstream controls, other means must be used to determine the tailwater elevation. Downstream controls can include such things as natural stream constrictions, downstream obstructions, or backwater from another stream or water body. If it is determined that a downstream control exists, a method such as a backwater analysis, a study of the stage-discharge relationship of another stream into which the stream in question flows, or the securing of data on reservoir storage elevations or tidal information may be involved in determining the tailwater elevation during the design flow. If a field inspection reveals the likelihood of a downstream control, contact the State Hydraulics Office for additional guidance.
