The installation of a culvert through a highway embankment may significantly constrict the floodplain. Therefore, pre-construction data should be collected to predict the consequences of the culvert alteration. Refer to Chapters 4, 5, and 7 for information on site surveys and data collection, hydrology, NFIP criterion, and channel properties.

The longitudinal slope of the existing channel in the vicinity of the proposed culvert should be determined in order to establish culvert vertical profile and to define flow characteristics in the natural stream. Often, the proposed culvert can be positioned at the same longitudinal slope as the streambed.

The existing channel must be evaluated for downstream obstructions that will affect the tailwater. Obstructions may include a narrowing of the channel or another roadway crossing or railroad crossing. Other phenomena which may affect the tailwater may be represented by a high roughness coefficient such as vegetation or excessive sinuosity, usually expressed as Mannings "n" (See Chapter 6, Roughness Coefficients). Other sources may include a decreasing channel slope, or water backed up from another source. The tailwater elevation will affect culvert capacity under outlet control conditions or may even drive a culvert into outlet control which may otherwise operate as inlet control.

The storage capacity upstream of (behind) the culvert may need to be considered, especially if the culvert is to be altered in a way that may increase the headwater.

The channel must be scrutinized downstream for adequate capacity, especially ifa culvert will be replaced with a larger barrel. Increased flow through the larger barrel could be perceived as causing flooding that had not occurred before. Although rare, the situation has been known to occur.