Energy is required to force flow through a culvert. Energy takes the form of an increased water surface elevation on the upstream side of the culvert. The depth of the upstream water surface measured from the invert at the culvert entrance is generally referred to as headwater depth.

The headwater subtended by a culvert is a function of several parameters, including the culvert geometric configuration. The culvert geometric configuration elevation consists of the number of barrels, barrel dimensions, length, slope, entrance characteristics, and barrel roughness characteristics.

Selection of a design flood and allowable design headwater elevation are restricted by the potential for damage to adjacent property, damage to the culvert and the roadway, traffic interruption, hazard to human life, and damage to stream and floodplain environment. Potential damage to adjacent property or inconvenience to owners should be of primary concern in the design of all culverts. By definition, the allowable headwater associated with the design discharge must also be below the roadway, that is, the roadway must be passable. Other possible critical elevations on the highway itself include edge of pavement, sub-grade crown, and top of headwall. In addition, the allowable change in headwater of the 1% AEP should be limited to 1.0 foot if at all practicable. For roadways encroaching on a FEMA-designated floodplain or Special Flood Hazard Area, refer to Chapter 5 for information on FEMA NFIP criteria and procedures.

Culvert installations under high fills may present an opportunity to use a high headwater or to attenuate flood peaks. The possibility of catastrophic failure should be investigated prior to considering deep ponding because a breach in the highway fill could be quite similar to a dam failure:

Culverts should be located and designed for the least disruption of the existing flow distribution. Culvert headwater study should include verification that watershed divides are higher than design headwater elevations. If the divides are not sufficiently high to contain the headwater, if at all possible, culverts of lesser depths or earthen training dikes should be used to avoid diversion across watershed divides. In flat terrain, watershed divides are often undefined or nonexistent.