Quantifying Culvert Exit Loss

According to the Federal Highway Administration’s Hydraulic Design of Highway Culverts (HDS-5) Manual and the Hydrologic Engineering Center River Analysis System (HEC-RAS) Reference Manual, the exit loss associated with a culvert discharging into a downstream channel is equal to the change in culvert and channel velocity heads or a loss coefficient multiplied by the culvert velocity head. For a short culvert, the calculated exit loss often represents the largest single system energy loss component. To investigate the apparent dominance of exit loss in outlet control culvert hydraulics, a laboratory culvert exit loss study was conducted using prototype-scale culverts, with projecting end treatments, discharging into a downstream channel where all of the channel discharge was supplied by the culvert. The experimentally determined exit losses were compared with predicted exit loss values using traditional exit loss equations and the Borda–Carnot minor loss expression, which is traditionally used to quantify energy loss at sudden expansions in pressurized pipe flow. The Borda–Carnot expression proved to be significantly more accurate, relative to traditional methods, for the conditions tested.