Hydraulic Design of Broken-Back Culverts

A broken-back culvert has one or more breaks in profile slope and is most often used in areas of high relief and steep topography. The usual broken-back configuration begins with a relatively flat slope followed by a steep slope and a very flat "runout" section. Because of the steeply sloped portion of the culvert, outlet velocities in the absence of a hydraulic jump are very high. Past designs have simply assumed that a hydraulic jump will form in the runout section. Until recently there have been no adequate design tools to address this unique culvert. A computer program called BCAP (Broken-back Culvert Analysis Program) allows for the design of such culverts. BCAP is based on the energy equation (direct step method) and momentum equation (sequent depth equation) for classical jumps. Algorithms compare the subcritical sequent depth inside the culvert to the tailwater elevation — if the tailwater elevation is higher, a hydraulic jump will occur. If a jump forms, the location and length jump length are summarized. BCAP is a Windows-based program that uses the same equations found in HY8 for predicting headwater elevations. Water surface profiles are plotted for the entire culvert length for ten discharges. Rating curves for headwater, outlet velocity, and outlet depth allow the designer to determine the range of flows for which hydraulic jumps occur. If desired, the material in the runout section may be altered (e.g. to increase the frictional losses) to increase the likelihood of hydraulic jump formation. An optional feature is to link directly to the HY8 energy dissipation module, usually necessary if a hydraulic jump does not form. BCAP results have been tested in a laboratory flume and applied to several projects in the Midwest.