The Hydraulic Impact of the Lateral Dimension of a Flush Headwall on Culvert Inlet Loss

The Saskatchewan Ministry of Highways has developed a means of reinforcing the inlet of thin-walled pipes, such as that of corrugated steel pipe culverts, so as to minimize the likelihood of an uplift failure and also that of impact damage to the inlet from ice and other debris. For large diameter corrugated steel pipe culverts, the Ministry’s design requires the use of a concrete collar, while for smaller diameters of pipe a steel C-channel is specified. In each case, the end result is akin to that of a flush headwall of ‘modest’ dimension. The Ministry’s current design requirements are that an inlet loss coefficient corresponding to that for a projecting inlet be used, which the Ministry indicates is 0.9, regardless of the use of either the concrete or steel collar. The intent of this work is to learn what minimum dimension of flush headwall is required to allow use of the lower standard flush headwall inlet loss coefficient of 0.5. Any reduction in the inlet loss coefficient may allow for the use of a smaller diameter pipe at a particular installation or provide assurance that the factor of safety against overtopping of the embankment is greater than is currently assumed. The work presented in this study briefly summarizes the findings of a physical model study used to assess the minimum dimension of lateral extension required to yield an inlet loss coefficient typically adopted for a standard flush headwall.