LOCAL SCOUR DOWNSTREAM OF BOX-CULVERT OUTLETS

This laboratory study investigates the local scouring phenomenon in sand that occurs downstream of box culverts. Although culvert hydraulics can include special cases, such as culverts on hydraulically steep slopes for which special energy dissipators at an outlet must be provided to handle supercritical flows, this study was restricted to culverts on horizontal or mild slopes that operate freely and do not require special energy dissipators at their outlets. The principal factors governing this form of local scouring were found to be the discharge rate, the culvert width, the tailwater depth, the downstream channel width, and the bed-material properties. The experimental program investigated the effects of these principal variables on the local scour hole characteristics. The investigation also reviewed some well-known empirical formulas for the prediction of maximum scour depth, under a variety of hydraulic conditions. One such relationship for maximum scour depth in a stone bed downstream of a culvert outlet, was modified in this study to make it applicable to scour in sand beds.