ABSTRACT
Culvert flow is one of the most complex forms of hydraulics an engineer will encounter. Easy-to-understand nomographs and “cookbook” procedures can belie the reality that control of flow in a culvert can shift dramatically and unpredictably between inlet control (weir or orifice), barrel control, and outlet control, causing relatively sudden rises in headwater and, in the worst case in larger culverts, structural damage or failure due to sudden pressure surges. Certain types of culverts on certain slopes can exhibit a looped rating curve depending on whether the flow is rising or falling. Therefore, the most critical aspect of culvert design is to determine stable and predictable performance for all expected flow levels. When the type of flow is known, the well-known equations for orifice, weir, or pipe flow and backwater profiles can be applied to determine the relationships between head and discharge (Blaisdell, 1966). The good news is that modern culvert design nomographs, computer programs, and instructions are based on sound theory and extensive laboratory and field studies, and can be relied on in all but the most uncommon situations.
