Culverts by their nature can be quite tricky to analyze, model, & design... there are an array of variables to consider (headwater and tailwater conditions, inlet and outlet configurations/losses, friction loss through the pipe itself, etc.), and depending on these the culvert may behave more like an open channel or more like an orifice, may have a hydraulic jump or not, etc. Even in practice with computer programs like HY-8 and HEC-RAS to do the heavy number crunching, a lot of simplifying assumptions are very much the norm.
I agree that the CERM does a fairly good job in covering this topic, though rest assured that if there is anything culvert-related in the a.m. breadth portion, it's most likely going to be a simple definition or concept type question. "Culvert design" is a bit of a stretch for an a.m. syllabus topic as I wouldn't (and the good people at NCEES shouldn't) expect a non-hydro engineer to be able to "design" a culvert.