Most culverts inlets are standard concrete flared end section. But I cannot figure out which HY8 inlet category this fits into. Is it a conventional inlet or a side tapered inlet?
Standard precast type inlets are all considered conventional.
Side and slope tapered inlets are very specific design situations using very specific design standards. Thesze topics are covered in various federal (FHWA) publications whihc are free to download from the FHWA website.