My preference is to install a pipe (when water is low) that has the steepest slope that is practical and use an anti-siphon device (so it carries the most water when full). I also have an emergency spillway at an elevation above the top of the pipe that only gets used during severe flooding. Having an earthen spillway, even with rocks, can get eroded if used continuously or excessively. The pipe needs to be packed well with clay soil to prevent leaks around it. I personally spend the extra money on gasketed schedule 35 PVC sewer pipe, but many use double-wall plastic culverts. A trash guard will help prevent clogging.