As stated use a continuous model, like HSPF. Also don't forget about the possibility of groundwater input/output and evaporation. SWAT or BASINS may also be models that you might want to look into.
As a general rule of thumb, try and locate the spillway over native material and not over the principal spillway. The embankement will likely have differences in compaction that could cause weak points, and will tend to erode more quickly. The embankment can also have differential settlement...