The Chair handles all grade complaints from students. The student will be urged to try to reach resolution with the faculty member first. If agreement cannot be reached, the student will then be asked to put the complaint in writing. At that point, the Chair usually rules on the allegation. If the Chair thinks there is reason to investigate the complaint, a grievance committee of three faculty members will be appointed by the Chair to hear both sides of the conflict.
Recent experience with grade complaints strongly suggests that you should put down in writing on your syllabus every factor that will affect students' grade, including exactly what percentage of the grade each assignment will be worth. You should be especially explicit about how portfolio grading will be done, since many recent complaints have concerned student confusion over work that was not graded at first but eventually counted in the final grade.