Sewer Manholes Collapse (SMC) is one of the difficult issues facing the sewerage works industry today. The system for evaluating the reasons which causing SMC in sewer system used in this paper, is based on two parameters: (i) Manhole outflow-inflow approximate hydrodynamic hypotheical model; and (ii) A real sewer system over 48-hrs extended period simulation analysis. The system applied on Balks sewer system is used for analyzing the reasons of manhole collapse as a case study. The study concluded that, the most important reasons for manhole collapse as follows: (1) Bad construction in manholes and pipe connection, (2) Leakage of fine sand and silty sand inside the manholes resulting in high ground water table, (3) Disturbance in working of pumping station, and (4) Leakage of drinking water from supply pipe lines.