Sewer systems are undisputable part of environmental protection system. Design, construction and correct management of sewer systems eliminates or at least mitigates continual infiltration of sewage water to the environment and ground water. Verification of correct function of sewer system is done by the assessments. This study focuses on impact of terrain topology, placement and geometry of structures in area, precision and actuality of maps that serve as input data in calculation of runoff coefficients, creation of sub-catchments and the choice of model hectare. Those values have direct impact on the calculation of the amount of rain water in sewer system and thus help determine possible flooding of system. Study was conducted on existing sewer system in town of Vrable as a part of greater thesis dealing with verification of boundary conditions that serve as data in sewer system assessments.