[ACCESS RESTRICTED TO THE UNIVERSITY OF MISSOURI AT REQUEST OF AUTHOR.] Modeling rainfall runoff can help to understand what is happening throughout the system, how can control of water to prevent flood, and how much the quality of runoff can change in urban area. Modeling sewer system help decision maker to indicate best strategy to prevent flooding, reduce runoff pollution, reduce cost of wastewater treatment and determine best suitable LID to an urban area. This dissertation first starting with analysis the complexity of model necessary to get accepted result in term peak flow, runoff volume, numbers of flooding nodes and time to peak. Modeling requirements may lead to system simplification, as in limiting the size of the pipes included in the analyses. Researchers analyzed the combined sewer system of the urbanized Town Fork Creek catchment in Kansas City, Missouri using PCSWMM to assess the potential impact of the simplifications on hydraulic results.