Storm Sewer System Analysis in Urban Areas and Flood Risk Assessment Cvetanka Popovska, Milorad Jovanovski, Dragan Ivanoski, Igor Peševski University of Ss. Cyril and Metodius, Faculty of Civil Engineering, 1000 Skopje, MACEDONIA Abstract The urbanization changes the hydrological regime of urban streams as sites are cleared and natural vegetation is replaced by impervious cover such as rooftops, roadways, parking lots, sidewalks, and etc. One of the consequences is that more of a stream’s annual flow is delivered as storm water runoff rather than baseflow. Depending of the watershed impervious cover, the annual volume of storm water runoff can increase by up to 16 times that for natural areas (Schueler, 1995). Together with the human activities related to land use practices, improper maintenance and management, as well as climate change impacts, the flood issue is recognized as complex problem in flood control and protection measures in urban sites and associated ecosystems. This paper will present a case study on frequent flooding, hydrological analysis, and hydraulic modeling of existing storm sewer system of Prishtina airport in Kosovo. Availability, suitability, and quality of hydrological, meteorological, geomorphic, topographic, maintenance and management data are discussed. Some results from frequency analysis of storm events in the region, as well as the results obtained by hydraulic modeling using HEC-RAS are presented. Flood risk maps have been created, flood risk analysis was preformed and measures against flooding of the runway and surrounding terrain were proposed. Keywords: watershed, runoff, storm drainage system, culvert, hydraulic modeling, flood mapping, flood risk assessment Introduction The flood damage caused by heavy rainfall is one of the most important natural disasters and affects human life and social development. Moreover, the frequency of its occurrence and disaster risk are considered to increase recently with global warming. Therefore, the study on risk assessment and zoning of flood damage caused by heavy rainfall is very important to make strategies for preventing and mitigating flood damage. Risk assessment of natural disasters is defined as the assessment on both the probability of natural disaster occurrence and the degree of danger caused by natural disasters. It can be assumed that natural disasters result from the interaction of both physical impact and human and environmental vulnerability. The most appropriate procedure in flood risk assessment is: estimating of flood levels, generating flood risk maps, selecting methodology for risk assessment and zoning of flood damage. This paper will present the results from estimation of flood and flood risk mapping for the case study of Prishtina Airport in Kosovo. The main steps in flood risk mapping (Evans, Moore, Butcher, 2002) are: a) identify and prioritize “hotspot areas”, b) produce interception report on availability, suitability and quality of hydrological, meteorological and topographic data and selection of the most appropriate methodology in establishing flood levels, c) carry out hydrological analysis, including determining design flows and flood hydrographs, d) carry out hydraulic modeling on steady and unsteady state flow, e) produce flood risk maps by use of GIS based models, f) create a flood information database, and g) dissemination of flood risk information. Urban system interaction elements Flood risk assessment and mapping is extremely difficult for urban areas. Urban areas can be described by geographical position, climate, morphology, population, social relationship and economic activities. Describing the interactions between the urban system, environment and risk management is a difficult task because of the complex relationships in cities between physical, social and economic variables. In general, the urban systems gather the following five elements: nature, human, community, cores and networks (Figure 1). The nature is represents by the water, air, temperature, precipitation, land use, pollution and protection. The human element is describing by space, biological needs, emotional needs and relationships, while the community is representing by the laws, management, education, health, and economical and industrial development. Housing, public service, cultural and recreational centers are sub-elements in cores. Infrastructural systems such as roads,