- 1 - APPLICATION OF HYDRAULIC, HYDROLOGIC AND DIGITAL TERRAIN MODELING IN FLOOD RISK AREA MAPPING Carla Voltarelli Franco da Silva 1 , Cristiano de Pádua Milagres Oliveira 2 , André Sandor Kajdacsy Balla Sosnoski 3 , Arisvaldo Vieira Méllo Junior 4 Hydraulic and Environmental Engineering Department, University of São Paulo Av. Prof. Almeida Prado, 83 trav.2 – Cidade Universitária – 05508-900, São Paulo - SP, Brazil 1 carla.voltarelli@usp.br , 2 andre.sosnoski@fcth.br , 3 cpmoliveira@usp.br , 4 arisvaldo@usp.br ABSTRACT Current patterns of land use in Brazilian cities have intensified the difficulties faced by the urban population during flood events. In this context, a thorough analysis of these extreme events are an important strategy for identifying areas most susceptible to the impacts caused by floods and the consequent development of local urban policies aiming at proper water resources management. The purpose is therefore to apply a set of measures for assessment and treatment of the hydrologic data available, as a way of mapping the flooding areas, through the application of mathematical simulation models, combined with digital terrain modeling tools and additional data collected in field. For the application of the methodology, specific critical events with disastrous consequences that took place between the months of December 2009 and February 2010, and again in January 2011, have been chosen, located near the Atibaia River, state of São Paulo, downstream regionally important reservoirs. The hydrological analysis implies that even low-recurrence interval floods may cause potential harm and the resulting inundation mapping process reveals a consistent response to field observations, despite the lack of gauging data. Keywords: flood risk areas, hydraulics, hydrology INTRODUCTION Since the 1950s, the world urban population has been increasing and it is predicted that in 2020 this urban parcel will correspond to 56% (UNESCO, 2006). The challenge to be faced then is the suitable water resources management through supply and drainage sanitary systems and solid waste mechanisms. Urban development exacerbates the effects of extreme meteorological phenomena, leading to major social and economic impacts. In regard to flooding processes, urbanization contributes to the infiltration rate decline, increases surface runoff and accelerates the rise of hydrograph peak flows (Hirsch et al., 1990; McCuen, 1998; Rose and Peters, 2001; apud Burns et al., 2005). The situation described quite often occurs in countries with high growth rates of population concentrated in urban areas, such as Brazil. The city of Atibaia, located on the banks of the Atibaia River, State of São Paulo, Brazil, has a population of 126,603 inhabitants, 91% of whom live in urban areas (IBGE, 2011b), which represents a population increase of 46.6 % since 1991 (IBGE, 2011a). Ascendant urbanization may be followed by the occupation of riparian areas. Such improper land use might also be related to a previous period featured by below-average rainfall, observed from 1997 to 2006 (LabSid, 2007), which could have caused areas that previously belonged to the river banks and were usually affected by the natural floods of the Atibaia River to be occupied, due to the non-occurrence of great significance events for a while. Therefore, owing to the fact that the areas were in use and the precipitation conditions achieved average and higher rates, the constructions began to be affected by these average or higher flow volumes from the Atibaia River. Flood events occurred between December 2009 and February 2010, again in January 2011, and displaced 450 families giving rise to great damage to the local society (Zanchetta, 2011). The analysis of extreme events in order to identify the most susceptible areas to the impacts caused by floods is required in support of the development of local urban policies designed to properly manage the water resources. In this context, flood hazard area mapping is essential for formulating emergency plans and also for planning the regional development in the long term (Rabindra et al., 2008). According to the US Federal Emergency Management Agency (2009), river flood analysis is constituted of two steps: (1) hydrological analysis to determine the occurrence probability of events and (2) hydraulic analysis to identify the extent of the flooded area and the water levels associated with each event. The mapping process basically consists in defining the area covered by water, based on crossing data from the terrain surface and water level set for a particular event (Merwade et al., 2008). A variety of methods and programs are available for the establishment and analysis of floodplains through computer modeling and the software set developed by the Hydrologic Engineering Center (HEC) of the U.S. Army Corps of Engineers is a case in point. The package, internationally recognized for its application in river floodplain assessment and protection against flooding (Azevedo, 2002), includes the Hydrologic