Flood is one of disasters leads to annual financial losses, especially in our country it causes several billions rials as well as hundreds victims. With increasing floods, the funding necessary to control and cope with it is increased. On one hand, because of financial limitations, the amount of funds to deal with and control flood may not be enough, so to better flood-affected areas management, to plan and to reduce the damage caused by this disaster, identification of areas prone to flood risk and to explore options to reduce flood damage with the highest efficiency is essential. In this study, Khansar catchment as areas prone to flooding was considered to be a study area to assess the flood damage. The first step in flood damage assessment is to identify and mapping high-risk areas to flood. to mapping flood, it is necessary to simulate river channel and river morphology to extract cross sections using the software HEC- GEO RAS and HEC-RAS for different return periods. Software HEC- GEO RAS is installed as a accessory to software Arc map. Second step is to assess urban damage related to various options. To this end, Hydrologic Engineering Center of U.S. –Flood Damage Assessment was used. The software HEC-FDA estimates expected annual damage based on the flooded different user, by considering two cases, with and without the implementation of flood control, using Monte Carlo simulation as a numerical integration theory. Given that this software takes uncertainty of parameters to estimate the expected annual losses into account, to involve these uncertainties, it is necessary to apply numerical integration theory. As software HEC-FDA is developed to assess urban damage but it can be used to assess damage to agriculture indirectly, so that agriculture should be introduced in residential land use with their characteristics in the software. Third step is to economic analysis of losses for various return periods. Evaluating effect of uncertainty and increasing iterations of Monte Carlo simulations show that, as damage to the flood plain decreases, uncertainty gets more importance and the more number of iterations, the less error in the Monte Carlo simulation but much error reduction rate in the initial steps will be. Keywords: HEC-FDA flood, HEC- GEO RAS, HEC-RAS, the annual expected damage, Khansar
