RESUMEN
In recent decades, flood occurrences result in large casualties in Iran. Flood plains include at risk fertile farm lands. Flood map can be very useful in flood plains management studies. Nowadays, these maps are considered as basic and vital information for civil plans and assessed carefully to avoid losses before any investments or execution of developing plans in flood plains. Several factors can be considered in speed, depth and force of water stress during flood is one of them. Since there is no simple theorem relation between water cutting stress, slope and speed, experimental equations can be useful. Manning equation is used to calculate water speed as well as anticipate hydraulic behavior of rivers. There are various approaches to determine coefficient of [n]. In Manning Method those coefficients which are dependent to river situation like grain of river-bed, plant coverage and river route curvature degree obstructs and varies. Manning coefficient varies in a river section with plant coverage variation in different seasons. This variation may affect on water speed distribution, water height and flood area in different return periods. Also, inaccuracy in Manning coefficient estimation can result in incorrect answers. Tite et al., proposed an approach to increase output accuracy of HEC-RAS software in geographic data system by matching land mapping data, river geometry and control structures with available land model in geographic data system. Motiie et al, mapped Sefid-Roof river flood by HEC- GeoRAS and HEC-RAS. Andem in his thesis named [Comparison of Jungle and Non-jungle Rivers Regimes], used HEC- GeoRAS and HEC-RAS to study speed variations and descent numbers of the two types of rivers. Also, he compared effects of plant coverage on regimes and physical behavior of flow, finally he conclude that HEC-RAS can be useful in producing appropriate values to study regime and the other hydraulic characteristics of river. Islam and Kimitro used remote sensing and GIS data to produce flood risk map of Bangladesh in 1988. Yazdandoost and Nasiri determined at risk regions of Islamshar with different return periods. Safari mapped at risk areas of Neka river in east of Mazandaran province. Various studies performed to determine flood areas in different return periods in Iran and other countries, but programming to overcome natural hazards should be based on the most acute situations to be effective. The present study is performed to assess Manning coefficient season effect on flood hydraulic behavior anticipation