Determining the Flooding Zone Using GIS and HEC-RAS Hydraulic ModelCase Study: Goharrood River, Rasht

Flood zoning determines flood advance, elevation, and characteristics in different return periods. Flood zoning tends to subdivide all areas around the river and flood plains into different hazardous areas in order to control land use and land development. These areas are used to determine land use, identify the area in flood insurance, and establish mandatory restrictions in hazardous areas. Rasht, with an annual precipitation of nearly 1,400 mm, is one of the rainiest areas in Iran. In high intensity and in short time, these precipitations cause river flood in ranges of the river and flooded areas of the city. Flooded streets, houses and urban roads, slowing down of vehicle movements, destruction of urban facilities and many other problems are all results of these precipitations. Using mathematical models, digital elevation models (DEMs), GIS software and HECGeo-RAS extension, flooding zone map is discussed in a part of the main range of the Goharrood River at the beginning of the entrance to its outlet out of Rasht. The studied area was a range of the Goharrood River, one of the main branches of the Pirbazar River in Rasht, north of Iran. According to objectives of the study and investigation of the flood zone at the Goharrood riverside, the data included hydrometric statistics of the Lakan station during the statistical period of 1989-2013 (25 years) and river hydrometric characteristics using a 1: 2000 map and field surveys and layers were required to plot a flood zoning map. Using mapped data of the area, including river planning maps at a scale of 1: 2000, bed conditions, such as main flow line of the river, sides and cross sections, etc., geometric data required for simulation determined by TIN map was obtained; then, data were inserted into the HEC-RAS model. Subsequently, flow data and boundary conditions were included in the system and hydraulic calculations were carried out. The results were presented in the form of input formats into the GIS environment and the necessary processing was done using HEC-GeoRas extension. Finally, maps of water depth, water velocity, shear stress and flow power along the river range were plotted. These maps were presented in both GIS and Google Earth for better clarity. These maps precisely presented flood zone with different return periods. Moreover, the ranges of the river in which flood spread and caused watering were identified with the depth of watering and areas with low, medium and high risk of flood. To determine flood status of Rasht and Pirbazar Basin, a precipitation map of Rasht was plotted in GIS environment. The results show that the average annual precipitation is 1350 mm in Rasht which can cause flooding and problems such as flooding of streets due to precipitation regime and severe rainfall. Using statistics of the Lakan hydrometric station, maximum instantaneous discharges were determined by Smada software; after analyzing the distribution of Log Pearson Type III, the best statistical distribution was determined to estimate maximum instantaneous discharges in different return periods. Using geographic information system (HEC-GeoRAS), the river planning maps were plotted for physical model of the bed and riversides. Based on mathematical simulations performed by HEC-RAS software and results of the obtained data, the model was developed to determine hydraulic conditions. For this purpose, sections of the river were first identified in the urban environment by considering longitudinal and transverse profiles of the river as well as field observations. Next, the sections were modified according to the shape of adjacent sections and engineering judgment. Finally, the model was run for this condition and the results and output of hydraulic parameters of the model were obtained for discharge with a return period of 50 years. The files generated in the GIS environment using the HECGeoRAS extension included flood maps with a 50-year return period of discharge, indicating a river flood in a 500-meter range in the northern part of the city called Siah Estalakh. This hazardous area is located where the river passes the Shohadaye Gomnam Blvd. Extreme changes in the river's width in this area indicate a flood in this part of the river range. Plotting of flood zoning maps for identifying hazardous areas is one of the first tasks of responsible organizations to deal with floods. Data analysis and results of the model show that the only part of the 15 km range of the Goharrood River which is at risk of flooding with a 50-year return period of discharge is a 500 m range of the river after the Shohadaye Gomnam Blvd northward in Siah Estalakh. Considering land uses of the area, flood-exposed regions were identified as low-risk, moderate-risk and high-risk regions. Thus, any structural attempt on this river should be prioritized in this area.