Among the natural disasters, flood causes heavy losses on the agriculture, fishery, housing and infrastructures. It also strongly affects the economic and social activities. Using of modern science, especially remote sensing and GIS techniques helps the planners to assess the risk map of natural hazards such as floods for a region in the shortest time possible. Area of land adjacent to a river is an rivers are appropriate locations for economic activities due to specific situations. Nonetheless, This área is situted in the danger zone are in jeopardy because of flood risk. Consequently, determining the amount of flood progress, height and features in diverse returning periods which occur as flood zoning, are so essential. The Nekaroud River, according to its basin specific situations and the Sharp transation of the slope from The mountain to the coastal plain, makes catastrophic floods in last decades which give rise to irreparable economic damages in the area. The aim of this study is preparing the flood risk map for part of the Nekaroud River.
Materials and methods
Initial data which is used in this study as well as topographic and land use map daily maximum debit statics of last 30 years of the Abloo hydrometric station ere used. Manning roughness coefficients related to the main channel of the river and floodplains were measured according to the land use map as well as field visits and also by the help of the Chow proposed table. In order to prepare the depth zoning map and the flood velocity, three steps are applied on the initial data. In the preprocessing step, so as to determine geometric and morphological characteristics of the river and floodplains such as the flow central line in the river channel, flow path lines, shorelines and cross sections, Arc GIS environment was utilized by using HEC-GeoRAS in order to determine the digital elevation model (DEM), in suitable seasons. In the hydraulic step, hydraulic and hydrological characteristics of the river flow such as cross-path structures, energy loss coefficients, boundary conditions of the studied area, the type of the flow regime, the river debit with different return periods and Manning coefficients, are defined for the hydraulic model of HEC-RAS. In this model, height, level, depth and velocity of the water in each section of the studied area for the flood with various return periods are calculated. The obtained output of the hydraulic model is defined as the GIS input. Then, by using digital depth model and the water level velocity in each of the cross sections as well as macro postprocessors of HEC-GeoRAS in the Arc GIS environment, an exchange file of the digital elevation model which can indicate the level of the flooded, is prepared. Then, by using the spatial analysis capability of the Arc GIS in combining the level of the flooded with the land elevation model, the depth zoning map as well as the flood velocity for various return periods are produced. For this purpose, the combination of hydrologic engineering center-river analysis system (HEC-RAS) and geographical information system were utilized to model the deep zone and the flood speed of this river for returning periods of 5,10, 25, 50, 100 and 200 years. In order to achieve a suitable function for flow risk zoning, the flow energy feature as a function of depth and speed changes was used. Then, the flood risk map was obtained and areas with different degrees of risk were classified and analyzed in terms of the flow energy.
Results and discussion
Obtained results indicate that the flood velocity is different in various return periods and by increasing the return period time, the flood velocity increases. The flood velocity is different in various parts of the riverbed and the floodplains. Depending on the river deviation degree as well as the riverbed and floodplains topographic conditions, the type of the surface vegetation and the Manning roughness coefficient are variable. Flood height maps for various return periods imply that the maximum flood height for return periods of 5 and 200 years are 7 and 10.5m, respectively. The river channel is unable to transfer the heavy water flow during the flood and by increasing the flood return period time, the importance of this issue increases. Results imply the vulnerability of the area against the flood by returning area of above 25 years. By increasing the returning time periods, the area of the affected location by the flood increase. By using the obtained risk function, flood affected areas were classified into 5 risk categories concluding low, medium, high, very high, excessive. In all returning periods, the maximum covered area in terms of the risk, belong to excessive risk category.
Results actually emphasize the importance of planning and protective strategies management in order to diminish flood damages. This research evidently indicates that the geographical information system can properly create a situation for preparing and analyzing the flood risk map.
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