Preparing Flood Hazard Mapping by Using Hydraulic Model and Geomorphologic Data Set- Case Study “Ferizy Alluvial fan”

Due to the uncertainty predicting the location of flow path on active alluvial fans, alluvial fan floods could be more dangerous than riverine floods. Utilizing the alluvial fan characteristics, the United States Federal Emergency Management Agency (FEMA) developed a method to assess the flood risk of alluvial fans. The FEMA guidelines allow for a number of delineation methodologies that include geomorphological method, one – and two- dimensional fixed-bed hydraulic modeling, and composite methods that combine engineering and geological approach. The United States Federal Emergency Management Agency (FEMA) used a simple stochastic model named FAN for this purpose, which has been practiced for many years. In fact, the first attempt to address the alluvial fan flood complexities was performed by Dawdy, who developed a probability- based model. The model was based on a mathematical formation that was developed after a series of catastrophic alluvial fan floods and debris flows in the 1970s. FEMA applied directly this approach directly in a number of alluvial fan flood plain delineation studies in the 1980s, and thereafter, the FAN model was developed. The FAN model is a DOS-based software package that uses Dawdy's basic equations as well as a modification proposed by DMA consulting Engineers to predict flow depths and velocities on alluvial fans, on a regular basis. Also, it has been found that geological and geomorphological data have great impact on estimation of alluvial fan flooding. To sum up, geological maps could be a powerful tool for better analyzing flood hazard on alluvial fan, due to incorporating the effects of erosion and sedimentation. In fact, using geomorphological analysis can provide a context for: (i) understanding the basic system process, (ii) realizing the past up to present, and (iii) calibrating or verifying hydraulic modeling results. The goal of this study was to examine the applicability of overlaying layers of land/ground susceptible to erosion, water erosion potential, and hydraulic flooding zones in alluvial fans with the least needed data. This study was conducted on a fan located in Iran using a combination of the FAN model, the hydraulic portion of the HEC-RAS, and geomorphological information. The Ferizy fan in Khorasan Province, which is located in arid regions in the eastern part of Iran, was considered for this study. In this paper, a new method is introduced and applied for each data layer and the results are discussed. Initial stages included three steps: (a) identifying the alluvial fans landforms, (b) determining the active and inactive areas of alluvial fans, and (c) delineating 100-year flood within these selected areas. This information was used as an input in the mentioned three approaches of the (i) HEC-RAS model, (ii) geomorphological method, and (iii) FAN model. In this study, three steps have been followed to achieve the goal: 1. The FAN model was established using the 100 year return period hydrograph and the average slope which was extracted from topographical maps; 2. The HEC-RAS model was executed by using the 100-year return period hydrograph and topographical maps; 3. Active and inactive areas were distinguished by considering topographical maps, field investigations and geologic data; 4. Thereafter, all the models, results, and field investigations were georeferenced and overlapped in GIS as separated layers and each layer was appointed a score using a scoring system; 5. Finally, the score of layers was multiplied and afterwards the pixels with the highest value and pixels with a score of 0 were considered to be the highest and the lowest hazard zones, respectively. Thereafter, the results of each model were obtained and geographical information (GIS) layers were created and overlaid. Afterwards, using a scoring system, the results were evaluated and compared. In this paper, a simple but effective solution to estimate flood hazard is proposed. The present approach suggests a combination of the FAN model, geomorphological approach and a simple portion of the hydraulic model of HEC-RAS. It was concluded that the integrated method proposed in this study is superior at projecting alluvial fan flood hazards with minimum required input data, simplicity, and affordability, which are considered the primary goals of such comprehensive studies. These advantages are more highlighted in underdeveloped and developing countries, which may well lack detailed data and financially cannot support such costly projects. Furthermore, such a highly cost-effective method could be greatly advantageous and pragmatic for developed countries.