Dynamic Analysis of Flood Hazard in Active Alluvial fan using the HEC-RAS Hydrodynamic Model and GIS Technique (Case Study: Leilan Alluvial fans, Northwest of Iran)

Introduction
The Leilan alluvial fan in the northwest of Iran has been exploited by humans due to its topographical, geological and climatic conditions and has established many cities (Leilan and Miandoab) and many villages on its surface. Over-use of alluvial fan capacity by its residents over many years has made the alluvial fan vulnerable to geomorphic hazards. The most important geomorphic hazards that threaten the alluvial fan are flood risk. The development of agriculture on the alluvial fan surface and river encroachment has increased the risk of flooding on the margins and floodplains of the Leilan Chai River. The flooding of the Leilan River in the spring causes a lot of damage to the farmland around the river. The Leilan alluvial fan is also at risk of flooding the Leilan Chai Seasonal River. The purpose of this study was to evaluate the flood risk dynamics at the surface of this alluvial fan. Therefore, the flood hazard assessment of the Leilan Chai River with a length of 15.4 km was carried out using HEC-RAS hydraulic model.
Methodology
The study area is located in thewestern and eastern Azerbaijan provinces, and the cities of Leilan and part of the Miandoab city are located on it. This alluvial fan is located between 36o 55′ 10″ to 37o 00′ 40″ N and 46o 06′ 17″ to 46o 17′ 17″ E. The following data, software, and methods were used to dynamically analysis flood risk and prepare flood risk maps with different return periods in the study area: - Landsat 8 OLI satellite image for 2019 (path and row 168-134). - Topographic maps 1: 20000 and 1: 25000 of the study area. - Digital Elevation Model (DEM) with 12.5 m spatial resolution. - Shirin Kand hydrometer station data including daily and monthly discharge. - HEC-RAS hydrodynamic software for simulating flood in alluvial fan surface. - HEC-GeoRAS extension to provide the required parameters for the HEC-RAS model. - ENVI software for land use mapping. - ArcGIS software to generate maps.
Results and discussion
HEC-GeoRAS extension was used in ArcGIS software to determine the geometric characteristics of the Leilan Chai River and The layers required for model implementation such as centerline, river banks (right and left bank), flood plain, cross-sections, bridges, and floodgates were prepared. Due to the floodgate on the river to perform flood simulation, the Leilan Chai River is divided into two reachs (upstream and downstream of the floodgate). In this research, after field surveys and gathering the required information, 151 cross-sections with a distance of 100 m and a width of 1000 m were created on Leilan Chai in ArcGIS software. It was also found in field surveys that in the second reach most of the rivers were occupied by humans and converted to agricultural land and as a result, the river width in this area was significantly lower than the first. Land use map and field studies were used to determine the coefficient of roughness in the bed and shores of the Leilan Chai river.
Conclusion
The present study, using the HEC-RAS hydrodynamic model, simulates the Leilan chai River flow on the Leilan alluvial fan during different return periods. The peak daily discharge data recorded at Shirin Kand hydrometer station were used to predict peak discharge values. Study of the flood maps show that flood risk does not pose a risk of flooding during the 2 and 5 year return periods of agricultural lands and residential areas around the river. But in return periods of 10 years or more, we see flooding of agricultural lands and residential areas around Leilan Chai. During the 25-year return period, 120.2 hectares of agricultural land and about 9.1 hectares of residential areas along the river are affected by the risk of flooding. The village of Mullah Shahabuddin is one of the areas developed along the river and as a result is at risk of flooding in the event of heavy rainfall. The most vulnerable areas are in the second reach due to the occupancy of the river bed by residents of the region and converting their marginal land into agricultural land and residential areas. Therefore, it is necessary to carry out a serious review of the riverbed in order to prevent accidents and the possible financial and casualties in the event of flooding in the Leilan Chai River, and that the river's privacy should not be further confiscated. It was also found in field surveys that in the second reach most of the rivers were occupied by humans and converted to agricultural land and as a result, the river width in this area was significantly lower than the first. Land use map and field studies were used to determine the coefficient of roughness in the bed and shores of the Leilan Chai river.