Spatial Analysis of Flood Hazard in Ajabshir Ghaleh Chay Catchment using GIS and HEC-HMS Model

The most common type of natural hazard worldwide is flooding; as it accounts for about 40% of natural disasters (Warner, 2011: 1). The frequency and consequences of extreme flood events have increased rapidly worldwide in recent decades and climate change is likely to exacerbate this trend in the near future. The key factors for this increase in flood risk are: climate variability and extremes; global population growth and the increase in socio-economic activities in flood prone areas, together with their growing interdependency on flood protection and drainage infrastructure of which a significant part is of unknown or poor condition (Van Herk, 2014: 2). Therefore, supporting flood risk reduction strategies by increasing understanding of the spatial distribution of flood risk is of paramount importance (Makango Malcollo, 2016: 2). In this regard, the preparation of flood risk maps is an effective tool for flood risk assessment and management (Alcántara-Ayala and Goudie, 2010: 116). In the present study, the risk of flood in the Ghaleh Chay catchment, located in Ajabshir County, was analyzed spatially.

Eight important variables including elevation, slope, depth of valley, drainage density, convexity of land surface, distance from river, vegetation and land use were contributed to zoning flood risk in Ghaleh Chay catchment. Important part of the required data for flood risk zoning in the study area were obtained using digital elevation model (DEM) images. Fuzzy overlay in GIS consists of two basic steps. In the first step, the thematic layers must be transformed into dimensionless using different fuzzy functions. In the second step, the layers were overlaid using different fuzzy operators. In the present study, fuzzy gamma has the best performance, so combination with this function was selected as the final flood risk zoning map. After zoning of flood risk and identification of flood zones, in order to control and manage flood risk, it is necessary to identify the areas with the highest runoff production. In the present study, to evaluate runoff production in different areas of Ghaleh Chay catchment, HEC-HMS software and HEC-GeoHMS extension in ArcGIS software was used in order to precipitation-runoff simulation. Required data for the implementation of this model include digital elevation model (DEM), land use, soil hydrological groups, hydrographic and hyetograph data, which were provided by the Regional Water Organization of East Azerbaijan Province. This mathematical model simulates precipitation-runoff and routing processes in natural or controlled catchments.

According to the results of flood risk zoning map using fuzzy gamma, about 7.7% and 13% of the catchment area of Ghaleh Chay is in the very high risk and high risk class, respectively. Very high-risk zones are mainly located in floodplains within the main valleys of the region. These floodplains are not very developed due to the deep and narrow valleys of the region; however, they are considered the basis for the human settlements and agricultural activities. About 60% of the residential areas of the region was located in these very dangerous areas. Also, up to 24% of the agricultural lands and orchards of the region correspond to these zones. In addition, up to 26% of the residential land uses and 29% of the agricultural irrigated lands and orchards are at high-risk class.Mentioned items indicate the high risk of Ghaleh Chay catchment. The results of precipitation-runoff simulation with HEC-HMS model show that maximum 24-hour precipitation with a return period of less than 10 years does not pose much risk in the catchment area of Ghaleh Chay catchment. With increase in precipitation, the hydrological behavior of the study catchment becomes more risky; for precipitation of 32 mm, the peak discharge of the basin reaches about 128 cubic meters per second. This volume of flood can locally pose hazards to the settlements and agricultural lands along rivers. The peak discharge of the sub-basins for this volume of precipitation also increases significantly. Also, the peak discharge of the basin is more than 334 cubic meters per second when the precipitation rate is about 50 mm. This amount of discharge is significant for the relatively small basin of Ghaleh Chay and will undoubtedly pose serious risks to settlements and facilities adjacent to the river. If the variable of area is also included in the prioritization of flooding (runoff production) of the sub-basins, sub-basins 28, 12, 13, 6, 10, 5, 11 and 18 have higher flooding.

The results show that flood can be considered as one of the most serious environmental hazards in the region. According to the flood risk zoning map using fuzzy gamma, about 7.7% of the basin area has a very high risk of floods. These areas mainly correspond to the flood plains along the Ghaleh Chay River, which are the realm of habitation and human activities in the region. This increases the flood hazard in the study basin and greatly increases the vulnerability of communities living in the basin. Rainfall-runoff simulations show that precipitation with a higher return period (especially above 25 years) can cause flood discharges. The middle sub basins have high flooding due to factors such as poor vegetation, large area of group D soils, high curve number (CN), rocky outcrops, high slope, low elongation, and low concentration and delay time. In fact it, a significant portion of floods in the basin originate from these areas.