Evaluation of Morphological Changes and Flood Hazard of Kivi Chay River Using Geomorphometric Indices and HEC-RAS Model

Floods are one of the most abundant and destructive natural disasters that every year are caused heavy losses of life and property. Due to human activity in river systems and construction in rivers, flood damage has an upward trend. At the same time, a flood is a very complex phenomenon that connects the natural environment, people, and the social systems of their organization. Kivi Chay River is one of the flood rivers in Ardabil province, which requires comprehensive studies on flood hazards and morphological changes channel. In this research, the morphology and flood risk along the Kivi Chay River located in the south of Ardabil province are evaluated. In this regard, geomorphic quantitative indices and HEC-RAS model are used.

The most important data of the present study include topographic map scale of 1: 2000 (Ardabil Regional Water Authority), Topographic map scale of 1: 50,000, geological maps scale of 1: 100,000, digital elevation model (DEM) 12.5 meter from ALOS – PALSAR satellite, Aster Satellite images 2002 (Spatial resolution of 15 meter) Sentinel Satellite images 2020 (Spatial resolution of 10 meter) and Google Earth. In this study, GIS & RS software includes Google Earth, ENVI and Arc GIS software with HEC-GeoRAS and Planform Statistics extensions was used. Also, hydrometric stations data from Istiso, Firoozabad and synoptic stations data of Khalkhal, Firoozabad were used. In order to evaluate the planform and lateral channel changes of Kivi Chay River channel, indices of curvature coefficient, central angle and transect method were used. The transect method was also used to evaluate the lateral change of the river channel. The HEC-RAS model could calculate the water surface profile in stable flow gradual variable in rivers and artificial channels in the subcritical, supercritical and complex regimes. The calculation of water surface profile carried out from one cross section to other cross, step by step, solving energy equation in standard way(HEC, 2010).

Evaluation of Kivi Chay River channel planform using morphometric indicesIn this study Kivi Chay River was divided into four reaches. In order to evaluate the planform and lateral channel changes of Kivi Chay River channel, indices of curvature coefficient, central angle were used. The average central angle for the Kivi Chay River channel was calculated to be 103.5 degrees for 2002 and 105.5 degrees for 2020. The length of the river was calculated to be 77.12 km for 2002 and 78.43 km for 2020.The average curvature index was calculated to be 1.28 for 2002 and 1.31 for 2020. -Evaluation of lateral channel changes of Kivi Chay River using transect methodThe average value of transect index in reach (1) is about 0.579 hectares. The area of Land degradation within this reach during the period 2002 to 2020 is about 6.95 hectares. In reach (2), the average change is about 0.406 hectares. Interval (3) is the most dynamic reach of Kivi Chay River in terms of lateral changes. In reach (4) the least amount of lateral changes occurred. The average lateral change of the channel along this reach is about 0.24 hectares.-Evaluation of flood risk in Kivi Chay RiverThe results show that floods with a return period of less than 10 years do not a serious risk to human communities living in the Kivi Chay River banks. These floods mainly affect the agricultural lands along the river banks. The results also show that the parts of Khalkhal City center could be affected by floods with a return period of 25 years.

The results of morphological indices show that the planform of Kivi Chay River channel is developed meanders type. However, based on geological, geomorphological and anthropogenic characteristics, there is a lot of spatial variability in the river palanform. The palnform of the river in the first reache is mainly in the control of human factors. In this reache, the lateral change average was about 0.406 hectares. The river planform in the second reache is also of developed meanders type. Over the past 18 years, the values of the curvature coefficient and the central angle have increased in this reache. In this reache, the lateral change average during the years 2002 to 2020 was about 0.406 hectares. In the Third reache, The width of floodplain is significantly increased. In this reache, the lateral change average was about 1.319 hectares. Lateral change in the fourth reache, due to the limited width of the valley and bankful materials resistance, it has the least lateral mobility. In this research, ArcGIS software was used for spatial modeling of the Kivi Chay River from the HEC-GeoRAS extension. Simulation of Kivi Chay River flood using HEC-RAS hydrodynamic model shows very high spatial variability of flood risk along this river.