Dam Break Modelling, Flood Simulation and Floodplain Mapping (A case study: Bafrajard Earth Dam)

Background and Objectives Although a dam’s structure has an important role in suppressing floods and providing water for activities such as agriculture, human consumption, industrial use, electricity generation and pisciculture, its break has an immense damage and loss of life when it occurs. In order to reduce the damaging effects of the dam break on the downstream places, it is necessary to provide the flood hazard map. There are few studies on applying HEC-RAS two-dimensional model in the field of dam break analysis. For example, Hassanzadeh et al. (2019b) computed the output breach hydrograph of the Alavian dam by the BREACH model under the overtopping scenario. Also, the breach hydrograph was routed through the downstream of the river by using HEC-RAS two-dimensional model. Finally, the ArcGIS software was utilized to prepare the flood mapping. The results of their study illustrated that the Alavian dam is breached with the time of failure of 46 min and the peak discharge of about 66000 m3 s-1. Furthermore, according to the flood hazard map, most of the downstream inundation zones due to Alavian failure dam have been considered as the high hazard area.   Methodology Two-dimensional (2D) numerical simulation proved to be an important tool for understanding flood events. The HEC-RAS model is one of the most popular hydraulic models. In 2016 a new version of HEC-RAS (HEC-RAS-v5) was released including 2D capabilities. HEC-RAS software supports special capability to correct waterway and computing necessary ground-related operations. The capabilities of HEC-RAS are: 1) Developing a 2D unsteady flow model; 2) Modeling the 2D shallow water equations or the 2D diffusion wave equations; 3) Solving implicit finite volume algorithm; 4) Developing a terrain model and geospatial layers. The HEC-RAS software also enjoys connect-ability to ArcGIS software via an adaptor called HEC-GeoRAS to perform pre- and post-processing in geographical information system (GIS). The threat to personal safety and to gross structural damage caused by floods depends largely upon the speed and depth of floodwaters. The greater these factors become, the greater the danger to people and property. The depth-velocity hazard classification diagram considered in this study is based on the Garcia and Lopez (2005), in which hazard categories are broken down into high, medium and low hazard for each hydraulic category. These can be defined as: 1) High hazard: possible danger to personal safety; evacuation by trucks difficult; able-bodied adults would have difficulty in wading to safety, potential significant structural damage to buildings; 2) Low hazard: should it be necessary, truck can evacuate people and their possessions; able-bodied adults would have little difficulty in wading to safety; 3) Medium hazard: In the transition zone highlight by the median color, the degree of hazard is dependent on site conditions and the nature of the proposed development.   Findings In this research, the dam break problem has been presented from the viewpoints of importance, mechanism, spatial and temporal analyses as well as governing mathematical equations. For this purpose, the output hydrographs of the Bafrajard dam site have been computed by breach model under a fictitious dam break scenario (overtopping) as the first step. In the next step, the output hydrographs have been routed through the downstream river by using HEC-RAS model. Then water levels and wave front arrival times have been computed at all cross sections. Finally, the obtained results have been imposed to Geographic Information Systems in order to obtain the presentation of the results to develop floodplain maps.   Conclusion The results of the present study demonstrated that the Bafrajard dam is breached with the time of failure of 80 min after the upstream erosion and the peak discharge of about 4000 m3 s-1. Also, Khanghah-e Bafrajard village, Aznav tourist area and Khalkhal city are affected by floods with inundation percentages of about 35, 10 and 20%, respectively. Furthermore, the wave front arrival time at the mentioned areas is about 10, 30 and 40 minutes, respectively, so there will be very short time available for various emergency actions.