Coupling GIS with HEC-RAS Model to Develop Return Period Based Flood Hazard Map: A Case Study of Lower Mekong River, Cambodia

Abstract

Over the last decade, societies have been exposed to the danger of natural events such as earthquakes, droughts, and floods that cannot be avoided completely even with modern-day scientific and technological facilities, preparedness, mitigation, and early warning systems. The most hazardous extreme natural event is the occurrence of floods not only due to the effects of intensive rainfall, but more significantly, due to human settlement in areas that could be affected by flooding such as floodplains, adjacent to riverbanks, and valleys. Flood models are essential to assess human, economic, and financial impacts of flood inundations.

In order to achieve successful measures to reduce the danger of flooding, it is essential to know the fundamental aspects of flooding, the definition of flooding, and the generation processes. This paves the way for methodological procedures to predict and prepare for possible future events and to take the precautions necessary to mitigate future events. Moreover, the coordination of emergency activities during flood events can be positively affected by the correct use of flood extent information. It is also very important for the calibration and validation of hydraulic models to reconstruct what happened during the flood and to determine and monitor the extent of flooded areas. These descriptions are either empirically, or physically based, or combined conceptual physically-based descriptions of the physical processes involved. Although, in general, the conceptualizations may neglect or simplify some of the underlying hydrologic transport processes, the resulting models are quite useful in practice because they are simple and provide adequate estimates of flood hydrographs. GIS and HEC-RAS hydraulic models are a powerful tool in river flood applications like flood hazard analysis and mapping at various scales.

This study indicates the use of flood frequency analysis integrating with 1-D HEC-RAS hydraulic model and Geographic Information System (GIS) to prepare flood hazard maps of different return periods in the Lower Mekong River, Cambodia. Log-Pearson type III, Log-Normal, Normal and Gumbel distribution were used to calculate the peak flood of multi return periods, namely, 10, 20, 50, and 100-years. The peak flood from frequency analysis is entered into the HEC-RAS model to find the corresponding flood level and extents in the study area. The model results are used in integrating with ArcGIS to generate flood hazard maps. Flood depths and extents could be identified through flood hazard maps. Nevertheless, the coupling between HEC-RAS and GIS provides a capability to flood mapping for the study along the river. The flood hazard map from 10, 20, 50, and 100-year return periods provides satisfactory samples for this process. As well as, the calibration of the resulting model requires knowledge of the study area are used NSE, RSR, R2, and PBIAS by computed using daily average flow. In short, this study is in proving the necessity of GIS and HEC-RAS hydraulic modeling before flood mitigation measures realization in the territory. Because both models are improving the prediction of flood events and flood prevention system, the aims were to reduce damage from floods and providing a better quality of life along the community river.

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