Analysis of urban expansion and flood risk change in Da Nang city in Central Vietnam

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  • ヴェトナム中部、ダナン市における都市化と洪水リスクの変遷に関する検討

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Flood is widely considered to be the most hazardous, frequent, and widespread source of disaster risk throughout the world. Urban expansion into flood zone areas and the possible effects of climate change are further elevating the risk of flooding. In Vietnam, Da Nang is one of the cities with the highest rate of urbanization. However, during the last 10 years, Da Nang has been faced with flood disasters that have caused loss of life and damaged livelihoods and infrastructure, as well as disrupted economic activities. This study analyses the urbanization process and flood risk, and their relationship, using remote sensing and geographic information system techniques. Time series Landsat TM/ETM images and multi-seasonal ALOS images were analyzed to generate temporal land use/cover maps (for 1990, 2001, 2007, and 2010), which were then utilized to analyze the urban expansion process. Flow direction characteristics derived from the Aster GDEM (30 m resolution) and the past flood experiences obtained from ALOS PALSAR image was integrated to analyze and rank the potential flood hazard zones. Flood risk was then obtained by evaluating the flood hazard and demographic vulnerability with a ranking matrix in two-dimensional multiplication model. The results show that Da Nang has experienced a high rate of urbanization over the past 20 years, the approximate rate of increased built-up in the area was 220 %. The main directions of urbanization are seen in the West, Northwest, South, and Southeast and along the coastal line. The flood risk analysis represents that the most of the major high and moderate risk areas are located in the depression lowlands as well as along the banks of river channels. By overlaying expanded urban/settlements during 20 years (from 1990 to 2010) with those flood risk areas, we identified that some of urbanization have clearly invaded into the higher risk areas of flood. The potential risk revealed by such urban/settlement expansion into the relatively high flood risk areas increased from 1.9 to 3.5 % (nearly twofold) in the urbanization periods of 1990-2001 and 2007-2010, respectively.

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