Applicability of Stormwater – River Flood Combined Model in Low-Lying Delta: Disaster Survey in Hoi an and Model Assessment of 2017 Typhoon Damrey

Abstract

Flood maps are an indispensable tool for risk assessment in order to assist the protection of population and economical assets. In 2017, Typhoon Damrey triggered extensive flooding in the South-central coast of Vietnam. According to NASA, the typhoon caused maximum wind speeds of 130 km/h, triggering in total precipitations of up to 500 mm. The authors attempted to understand the characteristics of the flooding in the historical city of Hội An through a field/interview survey of the flood extent and inundation depth. Survey results show water depth up to 2.9 m from ground level at certain locations close to the Thu Bồn River. Risk awareness and evacuation behavior questionnaires were also conducted by enquiring local residents. More than half of the population did not evacuate despite high water depth, though more than half of the respondents had previous experience of flood disaster before the Damrey. About three quarter of the interviewees felt a strong fear of flood and took preventive measures against potential disasters. Computer based hydrodynamics modelling are the most cost-efficient tools to estimate flood extent and depth where hydrological data measured by ground sensors are not available. HEC-RAS software is used for modeling estuarine one-dimensional hydrodynamic unsteady flow based on Saint-Venant equations, coupled with rainfall-runoff for the city of Hội An, Vietnam. Flood extent and inundation depth results are validated using data from the field survey and gauge station. This study confirms that the HEC-RAS model combined with runoff is applicable to flood / inundation simulation induced by a typhoon even in a low-lying delta such as Thu Bồn river basin.