Vulnerability and flood risk management in the lower Gianh River

International audience Problems associated with flood risk definitely represent a highly topical issue in Vietnam. The case of the lower Gianh River in the Central region of Vietnam with an area of 353 km² which is regularly subject of flood risks, whose the scientific question is strongly linked to flood risk management and the evaluation of the vulnerability to limit its risks. In this study, flood risk are defined as the product/result of the hydrological risk of an event and the damages related to this event. They are determined by the combination between the hazard and the vulnerability. Furthermore, as a developing country, Vietnam often confronts to the rarity of lands. The urbanisation has encroached on agricultural lands situated in highly exposed areas, which raises the vulnerability. In view of this, this article presents first of all the first approaches of the vulnerability in the lower Gianh River to identify, characterize and organize into a hierarchy the spaces to spread the vulnerability. Then, the second part is about the flood management to orientate the risk prevention policy and to help decision makers develop a floodplain for urban planning and to reduce vulnerability of the area which are based on the risk map and the flood hazard. In addition, the vulnerability in the flood areas was caused by the flood of 2013. The area where the vulnerability is very high increased from 1656 ha in 1989 to 4453 ha in 2013, from 178 ha in 1989 to 947 ha in 2013 in high, from 8259 ha to 8520 ha in medium, and decreased from 1691 ha to 243 ha in low and from 2905 ha to 1354 ha in very low. With floods of 2013 constituted an extreme flood hazard for 11.39%, a very high for 10.60%, a high for 30.79%, a medium for 31.9%and a light for 15.32% of the flood area. Moreover, the urbanized area increased by 200 ha from 1989 to 2013 in the extreme flood hazard area; by nearly 200 ha in the very high hazard area and 800 ha in the high hazard area. Besides, according to the risk map of 2020 of twenty-seven Communities in the downstream and seven Communities in the upstream in the flood area caused in 2013: 2138 ha of the territory will be in the very high risk area, 5886 ha in the high risk area, 2766 ha in the average risk area, 1369 ha in the low risk area and 111 ha in the very low risk area.