STUDY ON THE COASTAL EROSION PROCESS HOI AN AND THE MEASURES

The project "Study on the coastal erosion process Hoi An and the measures" was funded by the French AFD and the People's Committee of Quang Nam Province. It was set up from July 2016 under the guidance of Professor Nguyen Kim Dan (University Paris-Est, France) - who has 35 years experience in the development of 2D-3D hydrodynamic and sediment transport numerical models for Rivers and coastal areas.

The objective of the project is to investigate the mechanism behind the coastal erosion processes in Hoi An; to find solutions to coastal protection in a sustainable manner without causing negative effects on the natural landscape; to build a strong scientific base for the Hoi An Integrated Beach Management Program in the future. Many Vietnamese and international scientists have participated in this project. On the side of the Hochiminh city University of Technology (HCMUT-VNU) and the Asian Center for Water Research, HCMUT (CARE); there are the contribution of Prof. Nguyen Thong with more than 30 years of experience in the field of computational dynamics and morphological estuary (Department of Water Resources - Faculty of Civil Engineering), Prof. Huynh Cong Hoai - expert on the numerical model applied in fluviomaritime works (Fluid Mechanics Department - Faculty of Civil Engineering), Dr. Patrick Marchesiello - expert on 3D models on coastal dynamics and erosion. Dr. Patrick Marchesiello from Toulouse, France is currently working at CARE and Dr. Nguyen Nguyet Minh doing her post-doc at CARE. On May 25, 2017; a workshop on project outcomes was held in Hoi An with the participation of Mr. Dinh Van Thu - Chairman of the People's Committee of Quang Nam, Mr. Hoang Van Thang - Deputy Minister of Agriculture and Rural Development, Mr. Bertrand Lortholary - French Ambassador to Vietnam, Mr. Bruno Angelet - Ambassador of the European Union Delegation to Vietnam with many Vietnamese and international scientists and representatives of relevant ministries. The team took the cause of the erosion of the north of the coast of Hoi An as follows:

(1) Downstream sediment from the river source (due to hydroelectric dams on the river network Vu Gia - Thu Bon, more than half the amount of sediment was trapped in the Dak Mi 4 and Song Tranh 2 reservoirs, and sand mining on the Thu Bon River) when the sediment from the river giving is not enough to compensate the amount of sediment transported by waves and currents, erosion will increase

(2) Due to the construction of hotels and resorts too close to the coast, inherent exchanges between dunes on the coast and sand under - marine are lost as they are - it is the submarine sand strips are factors that reduce the impact of the waves on the shore;

(3) The flow of Cua Dai tends to drift further south. On the one hand, they conserve sediments in the estuaries and, on the other hand, transport sediments to the south, which makes the northern region more and more diminished by sediment (this difference in fluxes caused by the asymmetric morphology of the estuary)

(4) The results of the study also showed that the probability of a high wave height appear more frequently in recent years, in winter (high season waves) waves coming perpendicular to the shore have caused severe erosion

(5) Construction of the defense station increases erosion in adjacent areas. Based on these causes, the team proposed various protection measures using the numerical model and examined the effectiveness of each measure. The best measure chosen is to feed beaches in combination with dikes underwater that can both protect nature and significantly decrease waves.

The nourished beach zone extends from the Cua Đai to the Boutique Hoi An resort; with 6,500 meters in length combined with the submerged dikes at 1.5 m height and about 200 m from the coast (vary depending on depth of seabed) to keep the beach. In order to avoid local scouring around breakwater foots, rock and gravel filters should be set-up. In order to increase sediment fluxes from the Thu-Bon Estuary to the Northern coast, dredging a channel of 200 meter of wide and 8 meter of depth, along the bank of the Cua Dai Estuary (nearby Vinpearl Resort) has been proposed.

This will be a sediment resource needed for beach nourishment. In addition to protective measures, experts suggest that Quang Nam province should prohibit all sand mining along the Thu Bon River at Cua Dai Beach and around the Hoi An Sea. Sediment discharge should be prohibited from reservoir upstream of pollutants trapped in sediments in the reservoir (Due to contaminants in sediments trapped in upstream reservoirs, it is not recommended to flush sediment from these reservoirs downstream). At the same time, the prohibition of the construction of dykes to protect local property is advised because these works destabilize the entire coast of Hoi An; the construction of other buildings at Hoi An Beach should also be banned, public awareness should be raised about participation in coastal protection.   CARE-Rescif scientists in the National Workshop, Hoi An  CARE-Rescif scientists in the National Workshop, Hoi An