HOI-AN project: study of erosion processes and protection measures for Hoi-An beaches
Collaborators: Nguyen Kim Dan (coordinator), Nguyen Thong (HCMUT/CARE), Nguyen Nguyet Minh (USTH/CARE), Nguyen Trung Viet (WRU), Rafael Almar (LEGOS)
Hoi An, located in central Viet Nam, around 25 kilometres south of Danang (Figure 1), is one of the most beautiful cities of Vietnam. The old town of Hoi An is listed as a UNESCO World Heritage site. Since October 2004, this well-known tourist site is under threat by severe erosion. Cua Dai Beach, north of the Thu Bon river mouth, has retreated by almost 200m and the river channel itself has shifted south by 250m. Several construction sites at tourist resorts were swallowed by waves, bringing work to a complete standstill. The erosion process becomes increasingly threatening, possibly for the old town itself. Current solutions for coastal erosion are temporary and would not achieve sustainable outcomes, and might in fact do the opposite.
From Tanaka et al. (2016)
A few studies (Ponsioen, 2015; Tanaka et al., 2016) have been conducted by Vietnamese, European and Japanese researchers, who proposed the following possible erosion processes:
There are eight hydropower plants in the upstream, starting from Ngoc Linh Mount in Quang Nam Province's Nam Tra My District. Hydropower reservoirs obstructs a huge volume of sand and mud, and thus decreases the sediment fluxes that go seaward. In addition, there is sand extraction at 15 sites along the Thu-Bon River operating at full capacity to supply materials to the building construction in the region. They largely modify the river morphology and decreases sand resources. As a consequence, sediment balance is lost in the coastal zone and the sediment flux deficit is suspected to be the primary cause of erosion at Cua-Dai Beach.
The sediment balance in the coastal zone is also relying on high-energy tides and waves, particularly during winter monsoon surges and typhoon periods. According to Huynh Cong Hoai and Le Duc Vinh (2015) the presence of the Cu-Lao Cham Island has a complex effect on the wave field and wave-induced circulation and transports in the coastal zone. What exactly is the effect of this complex and variable wave field is unknown.
- The sea level rise due to climate change or long-term variability is estimated to be about 30 cm at the Cua-Dai Mouth; This is probably not a primary cause for the present erosion problem but would need better evaluation.
Generally, these studies remain qualitative, based on aerial and satellite images of the site. Quantitative assessments using in-situ observations and numerical models are lacking, although they are needed in order to firmly endorse any of the proposed assumptions. This is what the present project is proposing.
My contribution (with Nguyen Nguyet Ming as postdoc) will be to provide the large scale forcing environment of the study site (waves, tides, winds), using global solutions and produce local coupled wave-current-sediment 3D model solutions (using ROMS/CROCO). This task is complementary to that of Thong Nguyen who will provide very high-resolution 2D model solutions of circulation and sediment transport for the nearshore zone and estuary (using TELEMAC2D). These models will be used to expose the alongshore and cross-shore processes at work and test changes in sediment balance associated with changes in upstream sediment sources and various coastal protection measures.
This work will build on extensive field survey and specifically deployed video cameras under supervision by Nguyen Trung Viet and Rafael Almar.