Author(s): Doan Nguyen Luyen Phuong; Binh Doan Van; Sameh A. Kantoush; Trung La Vinh; Mai Nguyen Thi Phuong; Tetsuya Sumi; Tuan Luc Anh

Linked Author(s): Sameh A. Kantoush

Keywords: Sediment; Morphology; VMD; River damming

Abstract: The Vietnamese Mekong Delta (VMD) has been suffered from coastal erosion, morphological changes, and sediment reduction caused by climate change, sand mining, and river damming. Therefore, the main objectives of this work are to study sediment dynamics and morphological changes to address past and present challenges and subsequently suggest adaptive policies. To achieve our objectives, two field trips were conducted and a two-dimensional (2-D) module was employed to examine the spatiotemporal dynamics of suspended sediment concentration (SSCs) and riverbed incision in the VMD’s major rivers. From August 2017 to April 2018, two field trips were conducted in 570-km-long major rivers in the VMD, namely the Tien (Mekong) – the Hau (Bassac) – the Vam Nao diversion channel, under the framework of the JASTIP project (Japan-ASEAN Science, Technology, and Innovation Platform). In the field surveys, we measured river depths, discharges, velocities, turbidity, and water levels by using various instruments, namely Garmin GPS, Trimble GPS-equipped ADCP, and turbidity meters. In the JASTIP project, we also installed three monitoring stations to measure the sub-daily SSCs in the above-mentioned main rivers. Besides, monitored flow, sediment, and morphology data are also collected from the Mekong River Commission (MRC). The measured and collected data are used to elucidate spatiotemporal hydro-sediment dynamics and morphological changes. To project future morphological changes resulting from sediment reduction from the Mekong River due to river damming, serving for sustainable river system management, a 2-D numerical model was established. In general, the SSCs in the VMD are seasonally changeable. In the high-flow season (i.e., August 2017), the highest SSC value was recorded as much as 1,300 g/m3 at Tan Chau (under JASTP project) and decreased remarkably to 5-70 g/m3 in the low-flow season in March-April 2018. Regarding sediment-morpho dynamics, the Tien and Hau Rivers are divided into two reaches. The rivers have high SSCs and deep bad elevations in the upper reach, decreasing seaward while they have lower SSCs and shallower bed elevations in the lower reach, increasing seaward. Additionally, the SSCs in the Tien River are higher than that in the Hau River in the high-flow in August 2017, while those in the two rivers are relatively balanced in the flow-flow season from March and April 2018. Moreover, the Tien River annually diverts about 40-50% of its suspended sediment to the Vam Nao Channel, which combined with sediments incised from the riverbed of this channel contributes some 60-80% to the suspended sediment budgets of the Hau River. Thus, interregional research against negative impacts of spatiotemporal variations and SSCs reduction in the entire channel-river systems of the VMD should be conducted to understand and suggest adaptive measures to protect this Delta under future stresses from climate change and intensified anthropogenic pressures.

DOI: https://doi.org/10.3850/IAHR-39WC2521711920221687

Year: 2022