Based on a new dataset of high temporal resolution of water discharge (hourly frequency) and suspended sediment concentration (twice daily frequency at ebb and flood tides) at Can Tho and My Thuan stations during the 2009-2016 period, monthly and annual flow and suspended sediment flux of the lower Mekong River were calculated. The present water discharge of the Mekong River to the sea can be estimated to be 400 km 3 yr −1 , +/− 100 km 3 yr −1 depending on El Niño Southern Oscillation (ENSO), and the present sediment supply to the sea can be estimated to be 40 Mt yr −1 , +/− 20 Mt yr −1 depending on ENSO. The ENSO influence (proxied by the Southern Oscillation Index-SOI) on Q (water discharge) and Qs (sediment flux) is at maximum at a time lag of 8-9 months. The 2010-2011 La Niña event increased the water supply by almost 30% and the sediment supply by 55% in 2011. The 2015-2016 El Niño event decreased the water supply by 20% and the sediment supply by 50% in 2015-2016. The average net water discharge was 12,550 m 3 s −1 in neutral years. The Tien River at My Thuan accounted for~52%, and the Hau River at Can Tho for 48% of the total Mekong River discharge, with small variations (of the order 1%) amongst years. In terms of suspended sediment delivery to the sea, the Tien River accounted for a more variable portion of 55% in neutral years, 57.3% in 2015-2016 (El Niño) and 62% in 2011 (La Niña). 80% of water discharge occurred during the flood season (July-December) and 20% occurred in the low flow season (January-June), the proportion being 91% and 9% for the sediment supply. Previous estimates of annual sediment supplies to the sea (145-160 Mt yr −1) had been established before 2003. Possible origins of the recent reduction by~75% of sediment supply are presented and discussed.

Abstract. Building high dykes is a common measure to cope with floods and plays an important role in agricultural management in the Vietnamese Mekong Delta. However, the construction of high dykes cause considerable changes in hydrodynamics of the Mekong River. Therefore, this paper aims to assess the impacts of the high dyke system on water level fluctuation and tidal propagation on the Mekong River branches using a modelling approach. In order to consider interaction between rivers and seas, an unstructured modelling grid was generated, with 1D–2D coupling, covering the Mekong Delta and extending to the East (South China Sea) and West (Gulf of Thailand) seas. The model was manually calibrated for the flood season of the year 2000. To assess the role of floodplains, scenarios consisting of high dykes built in different regions of the Long Xuyen Quadrangle (LXQ), Plains of Reeds (PoR) and TransBassac were carried out. Results show that the percentage of river outflow at Dinh An sharply increases in the dry season in comparison to the flood season while the other Mekong estuarine outflows rise slightly. In contrast, the lateral river flows of the Mekong River system to the seas by the Soai Rap mouth and the LXQ decrease somewhat in the dry season compared to the flood season due to overflow reduction at the Cambodia–Vietnam border. Additionally, the high dykes in the regions that are directly connected to a branch of the Mekong River, not only have an influence on the hydrodynamics in their own branch, but also on other branches because of the connecting channel of Vam Nao. Moreover, the high dykes built in the PoR, LXQ and TransBassac regions are the most important factor for changing water levels at Tan Chau, Chau Doc and Can Tho, respectively. The LXQ high dykes result in an increase of daily mean water levels and a decrease of tidal amplitudes on the Song Tien (downstream of the connecting channel of Vam Nao). A similar interaction is also found for the the PoR high dykes and the Song Hau.

Abstract. Building high dykes is a common measure of coping with floods and plays an important role in agricultural management in the Vietnamese Mekong Delta. However, the construction of high dykes causes considerable changes in hydrodynamics of the Mekong River. This paper aims to assess the impact of the high-dyke system on water level fluctuations and tidal propagation in the Mekong River branches. We developed a coupled 1-D to 2-D unstructured grid using Delft3D Flexible Mesh software. The model domain covered the Mekong Delta extending to the East (South China Sea) and West (Gulf of Thailand) seas, while the scenarios included the presence of high dykes in the Long Xuyen Quadrangle (LXQ), the Plain of Reeds (PoR) and the Trans-Bassac regions. The model was calibrated for the year 2000 high-flow season. Results show that the inclusion of high dykes changes the percentages of seaward outflow through the different Mekong branches and slightly redistributes flow over the low-flow and high-flow seasons. The LXQ and PoR high dykes result in an increase in the daily mean water levels and a decrease in the tidal amplitudes in their adjacent river branches. Moreover, the different high-dyke systems not only have an influence on the hydrodynamics in their own branch, but also influence other branches due to the Vam Nao connecting channel. These conclusions also hold for the extreme flood scenarios of 1981 and 1991 that had larger peak flows but smaller flood volumes. Peak flood water levels in the Mekong Delta in 1981 and 1991 are comparable to the 2000 flood as peak floods decrease and elongate due to upstream flooding in Cambodia. Future studies will focus on sediment pathways and distribution as well as climate change impact assessment.

Strong currents induced by tidal oscillations have been overlooked in previous flood-risk assessments of the Mekong River. To discuss the potential disaster risk associated with tidal-flow intensification, this study applied ocean tidal modelling to a typical dry season when the tidal regime is predominant compared to the fluvial regime. The model can forecast tidal levels and velocities at a given time and location in the urban area of the Mekong Delta, which is characterized by many tributaries, channels, and low-lying lands, in addition to the main stream of the Mekong River. Numerical simulations revealed that a rapid flow, which could exceed 1 m/s in both the ebb and flood tidal phases, likely occurred along the tributary. Although this study only focused on the tidal regime, such locally intensified flows can be further amplified by a high river discharge or storm surge, having potentially dangerous consequences, such as difficulties in handling ships and small ships being capsized.