“…Tidal propagation speed has increased upstream of Can Tho and tidal discharge amplitude has increased at both Can Tho and Chau Doc. Therefore, we conclude that channel bed levels have deepened substantially upstream of Can Tho but not significantly downstream of Can Tho, which is also in-line with recent bathymetric surveys 51 that find limited sources of modern sediment in lower Hau River. Hereafter, we show that the combined effect of deeper channels and amplified tides leads to increased SWI.Figure 4Cross-sections measured in 2014 versus 2017, showing channel deepening at various locations along the Tien River.…”
Natural resources of the Mekong River are essential to livelihood of tens of millions of people. Previous studies highlighted that upstream hydro-infrastructure developments impact flow regime, sediment and nutrient transport, bed and bank stability, fish productivity, biodiversity and biology of the basin. Here, we show that tidal amplification and saline water intrusion in the Mekong Delta develop with alarming paces. While offshore M2 tidal amplitude increases by 1.2–2 mm yr−1 due to sea level rise, tidal amplitude within the delta is increasing by 2 cm yr−1 and salinity in the channels is increasing by 0.2–0.5 PSU yr−1. We relate these changes to 2–3 m bed level incisions in response to sediment starvation, caused by reduced upstream sediment supply and downstream sand mining, which seems to be four times more than previous estimates. The observed trends cannot be explained by deeper channels due to relative sea level rise; while climate change poses grave natural hazards in the coming decades, anthropogenic forces drive short-term trends that already outstrip climate change effects. Considering the detrimental trends identified, it is imperative that the Mekong basin governments converge to effective transboundary management of the natural resources, before irreversible damage is made to the Mekong and its population.
“…Tidal propagation speed has increased upstream of Can Tho and tidal discharge amplitude has increased at both Can Tho and Chau Doc. Therefore, we conclude that channel bed levels have deepened substantially upstream of Can Tho but not significantly downstream of Can Tho, which is also in-line with recent bathymetric surveys 51 that find limited sources of modern sediment in lower Hau River. Hereafter, we show that the combined effect of deeper channels and amplified tides leads to increased SWI.Figure 4Cross-sections measured in 2014 versus 2017, showing channel deepening at various locations along the Tien River.…”
Natural resources of the Mekong River are essential to livelihood of tens of millions of people. Previous studies highlighted that upstream hydro-infrastructure developments impact flow regime, sediment and nutrient transport, bed and bank stability, fish productivity, biodiversity and biology of the basin. Here, we show that tidal amplification and saline water intrusion in the Mekong Delta develop with alarming paces. While offshore M2 tidal amplitude increases by 1.2–2 mm yr−1 due to sea level rise, tidal amplitude within the delta is increasing by 2 cm yr−1 and salinity in the channels is increasing by 0.2–0.5 PSU yr−1. We relate these changes to 2–3 m bed level incisions in response to sediment starvation, caused by reduced upstream sediment supply and downstream sand mining, which seems to be four times more than previous estimates. The observed trends cannot be explained by deeper channels due to relative sea level rise; while climate change poses grave natural hazards in the coming decades, anthropogenic forces drive short-term trends that already outstrip climate change effects. Considering the detrimental trends identified, it is imperative that the Mekong basin governments converge to effective transboundary management of the natural resources, before irreversible damage is made to the Mekong and its population.
“…This may partly explain the differences compared to this study. The fact that lower Hau estuarine system is morphologically stable [60], may explain why we observe less difference in Hau and larger differences in the Tien tributaries. Table 5.…”
Section: Temporal and Cumulative And Temporal Discharge Divisionmentioning
confidence: 63%
“…Geometry is perhaps the most vital input in modeling the VMD and is likely to have changed in the past 10 years, except the Hau River, downstream of Can Tho, where the most recent measurements were used and the overall morphological changes are negligible [60]. Note that due to the quality of observed data in Cambodia and the old morphology of the Cambodian channels, model evaluation in the Cambodian territories becomes a qualitative exercise.…”
The Mekong Delta constitutes a complicated multi-channel estuarine system, exchanging water with a delta-wide irrigation system. A 1D–2DH coupled numerical domain is calibrated and validated for water level and discharge during the dry season. This approach benefits from the simplicity of a 1D network within the estuarine and irrigation systems, while maintaining the interaction with the spatial tidal dynamics of the 2DH coastal domain. First, the role of the irrigation system on tidal dynamics is quantified; then, tidal propagation, freshwater budget, and the effect of offshore subtidal water level on discharge division are investigated. The results show that the complex irrigation system, in a friction-like manner, reduces the tidal amplitude up to 25%. The channels aggregate to 1% of the total water volume in the delta, while accommodating up to 10% of the tidal prism. Tidal amplitude reduces upstream, while subtidal water level is highly sensitive to upstream discharge, spring–neap cycles, and wind-generated offshore surge. Although cumulative discharge division within the estuarine network is consistent, temporal discharge division can be significantly sensitive to offshore wind-surge. During the dry season, it can reverse the expected subtidal discharge division within the time-scale of a few days and potentially influence salt intrusion.
“…Despite all exposed uncertainties, it must also be highlighted that this is the first estimate of sand mining volumes for the Mekong River using high-resolution bathymetric maps of sand mining sites and the analysis of their morphological evolution. Previous estimates relied on remote sensing techniques and questionnaires 22 or low resolution datasets 23 , which have ever since been put into question 51 .…”
The delta of the Mekong River in Vietnam has been heavily impacted by anthropogenic stresses in recent years, such as upstream dam construction and sand mining within the main and distributary channels, leading to riverbank and coastal erosion. Intensive bathymetric surveys, conducted within the Tien River branch during the dry and wet season 2018, reveal a high magnitude of sand mining activities. For the year 2018, an analysis of bathymetric maps and the local refilling processes leads to an estimated sand extraction volume of 4.64 0.31 Mm/yr in the study area, which covered around 20 km. Reported statistics of sand mining for all of the Mekong’s channels within the delta, which have a cumulative length of several hundred kilometres, are 17.77 Mm/yr for this period. Results from this study highlight that these statistics are likely too conservative. It is also shown that natural sediment supplies from upper reaches of the Mekong are insufficient to compensate for the loss of extracted bed aggregates, illustrating the non-sustainable nature of the local sand mining practices.
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