– Mekong Sediment from the Mekong River Commission Study Summary

This technical note presents the results of modelling changes in sediment loads in the Mekong River Basin by employing water resource modelling to support assessment of proposed dams on sediment loads entering the Mekong delta.


Introduction
The Mekong River, one of the largest rivers in Southeast Asia, flows southward from the Tibetan Plateau to the South China Sea through the Indochina Peninsula, forming a Delta at its mouth.The Mekong River Commission Member Countries -Cambodia, Lao PDR, Thailand and Viet Nam -have plans for development, and significant water-related infrastructure has already been built or under construction in major tributaries and the upper reaches of the mainstream.The development of water-related infrastructure is in response to rapidly growing populations and economic development, with associated increases in demand for, and the development of water resources.As a result of this development, economic growth would follow and bring significant benefits to the region.However, if development is not properly planned and designed, it could have serious consequences for the downstream reaches.
The Mekong River serves as the heart and circulatory system for all the nutrients and building blocks of life; the pure water is the lifeblood that carries the fine sediment that conveys the nutrients.Water resources development in the basin is bringing significant changes to the river, including deteriorating water quality, the trapping of sediments, and the creation of barriers for migrating fish.
The MRC Council Study (Study on Sustainable Management and Development of the Mekong River with Impacts of Hydropower Projects in the Mainstream) aims to enhance the ability of the MRC to advise Member Countries on the positive and negative impacts of water resources development on the people, the economy and the environment of the Mekong Basin1 .Six selected developments are modelled in the study, such as hydropower, irrigation, agriculture and land-use changes, domestic and industrial water use, navigation, and flood protection.Substantial changes in sediment can be expected in the Mekong River due to the cumulative effect of dams in the Basin (see Figure 1).
A scenario-based approach is being used to consider various future conditions: the near future (2020), a longer-term planning horizon (2040) and an early development condition (2007) are compared as seen in Table 1.

Objectives
The general objective of this paper is to provide key findings on sediment reduction in the Mekong River emerging from the study results.Three specific goals include: • Sediment contribution by mainstream and tributaries • Sediment trapped by hydropower cascade on the Mekong Basin • Trends in sediment reduction from scenario comparison analysis

Materials and Methods
This section describes the approaches used in the sediment reduction calculations.

Model and Data
The MRC Modelling system has been used extensively to support the CS together with additional modelling tools for agriculture and floodplain sedimentation.The MRC Decision Support Framework (DSF) was considered as a state-of-the-art computer-based system in 2001, which has been developed through a thorough consultation process involving the four MRC member Countries.The DSF includes a suite of models that performs the water quantity and quality simulations required to support the impact assessments.
Due to current limitations of the MRC DSF, it is integrated with the eWater Source to efficiently carry out sediment transport and trapping for scenario modelling structural change.The methodology used in the Council Study integrates the results of the IQQM and SWAT models and produces outputs which are required by the ISIS model.
All data transfers between models are facilitated by the Data Transfer Tool (DTT), which passes data between models via a central database called the Knowledge Base (KB) (see Figure 2).Table 2 summarises the data availability.There are numerous issues with the data, including limited spatial and temporal range, discontinuous records, low sample numbers, and unreliable and inappropriate sampling methods.

Approaches to estimate sediment trapping in reservoir and sediment contribution
Sediment trapping efficiency (TE) is the proportion of sediment deposited in a reservoir relative to the incoming load.
Two approaches are used in the Council Study; full modelling of the hydraulics of sediment movement for the mainstream using a 1D hydrodynamic model, and full basin-wide simulations of all tributaries, the upper Lancang and lower mainstream dams using Source.In ISIS, the movement of sediment is calculated using sediment transport formula according to grain size.In Source, trapping is estimated via Brune (1953)3 .The Brune algorithm is a widely used, well-established empirical model, which requires two key inputs to calculate TE.These are the reservoir capacity and average total annual inflow.

Sediment Contribution from tributaries
Results of the calculations are presented in Figure 3, which depicts each scenario for 62 tributaries that have direct sediment into the Mekong mainstream and two associated areas in the Viet Nam Delta.The total catchment area of the Mekong is 822,206 km 2 .The contributions of tributaries were represented as total sediment load yearly into the Mekong mainstream.

Trends in sediment reduction from scenario comparison analysis
Altering sediment transport and deposition in the Mekong River due to other infrastructure development is also provided in this paper.It was found that the primary effect is due to hydropower dam development in the basin.Figure 4 shows the changes in sediment loads along the course of the Mekong River for each of the main scenarios.We note a large reduction in the amount of sediment load transported downstream in the scenarios.The sediment trapped by the Lao mainstream hydropower cascade in 2020 is 10.2 Mt/annum (million tons per annum).Figure 6a shows that sediment trapping by reservoirs will be 67%, implying that the sediment load reaching the Delta will be 33% of its 2007 level.The amount trapped by the Lao and Cambodian mainstream hydropower cascade in 2040 is 47.1 Mt/annum.Figure 6b illustrates that about 97% of the sediment load will be trapped, which indicates that the sediment load reaching the Delta will be 3% of its 2007 level after sediment stored in the channel is lost.

Conclusion
The temporal and spatial changes in the sediment load were investigated for the entire Mekong Basin.Due to the number of development projects in the Mekong Basin, results of the models demonstrate that without mitigation, hydropower development will have major impacts on the sediment balance in the Mekong Delta because the reduction in sediment loads is around 97% under the 2040 scenario.With this magnitude of potential sediment starvation on the Mekong River system, mitigation strategies of all planned dams should be implemented to improve sediment loads reaching the Mekong Delta.
If mitigation strategies involving the implementation of sediment passage structures for mainstream dams such as fish passage structures and sediment sluicing operations in the Lower Mekong Basin are implemented, it would be possible to reduce the serious impacts on the river channel and delta landforms, floodplain fertility, and productivity of the ecosystem, including the extraordinary Mekong River fisheries.However, mitigation measures are only likely to be effective if they are implemented in a coordinated way for all dams in the cascade.

Figure 1 .
Figure 1.Hydropower dams in 2007, 2020 and 2040 The available sediment data comes from three sources within the MRCS 2 :  Mean suspended sediment concentration (SSC) from the Hymos Database  Total suspended solids (TSS) from the EP Database 2 MRC.(2015).Strategic Plan 2011-2015.Vientiane: Mekong River Commission Secretariat.

Figure 2 .
Figure 2. Relationship between the SWAT, IQQM, Source and ISIS models.Arrows show the direction of information flows

Figure 3 .
Figure 3. Annual sediment contribution by tributaries in Mt in each scenario

Figure 5 .Figure 4 .
Figure 5. Mass of Sediment Trapped by mainstream LMB hydropower cascade on the Mekong

Figure 6 .
Figure 6.The average reduction of sediment of the M2 (a) and M3 (b)relative to the natural load

Table 1 . Summary of the baseline and main development scenarios for the study
The Brune method was checked for Mekong conditions for the Manwan and Dachaoshan Dams in China.The results were compared with other literature reviews such asFu et al. (2007)  4andKummu et al.  (2010)  5.It was found that the implementation of the Brune TE methodology in eWater Source produced comparable results as those obtained byFu et al. (2007)  6 .As a ressult, TE for Xiaowan and Nuozhadu Dams, and other dams in China and the Lower Mekong Basin (LMB) were estimated using the same approach.eWater Source routes the sediment through tributaries to the Mekong River, including trapping in reservoirs.To validate the accuracy of the hydrodynamic model, modelling results are compared against monitoring data from MRC datasets, though as yet there is no data for mainstream reservoir trapping as the first dams at Xayaburi and DonSaHong are under construction.