DMS project: Origin, fate and impacts of the Mekong sediments
by Juha Sarkkula, Jorma Koponen, Hannu Lauri and Markku Virtanen (2010)
Main conclusions about sediment trapping and morphological changes
Dam sediment trapping will decrease sediment input to the Cambodian and Vietnamese Delta around 50%, and possibly even 90% if mainstream dams are implemented.
Although morphological changes linked to coarser sediments take time to be realised downstream, the fine suspended sediment impacts can be seen immediately.
Downstream sediment load is quite sensitive to mainstream dam sediment trapping: only 5% mainstream dam efficiency compared to the maximal theoretical trapping causes trapped sediment to increase from 50% to 70%.
In the long term river channels will suffer increased erosion and morphological changes due to trapping of coarser sediments and decreased bedload; the time scales or impact magnitude are difficult to estimate; more research is needed on this issue.
In the Delta channel changes might affect distribution of flows the distributaries; this would have implications for navigation, aquaculture, irrigation, water quality, coastal fisheries, habitats etc.
Coastal erosion will increase because sediment input to the coast will decrease; estimation of the magnitude of the impact requires further research.
Delta has been formed quite fast in geological terms, this indicates possibility of relatively fast morphological changes in the Delta.
There are no plausible compensation mechanisms for decreasing sediment input to the system; especially river bottom coarse sediments (sand and gravel) can't compensate for finer suspended sediments.
Reservoir sedimentation has large consequences both in terms of impacts, reservoir economics (decrease of storage volume) and reservoir fish production; because of the large unknowns reservoir sediment modelling is needed.
Reservoir sediment management (sediment flushing) will probably result in high sediment pulses that can be harmful to organisms and will have different impact downstream productivity than more even sediment input.
Large water level fluctuations will stress river banks through wetting and drying and ground water level changes causing increased bank erosion.
The nutrient, productivity and socio-economic consequences of the hydropower sediment trapping are discussed in the DMS Productivity and Socio Economics reports.
Knowledge gaps and additional considerations
The knowledge gaps related to sediments can be summarise as:
Delta processes are poorly understood; there is need for systematic and comprehensive modelling and monitoring effort
coastal areas need to modelled for erosion, water quality and productivity
detailed reservoir modelling is required for impact analysis, economic evaluation, fisheries production, guidance for sediment management, multi-purpose optimal operation etc.
sediment sources, especially the 3S, need to be verified
sediment nutrient sources and fates need to be clarified with field studies and modelling
suspended and bed load grain size changes need to be monitored and modelled
long-term river morphological changes need to be estimated and modelled
linkage between socio-economic analysis and modelling need to be strenghtened
economics of hydropower development require comprehensive and critical approach.
Nutrient and productivity related issues and knowledge gaps are discussed in detail in the DMS productivity report.