“Nature and Culture Conservation Forum for
The sustainable development on the Mekong Delta”
5th Forum: Maintaining Ecosystem Services in the Mekong Delta of Vietnam
Sediment budget of the Mekong basin: transfer processes and negative
impacts of dams and extractions
Prof. Jean Paul Bravard, University of Lyon, France
Marc Goichot, WWF-Greater Mekong, Vientiane
1. What is the sediment discharge of the Mekong River?
The average yearly sediment discharge of the Mekong River at its mouth averages 150
+/- 10 million tons according to scientific literature and this value has not been
discussed before 2005. This rough evaluation, considered as a constant through years,
used results from suspended sediment monitoring undertaken in the early 1960’s at
different stations of the Lower Mekong basin (Harden & Sunborg, 1992; Ta et al., 2002).
Data of suspended sediment load from monitoring stations (Chiang Saen, Luang
Prabang, Chiang Khan, Mukdahan, Khong Chiam, Pakse, Kratie) allowed scientists to
model the impact of the Lancang chain of dams and climate change (Fu et al., 2006;
Kummu & Varis, 2007; Wang et al., 2011; Liu et al., 2012).
However Walling (2005) warned the MRC (Mekong River Commission) to be very
cautious regarding the field sampling methods and the results obtained to date. Walling
pointed out that sand had been poorly taken into account. Geomorphic studies have
described the presence of sand in the channel without connecting landforms issuing
from flood deposition with the results of suspended monitoring (Gupta et al., 2002;
Carling, 2009). The average value of sediment transport may then have been
underestimated in a proportion that remains unknown today.
2. New perspectives for sand transport processes during floods
The field work for this study was performed in 2011-2012 by WWF (Jean-Paul Bravard
and Marc Goichot) in the main stem of the Lower Mekong channel from Chiang Saen to
the upper Mekong delta. This study first aimed at bridging the gap between landforms of
the banks, made of sand, and transport processes. Indeed sand is continuously present
along the 2400 km stretch of the Lower Mekong, but at different elevations above the
low flow level. This study was based on field sampling, application of the so-called CM
image method (Passega, 1964), and computation of unit stream power (a value of
stream energy during small floods).
The results are summarized in fig. 1. which displays the long profile of the Mekong at
low flow, the values of energy  (function of slope, of the 1 in 2 yrs flood and channel
width), and the succession of processes involved in sediment transport:
- Coarse bed load is a discontinuous process occurring in steep reaches controlled
by bedrock. It is important to note that fine gravel is present as far downstream
as the upper delta.
- Sand is continuously transported as bed load, from the China border to the sea
and the delta shore,
-
-
-
Sand is also transported in suspension along most of the river reaches, except in
the Mukdahan reach and downstream of the Cambodia-Vietnam border where 
has very low values. Sand displays a vertical gradient on the riverbanks (fine
sand on upper banks, coarse sand closer to the bottom), as it does in the water
body during floods (graded suspension): this is due to turbulence controlled by
velocity and roughness. Sand is transported mostly during the sharp spates of the
annual flood,
When sand is not transported as graded suspension, it moves as bed load, but the
two processes are usually simultaneous, with relative proportions that change
depending on the level of the flood. Note that sand transported in suspension
travels faster than sand transported as bed load.
Silt and clay are transported as wash load (or uniform suspension) during floods
whatever energy may be.
Fig. 1: Graph showing the long profile of the Mekong water level, the downstream variation of energy
(black squares) and the types of transport processes.
3. What is the practical consequence of the reevaluation of sand transport for
the Mekong sediment budget?
The reason why sand has been underestimated in the Mekong sediment budget (and in
some cases completely forgotten) is due to the fact that hydro-meteorological stations
did not use sampling methods that allowed accurate monitoring during flood spates.
Reappraisal of suspended transport of sand by the MRC Information and Knowledge
Management Programme (IKMP) only very recently confirmed that sand does constitute
a significant part of sediment transport (Koehnken, 2012). As a consequence sediment
budget of the Mekong should be revalued upward. To date it is not possible to provide a
revised average value, but we estimate the existing figures of 160 Mt per year could be
increased by a value between 10 and 30 millions.
4. Why is the delta shoreline retreating? Dams and upstream sediment
storage?
Several causes have been put forward to explain the delta shoreline retreat over the past
15 years. Deltas are highly dependant on sediment replenishment from the rivers that
have created them, thus reduction of sediment input from upstream are prime causes to
explain coastal erosion on delta fronts. We suggest two significant causes.
The first one relates to dam impacts.
4.1. Several papers have held the Lancang dams liable for downstream sediment
shortage at the river mouth (Lu & Siew, 2006; Liu et al., 2012). If it may be true for the
silt-clay component, as part of the total load is being trapped in the large hydropower
reservoirs on the Chinese section of the main stem of the Mekong. Most of the sand is
probably also trapped in those large reservoirs But the amount of sand stored in the
channel all along the river course make it dubious that sand shortage at the mouth may
be due to the Lancang dams. The impact of the Chinese hydropower dams will need
more time to affect the coast of Vietnam, as it will occur only after the stock of sand in
northern Laos is depleted. This might take 20 to 50 years.
4.2. It is estimated that the “Three S” rivers provided about 10% of the overall
load before 1993 (Sarkkula et al., 2010), or 35-40% of the present sediment supply from
the Mekong tributaries. The main supplier of sand is the Se San River which has been
dammed in Vietnam and which will be dammed downstream in Cambodia. This means
this important source of sand will be blocked. Considering the downstream position of
this tributary in the Mekong basin, the existing hydropower dam projects probably play
an important role in the sediment shortage at the mouth.
4.3. The WWF study has considered the expected impacts of the Xayabury dam, in
Lao PDR. The 80 km long reservoir behind the 35 m high dam will trap a significant part
of the sand stored downstream of the lowest Lancang dams, but should let most of wash
load (silt and clay) pass through the dam (considering turbulence in the reservoir during
floods). However WWF stressed the importance of the compaction of mixed sediment
because it will decrease the ability of floods to rework deposits on the bedrock lateral
platforms of the channel bottom. Trapping of silty sand will affect the downstream
reaches with erosion of sand banks and bars, due to the reduction of sediment
concentration during floods.
4.4. Also, the WWF study considered flushes that have been proposed by the
Government of Lao PDR as a mitigation measure. Flushes are really implemented for
“cleaning” the reservoir, but they will have detrimental impacts on habitat (clogging of
substrate), and on fauna (lethal concentrations of suspended sediment). It is dubious
that the operator will be able to follow the instructions and provide the required low
concentrations. The dilemma will therefore be to choose between the detrimental
sediment trapping or damaging flushes. In both cases the Mekong River will be
significantly altered.
In conclusion of point 4, WWF stresses that dams have already an unmeasured impact
on the Mekong sediment load. Sediment shortage impacts, which are non-reversible, will
increase in the future when those impacts will have prograded to downstream. It will be
all the more severe if planned dams are built up on the main steam and on the
tributaries that provide an important supply of sand.
5. The severe detrimental impacts of sand and gravel harvesting
WWF launched a study in the four countries of the Lower Mekong main stem in order to
quantify sand and gravel extractions, the places of extraction and the trends. The volumes per
category of grain-size are presented in table 1:
Extractions (1000 m3/yr)
Country
Sand
Gravel
Pebbles
Total
%
904.1
10
454.5
1 369
4%
Thailand
3 677.2
857.75
0
13%
Cambodia
18 748.5
2 045
0
4 535
20
793.45
7 750
0
0
7 750
22%
31 079.8
2 912.7
454.5
34 447
100%
90%
8%
1%
100
Lao
Vietnam
TOTAL
%
60%
Table 1: Volumes and percentage of grain-size categories per country (2010-2011)
In 2010-2011, most of the extractions were located in Cambodia but in part because
sediment resource is exhausted in the branches of the delta in Vietnam. Out of the 118
sites operated, 88 are less than 3 years old. The trend is an increase of volumes
extracted from Savannakhet to the delta.
The 34.4 millions m3 extracted is probably an underestimate of the effective extraction
as, the surveys was not exhaustive (only targeting the largest operators), were only
targeting the main stem, thus not encompassing extraction activities on tributaries, and
finally volumes have probably been minimized by the interviewed dredgers. This figure
corresponds to 56.6 millions tons of sand and gravel and is probably far higher. We
suggest that current rates of sand extraction exceeds the values of annual transit, thus
extraction rates exceed replenishment rates.
Conclusion
Sand and gravel extraction have probably been so far the most important factors of the
sediment depletion monitored along the Mekong delta shoreline. The second factor,
sediment trapping in dam reservoirs, will have increasing impacts in the future because
the impacts of the Lancang reach are partly delayed and more dams are constructed and
planed. It is important to note that the impact of sand and gravel extraction can be
considered as much easier to reverse than the impact of hydropower.
Extractions should be considered in the framework of a master plan taking the
total sediment balance into account and dams should be built up only on
tributaries which do not provide significant quantities of sand and gravel to the
downstream course and delta, and not on the main stem of the Lower Mekong.
References
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This reseach was made possible with the financial support of Fond Francais pour
l’Enviornnement Mondial (FFEM) and was conducted in the frame of a project coimplemented with the Information and Knowledge Management Programme of the
Mekong River Commission Secretariat.