ANTICIPATED GEOMORPHIC IMPACTS FROM MEKONG BASIN DAM CONSTRUCTION

By Z.K RUBIN, G.M. KONDOLF, AND P. CARLING (2014)

Dam locations are indicated for two scenarios: definite future, and full-build. 
Main stream dams are included in full-build scenario, but represented separately.


Bedrock channel of the Mekong River with surficial sand deposits 1-2 m thick, near Xayaburi, Laos. (photo by Kondolf, January 2012)

Abstract

The Mekong River is undergoing rapid dam construction. Seven main stream dams are under construction in China and 133 proposed for the Lower Mekong River Basin (LMRB). We combined geomorphic assessments of the Mekong channel and delta with models of sediment trapping by reservoirs. We expect the biggest geomorphic changes to occur along alluvial reaches, though stripping of thin sediment deposits in bedrock reaches may also have significant consequences for benthic invertebrates, fishes, and other aquatic organisms dependent on the presence of alluvium in the channel. If all dams are built as proposed, the resulting 96% reduction in sediment supply would have profound consequences on productivity of the river and persistence of the Delta landform itself. Strategies to pass sediment past dams should be explored to reduce the magnitude of sediment starvation and resulting impacts.

Key Words: Mekong River, sediment load, reservoir sedimentation, channel change, dam construction

1. Introduction

The Mekong River basin is undergoing rapid and widespread dam construction (Grumbine and Xu, 2011). On its upper reaches in China (the Lancang), seven dams have been constructed or are under construction on the mainstem. In the Lower Mekong River Basin in Laos, Thailand, Cambodia, and Vietnam, another 38 tributary dams are considered certain to be built, with an additional 95 dams at some level of planning for tributaries and the mainstem. How will these dams alter the sediment load of the Mekong and how will the dams change the morphology of the downstream channel and delta?

The Delta is currently home to 20 million people whose lives and economy are at risk from subsidence increased flood risk, and other changes in the delta. Previous authors have estimated the average annual suspended load of the entire Mekong as 160 million tonnes per year (Mt y-1), and observed that about half of this amount was produced by the upper 20% of the basin, the Lancang drainage in China (Gupta and Liew, 2007; Walling, 2005). These values reflected conditions prevailing prior to ongoing construction of a cascade of seven dams on the Lancang, now mostly completed, and which will trap about 83% of the Lancang basin sediment when complete (Kondolf et al submitted). Thus, almost half of the natural sediment load of the Mekong will be lost in the reservoirs of the Lancang, and the sediment load of the Lower Mekong River will consist largely of sediment derived from sources within the LMRB itself. The hotspot of the river’s extraordinary fish production is the Tonle Sap system. This tributary to the Mekong receives monsoon-driven, seasonal backwater flooding from the Mekong River mainstem, during which time fine-grained sediments deposit across the Ton Sap basin, making the Tonle Sap tributary basin net depositional (Tsukawaki 1997, Kummu et al. 2005) and potentially vulnerable to reduced sediment supply (Baran and Guerin 2012).

Most large rivers in the world are experiencing decreased sediment loads due to dam- induced sediment starvation. In the two millennia prior to widespread dam construction, human activities such as forest clearing and cultivation increased erosion and sediment delivery to the oceans ((Leopold 1921, Leopold 1923, Wolman and Schick 1967, Milliman and Syvitski 1992, Syvitski 2008). Worldwide, widespread dam construction has reversed this historical trend, and substantial reductions in the delivery of sediment to the oceans are now occurring in many of the world’s rivers (Milliman and Syvitski, 1992). The Mekong, however, differs from other large Asian rivers, having produced a relatively consistent sediment yield over the past three thousand years (Ta et al., 2002), reflecting relatively modest levels of development that prevailed until very recently.

Reservoirs trap all the bedload and a percentage of the suspended load carried by a river. The supply of sediment to the river downstream is reduced by the amount trapped.

Depending on the relative changes in sediment supply and transport capacity (Schmidt and Wilcock, 2008) erosion or deposition can occur, but most commonly the reach downstream of the dam is characterized by sediment-starved, or ‘hungry’ water, which can erode the bed and banks (Kondolf, 1997). These erosive flows threaten infrastructure and coarsen bed material, fundamentally altering physical habitat and aquatic food webs (Power et al., 1996). Sediment starvation typically affects downstream river channels and deltas by causing incision and bank erosion, habitat loss, and increased rates of land loss along the coast by reducing sediment replenishment.

The consequences of delta subsidence, both natural and accelerated, in combination with discharge control, sediment-load reduction, and channel stabilization, is to accelerate shoreline erosion, threaten the health and extent of mangrove swamps and wetlands, increase salinization of cultivated land, and put human populations at risk of costly disasters (Syvitski, 2008). Whereas eustatic sea level rise associated with global warming has received much focus and interest in recent years, the very land surface that meets the water has been subsiding more rapidly in recent years, as dam building reduces sediment supply needed for deposition on the delta plain, distributary channels are stabilized and dyked so that sediment-laden floodwaters can no longer disperse over the floodplain, and petroleum and groundwater extraction induce subsidence. Deltas that develop dense cities and industrial infrastructure become less resilient to tsunamis and hurricane-induced coastal surges. Lives and wetlands at risk today in coastal regions will be even more at risk in the future (Syvitski, 2008). The cumulative impacts of sea-level rise, sediment starvation from reservoir trapping and instream mining of construction aggregate, channelization of delta distributary channels, and groundwater extraction are common to many of the world’s major rivers (Bucx et al., 2010), and have consequences that are broadly predictable (Table 1).

Prior Work on the Mekong River and Delta

Gupta et al. (2002), Gupta and Liew (2007), Carling (2005), Gupta (2008), and Carling (2009a) described the geomorphic framework of the Mekong River (Table 2), noted differences in geomorphic characteristics of reaches of the Lower Mekong, and to some extent, explored how reduced sediment supplies might affect different reaches.

Following on Kummu et al.’s (2010) initial estimates of sediment reduction from planned dams, Kondolf et al. (submitted) developed geomorphically-based sediment yield estimates for the LMRB, calculated trap efficiencies for individual reservoirs, and compiling total storage capacity data for more accurate trap efficiency calculations. They then applied the 3W model (Minear and Kondolf 2009) to calculate cumulative sediment deficit from individual reservoirs under different reservoir development scenarios, accounting for reduced trap efficiencies as reservoirs fill, and accounting for multiple reservoirs in a given river basin. For the MRC’s ‘definite-future’ scenario of 38 dams already constructed, under construction, or certain to be built, the sediment load reaching the Delta will be about half of its pre-1990 level. However, with full build of dams in the Lower Mekong River basin (Figure 1), including mainstem dams, the cumulative sediment trapping by dams will be ~96% of its pre-1990 load. Sediment starvation would actually be more severe owing to the mining of about 27 Mt y-1 of sand and gravel from the river channel, mostly in Cambodia (Bravard and Goichot 2012).

Flow alteration from existing and proposed dams is expected to be more modest than the sediment trapping. Typical of tropical rivers, Mekong River flow is seasonal, with a monsoon-driven high flow period from July to October that is responsible for 75% of the annual flow (Piman et al. 2013). Under a 41-dam “definite future” scenario, and a full- build scenario of 136 dams in the lower Mekong, Piman et al. (2013), predicted a dry season flow increase of 22% and 29% for definite future and full build scenarios respectively at the Kratie station. Wet season flows were predicted to decrease 4% and 13% for definite future and full build scenarios respectively. Similar estimates of hydrological changes were also presented by the Mekong River Commission (2010).

These changes in flow regime may have significant impacts for the aquatic ecosystem and especially the fishery of Tonle Sap (Baran and Myschowoda 2009, Lamberts and Koponen 2008), but the small reduction in wet-season flow is unlikely to change the transport capacity of the Mekong. As such, the river will still have the capacity to transport a similar quantity of sediment with future hydropower development. Thus, sediment trapping by reservoirs is arguably the most important consequence of dams for the downstream channel.

Anthony et al. (2012) used sequential satellite images to analyze coastal retreat in the Mekong Delta from 2003 and 2011, finding an average of 4.4 m y-1 of coastline retreat across the entire delta, with higher rates of 12 m y-1 on the Ca Mau peninsula. Given the stable sediment supply and the growth of the Delta during the last ~6,000 years, Anthony et al. (2012) attribute this recent coastal retreat to the reduced sediment supply caused by massive extraction of sand and gravel from the river channel for construction aggregate (estimated to exceed 43 Mt y annually by Bravard and Goichot 2012), and by levees and channel straightening in the delta, which increase flow velocity and offshore sediment transport. To date, there has been a lack of analysis (and for that matter, a lack of data. To date, there has been a lack of analysis (and for that matter, a lack of data upon which to base analysis) to understand how the Mekong Delta is likely to respond to future sediment starvation. With better predictions of sediment starvation now available, it is clear that sediment starvation effects are likely to be severe, and thus there is an urgent need to draw upon available information for the Mekong Delta and analogous systems to make initial projections of likely impacts and identify critical data needs.

2. Methods

Our approach was twofold. We drew upon prior geomorphic work on the Mekong River basin by Adamson (2001), Gupta (2004) Gupta and Liew (2007), Gupta (2008), and Carling (2009a) to characterize channel reaches in terms of their likely response to sediment starvation We analyzed available data for other deltas as reported in the literature, and systematically compiled data such as degree of hydrologic alteration, percentage reduction in sediment supply, documented historical subsidence rates, and wave energy. Based on these analogous case studies, we made initial predictions for probable response of the Mekong Delta to the virtual elimination of its sediment supply.

3. Results

Although the influence of reservoir-induced sediment starvation on downstream channel change will clearly be complex and varied, fundamental principles such as Lane’s Balance (Lane, 1955) and the presence or absence of geologic controls can be used as preliminary predictive tools. The results of Kondolf et al. (submitted) suggest that sediment trapping will be substantial while reductions in high flows will be minimal (Mekong River Commission 2010). Therefore, the Mekong River will continue to have the capacity to transport sediment in large quantities, but the supply of sediment for transport will be reduced. Channel adjustment will be limited primarily by geologic controls.

3.1  Delineation of Reaches

The Upper Bedrock reach extends from the Chinese border downstream to about 5 km upstream of Vientiane. In this reach the Mekong River channel is bedrock controlled, with limited, and presumably transient, sediment storage (Figure 2). The channel gradient averages 0.0003, and channel width ranges from 200-2000m. This reach includes many wide, bedrock-floored reaches where bedrock is discontinuously overlain by a thin (ca 1-2 m) veneer of sand (Figure 3). The Middle Alluvial Reach extends downstream from Vientiane to Savannakhet. It is alluvial, with both single-channel and island-bar sections. Channel gradient averages 0.0001, and the channel is 800 to 1300m wide.

From Savannakhet downstream to Kratie, the Middle Bedrock Reach is again bedrock controlled. This reach includes a wide range of channel forms, as reflected in Gupta and Liew (2007) having broken this section of river into 4 reaches (their reaches 3, 4, 5, and part of 6). For our analysis, the key attribute of all these reaches is bedrock control (and thus we consider it a single reach), though a variety of sedimentary forms are present including sections with alluvial banks, anastamosed channels with rock-core islands covered with a relatively thin veneer of sand and silt (Meshkova and Carling 2012). For example, from Sambor to Kratie, the bedrock control is largely buried, so the river here displays many alluvial features. However, the underlying bedrock limits its potential response to sediment starvation. Channel gradient in the upper portion of Reach 3 is approximately 0.00006 and decreases downstream. Channel width ranges from 750 to 5000 m. The Cambodian Alluvial Reach extends from Kratie downstream to Phnom Penh. Here, the Mekong is again alluvial, crossing the wide floodplain of Cambodia to enter the depositional reaches of the delta. Channel gradient is 0.000005 and widths range from 3000 to 4000m. Some large-scale structural control is provided by bedrock, but channel planform and position is primarily set by channel migration through alluvium. Downstream of Phnom Penh is the Mekong Delta, by definition a reach of net deposition. The delta occupies an area of ~94,000 km2 making it the third largest delta in the world (Coleman and Wright 1975). The delta begins ~330 km from the sea where the Bassac, the first deltaic distributary, separates from the mainstem. The two channels flow parallel for 200 km without additional distributaries or connecting channels. Ultimately, there are four main channels that reach the sea. As a result of groundwater extraction and limited sediment starvation, the Mekong is categorized as a “Delta in Peril” with late 20th Century aggradation at less than 0.5 mm y-1 and relative sea level rise occurring at approximately 6 mm y -1 (Syvitski et al., 2009). Sediment trapping under future dam building scenarios will further limit sediment delivery and distribution in the delta.

3.2  Potential effects on channel Reaches

Definite Future Scenario

Under the “definite future scenario”, the Upper Bedrock Reach will have an 83% reduction in sediment at the upstream end of the reach, though as less regulated tributaries enter the reach the cumulative trapping decreases to 64% at the downstream end of Reach 1 (Kondolf et al. submitted). The relative reduction in sediment supply in this reach is the greatest of any reach in the definite future scenario, yet because the reach is bedrock controlled we anticipate only modest channel adjustment. Loose sediment deposits over bedrock as described by Carling 2009a (including slack-water deposits on bars, islands, inset floodplains and banks) (Figure 3) will likely be swept away in the first competent floods post-dam. Changes in bed-level will likely be confined to accelerated scouring of pools (Carling, 2009b)

In the Middle Alluvial Reach, the sediment reduction decreases to 52% at the downstream end of the reach while sediment reductions in the Middle Bedrock and Cambodian Floodplain Reaches fluctuate between 51-56%. Under current conditions, bank erosion is not excessive (Darby et al., 2010) but we anticipate the most substantial post-dam erosion and channel adjustment in the Middle Alluvial reach and Cambodian Alluvial reaches, where bank erosion is currently occurring and where coarse bed sediment is exposed, suggesting Holocene incision (Carling, 2009a). Large island features are believed to result from chute cutoffs and suggest a dynamic river system (Carling, 2009a). Because upstream reservoirs will have a limited influence on flow regime, but will trap more than half of the total sediment load, we expect channel widening in alluvial reaches as the river seeks to recover its sediment load by eroding the channel margin, as commonly observed in sediment-starved rivers (Kondolf, 1997).

Incision is also likely except where the bed elevation is controlled by bedrock. The Middle Bedrock reach has single-thread, bedrock-confined reaches, anastomosed reaches of bedrock islands, and also includes the base-level control of Khoné Falls (a Holocene lava flow that crosses the river) (Carling, 2009a). Future high flows of sediment-starved water may erode alluvium on bars, banks, and islands without replacing. Erosion into bedrock is not expected on the timescale of decades. The Cambodian Alluvial reach is a floodplain river with active meandering in anabranching and anastomosed sections.

Individual islands are transient features, though the island complex is relatively stable (Carling, 2009a). Without replenishment, new islands will be less likely to develop and loss of the island features will likely occur. Erosion of the main channel bed and banks is also expected. The Mekong Delta will receive about half of its natural sediment load, and can be expected to experience accelerated subsidence and coastal erosion. Further research is needed on the size distribution of sediment transported by the river, and the size fractions most affected by the dams, but we expect the dams to disproportionately affect bed material load, notably sand, which is most important for building beaches and nourishing the coast. See further discussion of the Delta response to reduced sediment supply below.

Full Build Scenario

With all proposed dams constructed (full build scenario) and cumulative sediment reduction ranging from 83% below the Chinese boarder to 96% in Vietnam, we expect the most dramatic response in the alluvial reaches from Vientiane to Savannakhet and from Kratie downstream, where channel bed and banks will be susceptible to erosion.

While the bedrock-controlled reaches above Vientiane and from Savannakhet to Kratie will not down cut (except to remove any layers of erodible alluvium overlying bedrock, and/or to deepen pools) and will not have dramatic occurrences of erosion or channel instability, the extensive existing sediment deposits (bars, islands, inset floodplains and banks) will be stripped away, and bed material size will coarsen, all with potentially important ecological consequences. If the thin veneer of sediment in the bedrock reaches is removed, it can substantially alter the substrate, base flow channel roughness, and water velocity that influence fundamental elements of habitat availability for the benthic macroinvertebrates, fishes, and other aquatic biota. Since little is known about many Mekong species, it is difficult to predict their response to channel change and consequent loss of habitat.

If the main stream dams are constructed, they will inundate long reaches of the river and their backwater effects will extend further upstream. Within these reservoirs and back- water areas, rather than experiencing erosion from energetic flows, the channel will become a depositional zone. An important ecological feature of the river are the deep pools that provide essential habitat for native fishes and river dolphins (Poulsen and Vlabo-Jorgensen 2001, Baird and Flaherty 2005), and which are maintained by scour created by local hydraulics. Sediment starvation below dams is unlikely to negatively affect these pools through increased erosion. However, within the extensive zones of reservoir inundation and backwater, local hydraulics will change, likely eliminating the scouring currents that have maintained these features, and they will begin to fill with sediment and debris.

3.3    Potential effects on Mekong River delta from analogous cases

At present, approximately 21,000 km2 of land in the Mekong Delta is less than 2 m abovesea level and 37,000 km2 is regularly flooded (Syvitski et al., 2009). The pre-dam sediment accumulation rate across the Mekong Delta was ~0.5 mm yr-1 while relative sea level is rising at ~6 mm yr-1 (Syvitski et al., 2009). While sediment delivery to the Mekong Delta has remained relatively constant over the 20th century, recent decades have seen accelerated rates sea level rise and more rapid compaction due to groundwater extraction. Thus, even under the pre-dam sediment regime, the delta was submerging and flood-prone areas expanding. Future reductions in sediment discharge from the Mekong River or channelization in the Delta will exacerbate the rate of land loss.

Similar to “full-build” predictions for the Mekong River by Kondolf et al. (submitted), the Colorado, Ebro, Indus, Krishna, Nile, and Yellow River deltas have all experienced sediment reductions of 90% or more (Table 1). Since those deltas have comparable or lower rates of relative sea level rise than the Mekong and similar intensities of wave energy, they provide a reasonable framework for understanding the likely impacts of unmitigated dam construction. The Indus, Nile, and Yellow River deltas were all prograding prior to dam construction and subsequently were net erosional. For example, the Indus coast was prograding ~100 m y-1 before dam construction and retreating ~50 m y-1 in recent decades. Pre-dam delta growth is not reported for the Colorado, Ebro, and Krishna, but all are actively eroding in the post-dam period (see Table 1 for citations).

Rates range from 1-90 km2 y-1 of area lost per year and from 10 to 70 m y-1 of coastline retreat. Detailed modeling of the delta is required to make quantitative predictions of erosion, but experience from around the world suggests a high likelihood of widespread erosion unless sediment management practices are implemented for proposed Mekong dams.

4. Discussion

Deltas evolve through a complex interplay of river, tides, waves, biological and human fac systems. These are extremely difficult and important processes to understand, yet likely impossible to parameterize accurately given the unpredictable nature of storm events and other stochastic processes. However, given the magnitude of sediment starvation likely to occur in the near future, our review tors. For example, mangrove communities slow wave velocities, thereby efficiently trapping sediment, and improving water quality and preventing coastal erosion. The extent of mangrove ecosystems in the Mekong delta has remained stable in recent decades (Shearman et al., 2013). Elsewhere, mangroves are at risk from rapid sediment deposition (Ellison 1999) (not likely in the Mekong), as well as sea-level rise and increased storm intensity, aquaculture and water quality impacts, and sediment reductions from dams (Thampanya et al., 2006). Such interactions exemplify the challenges posed to modelers of deltaicof experiences elsewhere, combined with fundamental geomorphic principles, can provide an initial prediction of likely effects. In a data-limited, poorly understood system such as the Mekong, implementation of detailed models may be unrealistic due to the lack of long- term and/or reliable data for calibration. By relying on the global dataset we hope to inform decision makers and stakeholders in the Mekong River basin while advances until accurate modeling and forecasting tools become available.

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Acknowledgements

Tim Burnhill and George Annandale provided helpful background information and insightful comments. We also thank Tom Wild, Matti Kummu, Jorma Koponen, Chris Alford, and Steve Darby for helpful discussions and background information. This research was partially supported by the US Agency for International Development funded project “A Climate-Resilient Mekong: Preserving the Flows that Nourish Life”, the Mekong River Commission, and the Beatrix Farrand Endowment of the Department of Landscape Architecture at the University of California Berkeley.

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Table 1. Deltas of the World




References:
a)                Syvitski et al. 2009
b)                Hori and Saito 2008
c)                Luecke et al. 1999
d)                Rovira and Ibàñez 2007
e)                Gamage and Smakhtin 2009
f)                 Coleman and Huh 2003

  1. Encyclopedia Britannica Online 2013
  2. Grillas 2013
  3. Ibàñez, et al. 1996
  4. Less than 5 m above mean sea level
  5. Nilsson et al. 2005- flow regulation calculated as sum of live reservoir storage relative to virgin mean annual
  6. Gupta et al. 2012
  7. Alexander et al. 2012
  8. Dai et al. 2008
  9. Dang et al. 2010
  10. Yang et al. 2011
  11. Wang et al. 2010
  12. Syvitski and Saito 2007
  13. Kundzewicz, et al. 2009
  14. Schmidt 2008
  15. Luecke et al. 1999
  16. Jimenez and Sanchez-arcilla 1997
  17. Malini and Rao 2004
  18. Giosan el al. 2006
  19. Wells and Coleman 1984
  20. Couvillion el al. 2011
  21. aa)           Frihy el al. 1994
  22. bb)         Zhang el al. 2010
  23. cc)         Van Maren 2004
  24. dd)         Yang el al. 2003
  25. ee)           Yang el al. 2007
  26. ff) Yang el al. 2005
  27. gg)         Yang el al. 2006
  28. hh)         Peng el al. 2010
  29. ii) Saito el al. 2007
  30. jj) Lóczy 2007
  31. kk)         Panin, 1999
  32. ll) Sarwar and Woodroffe 2013 mm) Hedley et al. 2010
  33. nn) Coleman et 2008 oo) Sabatier et al., 2009
  34. pp)   Mertes and Magadzire 2008 qq)       Syvitski and Milliman 2007 rr) Anthony et al., 2012

 

Table 2. Reach characterization of the lower Mekong River

Figure 1. Dam locations are indicated for two scenarios: definite future, and full-build. Main stream dams are included in full-build scenario, but represented separately.

 
Figure 2. Reaches of predominantly bedrock vs alluvial channel, as generalized from Adamson (2001) Gupta (2004) and Carling (2009 a). Bedrock-controlled channel reaches are likely to experience rapid loss of surficial sediment deposits, but will not manifest large channel changes in response to reduced sediment loads, whereas alluvial reaches will likely incise and/or widen as they erode to compensate for reduced sediment supply.

 
Figure 3. Bedrock channel of the Mekong River with surficial sand deposits 1-2 m thick, near Xayaburi, Laos. (photo by Kondolf, January 2012)

Nguồn: Mekongblog

Posted by nhomlymha at 4:30 AM

 

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