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Author's personal copy Geomorphology 119 (2010) 181–197

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Geomorphology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g e o m o r p h

Basin-wide sediment trapping efficiency of emerging reservoirs along the Mekong M. Kummu a,⁎, X.X. Lu b, J.J. Wang b, O. Varis a a b

Water & Development Research Group, Aalto University, P.O. Box 15200, FIN-00076 Aalto, Finland Department of Geography, National University of Singapore, 119260 Singapore

a r t i c l e

i n f o

Article history: Received 28 September 2009 Received in revised form 5 March 2010 Accepted 17 March 2010 Available online 24 March 2010 Keywords: Reservoir Dam Reservoir sediment trapping efficiency Trapped sediment load Mekong Basin

a b s t r a c t The Mekong Basin has remained relatively intact, but the current plans for rapid development in the hydropower sector may threaten the riverine ecosystems. Should all the plans be materialized in tributaries and mainstream, the cumulative active storage capacity of the reservoirs would increase more than tenfold from the present level to around 20% of the annual discharge of the Mekong (505 km3). In this study a protocol is developed to estimate the trapping efficiency (TE) of the existing and planned reservoirs in the Mekong Basin based on Brune's method. The existing reservoirs have a basin TE of 15–18% and should all the planned reservoirs be built, this will increase to 51–69%. However, due to the high heterogeneity of the specific sediment yield in different parts of the basin, the trapped sediment load (TSL) is predicted to be much higher. The existing and planned mainstream dams in the Chinese part of the river have the largest impact on the river sediment load (SL) as more than 60% of the basin SL originates from this stretch of the river. The three existing reservoirs in that part of the basin have potential to trap annually approximately 32–41 Mt of sediment. If the entire cascade of eight dams is constructed, TE will increase to 78–81%, and potentially 70–73 Mt, i.e. more than 50% of the total basin sediment load (∼ 140 Mt) will be trapped annually. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Large river basins with a variety of ecological zones and corridors are important lifelines for unique ecosystems, housing innumerable aquatic and semi-aquatic species. The transport of sediment is a fundamental feature of the morphology and biochemistry of rivers (Vörösmarty et al., 2003). Sediments are also important in transporting nutrients from the continent to the ocean. Riverine suspended solids bring nutrients to floodplains and fuel aquatic production (Ouillon et al., 2004). Humanity has utilised water resources for millennia by modifying natural water courses through the construction of canals and dams. However, the construction of large reservoirs and dams really boosted after the Second World War, particularly in the United States and Europe. During the latter half of the 20th century the number of large dams higher than 15 m increased worldwide from 5000 to 45 000 (World Commission of Dams, 2000) and their current number is approximately 50 000 (ICOLD, 2007). Therefore around 10 800 km3 of water is impounded behind the registered dams (Chao et al., 2008), although actual storage capacity might be much larger if small reservoirs were included (Downing et al., 2006). This represents a

⁎ Corresponding author. Tel.: + 358 9 470 23833; fax: + 358 9 470 23856. E-mail address: matti.kummu@iki.fi (M. Kummu). 0169-555X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2010.03.018

manifold increase in the natural residence time of river water (Vörösmarty et al., 1997). Reservoir construction may currently exert the most important influence on land–ocean sediment fluxes (Walling and Fang, 2003). A notable threat to the sustainability of reservoirs is sedimentation. It is estimated that 0.5–1% of the existing storage volume in the world is lost each year (e.g. Walling, 2006). The sediment trapping efficiency (TE) of these reservoirs is high, with half of the reservoirs showing a local TE of 80% or more (Vörösmarty et al., 2003). TE stands for the ratio of sediment deposition in the reservoir and synchronous total sediment input to the reservoir. It is estimated that over 50% of the basin-scale sediment flux in regulated basins is trapped in artificial impoundments (Vörösmarty et al., 2003). Including all the basins, the interception of global sediment flux by registered reservoirs is estimated to be approximately 4–5 Gt yr−1 (Vörösmarty et al., 2003) or 25–30% of the total land–ocean sediment flux that is estimated to be 15–20 Gt yr−1 (e.g. Milliman and Meade, 1983; Milliman and Syvitski, 1992). TE of an individual reservoir depends on various aspects, such as the volume of the reservoir compared to the inflow discharge, type and properties of the dam and reservoir, and sediment properties (e.g. Heinemann, 1984; USACE, 1989; Morris and Fan, 1998). There are various ways to estimate TE. The most accurate methods for existing reservoirs are either a direct measurement of the changes in reservoir volume (e.g. Rausch and Heinemann, 1984) or observing the inflow and outflow suspended sediment loads and bedload

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M. Kummu et al. / Geomorphology 119 (2010) 181–197

entering to the reservoir in question (e.g. Heinemann, 1984). To estimate TE for a planned reservoir or a reservoir where no field work has been done, detailed numerical models (e.g. Campos, 2001) or empirical equations (e.g. Brown, 1944; Churchill, 1948; Brune, 1953) need to be used. Due to the multiple parameters involved in the actual TE of a reservoir, there are naturally many uncertainties involved in the models and equations. This is particularly the case when the basin-wide TE values of various reservoirs are being estimated. By changing the natural sediment fluxes through such activities as reservoir construction, humans have greatly influenced not only the sediment fluxes but the overall river morphology and ecosystem in multiple ways. The impacts include increased changes in river channel morphology (Lane and Richards, 1997; Fuller et al., 2003; Rinaldi, 2003; Li et al., 2007), nutrient transport (e.g. Humborg et al., 1997), carbon sequestration (Stallard, 1998; Smith et al., 2001), and trace gas emission due to decomposition of deposited organic material (St. Louis et al., 2000; Rosa et al., 2004; Varis et al., in press). The Mekong River in Southeast Asia is one of the few large river basins that has not been irreversibly modified by large scale infrastructure (Kummu and Sarkkula, 2008; Keskinen et al., in press). While the first dam in the Mekong mainstream as well as some major dams in tributaries have already been built, flow regimes in the lower reaches of the mainstream are still relatively natural (Mekong River Commission, 2005). Various large hydropower dams as well as diversions for irrigation are planned throughout the basin, some on the tributaries and others on the mainstream (e.g. King et al., 2007; Mekong River Commission, 2008). There are currently 28 dams in the Mekong Basin, three of which are located in the Mekong mainstream in China. There are extensive plans to build reservoirs in the tributaries within Lao PDR, Vietnam and Cambodia, and 16 mainstream dams in China, Lao PDR and Cambodia (King et al., 2007; Mekong River Commission, 2008; Keskinen et al., in press). The status of the planned dams, however, varies considerably. So far the research and impact assessment of the planned dams have mostly concentrated on the impact on water quantity (Adamson, 2001; World Bank, 2004; ADB, 2004, 2008; Kummu and Sarkkula, 2008) along other disciplines such as fisheries (e.g. Dugan, 2008). There do exist estimates of the impact of dams on sediment trapping (Lu and Siew, 2006; Fu and He, 2007; Kummu and Varis, 2007; Fu et al., 2008). The focus, however, has mainly been on the Yunnan cascade of dams in the upper Mekong or Lancang in China. Basin-wide sediment fluxes have been analysed and discussed by Walling (2005, 2008) and Wang et al. (2010) while the role of sediments in Tonle Sap Lake and its floodplains has been presented by Kummu et al. (2008a). However, a basin-wide picture of the impact of reservoirs on sediment trapping is lacking. Hence, the main aim of this study is to estimate the sediment trapping by the existing and planned reservoirs across the entire Mekong Basin. For this purpose, first a protocol to estimate the basinwide TE based on methods presented by Brune (1953) and Vörösmarty et al. (2003) is developed, and then used to estimate TE under five cases. We also compare the theoretical TE calculations to the theoretical trapped sediment load (TSL) calculations, as the specific sediment yield (SSY) varies considerably within the basin. At the end of the paper a sensitivity analysis is performed to analyse the confidence intervals for the TE and TSL calculations. This paper is, therefore, an important step forward in diagnosing the impact of existing and planned reservoirs on the sediments in the Mekong Basin. The developed approaches of estimating TE and performing the sensitivity analysis for Brune's method may be also applicable to other basins. This paper thus focuses on TE calculations and estimations of TSL behind the dams. The downstream impacts due to disturbed sediment fluxes and detailed estimations of the reservoirs impacts on sediment loads are beyond the domain of the paper and are, therefore, subject for future studies.

2. Study area The Mekong is the largest river in Southeast Asia with a basin area of 816 000 km2 (Kummu, 2008). The length of the river is estimated to be 4909 km (Liu et al., 2007). The Mekong originates from the Qinghai Province and Eastern Tibet, China. From there, the river crosses the Chinese province of Yunnan, flowing through narrow gorges in a very steep topography for most of its upper course. After leaving China, the Mekong marks the border between Myanmar and Lao PDR. Further downstream, the river runs through Lao PDR, Thailand, Cambodia, and Vietnam to the South China Sea (Fig. 1). With approximately 505 km3 of water discharge each year (Shiklomanov, 1999), the Mekong is the world's 10th largest river (Kummu, 2008). Varis et al. (2008, pp. 146) state that “The Mekong region is currently undergoing rapid transitions, socially, economically, and environmentally”. Water is related to these changes in a very profound manner, and the Mekong River and its tributaries are seeing escalating plans and interventions for development, most notably in the form of hydropower (Keskinen et al., in press). 3. Data This section describes the data used for the analysis, which can be roughly divided into reservoir characteristics data and sediment load data. 3.1. Sub-basin reservoirs The Mekong Basin was divided into sub-basins based on the tributary classification in the Lower Mekong Basin (LMB; Fig. 1 and Table 1). The Upper Mekong Basin (UMB) is considered to be one subbasin i.e., Lancang. TE was also calculated separately for each of the Lancang mainstream dams. The sub-basins under this study cover around 69% of the total basin area and are responsible for approximately 76% of the total discharge (Table 1). The amount of existing and planned reservoirs and the active storage capacities for each subbasin are also presented in Table 1. The location of each reservoir together with the active storage capacity is presented in Fig. 2. According to the compiled information of the existing and planned reservoirs (Fu and He, 2007; King et al., 2007; Kummu and Varis, 2007; Mekong River Commission, 2008), there were 28 large registered dams in the Mekong Basin by the end of year 2008. The total active storage capacity of existing reservoirs (∑Vi_e) is approximately 8.6 km3 or 1.7% of the annual discharge of the Mekong, 505 km3 based on Shiklomanov (1999). In addition, there are 14 reservoirs under construction and 92 planned for the sub-basins with the total active storage capacity (∑Vi_p) of 91.4 km3. Supposing that all the plans become materialised, the active storage would be more than tenfold compared to the current situation and it would represent approximately 20% of the annual discharge of the Mekong. The planned reservoirs are, however, in various stages and most probably not all the reservoirs will ever be built. 3.2. Mainstream reservoirs There are various plans to build mainstream dams in the LMB and UMB (Fig. 2 and Table 2). Eleven dams are proposed (nine in Lao PDR and two in Cambodia) in the LMB and five are under construction or designed in the UMB in addition to the three existing ones (Fig. 2). The total active planned storage capacity is 4.1 km3 in the LMB and 23.1 km3 in the UMB. Six dams of the 11 planned in the LMB are practically run-of-the-river dams with a small or no reservoir (active storage around 0.1 km3 or less). The Sambor reservoir, Cambodia, is the largest reservoir that is planned to be constructed in the LMB part of the mainstream (Table 2). The cumulative storage of all the existing and planned reservoirs in the basin, including those in sub-basins and in the LMB

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Fig. 1. Map of the Mekong basin with existing mainstream and tributary dams. The Mekong sub-basins with existing or planned dam(s) are indicated; the number in each sub-basin refers to Table 1, where the name and other characteristics are listed. Sources—GIS base layers: Mekong River Commission (2006) and USGS (2001); Location of dams: Mekong River Commission (2008).

mainstream is presented in Fig. 3. For some of the planned reservoirs the estimated construction year is not available. The growth in the total storage volume due to those reservoirs is presented separately in Fig. 3.

3.3. Sediment load The sediment data are rather scarce in the Mekong tributaries and thus, no detailed observations of river sediment load (SL) are available



Table 1 Sub-basins, their general characteristics and reservoir data. See location of each sub-basin in Fig. 1. Lancang indicates the Upper Mekong Basin (UMB) within China and Myanmar. (Sources: Fu and He, 2007; King et al., 2007; Kummu and Varis, 2007; Kummu et al., 2008b; Mekong River Commission, 2008; Wang et al., 2010). Sub-basin information

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

Reservoirs

Sub-basin

Asb km2

Qsb km3 yr−1

SSYa t km−2 yr−1

Huai Bang Lieng Lancang Nam Beng Nam Chi Nam Hinboun Nam Kam Nam Khan Nam Ma Nam Mang Nam Mun Nam Ngum Nam Nhiam Nam Nhiep Nam Ou Nam Pho Nam Phoul Nam Phuong Nam Sane Nam Suong Nam Tha Nam Theun Se Bang Fai Se Bang Hieng Se Bang Nouan Se Done Se Kong Se San Sre Pok St. Pursat St. Sangker St. Sen TOTAL % of basin

657 184,845 2193 49,067 2702 3506 7409 1128 1788 70,827 17,169 1952 4496 26,130 3431 2057 3420 2224 6687 8691 14,894 10,188 19,412 3085 7730 28,766 18,684 31,079 5920 4338 16,232 560,708 69%

1.7 73.6 0.9 29.5 1.1 4.7 4.2 1.0 3.2 4.6 20.7 3.2 6.2 19.5 2.9 1.2 5.4 2.9 2.8 5.1 15.4 31.9 17.4 2.3 7.1 29.6 41.1 29.0 5.3 5.0 7.0 385 76%

200 489 230 18 60 35 113 240 40 26 36 30 40 237 250 30 80 40 170 240 41 80 163 175 206 220 240 240 73 97 33 135

exs

Vreg_e km3

3

1.388

3

0.002

1

0.000

plan 1 5 1 2

1 3 1

0.045 0.002 4.700

3

0.016

1

0.000

1

0.015

2 1 5 3

0.001 0.649 0.794 0.951

28

8.56 1.6%

4 1 1 9 1 5 11 1 1 3 3 2 1 7 2 4 1 3 19 9 4 2 2 1 106

Vreg_p km3

tot

Vreg km3

0.035 23.092 0.098 0.000 1.250 0.000 2.239 0.000 0.506 0.000 5.686 0.339 1.269 3.769 2.738 0.499 0.040 2.080 2.102 0.676 10.971 0.624 1.464 1.477 1.756 10.418 6.060 7.149 0.985 1.150 2.890 91.42 18%

1 8 1 3 2 1 4 1 2 3 10 1 5 14 1 1 4 3 2 1 8 2 4 1 5 20 14 7 2 2 1 134

0.035 23.948 0.098 0.002 1.250 0.000 2.239 0.000 0.551 0.002 10.386 0.339 1.269 3.784 2.738 0.499 0.040 2.080 2.102 0.676 10.986 0.624 1.464 1.477 1.757 11.067 6.855 8.100 0.985 1.150 2.890 99.39 20%

Asb: sub-basin surface area. Qsb: annual discharge of a sub-basin. SSY: specific sediment yield. exs: number of existing dams. Vreg_e: active storage volume of existing reservoirs. plan: number of planned reservoirs. Vreg_p: active storage volume of planned reservoirs. tot: total number of reservoirs. Vreg: active storage volume of existing and planned reservoirs. a SSY figures in bold are based on observed data (details are in Online Supplement), and the italicised figures are estimations based on the observed values from neighbouring subbasins. See Fig. 4 for the location of the observed sub-basins.

for all the investigated sub-basins. There were, however, data available from literature for four sub-basins: i. Lancang sub-basin: Jiuzhou (basin area: 87,205 km2) and Gajiu (107,681 km2) stations (He and Hsiang, 1997; Fu et al., 2008) and Chiang Saen (184,845 km2) at the lowest point of the sub-basin (Wang et al., 2010) ii. St. Pursat, St. Sangker, and St. Sen (Kummu et al., 2008a). Further, we found enough data for SL calculations for another 13 subbasins, of which two are outside the sub-basins of this study, namely Nam Songkhram and Nam Lik; details are in Online supplement. The SL calculations for these 13 basins were done by using multipleyear rating curve between suspended sediment concentration (SSC) and water discharge for each site. Daily sediment load and water discharge data were used for the rating curves. Such a multiple-year rating curve was first built by using all of the samples during the entire period (1962–2005; details are in Table S1 of Online Supplement). Then the rating curve was applied to individual years during this period. The sediment load data were used to calculate SSY by using the catchment area of each observation point. The discharge and SSC data were collected from the Mekong River Commission database called

HYMOS (Mekong River Commission, 2007). The SL from literature and results from above described analysis are presented spatially for each sub-basin in Fig. 4 (details are in Table S2 of Online supplement). The quality of the SSC data collected by the Mekong River Commission is analysed and discussed by Walling (2005, 2008). Most of the constructed dams in the sub-basins with observed sediment load have nil TE (details are in Table S2 of Online supplement; see also Table 3). Constructed sub-basin reservoirs with TE N 0 exist only in one sub-basin namely Nam Ngum (with TE of 44%). Therefore, from that sub-basin we selected to use the Ban Na Luang station located above the existing Nam Ngum 1 reservoir, in order to get undisturbed SSY for the sub-basin. Thus, the observed sub-basin sediment loads can be handled as undisturbed SL. The observed SL values were then used to estimate SL for sub-basins where neither measurements nor information from literature were available. This was done by using simple method where the values were estimated based on the observed values in the neighbouring subbasins (Table 1). The observed sediment loads represent approximately 75% or 100 Mt of the total SL from the sub-basins included in the study (Table 4) being 71% of the annual sediment load of the entire basin.


185

Fig. 2. Locations and active storages of the existing and planned reservoirs in the Mekong Basin. (A) Sub-basin reservoirs. (B) UMB and LMB mainstream reservoirs. Sources: King et al. (2007), Kummu and Varis (2007), and Mekong River Commission (2008).

According to the SL calculations, majority of the Mekong sediment originates from two areas: Lancang sub-basin (65% of the basin SL) and so called 3S area that includes the Se Kong, Se San and Sre Pok sub-basins (approximately 13% of the basin SL) (Table 4). Together these two areas account almost 80% of the total estimated SL of the Mekong Basin. The LMB mainstream SL observations prior the Manwan Dam, i.e. prior the year 1993, were adapted from Wang et al. (2010). There were only three existing reservoirs during the analysis period of 1962–1992 above the lowest observation station of Khong Chiam (this is around the location where the Ban Kum mainstream dam is planned to be constructed—see location in Fig. 2). These existing reservoirs are located in the following sub-basins: Nam Ngum, Nam Kam and Nam Phuong. They have potential to trap around 0.3 MT of sediments annually (Table 3) or approximately 0.2% of the annual average observed sediment loads at Khong Chiam. Therefore, the

observed mainstream sediment loads are very close to the undisturbed sediment loads. 4. Approaches to estimate reservoir trapping efficiency The approaches used in the TE and TSL calculations are described in this section. At the end, the approach for the sensitivity analysis is presented. 4.1. Calculation of TE for each reservoir Due to the high number of reservoirs included in our study (143), we were not able to use a detailed mathematical model for each reservoir. Thus, we decided to use the approximation to predict individual reservoir TE developed by Brune (1953). The method predicts TE as a function of local residence time in years (ΔτR), defined



Table 2 Local TE results for the planned LMB mainstream dams (see Fig. 2 for locations). The existing reservoirs are bolded. General information Ams,k km2

TE results Qms,k km3 yr−1

Vk km3

Δτms,

TEms,

k

k

CI

Upper Mekong Basin (UMB) Reservoirs A Gonguoqiao B Xiaowan C Manwan* D Dachaoshan* E Nuozhadu F Jinghong* G Ganlanba** H Mengsong**

97,200 113,300 114,500 121,000 144,700 149,100 151,800 160,000

31.1 38.5 38.8 42.3 55.2 58.0 59.3 63.7

0.120 9.900 0.344 0.467 12.300 0.577 0.120 0.120

0.004 0.257 0.009 0.011 0.223 0.010 0.002 0.002

0.20 0.90 0.47 0.52 0.89 0.50 0.0 0.0

0.0–0.39 0.88–0.93 0.34–0.60 0.41–0.64 0.87–0.92 0.38–0.62 0.0–0.16 0.0–0.12

Lower Mekong Basin (LMB) Reservoirs I Pakbeng J Luang Prabang K Xayabuly** L Paklay M Sanakham** N Sangthong–Pakchom** O Ban Kum** P Latsua** Q Don Sahong** R Stung Treng** S Sambor Basin Mouth

218,000 230,000 272,000 283,000 292,000 295,500 418,400 550,000 553,000 635,000 646,000 815,000

100.0 120.2 125.8 127.1 131.2 138.3 288.5 302.7 325.1 432.5 439.9 505

0.442 0.734 0.225 0.384 0.106 0.012 0.000 0.000 0.115 0.070 2.000

0.004 0.006 0.002 0.003 0.001 0.000 0.000 0.000 0.000 0.000 0.005

0.21 0.35 0.0 0.09 0.0 0.0 0.0 0.0 0.0 0.0 0.29

0.02–0.43 0.20–0.52 0.0–0.10 0.0–0.31 – – – – – – 0.12–0.44

*Existing mainstream reservoir. **Run-of-the-river dam with very small reservoir compared to the annual discharge. Abas_k: drainage area at location of a mainstream reservoir k. Qms_k: discharge at a mainstream reservoir k. Vk: active storage volume of a mainstream reservoir k. Δτms,k: residence time of a mainstream reservoirs k. TEms,k: trapping efficiency of a mainstream reservoir k. CI: Confidence interval for the TE calculations.

by dividing the effective reservoir volume by local mean annual discharge:

ΔτR;i =

Vi Qi

ð1Þ

where ΔτR,i = approximated residence time of reservoir i, Vi = active storage (i.e. operation volume) of reservoir i, and Q i = discharge at reservoir i.

After the local residence time was calculated, Brune's method was used to estimate the individual reservoir trapping efficiency (TER,i): 0:05α TER;i = 1− pffiffiffiffiffiffiffiffiffi ΔτR

ð2Þ

where TER,i = trapping efficiency for reservoir i, and α = constant. In the TE calculations, the constant α was kept 1, representing the median curve in Brune's method (Brune, 1953). In sensitivity analysis

Fig. 3. Cumulative storage of the existing and planned reservoirs in the basin. (Sources: Fu and He, 2007; King et al., 2007; Kummu and Varis, 2007; Mekong River Commission, 2008) and temporal development of TSL by sub-basin reservoirs.


187

Fig. 4. SSY for the sub-basins. (A) Observed sub-basin SSY. (B) Estimated sub-basin SSY values with the observed sub-basin classifications (B) (see also Table 1). Sources—GIS base layers: Mekong River Commission (2006); Observed and estimated SSY: see Section 3.3.

the confidence interval (CI) was calculated with different values of α (see Section 4.5). We decided to use Brune's method over the other empirical methods (e.g. Brown, 1944; Churchill, 1948; Siyam et al., 2001) because of the following reasons: i. The method is widely used and found to provide reasonable estimates of long-term, mean TE (Morris and Fan, 1998; Vörösmarty et al., 2003). ii. The method is used in various studies in the region with reasonable results compared to the observed TE values (e.g. Fu and He, 2007; Jothiprakash and Garg, 2008; Hu et al., 2009). iii. To keep the comparability between the TE studies done in the Mekong (Fu and He, 2007; Kummu and Varis, 2007) and other river basins (e.g. Vörösmarty et al., 2003).

According to the available hydropower development plans (ADB, 2004; King et al., 2007; ADB, 2008; Powering Progress, 2009), the reservoirs included in those are or will be normal ponded reservoirs, to which Brune's curve is developed for. Further, there are no plans indicating that the dams would have so called sediment release tunnel to flush the trapped sediment during the flood (ADB, 2004; King et al., 2007; ADB, 2008; Powering Progress, 2009). It is, therefore, reasonable to assume that majority of the planned reservoirs will fall within the envelope around Brune's mean curve (Brune, 1953) with rather high confidence level. This envelope around Brune's mean curve addresses the issue of sediment particle size through an upper bound corresponding to highly flocculated and coarse sediments and a lower bound for colloidal, dispersed, fine-grained particles (Vörösmarty et al., 2003). For this reason we decided to use the lower and upper envelope



Table 3 Sub-basin TE and TSL for the existing reservoirs. The individual mainstream reservoirs in the Lancang sub-basin are presented in Table 2. Sub-basin information

2 4 6 9 10 11 14 17 21 25 26 27 28

Reservoirs

TE results

TSL results

Sub-basin

Asb km2

Qsb km3 yr−1

SLa Mt yr−1

tot

Vreg km3

Areg km2

Qreg km3 yr−1

Δτreg

TEreg

Δτsb

TEsb

TSL Mt yr−1

Lancang Nam Chi Nam Kam Nam Mang Nam Mun Nam Ngum Nam Ou Nam Phuong Nam Theun Se Done Se Kong Se San Sre Pok TOTAL % of basin

184,845 49,067 3506 1788 70,827 17,169 26,130 3420 14,894 7730 28,766 18,684 31,079 457,905 54%

73.6 29.5 4.7 3.2 4.6 20.7 19.5 5.4 15.4 7.1 29.6 41.1 29.0 283 52%

90.3 0.9 0.1 0.1 1.8 0.6 6.2 0.3 0.6 1.6 6.3 4.5 7.5 121 86%

3 3 1 1 3 1 3 1 1 2 1 5 3 28

1.388 0.002 0.000 0.045 0.002 4.700 0.016 0.000 0.015 0.001 0.649 0.794 0.951 8.6 1.7%

160,000 12,104 296 577 117,000 16,900 25,979 714 14,070 6360 9700 18,684 26,200

63.70 7.28 0.39 1.02 7.57 20.40 19.36 1.13 14.51 5.85 9.98 41.12 24.44

0.02 0.00 0.00 0.04 0.00 0.23 0.00 0.00 0.00 0.00 0.07 0.02 0.04

0.66

0.02

0.40

36.6

32-41

0.89

0.23

0.44

0.3

0.27-0.28

0.80 0.64 0.75

0.02 0.02 0.03

0.02 0.32 0.23

0.1 1.4 1.7 40.2 29%

0.1-0.1 1.3-1.6 1.6-1.9 35-45 25-32%

CI Mt yr−1

0.76

Asb: sub-basin surface area. Qsb: annual discharge of a sub-basin. SL: sediment load. tot: total number of reservoirs. Vreg: active storage volume of existing and planned reservoirs. Areg: area of a regulated part of a sub-basin. Qreg: discharge of a regulated part of a sub-basin. Δτreg: residence time of a regulated part of a sub-basin. TEreg: trapping efficiency of a regulated part of a sub-basin. Δτsb: residence time of a sub-basin. TEsb: trapping efficiency of a sub-basin. TSL: trapped sediment load in a sub-basin. CI: Confidence Interval for the TSL calculations. a SL figures in bold are based on observed data (details are in Online Supplement), and the italicised figures are estimations based on the observed values from neighbouring subbasins. See Fig. 4 for the location of the observed sub-basins.

curves in the sensitivity analysis to analyse the confidence interval of the TE calculations (see Section 4.5). 4.2. Calculation of TE for sub-basins A modification of the method presented by Vörösmarty et al. (2003) for basin-wide TE calculations was used to estimate the trapping efficiency for the regulated portion of the j-th sub-basin (TEreg,j). To enable this, the residence time for the regulated portion of the j-th sub-basin (Δτreg,j) was calculated by dividing the total active storage capacity (i.e. operation volume) with the discharge at the location of the lowest reservoir in that sub-basin (Qreg,j): nj

Δτreg;j =

∑ Vi 1

Q reg;j

ð3Þ

at Gonguoqiao in China (Fig. 2). All the sub-basins above the mainstream reservoir k were taken into account when calculating TE for the reservoir. For calculating TE of the next mainstream reservoir downstream, TE for the previous reservoir was handled as one of the sub-basin(s) above the reservoir. m

TEbas;k

Q ms;k − ∑ TEsb;j Q sb;j 1 = 1− 1−TEms;k Q ms;k

ð6Þ

where TEbas,k = approximated trapping efficiency for the proportion of the basin upstream of the mainstream reservoir k, TEms,k = approximated trapping efficiency of the mainstream reservoir k, and Qms,k = discharge at the mainstream reservoir k. The sub-basin(s) above the mainstream reservoir k are indicated with symbol j. 4.4. Calculation of trapped sediment load (TSL)

TEreg;j

0:05α = 1− qffiffiffiffiffiffiffiffiffiffiffiffiffiffi Δτreg;j

ð4Þ

TE for the whole sub-basin (TEsb,j) was also calculated based on the ratio of discharge from the regulated portion of the sub-basin to total sub-basin discharge (Q reg,j/Q sb,j): TEsb; j =

Q reg;j TE Q sb;j reg;j

ð5Þ

4.3. Calculation of basin-wide TE The UMB and LMB mainstream reservoirs were included into the basin-wide TE calculations, which started from the uppermost reservoir

The amount of potentially trapped sediment by sub-basins was estimated with the help of the observed and approximated specific sediment yield data (Table 1) combined with the TE calculations. TSL was first calculated separately for each sub-basin. By applying the basin TE protocol, TSL was estimated for the upper half of the basin by using the sub-basin calculations and the observed sediment loads for the mainstream stations (Wang et al., 2010). 4.5. Sensitivity analysis The aim of the sensitivity analysis is to test how sensitive the TE results are for the constant α. In the TE calculations, α was kept 1, representing the median curve in Brune's (1953) method as stated above. This was done as only one reservoir in the basin (Manwan in China) has a record of direct TE measurements (Fu and He, 2007).


189

Table 4 TE and TSL estimations for the sub-basins when all the existing and planned reservoirs are taken into account. Sub-basin information

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31

Reservoirs

TE results

TSL results

Sub-basin

Asb km2

Qsb km3 yr−1

SLa Mt yr−1

tot

Vreg km3

Areg km2

Qreg km3 yr−1

Δτreg

TEreg

Δτsb

TEsb

TSL Mt yr−1

CI Mt yr−1

Huai Bang Lieng Lancang Nam Beng Nam Chi Nam Hinboun Nam Kam Nam Khan Nam Ma Nam Mang Nam Mun Nam Ngum Nam Nhiam Nam Nhiep Nam Ou Nam Pho Nam Phoul Nam Phuong Nam Sane Nam Suong Nam Tha Nam Theun Se Bang Fai Se Bang Hieng Se Bang Nouan Se Done Se Kongb Se Sanb Sre Pokb St. Pursat St. Sangker St. Sen Total % of basin

657 184,845 2193 49,067 2702 3506 7409 1128 1788 70,827 17,169 1952 4496 26,130 3431 2057 3420 2224 6687 8691 14,894 10,188 19,412 3085 7730 28,766 18,684 31,079 5920 4338 16,232 560,708 69%

1.7 73.6 0.9 29.5 1.1 4.7 4.2 1.0 3.2 4.6 20.7 3.2 6.2 19.5 2.9 1.2 5.4 2.9 2.8 5.2 15.4 31.9 17.4 2.3 7.1 29.6 41.1 29.0 5.3 5.0 7.0 385 76%

0.1 90.3 0.5 0.9 0.2 0.1 0.8 0.3 0.1 1.8 0.6 0.1 0.2 6.2 0.9 0.1 0.3 0.1 1.1 2.1 0.6 0.8 3.2 0.5 1.6 6.4 4.5 7.5 0.4 0.4 0.5 133 95%

1 8 1 3 2 1 4 1 2 3 10 1 5 14 1 1 4 3 2 1 8 2 4 1 5 20 14 7 2 2 1 134

0.035 23.948 0.098 0.002 1.250 0.000 2.239 0.000 0.551 0.002 10.386 0.339 1.269 3.784 2.738 0.499 0.040 2.080 2.102 0.676 10.986 0.624 1.464 1.477 1.757 11.067 6.855 8.100 0.985 1.150 2.890 99.39 20%

80 160,000 1908 12,104 1380 296 7300 156 577 117,000 16,900 1700 3750 25,979 2837 1680 714 1429 5755 8990 14,070 6350 1415 474 6360 9700 18,684 26,200 2080 2135 10,540

0.20 63.70 0.80 7.28 0.58 0.39 4.10 0.14 1.02 7.57 20.40 2.76 5.17 19.36 2.41 0.95 1.13 1.83 2.41 5.30 14.51 19.87 1.27 0.35 5.85 9.98 41.12 24.44 1.85 2.48 4.57

0.17 0.38 0.12

0.88 0.92 0.86

0.02 0.33 0.11

0.11 0.80 0.75

0.01 71.8 0.38

0.01–0.01 70.2–73.3 0.36–0.39

2.17

0.97

1.11

0.49

0.08

0.07–0.08

0.55

0.93

0.54

0.92

0.77

0.75–0.78

0.54

0.93

0.17

0.30

0.02

0.01–0.03

0.51 0.12 0.25 0.20 1.13 0.53 0.04 1.14 0.87 0.13 0.76 0.03 1.15 4.25 0.30 1.11 0.17 0.33 0.53 0.46 0.63

0.93 0.86 0.90 0.89 0.95 0.93 0.73 0.95 0.95 0.86 0.94 0.72 0.95 0.98 0.91 0.95 0.88 0.91 0.93 0.93 0.94

0.50 0.11 0.20 0.19 0.94 0.43 0.01 0.73 0.75 0.13 0.72 0.02 0.08 0.65 0.25 0.37 0.17 0.28 0.19 0.23 0.41

0.92 0.75 0.75 0.88 0.79 0.76 0.15 0.61 0.81 0.89 0.89 0.45 0.07 0.15 0.75 0.32 0.88 0.77 0.33 0.46 0.61

0.57 0.04 0.13 5.46 0.68 0.05 0.04 0.05 0.93 1.86 0.54 0.36 0.22 0.08 1.19 2.03 3.94 5.74 0.14 0.19 0.33 97.6 70%

0.55–0.57 0.04–0.04 0.13–0.13 5.29–5.62 0.66–0.68 0.04–0.04 0.03–0.04 0.05–0.05 0.91–0.93 1.78–1.92 0.53–0.55 0.32–0.39 0.21–0.22 0.08–0.08 1.16–1.21 2.00–2.06 3.80–4.07 5.61–5.87 0.13–0.14 0.18–0.19 0.32–0.33 95–100

Asb: sub-basin surface area. Qsb: annual discharge of a sub-basin. SL: sediment load. tot: total number of reservoirs. Vreg: active storage volume of existing and planned reservoirs. Areg: area of a regulated part of a sub-basin. Qreg: discharge of a regulated part of a sub-basin. Δτreg: residence time of a regulated part of a sub-basin. TEreg: trapping efficiency of a regulated part of a sub-basin. Δτsb: residence time of a sub-basin. TEsb: trapping efficiency of a sub-basin. TSL: trapped sediment load in a sub-basin. CI: Confidence Interval for the TSL calculations. a SL figures in bold are based on observed data (details are in Online Supplement), and the italicised figures are estimations based on the observed values from neighbouring subbasins. See Fig. 4 for the location of the observed sub-basins. b Sub-basins that that are part of the so called 3S area.

The dam and reservoir type and sediment properties vary naturally between the reservoirs in the basin and thus, by using only the median curve for the final results would not be justified. Therefore, in the sensitivity analysis we calculated the TE values with three values of α, representing approximately the lower envelope curve (α = 1.24), median curve (α = 1.0) and the upper envelope curve (α = 0.76) in Brune's method. With this approach we were able to estimate CI for TE calculations:

CI =

TEupper

envelope −TElower envelope

TEmedian

ð7Þ

where CI = Confidence interval, TEmedian = TE calculated with the median curve, TEupper_envelope = TE calculated with the upper envelope curve, and TElower_envelope = TE calculated with the lower envelope curve.

Based on the observed trapping efficiency of 60.5% in the Manwan reservoir, Fu and He (2007) estimated the constant α to be 0.758. This falls on the upper envelope curve. 5. Results 5.1. TE for existing sub-basin reservoirs TE for the 12 sub-basins with existing reservoirs varies from nil in the majority of sub-basins to 44% in Nam Ngum, the largest existing reservoir in the Mekong (Table 3). The existing reservoirs have the potential to trap around 40.2 Mt yr−1 or 29% of the estimated sediment flux of the basin (Table 3). Most of that sediment flux (90%) is potentially trapped by the three reservoirs in the Lancang sub-basin with a TE value of 40%. The residence time (Eq. (3)) in the sub-basins varies from 0.01 to 0.23 yr, with a total basin-wide residence time of 0.017 yr, indicating



that the existing reservoirs had active storage capacity corresponding to 1.7% of the basin mean annual discharge (Table 3). 5.2. TE for all sub-basin reservoirs TE for the existing and planned sub-basin reservoirs varies greatly among the sub-basins (Table 4 and Fig. 5). In four sub-basins the active storage was estimated to be very small (nil) compared to the discharge (Table 4). Four other sub-basins have TE between 7% and 30%, and six other sub-basins have TE between 30% and 60% (Table 4). The majority (17/31) of the sub-basins have TE larger than 60%. In the Nam Ngum and Nam Khan sub-basins TE exceeds 90% (Table 4). The existing and planned sub-basin reservoirs have the potential to trap annually 97.6 Mt of sediment on average, representing around 70% of the total basin sediment flux (Table 4). The majority of that sediment would be trapped by the Lancang part of the basin with a

potential to trap around 72 Mt or over half of the total estimated basin SL. The reservoirs in the 3S area (see Section 3.3; Table 4) have also large impact on the total TSL by potentially trapping annually 11.7 Mt of sediment. The sub-basin residence time varies accordingly (Table 4 and Fig. 5). The majority of the sub-basins (19) have a residence time below 0.3 yr (Table 4). In six sub-basins the residence time is over 0.6 yr and in Nam Hinboun even more than one year (Table 4), meaning that the total active storage capacity exceeds the annual discharge originating from the sub-basin. 5.3. TE for mainstream reservoirs The mainstream reservoirs in the UMB have much larger TE values than in the LMB due to their larger storage and smaller discharge. TE of the largest reservoirs, Xiaowan and Nuozhadu, exceeds 89% while TE

Fig. 5. Trapping efficiency calculation results for sub-basins. (A) Residence time. (B) TE. Sources of GIS base layers: Mekong River Commission (2006).


for the other reservoirs in the UMB varies between 9% and 52% (Table 2). The residence times varies from 0.002 yr (Ganlanba and Mengsong) to 0.26 yr (Xiaowan). The estimated TE for the planned LMB mainstream dams varies from nil for the seven run-of-the-river dams (Xayabuly and from Sanakham to Stung Treng) to 36% in Luang Prabang. TE is around 25% for the Sambor and Pakbeng reservoirs while 9% for the Paklay (Table 2). The residence times for each of the planned reservoirs are rather low varying between 0.001 and 0.006 yr. 5.4. Basin-wide TE The basin-wide trapping efficiency was estimated for each planned LMB dam location. Five different cases were used for the analyses: A. existing reservoirs only B. case A and planned sub-basin reservoirs but excluding planned mainstream reservoirs in the UMB C. case A and planned sub-basin reservoirs and UMB planned mainstream reservoirs D. case A and planned mainstream reservoirs in the UMB and LMB E. case A and all the planned reservoirs for sub-basins and mainstream In case A, TE was highest in Jinghong (76%) and then it drops gradually towards the basin mouth (17%) (Fig. 6). When the calculated basin TE is compared to the estimated amount of sediment that the existing reservoirs may potentially trap (40 Mt yr−1 or 29% of the total basin sediment flux; see Table 3), the latter is significantly higher because the Lancang part of the basin has a much higher SSY than any other part of the basin (Fig. 4 and Table 1). TE is significantly higher in cases B and C than in case A, having a value of 50–70% between Pakbeng and Sangthong–Pakchom and then

191

dropping to 43–46% at the basin mouth (Fig. 6). If TE for case C is compared to TSL estimations for sub-basins (70%; Table 4), TE for the former is again significantly lower for the same reason as above. In case D the variation between the stations is rather high as TE is close to 100% in various parts of the UMB and around 70% in the upper parts of the LMB (Fig. 6). TE then drops to below 50% in the middle reaches of the mainstream, as there are no large mainstream reservoirs planned between those locations (Fig. 6). The Sambor reservoir has a large impact on TE, and at the basin mouth TE is 42%. The form of the TE curve for case E is similar to that of case D but have higher TE values, particularly in the LMB part of the basin, reaching 61% at the mouth of the Mekong (Fig. 6). In all the cases analysed, TE is higher in the upper reaches of the basin mainly due to the Yunnan cascade of dams. Particularly TE for cases D and E is close to 100% in the upper reaches of the LMB. Therefore, the local impacts due to the sediment trapping would be larger in that part of the basin than in downstream parts. 5.5. Basin-wide TE: temporal development In addition to the cases presented above, the temporal change in the basin TE was analysed for the following time periods: 2009 (with reservoirs existed when analysis was made), 2012, 2015, 2018, 2022 and when all planned reservoirs are installed. The results are presented in Fig. 7. There is not much change in TE from the existing reservoirs (case A) to the year 2012 but rather significant change from years 2012 to 2015, when the largest reservoirs in the Lancang will be in operation. From 2015 to 2018 there is large increase particularly in upper parts of the LMB. The situation in 2022 is rather similar to that with all the reservoirs (case E). The temporal development of trapped sediment by sub-basin reservoirs is presented in Fig. 3.

Fig. 6. Basin TE under five different cases for each of the existing and planned mainstream dam locations (some of those are named in the graph) and basin mouth (area = 816 000 km2). See Fig. 2 for the location of the mainstream dams.



Fig. 7. Temporal developing of basin TE for each of the existing and planned mainstream dams (some of those are named in the graph) and basin mouth (area = 816 000 km2). See Fig. 2 for the location of the mainstream dams.

5.6. TSL estimations for the upper half of the basin Due to the limited amount of sediment data in the Mekong, our understanding of the basin-wide sediment dynamics is not sufficient to allow basin-wide estimation of TSL. Nevertheless, for the upper half of the basin, i.e. upstream of the Khong Chiam station, rather good datasets for the mainstream sediment loads were available as presented in detail in Section 3.3. The impacts of the reservoirs were, therefore, estimated for the upper half of the basin by using the observed SL of several stations for the LMB and UMB mainstream reservoir locations with the SL data for the sub-basins (Table 4). The TSL percentage is presented together with the basin TE results under three cases (A, C, and E) in Fig. 8. The TE and TSL results at the Ban Kum reservoir (the lowest reservoir analysed for TSL) are presented in Table 5. The results confirm that the basin TE protocol gives significantly lower values compared to the trapped sediment load. In case A, the TSL percentage is more than double compared to TE (56% vs. 23%) at the Ban Kum. In the other analysed cases the difference is not as large but still significant, being e.g. 52% vs. 75% for case C and 61% vs. 84% for case E (Fig. 8 and Table 5). 5.7. TE sensitivity to constant α With sensitivity analysis we estimated the confidence interval first for sub-basins (Tables 2 and 3) and mainstream reservoirs (Table 2), and then for the basin-wide TE calculations. The higher CI is, the more sensitive the TE result is for the constant α (see Eq. (7)). For the reservoirs with the local residence time in years (ΔτR, Eq. (1)) lower than around 0.1 yr, CI is higher than 10%, and the

reservoirs with ratios lower than 0.01 yr have CI higher than 50% (Fig. 9). The majority of the sub-basins (24 out of 31) have CI below 10% while for three sub-basins CI is above 50%. For four sub-basins TE was nil and thus, no CI calculations were needed (Fig. 9). CI is much higher for the mainstream reservoirs as only two reservoirs out of 19 have CI lower than 10% while eight mainstream reservoirs have CI higher than 50% (Fig. 9). Nine mainstream reservoirs are very small compared to the annual discharge, resulting in nil TE. In the basin-wide TE calculations CI is rather low for cases A, B and C, being in the basin mouth between 6% and 12% (Fig. 10). CI is significantly higher for cases D (47%) and E (31%) (Fig. 10) due to high CI in the mainstream reservoirs in the LMB which are included in those cases. The CI values for the basin-wide TSL calculations are rather similar to the ones of the TE calculations for cases A, B and C (Table 5). The CI values of the TE calculations for cases D and E are, however, significantly lower than the ones for TSL calculations (Table 5).

6. Discussion The data used in our analyses were gathered from various sources and thus, a thorough quality control was not feasible. For instance, the possible future impacts are estimated on the basis of the current plans for hydropower development, but the plans keep changing due to various reasons. Although there are challenges in using these kinds of data, we tried to use the best available data. We also estimated the data quality to be sufficiently good for analysing the basin TE values. In the followings some relevant issues related to the data and methods are discussed in more details.


193

Fig. 8. Estimated SL in the UMB mainstream and observed SL at the LMB mainstream presented with TSL percentage within three cases (A, C and E) which are compared to the corresponding basin TE estimations. (Source for observed SL in the UMB: Fu et al., 2008; and that for the LMB: Wang et al., 2010).

estimating the loss of capacity of each reservoir due to siltation is undergoing and thus, not included in the calculations of this paper.

6.1. Storage values—active storage vs. total storage We used the active storage for the TE calculations (Eq. (1)). The average active storage is 51% of the total storage based on the 44 reservoirs included in this study for which the total storage information is available in literature (ADB, 2004; Kummu and Varis, 2007; ADB, 2008; Powering Progress, 2009). The dead storage was, therefore, not included in our analysis. This means that the TE values are underestimated at the beginning of reservoir life where little siltation has happened. However, by using the active storage in calculations ensures that the TE values are not impacted significantly on the siltation that fills up the reservoir within the time scale of approximately several decades. The time it requires to fill up the dead storage, i.e. the time after which the siltation actually starts to reduce the active storage, naturally depends on the reservoir. The study of Table 5 TE and TSL ratios and CI at Ban Kum (see Fig. 2 for location). Case

TE CITE TSL CISL

A

B

C

D

E

0.23 19% 0.56 20%

0.48 13% 0.65 18%

0.52 6% 0.75 4%

0.38 35% 0.76 12%

0.61 18% 0.84 8%

TE: trapping efficiency. CITE: Confidence interval for TE. TSL: trapped sediment load ratio. CISL : Confidence interval for TSL.

6.2. SSY calculations SSY data were not available for individual reservoirs and therefore, the sub-basin SSY data were used. This may either underestimate or overestimate TSL for each reservoir. Further, the method to estimate SSY for the sub-basins without measured SL data includes high uncertainty. It was, however, justified to estimate SSY based on neighbour sub-basins as there was rather clear trend in the observed SSY values across the basin (Fig. 4). The observed sub-basins account for approximately 75% of the estimated SL from the sub-basins included in the study (Table 4). Therefore, the errors due to the possible over-or underestimations of SSY for non-gauged sub-basins might be significant in the sub-basin level but not that high in the basin level. Nevertheless, the TSL estimations should be looked critically and not taken as ultimate figures. These calculations are, therefore, the first step towards better estimations. They should be improved in future studies with data from further field work, including that the Mekong River Commission has planned to do in the near future. 6.3. Uncertainty of the basin-wide TE calculations We used Brune's (1953) method for the TE calculations as it is widely used and found to provide reasonable estimates of long-term, mean trapping efficiency (Morris and Fan, 1998). As almost no



Fig. 9. Median and envelope curves for Brune's method presented with TE estimations for sub-basin and mainstream reservoirs. Confidence intervals are also presented.

suspended sediment characteristics (such as grain size) or detailed information on reservoir design was available, the median curve of the method was used with the sensitivity analysis for the constant α. According to the sensitivity analysis, the mainstream reservoirs in the LMB and the smallest reservoirs in the Lancang sub-basin have large uncertainty in the estimated TE figures. The sub-basin TE results, however, have rather low uncertainty as CI is less than 10% in 77% of the sub-basins (Fig. 10). Therefore, the basin-wide TE calculations with the LMB mainstream dams included have rather high uncertainty. The basin-wide SL calculations are, however, not as sensitive to constant α as TE calculations. This is because the largest reservoirs with low uncertainty are located in the UMB where SSY is also the highest and thus, most of the sediment is trapped there with low CI. Due to the assumptions and uncertainties in the data and approach used, the results should be considered primarily as indicative of possible reservoir impacts on the sediment load in the Mekong.

the upper half of the basin confirmed that there is a significant difference between the basin TE and the total sediment trapped (Fig. 9 and Table 5). At this stage, it is not possible to calculate TSL for the whole basin due to the limitations in the observed sediment data. According to the preliminary estimations, the basin wide TSL for case D, where all the existing and planned dams are within the calculations, would be closer to 90% instead of the estimated 50– 69% by using the basin TE protocol. It is worth noting that these estimates did not include the possible erosion and other changes in the sediment dynamics due to the sediment trapping by reservoirs. Therefore, even though TSL would be 60–90% of the current sediment load, it would not mean that the sediment loads were subject to decrease that much, as there would be increased bank and bed erosion due to the ‘sediment hungry’ water downstream of the reservoirs (e.g. Kummu and Varis, 2007; Kummu et al., 2008b).

6.4. Discrepancy of the basin-wide TE calculations

7. Conclusions

SSY in the Lancang sub-basin is around three to four times as high as the average SSY of the analysed sub-basins (Fig. 4). Consequently, the Lancang reservoirs have the potential to trap much more sediment than other reservoirs with the same potential. Accordingly, even though the active storage capacity of the Lancang reservoirs is only 24% of the existing and planned sub-basin storage capacity, they have the potential to trap 78% of the estimated TSL. Because of this high heterogeneity of the sediment loads per unit area between the sub-basins, the obtained TSL percentage should be used for the calculations whenever possible. The TSL calculations for

This study estimated the potential trapping efficiency of the existing and planned reservoirs in the Mekong Basin by using Brune's (1953) method. Further, a protocol to calculate the basin TE value was developed on the basis of method presented by Vörösmarty et al. (2003). The sensitivity analysis was also performed to understand the confidence intervals of the calculations. The existing reservoirs in the Mekong, and particularly those in the Lancang part of the basin, have already a significant impact on the sediment fluxes. The planned reservoirs will significantly increase this impact. The existing dams in the sub-basin have the potential to trap


195

Fig. 10. Confidence intervals (CI) for basin TE calculations of all the five cases. TE with negative and positive CI values are presented in the upper right corner.

annually 35–45 Mt of sediment. Should all the planned dams be built in the sub-basins, the amount of trapped sediment would reach 95– 100 Mt. The basin TE was calculated under five cases and results varied from 15–18% (case A) to 51–69% (case E). Due to the high variety of SSY in the basin, we found that the TE calculations underestimated significantly the TSL percentages. The percentages at Ban Kum vary from 49–60% (case A) to 79–85% (case E). These are 35–140% larger than the TE estimations at that

location. Therefore, by using the basin-wide TE to estimate the amount of basin-wide trapped sediments, one might either significantly under-or overestimate the reservoir impacts on sediment loads depending on SSY in the sub-basins. The sensitivity analysis showed that the smallest mainstream reservoirs have rather large uncertainty in TE calculations while the uncertainty is rather low for sub-basin TE estimations. This led to the large confidence intervals in the basin TE analysis under cases D and E



(47% and 31%) whereas CI is relatively small (6–13%) under other cases. For the TSL calculations at Ban Kum the opposite is the case as CI varies from 4–20% for cases A–C and 8–12% for cases D and E. Significant part of the Mekong's sediment will be trapped in the reservoirs if the plans to build them go ahead. This may have various consequences downstream of the reservoirs. The analysis of those consequences, such as impacts on sediment loads and downstream morphological changes, are not the focus of this paper. It is, however, extremely important to understand those impacts well and thus, these issues demand rapid attention. Particularly the impacts on the floodplains and delta area in Cambodia and Vietnam might be severe and should be analysed in details in future studies. Acknowledgements The authors would like to thank the Mekong River Commission for providing the database on the existing and planned reservoirs, and the sediment data for the SSY and SL analysis. The authors are grateful to Prof. em. Pertti Vakkilainen, Marko Keskinen and other staff at the Water Research Group of Aalto University for their support. Further, the long-term collaboration with Dr. Juha Sarkkula and Jorma Koponen for the Mekong studies is highly appreciated. Marika Makkonen is acknowledged for her comments on the manuscript and Paula Nieminen for language editing. The constructive comments of the two independent reviewers are also highly appreciated together with the useful comments and editing by Prof. Takashi Oguchi. The work has been funded by the Academy of Finland project 111672, Maa-ja vesitekniikan tuki ry, and National University of Singapore (R-109-000-086-646). The first author has also received postdoctoral funds from Aalto University. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.geomorph.2010.03.018. References Adamson, P.T., 2001. Hydrological perspectives on the Lower Mekong Basin—the potential impacts of hydropower developments in Yunnan on the downstream flow regime. International Water Power and Dam Construction 2001, 16–21 March. ADB, 2004. Cumulative Impact Analysis and Nam Theun 2 Contributions, Final Report. Prepared by NORPLAN and EcoLao for Asian Development Bank. 143 pp. ADB, 2008. Lao People's Democratic Republic: Preparing the Cumulative Impact Assessment for the Nam Ngum 3 Hydropower Project. Prepared by Vattenfall Power Consultant AB in association with Ramboll Natura AB and Earth Systems Lao. Asian Development Bank (ADB). 394pp. Brown, C.B., 1944. Discussion of sedimentation in reservoirs. Transactions American Society of Civil Engineer 109, 1080–1086. Brune, G.M., 1953. Trap efficiency of reservoirs. Transactions of the American Geophysical Union 34, 407–418. Campos, R., 2001. Three-dimensional reservoir sedimentation model, PhD Thesis, University of Newcastle, 143pp. Chao, B.F., Wu, Y.H., Li, Y.S., 2008. Impact of artificial reservoir water impoundment on global sea level. Science 320 (5873), 212–214. Churchill, M.A., 1948. Discussion of analysis and use of reservoir sedimentation data. In: Gottschalk, L.C. (Ed.), Proc. of Federal Interagency Sedimentation Conference, Denver, pp. 139–140. Downing, J.A.P., Prairie, Y.T., Cole, J.J., Duarte, C.M., Tranvik, L.J., Striegl, R.G., McDowell, W.H., Kortelainen, P., Caraco, N.F., Melack, J.M., Middelburg, J.J., 2006. The global abundance and size distribution of lakes, ponds and impoundments. Limnology and Oceanography 51, 2388–2397. Dugan, P., 2008. Examining the Barrier Effect of Mainstream Dams to Fish Migration in the Mekong, with an Integrated Perspective to the Design of Mitigation Measures, Conclusions from an independent Expert Group Meeting, Vientiane, Lao PDR, 22– 23 September 2008. Fu, K.D., He, D.M., 2007. Analysis and prediction of sediment trapping efficiencies of the reservoirs in the mainstream of the Lancang River. Chinese Science Bulletin 52, 134–140. Fu, K.D., He, D.M., Lu, X.X., 2008. Sedimentation in the Manwan reservoir in the Upper Mekong and its downstream impacts. Quaternary International 186, 91–99. Fuller, I.C., Large, A.R.G., Milan, D.J., 2003. Quantifying channel development and sediment transfer following chute-off in a wandering gravel-bed river. Geomorphology 54, 307–323.

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