SEDIMENT DATA FOR THE LOWER MEKONG

Transcription

1 SEDIMENT DATA FOR THE LOWER MEKONG A review of past monitoring activity and what it tells us about past and recent sediment fluxes and what lessons can be learnt for future monitoring DES WALLING DEPARTMENT OF GEOGRAPHY WWF / MRCS Workshop on Knowledge of sediment transport and discharges in relation to fluvial geomorphology for detecting the impact of large-scale hydropower projects 22-23rd May, 2012, Phnom Penh, Cambodia

2 THE WIDER CONTEXT Changes in the Sediment Loads of the World s Major Rivers

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4 750,000 km 2 CAUSES OF REDUCED SEDIMENT LOAD Wang et al. (2006) Soil Conservation 40% Climate Change 30% Reservoir Trapping 30%

5 Recent changes in the annual sediment loads of ten major Chinese rivers River Area (km 2 ) Period of Ratio of mean Record 2005 to record mean Songhua Liaohe Yongding Yellow Huaihe Yangtze Qiantang Minjiang Donjiang Xijiang Based on Liu et al. (2008) and Hu et al. (2008)

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7 IMPACTS? INCREASING SEDIMENT LOADS Problems Physical and Ecological Problems for Water Resource Development Coastal Problems: Mangroves etc DECREASING SEDIMENT LOADS Channel Incision, Changes in Groundwater Levels Reduced Nutrient Flux, Ecological Changes Coastal Problems: Delta destruction

8 THE MEKONG? Catchment area km 2 12th longest river in the world 8th largest annual discharge One of the world s most pristine large rivers A Stable Sediment Load BUT Land Use Change Reservoir Construction Climate Change?

9 THE SEDIMENT LOAD OF THE MEKONG RIVER Past monitoring programmes and their limitations. Lessons to be learnt Current understanding of sediment loads Recent changes in sediment flux caused by Chinese Dams

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11 Discontinuous records. Very few years with records for all stations Sampling frequency commonly low Raw concentration data only for main Mekong stations Some variations in practice between countries Chinese data unavailable since 1990 Lack of continuous annual time series Water Quality data low frequency and possibly unreliable Little reliable particle size data Little information on concentration profiles Limited information on sediment properties such as organic matter content No systematic measurement of bedload fluxes

12 The Available Data

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14 NUMBER OF DATA DAYS FOR INDIVIDUAL YEARS FOR THE SEDIMENT MEASURING STATIONS ON THE MEKONG RIVER

15 1961

16 NUMBER OF DATA DAYS FOR INDIVIDUAL YEARS FOR THE SEDIMENT MEASURING STATIONS ON THE MEKONG RIVER Focus on Annual Fluxes

17 (0.64)

18 Minimum Data Requirements and Uncertainty of Resulting Load Estimates

19 ESTIMATES OF UNCERTAINTY AT THE 95% LEVEL OF CONFIDENCE FOR ANNUAL LOAD ESTIMATES DERIVED USING SAMPLES COLLECTED AT DIFFERENT SAMPLING FREQUENCIES

20 NUMBER OF DATA DAYS FOR INDIVIDUAL YEARS FOR THE SEDIMENT MEASURING STATIONS ON THE MEKONG RIVER

21 ANNUAL SEDIMENT LOADS The values in cells coloured yellow and orange are judged to be reliable, whereas those printed in red italics involve greater uncertainty, due to the lack of a well-defined rating relationship

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23 Some Warning Messages Sampling Frequency Use of Water Quality Data

24 Sampling Frequency Data requirements - What data are needed? Required accuracy Reconstruction of actual concentration or flux time series OR Use of rating curves

25 Use of Water Quality Data Sampling frequency Reliability

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27 Mekong at Chiang Saen Water Quality Data Sediment Monitoring Data

28 Data for Lancang River based on Fu et al. (2006)

29 The Sediment Dynamics of the Mekong Basin

30 1961

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32 D D D

33 THE EFFECT OF THE CHINESE DAMS?

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37 Assessments of the Impact of the Manwan Dam Kummu & Varis (2007) Sharp decrease in the mean annual sediment load at Chiang Saen and Pakse Before ( ) After ( ) Chiang Saen 71 Mt 31 Mt Pakse 133 Mt 106 Mt Lu & Siew (2006) Major Reduction in the mean annual sediment load at Chiang Saen Mt Mt Walling (2008) No clear evidence of a reduction at Chiang Saen or further downstream

38 Based on reservoir surveys undertaken by the Chinese authorities between 1993 and 2005 it has been estimated that the Manwan Dam trapped ca. 490 Mm 3, which is equivalent to an annual sedimentation rate of ca. 50 Mt year -1.

39 1993

40 New Data

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43 Increase Decrease

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45 THE FUTURE? More Dams Climate change?

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47 CONCLUSIONS The Mekong River shows evidence of buffering of changes in its sediment load over the past 40 years. The apparent lack of change may, however, mask what are likely to have been significant changes in sediment transport. The difficulty of identifying temporal trends due to incomplete data and the interaction of increasing and decreasing loads due to different controls Uncertainty over the future impact of climate change

48 Thank You