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
THE WIDER CONTEXT Changes in the Sediment Loads of the World s Major Rivers
750,000 km 2 CAUSES OF REDUCED SEDIMENT LOAD Wang et al. (2006) Soil Conservation 40% Climate Change 30% Reservoir Trapping 30%
Recent changes in the annual sediment loads of ten major Chinese rivers River Area (km 2 ) Period of Ratio of mean 1996- Record 2005 to record mean Songhua 389769 1955-2005 0.74 Liaohe 120764 1954-2005 0.23 Yongding 43674 1963-2005 0 Yellow 752032 1952-2005 0.25 Huaihe 121330 1950-2005 0.57 Yangtze 1705383 1950-2005 0.68 Qiantang 18233 1977-2005 0.81 Minjiang 54500 1950-2005 0.39 Donjiang 25325 1954-2005 0.59 Xijiang 351535 1957-2005 0.69 Based on Liu et al. (2008) and Hu et al. (2008)
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
THE MEKONG? Catchment area 795000 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?
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
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
The Available Data
NUMBER OF DATA DAYS FOR INDIVIDUAL YEARS FOR THE SEDIMENT MEASURING STATIONS ON THE MEKONG RIVER
1961
NUMBER OF DATA DAYS FOR INDIVIDUAL YEARS FOR THE SEDIMENT MEASURING STATIONS ON THE MEKONG RIVER Focus on Annual Fluxes
(0.64)
Minimum Data Requirements and Uncertainty of Resulting Load Estimates
ESTIMATES OF UNCERTAINTY AT THE 95% LEVEL OF CONFIDENCE FOR ANNUAL LOAD ESTIMATES DERIVED USING SAMPLES COLLECTED AT DIFFERENT SAMPLING FREQUENCIES
NUMBER OF DATA DAYS FOR INDIVIDUAL YEARS FOR THE SEDIMENT MEASURING STATIONS ON THE MEKONG RIVER
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
Some Warning Messages Sampling Frequency Use of Water Quality Data
Sampling Frequency Data requirements - What data are needed? Required accuracy Reconstruction of actual concentration or flux time series OR Use of rating curves
Use of Water Quality Data Sampling frequency Reliability
Mekong at Chiang Saen Water Quality Data Sediment Monitoring Data
Data for Lancang River based on Fu et al. (2006)
The Sediment Dynamics of the Mekong Basin
1961
D D D
THE EFFECT OF THE CHINESE DAMS?
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 (1962-1992) After (1993-2002) 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 1962-1992 74 Mt 1993-2000 34.5 Mt Walling (2008) No clear evidence of a reduction at Chiang Saen or further downstream
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.
1993
New Data
Increase Decrease
THE FUTURE? More Dams Climate change?
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
Thank You