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1 Session: For more information:

2 Sediment management: mitigation options? Marc Goichot World Hydropower Congress Beijing, China 21 May 2015

3 A project jointly implemented by MRCS & WWF funded by FFEM/AFD Knowledge of sediment transport and discharges in relation to fluvial geomorphology for detecting the impact of large-scale hydropower projects in the basin of the Mekong River INSTITUTE OF OCEANOGRAPHY

4 High sediment supply = High rates of Holocene deltaic progradation (after Xiu et al., Marine Geology, 2010, v. 269, p )

5 Morphodynamics of Delta Channels Compared morphology of delta channels between 1998 and 2008 bathymetric surveys Mekong 59% of channel showed erosion 16% of channel showed accretion ~90Mm 3 loss of material Bassac 70% erosion 12% accretion ~110 Mm 3 loss No correlation With stream energy Attributed to sand mining Anthony et al., 2013, AGU, San Francisco

6 Delta costal investigations Examined ~2000 satellite images of delta 2003, 2007, 2011 Eastern sector 48% retreat, 22% advance Average = -4.4 m/yr Western sector Historically high advancement rates 70% retreat Avg=-12.2 m/yr Gulf of Thailand 36% retreat Avg=-3.7 m/yr Changes likely reflecting reduction of sediment supply Edward J. Anthony, Guillaume Brunier, Manon Besset, Marc Goichot, Philippe Dussouillez, Van Lap Nguyen 2015 Linking rapid erosion of the Mekong River delta with human activities

7 Bravard, J.P., Goichot, M., Gaillot, S., Geography of sand and gravel mining in the Lower Mekong River. First survey and impact assessment. EchoGéo. /13659). 6

8 Delta Plume Investigations Analysed ~2000 satellite images of Mekong River plume obtained between 2003 and 2011 Optical properties provide information about: Suspended Particulate Matter (SPM) Particle Size Distribution Phytoplankton / nutrient concentration Organic matter Light attenuation Trend analysis : net decrease of 4 to 5 % per year over 8 years in the prodelta area Not related to meteorological (waves, wind) processes Likely affecting freshwater & marine fisheries & agriculture productivity Consistent with the erosion of muddy coastline (East) Hubert Loisel, Antoine Mangin, Vincent Vantrepotte, David Dessailly, Dat Ngoc Dinh, Philippe Garnesson, Sylvain Ouillon, Jean-Pierre Lefebvre, Xavier Mériauxc, Thu Minh Phan Variability of suspended particulate matter concentration in coastalwaters under the Mekong's influence from ocean color (MERIS) remote sensing over the last decade. Remote Sensing of Environment 150 (2014)

9 The Mekong River, and especially its delta, is already responding to alterations in the basin, indicating high sensitivity to present levels of changes 1) MRC monitoring : suspended sediment budget reduction from 160 Mt/yr to 75Mt/yr (between 1992 and 2014) 1) The impacts of sand mining and dams are cumulative; they deeply affects the sediment continuity and most probably are the 2 key factors in the retreat of the delta coastline (climate change = aggravating factor) 1) The impacts of sand extractions are felt more rapidly, but may be reversible if sediment budget is properly managed. 1) Dams preferentially trap coarse sediment and in most of cases the impact is not reversible 5) More dams are being proposed: flushes are seen as the prime mitigation solution BUT

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11 CONCLUSIONS Flushes may be efficient from a reservoir management perspective & can achieve partial restoration of the sediment continuity, but they cause sever negative impacts to downstream ecosystems For for time being preferred strategy may be avoidance. i.e. selectively sitting dams to avoid blocking main sources of sediment. (In the Mekong preserving rivers that contribute sand) Carefully measure the risk of exporting to developing countries a technology not yet mastered Bravard, J.P.,Goichot, M., Tronchère, H., An assessment of sediment-transport processes in the Lower Mekong River based on deposit grain sizes, the CM technique and flow-energy data. Geomorphology 207,

12 Thank you for your attention