CHAPTER 7 I THE MEKONG DELTA IN THE FACE OF INCREASING CLIMATIC AND ANTHROPOGENIC PRESSURES I 357
IMHEN. When the GWL reaches 1.5°C, the dynamical experiments show rainfall increases in the range of 11.6–17.5% in the Mekong Delta, while the BCSD results shows slight rainfall decreases of less than 3% over most of the region. When the GWL rises to 2.0°C, the IMHEN experiments show a slightly larger increase in rainfall in the range of 13.6–18.5%; while the BCSD shows an increase of 3–5% and areas with increasing rainfall are projected to ex-
4. Anthropogenic pressures 4.1 Upstream dams and sand mining drive sediment starvation The flowing water and moving sediments of the Mekong River are fundamental to the existence of the Mekong Delta. Basin-wide alterations of natural water and sediment dynamics have had the biggest impact on the ecological and geophysical function of rivers and their deltas worldwide [ Grill et al., 2019 ]. Due to upstream impoundments and downstream interventions (e.g., channel fixing with levees, dyking populated areas), fluvial sediment supply to world deltas has decreased by 30% [ Vörösmarty et al., 2003; Syvitski et al., 2009; Syvitski and Kettner, 2011; Besset et al., 2019; Best, 2019 ], and 40–50 109 T/yr of global demand for sand [ UNEP, 2019 ] is incising rivers and estuarine systems [ Bravard et al., 2013; Brunier et al., 2014; Best, 2019 ]. Once one of the last uninterrupted rivers in the world, over the past three decades, the Mekong River has rapidly joined that trend, and sediment starvation has already caused irreparable damage along the river and
pand. For the higher GWLs of 3.0°C and 4.0°C, the increases in average rainfall obtained from the dynamical downscaling experiments in the Mekong Delta are 22.2% and 20.8%, respectively. Meanwhile, the BCSD results show respective increases of only 3.7% and 7.9%. At the 4.0°C GWL, both the statistical and dynamical experiments project increasing rainfall trends across the entire VMD.
within the VMD. The estimate [ Milliman and Farnsworth, 2011 ] of total sediment transported by the pristine Mekong river at the entrance of the delta was ~160 Mt yr1, but recent estimates show 40–90% reduction in fluvial sediment supply [ Kummu and Varis, 2007; Walling, 2009; Kummu et al., 2010; Koehnken, 2014; Lu et al., 2014; Fan et al., 2015; Manh et al., 2015; Darby et al., 2016; Dang et al., 2018 ]. The effects of sediment trapping by dams under foreseeable future scenarios are expected to have already reached or to reach the VMD within the next 10–20 years [ MRC, 2011 ]. The effect of sediment trapping on the the coarse sediment (sand and gravel), which is a very small fraction of over-all sediment load, takes a long time to travel downstream, but the effect of fine sediment trapping can almost immediately travel to the Delta. Furthermore, although sand export is banned in Cambodia and Viet Nam, domestic consumption has persisted. Projections to 2040 [ SIWRP, 2015 ] show over 1,500 M m3 demand within the VMD for infrastructure development. Current sand mining within the VMD and upstream in Cambodia, are estimated at 28 M m3/yr (40–50 Mt yr-1) [ Bravard et al., 2013; Eslami et al., 2019b; Jordan et al., 2019 ]. “Considering that this does not account for sand mining in Cambodia, and possible illegal sand mining, only in Viet Nam, this amount
