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2. Geological and hydrological characteristics of the Delta
2.1 Evolution and elevation of the Delta
Deltas are geologically young landforms that have been formed during the past millennia. A delta is created when the volume of sediments accumulated over time at a location where a river enters a sea exceeds the volume (i.e. accommodation or accommodation space) that is created by sediment compaction and relative sea level rise. The surplus of both fluvial and marine sediments creates a delta extending into the sea. The elevation of this delta plain is dynamic, and changes over time following sediment compaction and sea level fluctuations. When sea level rises, the elevation of a delta plain can be increased by deposition of new fluvial or marine sediments during floods, or by the production of organic sediments by vegetation on the delta plain.
The Mekong Delta was formed over the last 6,000–7,000 years, when the sea level in southeast Asia reached a relatively stable level following its rise after the last ice age [ Hanebuth et al., 2012 ]. Under the influx of large quantities of primarily fluvial sediments brought in by the Mekong River, the Mekong Delta started to form its apex in present-day Cambodia [ Ta et al., 2005; Tamura et al., 2012 ]. Due to the combination of the relatively shallow seafloor underlying the present delta and the large sediment influx [ Anthony et al., 2015 ], the
[ Figure 7.3 ] Shoreline changes of the Mekong Delta between 2003 and 2012
Delta rapidly extended during the last couple of thousand years to its present massive size, ranking it the third-largest delta plain on Earth [ Coleman and Roberts, 1989 ].
The Mekong Delta used to progress seaward at a mean rate of >30 m/year [ Liu et al., 2017 ], but this rate decreased over the past decades, to become negative after 2005. Currently, most of the coastline is eroding [ Figure 7.3 ], with high rates of up to 50 m/year, because of sediment starvation due to upstream dams and sand mining, subsidence (see Section 4 and Chapter 9), increased storms and sea level rise [ Anthony et al., 2015; Liu et al., 2017; Tamura et al., 2020 ].
Besides being one of the largest deltas, the Mekong Delta also has an extraordinarily flat and low-elevated delta plain. The vast majority of the Delta is elevated less than two meters above local mean sea level, with an estimated average delta plain elevation of some 80 cm [ Minderhoud et al., 2019 ]. Over a transect of more than 100 kilometres, running from the Cambodian border to the coast, the elevation of the delta plain changes by less than 2 metres (see Chapter 9). This makes the delta extremely vulnerable to relatively small changes in sea level or vertical land motion (e.g. land subsidence).
2.2 Hydrological regime
The Mekong River Basin (MRB) is delineated with its monsoon-driven seasonality. As a result, water discharge flowing into the Mekong Delta ranges from 1,700 m3/s to 40,000 m3/s, in dry and wet seasons respectively [ Wolanski et al., 1996; Le et al., 2007 ]. Hence, during the dry season (NE monsoon, December to May), the Mekong River only provides 5–10% of the yearly discharge to the VMD, while the flood pulse of the wet season (SW monsoon, July to October) inundates the lakes and nourishes the floodplains with sediments and nutrients, driving major fish recruitment along the river (see Figure 7.2 for the location of the flood zones).
The MRB is stressed by significant anthropogenic and climatic drivers. The Mekong fluvial discharge (seasonal and annual) is projected to increase under most climate scenarios [ Lauri et al. (2012); Thompson et al. (2013); Hoang et al. (2016); Hoang et al. (2019) ] and will be subject to disruptions in known frequencies of flood and drought events. Moreover, hydropower development has altered the hydrological regime of the river by moderating the flood pulse and controlling the dry season discharge [ ICEM, 2010 ], impacting the VMD’s ecological cycle.
The pristine (pre-dam) Mekong River Basin provided two sources of fresh water for the VMD: the Mekong river itself, and the Tonle Sap Lake (TSL) in Cambodia (see location on Figure 7.2). The flood pulse of the wet season filled the TSL , the largest freshwater body in south-east Asia, covering 3,500 km2 during dry seasons and 10,500 km2 during wet seasons [ MRC, 2005 ]. It played a crucial role in the hydrology of the VMD, acting as a retention area to feed the Delta along with the Mekong River during the dry season. Figure 7.4 shows the average water discharge (1997–2004) flowing in and out of the TSL at Prek Kdam station: the lake was filled with water from the Mekong River during flood seasons (June to October); then, after water levels at Kampong Luong (southern bank of the TSL) reached a peak of over 9 meters, the lake supplied water to the Delta, helping to maintain river flows and reduce salinity intrusion into coastal areas during dry seasons (October to May).
[ Figure 7.4 ] Average yearly water level and discharge variations of the Lake Tonle Sap
Daily water discharge at Prek KDam station (Tonle Sap River, downstream of the lake) and water level variation at Kampong Luong (Tonle Sap Lake) Source: Kummu et al., 2014. Average over 1997-2004.
However, while the Mekong dry season discharge has increased over the past three decades [ Figure 7.5 ], driven by climate change and hydropower operations [ Eslami et al., 2021b ], water levels nowadays are hardly ever capable of filling Tonle Sap Lake to capacity during the wet season, due to the decrease in the Mekong discharge [ Arias et al., 2014b; Cochrane et al., 2014 ]. Wet season water levels have followed a declining trend in the past two decades [ Eslami et al, 2021b ]. Therefore, the Delta is short of TSL as its second freshwater source in the dry season, and experiences longer periods of dry seasons, driving a major exposure especially during drought events
2.3 Tides
Tide is the most important hydrodynamic force in coastal areas. The Mekong Delta has complex interactions between the mixed diurnal-semidiurnal tide from the East Sea and the diurnal tide from the West Sea (Gulf of Thailand). The greatest tidal differences occur in the East Sea, where they can reach up to 3.8 m, and decline gradually in the south-west direction [ Eslami et al., 2019a ]. Tidal amplitudes in the West Sea are much smaller, ranging from 0.5 to 1 m [ Unverricht et al., 2013 ].
However, while the tidal ranges at Dinh An (the East Sea) are much larger than that at Rach Gia (the West Sea), the daily average water levels at Rach Gia are slightly higher. As a result, floods during the wet season are mainly drained to the East Sea.
During the flood season, tides still influence water levels in the coastal areas [ Lê Sâm, 1996 ]. Yet the high fluvial discharge dominates water levels further upstream, and the tide does not travel beyond the Cambodian
[ Figure 7.5 ] Trends of the Mekong River discharge and water level
Left axis: Trends of upstream (Kratie) dry season cumulative discharge and downstream (Tan Chau + Chau Doc); Right Axis: monthly 90th percentile of water level and in Oct. and Nov. in Phnom Penh Port, Cambodia (right axis). Source: Eslami; et al. (2021b).
border [ Gugliotta et al., 2017; Eslami et al., 2019b ]. On the other hand, during the dry season, tides are the main factor controlling water level variations in the Delta. Their effects can spread 190 km further inland in the estuary channels of the Mekong [ Takagi et al., 2016; Eslami et al., 2019a ].
