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Quantification of the effect of river-damming and sand mining on riverbed incision in theVietnamese Mekong Delta〇Doan Van BINH, Sameh KANTOUSH, Tetsuya SUMI, Nguyen Phuong MAI, La Vinh TRUNGINTRODUCTION
The hydrodynamics of the Vietnamese Mekong

substantial increases of riverbed incision and saltwater
intrusion in the VMD.

Delta (VMD) is controlled by the inflow discharge
from the Mekong River (MR) and the tide from theEast and West Seas of Vietnam. The VMD has been
formed by the abundant sediment supply from the MRof 166.7 Mt/yr (Binh et al., 2020), which supports theVMD to sustain from land subsidence, sea level rise,and saltwater intrusion.
Sand mounts in the estuaries resist tidal andsaltwater influences. However, the VMD’s riverbeds
have been significantly incised (Binh et al., 2020;
Eslami et al., 2019), leading to increasing saltwater
intrusion (Eslami et al., 2019). Brunier et al. (2014)
found large-scale morphological changes in theFig. 1 Spatial map of the VMD, including hydrological stations

VMD’s main rivers between 1998 and 2008, and they
attributed such changes were caused by sand mining.
Recently, Jordan et al. (2019) quantified the effect ofsand mining on riverbed incision in a 20-km reach of
the Tien river based on the data measured in 2018.
Although some studies were conducted, no study
investigates large-scale riverbed incision in the VMD
in recent years. Therefore, the aim of this study is toquantify riverbed incision in the VMD’ main rivers,
then to link the incision with upstream dams and sandmining.

In quantification of recent riverbed incision, aboat-based bathymetric survey was conducted in thehigh flow season of 2017 by ADCP (acoustic Doppler
current profiler) along the main rivers in the VMD.
The distance between two adjacent cross sections wasranging from 1 to 5 km. The 2017 bathymetric data
were compared with bathymetric data taken in 2014
and 1998 to quantify riverbed incision. The incision
volume was then linked with long-term sediment
discharge and sand mining volume in the VMD(obtained from Bravard et al., 2013 who estimated

DATA AND METHODS
Sixty-four large dams (>15 m) have been built inthe MR basin by 2015. Existing dams trapped asignificant volume of the incoming sediment from theMR (Binh et al., 2020). Inside the VMD (Fig. 1), sandmining is increasing. These two drivers have caused

sand mining in the entire VMD in 2012 being 7.75
Mm3). Moreover, to examine the effect of reduced
sediment load on riverbed incision in the VMD, acoupled TELEMAC-2D and Sisyphe model wasdeveloped for the upper part of the VMD (Fig. 2).

in the predam period (pre-1992) to 43 Mt/yr in theperiod of 2012-2015. Sand mining was also a main
driver of riverbed incision (Eslami et al., 2019).
Comparing the total sand mining volume in the entire
Tien and Hau rivers in 2012 of 7.75 Mm3 (Bravard etal., 2013) with the total incision volume of only theTien river from Tan Chau to My Thuan and the Vam
Nao channel of -157.5 Mm3, sand mining was
responsible for 14.8% of riverbed incision. Therefore,
we concluded that upstream dams caused large-scaleFig. 2 Study domain and geometry mesh of the model

riverbed incision in the VMD whereas sand mining
additionally caused riverbed incision at local scales.
The results of the coupled TELEMAC-2D andSisyphe model show that the mean VMD’s main rivers
may be increased by -2 m in 2026 if the sediment load of2017 decreased by 84.8% during 2017-2026. The mostsevere incision occurred in the Vam Nao channel,
followed by the Tien river. We concluded that riverbed
incision is one of the strongest drivers of increasing
saltwater intrusion in the VMD in recent years. Therefore,
the VMD is at a dangerous phase because of upstream
dam development and uncontrolled sand mining.
Fig. 3 Riverbed incision in the estuaries of the VMD

REFERENCESRESULTS AND DISCUSSIONS
The riverbeds of the Tien and Hau rivers near theestuaries were incised by a mean cross-sectional valueof -3.7 m and -4.3 m, respectively, between 1998 and2017 (Fig. 3), equivalent to the mean incision rates of-0.18 and -0.21 m/yr, respectively. In the upper VMDfrom Tan Chau to My Thuan (Fig. 1), the total net
riverbed

incisionvolumein

2014-2017

wasapproximately -157.5 Mm3, which was in a similarorder of the total incision volume of -200 Mm3 of theentire Tien and Hau rivers in 1998-2008 reported byBrunier et al. (2014). It indicates that recent riverbed
incision in the VMD was approximately threefold the past.
Such highly incised riverbeds in the VMD werecaused mainly by upstream dams which reduced thesediment load of the VMD by 74% from 166.7 Mt/yr

Binh, D.V., Kantoush, S., Sumi, T. (2020): Changes tolong-term discharge and sediment loads in theVietnamese Mekong Delta caused by upstream dams,Geomorphology, Vol. 353, 107011.
Bravard, J.-P. Goichot, M., Gaillot, S. (2013): Geographyof sand and gravel mining in the Lower MekongRiver: first survey and impact assessment, EchoGéo,Vol. 26, pp. 1-18.
Brunier, G., Anthony, E.J., Goichot, M., Provansal, M.,
Dussouillez, O. (2014): Recent morphologicalchanges in the Mekong and Bassac River channels,
Mekong Delta: the marked impact of river-bed
mining and implications for delta destabilisation,Geomorphology, Vol. 224, pp. 177-191.
Eslami, S., Hoekstra, P., Trung, N.T., Kantoush, S.A.,
Binh, D.V., Dung, D.D., Quang, T.T., van der Vegt,M., (2019): Tidal amplification and salt intrusion inthe Mekong Delta driven by anthropoginic sedimentstarvation, Scientific Reports, Vol. 9, 18746.
Jordan, C., Tiede, J., Lojek, O., Visscher, J., Apel, H.,
Nguyen, H.Q., Quang, C.N.X., Schlurmann, T.
(2019): Sand mining in the Mekong Delta revisited –current scales of local sediment deficits, ScientificReports, Vol. 9, 17823.