Fine sediment dynamics in the mekong river estuary, vietnam

See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/223311236 Fine-sediment Dynamics in the Mekong River Estuary, Vietnam Article in Estuarine Coastal and Shelf Science · November 1996 DOI: 10.1006/ecss.1996.0088 CITATIONS READS 104 1,605 5 authors, including: Eric Wolanski Nhan Huu Nguyen 310 PUBLICATIONS 9,696 CITATIONS 9 PUBLICATIONS 358 CITATIONS James Cook University SEE PROFILE University of Ulsan SEE PROFILE All content following this page was uploaded by Eric Wolanski on 18 August 2014 The user has requested enhancement of the downloaded file Estuarine, Coastal and Shelf Science (1996) 43, 565–582 Fine-sediment Dynamics in the Mekong River Estuary, Vietnam Eric Wolanskia, Nguyen Ngoc Huanb, Le Trong Daob, Nguyen Huu Nhanc and Nguyen Ngoc Thuyd a Australian Institute of Marine Science, PMB No 3, Townsville M.C., Queensland 4810, Australia, bHydrometeorological Service, Dang Thai Than Street, Hanoi, Vietnam, cHydrometeorological Service, 19 Nguyen Thi Minh Khai Street, 1st District, Ho Chi Minh City, Vietnam and dVietnam Marine Science & Technology Association, 36 Hoang Dieu Street, Hanoi, Vietnam Received 24 January 1995 and accepted in revised form 11 September 1995 Keywords: Mekong Delta; sediments; salt wedge; turbidity maximum; siltation; salinity intrusion Field studies of fine-sediment transport were carried out in the Mekong River estuary, Vietnam, in the high-flow season of November 1993 In the freshwater region, erosion and deposition of suspended sediment occurred at tidal frequency with a strong down-river transport at a mean velocity of m s and mean suspended-solid concentration of about 250 mg l Also, the suspended sediment was mostly fine silt, and the clay fraction accounted for only 15% The suspended sediment was transported either as individual particles or agglomerated with organic detritus At the mouth of the estuary, a salt wedge was present but was flushed out of the estuary at low tide Flow reversal occurred across the pycnocline A turbidity maximum zone was present at the toe of the salt wedge at flood tide Most of the suspended sediment was coagulated with little organic matter The bulk of the suspended sediment was exported to coastal waters, but some sediment returned to the estuary in the salt wedge Data for the low-flow season are sparse but suggest that partially well mixed estuarine conditions prevail, with salinity penetrating about 40 km inland, carrying fine-sediment up-river to a turbidity maximum zone Most of this sediment would have been deposited in shallow coastal waters in the previous high-flow seasons The construction of large hydro-electric dams further upstream on the Mekong River may exacerbate siltation patterns in the  1996 Academic Press Limited estuary Introduction The Mekong River (Figure 1) is 4200 km long, it drains an area of 0·79 106 km2 with a mean annual water discharge of 470 km3 (Borland, 1973; Milliman & Meade, 1983) The discharge of the lower Mekong River at Phnom Penh, Cambodia, varies seasonally (United Nations, 1957; Gagliano & McIntire, 1968); it is smallest (typically 1700 m3 s 1) in May and largest in October (typically 39 000 m3 s 1) The estuary has a few major channels, shown in Figure 1, and a large number of smaller channels (not shown), and these form a vast delta (Figure 1) The thalweg in the estuary is typically 0272–7714/96/050565+18 $25.00/0  1996 Academic Press Limited 566 E Wolanski et al 106°E 106°E Phnom Penh 50 km Can Tho Mekong River Delta 11 Vung Tau South China Sea 9°N 20 ea 10 South China Sea Ca Loc 9°N am tn 10 China e Vi 10° Thailand 10 Can Tho 10 km 20 21 22 23 24 S Sou th China 106°E Figure Location map and sampling sites ( ), Mekong River estuary 10 m deep in the freshwater region between 30 and 140 km from the mouth; closer to the mouth in the dry season saline-intrusion region, the depth decreases to m typically; even shallower waters are found at the mouth (Figure 2) Coastal waters are shallow, the 20-m depth contour is located 30 km from the coast (Figure 1) Mixed, macro-tides prevail with a strong diurnal inequality (Nguyen Ngoc Thuy, 1979) At the mouth, the mean maximum and average tidal range are, respectively, c 3·2 m and 2·2 m The average tide range decreases with distance upstream (Gagliano & McIntire, 1968; Nguyen Ngoc Thuy, 1988a,b) Indeed at Can Tho (123 km), it is only 1·9 m in the low-flow season and 0·7 m in the high-flow season; at Chau Doc (228 km) it is only 0·49 m in the low-flow season and 0·0 m in the high-flow season The tidal dynamics can be modelled as friction-damped, progressive waves in branched, onedimensional channels, with the tidal amplitude decreasing and the time lag increasing with distance from the river mouth (Nguyen Ngoc Huan, 1987a,b; Tingsanchali & Lien, 1987; Nguyen Ngoc Thuy, 1989) Data on sediment discharge in the lower Mekong River are even more sparse, especially over the last 30 years The sediment discharge may be about 160 106 tonnes year (Milliman & Meade, 1983; Milliman & Syvitski, 1992) To place the Mekong in perspective with other major rivers in the world, the Mekong River has (see Table 1) a smaller drainage area than the Yangtze (41%), the Amazon (12%), the Mississippi (24%), and the Ganges/Brahmaputra (53%) Rivers The sediment load is about the same as that of the Mississippi River; however, its sediment yield is about twice that of the Mississippi The Mekong River sediment yield is equal to about one-seventh that of the Fly and Ganges/Brahmaputra Rivers, 85% that of the Yangtze River, and it is about 12% larger than that of the Amazon River Data on salinity are sparse and have been collected mainly near the surface and in the dry season, in view of the importance of salinity for irrigation The surface salinity varies seasonally, being maximum in the low-flow season and minimum in the high-flow season In the low-flow season, the maximum salinity at 21 km is 20 and at 45 km is In the high-flow season, the water is fresh nearly up to the mouth of the main channels 567 Fine-sediment dynamics 240 200 Distance (km) 80 160 120 40 Depth (m) 10 20 River mouth 30 40 (a) 7 No data No data km (b) Figure Depth in m (a) in the thalweg (adapted from Gagliano & McIntire, 1968) and (b) at the mouth (Nguyen Ngoc Thuy, unpubl data) of the estuary (Gagliano & McIntire, 1968; Nguyen Ngoc Thuy, 1988a) Data on suspended sediment are sparse in the estuary (Gagliano & McIntire, 1968; Nguyen Ngoc Thuy, unpubl data) and in coastal waters (Anikiyev et al., 1986) About 32 106 people live in the delta in Vietnam and are engaged mainly in rice farming, artisanal fishing and aquaculture The estuary is important for transport and shipping, including the export of rice through ports such as Can Tho but this is hindered by siltation especially in the saline-water region (Figure 2) Irrigation for rice farming is hindered by salt intrusion in the estuary, particularly in the low-flow season The delta is an important mangrove area (Phan Nguyen Hong, 1991; Le Duc An & Phan Trung Luong, 1993), and these mangroves are used extensively for artisan fisheries and wood Important questions have been asked about the feasibility of dredging for a seaport (e.g at Can Tho), the effect of a sea-level rise on the delta, and the effects on the delta from the construction of proposed, major hydro-electric dams in Cambodia, Laos and Thailand (Lohmann, 1991) 568 E Wolanski et al T Comparison of the drainage area, sediment load and yield for the Yangtze, Amazon, Mississippi, Ganges/Brahmaputran, Yangtze, Mekong and Fly Rivers (adapted from Milliman & Meade, 1983; Milliman & Syvitski, 1992; Wolanski & Gibbs, 1995) River Yangtze Amazon Mississippi Ganges/Brahmaputra Mekong Fly Area (106 km2) 1·9 6·1 3·3 1·48 0·79 0·076 Load (106 tonne year 480 1200 210 2180 170 116 ) Yield (tonne km year ) 252 190 120 1670 215 1500 The available hydrodynamic data necessary to help answer some of these questions remain inadequate In view of the sparse baseline data, the authors undertook this study to gain some insight on currents, salinity and suspended-sediment dynamics in the estuary in the high-flow season, to help answer ultimately some of these important management questions Methods During 15–20 November 1993, vertical profiles of temperature, salinity and suspendedsediment concentration were obtained at stations shown in Figure 1, using a Seabird CTD This CTD was fitted with an Analite optical fibre nephelometer which is more sensitive than the Seabird nephelometer Position fixing was by dead reckoning At Stations 11 and 24, a 13-h station was maintained from a small, wooden vessel with hourly profiles using the CTD In addition at these two stations, vertical profiles of currents were measured at about m intervals using a Vertusca current meter suspended from a vessel The nephelometers were calibrated for suspended-sediment concentration using water samples taken in situ and later filtered through 0·45- m filters None of the water collected using a 5-l Niskin bottle contained sand, although sand was the dominant sediment on the bottom, so that the nephelometer data were not aliased (Ludwig & Hanes, 1990); for the observed variation in floc size, the nephelometer calibration is expected to vary by 20% only (Gibbs & Wolanski, 1992) In situ floc cameras were unavailable and could not have been used because the strong tidal currents caused floc breakage around windows, and the high concentration caused excessive floc overlap (Eisma et al., 1990) Instead, the technique of Gibbs and Konwar (1986) and Gibbs et al (1989), modified by Wolanski and Gibbs (1995), was used to measure floc size In particular, a specially modified, wide mouth 5-l Niskin bottle was used, the water was sampled directly in the bottle using a slide with well, avoiding floc breakage in other sampling techniques such as pipettes, pumps, analysers and Niskin bottle ports The sample was capped underwater by a glass, and viewed and photographed using an Olympus inverted microscope with a range of magnification from to 800 For each water sample, several images at different magnifications were used to sample the full range of floc sizes The photographs were scanned on an IBM-compatible computer and the digitized data were used to calculate the floc-size distribution using an image analysis software package The system was calibrated using ragweed pollen and latex particles of 569 Fine-sediment dynamics Sea level (m) 15 16 17 18 19 Date (November 1993) 20 21 Figure Time-series plot of the sea level at Vung Tau during the field study median size 17·5 and 40 m, respectively Most samples were taken from m above the bottom, and a few samples were taken from near the surface Water samples were also collected from the Niskin bottles and used to measure the primary (not flocculated) particle-size distributions using a Horiba CAPA-300 gravitational/centrifugal particlesize analyser after pre-processing the samples by treatment with an ultrasonic bath and Calgon-T dispersant and visual examination on a microscope Results Freshwater region The freshwater region starts at c 10 km from the mouth Spring tides prevailed (Figure 3) Freshwater was found throughout the estuary except at stations 21–24 near the mouth The tides reversed the currents at least up to Can Tho [123 km; Figure 4(a)] A strong inequality of the tidal currents was apparent, the ebb and flood currents peaking at 1·2 and 0·4 m s 1, respectively A friction-induced vertical velocity shear existed in the bottom m of the water column The temperature (not shown) was homogeneous vertically, varying in time between 29·4 and 29·6 C The suspended-sediment concentration varied with tidal frequency, and was higher near the bottom where it reached 0·6 g l at peak ebb currents [Figure 4(b)] There also appeared to be a background concentration of 0·15 g l The reason for this background concentration is to be found in the nature of the suspended sediment in the freshwater region The particle-size distribution of the suspended sediment in the freshwater region of the estuary (Figure 5) varied little with site, depth and tide phase, the median particle size, d50, varying only between 2·5 and 3·9 m, characteristic of fine silt The clay fraction (particle size 0·5 m s 1, thus the suspended-sediment concentration fluctuated with the tidal currents; on the other hand, the non-flocculated sediment, being very fine, had no time to settle at slack tide, hence the presence of a background concentration No evidence for a net (after tidal-averaging) along-channel gradient of suspendedsediment concentration was found, any such gradients being much smaller than the temporal changes due to erosion and settling at tidal frequency Hence, no turbiditymaximum zone was present in the freshwater region of the estuary This finding and the presence of a deep (10 m, excluding many shoals, in the Can Tho area) thalweg in the freshwater region of the estuary suggest that during high-flow conditions, little siltation occurs in the main channel Saltwater region This region extends only over 10 km or so from the mouth Brackish water was only found near the mouth and then only near the bottom, around high tide and in the thalweg, respectively (Figure 7); this water was also colder (by 0·4 C) and more turbid (by 0·2 g l 1), than in the freshwater region The top m of the water column were freshwater, and the pycnocline below was sharp The estuary was thus highly stratified in the bottom m, the situation approaching that of a salt wedge Time-series data over one tidal cycle on 19–20 November 1993, at Station 24, show tidally-reversing currents throughout the water column, with peak flood and ebb tidal velocities of comparable magnitude [Figure 8(a)] However, near 29 h in the time series (the morning flood tide 571 Fine-sediment dynamics 100 (a) 80 60 20 500–1000 250–500 125–250 62.5–125 31.3–62.5 15.6–31.3 7.81–15.6 3.91–7.81 1.95–3.91 0.98–1.95 0.49–0.98 (b) 0.24–0.49 70 60 50 40 30 20 10 0–0.24 Frequency (%) 40 Size (µm) Figure For the suspended sediment in the freshwater region of the Mekong River estuary, typical distribution of (a) particle size at various locations, depths and tide phases, and (b) floc size at the surface ( ) and near the bottom ( ) of 20 November 1993), a strong velocity shear was present [Figure 8(a)] at the elevation of a sharp pycnocline as evidenced by saltier [by 15; Figure 8(b)] and colder [by 0·5 C; Figure 8(c)] water near the bottom A salt-wedge estuarine situation was present Coastal waters are not much saltier than the near-bottom values observed at Station 24 at 30–31 h (Anikiyev et al., 1986) While a salt wedge was present, it was unsteady, moving back and forth with the tides; at low tide, it was pushed out of the estuary completely [Figure 8(b)] The suspended-sediment concentration varied with tidal frequency, again with a background of about 0·15 g l [Figure 9(a)] When the salt wedge was absent, the salinity stratification was weak and a strong relationship was apparent between velocity and concentration, with a short phase lag due to erosion and deposition However, when the salt wedge was present, the salinity stratification, by inhibiting turbulence, limited the resuspension to below the pycnocline Most noticeable was the high concentration at the toe of the salt wedge (27–29 h) The near-bottom particle size-distribution [Figure 9(b)] fluctuated slightly with the tides; the median particle size, d50, was slightly larger than in the freshwater region, varying between and 3·5 m [Figure 10(b)], with an unexplained occurrence of a very small d50 of m at 21 h Except for that event, the clay fraction (size uc =0, if u