DSpace at VNU: Sediment distribution and transport at the nearshore zone of the Red River delta, Northern Vietnam

Journal of Asian Earth Sciences 29 (2007) 558–565 www.elsevier.com/locate/jaes Sediment distribution and transport at the nearshore zone of the Red River delta, Northern Vietnam Do Minh Duc a,* , Mai Trong Nhuan a, Chu Van Ngoi a, Tran Nghi a, Dao Manh Tien b, Tj C.E van Weering c, G.D van den Bergh c a Faculty of Geology, University of Science, Vietnam National University, Hanoi, Viet Nam Division for Marine Geology, Department of Geology and Mineral Resources, Viet Nam Royal Netherlands Institute for Sea Research (NIOZ), P.O Box 59, 1790, AB Den Burg, Texel, The Netherlands b c Received 22 August 2003; received in revised form 27 February 2005; accepted March 2006 Abstract The coast between Ngason and Haiphong is largely formed by accretion of the Red River system In the region, five main surface sediment types (sand, sandy silt, silt, mud and sand at shoals) could be defined, which differ from one another in their sedimentary characteristics Sand dominates along the shoreline between and 15 m water depth Down to a water depth of about 25–30 m, the sediment is dominantly silt Further offshore the surface sediments are mainly sandy silt and sand of older units (Early-Middle Holocene, Late Pleistocene) Net sediment transport directions are defined by grainsize analysis according to the method of Gao and Collins [Gao, S., Collins, M., 1992 Net sediment transport patterns inferred from grain-size trends, based upon definition of transport vectors Sedimen Geol 80, 47–60, 1992] At river mouths, directions of sediment transport are variable where the depths are shallower than m From to 10 m water depth, sediments are mainly transported southeastward at the Ba Lat, Lach and Day mouths, northeastward at the Tra Ly mouth and eastward at the Thai Binh mouth Recently, the Hai Hau erosional shoreline is not supplied with sediment from the Ba Lat mouth and sediments are transported southwestward alongshore in the region shallower than m The region of depths from 10 to 30 m is specified by southward sediment transport Ó 2007 Elsevier Ltd All rights reserved Keywords: Grain size; Surface sediment; Sediment transport; Red River delta; Vietnam Introduction The Ngason-Haiphong area forms a part of the west coast of the South China Sea (Fig 1) and has been largely formed by accretion of the Red River delta system The annual amount of sediment transported by the Red River system into the South China Sea is about 82 · 106 m3 In the wet season (from June to January), about 90% of the annual sediment supply is transported through the various distributaries (Nhuan et al., 1996) Of the total amount of sediment supplied, 11.7% passes through the Van Uc and * Corresponding author E-mail address: ducdm@fpt.vn (D.M Duc) 1367-9120/$ - see front matter Ó 2007 Elsevier Ltd All rights reserved doi:10.1016/j.jseaes.2006.03.007 Thai Binh river mouths, 11.8% through the Tra Ly river mouth, 37.8% through the Red River (Ba Lat) mouth and 23.7% through the Day river mouth These major river mouths represent very rapid accretion zones where sediment accumulation rates exceed sea level rise (1–2 mm/ year) and tectonic subsidence (2 mm/year, Ngoi et al., 2000) The northern part of the Ngason-Haiphong coast (from Ba Lat to Haiphong) has a diurnal tidal regime with an average amplitude of 2.5–3.5 m In the southern part, from Ba Lat to Ngason, the tide is mixed with a diurnal dominance The average tidal amplitude is 2–3 m Waves usually have a dominant direction from the east-northeast during the dry season and from east-southeast during the wet season The average and maximum wave heights are 0.7–1.3 m D.M Duc et al / Journal of Asian Earth Sciences 29 (2007) 558–565 559 106o 30’ o 106 00’ HAIPHONG Va n o 20 40’ Thai B Uc mo inh m uth outh THAI THUY Diem Dien mou th 20o30’ Tr a Ly mouth Re dR 20o20’ NAM DINH province THAI BINH TIEN HAI province ive Lan mouth r Vanh shoal Ngo Dong River Ba Ninh Co Giao Phong Den lat m River ou shoal Hai Loc th 20o10’ Da y mo ut h NGASON HAI HAU LEGEND Accretionary shoreline Hai Thinh Erosional shoreline Lach mouth 106o 00’ 10 10 20 km o 106 30’ Fig Location of the study area and 3.5–4.5 m, respectively, but in severe storms wave heights can reach over m (Nhuan et al., 1996) The shorelines at the major river mouths are currently expanding at a rate of about 15–100 m/year The newly formed land constitutes a useful environment for aquatic cultures and mangrove development The rapid accretion in front of the river mouths causes widespread difficulties for navigation On the other hand, sediment deficits in the adjacent areas lead to shoreline erosion The coastal erosion causes the loss of land, demolishment of infrastructure and expansion of saline intrusions The coastal zone where rapid erosion prevails is from south of the Ba Lat mouth (Giao Long) to the Hai Thinh commune (Hai Hau district, Nam Dinh province) (Fig 1) The shoreline regression in this region can reach 10–15 m/year Erosion of the shoreline also takes place north of the Van Uc river (Haiphong city) with lower intensity 234Th and 210Pb analysis on a number of boxcores and gravity cores in front of the Red River mouth indicates that the main deposition takes place south of the river mouth Toward the north the sea bed at a depth of 20–25 m is eroded during the dry season (van den Bergh et al., this volume) There is a regular southwest to southward bottom current in the study area at a depth of 15–25 m (Nhuan et al., 1996; Dankers, 2001), enabling transport of fine-grained sediments in front of the Red River mouth towards the south Detailed knowledge concerning the sediment distribution and transport pathways is very important in understanding the accretionary and erosional patterns in the coastal zone This paper presents some results of sediment distribution and transport analysis along the Ngason-Haiphong coastal zone between a depth of and 30 m, in order to make a contribution in solving questions related to forecasting shoreline changes, landuse planning and mitigation of hazards caused by erosion and accretion Methods and materials Sediment samples were collected and obtained during the national research projects on sedimentology and geoenvironment of the coastal zone of Northern Vietnam, carried out by Hanoi University of Science in 1996 and by the Vietnam – Netherlands ‘‘Red River Delta’’ joint research project in 2000 During the fieldwork of these projects, small ships were used The position of sampling stations in 1996 and 2000 was determined using a GPS with an accuracy of and 100 m, respectively In 1996, 564 surface sediment samples were taken with a grab sampler The distance between stations was 2.5 km in shallow areas ( 2.5), indicating quite unstable hydrodynamic conditions of sediment deposition The recent silt is dominated on average by 70% silt, similar as in the old sedimentary units The average contents of clay and sand are 22% and 2%, respectively D50 varies from 0.018 to 0.051 mm Five main sediment types could be distinguished, and a map of the surface sediment distribution was drawn (Fig 3) Around the 30 m isobath in the central part of the study area the sediment changes from silt to sandy silt in offshore direction The sandy silt samples in the region usually contain abundant shell fragments and have greenish grey colour To the contrary, sediment from the near coastal zone has a dominant red colour The coefficient of cation exchange (Kt = (Na+ + K+)/(Ca2+ + Mg2+)) of greenish sediments is always above and the pH varies between 7.2 and 8.0 These values are typical for a shallow marine environment and confirm the conclusions of Nghi et al (1996) that silt, sandy silt and sand along the eastern margin represent old sediment units of Early-Middle Holo2 cene ðQ1–2 IV Þ and Late Pleistocene ðQIII Þ The characteristics of the most widespread old (Q2III , 1–2 QIV ) and recent ðQ3IV Þ sediment units are shown in Tables and 3, respectively 3.1 Sand 1–2 The old units Q2III and QIV cover large sandy areas along the eastern margin of the study area (Fig 3) The old sand unit is restricted to water depths of more than 25 m Most of the old sand unit is well sorted with an average So of 1.297 Only three samples show moderate sorting (So up to 2.516) This sand unit contains between 90 and 100% sandy particles The remaining part is silt D50 is rather variable, with a maximum of 0.713 mm Recent sand is distributed along the shoreline in water depths of 3–5 m, except to the southeast of the Red River mouth, where sand extends down to the water depth of 15 m The recent sand is very well sorted (So = 1.100– 1.318) and consists on average for 98.5% of sandy and 1.5% of silty particles D50 varies between 0.085 and 0.406 mm with an average of 0.135 mm, which is finer grained than sand from the old sandy units Sand at the Den and Vanh shoals in front of the Red River mouth is characterized by very good sorting (So = 1.116–1.287) and rather uniform D50 (0.125– 0.196 mm) 3.3 Silt 3.4 Mud Recent mud covers small patches in a water depth between and 16 m The sediment is moderately to poorly sorted It consists mainly of silt (52%) and clay (44%) with minor amounts of sand (